WEBVTT 00:00:00.000 --> 00:00:00.985 align:middle line:90% 00:00:00.985 --> 00:00:01.485 align:middle line:90% [SQUEAKING] 00:00:01.485 --> 00:00:02.475 align:middle line:90% [CLICKING] 00:00:02.475 --> 00:00:04.455 align:middle line:90% [RUSTLING] 00:00:04.455 --> 00:00:11.390 align:middle line:90% [CLICKING] 00:00:11.390 --> 00:00:13.520 align:middle line:84% DAVID KAISER: Welcome to our final class session 00:00:13.520 --> 00:00:17.690 align:middle line:90% for 8.225 STS042. 00:00:17.690 --> 00:00:19.160 align:middle line:90% Kind of hard to believe-- 00:00:19.160 --> 00:00:21.530 align:middle line:90% hard for me to believe at least. 00:00:21.530 --> 00:00:24.440 align:middle line:84% But we've made it to the end of the semester, this very 00:00:24.440 --> 00:00:26.610 align:middle line:90% strange and unusual semester. 00:00:26.610 --> 00:00:28.100 align:middle line:90% So I hope you're all doing OK. 00:00:28.100 --> 00:00:32.060 align:middle line:84% It's I know it's crunch time, even more than usual, 00:00:32.060 --> 00:00:35.070 align:middle line:84% I think, with lots of final projects and end of term stuff. 00:00:35.070 --> 00:00:38.240 align:middle line:84% So I hope you're all hanging in there, doing OK. 00:00:38.240 --> 00:00:41.403 align:middle line:84% There's a little while left on your paper three 00:00:41.403 --> 00:00:43.070 align:middle line:84% assignment for this class, and hopefully 00:00:43.070 --> 00:00:44.862 align:middle line:84% the other assignments for your other things 00:00:44.862 --> 00:00:46.770 align:middle line:90% are well in hand as well. 00:00:46.770 --> 00:00:50.870 align:middle line:84% Any final logistical questions about the assignments, 00:00:50.870 --> 00:00:52.280 align:middle line:90% or really anything like that? 00:00:52.280 --> 00:00:55.510 align:middle line:90% 00:00:55.510 --> 00:00:56.910 align:middle line:90% If not, that's OK. 00:00:56.910 --> 00:00:58.140 align:middle line:90% I will jump in. 00:00:58.140 --> 00:01:02.880 align:middle line:84% I do have one set of lecture slides for today 00:01:02.880 --> 00:01:06.420 align:middle line:90% and a kind of wrap-up. 00:01:06.420 --> 00:01:09.310 align:middle line:84% We won't wrap up every last question in all of physics, 00:01:09.310 --> 00:01:10.350 align:middle line:90% I don't think. 00:01:10.350 --> 00:01:14.760 align:middle line:84% But we'll explore some ideas that kind of build 00:01:14.760 --> 00:01:16.987 align:middle line:84% on, or follow the story forward in time, 00:01:16.987 --> 00:01:19.320 align:middle line:84% compared to some of the recent things we've been looking 00:01:19.320 --> 00:01:24.270 align:middle line:84% at, especially in this relatively recent overlap 00:01:24.270 --> 00:01:26.580 align:middle line:84% space-- conceptual overlap space-- 00:01:26.580 --> 00:01:30.120 align:middle line:84% between very high-energy theory, particle physics, 00:01:30.120 --> 00:01:33.480 align:middle line:84% and studies of cosmology, this area 00:01:33.480 --> 00:01:36.210 align:middle line:84% that we now call particle cosmology. 00:01:36.210 --> 00:01:39.123 align:middle line:84% And so for today, I want to talk about some things that you 00:01:39.123 --> 00:01:40.290 align:middle line:90% might well have heard about. 00:01:40.290 --> 00:01:42.540 align:middle line:84% Some of you might have already had a chance to read up 00:01:42.540 --> 00:01:43.800 align:middle line:90% on this in some detail. 00:01:43.800 --> 00:01:45.300 align:middle line:84% But there are things that are often 00:01:45.300 --> 00:01:47.370 align:middle line:84% described in popular culture, there 00:01:47.370 --> 00:01:51.570 align:middle line:84% are ideas that are still clearly speculative. 00:01:51.570 --> 00:01:55.037 align:middle line:84% These are not proven by any possible measure, 00:01:55.037 --> 00:01:57.370 align:middle line:84% but they are, at least to some members of the community, 00:01:57.370 --> 00:01:58.152 align:middle line:90% well-motivated. 00:01:58.152 --> 00:02:00.610 align:middle line:84% There's reasons why lots of people think about these things 00:02:00.610 --> 00:02:04.600 align:middle line:84% and take them seriously, even though we, the community, 00:02:04.600 --> 00:02:08.479 align:middle line:84% certainly don't have any kind of consensus yet, 00:02:08.479 --> 00:02:13.750 align:middle line:84% let alone a lot of reliable, empirical or observational 00:02:13.750 --> 00:02:17.170 align:middle line:84% information to help us sift through competing ideas. 00:02:17.170 --> 00:02:22.510 align:middle line:84% So we're in this kind of awkward state where lots of folks 00:02:22.510 --> 00:02:25.420 align:middle line:84% are pursuing interesting and sometimes very strange-sounding 00:02:25.420 --> 00:02:28.300 align:middle line:84% questions, so they get actually stranger and stranger sounding, 00:02:28.300 --> 00:02:29.200 align:middle line:90% I think. 00:02:29.200 --> 00:02:32.920 align:middle line:84% And so I wanted to end our semester this term 00:02:32.920 --> 00:02:36.737 align:middle line:84% with this taste of some small set of the open questions 00:02:36.737 --> 00:02:39.070 align:middle line:84% that members of the community are still really wrestling 00:02:39.070 --> 00:02:40.370 align:middle line:90% with and grappling with today. 00:02:40.370 --> 00:02:43.377 align:middle line:84% So I'll focus on ideas about string theory-- 00:02:43.377 --> 00:02:44.710 align:middle line:90% some ideas about string theory-- 00:02:44.710 --> 00:02:46.270 align:middle line:90% and the multiverse. 00:02:46.270 --> 00:02:48.940 align:middle line:84% And there's lots and lots and lots written about this, often 00:02:48.940 --> 00:02:51.460 align:middle line:90% for a non-technical audience. 00:02:51.460 --> 00:02:54.960 align:middle line:84% And part of also what I wanted to do with this class was just 00:02:54.960 --> 00:02:57.960 align:middle line:84% give you some pointers for some books that I really 00:02:57.960 --> 00:03:01.320 align:middle line:84% like, some accessible, really very well-written 00:03:01.320 --> 00:03:05.610 align:middle line:84% popular books, by physicists with a range of opinions 00:03:05.610 --> 00:03:06.460 align:middle line:90% about this stuff. 00:03:06.460 --> 00:03:10.140 align:middle line:84% And so some of them are written by arch rivals conceptually, 00:03:10.140 --> 00:03:12.270 align:middle line:84% and they, together, I think can help 00:03:12.270 --> 00:03:13.830 align:middle line:84% us get a reasonable sense of at least 00:03:13.830 --> 00:03:16.980 align:middle line:84% what seems to be at stake, what are some of our colleagues so 00:03:16.980 --> 00:03:18.660 align:middle line:84% exercised about, and why do they have 00:03:18.660 --> 00:03:21.630 align:middle line:84% competing visions for next big steps 00:03:21.630 --> 00:03:25.650 align:middle line:84% forward on this intellectual journey we've been marching 00:03:25.650 --> 00:03:27.480 align:middle line:90% along together this term. 00:03:27.480 --> 00:03:29.952 align:middle line:84% Not all of it is in the Complete Idiot's Guide, 00:03:29.952 --> 00:03:31.410 align:middle line:84% though I do actually like this book 00:03:31.410 --> 00:03:32.702 align:middle line:90% by my colleague, George Musser. 00:03:32.702 --> 00:03:34.890 align:middle line:84% He's an award-winning science writer 00:03:34.890 --> 00:03:37.320 align:middle line:90% and a PhD in astrophysics. 00:03:37.320 --> 00:03:39.565 align:middle line:84% He actually is, himself, not an idiot, 00:03:39.565 --> 00:03:41.940 align:middle line:84% but he wrote this fun book for the Complete Idiot's Guide 00:03:41.940 --> 00:03:44.260 align:middle line:84% series on string theory, and of course, other books as well. 00:03:44.260 --> 00:03:46.052 align:middle line:84% So partly what I'll do with these slides is 00:03:46.052 --> 00:03:48.547 align:middle line:84% give a little shout-outs with some book covers for some 00:03:48.547 --> 00:03:50.130 align:middle line:84% of these books that I enjoy, you might 00:03:50.130 --> 00:03:53.490 align:middle line:84% enjoy, after the dust has settled on this crazy semester, 00:03:53.490 --> 00:03:57.440 align:middle line:84% if you're looking for some virtual beach reading for IAP. 00:03:57.440 --> 00:03:59.200 align:middle line:90% These might be of interest. 00:03:59.200 --> 00:04:01.500 align:middle line:84% So as usual, of course, three main parts 00:04:01.500 --> 00:04:04.560 align:middle line:90% for the discussion today. 00:04:04.560 --> 00:04:06.780 align:middle line:84% I want to turn the clock back and look 00:04:06.780 --> 00:04:11.340 align:middle line:84% at this really 100-year long quest, 100-year long challenge, 00:04:11.340 --> 00:04:14.880 align:middle line:84% to try to find a quantum mechanical description 00:04:14.880 --> 00:04:15.840 align:middle line:90% of gravity. 00:04:15.840 --> 00:04:17.339 align:middle line:90% It's still an elusive challenge. 00:04:17.339 --> 00:04:20.190 align:middle line:84% And we'll see what were some of the earliest ideas about that 00:04:20.190 --> 00:04:24.570 align:middle line:84% and why has this approach taken such a different turn, compared 00:04:24.570 --> 00:04:28.560 align:middle line:84% to other efforts to describe fundamental forces of nature 00:04:28.560 --> 00:04:29.520 align:middle line:90% quantum mechanically. 00:04:29.520 --> 00:04:32.987 align:middle line:84% So we'll look at, in some sense, where string theory comes from, 00:04:32.987 --> 00:04:35.070 align:middle line:84% and why some people have been so excited about it, 00:04:35.070 --> 00:04:37.103 align:middle line:90% compared to other approaches. 00:04:37.103 --> 00:04:39.270 align:middle line:84% Then in the second part, we'll look again, just very 00:04:39.270 --> 00:04:42.690 align:middle line:84% briefly, at some of the collisions, some 00:04:42.690 --> 00:04:46.170 align:middle line:84% of the possible implications of thinking about these string 00:04:46.170 --> 00:04:49.710 align:middle line:84% theory ideas in the context of cosmology, 00:04:49.710 --> 00:04:53.230 align:middle line:84% getting a little closer to work that I've been involved with, 00:04:53.230 --> 00:04:55.480 align:middle line:84% or some of my colleagues have been more involved with. 00:04:55.480 --> 00:04:59.080 align:middle line:84% So when we take this kind of particle cosmology view, 00:04:59.080 --> 00:05:01.330 align:middle line:84% and take on board, or try to take seriously, 00:05:01.330 --> 00:05:04.960 align:middle line:84% some of the ideas or possible implications of the high energy 00:05:04.960 --> 00:05:06.880 align:middle line:84% theory part-- the string theory part-- 00:05:06.880 --> 00:05:09.070 align:middle line:84% what does that lead us, at least lead some people, 00:05:09.070 --> 00:05:11.500 align:middle line:84% to wonder about, in this more cosmological setting. 00:05:11.500 --> 00:05:12.370 align:middle line:90% That's part two. 00:05:12.370 --> 00:05:15.190 align:middle line:84% And then part three, just very briefly, we'll 00:05:15.190 --> 00:05:17.092 align:middle line:84% zoom out and again, remind ourselves 00:05:17.092 --> 00:05:18.550 align:middle line:84% that like all the work we've looked 00:05:18.550 --> 00:05:21.880 align:middle line:84% at together over this entire semester, none of these ideas 00:05:21.880 --> 00:05:24.310 align:middle line:84% are unfolding in a vacuum, that I 00:05:24.310 --> 00:05:27.610 align:middle line:84% think we can make sense of some of the rhythms of change, 00:05:27.610 --> 00:05:29.800 align:middle line:84% if not the particular ideas that come forward, 00:05:29.800 --> 00:05:32.830 align:middle line:84% by asking about who's doing this work in what settings, 00:05:32.830 --> 00:05:37.150 align:middle line:84% and some elements of a larger context that might help us make 00:05:37.150 --> 00:05:39.070 align:middle line:90% sense of the twists and turns. 00:05:39.070 --> 00:05:39.730 align:middle line:90% OK. 00:05:39.730 --> 00:05:42.820 align:middle line:84% So as I mentioned this century-long challenge, 00:05:42.820 --> 00:05:45.070 align:middle line:84% a kind of grand challenge for the field, 00:05:45.070 --> 00:05:48.310 align:middle line:84% has been to find some internally, self-consistent 00:05:48.310 --> 00:05:50.740 align:middle line:84% description of gravity that would 00:05:50.740 --> 00:05:53.860 align:middle line:84% be in the framework of quantum theory. 00:05:53.860 --> 00:05:56.840 align:middle line:84% We've seen, at least in some we looked at in some detail, 00:05:56.840 --> 00:06:00.490 align:middle line:84% others we had kind of hints at, that over the mostly second 00:06:00.490 --> 00:06:03.070 align:middle line:84% half of the 20th century especially, 00:06:03.070 --> 00:06:06.400 align:middle line:84% each of the other three main known physical forces 00:06:06.400 --> 00:06:09.730 align:middle line:84% of nature-- electromagnetism, the strong nuclear force, 00:06:09.730 --> 00:06:11.350 align:middle line:90% and the weak nuclear force-- 00:06:11.350 --> 00:06:15.657 align:middle line:84% each of those has been given a quantitative treatment 00:06:15.657 --> 00:06:17.740 align:middle line:84% in terms of quantum theory, in terms in particular 00:06:17.740 --> 00:06:19.120 align:middle line:90% of quantum field theory. 00:06:19.120 --> 00:06:22.210 align:middle line:84% We looked a little bit at quantum electrodynamics, 00:06:22.210 --> 00:06:25.390 align:middle line:84% the quantum mechanical version of electromagnetism, 00:06:25.390 --> 00:06:29.200 align:middle line:84% associated with people like Tomonaga and Julian Schwinger 00:06:29.200 --> 00:06:31.250 align:middle line:90% and Feynman and others. 00:06:31.250 --> 00:06:33.500 align:middle line:84% Likewise, we looked briefly at quantum chromodynamics. 00:06:33.500 --> 00:06:35.920 align:middle line:84% There's been this remarkable success, often 00:06:35.920 --> 00:06:41.620 align:middle line:84% in very close dialogue, between experimental inputs 00:06:41.620 --> 00:06:43.210 align:middle line:90% and theoretical advances. 00:06:43.210 --> 00:06:46.300 align:middle line:84% And yet gravity, the fourth known force of nature, 00:06:46.300 --> 00:06:48.560 align:middle line:90% has been this stubborn holdout. 00:06:48.560 --> 00:06:50.663 align:middle line:84% Turns out this has been going on for-- the effort 00:06:50.663 --> 00:06:52.330 align:middle line:84% has been going on for a long, long time. 00:06:52.330 --> 00:06:54.590 align:middle line:84% One of the earliest people to try 00:06:54.590 --> 00:06:56.660 align:middle line:84% to build a quantum theory of gravity 00:06:56.660 --> 00:06:59.630 align:middle line:84% is this young person shown here, Matvei Bronstein, 00:06:59.630 --> 00:07:03.000 align:middle line:84% who was working in the Soviet Union in the mid 1930s. 00:07:03.000 --> 00:07:05.510 align:middle line:84% He was quite young, as you can see in the photo. 00:07:05.510 --> 00:07:07.490 align:middle line:84% Bronstein was writing papers, mostly 00:07:07.490 --> 00:07:09.710 align:middle line:84% in Russian in some of the Russian journals-- 00:07:09.710 --> 00:07:11.150 align:middle line:90% a few of them in German. 00:07:11.150 --> 00:07:13.820 align:middle line:84% Not very well known outside of his immediate circle 00:07:13.820 --> 00:07:16.490 align:middle line:84% at the time, but he was doing what, in hindsight, 00:07:16.490 --> 00:07:18.740 align:middle line:84% we see was really pretty advanced stuff 00:07:18.740 --> 00:07:22.160 align:middle line:84% and that has held up, I think, quite well over time, 00:07:22.160 --> 00:07:24.200 align:middle line:84% where he recognized that if one tried 00:07:24.200 --> 00:07:27.200 align:middle line:84% to make a quantum mechanical treatment of Einstein's 00:07:27.200 --> 00:07:29.150 align:middle line:84% general theory of relativity, the reigning 00:07:29.150 --> 00:07:32.210 align:middle line:84% description of gravity, then you could 00:07:32.210 --> 00:07:36.110 align:middle line:84% try to formulate it on a model of exchange 00:07:36.110 --> 00:07:37.670 align:middle line:90% of virtual particles. 00:07:37.670 --> 00:07:39.170 align:middle line:84% The virtual particles, in this case, 00:07:39.170 --> 00:07:40.670 align:middle line:84% would have a little more complicated 00:07:40.670 --> 00:07:41.930 align:middle line:90% mathematical structure. 00:07:41.930 --> 00:07:43.460 align:middle line:84% That might not surprise us, given 00:07:43.460 --> 00:07:46.310 align:middle line:84% how much we saw even Einstein's own version 00:07:46.310 --> 00:07:48.950 align:middle line:84% of general relativity was so mathematically involved, 00:07:48.950 --> 00:07:50.960 align:middle line:90% this warping spacetime. 00:07:50.960 --> 00:07:52.790 align:middle line:84% Bronstein showed that you could actually 00:07:52.790 --> 00:07:55.940 align:middle line:84% reproduce the basic structure of general relativity 00:07:55.940 --> 00:08:01.010 align:middle line:84% as arising from the exchange of a certain kind of particle, 00:08:01.010 --> 00:08:03.110 align:middle line:84% a certain kind of force-carrying particle. 00:08:03.110 --> 00:08:06.260 align:middle line:84% Instead of the photon being the force-carrying particle 00:08:06.260 --> 00:08:09.740 align:middle line:84% for a quantum mechanical version of electromagnetism-- 00:08:09.740 --> 00:08:11.720 align:middle line:84% for quantum electrodynamics-- there 00:08:11.720 --> 00:08:14.930 align:middle line:84% would be a hypothetical graviton, a particle 00:08:14.930 --> 00:08:17.540 align:middle line:84% force-carrier for gravity, and it 00:08:17.540 --> 00:08:19.760 align:middle line:84% would be kind of analogous to the photon. 00:08:19.760 --> 00:08:22.820 align:middle line:84% It would have zero mass, just as the photon has zero mass. 00:08:22.820 --> 00:08:25.250 align:middle line:84% But it would have two units of spin, 00:08:25.250 --> 00:08:27.650 align:middle line:84% whereas the photon has one unit of spin, 00:08:27.650 --> 00:08:30.470 align:middle line:84% and most ordinary matter has half integer unit 00:08:30.470 --> 00:08:32.840 align:middle line:84% of spin, like electrons and quarks, or even protons 00:08:32.840 --> 00:08:33.659 align:middle line:90% and neutrons. 00:08:33.659 --> 00:08:38.210 align:middle line:84% So one could try to reformulate Einstein's version of gravity 00:08:38.210 --> 00:08:40.880 align:middle line:84% as arising from the exchange of a particular kind 00:08:40.880 --> 00:08:44.270 align:middle line:84% of force-carrying particle, a massless spin-2 particle, 00:08:44.270 --> 00:08:45.920 align:middle line:90% dubbed the graviton. 00:08:45.920 --> 00:08:48.200 align:middle line:84% That was done mostly in obscurity. 00:08:48.200 --> 00:08:49.970 align:middle line:84% And quite tragically, Bronstein was 00:08:49.970 --> 00:08:51.983 align:middle line:90% murdered about a year later. 00:08:51.983 --> 00:08:54.650 align:middle line:84% He never had a chance to explore this, or frankly anything else. 00:08:54.650 --> 00:08:59.960 align:middle line:84% He fell afoul, as many, many people did, of Joseph Stalin's 00:08:59.960 --> 00:09:01.370 align:middle line:90% purges in the Soviet Union. 00:09:01.370 --> 00:09:05.978 align:middle line:84% He had some political ideas that suddenly put him on the outs 00:09:05.978 --> 00:09:07.520 align:middle line:84% with the reigning authorities, and he 00:09:07.520 --> 00:09:11.480 align:middle line:84% was rounded up, found guilty on a so-called show trial, 00:09:11.480 --> 00:09:13.070 align:middle line:90% and executed the very same day. 00:09:13.070 --> 00:09:14.930 align:middle line:90% There's no chance for an appeal. 00:09:14.930 --> 00:09:18.740 align:middle line:84% His quest, in many ways, ended tragically, tragically early. 00:09:18.740 --> 00:09:20.880 align:middle line:84% Many, many years later, other colleagues, 00:09:20.880 --> 00:09:23.270 align:middle line:84% including many in Western Europe and North America, 00:09:23.270 --> 00:09:26.060 align:middle line:84% kind of rediscovered insights that Bronstein had put 00:09:26.060 --> 00:09:28.760 align:middle line:90% together way back in the 1930s. 00:09:28.760 --> 00:09:31.040 align:middle line:84% Much more famously, much, much better known, 00:09:31.040 --> 00:09:33.980 align:middle line:84% especially in the US and Europe, Richard Feynman 00:09:33.980 --> 00:09:36.260 align:middle line:84% followed along very similar thought paths 00:09:36.260 --> 00:09:39.560 align:middle line:84% in the early 1960s, when he himself got interested 00:09:39.560 --> 00:09:41.570 align:middle line:84% in gravity, not only particle physics. 00:09:41.570 --> 00:09:44.360 align:middle line:84% And Feynman also gave these now famous lectures 00:09:44.360 --> 00:09:46.188 align:middle line:84% that were published many years later, 00:09:46.188 --> 00:09:48.230 align:middle line:84% his lectures on gravitation, where he also taught 00:09:48.230 --> 00:09:50.270 align:middle line:84% his students that one could capture 00:09:50.270 --> 00:09:52.700 align:middle line:84% the mathematical structure of general relativity 00:09:52.700 --> 00:09:55.130 align:middle line:84% as arising from the exchange of these particular kinds 00:09:55.130 --> 00:09:56.390 align:middle line:90% of force-carrying particles. 00:09:56.390 --> 00:09:57.343 align:middle line:90% It's just classic. 00:09:57.343 --> 00:09:59.510 align:middle line:84% Now you might be saying, well, don't we already know 00:09:59.510 --> 00:10:00.560 align:middle line:90% that gravitons exist? 00:10:00.560 --> 00:10:02.930 align:middle line:84% After all, our very good friends at LIGO 00:10:02.930 --> 00:10:05.270 align:middle line:84% have found gravitational radiation. 00:10:05.270 --> 00:10:09.260 align:middle line:84% They certainly have, using these enormous detectors in Hanford, 00:10:09.260 --> 00:10:11.960 align:middle line:84% Washington, in Louisiana, now similar devices 00:10:11.960 --> 00:10:14.840 align:middle line:84% throughout Europe, and others under construction elsewhere. 00:10:14.840 --> 00:10:18.320 align:middle line:84% But this is really finding classical gravitational 00:10:18.320 --> 00:10:18.830 align:middle line:90% radiation. 00:10:18.830 --> 00:10:21.050 align:middle line:84% It is not identifying individual gravitons. 