WEBVTT 00:00:00.000 --> 00:00:01.924 align:middle line:90% [SQUEAKING] 00:00:01.924 --> 00:00:03.367 align:middle line:90% [RUSTLING] 00:00:03.367 --> 00:00:05.291 align:middle line:90% [CLICKING] 00:00:05.291 --> 00:00:11.560 align:middle line:90% 00:00:11.560 --> 00:00:13.180 align:middle line:84% DAVID KAISER: So today we're going 00:00:13.180 --> 00:00:16.750 align:middle line:84% to be talking about the kind of invention 00:00:16.750 --> 00:00:20.710 align:middle line:84% or the hybridization of a whole new subfield 00:00:20.710 --> 00:00:24.760 align:middle line:84% within physics that now is often called particle cosmology. 00:00:24.760 --> 00:00:27.730 align:middle line:84% It happens to be the field where I spend a lot of my time. 00:00:27.730 --> 00:00:30.250 align:middle line:90% I think it's just fantastic. 00:00:30.250 --> 00:00:32.770 align:middle line:84% But it's actually a fairly new invention. 00:00:32.770 --> 00:00:37.630 align:middle line:84% This whole field of study came together fairly recently 00:00:37.630 --> 00:00:39.410 align:middle line:90% in historical terms. 00:00:39.410 --> 00:00:40.840 align:middle line:84% And so it gives us an opportunity 00:00:40.840 --> 00:00:43.168 align:middle line:84% to ask both about the ideas that help 00:00:43.168 --> 00:00:44.710 align:middle line:84% drive this kind of scientific change, 00:00:44.710 --> 00:00:46.390 align:middle line:84% but also, as we've been doing really 00:00:46.390 --> 00:00:50.350 align:middle line:84% throughout the whole semester, the kinds of factors 00:00:50.350 --> 00:00:53.260 align:middle line:84% or ingredients beyond only cool ideas 00:00:53.260 --> 00:00:57.640 align:middle line:84% or new experiments that also can play a very substantial role. 00:00:57.640 --> 00:01:00.490 align:middle line:84% So we'll talk about institutions, and politics, 00:01:00.490 --> 00:01:03.340 align:middle line:84% and some broader shifts in the field, 00:01:03.340 --> 00:01:06.460 align:middle line:84% out of which come this new now quite flourishing subfield. 00:01:06.460 --> 00:01:09.110 align:middle line:90% So that's our job for today. 00:01:09.110 --> 00:01:12.730 align:middle line:84% So particle cosmology today is, objectively speaking, 00:01:12.730 --> 00:01:14.410 align:middle line:84% just the coolest part of physics. 00:01:14.410 --> 00:01:15.350 align:middle line:90% That's obvious. 00:01:15.350 --> 00:01:17.140 align:middle line:90% I don't have to belabor that. 00:01:17.140 --> 00:01:19.210 align:middle line:84% It is actually very cool, and it's also 00:01:19.210 --> 00:01:21.520 align:middle line:90% flourishing by any measure. 00:01:21.520 --> 00:01:25.210 align:middle line:84% It studies the smallest units of matter, the fundamental forces 00:01:25.210 --> 00:01:28.210 align:middle line:84% and elementary constituents of matter. 00:01:28.210 --> 00:01:33.040 align:middle line:84% And it asks about what role they might have in shaping, really, 00:01:33.040 --> 00:01:34.490 align:middle line:90% the fate of the entire universe. 00:01:34.490 --> 00:01:39.340 align:middle line:84% So it's this melding of the very, very tiny elements 00:01:39.340 --> 00:01:41.290 align:middle line:84% of matter and the role they might 00:01:41.290 --> 00:01:44.110 align:middle line:84% play on literally cosmological scales, 00:01:44.110 --> 00:01:46.060 align:middle line:90% from the Big Bang to today. 00:01:46.060 --> 00:01:46.922 align:middle line:90% Here's an example. 00:01:46.922 --> 00:01:48.880 align:middle line:84% We'll talk more about images like this actually 00:01:48.880 --> 00:01:51.580 align:middle line:84% in the next class session on next Monday. 