00:10:21.050 --> 00:10:24.980 align:middle line:84% It's like the difference between finding classical Maxwell 00:10:24.980 --> 00:10:27.890 align:middle line:84% waves, like radial waves, electromagnetic waves that 00:10:27.890 --> 00:10:29.960 align:middle line:84% are continuous and classical and extended 00:10:29.960 --> 00:10:32.060 align:middle line:90% through space and time. 00:10:32.060 --> 00:10:35.150 align:middle line:84% It's a similar gulf between that and finding evidence 00:10:35.150 --> 00:10:38.210 align:middle line:84% of individual photons, individual quantized 00:10:38.210 --> 00:10:40.920 align:middle line:84% force-carriers of a quantum mechanical force. 00:10:40.920 --> 00:10:42.440 align:middle line:90% So whereas now we have-- 00:10:42.440 --> 00:10:45.800 align:middle line:84% thank goodness-- really quite compelling evidence 00:10:45.800 --> 00:10:48.890 align:middle line:84% about classical, extended, wave-like features 00:10:48.890 --> 00:10:50.870 align:middle line:84% from gravity, this still is not evidence 00:10:50.870 --> 00:10:52.010 align:middle line:90% of individual gravitons. 00:10:52.010 --> 00:10:54.760 align:middle line:84% A challenge on that score still elusive. 00:10:54.760 --> 00:10:57.940 align:middle line:84% Now even though there's not compelling experimental inputs 00:10:57.940 --> 00:11:00.100 align:middle line:84% about the behavior of gravitons, one could still-- 00:11:00.100 --> 00:11:02.110 align:middle line:84% and people have over the century-- 00:11:02.110 --> 00:11:05.860 align:middle line:84% try to build a kind of quantum mechanical treatment 00:11:05.860 --> 00:11:08.080 align:middle line:84% of these hypothetical force-carriers-- 00:11:08.080 --> 00:11:10.690 align:middle line:84% these gravitons, massless spin-2 particles. 00:11:10.690 --> 00:11:13.930 align:middle line:84% And that's the kind of work that Bronstein began all the way 00:11:13.930 --> 00:11:16.120 align:middle line:84% back in the mid 1930s, and people like Feynman 00:11:16.120 --> 00:11:18.340 align:middle line:84% and many of his students tried to do head 00:11:18.340 --> 00:11:20.450 align:middle line:84% on throughout the 1960s, and so on. 00:11:20.450 --> 00:11:25.570 align:middle line:84% So you can basically say there's a kind of gravitational field, 00:11:25.570 --> 00:11:27.390 align:middle line:84% a kind of warping spacetime of the sort 00:11:27.390 --> 00:11:28.690 align:middle line:90% that Einstein had described. 00:11:28.690 --> 00:11:31.270 align:middle line:84% You can imagine the little perturbations 00:11:31.270 --> 00:11:33.190 align:middle line:90% around some average value. 00:11:33.190 --> 00:11:36.280 align:middle line:84% You can try to treat that as some quantum mechanical object, 00:11:36.280 --> 00:11:40.270 align:middle line:84% all in direct analogy to the treatment of, say, the Maxwell 00:11:40.270 --> 00:11:42.430 align:middle line:90% field as composed of photons. 00:11:42.430 --> 00:11:44.950 align:middle line:84% But unfortunately, as people like Bronstein 00:11:44.950 --> 00:11:49.720 align:middle line:84% were finding early on, this does not lead to a clear way 00:11:49.720 --> 00:11:50.290 align:middle line:90% forward. 00:11:50.290 --> 00:11:52.150 align:middle line:84% This approach leads to infinities, 00:11:52.150 --> 00:11:54.980 align:middle line:84% when you try to account quantitatively 00:11:54.980 --> 00:11:56.897 align:middle line:84% for the behavior of virtual gravitons, 00:11:56.897 --> 00:11:58.730 align:middle line:84% much as one would do with, say, the exchange 00:11:58.730 --> 00:12:00.020 align:middle line:90% of virtual photons. 00:12:00.020 --> 00:12:03.770 align:middle line:84% But unlike the case of QED, or as people now know, 00:12:03.770 --> 00:12:07.370 align:middle line:84% even unlike the case of quantum chromodynamics or the others, 00:12:07.370 --> 00:12:11.330 align:middle line:84% there's no way to get rid of or to self-consistently 00:12:11.330 --> 00:12:13.185 align:middle line:90% absorb these infinities. 00:12:13.185 --> 00:12:15.560 align:middle line:84% And we can get a sense for that by going all the way back 00:12:15.560 --> 00:12:17.393 align:middle line:84% to one of the very first lectures this term. 00:12:17.393 --> 00:12:19.910 align:middle line:84% Even classically, we can get a sense, at least, 00:12:19.910 --> 00:12:23.210 align:middle line:84% for why there might be a disanalogy between, say, 00:12:23.210 --> 00:12:25.550 align:middle line:84% electromagnetism and gravity, when we try to treat 00:12:25.550 --> 00:12:27.780 align:middle line:90% them each quantum mechanically. 00:12:27.780 --> 00:12:29.780 align:middle line:84% We go back to things that some of our favorite 00:12:29.780 --> 00:12:31.730 align:middle line:84% wranglers-- my favorite wranglers were super 00:12:31.730 --> 00:12:34.100 align:middle line:84% excited about back in the 1840s and 50s, 00:12:34.100 --> 00:12:36.710 align:middle line:84% people like William Thompson and James Clerk Maxwell. 00:12:36.710 --> 00:12:39.470 align:middle line:84% They were working out this electromagnetic-- or sorry-- 00:12:39.470 --> 00:12:41.450 align:middle line:84% this mechanical worldview, where they 00:12:41.450 --> 00:12:44.960 align:middle line:84% found these mathematical analogies between, say, 00:12:44.960 --> 00:12:47.540 align:middle line:84% electrostatics and Newtonian gravity. 00:12:47.540 --> 00:12:48.980 align:middle line:84% They can each be written in terms 00:12:48.980 --> 00:12:53.150 align:middle line:84% of some potential, some function that extends throughout space 00:12:53.150 --> 00:12:54.530 align:middle line:90% and changes over time. 00:12:54.530 --> 00:12:58.400 align:middle line:84% In the electromagnetic case, the electromagnetic potential 00:12:58.400 --> 00:13:01.550 align:middle line:84% could be understood as responding to, as we'd say, 00:13:01.550 --> 00:13:05.480 align:middle line:84% sourced by, where the electric charges are. 00:13:05.480 --> 00:13:08.840 align:middle line:84% So we could look at the variation, 00:13:08.840 --> 00:13:13.370 align:middle line:84% the gradient squared, of the electric potential 00:13:13.370 --> 00:13:18.390 align:middle line:84% as arising from the charge density, the charge per volume. 00:13:18.390 --> 00:13:21.115 align:middle line:84% And if we just do some dimensional analysis-- 00:13:21.115 --> 00:13:23.240 align:middle line:84% just some quick dimensions without trying to really 00:13:23.240 --> 00:13:24.810 align:middle line:90% calculate things carefully-- 00:13:24.810 --> 00:13:27.680 align:middle line:84% we can see that the charge density goes 00:13:27.680 --> 00:13:30.270 align:middle line:84% like the electric charge per volume, 00:13:30.270 --> 00:13:31.910 align:middle line:84% so it's like some charge, whether it's 00:13:31.910 --> 00:13:34.880 align:middle line:84% the charge electron or some larger collection of charges, 00:13:34.880 --> 00:13:36.560 align:middle line:90% divided by some volume. 00:13:36.560 --> 00:13:39.890 align:middle line:84% Also remember, as the wranglers certainly well knew, 00:13:39.890 --> 00:13:43.130 align:middle line:84% that this all important quantity-- the wave number-- 00:13:43.130 --> 00:13:46.760 align:middle line:84% goes inversely like some characteristic length. 00:13:46.760 --> 00:13:49.880 align:middle line:84% So wave number is inverse to wavelength. 00:13:49.880 --> 00:13:53.300 align:middle line:84% And so the charge density in appropriate units 00:13:53.300 --> 00:13:55.670 align:middle line:90% goes like the wave number cubed. 00:13:55.670 --> 00:13:58.700 align:middle line:84% As I mentioned at least briefly some lectures ago, 00:13:58.700 --> 00:14:00.410 align:middle line:84% the electric charge, if we choose 00:14:00.410 --> 00:14:03.710 align:middle line:84% to measure it in so-called natural or appropriate units, 00:14:03.710 --> 00:14:05.720 align:middle line:84% the electric charge is dimensionless. 00:14:05.720 --> 00:14:09.620 align:middle line:84% In fact, it's the charge squared goes like 1 over 137. 00:14:09.620 --> 00:14:11.850 align:middle line:84% It has a very particular-- just a number. 00:14:11.850 --> 00:14:13.500 align:middle line:84% It's not a dimensionful quantity. 00:14:13.500 --> 00:14:17.330 align:middle line:84% So dimensionally, the charge density 00:14:17.330 --> 00:14:20.780 align:middle line:84% to which the electrostatic potential is sensitive 00:14:20.780 --> 00:14:23.640 align:middle line:84% scales dimensionally like the cube of the wave number. 00:14:23.640 --> 00:14:25.250 align:middle line:84% Now let's try to follow that analogy 00:14:25.250 --> 00:14:26.700 align:middle line:90% for the gravitational side. 00:14:26.700 --> 00:14:29.780 align:middle line:84% Even for Newtonian gravity, the very similar scaling 00:14:29.780 --> 00:14:32.960 align:middle line:84% holds even for Einstein's more complicated version. 00:14:32.960 --> 00:14:36.170 align:middle line:84% Remember that the gravitational potential 00:14:36.170 --> 00:14:38.480 align:middle line:84% is sourced not by the electric charges, 00:14:38.480 --> 00:14:41.000 align:middle line:84% but by masses, or eventually in Einstein's version, 00:14:41.000 --> 00:14:42.300 align:middle line:90% by mass energy. 00:14:42.300 --> 00:14:44.930 align:middle line:84% So we have to worry about the dimensions of this source 00:14:44.930 --> 00:14:46.790 align:middle line:90% over here, the mass density. 00:14:46.790 --> 00:14:49.040 align:middle line:84% Well, that goes by some characteristic mass 00:14:49.040 --> 00:14:50.570 align:middle line:90% divided by volume. 00:14:50.570 --> 00:14:51.740 align:middle line:90% It's a density. 00:14:51.740 --> 00:14:53.630 align:middle line:84% The volume factor we just convinced 00:14:53.630 --> 00:14:56.210 align:middle line:84% ourselves scales like the cube of some wave number, 00:14:56.210 --> 00:14:58.220 align:middle line:84% but the mass, unlike the electric charge, 00:14:58.220 --> 00:15:00.170 align:middle line:90% the mass is also dimensionful. 00:15:00.170 --> 00:15:02.660 align:middle line:84% And so something that was not known to either William 00:15:02.660 --> 00:15:06.080 align:middle line:84% Thompson or James Clark Maxwell, but has become really 00:15:06.080 --> 00:15:09.650 align:middle line:84% clear to theorists over the course of the 20th century, 00:15:09.650 --> 00:15:12.170 align:middle line:84% is that dimensionful quantity mass actually 00:15:12.170 --> 00:15:13.670 align:middle line:90% matters quite a lot. 00:15:13.670 --> 00:15:16.910 align:middle line:84% Thinking just from relativity, we saw, thanks to Einstein, 00:15:16.910 --> 00:15:19.370 align:middle line:84% that energy, mass, and momentum are all 00:15:19.370 --> 00:15:21.120 align:middle line:90% basically interchangeable. 00:15:21.120 --> 00:15:23.600 align:middle line:84% The famous expression E equals MC squared, 00:15:23.600 --> 00:15:26.150 align:middle line:84% we saw briefly, is really more generally 00:15:26.150 --> 00:15:29.630 align:middle line:84% a relation between energy, mass, and momentum, and they're 00:15:29.630 --> 00:15:33.170 align:middle line:84% all interchangeable with some kind of change of units, 00:15:33.170 --> 00:15:35.220 align:middle line:90% given by the speed of light. 00:15:35.220 --> 00:15:36.800 align:middle line:84% But if we measure these quantities 00:15:36.800 --> 00:15:39.650 align:middle line:84% in appropriate units, they're all dimensionful-- energy, 00:15:39.650 --> 00:15:41.990 align:middle line:84% mass, and momentum-- and in fact, they're all really 00:15:41.990 --> 00:15:43.280 align:middle line:90% kind of interchangeable. 00:15:43.280 --> 00:15:47.000 align:middle line:84% There's not a sharp conceptual divide anymore, 00:15:47.000 --> 00:15:49.995 align:middle line:84% in relativity, between mass, energy, and momentum. 00:15:49.995 --> 00:15:51.120 align:middle line:90% That's the relativity side. 00:15:51.120 --> 00:15:53.520 align:middle line:84% Then from quantum theory, we saw, thanks to people 00:15:53.520 --> 00:15:57.240 align:middle line:84% like Louis de Broglie, and built into the Schrodinger equation 00:15:57.240 --> 00:16:00.390 align:middle line:84% too much else, that momentum and wave number themselves 00:16:00.390 --> 00:16:01.590 align:middle line:90% are directly related. 00:16:01.590 --> 00:16:04.950 align:middle line:84% In fact, they're related by a different constant of nature-- 00:16:04.950 --> 00:16:05.980 align:middle line:90% Planck's constant. 00:16:05.980 --> 00:16:08.070 align:middle line:84% So if we go back and find a scaling-- 00:16:08.070 --> 00:16:10.530 align:middle line:84% even classically for Newtonian gravity, 00:16:10.530 --> 00:16:14.490 align:middle line:84% that the source for these gravitational disturbances, 00:16:14.490 --> 00:16:17.640 align:middle line:84% which we might choose to quantize-- if they're sourced 00:16:17.640 --> 00:16:19.890 align:middle line:84% by a mass density, that actually has 00:16:19.890 --> 00:16:22.260 align:middle line:84% a different scaling with wave number, 00:16:22.260 --> 00:16:25.990 align:middle line:84% and therefore with momentum, than in the electrostatic case. 00:16:25.990 --> 00:16:29.850 align:middle line:84% There's a disanalogy between, say, quantum electrodynamics 00:16:29.850 --> 00:16:32.070 align:middle line:84% and a putative quantum theory of gravity, 00:16:32.070 --> 00:16:34.410 align:middle line:84% that has everything to do with insights from relativity 00:16:34.410 --> 00:16:35.460 align:middle line:90% and quantum theory-- 00:16:35.460 --> 00:16:38.170 align:middle line:84% E equals MC squared and the de Broglie wavelength. 00:16:38.170 --> 00:16:40.770 align:middle line:84% And so what that means, when you try to just follow 00:16:40.770 --> 00:16:44.190 align:middle line:84% an analogy to these otherwise very successful quantum field 00:16:44.190 --> 00:16:47.370 align:middle line:84% theories, is that those divergences become 00:16:47.370 --> 00:16:50.970 align:middle line:84% more severe in quantum gravity than any of the analogous 00:16:50.970 --> 00:16:52.710 align:middle line:90% ones for the other forces. 00:16:52.710 --> 00:16:55.200 align:middle line:84% Because gravity is sourced by mass, 00:16:55.200 --> 00:16:57.060 align:middle line:84% this dimensionful quantity, which itself 00:16:57.060 --> 00:17:00.090 align:middle line:84% is akin to a kind of momentum, the integrals-- 00:17:00.090 --> 00:17:05.190 align:middle line:84% when we try to count up the effects of virtual gravitons-- 00:17:05.190 --> 00:17:07.740 align:middle line:84% the integrals diverge with momentum more 00:17:07.740 --> 00:17:11.560 align:middle line:84% violently than even the ones do in for the other forces. 00:17:11.560 --> 00:17:14.160 align:middle line:84% And it really comes down to these very elementary notions 00:17:14.160 --> 00:17:15.900 align:middle line:84% from both relativity and quantum theory. 00:17:15.900 --> 00:17:17.829 align:middle line:90% The scaling is different. 00:17:17.829 --> 00:17:20.530 align:middle line:84% What does that mean in more in slightly more practical terms? 00:17:20.530 --> 00:17:24.150 align:middle line:84% We saw-- again, going back to these insights from Sin-Itiro 00:17:24.150 --> 00:17:28.440 align:middle line:84% Tomonaga, during the war in Tokyo and then independently 00:17:28.440 --> 00:17:32.520 align:middle line:84% rediscovered soon after the Second World War by people like 00:17:32.520 --> 00:17:35.430 align:middle line:84% Julian Schwinger and Richard Feynman-- 00:17:35.430 --> 00:17:37.380 align:middle line:84% that for quantum electrodynamics, 00:17:37.380 --> 00:17:40.470 align:middle line:84% they could always arrange their equations so that they never 00:17:40.470 --> 00:17:43.260 align:middle line:90% had a bare infinity on its own. 00:17:43.260 --> 00:17:46.290 align:middle line:84% They could always organize things as these input output 00:17:46.290 --> 00:17:47.940 align:middle line:84% relations-- this was the heart of what 00:17:47.940 --> 00:17:49.680 align:middle line:84% became known as renormalization-- 00:17:49.680 --> 00:17:52.440 align:middle line:84% and that you could always have these combinations which 00:17:52.440 --> 00:17:55.170 align:middle line:84% remained finite, even though each of these, on their own, 00:17:55.170 --> 00:17:58.810 align:middle line:84% would diverge usually logarithmically with momentum. 00:17:58.810 --> 00:18:02.040 align:middle line:84% So each of these, if you try to calculate them on their own, 00:18:02.040 --> 00:18:04.170 align:middle line:84% would be formally infinite, but their sum 00:18:04.170 --> 00:18:06.150 align:middle line:90% was well-defined and finite. 00:18:06.150 --> 00:18:09.370 align:middle line:84% And what people like Freeman Dyson and others showed 00:18:09.370 --> 00:18:12.640 align:middle line:84% is it that trick holds, order by order, in perturbation theory. 00:18:12.640 --> 00:18:15.070 align:middle line:84% You can make arbitrarily precise calculations 00:18:15.070 --> 00:18:18.430 align:middle line:84% and these calculations hold all the way through. 00:18:18.430 --> 00:18:20.230 align:middle line:84% What it means for quantum gravity, 00:18:20.230 --> 00:18:22.300 align:middle line:84% to have these more violent divergences, 00:18:22.300 --> 00:18:25.120 align:middle line:84% the stronger growth with momentum 00:18:25.120 --> 00:18:27.490 align:middle line:84% for these virtual graviton processes 00:18:27.490 --> 00:18:30.340 align:middle line:84% is that you need actually an infinite set 00:18:30.340 --> 00:18:34.120 align:middle line:84% of these absorbing coefficients, that you can't arrange things 00:18:34.120 --> 00:18:35.920 align:middle line:90% just in the input output way. 00:18:35.920 --> 00:18:39.940 align:middle line:84% You never have a finite number of these absorbing infinities. 00:18:39.940 --> 00:18:42.070 align:middle line:84% So you can never absorb or cancel 00:18:42.070 --> 00:18:46.540 align:middle line:84% the infinite divergences, at least from this first approach 00:18:46.540 --> 00:18:47.780 align:middle line:90% to quantum gravity. 00:18:47.780 --> 00:18:50.600 align:middle line:90% So this leads to a hard problem. 00:18:50.600 --> 00:18:53.350 align:middle line:84% And one of the first books that I'll recommend today 00:18:53.350 --> 00:18:56.590 align:middle line:84% is this really lovely book that came out 20 years ago 00:18:56.590 --> 00:18:58.810 align:middle line:90% by physicist Lee Smolin. 00:18:58.810 --> 00:19:01.480 align:middle line:84% It's, I think, a very accessible, very nice book. 00:19:01.480 --> 00:19:04.360 align:middle line:84% He is a leading theoretical physicist. 00:19:04.360 --> 00:19:06.190 align:middle line:90% He's also a very gifted author. 00:19:06.190 --> 00:19:09.140 align:middle line:84% This book is called The Three Roads to Quantum Gravity. 00:19:09.140 --> 00:19:11.780 align:middle line:84% And he really goes through some of these early arguments 00:19:11.780 --> 00:19:15.800 align:middle line:84% as to why the analogies failed, time and time again 00:19:15.800 --> 00:19:18.080 align:middle line:84% over decades, for generations when 00:19:18.080 --> 00:19:20.660 align:middle line:84% people tried to apply insights from something like quantum 00:19:20.660 --> 00:19:24.110 align:middle line:84% electrodynamics to try to just do the same thing for quantum 00:19:24.110 --> 00:19:24.900 align:middle line:90% gravity. 00:19:24.900 --> 00:19:27.230 align:middle line:84% So what now we can call, somewhat tongue-in-cheek, 00:19:27.230 --> 00:19:31.520 align:middle line:84% old school, quantum gravity is not subject to renormalization. 00:19:31.520 --> 00:19:34.160 align:middle line:84% You can't get any sensible, finite answers 00:19:34.160 --> 00:19:36.410 align:middle line:84% from trying to deal with the zigzagging 00:19:36.410 --> 00:19:38.450 align:middle line:84% of virtual gravitons, because there's 00:19:38.450 --> 00:19:42.140 align:middle line:84% stronger dependence of gravity on things 00:19:42.140 --> 00:19:43.560 align:middle line:90% like mass and momentum. 00:19:43.560 --> 00:19:44.060 align:middle line:90% OK. 00:19:44.060 --> 00:19:45.200 align:middle line:90% So read Lee's book. 00:19:45.200 --> 00:19:46.617 align:middle line:84% He'll explain it better than I do. 00:19:46.617 --> 00:19:48.950 align:middle line:84% But I think that's, in brief, some of what's going on 00:19:48.950 --> 00:19:51.170 align:middle line:84% was going on over, really, decades 00:19:51.170 --> 00:19:56.870 align:middle line:84% in this first wave of effort to combine gravitation and quantum 00:19:56.870 --> 00:19:58.220 align:middle line:90% theory. 00:19:58.220 --> 00:20:01.850 align:middle line:84% While all that was unfolding and becoming clarified-- 00:20:01.850 --> 00:20:03.800 align:middle line:84% the dead end was becoming clarified-- 00:20:03.800 --> 00:20:05.810 align:middle line:84% in a quite different branch of physics, 00:20:05.810 --> 00:20:09.530 align:middle line:84% now going back to ideas about the strong nuclear force, 00:20:09.530 --> 00:20:12.020 align:middle line:84% high energy nuclear physics and particle physics, 00:20:12.020 --> 00:20:14.660 align:middle line:84% in the late 1960s, a number of theorists-- 00:20:14.660 --> 00:20:17.360 align:middle line:84% some of them actually, at the time, based at MIT-- 00:20:17.360 --> 00:20:19.490 align:middle line:84% were trying to make sense of this. 00:20:19.490 --> 00:20:21.680 align:middle line:84% One of these main approaches we looked at, 00:20:21.680 --> 00:20:24.080 align:middle line:84% from the middle decades of the 20th century, 00:20:24.080 --> 00:20:25.160 align:middle line:90% for the strong force-- 00:20:25.160 --> 00:20:27.350 align:middle line:84% Geoffrey Chew's so-called S-matrix force. 00:20:27.350 --> 00:20:29.900 align:middle line:84% He was flipping these Feynman diagrams around, 00:20:29.900 --> 00:20:34.010 align:middle line:84% finding these self-consistent, dynamical solutions where 00:20:34.010 --> 00:20:35.450 align:middle line:84% basically nuclear particles could 00:20:35.450 --> 00:20:37.570 align:middle line:84% make the force-carrying particles that 00:20:37.570 --> 00:20:39.320 align:middle line:84% would bring the particles together to make 00:20:39.320 --> 00:20:40.612 align:middle line:90% those force-carrying particles. 