00:01:51.580 --> 00:01:53.980 align:middle line:84% This is a series of images of what's 00:01:53.980 --> 00:01:56.080 align:middle line:84% called the Cosmic Microwave Background 00:01:56.080 --> 00:01:58.600 align:middle line:84% radiation, or the CMB, and we'll talk more about that. 00:01:58.600 --> 00:02:02.590 align:middle line:84% It's one of the best examples we have of how we now 00:02:02.590 --> 00:02:06.370 align:middle line:84% use our tools about quantum theory and elementary particle 00:02:06.370 --> 00:02:09.410 align:middle line:84% physics to try to make sense of and, in turn, 00:02:09.410 --> 00:02:14.443 align:middle line:84% be constrained by measurements of characteristics 00:02:14.443 --> 00:02:15.860 align:middle line:84% of the entire observable universe, 00:02:15.860 --> 00:02:18.560 align:middle line:84% this fascinating interplay between the very large 00:02:18.560 --> 00:02:20.090 align:middle line:90% and the very small. 00:02:20.090 --> 00:02:23.690 align:middle line:84% The field is doing pretty well these days by other measures. 00:02:23.690 --> 00:02:26.780 align:middle line:84% Its annual budget just within the United States 00:02:26.780 --> 00:02:29.622 align:middle line:84% is on the order of $1 billion a year, roughly. 00:02:29.622 --> 00:02:31.580 align:middle line:84% That's if you combine spending in this subfield 00:02:31.580 --> 00:02:34.320 align:middle line:84% from the National Science Foundation, 00:02:34.320 --> 00:02:39.480 align:middle line:84% the Department of Energy, NASA, and other federal grants. 00:02:39.480 --> 00:02:43.980 align:middle line:84% I think a more telling measure of its kind of state of health 00:02:43.980 --> 00:02:48.570 align:middle line:84% these days is that there are on average two new physics papers, 00:02:48.570 --> 00:02:51.240 align:middle line:84% two new preprints posted to the central preprint 00:02:51.240 --> 00:02:54.720 align:middle line:84% server archive.org every hour of every day 00:02:54.720 --> 00:02:55.830 align:middle line:90% just in this subfield. 00:02:55.830 --> 00:02:57.510 align:middle line:84% So during this class session, there 00:02:57.510 --> 00:03:00.192 align:middle line:84% will be, on average, three new preprints posted just 00:03:00.192 --> 00:03:00.900 align:middle line:90% in this subfield. 00:03:00.900 --> 00:03:03.150 align:middle line:84% And that doesn't take a break for nights and weekends. 00:03:03.150 --> 00:03:05.740 align:middle line:84% That's averaging over 24 hours a day, seven days a week. 00:03:05.740 --> 00:03:09.460 align:middle line:84% So it is really a booming, booming subject of study. 00:03:09.460 --> 00:03:11.760 align:middle line:84% And I find that all the more astonishing since this 00:03:11.760 --> 00:03:16.330 align:middle line:84% field literally didn't even exist 45 years ago. 00:03:16.330 --> 00:03:21.150 align:middle line:84% So this is a $1 billion area of study 00:03:21.150 --> 00:03:23.590 align:middle line:84% with devoted colleagues all over the world, 00:03:23.590 --> 00:03:25.080 align:middle line:84% not just here in the United States. 00:03:25.080 --> 00:03:28.050 align:middle line:84% And yet, it's a fairly recent vintage 00:03:28.050 --> 00:03:30.030 align:middle line:84% in terms of the kinds of time scales 00:03:30.030 --> 00:03:32.070 align:middle line:84% we've been looking at this whole semester. 00:03:32.070 --> 00:03:33.940 align:middle line:90% So how did this come to be? 00:03:33.940 --> 00:03:38.050 align:middle line:84% There's a very compelling story and the story that we will take 00:03:38.050 --> 00:03:42.130 align:middle line:84% some time to look at today that this new field really emerged 00:03:42.130 --> 00:03:46.480 align:middle line:84% in the mid-1970s because of changes in ideas, 00:03:46.480 --> 00:03:51.160 align:middle line:84% because of new research insights coming mostly from high-energy 00:03:51.160 --> 00:03:54.490 align:middle line:84% physics or particle physics and that it somehow this set 00:03:54.490 --> 00:03:58.030 align:middle line:84% of ideas that bubbled up in the mid-1970s of that we'll look 00:03:58.030 --> 00:03:59.050 align:middle line:90% at today-- 00:03:59.050 --> 00:04:02.680 align:middle line:84% that that compelled researchers to change their whole field 00:04:02.680 --> 00:04:06.100 align:middle line:84% of study and in the process to invent this new subfield, 00:04:06.100 --> 00:04:09.100 align:middle line:84% that it was somehow natural to start asking questions 00:04:09.100 --> 00:04:12.910 align:middle line:84% at the interface between particle physics and cosmology. 00:04:12.910 --> 00:04:15.940 align:middle line:84% And so there's a lot going for that explanation. 00:04:15.940 --> 00:04:18.940 align:middle line:84% It's not flagrantly false, but it's also, 00:04:18.940 --> 00:04:21.310 align:middle line:84% I think, really, really quite incomplete. 00:04:21.310 --> 00:04:23.890 align:middle line:84% And so if we start using the kinds of tools 00:04:23.890 --> 00:04:26.590 align:middle line:84% that we've been working on together this whole semester, 00:04:26.590 --> 00:04:28.870 align:middle line:84% I think we can dig in a bit more and try 00:04:28.870 --> 00:04:32.470 align:middle line:84% to uncover some of the additional factors that really 00:04:32.470 --> 00:04:36.297 align:middle line:84% were at play in especially in the early years of this field. 00:04:36.297 --> 00:04:38.880 align:middle line:84% So of the sort that we've been looking at throughout the term, 00:04:38.880 --> 00:04:41.430 align:middle line:84% we'll be talking about some changes in institutions, 00:04:41.430 --> 00:04:44.820 align:middle line:84% in training, in broader politics, 00:04:44.820 --> 00:04:48.720 align:middle line:84% and the support for science, and that these shifts were 00:04:48.720 --> 00:04:50.220 align:middle line:90% deeply consequential. 00:04:50.220 --> 00:04:54.060 align:middle line:84% And they weren't important to the exclusion of ideas 00:04:54.060 --> 00:04:56.880 align:middle line:84% from particle theory, but the ideas from particle theory, 00:04:56.880 --> 00:04:59.790 align:middle line:84% as cool and awesome as they are, I think, 00:04:59.790 --> 00:05:01.390 align:middle line:90% really were not the whole story. 00:05:01.390 --> 00:05:04.543 align:middle line:84% We were pretty insufficient to get us to where we are today. 00:05:04.543 --> 00:05:06.210 align:middle line:84% So we're going to, as usual, try to keep 00:05:06.210 --> 00:05:08.950 align:middle line:84% all these different kinds of facets in mind. 00:05:08.950 --> 00:05:11.650 align:middle line:84% So our three steps for today's class-- 00:05:11.650 --> 00:05:14.310 align:middle line:84% the first is we want to set up what 00:05:14.310 --> 00:05:17.370 align:middle line:84% were the kinds of questions that were occupying physicists 00:05:17.370 --> 00:05:23.070 align:middle line:84% in various branches of the field before this merger took place, 00:05:23.070 --> 00:05:26.310 align:middle line:84% before the emergence of what we would now recognize as particle 00:05:26.310 --> 00:05:27.450 align:middle line:90% cosmology? 00:05:27.450 --> 00:05:29.835 align:middle line:84% And so as we'll see, that really comes around 00:05:29.835 --> 00:05:31.830 align:middle line:84% to one clever, one helpful way to think 00:05:31.830 --> 00:05:34.840 align:middle line:84% about that is to focus on the question of mass. 00:05:34.840 --> 00:05:39.430 align:middle line:84% How did physicists in different subfields in the 1950s and '60s 00:05:39.430 --> 00:05:42.670 align:middle line:84% think about this very common seeming notion of mass? 00:05:42.670 --> 00:05:45.640 align:middle line:84% Why do objects, say, resist changes in their motion? 