00:20:40.612 --> 00:20:42.440 align:middle line:84% Could you find one self-consistent set 00:20:42.440 --> 00:20:48.770 align:middle line:84% of relationships among the particle zoo? 00:20:48.770 --> 00:20:51.530 align:middle line:84% And so younger theorists were trying to play with that 00:20:51.530 --> 00:20:54.200 align:middle line:84% self-consistent swapping structure, 00:20:54.200 --> 00:20:57.260 align:middle line:84% and they kept finding these extended objects-- 00:20:57.260 --> 00:20:59.540 align:middle line:84% geometrical objects-- in space, where 00:20:59.540 --> 00:21:03.350 align:middle line:84% a lot of the force among these particles would be spread out, 00:21:03.350 --> 00:21:06.980 align:middle line:84% splayed out in a cylinder or tube, a kind 00:21:06.980 --> 00:21:09.290 align:middle line:90% of a string-like, flux tube. 00:21:09.290 --> 00:21:11.390 align:middle line:84% And that really was, as far as I'm concerned, 00:21:11.390 --> 00:21:13.880 align:middle line:84% what might explain the nuclear force that keeps things 00:21:13.880 --> 00:21:17.060 align:middle line:84% like protons and neutrons bound within atomic nuclei. 00:21:17.060 --> 00:21:20.180 align:middle line:84% That was being published starting in 1967, '68 00:21:20.180 --> 00:21:22.820 align:middle line:84% by a small little circle of specialists. 00:21:22.820 --> 00:21:24.608 align:middle line:90% It was curious. 00:21:24.608 --> 00:21:25.400 align:middle line:90% It was interesting. 00:21:25.400 --> 00:21:27.320 align:middle line:84% And it was very quickly overshadowed 00:21:27.320 --> 00:21:29.810 align:middle line:84% for the reasons that I think we can make sense of, 00:21:29.810 --> 00:21:33.830 align:middle line:84% given what we saw together, the kind of gathering force, 00:21:33.830 --> 00:21:37.100 align:middle line:84% the kind of choppy road toward the quark model 00:21:37.100 --> 00:21:39.650 align:middle line:84% and ultimately quantum chromodynamics. 00:21:39.650 --> 00:21:41.840 align:middle line:84% So we saw, although the idea of quarks 00:21:41.840 --> 00:21:44.180 align:middle line:84% had been introduced as early as 1964, 00:21:44.180 --> 00:21:47.630 align:middle line:84% it took really a solid decade, into the mid 1970s, 00:21:47.630 --> 00:21:49.790 align:middle line:84% before even the quark model proponents 00:21:49.790 --> 00:21:51.710 align:middle line:84% themselves, like Murray Gell-Man, 00:21:51.710 --> 00:21:55.260 align:middle line:84% were saying that these are physical entities in the world. 00:21:55.260 --> 00:21:59.090 align:middle line:84% And it was really only after the successes of this very specific 00:21:59.090 --> 00:22:00.860 align:middle line:84% quantum mechanical field theory-- 00:22:00.860 --> 00:22:03.470 align:middle line:90% quantum chromodynamics, or QCD-- 00:22:03.470 --> 00:22:06.470 align:middle line:84% where the various pieces came together and also got 00:22:06.470 --> 00:22:11.320 align:middle line:84% some helpful, bolstered by some new experimental inputs. 00:22:11.320 --> 00:22:14.140 align:middle line:84% So by the mid 1970s, it looked like people 00:22:14.140 --> 00:22:17.050 align:middle line:84% had this strong nuclear force well in hand. 00:22:17.050 --> 00:22:20.800 align:middle line:84% And all these kind of weird, funky, geometrical, flux tube 00:22:20.800 --> 00:22:24.310 align:middle line:84% things from the Geoffrey Chew-inspired work, 00:22:24.310 --> 00:22:25.480 align:middle line:90% that kind of faded away. 00:22:25.480 --> 00:22:27.040 align:middle line:84% It wasn't proven to be wrong, it just 00:22:27.040 --> 00:22:29.350 align:middle line:84% seemed like it was no longer the best road forward. 00:22:29.350 --> 00:22:31.870 align:middle line:84% The quark model finally seemed to be 00:22:31.870 --> 00:22:35.650 align:middle line:84% scoring success after success, and that was built, as we saw, 00:22:35.650 --> 00:22:38.530 align:middle line:84% in much closer analogy to quantum electrodynamics. 00:22:38.530 --> 00:22:41.230 align:middle line:84% You imagine elementary point particles with certain kinds 00:22:41.230 --> 00:22:43.630 align:middle line:84% of charges-- in this case, quarks and gluons-- 00:22:43.630 --> 00:22:46.900 align:middle line:84% and you trace the virtual particles among them, 00:22:46.900 --> 00:22:49.360 align:middle line:84% and all that in a framework, much more 00:22:49.360 --> 00:22:50.860 align:middle line:90% like quantum electrodynamics. 00:22:50.860 --> 00:22:53.900 align:middle line:84% So the early string ideas really kind of fell away, 00:22:53.900 --> 00:22:55.900 align:middle line:84% not because it was just proven, but because they 00:22:55.900 --> 00:22:58.840 align:middle line:84% seemed like maybe not the best way forward 00:22:58.840 --> 00:23:00.640 align:middle line:90% to understand the strong force. 00:23:00.640 --> 00:23:03.790 align:middle line:84% Before people stopped paying attention though, 00:23:03.790 --> 00:23:07.450 align:middle line:84% a few of this small circle recognized something curious 00:23:07.450 --> 00:23:09.110 align:middle line:84% in their equations for the Geoffrey 00:23:09.110 --> 00:23:13.640 align:middle line:84% Chew-like for the string approach to the nuclear force. 00:23:13.640 --> 00:23:15.680 align:middle line:84% If you look at the low energy limit 00:23:15.680 --> 00:23:18.260 align:middle line:84% of these complicated, self-consistent, highly 00:23:18.260 --> 00:23:22.580 align:middle line:84% nonlinear equations, it looks like, in low energies, 00:23:22.580 --> 00:23:25.280 align:middle line:84% this seemingly nuclear type interaction 00:23:25.280 --> 00:23:29.420 align:middle line:84% includes the exchange of a massless spin-2 particle. 00:23:29.420 --> 00:23:31.293 align:middle line:84% And some people said, wait a minute. 00:23:31.293 --> 00:23:32.960 align:middle line:84% I remember hearing something about that. 00:23:32.960 --> 00:23:36.950 align:middle line:84% A massless spin-2 particle maybe comes up in other contexts, 00:23:36.950 --> 00:23:38.420 align:middle line:90% like in gravity. 00:23:38.420 --> 00:23:43.790 align:middle line:84% And so a few groups who knew about the kind of strong force, 00:23:43.790 --> 00:23:48.560 align:middle line:84% stringy work said, maybe these insights about strings 00:23:48.560 --> 00:23:52.430 align:middle line:84% extended 1D structures, maybe it has nothing particularly 00:23:52.430 --> 00:23:54.890 align:middle line:84% to do with the strong force, but maybe that 00:23:54.890 --> 00:23:58.250 align:middle line:84% was a theory about gravity and not about nuclear particles. 00:23:58.250 --> 00:24:01.550 align:middle line:84% If so, then the typical length scale for these things 00:24:01.550 --> 00:24:04.160 align:middle line:84% would not be the size of a single nuclear particle 00:24:04.160 --> 00:24:07.130 align:middle line:84% like a proton, it wouldn't be femtometers-- 00:24:07.130 --> 00:24:08.790 align:middle line:90% 10 to the -15 meter-- 00:24:08.790 --> 00:24:13.950 align:middle line:84% it might be the Planck length, the smallest possible length 00:24:13.950 --> 00:24:16.350 align:middle line:84% longer than which one should be able to ignore 00:24:16.350 --> 00:24:18.480 align:middle line:84% quantum gravitational corrections. 00:24:18.480 --> 00:24:23.850 align:middle line:84% Maybe the actual fabric of Einstein's warping spacetime 00:24:23.850 --> 00:24:27.180 align:middle line:84% consists at these unimaginably tiny scales 00:24:27.180 --> 00:24:30.300 align:middle line:84% of a play of these extended one dimensional objects. 00:24:30.300 --> 00:24:33.870 align:middle line:84% Maybe the strings are what give rise to the effects of gravity, 00:24:33.870 --> 00:24:38.260 align:middle line:84% not that bound quarks within a proton, for example. 00:24:38.260 --> 00:24:40.770 align:middle line:84% So because the low energy behavior 00:24:40.770 --> 00:24:42.720 align:middle line:84% of these stringy structures gave rise 00:24:42.720 --> 00:24:44.820 align:middle line:84% to just the kind of particle needed 00:24:44.820 --> 00:24:50.220 align:middle line:84% to make sense of gravity, in terms of particle exchange, 00:24:50.220 --> 00:24:52.020 align:middle line:84% the ideas about string theory became 00:24:52.020 --> 00:24:54.030 align:middle line:84% very interesting to a small circle 00:24:54.030 --> 00:24:57.420 align:middle line:84% as a possible road toward quantum gravity-- quite 00:24:57.420 --> 00:25:00.480 align:middle line:84% different from the kind of particle scattering QED type 00:25:00.480 --> 00:25:01.230 align:middle line:90% model. 00:25:01.230 --> 00:25:03.510 align:middle line:84% In fact, it looked even more promising, again, 00:25:03.510 --> 00:25:05.430 align:middle line:84% as many of these folks began to put together 00:25:05.430 --> 00:25:10.040 align:middle line:84% by the early and mid 1980s, roughly 10 years later. 00:25:10.040 --> 00:25:12.680 align:middle line:84% Then it looked like calculating effects-- 00:25:12.680 --> 00:25:17.150 align:middle line:84% quantum virtual effects-- in this string theory of gravity 00:25:17.150 --> 00:25:20.570 align:middle line:84% might actually avoid all these messy infinities altogether. 00:25:20.570 --> 00:25:21.950 align:middle line:90% Why was that? 00:25:21.950 --> 00:25:25.160 align:middle line:84% The infinities keep arising in these quantum field theories 00:25:25.160 --> 00:25:29.060 align:middle line:84% because people integrate up to an infinite virtual particle 00:25:29.060 --> 00:25:29.570 align:middle line:90% momentum. 00:25:29.570 --> 00:25:33.080 align:middle line:84% That's like saying-- that's like imagining that we could model 00:25:33.080 --> 00:25:37.430 align:middle line:84% the exchange, the scattering of, say, a virtual force carrying 00:25:37.430 --> 00:25:40.520 align:middle line:84% photon an electron, that they meet literally 00:25:40.520 --> 00:25:42.170 align:middle line:90% at a mathematical point. 00:25:42.170 --> 00:25:45.650 align:middle line:84% They meet in a region of infinitesimally, tiny region 00:25:45.650 --> 00:25:46.730 align:middle line:90% of space. 00:25:46.730 --> 00:25:49.640 align:middle line:84% That suggests that they could be scattering off 00:25:49.640 --> 00:25:51.680 align:middle line:84% each other with a borrowed momentum, 00:25:51.680 --> 00:25:54.710 align:middle line:90% up to infinity, up to 1 over 0. 00:25:54.710 --> 00:25:57.050 align:middle line:84% The delta r could shrink to zero. 00:25:57.050 --> 00:25:59.950 align:middle line:84% That means that the momentum exchange could, in fact, become 00:25:59.950 --> 00:26:02.450 align:middle line:84% infinite, which is why you have integrate those integrals up 00:26:02.450 --> 00:26:03.360 align:middle line:90% to infinity. 00:26:03.360 --> 00:26:05.120 align:middle line:84% Well, these string objects are actually 00:26:05.120 --> 00:26:08.420 align:middle line:84% sweeping out not world lines through space and time, 00:26:08.420 --> 00:26:09.960 align:middle line:90% but actually world sheets. 00:26:09.960 --> 00:26:14.220 align:middle line:84% They are extended objects, so the analog Feynman diagrams 00:26:14.220 --> 00:26:17.820 align:middle line:84% never pinch down to a single mathematical point. 00:26:17.820 --> 00:26:21.060 align:middle line:84% In fact, you have these kind of pants legs structures, 00:26:21.060 --> 00:26:25.500 align:middle line:84% where you have one kind of extended structure 00:26:25.500 --> 00:26:27.750 align:middle line:84% might blend into others, but they never pinch down 00:26:27.750 --> 00:26:29.560 align:middle line:90% into a mathematical point. 00:26:29.560 --> 00:26:31.980 align:middle line:84% So if you never have these point like vertices 00:26:31.980 --> 00:26:35.550 align:middle line:84% in the kind of scattering among these putative strings, 00:26:35.550 --> 00:26:37.170 align:middle line:84% then you actually never integrate up 00:26:37.170 --> 00:26:39.130 align:middle line:90% to literally infinite momentum. 00:26:39.130 --> 00:26:42.090 align:middle line:84% So maybe virtual processes involving strings 00:26:42.090 --> 00:26:45.480 align:middle line:84% would naturally avoid all these divergence problems 00:26:45.480 --> 00:26:49.800 align:middle line:84% that seem to hang up the old school or classic approach 00:26:49.800 --> 00:26:50.910 align:middle line:90% to quantum gravity. 00:26:50.910 --> 00:26:53.700 align:middle line:84% So maybe you could have a finite theory of gravity 00:26:53.700 --> 00:26:55.740 align:middle line:84% that has the right particle exchange 00:26:55.740 --> 00:26:59.160 align:middle line:84% in the appropriate limit and avoids infinities. 00:26:59.160 --> 00:27:01.950 align:middle line:90% So why weren't we done in 1984? 00:27:01.950 --> 00:27:04.920 align:middle line:84% Well, because the same scholars found a pretty interesting 00:27:04.920 --> 00:27:05.610 align:middle line:90% catch. 00:27:05.610 --> 00:27:09.240 align:middle line:84% Some call it a bug, few would call it a feature. 00:27:09.240 --> 00:27:11.730 align:middle line:84% The simplest models that could be written down, 00:27:11.730 --> 00:27:15.300 align:middle line:84% that could be applied to a gravitational model, 00:27:15.300 --> 00:27:18.030 align:middle line:84% where these strings could be dancing around at the Planck 00:27:18.030 --> 00:27:21.780 align:middle line:84% length and giving rise, in the appropriate limit, 00:27:21.780 --> 00:27:24.630 align:middle line:84% to Einstein's general theory of relativity. 00:27:24.630 --> 00:27:26.520 align:middle line:84% They only could live mathematically 00:27:26.520 --> 00:27:29.700 align:middle line:90% in 26 spacetime dimensions. 00:27:29.700 --> 00:27:31.770 align:middle line:84% I put that in bold and italics, so we'll 00:27:31.770 --> 00:27:33.160 align:middle line:90% linger on that for a second. 00:27:33.160 --> 00:27:35.460 align:middle line:84% The earliest self-consistent models, 00:27:35.460 --> 00:27:38.748 align:middle line:84% when applied to gravity to get the right limit at low 00:27:38.748 --> 00:27:41.040 align:middle line:84% energies, to make it look like Einstein's theory at low 00:27:41.040 --> 00:27:44.280 align:middle line:84% energies, these strings weren't dancing around in three 00:27:44.280 --> 00:27:46.470 align:middle line:84% dimensions of space, at one of time-- 00:27:46.470 --> 00:27:50.430 align:middle line:84% XYZ plus T. They could only dance around 00:27:50.430 --> 00:27:52.470 align:middle line:84% in a mathematically, self-consistent way 00:27:52.470 --> 00:27:57.120 align:middle line:84% if they were living in a 26 dimensional spacetime. 00:27:57.120 --> 00:27:59.700 align:middle line:84% It seems pretty clear that we live 00:27:59.700 --> 00:28:04.080 align:middle line:84% in a four-dimensional spacetime, so what's going on? 00:28:04.080 --> 00:28:06.960 align:middle line:84% A little while later, some of these physicists 00:28:06.960 --> 00:28:09.400 align:middle line:84% found that they could actually make 00:28:09.400 --> 00:28:11.530 align:middle line:84% these mathematically, self-consistent string 00:28:11.530 --> 00:28:13.130 align:middle line:90% models of gravity. 00:28:13.130 --> 00:28:15.490 align:middle line:84% If they incorporated a particular symmetry-- 00:28:15.490 --> 00:28:18.490 align:middle line:84% strictly hypothetical symmetry-- for which even, to this day, 00:28:18.490 --> 00:28:20.710 align:middle line:84% there is no empirical evidence at all, 00:28:20.710 --> 00:28:24.190 align:middle line:84% a symmetry called supersymmetry, which suggests that for every 00:28:24.190 --> 00:28:28.960 align:middle line:84% known particle, every quark, every electron, every neutrino, 00:28:28.960 --> 00:28:31.360 align:middle line:84% and all the rest, there is a partner-- 00:28:31.360 --> 00:28:36.610 align:middle line:84% a super partner particle-- with the same mass and same charge, 00:28:36.610 --> 00:28:39.170 align:middle line:90% but one half unit more of spin. 00:28:39.170 --> 00:28:41.590 align:middle line:84% So for every spin, one half particle, like quarks, 00:28:41.590 --> 00:28:43.540 align:middle line:84% and electrons, and neutrinos, there 00:28:43.540 --> 00:28:47.210 align:middle line:84% would be integer spin superpartners and vice versa. 00:28:47.210 --> 00:28:50.260 align:middle line:84% So for every integer spin photon, 00:28:50.260 --> 00:28:52.930 align:middle line:84% there would be a half integer or three halves integer 00:28:52.930 --> 00:28:55.000 align:middle line:90% superpartner, like a photino. 00:28:55.000 --> 00:28:58.280 align:middle line:84% For every quark, there would be a squark, and so on. 00:28:58.280 --> 00:29:01.910 align:middle line:84% So at the cost of doubling every single known form of matter, 00:29:01.910 --> 00:29:04.000 align:middle line:84% these superstring theories could be 00:29:04.000 --> 00:29:08.290 align:middle line:84% formulated in only 10 dimensions of spacetime instead of 26. 00:29:08.290 --> 00:29:10.460 align:middle line:84% That's getting more than halfway there, 00:29:10.460 --> 00:29:12.590 align:middle line:84% but it still means a lot more structure 00:29:12.590 --> 00:29:14.930 align:middle line:84% than what we seem to measure or just 00:29:14.930 --> 00:29:16.800 align:middle line:90% observe in our daily lives. 00:29:16.800 --> 00:29:20.060 align:middle line:84% So these early superstring theories for gravity 00:29:20.060 --> 00:29:21.950 align:middle line:90% had some unbelievable promise. 00:29:21.950 --> 00:29:25.207 align:middle line:84% These people weren't of deluding themselves 00:29:25.207 --> 00:29:26.540 align:middle line:90% that these could be of interest. 00:29:26.540 --> 00:29:29.112 align:middle line:84% But it wasn't such a straightforward deal. 00:29:29.112 --> 00:29:31.070 align:middle line:84% So we can then go to some of the kinds of plots 00:29:31.070 --> 00:29:33.230 align:middle line:84% that I love to make and start counting stuff. 00:29:33.230 --> 00:29:36.980 align:middle line:84% So the blue curve shows all physics articles 00:29:36.980 --> 00:29:40.520 align:middle line:84% as covered by something called physics abstracts, a kind 00:29:40.520 --> 00:29:43.340 align:middle line:84% of worldwide consortium of physics-trained librarians 00:29:43.340 --> 00:29:45.980 align:middle line:84% in many parts of the world, trying to literally count up 00:29:45.980 --> 00:29:49.365 align:middle line:84% and categorize, by subtopic, all the articles 00:29:49.365 --> 00:29:51.740 align:middle line:84% by professional physicists in the peer reviewed journals, 00:29:51.740 --> 00:29:55.160 align:middle line:84% in as many languages as they could as they could master. 00:29:55.160 --> 00:29:56.460 align:middle line:90% So that's the blue curve. 00:29:56.460 --> 00:30:00.540 align:middle line:84% And you see it's growing fairly steadily over this period. 00:30:00.540 --> 00:30:03.890 align:middle line:84% And the red curve-- and I've renormalized it myself-- 00:30:03.890 --> 00:30:07.047 align:middle line:84% the red curve is then all the articles on string theory, 00:30:07.047 --> 00:30:09.380 align:middle line:84% including superstring theory, over the same time period. 00:30:09.380 --> 00:30:12.560 align:middle line:84% To fit them on one curve, I've actually divided the blue one 00:30:12.560 --> 00:30:14.070 align:middle line:90% by a factor of 100. 00:30:14.070 --> 00:30:18.560 align:middle line:84% So by the end of this period, the red curves are about 1% 00:30:18.560 --> 00:30:20.780 align:middle line:84% of all the publications in all of physics. 00:30:20.780 --> 00:30:22.910 align:middle line:84% Let's say they're almost equal, but I've 00:30:22.910 --> 00:30:25.160 align:middle line:84% cheated by making the blue curve changes 00:30:25.160 --> 00:30:27.180 align:middle line:90% amplitude by a factor of 100. 00:30:27.180 --> 00:30:29.480 align:middle line:84% So you see these very distinct structures 00:30:29.480 --> 00:30:32.480 align:middle line:84% in the effort expended on superstring theory, 00:30:32.480 --> 00:30:35.420 align:middle line:84% compared to global efforts on every branch of physics 00:30:35.420 --> 00:30:36.830 align:middle line:90% altogether. 00:30:36.830 --> 00:30:40.310 align:middle line:84% You see virtually no attention at all to string theory. 00:30:40.310 --> 00:30:43.700 align:middle line:84% It really is a very curious little specialty area 00:30:43.700 --> 00:30:45.920 align:middle line:90% until the mid 1980s. 00:30:45.920 --> 00:30:49.790 align:middle line:84% What proponents of this work later called the first string 00:30:49.790 --> 00:30:51.860 align:middle line:84% revolution-- which you have to know they only 00:30:51.860 --> 00:30:53.780 align:middle line:84% named after a second string evolution, right? 00:30:53.780 --> 00:30:57.020 align:middle line:84% No one called World War I World War I until World War II. 00:30:57.020 --> 00:30:59.390 align:middle line:84% So what later became called the first string 00:30:59.390 --> 00:31:02.690 align:middle line:84% evolution, in hindsight, was identified with this work that 00:31:02.690 --> 00:31:05.390 align:middle line:84% was really published by a few independent small groups 00:31:05.390 --> 00:31:08.540 align:middle line:84% around 1984, where they began showing 00:31:08.540 --> 00:31:11.750 align:middle line:84% that the stringy stuff could include 00:31:11.750 --> 00:31:15.320 align:middle line:84% the structure of gravity, if you imagine 00:31:15.320 --> 00:31:19.070 align:middle line:84% that string-like filament to be describing 00:31:19.070 --> 00:31:21.170 align:middle line:84% gravitational interactions at the Planck length, 00:31:21.170 --> 00:31:23.270 align:middle line:90% as opposed to nuclear physics. 00:31:23.270 --> 00:31:26.690 align:middle line:84% And indeed, there were several different, self-consistent, 00:31:26.690 --> 00:31:30.470 align:middle line:84% supersymmetric models published all in 1984, '85, 00:31:30.470 --> 00:31:32.630 align:middle line:84% and that led to an enormous burst-- 00:31:32.630 --> 00:31:35.360 align:middle line:84% an exponential rise of interest-- in string theories, 00:31:35.360 --> 00:31:38.360 align:middle line:84% coming very soon after that so-called first revolution. 00:31:38.360 --> 00:31:40.970 align:middle line:84% Then there's a kind of plateau for about a decade. 