00:05:45.640 --> 00:05:49.210 align:middle line:84% The m that's in Newton's second law of F equals ma. 00:05:49.210 --> 00:05:51.580 align:middle line:84% Was there a deeper origin or way to make 00:05:51.580 --> 00:05:53.920 align:middle line:84% sense of this very common notion of mass? 00:05:53.920 --> 00:05:56.800 align:middle line:84% And as we'll see, two different communities or sub communities 00:05:56.800 --> 00:05:59.860 align:middle line:84% were really focusing on that in quite different ways 00:05:59.860 --> 00:06:01.490 align:middle line:90% in the 1950s and '60s. 00:06:01.490 --> 00:06:05.140 align:middle line:84% And that helps us trace the precursors 00:06:05.140 --> 00:06:08.770 align:middle line:84% for this overlap field of particle cosmology. 00:06:08.770 --> 00:06:11.620 align:middle line:84% Then we'll take a look at some of the broader 00:06:11.620 --> 00:06:13.870 align:middle line:84% institutional shifts that were also 00:06:13.870 --> 00:06:16.630 align:middle line:84% coming to be very dramatic, and some of them 00:06:16.630 --> 00:06:20.200 align:middle line:84% quite unexpected, by the late '60s and early 1970s 00:06:20.200 --> 00:06:23.980 align:middle line:84% that also helped propel this new merger of fields. 00:06:23.980 --> 00:06:26.710 align:middle line:84% And then we'll zoom back in to see 00:06:26.710 --> 00:06:29.590 align:middle line:84% what were some examples of the kinds of research questions 00:06:29.590 --> 00:06:33.820 align:middle line:84% that now seemed obvious or natural to ask for members 00:06:33.820 --> 00:06:37.150 align:middle line:84% of this new hybrid area in the wake 00:06:37.150 --> 00:06:40.040 align:middle line:84% both of the intellectual changes that we'll look at here, 00:06:40.040 --> 00:06:42.310 align:middle line:84% as well as some of these broader institutional shifts. 00:06:42.310 --> 00:06:45.920 align:middle line:84% So that's our three steps for today. 00:06:45.920 --> 00:06:49.530 align:middle line:84% So as I mentioned, I find it helpful to think 00:06:49.530 --> 00:06:52.650 align:middle line:84% about the precursors for particle cosmology 00:06:52.650 --> 00:06:56.010 align:middle line:84% by asking about this very humdrum topic of mass. 00:06:56.010 --> 00:06:57.210 align:middle line:90% Why do objects have mass? 00:06:57.210 --> 00:06:59.910 align:middle line:84% Why do objects resist changes in their motion? 00:06:59.910 --> 00:07:02.910 align:middle line:84% And this actually became a very live question, 00:07:02.910 --> 00:07:06.570 align:middle line:84% a very challenging question, but in pretty distinct ways 00:07:06.570 --> 00:07:10.090 align:middle line:84% during the middle decades of the 20th century. 00:07:10.090 --> 00:07:13.080 align:middle line:84% So in one of these subfields, a pretty tiny 00:07:13.080 --> 00:07:14.310 align:middle line:90% subfield at the time-- 00:07:14.310 --> 00:07:16.800 align:middle line:84% experts in gravitation and cosmology-- 00:07:16.800 --> 00:07:20.280 align:middle line:84% the question about the origin or the impact of mass 00:07:20.280 --> 00:07:22.170 align:middle line:84% was often framed in terms of what 00:07:22.170 --> 00:07:23.670 align:middle line:90% was called Mach's principle. 00:07:23.670 --> 00:07:25.590 align:middle line:84% That's the name for that same Ernst Mach 00:07:25.590 --> 00:07:28.170 align:middle line:84% whose work we looked at way back in the early part 00:07:28.170 --> 00:07:32.610 align:middle line:84% of this semester, the 19th century polymath, whose work 00:07:32.610 --> 00:07:35.130 align:middle line:84% actually helped very directly inspire the young Albert 00:07:35.130 --> 00:07:35.730 align:middle line:90% Einstein. 00:07:35.730 --> 00:07:37.672 align:middle line:84% It was actually Einstein who named this what 00:07:37.672 --> 00:07:38.880 align:middle line:90% we'll talk about in a moment. 00:07:38.880 --> 00:07:41.340 align:middle line:84% It was Einstein who named it Mach's principle 00:07:41.340 --> 00:07:44.080 align:middle line:84% because Einstein was inspired by some of Mach's writings 00:07:44.080 --> 00:07:44.580 align:middle line:90% on this. 00:07:44.580 --> 00:07:46.680 align:middle line:84% So Mach himself didn't call it Mach's principle. 00:07:46.680 --> 00:07:49.500 align:middle line:84% But it was attributed to Mach by Albert Einstein 00:07:49.500 --> 00:07:51.240 align:middle line:90% as early as 1918-- 00:07:51.240 --> 00:07:54.640 align:middle line:84% so from the early days of the study of general relativity. 00:07:54.640 --> 00:07:57.060 align:middle line:84% So the idea was-- it was often we 00:07:57.060 --> 00:07:58.920 align:middle line:90% could frame it as a question. 00:07:58.920 --> 00:08:02.220 align:middle line:84% Do the local effects that we attribute to mass-- 00:08:02.220 --> 00:08:05.400 align:middle line:84% do the local inertial effects resistances to changes 00:08:05.400 --> 00:08:07.770 align:middle line:90% in motion for everyday objects-- 00:08:07.770 --> 00:08:10.470 align:middle line:84% do those local inertial effects actually 00:08:10.470 --> 00:08:13.980 align:middle line:84% arise from very distant gravitational interactions? 00:08:13.980 --> 00:08:17.010 align:middle line:84% Do we have to think about the distribution of matter 00:08:17.010 --> 00:08:19.170 align:middle line:84% throughout the whole universe in order 00:08:19.170 --> 00:08:22.260 align:middle line:84% to make sense for why this block slides down this inclined 00:08:22.260 --> 00:08:24.120 align:middle line:90% plane at a certain rate? 00:08:24.120 --> 00:08:27.420 align:middle line:84% Local inertial effects, are they somehow tied 00:08:27.420 --> 00:08:30.420 align:middle line:84% to cosmological distributions of all the other stuff? 00:08:30.420 --> 00:08:33.960 align:middle line:84% After all, gravity is a long-range force, Both. 00:08:33.960 --> 00:08:36.360 align:middle line:84% Newton's gravity and even in Einstein's framework. 00:08:36.360 --> 00:08:41.190 align:middle line:84% And so the idea was should we be constantly paying attention 00:08:41.190 --> 00:08:45.330 align:middle line:84% to the global distribution of matter and energy 00:08:45.330 --> 00:08:49.080 align:middle line:84% when we try to make sense of local phenomena associated 00:08:49.080 --> 00:08:50.010 align:middle line:90% with mass? 00:08:50.010 --> 00:08:52.110 align:middle line:84% So that was a very challenging question, 00:08:52.110 --> 00:08:53.890 align:middle line:84% and we'll say more about it soon. 00:08:53.890 --> 00:08:57.540 align:middle line:84% But that was one of the ways that the problem of mass 00:08:57.540 --> 00:09:00.600 align:middle line:84% took form, was given a concrete form 00:09:00.600 --> 00:09:02.580 align:middle line:84% for these specialists who were pursuing things 00:09:02.580 --> 00:09:05.910 align:middle line:84% like Einstein's general theory of relativity in the 1950s 00:09:05.910 --> 00:09:07.450 align:middle line:90% and '60s. 00:09:07.450 --> 00:09:09.450 align:middle line:84% There was quite a different set of conversations 00:09:09.450 --> 00:09:11.600 align:middle line:90% happening around the same time. 00:09:11.600 --> 00:09:13.810 align:middle line:84% But now among the community that was 00:09:13.810 --> 00:09:15.790 align:middle line:84% focused on nuclear and particle physics, 00:09:15.790 --> 00:09:18.700 align:middle line:84% what we'd soon come to call simply high-energy physics. 00:09:18.700 --> 00:09:22.150 align:middle line:84% This had nothing to do, at least on the surface, 00:09:22.150 --> 00:09:25.510 align:middle line:84% with Mach's principle or even with Einstein's general theory 00:09:25.510 --> 00:09:26.650 align:middle line:90% of relativity. 