00:31:40.970 --> 00:31:42.770 align:middle line:84% And then there was what the proponents 00:31:42.770 --> 00:31:46.130 align:middle line:84% came to call the second string revolution, almost right 00:31:46.130 --> 00:31:47.810 align:middle line:90% on schedule 10 years later. 00:31:47.810 --> 00:31:49.880 align:middle line:84% What happened with the second string revolution, 00:31:49.880 --> 00:31:52.340 align:middle line:84% in brief, was that a few of these specialists 00:31:52.340 --> 00:31:55.010 align:middle line:84% found that what looked like five totally 00:31:55.010 --> 00:31:58.280 align:middle line:84% separate distinct, but self-consistent, superstring 00:31:58.280 --> 00:31:59.007 align:middle line:90% models. 00:31:59.007 --> 00:32:00.590 align:middle line:84% They looked different from each other. 00:32:00.590 --> 00:32:02.870 align:middle line:84% They all were mathematically self-consistent, 00:32:02.870 --> 00:32:05.900 align:middle line:84% even though they looked like they were different candidates. 00:32:05.900 --> 00:32:08.900 align:middle line:84% In the second wave, people like Edward Witten 00:32:08.900 --> 00:32:12.240 align:middle line:84% and other colleagues, found kind of one to one mapping. 00:32:12.240 --> 00:32:14.240 align:middle line:84% So in fact, these five different models 00:32:14.240 --> 00:32:17.900 align:middle line:84% might all be ways of expressing a single shared model. 00:32:17.900 --> 00:32:21.500 align:middle line:84% These might be almost like coordinate transformations 00:32:21.500 --> 00:32:24.470 align:middle line:84% of a single underlying model, and that became 00:32:24.470 --> 00:32:26.120 align:middle line:90% known as the second revolution. 00:32:26.120 --> 00:32:29.300 align:middle line:84% So if one performed what became known as dualities, 00:32:29.300 --> 00:32:32.210 align:middle line:84% it really is analogous to coordinate transformations. 00:32:32.210 --> 00:32:34.790 align:middle line:84% You can actually map, feature by feature, 00:32:34.790 --> 00:32:38.180 align:middle line:84% those seemingly five distinct models into a single structure. 00:32:38.180 --> 00:32:39.930 align:middle line:90% That got people excited again. 00:32:39.930 --> 00:32:42.180 align:middle line:84% You see another exponential takeoff of interest. 00:32:42.180 --> 00:32:45.630 align:middle line:84% So that by the early 2000s, efforts on string theory, 00:32:45.630 --> 00:32:51.260 align:middle line:84% which had once been vanishingly tiny, occupy nearly 1% 00:32:51.260 --> 00:32:54.690 align:middle line:84% of all the physics output of all the physicists on the planet. 00:32:54.690 --> 00:32:56.360 align:middle line:84% So let me pause there, see if there's 00:32:56.360 --> 00:32:59.480 align:middle line:84% any questions on that kind of alternate approach to an effort 00:32:59.480 --> 00:33:00.725 align:middle line:90% to try to quantize gravity. 00:33:00.725 --> 00:33:05.680 align:middle line:90% 00:33:05.680 --> 00:33:08.230 align:middle line:90% Any questions on that? 00:33:08.230 --> 00:33:09.320 align:middle line:90% It's so straightforward. 00:33:09.320 --> 00:33:10.195 align:middle line:90% You're all convinced. 00:33:10.195 --> 00:33:12.460 align:middle line:84% You're all now card-carrying string theorists. 00:33:12.460 --> 00:33:14.140 align:middle line:90% No lingering questions? 00:33:14.140 --> 00:33:15.670 align:middle line:90% Julie is in. 00:33:15.670 --> 00:33:17.430 align:middle line:90% Thank you, Julie. 00:33:17.430 --> 00:33:18.540 align:middle line:90% I'll tell Andy Strominger. 00:33:18.540 --> 00:33:20.100 align:middle line:90% He'll be glad. 00:33:20.100 --> 00:33:23.360 align:middle line:90% Any other questions? 00:33:23.360 --> 00:33:24.640 align:middle line:90% If not, that's OK, of course. 00:33:24.640 --> 00:33:27.040 align:middle line:90% But I'll press on. 00:33:27.040 --> 00:33:29.290 align:middle line:84% Because as if that weren't strange enough, boy, have I 00:33:29.290 --> 00:33:30.110 align:middle line:90% got news for you. 00:33:30.110 --> 00:33:31.270 align:middle line:90% You think that's strange-- 00:33:31.270 --> 00:33:34.570 align:middle line:84% we live in 10 dimensions-- let's see where people take this, 00:33:34.570 --> 00:33:36.280 align:middle line:84% since they're now paying attention. 00:33:36.280 --> 00:33:38.550 align:middle line:90% OK. 00:33:38.550 --> 00:33:42.280 align:middle line:84% Now I very blithely told you-- and none of you seem to object, 00:33:42.280 --> 00:33:43.950 align:middle line:84% so clearly it's the end of the term-- 00:33:43.950 --> 00:33:45.870 align:middle line:84% that these superstring theories can only 00:33:45.870 --> 00:33:50.040 align:middle line:84% be formulated in a minimum of 10 spacetime dimensions. 00:33:50.040 --> 00:33:52.110 align:middle line:84% They still only involve one timelike dimension. 00:33:52.110 --> 00:33:54.570 align:middle line:84% That means nine dimensions of space-- 00:33:54.570 --> 00:33:57.870 align:middle line:84% height, width, breadth-- and six others 00:33:57.870 --> 00:33:59.250 align:middle line:90% all at right angles to them. 00:33:59.250 --> 00:34:01.590 align:middle line:84% I mean, I look at where the walls meet in my house 00:34:01.590 --> 00:34:03.180 align:middle line:84% and I can't find where the other six 00:34:03.180 --> 00:34:05.320 align:middle line:90% right angles would come from. 00:34:05.320 --> 00:34:07.620 align:middle line:84% So it's not just a failure of my own imagination, 00:34:07.620 --> 00:34:09.210 align:middle line:90% which fails all the time. 00:34:09.210 --> 00:34:12.600 align:middle line:84% This leads to some pretty strange-sounding 00:34:12.600 --> 00:34:14.949 align:middle line:90% possible implications. 00:34:14.949 --> 00:34:16.920 align:middle line:84% Let's take a very simple one, going back again, 00:34:16.920 --> 00:34:22.620 align:middle line:84% right to the very first class sessions from this semester. 00:34:22.620 --> 00:34:26.070 align:middle line:84% Think about, say, Faraday's lines of force. 00:34:26.070 --> 00:34:28.239 align:middle line:84% Unless something very strange is happening, 00:34:28.239 --> 00:34:32.100 align:middle line:84% we really can't live in a 10-dimensional world. 00:34:32.100 --> 00:34:34.440 align:middle line:90% If you imagine a single mass-- 00:34:34.440 --> 00:34:36.449 align:middle line:84% some source of gravitation, some ball, 00:34:36.449 --> 00:34:38.469 align:middle line:84% with some mass, a spherically symmetric one, 00:34:38.469 --> 00:34:39.639 align:middle line:90% to keep things simple-- 00:34:39.639 --> 00:34:42.760 align:middle line:84% set it down at the origin of our coordinate system. 00:34:42.760 --> 00:34:44.830 align:middle line:84% Then just like a single electric charge 00:34:44.830 --> 00:34:49.420 align:middle line:84% that got Michael Faraday so excited in the 1820s and 30s, 00:34:49.420 --> 00:34:52.960 align:middle line:84% we can try to imagine how lines of force would emanate, 00:34:52.960 --> 00:34:55.060 align:middle line:84% in this case very simply, very spherically 00:34:55.060 --> 00:34:57.760 align:middle line:84% symmetrically, lines of gravitational force 00:34:57.760 --> 00:35:01.420 align:middle line:84% emanating from that source, that mass M at the center. 00:35:01.420 --> 00:35:03.820 align:middle line:84% Well, we can then try to do like Faraday 00:35:03.820 --> 00:35:07.840 align:middle line:84% did and an envelop, kind of surround our source, 00:35:07.840 --> 00:35:09.760 align:middle line:90% with an imagined sphere. 00:35:09.760 --> 00:35:11.965 align:middle line:84% We can change the radius of the spheres, 00:35:11.965 --> 00:35:13.840 align:middle line:84% we can imagine how the strength of that force 00:35:13.840 --> 00:35:15.850 align:middle line:90% should fall off with distance. 00:35:15.850 --> 00:35:21.040 align:middle line:84% Now again, as you know, the surface area of some sphere 00:35:21.040 --> 00:35:24.370 align:middle line:84% will scale as one power less of the radius 00:35:24.370 --> 00:35:25.810 align:middle line:90% compared to the volume. 00:35:25.810 --> 00:35:27.610 align:middle line:90% That holds in three dimensions. 00:35:27.610 --> 00:35:31.000 align:middle line:84% The surface area of a sphere goes like R squared, 00:35:31.000 --> 00:35:32.860 align:middle line:90% the volume goes like R cubed. 00:35:32.860 --> 00:35:35.140 align:middle line:84% That same geometrical relation holds 00:35:35.140 --> 00:35:38.800 align:middle line:84% for any dimensions of space, any hypersphere, in particular one 00:35:38.800 --> 00:35:42.490 align:middle line:84% that's in, say, nine dimensions of space, the surface area we 00:35:42.490 --> 00:35:47.230 align:middle line:84% can vary self-consistently project an appropriate sub 00:35:47.230 --> 00:35:49.900 align:middle line:84% manifold, to be all super fancy geometrical about it. 00:35:49.900 --> 00:35:53.200 align:middle line:84% We can calculate the surface area of a nine sphere, 00:35:53.200 --> 00:35:56.150 align:middle line:84% and the area will grow like the radius to the eighth power, 00:35:56.150 --> 00:35:57.430 align:middle line:90% to D minus 1. 00:35:57.430 --> 00:36:00.820 align:middle line:84% So if we just redo Michael Faraday's really lovely, 00:36:00.820 --> 00:36:03.700 align:middle line:84% ingenious in fact, geometrical scaling, 00:36:03.700 --> 00:36:06.010 align:middle line:84% then if we really think gravity lives 00:36:06.010 --> 00:36:08.560 align:middle line:84% in nine dimensions of space, then we 00:36:08.560 --> 00:36:11.890 align:middle line:84% would expect Newton's law to go like 1 over R to the eighth, 00:36:11.890 --> 00:36:14.410 align:middle line:90% instead of 1 over R squared. 00:36:14.410 --> 00:36:16.780 align:middle line:84% Why are we finding inverse square laws even 00:36:16.780 --> 00:36:19.030 align:middle line:84% in the context of Einstein's general theory 00:36:19.030 --> 00:36:21.370 align:middle line:84% of relativity, very high accuracy? 00:36:21.370 --> 00:36:24.910 align:middle line:84% Whereas if we just have gravity spilling 00:36:24.910 --> 00:36:28.450 align:middle line:84% out across nine co-equal dimensions of space, 00:36:28.450 --> 00:36:30.940 align:middle line:84% we would expect a completely different behavior 00:36:30.940 --> 00:36:32.150 align:middle line:90% for the force of gravity. 00:36:32.150 --> 00:36:35.170 align:middle line:84% So what's going on when you say that this theory of gravity 00:36:35.170 --> 00:36:38.290 align:middle line:84% is formulated in nine dimensions of space? 00:36:38.290 --> 00:36:40.360 align:middle line:84% One of the earliest efforts to address 00:36:40.360 --> 00:36:44.230 align:middle line:84% that is called compactification, which really just means 00:36:44.230 --> 00:36:47.000 align:middle line:84% make the extra dimensions super tiny. 00:36:47.000 --> 00:36:51.670 align:middle line:84% So the idea is that for, as yet unexplained or unknown reasons, 00:36:51.670 --> 00:36:54.160 align:middle line:84% the idea is that these extra dimensions of space 00:36:54.160 --> 00:36:56.470 align:middle line:90% are not arbitrarily extended. 00:36:56.470 --> 00:36:57.820 align:middle line:90% They're not macroscopic. 00:36:57.820 --> 00:37:02.770 align:middle line:84% I can't take a walk in dimensions numbers 4 through 9, 00:37:02.770 --> 00:37:05.170 align:middle line:84% because, at least hypothetically, they've 00:37:05.170 --> 00:37:07.060 align:middle line:90% been curled up onto each other. 00:37:07.060 --> 00:37:09.370 align:middle line:84% And the way the analogy is always made, 00:37:09.370 --> 00:37:12.220 align:middle line:84% and you might have heard before, we think about a soda 00:37:12.220 --> 00:37:14.080 align:middle line:90% straw or a garden hose. 00:37:14.080 --> 00:37:17.200 align:middle line:84% Now if you're very close to that straw or hose, 00:37:17.200 --> 00:37:19.150 align:middle line:84% you can see it's an extended object. 00:37:19.150 --> 00:37:22.770 align:middle line:84% It has both a length and a width to it. 00:37:22.770 --> 00:37:25.260 align:middle line:84% But if you look at that object from very far away, 00:37:25.260 --> 00:37:28.040 align:middle line:84% it looks indistinguishable from a dimensionless line, 00:37:28.040 --> 00:37:32.250 align:middle line:84% an arbitrarily thin line extended only in one dimension. 00:37:32.250 --> 00:37:34.790 align:middle line:84% So the idea was, what if these extra dimensions of space 00:37:34.790 --> 00:37:37.820 align:middle line:84% are not macroscopically large, but in fact, 00:37:37.820 --> 00:37:39.630 align:middle line:90% are curled up on each other? 00:37:39.630 --> 00:37:41.600 align:middle line:84% They have a kind of internal radius 00:37:41.600 --> 00:37:45.380 align:middle line:84% that never becomes large, that's controlled by some, 00:37:45.380 --> 00:37:48.660 align:middle line:84% as yet, hypothetical kinds of dynamics, 00:37:48.660 --> 00:37:52.670 align:middle line:84% so that from our clumsy human sized scales what is actually 00:37:52.670 --> 00:37:55.310 align:middle line:84% a garden hose at an impossibly tiny radius 00:37:55.310 --> 00:37:57.770 align:middle line:84% looks to us indistinguishable from being 00:37:57.770 --> 00:38:00.110 align:middle line:90% merely a line of zero width. 00:38:00.110 --> 00:38:01.970 align:middle line:84% That's the kind of hand-waving version 00:38:01.970 --> 00:38:05.300 align:middle line:84% of how you could compactify, in this case, one extra dimension. 00:38:05.300 --> 00:38:08.450 align:middle line:84% So we might live in a universe with X,Y and Z, 00:38:08.450 --> 00:38:11.810 align:middle line:84% macroscopic height, width, and breadth, and then W, 00:38:11.810 --> 00:38:14.190 align:middle line:84% let's say some fourth dimension of space. 00:38:14.190 --> 00:38:17.090 align:middle line:84% But that one is a tiny little circle 00:38:17.090 --> 00:38:20.630 align:middle line:84% that we don't probe at low energies, 00:38:20.630 --> 00:38:23.445 align:middle line:84% and so we never see this structure, 00:38:23.445 --> 00:38:25.320 align:middle line:84% because we're never measuring it on the right 00:38:25.320 --> 00:38:26.700 align:middle line:90% on the right length scales. 00:38:26.700 --> 00:38:30.330 align:middle line:84% If the curvature of that garden hose, of the radius, 00:38:30.330 --> 00:38:31.950 align:middle line:84% were of order the Planck length, we'd 00:38:31.950 --> 00:38:34.020 align:middle line:84% have no way to know it directly, because we 00:38:34.020 --> 00:38:38.290 align:middle line:84% can't measure anything with such high spatial resolution. 00:38:38.290 --> 00:38:40.770 align:middle line:84% So that's the idea to compactify these extra dimensions. 00:38:40.770 --> 00:38:42.480 align:middle line:84% That works pretty straightforwardly 00:38:42.480 --> 00:38:44.610 align:middle line:84% if you only have one extra dimension of space 00:38:44.610 --> 00:38:46.680 align:middle line:90% to get rid of. 00:38:46.680 --> 00:38:48.900 align:middle line:84% But remember, these superstring theories 00:38:48.900 --> 00:38:52.200 align:middle line:84% require at least six extra dimensions, nine dimensions 00:38:52.200 --> 00:38:54.630 align:middle line:90% of space, plus one of time. 00:38:54.630 --> 00:38:57.780 align:middle line:84% So we have three macroscopic ones that we can self-evidently 00:38:57.780 --> 00:38:58.770 align:middle line:90% move around in. 00:38:58.770 --> 00:39:01.920 align:middle line:84% So you have to compactify six extra dimensions, not just one. 00:39:01.920 --> 00:39:03.480 align:middle line:84% It turns out, again, as people began 00:39:03.480 --> 00:39:06.240 align:middle line:84% finding, as early as the mid 1980s, 00:39:06.240 --> 00:39:09.000 align:middle line:84% that that becomes an unbelievably complicated 00:39:09.000 --> 00:39:12.240 align:middle line:84% problem in topology, that there are estimated 00:39:12.240 --> 00:39:16.830 align:middle line:84% to be about 100,000 topologically distinct ways 00:39:16.830 --> 00:39:20.850 align:middle line:84% to curl up, or fold up, these six extra dimensions. 00:39:20.850 --> 00:39:25.400 align:middle line:84% So at every point in XYZ that we could walk along, 00:39:25.400 --> 00:39:28.040 align:middle line:84% there would be curled up, maybe in some different pattern, 00:39:28.040 --> 00:39:32.120 align:middle line:84% six extra dimensions of space, with a characteristic scale 00:39:32.120 --> 00:39:35.210 align:middle line:84% of order the Planck scale, 10 to the minus 35th of the meter. 00:39:35.210 --> 00:39:37.790 align:middle line:84% And yet the actual curling up-- the shape of it, 00:39:37.790 --> 00:39:40.740 align:middle line:84% really, the topology-- could be different from one location 00:39:40.740 --> 00:39:41.240 align:middle line:90% to the next. 00:39:41.240 --> 00:39:44.270 align:middle line:84% So people start talking about these so-called Calabi-Yau 00:39:44.270 --> 00:39:46.370 align:middle line:84% manifolds, a topic of great interest 00:39:46.370 --> 00:39:49.490 align:middle line:84% to pure mathematicians, and some of the string theorists 00:39:49.490 --> 00:39:53.090 align:middle line:84% wondered would that kind of topology accounting 00:39:53.090 --> 00:39:55.490 align:middle line:84% be useful for making sense of these shrunken 00:39:55.490 --> 00:39:57.230 align:middle line:90% down, compactified dimensions. 00:39:57.230 --> 00:40:00.410 align:middle line:84% My next book club suggestion is a lovely book 00:40:00.410 --> 00:40:04.310 align:middle line:84% by a colleague, Lisa Randall, who's teaches at Harvard. 00:40:04.310 --> 00:40:07.467 align:middle line:84% She used to teach at MIT, but Harvard convinced 00:40:07.467 --> 00:40:09.050 align:middle line:84% her to move all the way across campus, 00:40:09.050 --> 00:40:12.530 align:middle line:84% and a really lovely book called Warped Passages, which 00:40:12.530 --> 00:40:14.270 align:middle line:84% Lisa walks through some of the reasoning 00:40:14.270 --> 00:40:16.070 align:middle line:84% behind these extra dimensions of space, 00:40:16.070 --> 00:40:18.230 align:middle line:84% what are different ways to try to make them go away 00:40:18.230 --> 00:40:19.610 align:middle line:90% from our everyday experience. 00:40:19.610 --> 00:40:21.690 align:middle line:84% So she claims that, I think, really very nicely. 00:40:21.690 --> 00:40:24.480 align:middle line:84% So the first main conceptual approach 00:40:24.480 --> 00:40:26.370 align:middle line:84% to these extra dimensions is curl them up, 00:40:26.370 --> 00:40:29.590 align:middle line:84% even though it turns out that's pretty complicated to do. 00:40:29.590 --> 00:40:31.133 align:middle line:90% It got even worse. 00:40:31.133 --> 00:40:32.050 align:middle line:90% I would call it worse. 00:40:32.050 --> 00:40:35.020 align:middle line:84% It got more complicated in the early 2000s, 00:40:35.020 --> 00:40:38.500 align:middle line:84% and we're getting kind of close to where we are today. 00:40:38.500 --> 00:40:40.900 align:middle line:84% Other string theorists began to realize you don't only 00:40:40.900 --> 00:40:44.360 align:middle line:84% have to worry about the actual kind of dimensions of space, 00:40:44.360 --> 00:40:46.390 align:middle line:84% which can be topologically complicated 00:40:46.390 --> 00:40:48.820 align:middle line:84% in those so-called Calabi-Yau manifolds, 00:40:48.820 --> 00:40:50.320 align:middle line:84% but you actually have to worry about 00:40:50.320 --> 00:40:52.990 align:middle line:84% self-consistent configurations of, say, 00:40:52.990 --> 00:40:56.140 align:middle line:84% these strings within and among those dimensions. 00:40:56.140 --> 00:40:58.960 align:middle line:84% So you don't only have topologically distinct 00:40:58.960 --> 00:41:01.600 align:middle line:84% configurations of geometry, you actually 00:41:01.600 --> 00:41:05.270 align:middle line:84% have some physical degrees of freedom dancing around on them. 00:41:05.270 --> 00:41:09.910 align:middle line:84% And as you can imagine you get quite a different solution 00:41:09.910 --> 00:41:13.840 align:middle line:84% if your string is wrapped around a sphere, versus if it's 00:41:13.840 --> 00:41:17.050 align:middle line:84% wrapped around a torus, versus if it's wrapped around three 00:41:17.050 --> 00:41:18.290 align:middle line:90% torus or whatever else. 00:41:18.290 --> 00:41:22.210 align:middle line:84% So when they began counting up the actual number of low energy 00:41:22.210 --> 00:41:25.120 align:middle line:84% string theory states, we worry not only 00:41:25.120 --> 00:41:27.880 align:middle line:84% about the geometry of the dimensions of spacetime, 00:41:27.880 --> 00:41:31.070 align:middle line:84% but actually how these physical degrees of freedom-- 00:41:31.070 --> 00:41:33.920 align:middle line:84% these string-like states-- could be distributed 00:41:33.920 --> 00:41:35.420 align:middle line:90% within and among them. 00:41:35.420 --> 00:41:38.660 align:middle line:84% They came up not with 10 to the 5, 100,000, 00:41:38.660 --> 00:41:40.790 align:middle line:84% they came up with 10 to the 500, which 00:41:40.790 --> 00:41:43.140 align:middle line:90% is more than a rounding error. 00:41:43.140 --> 00:41:45.200 align:middle line:84% So they began to be concerned that there 00:41:45.200 --> 00:41:50.490 align:middle line:84% seemed to be 10 to the 500 distinct low energy string 00:41:50.490 --> 00:41:51.690 align:middle line:90% states-- 00:41:51.690 --> 00:41:54.120 align:middle line:90% 10 to the 500. 00:41:54.120 --> 00:41:56.640 align:middle line:84% I won't ask for a show of hands but it's pretty hard 00:41:56.640 --> 00:41:58.980 align:middle line:84% to come up with numbers like 10 to the 500. 00:41:58.980 --> 00:41:59.580 align:middle line:90% I've tried. 00:41:59.580 --> 00:42:02.130 align:middle line:84% Let me give you some of my mundane examples. 00:42:02.130 --> 00:42:04.440 align:middle line:84% If internet accounts are to be believed, 00:42:04.440 --> 00:42:07.385 align:middle line:84% then Jeff Bezos has 10 to the $5 for every buck 00:42:07.385 --> 00:42:08.760 align:middle line:84% that I can lay claim to, and that 00:42:08.760 --> 00:42:10.500 align:middle line:84% means leveraging everything-- sell the house, 00:42:10.500 --> 00:42:12.375 align:middle line:84% the kids never go to college, forget the car. 00:42:12.375 --> 00:42:14.250 align:middle line:84% Every single cent I could rub together, 00:42:14.250 --> 00:42:17.340 align:middle line:84% Bezos has more than $100,000 for every dollar 00:42:17.340 --> 00:42:18.570 align:middle line:90% I could scrape together. 