00:09:26.650 --> 00:09:29.980 align:middle line:84% These experts were focused on a very different set of puzzles 00:09:29.980 --> 00:09:32.085 align:middle line:84% or challenges, and this picks up more directly 00:09:32.085 --> 00:09:33.460 align:middle line:84% from the material we were talking 00:09:33.460 --> 00:09:36.160 align:middle line:84% about just at the very end of Monday's class. 00:09:36.160 --> 00:09:40.150 align:middle line:84% Already by the late 1950s and with greater acceleration 00:09:40.150 --> 00:09:44.770 align:middle line:84% throughout the 1960s, a number of high-energy theorists 00:09:44.770 --> 00:09:48.340 align:middle line:84% were trying to put together these highly symmetric models 00:09:48.340 --> 00:09:50.440 align:middle line:84% to account for things like the nuclear forces. 00:09:50.440 --> 00:09:52.840 align:middle line:84% We looked at one example of that at the end 00:09:52.840 --> 00:09:56.650 align:middle line:84% of class last time, Quantum Chromodynamics, or QCD, 00:09:56.650 --> 00:09:59.930 align:middle line:84% which was really coming together in the early to mid 1970s. 00:09:59.930 --> 00:10:03.190 align:middle line:84% There were other instances of that, cousin models, 00:10:03.190 --> 00:10:06.310 align:middle line:84% similar kinds of models that were getting a lot of attention 00:10:06.310 --> 00:10:11.050 align:middle line:84% throughout the 1960s for the other main nuclear force, 00:10:11.050 --> 00:10:14.080 align:middle line:84% for what's called the weak force rather than the strong force. 00:10:14.080 --> 00:10:15.580 align:middle line:84% That is to say the force that causes 00:10:15.580 --> 00:10:17.440 align:middle line:90% things like radioactive decay. 00:10:17.440 --> 00:10:22.510 align:middle line:84% There was a lot of ideas and early suggestive experimental 00:10:22.510 --> 00:10:27.310 align:middle line:84% evidence already by the 1960s that the weak nuclear force was 00:10:27.310 --> 00:10:33.670 align:middle line:84% somehow mediated was what arose from particles exchanging 00:10:33.670 --> 00:10:37.030 align:middle line:84% certain kinds of force-carrying particles-- again, 00:10:37.030 --> 00:10:38.500 align:middle line:90% analogies to the photon-- 00:10:38.500 --> 00:10:41.230 align:middle line:84% in this case, what we would now call the W and Z bosons. 00:10:41.230 --> 00:10:43.390 align:middle line:84% The point is there were new kinds of matter, 00:10:43.390 --> 00:10:44.950 align:middle line:90% at least hypothesized. 00:10:44.950 --> 00:10:47.740 align:middle line:84% And then when particles tossed these back and forth, 00:10:47.740 --> 00:10:51.410 align:middle line:84% that would give rise to things like radioactive decay. 00:10:51.410 --> 00:10:54.820 align:middle line:84% But the challenge became very clear very quickly. 00:10:54.820 --> 00:10:58.690 align:middle line:84% These nuclear forces are self-evidently of short range, 00:10:58.690 --> 00:11:01.660 align:middle line:84% unlike gravitation or electromagnetism, which, 00:11:01.660 --> 00:11:06.340 align:middle line:84% in principle, can extend arbitrarily long distances, 00:11:06.340 --> 00:11:08.950 align:middle line:84% the nuclear forces really exert themselves 00:11:08.950 --> 00:11:12.390 align:middle line:84% across nuclear dimensions, very tiny fraction 00:11:12.390 --> 00:11:14.370 align:middle line:84% of the size even of a single atom, let 00:11:14.370 --> 00:11:16.290 align:middle line:90% alone macroscopic scales. 00:11:16.290 --> 00:11:20.040 align:middle line:84% And one of the most obvious ways to account for that at the time 00:11:20.040 --> 00:11:23.130 align:middle line:84% was to assume that the force-carrying particles that 00:11:23.130 --> 00:11:26.340 align:middle line:84% were responsible for those interactions were very massive. 