00:42:18.570 --> 00:42:22.290 align:middle line:84% So that's either impressive or depressing. 00:42:22.290 --> 00:42:23.280 align:middle line:90% It's mostly depressing. 00:42:23.280 --> 00:42:26.490 align:middle line:84% But 10 to the 5 is nowhere near 10 to the 500. 00:42:26.490 --> 00:42:29.400 align:middle line:84% OK, so now let's go cosmo-- let's take the whole universe. 00:42:29.400 --> 00:42:31.420 align:middle line:84% Let's put the universe to work for us. 00:42:31.420 --> 00:42:35.070 align:middle line:84% If we try to measure the age of our universe in seconds, 00:42:35.070 --> 00:42:37.950 align:middle line:90% we get a measly 10 to the 17. 00:42:37.950 --> 00:42:40.230 align:middle line:84% If we're a little fancier and say, 00:42:40.230 --> 00:42:43.170 align:middle line:84% let's measure the age of the universe with the shortest 00:42:43.170 --> 00:42:46.830 align:middle line:84% measurable unit of time that has been measured so far in quantum 00:42:46.830 --> 00:42:50.310 align:middle line:84% optics, that's about a femtosecond, so 10 to the minus 00:42:50.310 --> 00:42:51.060 align:middle line:90% 15th of a second. 00:42:51.060 --> 00:42:53.770 align:middle line:84% That only gets us to something like 10 to the 32. 00:42:53.770 --> 00:42:55.360 align:middle line:90% We're still scraping. 00:42:55.360 --> 00:42:57.440 align:middle line:84% That's nowhere near 10 to the 500. 00:42:57.440 --> 00:43:00.430 align:middle line:84% Now let's compare, say, the mass of the entire Milky Way 00:43:00.430 --> 00:43:03.550 align:middle line:84% galaxy to the mass of a single electron. 00:43:03.550 --> 00:43:04.480 align:middle line:90% That's really big. 00:43:04.480 --> 00:43:05.530 align:middle line:90% It's a big galaxy. 00:43:05.530 --> 00:43:06.670 align:middle line:90% Electrons are tiny. 00:43:06.670 --> 00:43:08.260 align:middle line:90% That ratio is 10 to the 71. 00:43:08.260 --> 00:43:09.490 align:middle line:90% You get the idea, right? 00:43:09.490 --> 00:43:12.580 align:middle line:84% 10 to the 71 is basically equal to zero 00:43:12.580 --> 00:43:15.590 align:middle line:84% when we're comparing it to a number like 10 to the 500. 00:43:15.590 --> 00:43:19.300 align:middle line:84% So what became either interesting or horrifying 00:43:19.300 --> 00:43:22.398 align:middle line:84% to me, I think-- horrifying-- in the early 2000s 00:43:22.398 --> 00:43:24.190 align:middle line:84% was that a number of these string theorists 00:43:24.190 --> 00:43:27.070 align:middle line:84% realized that there were unfathomably 00:43:27.070 --> 00:43:30.880 align:middle line:84% large numbers of what looked to be self-consistent, low energy 00:43:30.880 --> 00:43:36.010 align:middle line:84% string states within these superstring theory models. 00:43:36.010 --> 00:43:38.950 align:middle line:84% Now why should we care, because at least within this 00:43:38.950 --> 00:43:43.090 align:middle line:84% superstring theory framework, every single quantity with 00:43:43.090 --> 00:43:45.790 align:middle line:84% which we might try to characterize our universe-- 00:43:45.790 --> 00:43:48.550 align:middle line:84% the masses and charges of every kind of particle, 00:43:48.550 --> 00:43:51.520 align:middle line:84% every electron, quark, gluon, everything-- 00:43:51.520 --> 00:43:55.740 align:middle line:84% would depend, in principle, on the particular one out 00:43:55.740 --> 00:43:58.470 align:middle line:84% of those 10 to the 500 string states in which 00:43:58.470 --> 00:44:01.840 align:middle line:90% our universe happens to land. 00:44:01.840 --> 00:44:05.280 align:middle line:84% So even for elementary particles, this is a huge mess. 00:44:05.280 --> 00:44:08.920 align:middle line:84% Why is a charged electron that versus something else? 00:44:08.920 --> 00:44:12.190 align:middle line:84% Even for bulk or astrophysical properties with which we 00:44:12.190 --> 00:44:14.770 align:middle line:84% characterize our universe, things like the expansion 00:44:14.770 --> 00:44:17.800 align:middle line:84% rate-- that's the letter H, the Hubble expansion parameter-- 00:44:17.800 --> 00:44:20.410 align:middle line:84% the rate at which galaxies recede from each other, 00:44:20.410 --> 00:44:23.020 align:middle line:84% or things like the value of so-called dark energy, 00:44:23.020 --> 00:44:25.603 align:middle line:84% or the cosmological constant, even the bulk properties 00:44:25.603 --> 00:44:27.520 align:middle line:84% with which we try to characterize our universe 00:44:27.520 --> 00:44:31.600 align:middle line:84% at large, each of those numbers, the actual quantitative value 00:44:31.600 --> 00:44:34.630 align:middle line:84% would, in principle, depend on which one out of these 10 00:44:34.630 --> 00:44:37.460 align:middle line:84% to the 500 distinct string vacuum states. 00:44:37.460 --> 00:44:40.000 align:middle line:84% So how do we make any predictions or how 00:44:40.000 --> 00:44:43.960 align:middle line:84% to make any sense at all, empirically or quantitatively, 00:44:43.960 --> 00:44:46.510 align:middle line:84% out of the universe we live in, either from data 00:44:46.510 --> 00:44:48.760 align:middle line:84% like the Large Hadron Collider or from all 00:44:48.760 --> 00:44:51.250 align:middle line:84% of our astrophysics cosmology, if suddenly we 00:44:51.250 --> 00:44:53.920 align:middle line:84% have no way, or at least no practical way, 00:44:53.920 --> 00:44:56.800 align:middle line:84% it seems, to relate this kind of seemingly 00:44:56.800 --> 00:44:59.200 align:middle line:84% fundamental description of nature 00:44:59.200 --> 00:45:01.450 align:middle line:84% at these very tiny string scales to the things 00:45:01.450 --> 00:45:04.150 align:middle line:84% we can actually probe, measure, and wonder about? 00:45:04.150 --> 00:45:07.450 align:middle line:84% That became this new twist in the string theory saga 00:45:07.450 --> 00:45:09.110 align:middle line:90% in the early 2000. 00:45:09.110 --> 00:45:12.500 align:middle line:84% So here, once again, is Alan Guth. 00:45:12.500 --> 00:45:16.010 align:middle line:84% That idea about the so-called string landscape-- 00:45:16.010 --> 00:45:20.570 align:middle line:84% these 10 to the 500 equally self-consistent, 00:45:20.570 --> 00:45:22.490 align:middle line:84% low energy states of string theory-- 00:45:22.490 --> 00:45:24.530 align:middle line:84% that set of ideas kind of collided 00:45:24.530 --> 00:45:28.130 align:middle line:84% with some cosmological ideas which had first been 00:45:28.130 --> 00:45:29.630 align:middle line:90% developed quite independently. 00:45:29.630 --> 00:45:31.790 align:middle line:84% So we talked, in the most recent class session, 00:45:31.790 --> 00:45:35.480 align:middle line:84% about cosmic inflation, a set of ideas that I dearly love. 00:45:35.480 --> 00:45:37.490 align:middle line:84% It turns out early on in thinking 00:45:37.490 --> 00:45:39.230 align:middle line:84% about inflation, people, including 00:45:39.230 --> 00:45:41.570 align:middle line:84% Alan and Paul Steinhardt, and actually a number of folks 00:45:41.570 --> 00:45:42.903 align:middle line:90% we mentioned briefly last time-- 00:45:42.903 --> 00:45:46.010 align:middle line:84% Andrei Linde-- began to recognize something curious 00:45:46.010 --> 00:45:48.620 align:middle line:90% about these inflationary models. 00:45:48.620 --> 00:45:51.110 align:middle line:84% Basically, each inflationary patch-- 00:45:51.110 --> 00:45:53.720 align:middle line:84% each inflationary phase, let's say-- 00:45:53.720 --> 00:45:56.630 align:middle line:84% has some kind of natural lifetime. 00:45:56.630 --> 00:45:58.610 align:middle line:84% It's almost like a radioactive decay, 00:45:58.610 --> 00:46:00.590 align:middle line:84% that the configuration of those Higgs-like 00:46:00.590 --> 00:46:03.260 align:middle line:84% fields that temporarily drive this very 00:46:03.260 --> 00:46:06.650 align:middle line:84% rapid, accelerating expansion of space and time, 00:46:06.650 --> 00:46:10.040 align:middle line:84% they have a kind of natural decay, or natural lifetime. 00:46:10.040 --> 00:46:10.790 align:middle line:90% It's not infinite. 00:46:10.790 --> 00:46:12.832 align:middle line:84% In fact, we know it was a very short-lived period 00:46:12.832 --> 00:46:13.700 align:middle line:90% of inflation. 00:46:13.700 --> 00:46:17.750 align:middle line:84% And in fact, we can estimate for a given model what 00:46:17.750 --> 00:46:21.480 align:middle line:84% a half life would be for how long that phase is expected 00:46:21.480 --> 00:46:21.980 align:middle line:90% to last. 00:46:21.980 --> 00:46:24.440 align:middle line:84% It's probabilistic, it's a quantum mechanical process, 00:46:24.440 --> 00:46:27.260 align:middle line:84% but it's very similar to calculating a radioactive half 00:46:27.260 --> 00:46:27.980 align:middle line:90% life. 00:46:27.980 --> 00:46:30.030 align:middle line:84% And what became clear is that for many, many, 00:46:30.030 --> 00:46:31.822 align:middle line:84% many of these models, that otherwise seemed 00:46:31.822 --> 00:46:33.900 align:middle line:84% to match predictions on our sky beautifully, 00:46:33.900 --> 00:46:37.550 align:middle line:84% they're models that seemed to have many nice features, 00:46:37.550 --> 00:46:41.970 align:middle line:84% that the half life, the decay rate, so to speak, 00:46:41.970 --> 00:46:44.450 align:middle line:84% is actually smaller than the expansion rate. 00:46:44.450 --> 00:46:47.030 align:middle line:84% That in regions that have not yet decayed out 00:46:47.030 --> 00:46:50.540 align:middle line:84% of that phase, space should be stretching exponentially 00:46:50.540 --> 00:46:53.510 align:middle line:84% quickly, more quickly than the rate 00:46:53.510 --> 00:46:55.280 align:middle line:84% at which neighboring patches of space 00:46:55.280 --> 00:46:57.650 align:middle line:84% will fall out of that inflating phase. 00:46:57.650 --> 00:47:00.440 align:middle line:84% This became called eternal inflation. 00:47:00.440 --> 00:47:04.100 align:middle line:84% And the idea is that-- at least it seems self-consistent 00:47:04.100 --> 00:47:06.680 align:middle line:84% to suppose, based on what we otherwise know-- 00:47:06.680 --> 00:47:09.840 align:middle line:84% that if inflation begins anywhere in the universe, 00:47:09.840 --> 00:47:10.910 align:middle line:90% it never stops. 00:47:10.910 --> 00:47:12.710 align:middle line:90% It will stop at that location. 00:47:12.710 --> 00:47:14.570 align:middle line:84% It's not inflating for us right now. 00:47:14.570 --> 00:47:16.610 align:middle line:84% It'll stop at that location, but there'll 00:47:16.610 --> 00:47:18.950 align:middle line:84% be some other regions of spacetime that are still 00:47:18.950 --> 00:47:20.930 align:middle line:90% growing exponentially quickly. 00:47:20.930 --> 00:47:23.810 align:middle line:84% And that's because there's this competition 00:47:23.810 --> 00:47:27.920 align:middle line:84% between the rate for any given location in space to fall out 00:47:27.920 --> 00:47:28.740 align:middle line:90% of inflation. 00:47:28.740 --> 00:47:30.200 align:middle line:84% That's the kind of capital gamma, 00:47:30.200 --> 00:47:32.572 align:middle line:84% the kind of lifetime of inflating phase, 00:47:32.572 --> 00:47:35.030 align:middle line:84% and the rate at which the volume of some neighboring region 00:47:35.030 --> 00:47:36.330 align:middle line:90% continues to grow. 00:47:36.330 --> 00:47:38.450 align:middle line:84% So even though it's perfectly self-consistent 00:47:38.450 --> 00:47:40.460 align:middle line:84% for our universe that we see around us-- 00:47:40.460 --> 00:47:43.430 align:middle line:84% our observable universe-- not to be in that inflating phase 00:47:43.430 --> 00:47:46.730 align:middle line:84% right now, or to have phonon out of the primordial one, 00:47:46.730 --> 00:47:50.040 align:middle line:84% for all we know it looks more likely than not, 00:47:50.040 --> 00:47:52.917 align:middle line:84% at least in this scheme, that some regions so far 00:47:52.917 --> 00:47:54.500 align:middle line:84% away from us, we haven't received even 00:47:54.500 --> 00:47:56.630 align:middle line:84% a single light beam yet, they could still 00:47:56.630 --> 00:47:58.160 align:middle line:90% be in an inflating phase. 00:47:58.160 --> 00:48:01.400 align:middle line:84% And that could happen forever, because when that neighboring 00:48:01.400 --> 00:48:03.800 align:middle line:84% point falls out of inflation, some other volume of space 00:48:03.800 --> 00:48:06.230 align:middle line:90% will grow quicker still. 00:48:06.230 --> 00:48:08.300 align:middle line:84% So that led to this idea that Andrei Linde 00:48:08.300 --> 00:48:11.150 align:middle line:84% in particular really championed, and Alex Vilenkin at Tufts-- 00:48:11.150 --> 00:48:14.450 align:middle line:84% many scholars who were interested in cosmology-- 00:48:14.450 --> 00:48:17.510 align:middle line:84% began wondering about a so-called multiverse. 00:48:17.510 --> 00:48:19.435 align:middle line:84% Could it be that our observable universe-- 00:48:19.435 --> 00:48:21.560 align:middle line:84% everything we've seen, everything we could measure, 00:48:21.560 --> 00:48:23.120 align:middle line:90% the CMDB and all the rest-- 00:48:23.120 --> 00:48:26.330 align:middle line:84% is that actually just one self-contained 00:48:26.330 --> 00:48:32.495 align:middle line:84% region of space and time within an arbitrarily larger region 00:48:32.495 --> 00:48:34.370 align:middle line:84% in the midst of which other patches are still 00:48:34.370 --> 00:48:36.495 align:middle line:84% inflating exponentially and others have [INAUDIBLE] 00:48:36.495 --> 00:48:37.020 align:middle line:90% out as well? 00:48:37.020 --> 00:48:40.010 align:middle line:84% Could you have an infinite multiverse? 00:48:40.010 --> 00:48:43.700 align:middle line:84% If inflation never ends, if it really could happen towards T 00:48:43.700 --> 00:48:45.880 align:middle line:84% equals infinity, then in principle, you 00:48:45.880 --> 00:48:48.170 align:middle line:84% have an infinite volume of space. 00:48:48.170 --> 00:48:50.900 align:middle line:84% We occupy, we can observe around us, 00:48:50.900 --> 00:48:54.290 align:middle line:84% a very tiny, self-contained bubble 00:48:54.290 --> 00:48:55.850 align:middle line:90% within an infinite bathtub. 00:48:55.850 --> 00:48:59.510 align:middle line:84% We're like a soap bubble within a bathtub that actually 00:48:59.510 --> 00:49:02.100 align:middle line:90% is infinitely large in volume. 00:49:02.100 --> 00:49:04.720 align:middle line:84% So that was the idea of eternal inflation, 00:49:04.720 --> 00:49:06.470 align:middle line:84% suggesting-- not proving, but suggesting-- 00:49:06.470 --> 00:49:10.790 align:middle line:84% there could exist a multiverse, a huge collection of pocket 00:49:10.790 --> 00:49:13.010 align:middle line:84% universes well beyond our own with which 00:49:13.010 --> 00:49:15.260 align:middle line:90% we can gather information. 00:49:15.260 --> 00:49:17.660 align:middle line:84% And that idea then collided with, 00:49:17.660 --> 00:49:19.790 align:middle line:84% or was merged with, this series of ideas 00:49:19.790 --> 00:49:22.100 align:middle line:84% from string theory in the early 2000s, 00:49:22.100 --> 00:49:24.380 align:middle line:84% in particular the so-called string landscape. 00:49:24.380 --> 00:49:27.410 align:middle line:84% If there are 10 to the 500 distinct, 00:49:27.410 --> 00:49:30.290 align:middle line:84% self-consistent, low energy string states, 00:49:30.290 --> 00:49:33.260 align:middle line:84% and you have an infinite number of chances for them 00:49:33.260 --> 00:49:36.110 align:middle line:84% to be realized, because you have an infinite multiverse 00:49:36.110 --> 00:49:38.930 align:middle line:84% within which you can have these self-consistent pocket 00:49:38.930 --> 00:49:41.510 align:middle line:84% universes constantly inflating and then falling out 00:49:41.510 --> 00:49:44.120 align:middle line:84% of inflation, while neighboring regions inflate, 00:49:44.120 --> 00:49:48.020 align:middle line:84% could it be that every single string state is actually 00:49:48.020 --> 00:49:50.430 align:middle line:84% realized an infinite number of times? 00:49:50.430 --> 00:49:52.100 align:middle line:90% So 10 to the 500 is big. 00:49:52.100 --> 00:49:54.450 align:middle line:84% It's infinitely smaller than infinity, right? 00:49:54.450 --> 00:49:56.310 align:middle line:90% You can see where-- 00:49:56.310 --> 00:49:58.590 align:middle line:84% I hope it's clear that I'm not endorsing these ideas. 00:49:58.590 --> 00:50:02.030 align:middle line:84% I find these ideas pretty unusual, 00:50:02.030 --> 00:50:03.350 align:middle line:90% is the nicest way I can put it. 00:50:03.350 --> 00:50:05.900 align:middle line:84% But I can at least appreciate why so many of my colleagues 00:50:05.900 --> 00:50:07.440 align:middle line:84% have been led to wonder about them. 00:50:07.440 --> 00:50:09.470 align:middle line:84% You can see the thought train that 00:50:09.470 --> 00:50:11.630 align:middle line:84% leads to this as the next question 00:50:11.630 --> 00:50:13.795 align:middle line:90% or tentative conclusion. 00:50:13.795 --> 00:50:15.920 align:middle line:84% You can also see how it's getting harder and harder 00:50:15.920 --> 00:50:19.550 align:middle line:84% to get the reinforcement, or the input, or the kind of dialogue 00:50:19.550 --> 00:50:23.360 align:middle line:84% with empirical data, unlike, say, the early days of quark 00:50:23.360 --> 00:50:25.310 align:middle line:90% physics. 00:50:25.310 --> 00:50:29.480 align:middle line:84% I hope I can at least convey to you, as a collegial neighboring 00:50:29.480 --> 00:50:32.780 align:middle line:84% skeptic, why people ask these kinds of questions, 00:50:32.780 --> 00:50:35.210 align:middle line:84% this kind of merger of ideas from string theory 00:50:35.210 --> 00:50:38.390 align:middle line:90% and from things like inflation. 00:50:38.390 --> 00:50:39.590 align:middle line:90% OK. 00:50:39.590 --> 00:50:41.900 align:middle line:84% So if every one of the parameters that we measure 00:50:41.900 --> 00:50:44.180 align:middle line:84% empirically-- the masses and charges of elementary 00:50:44.180 --> 00:50:46.940 align:middle line:84% particles, larger properties like the Hubble expansion 00:50:46.940 --> 00:50:47.510 align:middle line:90% rate-- 00:50:47.510 --> 00:50:50.660 align:middle line:84% if they were slightly different, people began to wonder, 00:50:50.660 --> 00:50:52.900 align:middle line:84% would the universe, as we know it, 00:50:52.900 --> 00:50:54.410 align:middle line:84% would our observable universe have 00:50:54.410 --> 00:50:56.720 align:middle line:84% shown anything like the pattern of evolution 00:50:56.720 --> 00:50:59.000 align:middle line:84% that we have actually empirically measured? 00:50:59.000 --> 00:51:03.270 align:middle line:84% Could there have been galaxies that actually formed stably 00:51:03.270 --> 00:51:05.580 align:middle line:84% if the expansion rate were slightly larger? 00:51:05.580 --> 00:51:07.920 align:middle line:84% The idea was if the expansion rate were actually 00:51:07.920 --> 00:51:09.870 align:middle line:84% a little bit larger in early times, 00:51:09.870 --> 00:51:12.420 align:middle line:84% could galaxies never have overcome 00:51:12.420 --> 00:51:15.280 align:middle line:84% the expansive pull of gravity, where 00:51:15.280 --> 00:51:17.760 align:middle line:84% they've never coalesced to become 00:51:17.760 --> 00:51:20.370 align:middle line:90% gravitationally self bound. 00:51:20.370 --> 00:51:23.155 align:middle line:84% If the expansion rate were slightly larger, 00:51:23.155 --> 00:51:25.530 align:middle line:84% then it happened to have been in our observable universe, 00:51:25.530 --> 00:51:27.270 align:middle line:84% maybe there would never have been galaxies. 00:51:27.270 --> 00:51:28.620 align:middle line:84% If there's never galaxies, could there 00:51:28.620 --> 00:51:30.583 align:middle line:84% be things like stable solar systems or planets? 00:51:30.583 --> 00:51:32.250 align:middle line:84% You can play this game all the way down. 00:51:32.250 --> 00:51:34.290 align:middle line:84% Could there be life as we know it, 00:51:34.290 --> 00:51:38.247 align:middle line:84% if any of these seemingly fundamental parameters 00:51:38.247 --> 00:51:40.080 align:middle line:84% have been slightly different from the values 00:51:40.080 --> 00:51:41.490 align:middle line:90% that we happen to measure? 00:51:41.490 --> 00:51:44.100 align:middle line:84% Now it turns out historians can remind us, 00:51:44.100 --> 00:51:45.780 align:middle line:90% that's a very old argument. 00:51:45.780 --> 00:51:48.450 align:middle line:84% If that sounds familiar, especially to our historians 00:51:48.450 --> 00:51:50.700 align:middle line:84% of science in the group, you're right. 00:51:50.700 --> 00:51:55.020 align:middle line:84% That idea-- that if any parameters of our environment 00:51:55.020 --> 00:51:57.960 align:middle line:84% had been slightly different, then we wouldn't be here-- 00:51:57.960 --> 00:52:01.590 align:middle line:84% that goes back at least, in the European tradition, 00:52:01.590 --> 00:52:03.480 align:middle line:84% at least to the late 17th century. 00:52:03.480 --> 00:52:06.420 align:middle line:84% I'm sure we could find clever anticipations even earlier. 00:52:06.420 --> 00:52:09.030 align:middle line:84% But some of the most famous articulations of that 00:52:09.030 --> 00:52:12.840 align:middle line:84% come from this very charming book by Bernard de Fontenelle, 00:52:12.840 --> 00:52:16.620 align:middle line:84% a French natural philosopher, translated as Conversations 00:52:16.620 --> 00:52:17.737 align:middle line:90% on the Plurality of World. 00:52:17.737 --> 00:52:18.570 align:middle line:90% It's still in print. 00:52:18.570 --> 00:52:20.520 align:middle line:90% You can buy a cheap paperback. 