00:11:26.340 --> 00:11:29.910 align:middle line:84% It would be unlikely for them to travel very far 00:11:29.910 --> 00:11:32.370 align:middle line:84% as virtual particles because, after all, they 00:11:32.370 --> 00:11:33.900 align:middle line:84% have to pay back the energy they've 00:11:33.900 --> 00:11:35.160 align:middle line:90% borrowed from the vacuum. 00:11:35.160 --> 00:11:39.300 align:middle line:84% So if they can only travel so far-- 00:11:39.300 --> 00:11:41.550 align:middle line:84% I should say, if they have a large mass, 00:11:41.550 --> 00:11:43.860 align:middle line:84% they have to borrow a substantial amount of energy 00:11:43.860 --> 00:11:46.240 align:middle line:84% from the vacuum as virtual particles. 00:11:46.240 --> 00:11:49.050 align:middle line:84% So they have to have a correspondingly short delta 00:11:49.050 --> 00:11:50.820 align:middle line:90% t, over which they pay it back. 00:11:50.820 --> 00:11:52.410 align:middle line:90% They can only travel so far. 00:11:52.410 --> 00:11:57.000 align:middle line:84% So the idea was could have finite range nuclear forces 00:11:57.000 --> 00:11:59.100 align:middle line:84% if these force-carrying particles had 00:11:59.100 --> 00:12:00.690 align:middle line:90% a very large mass. 00:12:00.690 --> 00:12:03.090 align:middle line:84% That will make it very unlikely for that force 00:12:03.090 --> 00:12:06.300 align:middle line:84% to be felt across a very large distance because 00:12:06.300 --> 00:12:08.880 align:middle line:84% of the whole set of ideas about virtual particles 00:12:08.880 --> 00:12:10.360 align:middle line:90% and the uncertainty principle. 00:12:10.360 --> 00:12:12.220 align:middle line:84% All well and good, but the problem 00:12:12.220 --> 00:12:15.370 align:middle line:84% was with these new fancy highly symmetric models 00:12:15.370 --> 00:12:17.440 align:middle line:84% of the nuclear forces, like the weak force 00:12:17.440 --> 00:12:22.030 align:middle line:84% and also true as we saw in the strong force of quantum 00:12:22.030 --> 00:12:25.060 align:middle line:84% chromodynamics, the symmetries of these new particles 00:12:25.060 --> 00:12:29.500 align:middle line:84% are meant to enforce is violated if you give those particles 00:12:29.500 --> 00:12:30.220 align:middle line:90% a mass. 00:12:30.220 --> 00:12:32.030 align:middle line:84% So you can have one or the other. 00:12:32.030 --> 00:12:34.450 align:middle line:84% You could have a short-range nuclear force 00:12:34.450 --> 00:12:37.030 align:middle line:84% that does not have any of those fancy symmetries. 00:12:37.030 --> 00:12:39.895 align:middle line:84% So that broke half of the motivation for it. 00:12:39.895 --> 00:12:41.770 align:middle line:84% Or we should keep all those fancy symmetries, 00:12:41.770 --> 00:12:44.720 align:middle line:84% but break this notion of a short-range force. 00:12:44.720 --> 00:12:47.050 align:middle line:84% So this was a pretty substantial challenge. 00:12:47.050 --> 00:12:51.100 align:middle line:84% It got lots of theorists very exercised over the 1950s 00:12:51.100 --> 00:12:53.260 align:middle line:84% and especially 1960s, which is really 00:12:53.260 --> 00:12:55.700 align:middle line:84% like, why do these particles have mass at all? 00:12:55.700 --> 00:12:58.900 align:middle line:84% Can we account for mass of these elementary particles 00:12:58.900 --> 00:13:00.250 align:middle line:90% in a self-consistent way? 00:13:00.250 --> 00:13:02.680 align:middle line:84% Because there's other set of puzzles 00:13:02.680 --> 00:13:04.820 align:middle line:84% that didn't seem to fit together. 