00:52:20.520 --> 00:52:23.430 align:middle line:84% Very soon after that, Isaac Newton was writing letters 00:52:23.430 --> 00:52:25.383 align:middle line:84% privately to a theologian friend of his-- 00:52:25.383 --> 00:52:27.300 align:middle line:84% they were published soon after Newton's death, 00:52:27.300 --> 00:52:28.620 align:middle line:90% his friend Richard Bentley-- 00:52:28.620 --> 00:52:30.600 align:middle line:84% where he was musing on this as well. 00:52:30.600 --> 00:52:33.990 align:middle line:84% To these folks, this kind of fine-tuning argument 00:52:33.990 --> 00:52:36.990 align:middle line:84% was to them absolute, incontrovertible proof 00:52:36.990 --> 00:52:38.370 align:middle line:90% that God exists. 00:52:38.370 --> 00:52:42.240 align:middle line:84% And to them that was a fairly recognizable Christian God 00:52:42.240 --> 00:52:44.400 align:middle line:84% of the Bible, in their reckoning. 00:52:44.400 --> 00:52:47.370 align:middle line:84% The idea was that God must have made the world just for us. 00:52:47.370 --> 00:52:50.302 align:middle line:84% If the Earth were slightly further from the sun, 00:52:50.302 --> 00:52:52.260 align:middle line:84% it would be too cold for us to have flourished. 00:52:52.260 --> 00:52:54.600 align:middle line:84% If the Earth were slightly closer to the sun than it is, 00:52:54.600 --> 00:52:55.440 align:middle line:90% it would be too hot. 00:52:55.440 --> 00:52:57.210 align:middle line:84% These kinds of fine-tuning arguments 00:52:57.210 --> 00:53:00.186 align:middle line:84% have a very, very long history in astronomy and astrophysics. 00:53:00.186 --> 00:53:02.370 align:middle line:84% And for a lot of the early advocates, 00:53:02.370 --> 00:53:04.880 align:middle line:84% this was taken as literally proof 00:53:04.880 --> 00:53:08.210 align:middle line:84% of a biblical story of Genesis, that God made the universe 00:53:08.210 --> 00:53:09.710 align:middle line:90% literally for us. 00:53:09.710 --> 00:53:14.150 align:middle line:84% So is that how these later string theorists formulated? 00:53:14.150 --> 00:53:16.280 align:middle line:90% Not really-- not at all. 00:53:16.280 --> 00:53:19.190 align:middle line:84% So 2/3 of that fine-tuning argument 00:53:19.190 --> 00:53:22.160 align:middle line:84% sounds just like what Isaac Newton or de Fontenelle had 00:53:22.160 --> 00:53:24.170 align:middle line:90% been saying 300 years earlier. 00:53:24.170 --> 00:53:26.330 align:middle line:84% But some of these later, more recent advocates 00:53:26.330 --> 00:53:28.340 align:middle line:90% give it an interesting twist-- 00:53:28.340 --> 00:53:32.570 align:middle line:84% the appeal not to a religious or a kind of supernatural force, 00:53:32.570 --> 00:53:34.812 align:middle line:84% they appeal back to us in something 00:53:34.812 --> 00:53:36.770 align:middle line:84% called the anthropic principle, which again you 00:53:36.770 --> 00:53:38.420 align:middle line:90% might have read about. 00:53:38.420 --> 00:53:40.880 align:middle line:84% The idea, as I've mentioned before, as even 00:53:40.880 --> 00:53:43.310 align:middle line:84% Fontenelle and Newton had had recognized, 00:53:43.310 --> 00:53:47.030 align:middle line:84% is that the natural constants seem 00:53:47.030 --> 00:53:49.370 align:middle line:84% to have to be within very specific and actually 00:53:49.370 --> 00:53:53.300 align:middle line:84% narrow, fine-tuned ranges for anything like life 00:53:53.300 --> 00:53:57.180 align:middle line:84% as we know it to have survived, to have evolved and survived. 00:53:57.180 --> 00:54:00.050 align:middle line:84% So if those parameters were not within those finely-tuned 00:54:00.050 --> 00:54:02.740 align:middle line:84% ranges, then we wouldn't be here to ask these questions. 00:54:02.740 --> 00:54:05.730 align:middle line:84% So it comes back to what's the precondition for cosmologists, 00:54:05.730 --> 00:54:08.040 align:middle line:84% or any humans at all, to be asking 00:54:08.040 --> 00:54:09.910 align:middle line:90% questions about our universe? 00:54:09.910 --> 00:54:12.330 align:middle line:84% And so again, here are some other very nice, popular books 00:54:12.330 --> 00:54:14.730 align:middle line:84% that came out around 15 years ago or so. 00:54:14.730 --> 00:54:17.070 align:middle line:84% One by Alex Vilenkin, who teaches at Tufts. 00:54:17.070 --> 00:54:20.380 align:middle line:84% He's very nearby, real pioneer in this work. 00:54:20.380 --> 00:54:22.020 align:middle line:84% Another book by Leonard Susskind, 00:54:22.020 --> 00:54:23.280 align:middle line:90% who teaches at Stanford. 00:54:23.280 --> 00:54:27.120 align:middle line:84% And they're both discussing these more recent developments 00:54:27.120 --> 00:54:29.598 align:middle line:84% when the string landscape meets the multiverse. 00:54:29.598 --> 00:54:31.140 align:middle line:84% And you can see each of their answers 00:54:31.140 --> 00:54:33.360 align:middle line:84% are very clearly not appealing to a kind 00:54:33.360 --> 00:54:35.470 align:middle line:90% of religious explanation. 00:54:35.470 --> 00:54:38.190 align:middle line:84% In fact, they say the basic structure of the argument 00:54:38.190 --> 00:54:40.890 align:middle line:84% is if there are 10 to the 500 distinct string 00:54:40.890 --> 00:54:44.088 align:middle line:84% states, each of which has an infinite number of chances-- 00:54:44.088 --> 00:54:46.380 align:middle line:84% because this eternal inflation argument-- each of which 00:54:46.380 --> 00:54:49.470 align:middle line:84% is realized in an actual physical volume of space, 00:54:49.470 --> 00:54:51.870 align:middle line:84% an infinite number of times, then it just 00:54:51.870 --> 00:54:53.490 align:middle line:90% takes pure, random chance. 00:54:53.490 --> 00:54:56.130 align:middle line:84% It's almost kind of Darwinian, that we 00:54:56.130 --> 00:54:58.530 align:middle line:84% would evolve where we are to measure the constants we 00:54:58.530 --> 00:54:59.930 align:middle line:90% do because we had no choice. 00:54:59.930 --> 00:55:02.450 align:middle line:84% That is say, any difference in those constants, which 00:55:02.450 --> 00:55:05.660 align:middle line:84% might really be happening arbitrarily far away, 00:55:05.660 --> 00:55:07.160 align:middle line:84% people like us wouldn't have evolved 00:55:07.160 --> 00:55:09.060 align:middle line:90% there to ask those questions. 00:55:09.060 --> 00:55:11.030 align:middle line:84% So the explanation-- why do we measure 00:55:11.030 --> 00:55:12.770 align:middle line:84% this value for the expansion rate 00:55:12.770 --> 00:55:16.890 align:middle line:84% or that value for the mass electron-- is not, they argue, 00:55:16.890 --> 00:55:19.790 align:middle line:84% because we have to know why one special value out 00:55:19.790 --> 00:55:22.590 align:middle line:84% of those 10 to the 500 gets picked out, but simply 00:55:22.590 --> 00:55:23.990 align:middle line:90% to say they're all out there. 00:55:23.990 --> 00:55:26.000 align:middle line:84% Each of the 10 to the 500 is out there 00:55:26.000 --> 00:55:30.170 align:middle line:90% beyond our immediate vicinity. 00:55:30.170 --> 00:55:32.630 align:middle line:84% We should only ever expect to measure the constants 00:55:32.630 --> 00:55:36.170 align:middle line:84% that we do, because of the chain of causes 00:55:36.170 --> 00:55:38.652 align:middle line:84% that had to happen for life, at least as we currently 00:55:38.652 --> 00:55:40.610 align:middle line:84% understand it, to have been able to come around 00:55:40.610 --> 00:55:41.820 align:middle line:90% to ask those questions. 00:55:41.820 --> 00:55:44.680 align:middle line:84% So it's not that you pick out one string state physically. 00:55:44.680 --> 00:55:46.430 align:middle line:84% You say, we'd only ever be able to measure 00:55:46.430 --> 00:55:49.262 align:middle line:90% a certain very small subset. 00:55:49.262 --> 00:55:51.470 align:middle line:84% So some critics-- I'm going to say say critics within 00:55:51.470 --> 00:55:51.980 align:middle line:90% physics-- 00:55:51.980 --> 00:55:54.830 align:middle line:84% critics, Nobel laureates, within high energy theory for example, 00:55:54.830 --> 00:55:57.330 align:middle line:84% some of them really, really don't like this idea. 00:55:57.330 --> 00:56:00.620 align:middle line:84% I personally am still rather ambivalent, to put it mildly. 00:56:00.620 --> 00:56:02.300 align:middle line:84% Critics have called it, in print, 00:56:02.300 --> 00:56:05.010 align:middle line:84% dangerous, disappointing, a virus-- 00:56:05.010 --> 00:56:07.280 align:middle line:84% which was scary even before coronavirus, 00:56:07.280 --> 00:56:09.200 align:middle line:90% now it really makes me shudder-- 00:56:09.200 --> 00:56:12.110 align:middle line:84% an abdication of what physicists should be doing. 00:56:12.110 --> 00:56:14.660 align:middle line:90% This inspires strong reactions. 00:56:14.660 --> 00:56:18.095 align:middle line:84% Lenny Susskind, in this book here, 00:56:18.095 --> 00:56:19.220 align:middle line:90% knows that perfectly well. 00:56:19.220 --> 00:56:22.740 align:middle line:84% He's no stranger to making bold statements. 00:56:22.740 --> 00:56:25.610 align:middle line:84% So Susskind actually borrowed, for the epigraph 00:56:25.610 --> 00:56:28.850 align:middle line:84% of his book, the famous, and probably apocryphal, statement 00:56:28.850 --> 00:56:32.930 align:middle line:84% from Pierre Simon Laplace in answering Napoleon. 00:56:32.930 --> 00:56:36.470 align:middle line:84% Napoleon was so impressed by Laplace's 18th century, 00:56:36.470 --> 00:56:37.760 align:middle line:90% Newtonian cosmos. 00:56:37.760 --> 00:56:40.280 align:middle line:84% They could account for the motion of planets and comets 00:56:40.280 --> 00:56:42.020 align:middle line:84% and everything with such great precision. 00:56:42.020 --> 00:56:44.660 align:middle line:84% Napoleon supposedly had asked Laplace, 00:56:44.660 --> 00:56:46.760 align:middle line:84% where is the room for God in your theory? 00:56:46.760 --> 00:56:49.260 align:middle line:84% And Laplace, like Susskind, says, your Highness 00:56:49.260 --> 00:56:51.150 align:middle line:84% I have no need of that hypothesis. 00:56:51.150 --> 00:56:53.400 align:middle line:84% He doesn't need, as Newton seemed to need, 00:56:53.400 --> 00:56:57.870 align:middle line:84% an appeal to a kind of religious or biblical supernatural force. 00:56:57.870 --> 00:56:59.640 align:middle line:84% He says, you just have to have 10 00:56:59.640 --> 00:57:03.030 align:middle line:84% to the 500 distinct states and an infinite number of chances. 00:57:03.030 --> 00:57:05.970 align:middle line:84% And we would measure, around our universe, only 00:57:05.970 --> 00:57:07.620 align:middle line:84% those that would be consistent with us 00:57:07.620 --> 00:57:09.240 align:middle line:90% being there to measure them. 00:57:09.240 --> 00:57:10.860 align:middle line:84% Again, I'm not endorsing that view. 00:57:10.860 --> 00:57:14.130 align:middle line:84% I find that interesting, although I personally 00:57:14.130 --> 00:57:15.060 align:middle line:90% am rather ambivalent. 00:57:15.060 --> 00:57:18.030 align:middle line:84% But hopefully, you can see how the kind of chain of reasoning 00:57:18.030 --> 00:57:20.220 align:middle line:84% has unfolded to where this is now 00:57:20.220 --> 00:57:22.410 align:middle line:84% talked about with some great attention, 00:57:22.410 --> 00:57:26.190 align:middle line:84% and inspires great passions among physicists 00:57:26.190 --> 00:57:27.450 align:middle line:90% and cosmologists. 00:57:27.450 --> 00:57:28.680 align:middle line:90% So let me pause there again. 00:57:28.680 --> 00:57:30.840 align:middle line:84% Any questions on the string landscape, 00:57:30.840 --> 00:57:33.878 align:middle line:84% on eternal inflation, on anthropic principle, 00:57:33.878 --> 00:57:34.920 align:middle line:90% any of these juicy ideas? 00:57:34.920 --> 00:57:38.800 align:middle line:90% 00:57:38.800 --> 00:57:39.710 align:middle line:90% Very good, Steven. 00:57:39.710 --> 00:57:40.210 align:middle line:90% Thank you. 00:57:40.210 --> 00:57:42.050 align:middle line:90% That's a great question. 00:57:42.050 --> 00:57:44.950 align:middle line:84% The short answer is, in principle, yes. 00:57:44.950 --> 00:57:47.920 align:middle line:84% If we had a arbitrarily powerful microscope, 00:57:47.920 --> 00:57:50.260 align:middle line:84% if we could zoom in to length scales of order-- 00:57:50.260 --> 00:57:52.700 align:middle line:84% the Planck length, 10 to minus 35th of a meter, 00:57:52.700 --> 00:57:53.800 align:middle line:90% which we can't-- 00:57:53.800 --> 00:57:56.110 align:middle line:84% that it could be that each of those 00:57:56.110 --> 00:57:59.317 align:middle line:84% was separately realized at each point, 00:57:59.317 --> 00:58:01.525 align:middle line:84% that we had separately called X equals 1, X equals 2, 00:58:01.525 --> 00:58:04.000 align:middle line:84% X equals 3 of the macroscopic space 00:58:04.000 --> 00:58:07.690 align:middle line:84% that we otherwise can move around and measure. 00:58:07.690 --> 00:58:10.270 align:middle line:84% So it's not that they must have been, 00:58:10.270 --> 00:58:12.670 align:middle line:84% but actually that they could have been, 00:58:12.670 --> 00:58:15.010 align:middle line:84% that these are each self-consistent, viable 00:58:15.010 --> 00:58:19.600 align:middle line:84% solutions to this complicated set of superstring theory 00:58:19.600 --> 00:58:20.740 align:middle line:90% equations. 00:58:20.740 --> 00:58:23.590 align:middle line:84% And, in principle, each of them could independently 00:58:23.590 --> 00:58:24.460 align:middle line:90% be realized. 00:58:24.460 --> 00:58:27.070 align:middle line:84% Or it could be that there's some kind of coherent structure, 00:58:27.070 --> 00:58:28.862 align:middle line:84% where they all look the same in one region, 00:58:28.862 --> 00:58:32.050 align:middle line:84% but then in some macroscopically distant region, 00:58:32.050 --> 00:58:36.010 align:middle line:84% some other set of values might have taken place. 00:58:36.010 --> 00:58:38.420 align:middle line:84% When the idea gets combined with the string landscape, 00:58:38.420 --> 00:58:42.050 align:middle line:84% it's more the latter, that there are actually 00:58:42.050 --> 00:58:44.360 align:middle line:84% microscopic regions where there would have been one 00:58:44.360 --> 00:58:45.860 align:middle line:90% of those states versus another. 00:58:45.860 --> 00:58:48.740 align:middle line:84% But then something like the Higgs field 00:58:48.740 --> 00:58:52.400 align:middle line:84% or an inflation-causing field settles into one of those 00:58:52.400 --> 00:58:54.860 align:middle line:84% vacuum states versus another, and then the volume of that 00:58:54.860 --> 00:58:55.427 align:middle line:90% space-- 00:58:55.427 --> 00:58:57.260 align:middle line:84% the three dimensional volume of that space-- 00:58:57.260 --> 00:58:58.830 align:middle line:90% grows exponentially. 00:58:58.830 --> 00:59:01.430 align:middle line:84% So when you start combining the 10 to the 500 00:59:01.430 --> 00:59:03.662 align:middle line:84% with ideas about inflation, then it 00:59:03.662 --> 00:59:05.120 align:middle line:84% becomes more like we should only be 00:59:05.120 --> 00:59:08.810 align:middle line:84% able to measure one of those versions macroscopically, 00:59:08.810 --> 00:59:11.730 align:middle line:84% but all the others could still be realized elsewhere. 00:59:11.730 --> 00:59:13.820 align:middle line:84% So to the string theorist alone, the idea 00:59:13.820 --> 00:59:16.190 align:middle line:84% was each of those could be a self-consistent solution 00:59:16.190 --> 00:59:22.100 align:middle line:84% at any arbitrarily small location in the X, Y, Z 00:59:22.100 --> 00:59:23.430 align:middle line:90% space we use. 00:59:23.430 --> 00:59:26.540 align:middle line:84% But if you start thinking about combining 00:59:26.540 --> 00:59:30.830 align:middle line:84% that with inflation, the idea tends to be that all those are 00:59:30.830 --> 00:59:33.380 align:middle line:84% near each other, so to speak, at early times, 00:59:33.380 --> 00:59:36.170 align:middle line:84% but then whatever's going to start driving 00:59:36.170 --> 00:59:40.580 align:middle line:84% a macroscopic inflation for the dimensions that we live in, 00:59:40.580 --> 00:59:44.420 align:middle line:84% will actually stretch the space containing that one string 00:59:44.420 --> 00:59:46.850 align:middle line:90% state, rather than the neighbor. 00:59:46.850 --> 00:59:51.625 align:middle line:84% So I guess in an eternal inflation multiverse model, 00:59:51.625 --> 00:59:53.750 align:middle line:84% I think the idea would more be like we would expect 00:59:53.750 --> 00:59:56.930 align:middle line:84% to have a coherence, that if we could zoom in 00:59:56.930 --> 00:59:58.970 align:middle line:84% with our microscope we'd measure the same string 00:59:58.970 --> 01:00:01.492 align:middle line:84% state within our own macroscopic bubble, 01:00:01.492 --> 01:00:03.950 align:middle line:84% even though, in principle, there could be bubbles out there 01:00:03.950 --> 01:00:05.820 align:middle line:90% where it's different. 01:00:05.820 --> 01:00:07.493 align:middle line:90% So Alex asks-- oh, go ahead. 01:00:07.493 --> 01:00:08.160 align:middle line:90% Steve, go ahead. 01:00:08.160 --> 01:00:09.160 align:middle line:90% Do you have a follow up? 01:00:09.160 --> 01:00:11.843 align:middle line:90% 01:00:11.843 --> 01:00:13.260 align:middle line:84% I think, in principle, they could, 01:00:13.260 --> 01:00:15.427 align:middle line:84% but if you add on the dynamics, that you're actually 01:00:15.427 --> 01:00:18.780 align:middle line:84% expanding-- really stretching one region of space 01:00:18.780 --> 01:00:20.170 align:middle line:90% to the exclusion of another-- 01:00:20.170 --> 01:00:21.780 align:middle line:84% then you're stretching the space that was filled 01:00:21.780 --> 01:00:22.620 align:middle line:90% with one of those states. 01:00:22.620 --> 01:00:23.620 align:middle line:90% I think that's the idea. 01:00:23.620 --> 01:00:26.220 align:middle line:84% So Alex asks, of those 10 to 500 states before the infinities, 01:00:26.220 --> 01:00:27.450 align:middle line:90% how many are actually livable? 01:00:27.450 --> 01:00:27.660 align:middle line:90% Right. 01:00:27.660 --> 01:00:28.993 align:middle line:90% So that's a good question, Alex. 01:00:28.993 --> 01:00:30.957 align:middle line:84% And part of the question is, honestly, I don't 01:00:30.957 --> 01:00:32.040 align:middle line:90% think anyone really knows. 01:00:32.040 --> 01:00:35.545 align:middle line:84% Because we could take what we think 01:00:35.545 --> 01:00:37.170 align:middle line:84% we know about the origin of life-- what 01:00:37.170 --> 01:00:38.550 align:middle line:84% I know about the origin of life is approximately 01:00:38.550 --> 01:00:40.260 align:middle line:84% zero, but people who look at it more carefully-- 01:00:40.260 --> 01:00:41.820 align:middle line:84% or even something that may be closer 01:00:41.820 --> 01:00:44.760 align:middle line:84% to astrophysics, like what would it take for stable galaxies 01:00:44.760 --> 01:00:47.430 align:middle line:84% to form and not get ripped apart by a too fast 01:00:47.430 --> 01:00:48.630 align:middle line:90% expanding early universe? 01:00:48.630 --> 01:00:52.960 align:middle line:84% That's one that people looked at in more quantitative detail. 01:00:52.960 --> 01:00:55.600 align:middle line:84% So if you change one parameter, in this case, 01:00:55.600 --> 01:00:58.140 align:middle line:84% the early expansion rate, you either do or don't 01:00:58.140 --> 01:01:00.082 align:middle line:90% get stable galaxies to form. 01:01:00.082 --> 01:01:02.040 align:middle line:84% But we have a lot of parameters we could tweak. 01:01:02.040 --> 01:01:04.445 align:middle line:84% What if we tweaked the electromagnetic force, 01:01:04.445 --> 01:01:05.820 align:middle line:84% and the unit strength of gravity, 01:01:05.820 --> 01:01:07.560 align:middle line:84% and the expansion rate, and-- right? 01:01:07.560 --> 01:01:10.440 align:middle line:84% So who's to say, in this multi-dimensional system 01:01:10.440 --> 01:01:13.000 align:middle line:84% of constants, even the ones that we know about, 01:01:13.000 --> 01:01:16.720 align:middle line:84% that one couldn't find compensating solutions? 01:01:16.720 --> 01:01:18.730 align:middle line:84% Could we find galaxies for them if we 01:01:18.730 --> 01:01:20.920 align:middle line:84% did have the faster early expansion rate, 01:01:20.920 --> 01:01:24.190 align:middle line:84% but if we tweaked three other parameters of our choice? 01:01:24.190 --> 01:01:27.010 align:middle line:84% And that's, I think, a well-posed critique 01:01:27.010 --> 01:01:30.470 align:middle line:84% of this wiggle one parameter, find things do or don't work. 01:01:30.470 --> 01:01:32.300 align:middle line:84% And to be honest, I think a lot of the work 01:01:32.300 --> 01:01:34.870 align:middle line:84% so far has still been, understandably-- 01:01:34.870 --> 01:01:37.690 align:middle line:84% to make it computationally tractable-- understandably, 01:01:37.690 --> 01:01:40.210 align:middle line:84% it's still mostly been wiggle one constant what changes. 01:01:40.210 --> 01:01:42.922 align:middle line:90% 01:01:42.922 --> 01:01:45.380 align:middle line:84% If the value of the electric charge was slightly different, 01:01:45.380 --> 01:01:47.650 align:middle line:84% could you have stable hydrogen atoms, yes or no. 01:01:47.650 --> 01:01:50.140 align:middle line:84% OK, well, if I tweak that in five other things, 01:01:50.140 --> 01:01:51.310 align:middle line:90% maybe I can again. 01:01:51.310 --> 01:01:54.910 align:middle line:84% So the short answer, Alex, to your very well-posed question 01:01:54.910 --> 01:01:57.610 align:middle line:84% is, I don't know and, frankly, I don't think anyone knows. 01:01:57.610 --> 01:01:59.140 align:middle line:84% The way people have tried to pose 01:01:59.140 --> 01:02:02.043 align:middle line:84% that is in this somewhat simplified 01:02:02.043 --> 01:02:03.460 align:middle line:84% or, let's say, tractable approach. 01:02:03.460 --> 01:02:03.970 align:middle line:90% But I agree. 01:02:03.970 --> 01:02:05.500 align:middle line:84% If we're honest about it, I think 01:02:05.500 --> 01:02:07.815 align:middle line:90% it's a wide open question. 01:02:07.815 --> 01:02:09.190 align:middle line:84% Kay asks, is there any connection 01:02:09.190 --> 01:02:11.710 align:middle line:84% between the anthropic principle and the [INAUDIBLE] idea 01:02:11.710 --> 01:02:13.540 align:middle line:84% that quantum values don't exist until we choose to measure? 01:02:13.540 --> 01:02:14.665 align:middle line:90% There could be connections. 01:02:14.665 --> 01:02:15.910 align:middle line:90% It's a great question, Kay. 01:02:15.910 --> 01:02:18.570 align:middle line:84% One of my coauthors-- so now it's getting close to home, 01:02:18.570 --> 01:02:21.580 align:middle line:84% I've been tainted by all these crazy people--no-- 01:02:21.580 --> 01:02:25.270 align:middle line:84% a very dear friend has actually published on a quantum 01:02:25.270 --> 01:02:25.900 align:middle line:90% multiverse. 01:02:25.900 --> 01:02:28.300 align:middle line:84% What if the many worlds interpretation from quantum 01:02:28.300 --> 01:02:29.950 align:middle line:84% theory-- which I think is kind of like what you're referring 01:02:29.950 --> 01:02:30.940 align:middle line:90% to here, Kay-- 01:02:30.940 --> 01:02:33.190 align:middle line:84% what if the many worlds interpretation of quantum 01:02:33.190 --> 01:02:36.040 align:middle line:84% theory were actually realized in physically 01:02:36.040 --> 01:02:39.790 align:middle line:84% distinct but disjointed regions of space, 01:02:39.790 --> 01:02:41.930 align:middle line:90% like in the multiverse? 01:02:41.930 --> 01:02:43.930 align:middle line:84% So one can at least try to pursue this question. 01:02:43.930 --> 01:02:45.020 align:middle line:90% It's not required. 01:02:45.020 --> 01:02:48.700 align:middle line:84% So typically, these are actually separate kinds of many worlds 01:02:48.700 --> 01:02:50.720 align:middle line:90% that people talk about. 01:02:50.720 --> 01:02:53.230 align:middle line:84% And so typically, they're kept separate. 01:02:53.230 --> 01:02:55.030 align:middle line:84% But certainly, conceptually, one can 01:02:55.030 --> 01:02:57.100 align:middle line:84% try to build these self-consistent ideas 01:02:57.100 --> 01:02:59.600 align:middle line:84% and maybe even see if that leads to new predictions. 01:02:59.600 --> 01:03:00.985 align:middle line:90% So far I haven't seen any. 01:03:00.985 --> 01:03:02.860 align:middle line:84% But one can try to see if these sets of ideas 01:03:02.860 --> 01:03:04.450 align:middle line:84% could even fit together in principle, 01:03:04.450 --> 01:03:06.787 align:middle line:84% let alone do they help explain one another. 01:03:06.787 --> 01:03:08.120 align:middle line:90% So Kay, that's a great question. 01:03:08.120 --> 01:03:10.870 align:middle line:84% So people do work very squarely on that. 01:03:10.870 --> 01:03:12.880 align:middle line:84% Fisher asks, since these X dimensions are 01:03:12.880 --> 01:03:14.710 align:middle line:84% on the scale of the Planck length, 01:03:14.710 --> 01:03:16.127 align:middle line:84% I imagine there's no experiment we 01:03:16.127 --> 01:03:18.680 align:middle line:84% can perform to determine if they have any physical meaning? 01:03:18.680 --> 01:03:19.180 align:middle line:90% Good. 01:03:19.180 --> 01:03:22.600 align:middle line:84% So I should say, the limits, empirically, 01:03:22.600 --> 01:03:24.970 align:middle line:84% are that these extra dimensions can't be larger 01:03:24.970 --> 01:03:26.710 align:middle line:90% than about millimeter scale. 01:03:26.710 --> 01:03:29.423 align:middle line:84% And millimeter is a lot bigger than the Planck length. 01:03:29.423 --> 01:03:31.840 align:middle line:84% There's no compelling reason why they should be millimeter 01:03:31.840 --> 01:03:33.370 align:middle line:90% scale and not larger, right? 01:03:33.370 --> 01:03:35.770 align:middle line:84% So that would still take some explaining. 01:03:35.770 --> 01:03:38.560 align:middle line:84% But the high precision tests of gravity-- 01:03:38.560 --> 01:03:40.420 align:middle line:84% that's where these limits mostly come from. 01:03:40.420 --> 01:03:43.990 align:middle line:84% Testing gravity in a classical, general, relativistic 01:03:43.990 --> 01:03:46.480 align:middle line:84% framework, people have done pretty compelling 01:03:46.480 --> 01:03:50.320 align:middle line:84% experimental bounds, saying there's no measurable departure 01:03:50.320 --> 01:03:53.410 align:middle line:84% down from like millimeters out to tens of billions 01:03:53.410 --> 01:03:54.100 align:middle line:90% of light years. 01:03:54.100 --> 01:03:56.890 align:middle line:90% That is just unbelievable. 01:03:56.890 --> 01:04:00.220 align:middle line:84% So could it be that these extra dimensions were 01:04:00.220 --> 01:04:03.880 align:middle line:84% as large as the size of a proton, let alone the size 01:04:03.880 --> 01:04:04.870 align:middle line:90% of a millimeter? 01:04:04.870 --> 01:04:05.560 align:middle line:90% Maybe. 01:04:05.560 --> 01:04:08.440 align:middle line:84% So one way that people try to look for them, 01:04:08.440 --> 01:04:11.980 align:middle line:84% if these extra dimensions were small on human scale, 01:04:11.980 --> 01:04:15.420 align:middle line:84% but not so small, is with something 01:04:15.420 --> 01:04:17.665 align:middle line:90% that's called missing momentum. 01:04:17.665 --> 01:04:19.540 align:middle line:84% It's different from the missing mass problem. 01:04:19.540 --> 01:04:21.930 align:middle line:84% So at very high energy particle accelerators, 01:04:21.930 --> 01:04:26.790 align:middle line:84% like the large Hadron Collider, is there 01:04:26.790 --> 01:04:28.530 align:middle line:84% basically momentum that goes missing, 01:04:28.530 --> 01:04:32.580 align:middle line:84% in otherwise very careful momentum conservation balance? 01:04:32.580 --> 01:04:34.680 align:middle line:84% When you smash, for example, two protons together 01:04:34.680 --> 01:04:37.230 align:middle line:84% and collect all the junk that comes flying out, 01:04:37.230 --> 01:04:39.000 align:middle line:84% every time they've done it so far, 01:04:39.000 --> 01:04:41.730 align:middle line:84% the momentum balance is perfectly. 01:04:41.730 --> 01:04:44.880 align:middle line:84% We should think that's really great, that's great. 01:04:44.880 --> 01:04:47.460 align:middle line:84% If there had been some of these particles, in some sense, 01:04:47.460 --> 01:04:50.400 align:middle line:84% taking some of their journey, taking a kind of detour, 01:04:50.400 --> 01:04:52.650 align:middle line:84% like a shortcut through some of these extra dimensions 01:04:52.650 --> 01:04:54.235 align:middle line:84% of space, then the momentum balance 01:04:54.235 --> 01:04:55.860 align:middle line:84% that we measure in our three dimensions 01:04:55.860 --> 01:04:57.690 align:middle line:84% should, in principle, be off, and maybe 01:04:57.690 --> 01:05:00.900 align:middle line:84% by a measurable amount if those shortcuts 01:05:00.900 --> 01:05:06.300 align:middle line:84% were over long enough distances, like microns, 01:05:06.300 --> 01:05:07.530 align:middle line:90% not Planck length. 01:05:07.530 --> 01:05:11.350 align:middle line:84% So there is a program to use particle collision 01:05:11.350 --> 01:05:14.320 align:middle line:84% data to put limits on that version 01:05:14.320 --> 01:05:16.840 align:middle line:84% of a shrunken down, extra dimension. 01:05:16.840 --> 01:05:18.850 align:middle line:84% I should say, I mentioned Lisa Randall's book, 01:05:18.850 --> 01:05:19.900 align:middle line:90% Warped Passages. 01:05:19.900 --> 01:05:21.580 align:middle line:84% In that book, Lisa talks about one 01:05:21.580 --> 01:05:25.418 align:middle line:84% of her own very, very, very ingenious interventions, 01:05:25.418 --> 01:05:27.460 align:middle line:84% was what if these extra dimensions actually never 01:05:27.460 --> 01:05:29.270 align:middle line:90% got small? 01:05:29.270 --> 01:05:31.960 align:middle line:84% What if they are still macroscopically large? 01:05:31.960 --> 01:05:35.350 align:middle line:84% What she found with a colleague back in the late 1990s-- 01:05:35.350 --> 01:05:36.850 align:middle line:84% and I did some work on these models, 01:05:36.850 --> 01:05:39.610 align:middle line:84% but really Lisa was the driving force-- 01:05:39.610 --> 01:05:44.080 align:middle line:84% was could find self-consistent, gravitational configurations 01:05:44.080 --> 01:05:48.370 align:middle line:84% in which we would live on a 3 plus 1 dimensional slice. 01:05:48.370 --> 01:05:50.560 align:middle line:84% And in fact, quarks would stay stuck here. 01:05:50.560 --> 01:05:54.490 align:middle line:84% Even gravity would appear to be stuck on this kind of sliver 01:05:54.490 --> 01:05:57.040 align:middle line:90% self-consistently. 01:05:57.040 --> 01:05:59.680 align:middle line:84% It's almost like bound states for gravity. 01:05:59.680 --> 01:06:01.750 align:middle line:84% It's like an analogy to bound states. 01:06:01.750 --> 01:06:03.980 align:middle line:84% So the graviton might be exchanged, 01:06:03.980 --> 01:06:08.398 align:middle line:84% but might be exchanged only within a subspace of all 01:06:08.398 --> 01:06:09.190 align:middle line:90% that are out there. 01:06:09.190 --> 01:06:13.280 align:middle line:84% And you could find, again, simplified toy models 01:06:13.280 --> 01:06:15.020 align:middle line:84% in which gravity would behave that way, 01:06:15.020 --> 01:06:18.110 align:middle line:84% and Lisa Randall was really the pioneer for that, 01:06:18.110 --> 01:06:20.360 align:middle line:90% starting in, I think, '87, '98. 01:06:20.360 --> 01:06:24.320 align:middle line:84% So there are other ideas around to try to live with, 01:06:24.320 --> 01:06:27.020 align:middle line:84% at least conceptually, large extra dimensions, 01:06:27.020 --> 01:06:28.520 align:middle line:90% without having to curl them up. 01:06:28.520 --> 01:06:31.520 align:middle line:84% Now the challenge there was could one 01:06:31.520 --> 01:06:35.270 align:middle line:84% find a model that has the symmetries that we actually 01:06:35.270 --> 01:06:37.460 align:middle line:84% observe in our universe and not more? 01:06:37.460 --> 01:06:41.210 align:middle line:84% So those models work really very, very lovely, 01:06:41.210 --> 01:06:45.530 align:middle line:84% if you have very, very highly symmetric spaces 01:06:45.530 --> 01:06:47.450 align:middle line:84% for the ones that we would live on, 01:06:47.450 --> 01:06:50.810 align:middle line:84% that look either exactly like Minkowski space 01:06:50.810 --> 01:06:52.230 align:middle line:90% or something very similar. 01:06:52.230 --> 01:06:55.640 align:middle line:84% We live in a slightly less symmetric space 01:06:55.640 --> 01:06:58.740 align:middle line:90% time than Minkowski space. 01:06:58.740 --> 01:07:01.700 align:middle line:84% And it turns out, there was at least no 01:07:01.700 --> 01:07:04.280 align:middle line:84% mathematically demonstrated way to embed even 01:07:04.280 --> 01:07:06.530 align:middle line:84% that simplified toy model with one 01:07:06.530 --> 01:07:09.590 align:middle line:84% fewer symmetry group in that structure. 01:07:09.590 --> 01:07:11.460 align:middle line:84% That doesn't mean it couldn't be done. 01:07:11.460 --> 01:07:14.330 align:middle line:84% So that's an open question, but there are many ways to try 01:07:14.330 --> 01:07:16.250 align:middle line:90% to live with-- make peace with-- 01:07:16.250 --> 01:07:20.300 align:middle line:84% many extra dimensions, but they all require a pretty big leap. 01:07:20.300 --> 01:07:25.100 align:middle line:84% And none of them so far has been compelling, 01:07:25.100 --> 01:07:28.130 align:middle line:84% forced on anyone, based on new experimental observations 01:07:28.130 --> 01:07:29.070 align:middle line:90% or anything else. 01:07:29.070 --> 01:07:31.120 align:middle line:84% So it's still in the realm of the hypothetical. 01:07:31.120 --> 01:07:31.620 align:middle line:90% Good. 01:07:31.620 --> 01:07:32.798 align:middle line:90% Other questions. 01:07:32.798 --> 01:07:36.050 align:middle line:90% 01:07:36.050 --> 01:07:38.870 align:middle line:84% I think part of it is that, really, 01:07:38.870 --> 01:07:40.640 align:middle line:84% at least to its critics-- and frankly, 01:07:40.640 --> 01:07:43.040 align:middle line:84% I'm sympathetic with this critical view-- 01:07:43.040 --> 01:07:45.860 align:middle line:84% it seems to give up on the aim of trying 01:07:45.860 --> 01:07:49.010 align:middle line:84% to explain the universe that we live in, 01:07:49.010 --> 01:07:54.440 align:middle line:84% based on physical forces or laws that we could explain. 01:07:54.440 --> 01:07:56.870 align:middle line:84% It basically says that the universe that we live in 01:07:56.870 --> 01:07:59.180 align:middle line:84% is the outcome of, basically, random chance. 01:07:59.180 --> 01:08:01.130 align:middle line:84% It makes it more like the way we account 01:08:01.130 --> 01:08:03.800 align:middle line:84% for change in Darwinian natural selection, 01:08:03.800 --> 01:08:07.670 align:middle line:84% but now with infinities instead of just large numbers. 01:08:07.670 --> 01:08:11.270 align:middle line:84% It could just be random twists and turns of fate, 01:08:11.270 --> 01:08:13.730 align:middle line:84% and maybe that is the right answer. 01:08:13.730 --> 01:08:17.569 align:middle line:84% But that kind of grates against-- 01:08:17.569 --> 01:08:20.370 align:middle line:84% maybe it's a philosophical or aesthetic preference, 01:08:20.370 --> 01:08:23.120 align:middle line:84% it's not like it's a logical necessity-- 01:08:23.120 --> 01:08:26.990 align:middle line:84% but it grates against a kind of form of explanation 01:08:26.990 --> 01:08:32.060 align:middle line:84% that has, I think, otherwise been successful as a goal. 01:08:32.060 --> 01:08:36.470 align:middle line:84% It's aspirational for things from Newton's day, 01:08:36.470 --> 01:08:40.490 align:middle line:84% through Lisa Randall's day, for that matter. 01:08:40.490 --> 01:08:45.529 align:middle line:84% Efforts to try to account for a specific set of, let's say, 01:08:45.529 --> 01:08:47.569 align:middle line:84% fundamental forces, acting on a specific set 01:08:47.569 --> 01:08:49.460 align:middle line:84% of elementary constituents, whether they're 01:08:49.460 --> 01:08:51.210 align:middle line:84% quarks and gluons or superstrings 01:08:51.210 --> 01:08:54.120 align:middle line:84% or name your favorite, and being able to account 01:08:54.120 --> 01:08:56.189 align:middle line:84% for bulk properties that we could 01:08:56.189 --> 01:08:58.859 align:middle line:90% compare with observations. 01:08:58.859 --> 01:09:00.689 align:middle line:84% I think the thing that sticks in the craw, 01:09:00.689 --> 01:09:03.000 align:middle line:84% so to speak, for critics of the anthropic principle 01:09:03.000 --> 01:09:05.550 align:middle line:84% is that at the end of the day, people have to say, 01:09:05.550 --> 01:09:09.000 align:middle line:84% just random chance, and there's no kind 01:09:09.000 --> 01:09:11.698 align:middle line:84% of deeper explanation for why we measure the things we do. 01:09:11.698 --> 01:09:13.240 align:middle line:84% And they not only could be different, 01:09:13.240 --> 01:09:14.615 align:middle line:84% they actually might be different, 01:09:14.615 --> 01:09:16.710 align:middle line:84% and maybe even are different, infinitely different 01:09:16.710 --> 01:09:19.740 align:middle line:84% ways, out beyond areas where that are kind of 01:09:19.740 --> 01:09:21.540 align:middle line:90% outside our own causal horizon. 01:09:21.540 --> 01:09:26.069 align:middle line:84% So it's changing, so to speak, the explanatory goals. 01:09:26.069 --> 01:09:29.819 align:middle line:84% It's not that it's logically, internally inconsistent. 01:09:29.819 --> 01:09:34.649 align:middle line:84% It's a different kind of aspiration or aesthetic choice 01:09:34.649 --> 01:09:36.840 align:middle line:84% or philosophical aim for what one 01:09:36.840 --> 01:09:39.090 align:middle line:84% thinks the nature of explanation should 01:09:39.090 --> 01:09:41.970 align:middle line:84% be, in an area of science like physics. 01:09:41.970 --> 01:09:45.390 align:middle line:90% And they might be right. 01:09:45.390 --> 01:09:49.784 align:middle line:84% Let me press on for this last part much quicker. 01:09:49.784 --> 01:09:52.290 align:middle line:90% 01:09:52.290 --> 01:09:53.850 align:middle line:84% So I mentioned that string theory 01:09:53.850 --> 01:09:55.320 align:middle line:84% has some great features, at least features 01:09:55.320 --> 01:09:56.320 align:middle line:90% that got people excited. 01:09:56.320 --> 01:09:59.370 align:middle line:84% We could make sense of those bursts of enthusiasm 01:09:59.370 --> 01:10:01.140 align:middle line:90% on the publication pattern. 01:10:01.140 --> 01:10:03.480 align:middle line:84% But actually, I borrowed the term package deal 01:10:03.480 --> 01:10:06.730 align:middle line:84% from Lee Smolin, one of whose books I mentioned earlier. 01:10:06.730 --> 01:10:10.750 align:middle line:84% It's like when you want to go buy a used car off the lot. 01:10:10.750 --> 01:10:14.330 align:middle line:84% You don't get to choose exactly which features you want. 01:10:14.330 --> 01:10:17.500 align:middle line:84% And so, again, Lee Smolin uses this analogy. 01:10:17.500 --> 01:10:19.210 align:middle line:84% It's like, what if the car you really 01:10:19.210 --> 01:10:21.460 align:middle line:84% love-- this one on the used car lot-- 01:10:21.460 --> 01:10:24.740 align:middle line:84% has just the right kind of transmission 01:10:24.740 --> 01:10:27.520 align:middle line:84% you're looking for, but not the stereo system you want, 01:10:27.520 --> 01:10:28.840 align:middle line:90% or vice versa. 01:10:28.840 --> 01:10:30.800 align:middle line:90% It's like a package deal. 01:10:30.800 --> 01:10:35.890 align:middle line:84% And so string theory has many features like it includes 01:10:35.890 --> 01:10:38.377 align:middle line:84% massless spin two graviton, it seems 01:10:38.377 --> 01:10:40.210 align:middle line:84% like it could avoid some of these infinities 01:10:40.210 --> 01:10:42.910 align:middle line:84% from quantum electrodynamics, and yet it 01:10:42.910 --> 01:10:45.220 align:middle line:84% can only be formulated self-consistently 01:10:45.220 --> 01:10:47.480 align:middle line:84% with these as yet undetected symmetries, 01:10:47.480 --> 01:10:50.560 align:middle line:84% the so-called supersymmetry, doubling every known particle 01:10:50.560 --> 01:10:54.460 align:middle line:84% in the universe, and also many extra dimensions. 01:10:54.460 --> 01:10:57.640 align:middle line:84% And so critics like Lee Smolin have said, 01:10:57.640 --> 01:11:00.790 align:middle line:84% are the things we don't want starting to outweigh 01:11:00.790 --> 01:11:02.110 align:middle line:90% the things that we do want? 01:11:02.110 --> 01:11:05.750 align:middle line:84% Is this a is this a good deal in this package deal? 01:11:05.750 --> 01:11:07.550 align:middle line:84% And so again, for your reading list, 01:11:07.550 --> 01:11:09.140 align:middle line:84% these are now kind of classic books. 01:11:09.140 --> 01:11:12.500 align:middle line:84% Brian Greene's book came out first in 1999. 01:11:12.500 --> 01:11:15.290 align:middle line:84% Brian has been, for a long time, one of the most, 01:11:15.290 --> 01:11:19.010 align:middle line:84% I'd say, eloquent proponents of the superstring theory 01:11:19.010 --> 01:11:19.550 align:middle line:90% approach. 01:11:19.550 --> 01:11:21.830 align:middle line:84% And this book was a finalist for the Pulitzer Prize. 01:11:21.830 --> 01:11:24.650 align:middle line:84% It's a beautifully written book, very accessible. 01:11:24.650 --> 01:11:27.110 align:middle line:84% And then again, a very clear book by one 01:11:27.110 --> 01:11:29.210 align:middle line:84% of these now outspoken critics, Lee Smolin. 01:11:29.210 --> 01:11:32.960 align:middle line:84% This one came out in around 2005 or 2006, called The Trouble 01:11:32.960 --> 01:11:33.720 align:middle line:90% with Physics. 01:11:33.720 --> 01:11:35.360 align:middle line:84% And I think these two together give you 01:11:35.360 --> 01:11:38.810 align:middle line:84% a pretty nice range of, again, what the stakes are 01:11:38.810 --> 01:11:41.210 align:middle line:90% in these so-called string wars. 01:11:41.210 --> 01:11:43.070 align:middle line:84% What are the hopes that got people 01:11:43.070 --> 01:11:44.990 align:middle line:84% like Brian and so many people excited, 01:11:44.990 --> 01:11:46.790 align:middle line:84% and what's the kind of cold water? 01:11:46.790 --> 01:11:50.498 align:middle line:84% What are the cautions that some critics like Lee Smolin 01:11:50.498 --> 01:11:51.540 align:middle line:90% put on the table as well? 01:11:51.540 --> 01:11:53.450 align:middle line:90% Those are two you might enjoy. 01:11:53.450 --> 01:11:55.250 align:middle line:84% Let's go back to counting things. 01:11:55.250 --> 01:11:58.100 align:middle line:84% As I tried to indicate and we've seen many times in our course 01:11:58.100 --> 01:12:01.070 align:middle line:84% together, these rivalries or debates 01:12:01.070 --> 01:12:03.022 align:middle line:90% are not happening in a vacuum. 01:12:03.022 --> 01:12:04.730 align:middle line:84% They're not happening in a string vacuum, 01:12:04.730 --> 01:12:06.707 align:middle line:84% not even happening in a macroscopic vacuum. 01:12:06.707 --> 01:12:08.540 align:middle line:84% So let's go back and ask about what else has 01:12:08.540 --> 01:12:10.910 align:middle line:84% been going on that might help nudge 01:12:10.910 --> 01:12:13.910 align:middle line:84% or push and pull the state of discussion among, 01:12:13.910 --> 01:12:16.550 align:middle line:84% let's say, high energy physics, especially but not 01:12:16.550 --> 01:12:19.042 align:middle line:84% only in the United States in recent decades? 01:12:19.042 --> 01:12:20.750 align:middle line:84% So here is another thing I like to count. 01:12:20.750 --> 01:12:23.720 align:middle line:84% The blue is all-- with this axis here-- 01:12:23.720 --> 01:12:27.993 align:middle line:84% are all the PhDs per year in the US on any area of high energy 01:12:27.993 --> 01:12:30.410 align:middle line:84% theory-- high energy physics, including [? experiments, ?] 01:12:30.410 --> 01:12:32.330 align:middle line:84% all high energy physics, particle physics. 01:12:32.330 --> 01:12:34.412 align:middle line:84% And the red, is once again, dissertations now 01:12:34.412 --> 01:12:35.120 align:middle line:90% on string theory. 01:12:35.120 --> 01:12:37.910 align:middle line:84% It looks a lot like the curve of publications, not surprisingly. 01:12:37.910 --> 01:12:39.950 align:middle line:84% So again, you see a very different set 01:12:39.950 --> 01:12:44.120 align:middle line:84% of pattern of when you have a growing number of people 01:12:44.120 --> 01:12:46.910 align:middle line:84% in particle physics broadly, and then going into a decline-- 01:12:46.910 --> 01:12:49.160 align:middle line:84% a pretty clear trend line there-- 01:12:49.160 --> 01:12:51.620 align:middle line:84% starting in the early to mid 1990s. 01:12:51.620 --> 01:12:56.150 align:middle line:84% Meanwhile, a very steep climb, a kind of almost runaway growth, 01:12:56.150 --> 01:12:58.760 align:middle line:84% on the attention to string theory, 01:12:58.760 --> 01:13:00.380 align:middle line:84% and they're kind of out of phase. 01:13:00.380 --> 01:13:02.547 align:middle line:84% Overall particle physics starts to decline, 01:13:02.547 --> 01:13:04.130 align:middle line:84% while string theory starts another one 01:13:04.130 --> 01:13:06.950 align:middle line:90% of these large periods of rise. 01:13:06.950 --> 01:13:08.590 align:middle line:84% Well, that's not happening alone. 01:13:08.590 --> 01:13:11.090 align:middle line:84% One of the things that happens right around that pivot point 01:13:11.090 --> 01:13:14.240 align:middle line:84% is something that I never stop talking about. 01:13:14.240 --> 01:13:17.030 align:middle line:84% It was the cancellation of an enormous project 01:13:17.030 --> 01:13:21.260 align:middle line:84% called the Superconducting Super Collider, or SSC. 01:13:21.260 --> 01:13:25.310 align:middle line:84% This was a project approved in the US in 1985. 01:13:25.310 --> 01:13:27.740 align:middle line:84% It was under construction outside of Dallas, Texas, 01:13:27.740 --> 01:13:30.800 align:middle line:84% in a tiny town called Waxahachie, Texas, not too far 01:13:30.800 --> 01:13:31.610 align:middle line:90% from Dallas. 01:13:31.610 --> 01:13:34.190 align:middle line:84% It was actually going to be three times larger 01:13:34.190 --> 01:13:37.640 align:middle line:84% than the Large Hadron Collider, which at that point, 01:13:37.640 --> 01:13:41.810 align:middle line:84% had not yet been built. So it would have been a-- 01:13:41.810 --> 01:13:43.670 align:middle line:84% let's see-- a 52-mile circumference 01:13:43.670 --> 01:13:48.320 align:middle line:84% or 54-mile circumference ring to accelerate protons and smash 01:13:48.320 --> 01:13:51.380 align:middle line:84% them together with correspondingly three times 01:13:51.380 --> 01:13:52.898 align:middle line:90% larger interaction energies. 01:13:52.898 --> 01:13:55.190 align:middle line:84% The argument was that people could have hopefully found 01:13:55.190 --> 01:13:56.930 align:middle line:84% things like the Higgs boson, maybe 01:13:56.930 --> 01:13:59.510 align:middle line:84% evidence of supersymmetry, in the 80s and 90s. 01:13:59.510 --> 01:14:02.120 align:middle line:84% The problem was this was, as you might expect, 01:14:02.120 --> 01:14:03.740 align:middle line:90% unbelievably expensive. 01:14:03.740 --> 01:14:05.420 align:middle line:84% They actually wound up excavating 01:14:05.420 --> 01:14:08.930 align:middle line:84% almost the entire tunnel many, many, many hundreds of feet 01:14:08.930 --> 01:14:09.673 align:middle line:90% underground. 01:14:09.673 --> 01:14:11.840 align:middle line:84% They had not finished installing the superconducting 01:14:11.840 --> 01:14:13.100 align:middle line:90% magnets or all the rest. 01:14:13.100 --> 01:14:15.980 align:middle line:84% The cost was growing very quickly. 01:14:15.980 --> 01:14:18.950 align:middle line:84% It was approved with a budget of a couple billion dollars. 01:14:18.950 --> 01:14:21.470 align:middle line:84% By the time the project was canceled, 01:14:21.470 --> 01:14:24.470 align:middle line:84% the budget had ballooned to $15 billion. 01:14:24.470 --> 01:14:26.630 align:middle line:84% That's now-- on a US appropriations 01:14:26.630 --> 01:14:28.550 align:middle line:90% budget-- that's a lot of money. 01:14:28.550 --> 01:14:31.130 align:middle line:84% That kind of budget now conflicts with things 01:14:31.130 --> 01:14:36.030 align:middle line:84% like military expenditures, not basic science expenditures. 01:14:36.030 --> 01:14:39.260 align:middle line:84% So this project was heading for a collision. 01:14:39.260 --> 01:14:41.870 align:middle line:84% And in fact, after many fits and starts, 01:14:41.870 --> 01:14:45.410 align:middle line:84% the Congress canceled the SSC with one fateful vote 01:14:45.410 --> 01:14:47.133 align:middle line:90% in October of 1993. 01:14:47.133 --> 01:14:48.800 align:middle line:84% I remember the date very well, because I 01:14:48.800 --> 01:14:51.830 align:middle line:84% entered graduate school in September of 1993. 01:14:51.830 --> 01:14:53.280 align:middle line:90% So I don't take it personally. 01:14:53.280 --> 01:14:54.697 align:middle line:84% But the US Congress killed off one 01:14:54.697 --> 01:14:56.197 align:middle line:84% of the reasons I went to grad school 01:14:56.197 --> 01:14:58.110 align:middle line:84% one month after I entered graduate school. 01:14:58.110 --> 01:14:59.527 align:middle line:84% I don't think they had me in mind. 01:14:59.527 --> 01:15:02.570 align:middle line:84% So you can see here, look at US funding 01:15:02.570 --> 01:15:05.180 align:middle line:84% for high-energy physics in inflation-adjusted or constant 01:15:05.180 --> 01:15:06.023 align:middle line:90% dollars. 01:15:06.023 --> 01:15:08.190 align:middle line:84% Funding for the entire field-- not just for that one 01:15:08.190 --> 01:15:10.710 align:middle line:84% experiment, for the entire field of high-energy physics-- 01:15:10.710 --> 01:15:12.840 align:middle line:90% fell in half in one year. 01:15:12.840 --> 01:15:16.050 align:middle line:84% That was an even sharper fall than the otherwise sharp 01:15:16.050 --> 01:15:19.260 align:middle line:84% falls we looked at many times in this class in the early 1970s. 01:15:19.260 --> 01:15:21.720 align:middle line:84% In the early 70s, the funding for physics 01:15:21.720 --> 01:15:24.360 align:middle line:84% fell in half over the span of four years. 01:15:24.360 --> 01:15:26.730 align:middle line:84% This time, Congress was even more efficient 01:15:26.730 --> 01:15:31.170 align:middle line:84% and reduced the budget for energy physics in half 01:15:31.170 --> 01:15:32.490 align:middle line:90% in literally one year. 01:15:32.490 --> 01:15:35.670 align:middle line:84% And over the rest of the 90s, after that, 01:15:35.670 --> 01:15:39.330 align:middle line:84% funding for physics across the board continued to lose ground, 01:15:39.330 --> 01:15:42.810 align:middle line:84% both because of budget cuts and because of inflation. 01:15:42.810 --> 01:15:45.690 align:middle line:84% Many, many books that have looked at the troubled history 01:15:45.690 --> 01:15:47.280 align:middle line:90% of the SSC, in particular. 01:15:47.280 --> 01:15:50.020 align:middle line:84% There was even a novel about this by the novelist Herman 01:15:50.020 --> 01:15:50.520 align:middle line:90% Wouk. 01:15:50.520 --> 01:15:53.083 align:middle line:84% He's the same novelist who wrote The Caine Mutiny-- a very 01:15:53.083 --> 01:15:54.000 align:middle line:90% accomplished novelist. 01:15:54.000 --> 01:15:57.000 align:middle line:84% He wrote a novel about physicists in Texas 01:15:57.000 --> 01:15:58.530 align:middle line:90% watching the SSC get canceled. 01:15:58.530 --> 01:16:00.240 align:middle line:90% I took it to heart. 01:16:00.240 --> 01:16:02.100 align:middle line:90% Why did Congress cancel funding? 01:16:02.100 --> 01:16:04.650 align:middle line:84% In short, the Cold War had ended. 01:16:04.650 --> 01:16:07.960 align:middle line:84% There's many moving parts, but the overwhelming cause 01:16:07.960 --> 01:16:10.210 align:middle line:84% seems to have been, as many of these scholars 01:16:10.210 --> 01:16:12.610 align:middle line:84% have agreed, that the reason to keep 01:16:12.610 --> 01:16:14.920 align:middle line:84% spending very large amounts of money 01:16:14.920 --> 01:16:18.850 align:middle line:84% on so-called basic research in areas like high-energy physics 01:16:18.850 --> 01:16:21.670 align:middle line:84% had always been a competition with the Soviets, and the idea 01:16:21.670 --> 01:16:25.013 align:middle line:84% that if an outright warfare broke out, 01:16:25.013 --> 01:16:26.680 align:middle line:84% you'd have all these well-trained people 01:16:26.680 --> 01:16:28.150 align:middle line:90% with good equipment. 01:16:28.150 --> 01:16:32.680 align:middle line:84% And after the Soviet Union dissolved in summer of 1991, 01:16:32.680 --> 01:16:35.530 align:middle line:84% those arguments no longer held the same kind of sway 01:16:35.530 --> 01:16:37.330 align:middle line:84% that they had for generations before. 01:16:37.330 --> 01:16:39.130 align:middle line:84% And one of the most visible symbols 01:16:39.130 --> 01:16:41.770 align:middle line:84% of the end of the Cold War for science funding in the US 01:16:41.770 --> 01:16:45.100 align:middle line:84% was this cancellation of the huge accelerator. 01:16:45.100 --> 01:16:47.650 align:middle line:90% Let's go back to that plot of-- 01:16:47.650 --> 01:16:51.100 align:middle line:84% remember this is now all PhDs in particle physics, PhDs 01:16:51.100 --> 01:16:52.670 align:middle line:84% in the US strictly on string theory. 01:16:52.670 --> 01:16:54.580 align:middle line:84% And here, because green means money, 01:16:54.580 --> 01:16:57.490 align:middle line:84% green is now that budget curve I just showed in the last plot. 01:16:57.490 --> 01:16:59.890 align:middle line:84% Look at the amazing correlations here, 01:16:59.890 --> 01:17:02.740 align:middle line:84% that you stop getting more people entering particle 01:17:02.740 --> 01:17:06.097 align:middle line:84% physics when funding falls in half, meaning 01:17:06.097 --> 01:17:07.930 align:middle line:84% you don't replenish the people who graduate. 01:17:07.930 --> 01:17:10.180 align:middle line:84% So people graduate here, but not as many people 01:17:10.180 --> 01:17:11.000 align:middle line:90% enter afterwards. 01:17:11.000 --> 01:17:13.930 align:middle line:84% So you start seeing the characteristic, five-year time 01:17:13.930 --> 01:17:17.140 align:middle line:84% scale fall off that we can at least correlate 01:17:17.140 --> 01:17:20.050 align:middle line:84% with this very dramatic change in funding for the larger 01:17:20.050 --> 01:17:20.870 align:middle line:90% field. 01:17:20.870 --> 01:17:23.110 align:middle line:84% Meanwhile, string theory costs approximately 01:17:23.110 --> 01:17:26.650 align:middle line:84% pencils and espresso-- well, and health insurance-- all of which 01:17:26.650 --> 01:17:28.960 align:middle line:84% are important, none of which cost $15 billion 01:17:28.960 --> 01:17:30.370 align:middle line:90% for an atom smasher. 01:17:30.370 --> 01:17:32.260 align:middle line:84% So dissertations on string theory 01:17:32.260 --> 01:17:34.570 align:middle line:84% start growing exponentially just at the moment when 01:17:34.570 --> 01:17:37.960 align:middle line:84% other trends within the subfield are looking 01:17:37.960 --> 01:17:40.610 align:middle line:90% less and less fiscally viable. 01:17:40.610 --> 01:17:41.560 align:middle line:90% It's not only the US. 01:17:41.560 --> 01:17:44.672 align:middle line:84% This is a complicated plot that I like staring at. 01:17:44.672 --> 01:17:46.630 align:middle line:84% Let me just tell you briefly what it's showing. 01:17:46.630 --> 01:17:49.580 align:middle line:84% It's comparing the decade by decade averages. 01:17:49.580 --> 01:17:52.570 align:middle line:84% I was trying to capture lots of moving parts, 01:17:52.570 --> 01:17:55.360 align:middle line:84% across many countries in the world, where 01:17:55.360 --> 01:18:01.870 align:middle line:84% the blue curve shows the change between the decade 01:18:01.870 --> 01:18:05.530 align:middle line:84% of the mid 80s to mid 90s, the change of that decade compared 01:18:05.530 --> 01:18:06.370 align:middle line:90% to the next decade-- 01:18:06.370 --> 01:18:12.130 align:middle line:84% 90s to mid 2000s-- the change in proportion of the world physics 01:18:12.130 --> 01:18:15.160 align:middle line:84% literature, contributed by physicists in that country. 01:18:15.160 --> 01:18:16.968 align:middle line:84% That's a very close proxy for budgets. 01:18:16.968 --> 01:18:18.760 align:middle line:84% Other people looked at this more carefully. 01:18:18.760 --> 01:18:22.140 align:middle line:84% So you can really think of this as kind of budget trends. 01:18:22.140 --> 01:18:24.160 align:middle line:84% You can see the budget for physics research 01:18:24.160 --> 01:18:26.660 align:middle line:84% across the board fell in the United States 01:18:26.660 --> 01:18:27.910 align:middle line:90% after the end of the Cold War. 01:18:27.910 --> 01:18:29.910 align:middle line:84% That's consistent with what I showed you before. 01:18:29.910 --> 01:18:33.310 align:middle line:84% Likewise, for many of these former Cold War so-called 01:18:33.310 --> 01:18:34.900 align:middle line:90% superpowers-- 01:18:34.900 --> 01:18:38.950 align:middle line:84% the USSR and then the post-Soviet republics, 01:18:38.950 --> 01:18:40.030 align:middle line:90% same with Britain. 01:18:40.030 --> 01:18:42.850 align:middle line:84% Meanwhile, some areas of the world during this period 01:18:42.850 --> 01:18:45.490 align:middle line:84% were investing like crazy in the basic sciences 01:18:45.490 --> 01:18:48.130 align:middle line:84% after the end of the Cold War, countries like Brazil 01:18:48.130 --> 01:18:50.290 align:middle line:90% and China for some time. 01:18:50.290 --> 01:18:53.380 align:middle line:84% And you have a very clear anti-correlation 01:18:53.380 --> 01:18:57.130 align:middle line:84% between countries that stopped investing so much in physics 01:18:57.130 --> 01:19:00.100 align:middle line:84% overall and the proportion of physicists 01:19:00.100 --> 01:19:01.870 align:middle line:84% there who began working on string theory, 01:19:01.870 --> 01:19:03.400 align:middle line:84% versus the inverse, countries that 01:19:03.400 --> 01:19:05.680 align:middle line:84% were investing across the board more and more 01:19:05.680 --> 01:19:08.440 align:middle line:84% aggressively, across the full range of physics, 01:19:08.440 --> 01:19:11.200 align:middle line:84% and you see either a slower growing proportion 01:19:11.200 --> 01:19:14.920 align:middle line:84% or a falling proportion of physicists 01:19:14.920 --> 01:19:15.962 align:middle line:90% working on string theory. 01:19:15.962 --> 01:19:17.462 align:middle line:84% That doesn't mean, there's no reason 01:19:17.462 --> 01:19:19.150 align:middle line:84% to work on string theory intellectually. 01:19:19.150 --> 01:19:21.067 align:middle line:84% It does remind us that these decisions are not 01:19:21.067 --> 01:19:24.040 align:middle line:84% happening in a vacuum, that what are the resources available, 01:19:24.040 --> 01:19:26.290 align:middle line:84% with which to even ask certain questions. 01:19:26.290 --> 01:19:28.660 align:middle line:84% Those change not only based on arguments 01:19:28.660 --> 01:19:31.930 align:middle line:84% about string compactifications or eternal inflation, 01:19:31.930 --> 01:19:33.910 align:middle line:84% physics, then as now, is embedded 01:19:33.910 --> 01:19:36.490 align:middle line:84% in a pretty messy world, a world that, in this case, 01:19:36.490 --> 01:19:38.380 align:middle line:84% went through a pretty dramatic change 01:19:38.380 --> 01:19:39.940 align:middle line:90% with the end of the Cold War. 01:19:39.940 --> 01:19:41.950 align:middle line:90% So let me wrap up. 01:19:41.950 --> 01:19:44.380 align:middle line:84% Early in our own millennium, physics 01:19:44.380 --> 01:19:46.750 align:middle line:84% seems to be just as much in the flow, 01:19:46.750 --> 01:19:51.070 align:middle line:84% embedded in the world of people, culture, politics, geopolitics, 01:19:51.070 --> 01:19:52.690 align:middle line:84% and budgets and all the rest, as it 01:19:52.690 --> 01:19:54.190 align:middle line:84% has been throughout the whole period 01:19:54.190 --> 01:19:56.260 align:middle line:90% we've looked at this semester. 01:19:56.260 --> 01:20:00.250 align:middle line:84% We saw, for a good chunk of the term, 01:20:00.250 --> 01:20:03.220 align:middle line:84% there was a moment in the middle decades of the 20th century, 01:20:03.220 --> 01:20:06.060 align:middle line:84% largely coming out of the wartime projects, 01:20:06.060 --> 01:20:07.860 align:middle line:84% when physics, especially in the US, 01:20:07.860 --> 01:20:12.030 align:middle line:84% seemed to have a unlimited range of resources 01:20:12.030 --> 01:20:14.620 align:middle line:84% and a command of respect and so on, 01:20:14.620 --> 01:20:17.272 align:middle line:84% as well as heightened scrutiny with McCarthyism 01:20:17.272 --> 01:20:17.980 align:middle line:90% and all the rest. 01:20:17.980 --> 01:20:19.980 align:middle line:84% It was center stage, for good or bad. 01:20:19.980 --> 01:20:21.030 align:middle line:90% It wasn't all good. 01:20:21.030 --> 01:20:23.910 align:middle line:84% But it was a central player in a way 01:20:23.910 --> 01:20:28.650 align:middle line:84% that was unlike what had either come before or indeed since. 01:20:28.650 --> 01:20:30.990 align:middle line:84% I got my own taste of this about the changing 01:20:30.990 --> 01:20:32.820 align:middle line:84% fortunes of the cultural capital, 01:20:32.820 --> 01:20:36.300 align:middle line:84% let's say, of high-energy physics myself. 01:20:36.300 --> 01:20:40.410 align:middle line:84% About 15 years ago, Alan and I had written a very brief review 01:20:40.410 --> 01:20:41.285 align:middle line:90% article on inflation. 01:20:41.285 --> 01:20:42.743 align:middle line:84% In fact, it was one of the readings 01:20:42.743 --> 01:20:43.710 align:middle line:90% for the previous class. 01:20:43.710 --> 01:20:45.060 align:middle line:90% It was published in Science. 01:20:45.060 --> 01:20:47.850 align:middle line:84% One week later, there was a point-by-point rebuttal 01:20:47.850 --> 01:20:50.430 align:middle line:84% posted on a creationist website that someone 01:20:50.430 --> 01:20:52.870 align:middle line:90% emailed us to pay attention to. 01:20:52.870 --> 01:20:54.990 align:middle line:84% They went through and showed all the reasons 01:20:54.990 --> 01:20:58.110 align:middle line:84% why this inflation account and, even the standard Big Bang, 01:20:58.110 --> 01:21:01.290 align:middle line:84% was baloney and, in fact, instead, much as the Christian 01:21:01.290 --> 01:21:03.720 align:middle line:84% Bible seems to suggest, at least to some readers, 01:21:03.720 --> 01:21:05.950 align:middle line:84% the universe is only 6,000 years old. 01:21:05.950 --> 01:21:07.540 align:middle line:90% It's not 14 billion years old. 01:21:07.540 --> 01:21:10.450 align:middle line:84% And they wanted to show why every one of our arguments 01:21:10.450 --> 01:21:11.800 align:middle line:90% was wrong. 01:21:11.800 --> 01:21:15.025 align:middle line:84% They conclude, quite correctly, with the following. 01:21:15.025 --> 01:21:16.900 align:middle line:84% "We had to show you, in their own words, what 01:21:16.900 --> 01:21:19.750 align:middle line:84% these MIT eggheads are saying," I said, well, guilty. 01:21:19.750 --> 01:21:20.710 align:middle line:90% That one they nailed. 01:21:20.710 --> 01:21:22.750 align:middle line:90% That was empirically astute. 01:21:22.750 --> 01:21:25.740 align:middle line:84% "Guth and Kaiser need to take up truck driving," they wrote. 01:21:25.740 --> 01:21:26.470 align:middle line:90% I'm listening. 01:21:26.470 --> 01:21:28.510 align:middle line:84% "That would get them out of their ivory towers 01:21:28.510 --> 01:21:30.550 align:middle line:84% at MIT and into the real world, where 01:21:30.550 --> 01:21:33.070 align:middle line:84% they would be forced to look at trees, mountains, weather, 01:21:33.070 --> 01:21:33.880 align:middle line:90% ecology." 01:21:33.880 --> 01:21:36.040 align:middle line:84% Plus, what do you see on the interstate but Taco 01:21:36.040 --> 01:21:39.370 align:middle line:84% Bell and Motel 6-- all these wonderful features of nature 01:21:39.370 --> 01:21:44.290 align:middle line:84% that signal the evidence of design, in this case 01:21:44.290 --> 01:21:46.660 align:middle line:84% they meant design by a biblical God. 01:21:46.660 --> 01:21:49.457 align:middle line:84% "Realities that proclaim design purpose and intention." 01:21:49.457 --> 01:21:51.790 align:middle line:84% So I've had a wonderful time talking with you this term. 01:21:51.790 --> 01:21:53.260 align:middle line:84% I'm sorry we had to do it remotely. 01:21:53.260 --> 01:21:57.430 align:middle line:84% I hope that was, nonetheless, a reasonable experience for you. 01:21:57.430 --> 01:21:58.940 align:middle line:84% Please don't hesitate to email me. 01:21:58.940 --> 01:22:01.027 align:middle line:84% But for now, I got to head out because I 01:22:01.027 --> 01:22:02.110 align:middle line:90% got to get back to my rig. 01:22:02.110 --> 01:22:03.040 align:middle line:90% So it's been great. 01:22:03.040 --> 01:22:04.757 align:middle line:84% Good luck with the end of the term. 01:22:04.757 --> 01:22:06.340 align:middle line:84% And please don't hesitate to reach out 01:22:06.340 --> 01:22:07.690 align:middle line:84% if you have any questions, any time. 01:22:07.690 --> 01:22:08.510 align:middle line:90% But I'll stop there. 01:22:08.510 --> 01:22:09.427 align:middle line:90% Any questions on that? 01:22:09.427 --> 01:22:16.650 align:middle line:90% 01:22:16.650 --> 01:22:18.090 align:middle line:90% No questions about MIT, I guess. 01:22:18.090 --> 01:22:20.130 align:middle line:84% I guess that part was just self-evident. 01:22:20.130 --> 01:22:21.680 align:middle line:90% OK. 01:22:21.680 --> 01:22:36.000 align:middle line:90%