00:13:04.820 --> 00:13:06.610 align:middle line:84% So the question of mass turns out 00:13:06.610 --> 00:13:10.580 align:middle line:84% to have been on many specialists minds in the 1950s and '60s, 00:13:10.580 --> 00:13:13.340 align:middle line:84% but as embedded in quite different-sounding 00:13:13.340 --> 00:13:15.330 align:middle line:90% conversations. 00:13:15.330 --> 00:13:18.710 align:middle line:84% So let's look a bit more at some of the proposed solutions 00:13:18.710 --> 00:13:21.260 align:middle line:84% to this question of mass from within these two quite 00:13:21.260 --> 00:13:22.640 align:middle line:90% separated communities. 00:13:22.640 --> 00:13:25.630 align:middle line:90% 00:13:25.630 --> 00:13:29.620 align:middle line:84% So right around the same time, often in the same year, often, 00:13:29.620 --> 00:13:32.000 align:middle line:84% in fact, published in the same journals, 00:13:32.000 --> 00:13:34.690 align:middle line:84% members of these two quite different communities-- 00:13:34.690 --> 00:13:37.300 align:middle line:84% the gravitation and cosmology crowd on one side 00:13:37.300 --> 00:13:39.910 align:middle line:84% and the high-energy particle physics community on the other 00:13:39.910 --> 00:13:40.540 align:middle line:90% side-- 00:13:40.540 --> 00:13:44.830 align:middle line:84% they were proposing solutions, or at least 00:13:44.830 --> 00:13:47.950 align:middle line:84% hypothetical solutions to these problems of mass-- 00:13:47.950 --> 00:13:50.172 align:middle line:84% but, again, within their own idiom. 00:13:50.172 --> 00:13:51.880 align:middle line:84% So even though the ideas were bubbling up 00:13:51.880 --> 00:13:54.463 align:middle line:84% around the same time and often published in the same journals, 00:13:54.463 --> 00:13:56.890 align:middle line:84% they still were embedded in quite different research 00:13:56.890 --> 00:13:58.930 align:middle line:90% traditions and conversations. 00:13:58.930 --> 00:14:06.490 align:middle line:84% So on the gravity side, one of the most significant elements 00:14:06.490 --> 00:14:11.230 align:middle line:84% of research in this topic was put forward in 1961 by, 00:14:11.230 --> 00:14:13.060 align:middle line:84% at the time, a very young Carl Brans. 00:14:13.060 --> 00:14:15.430 align:middle line:84% This is part of Carl's PhD thesis 00:14:15.430 --> 00:14:18.770 align:middle line:84% that he worked on with his PhD advisor, Robert Dicke. 00:14:18.770 --> 00:14:21.790 align:middle line:84% So this became known as the Brans-Dicke theory of gravity, 00:14:21.790 --> 00:14:25.052 align:middle line:84% named for Carl Brans and Robert Dicke. 00:14:25.052 --> 00:14:26.260 align:middle line:90% And they published it in the. 00:14:26.260 --> 00:14:28.630 align:middle line:90% Physical Review in 1961. 00:14:28.630 --> 00:14:31.240 align:middle line:84% Their idea was actually to try to go back 00:14:31.240 --> 00:14:33.115 align:middle line:84% to this notion of Mach's principle 00:14:33.115 --> 00:14:36.160 align:middle line:84% and more thoroughly account for that 00:14:36.160 --> 00:14:39.020 align:middle line:84% within a quantitative theory of gravity. 00:14:39.020 --> 00:14:41.950 align:middle line:84% So they wanted to modify Einstein's general theory 00:14:41.950 --> 00:14:44.080 align:middle line:84% of relativity in a very specific way 00:14:44.080 --> 00:14:46.240 align:middle line:84% to try to address this question of mass 00:14:46.240 --> 00:14:49.630 align:middle line:84% as it had been articulated around Mach's principle. 00:14:49.630 --> 00:14:52.150 align:middle line:84% So their idea was to introduce a whole new kind 00:14:52.150 --> 00:14:57.040 align:middle line:84% of matter, a new kind of particle in nature. 00:14:57.040 --> 00:15:00.130 align:middle line:84% They labeled it with the Greek letter phi. 00:15:00.130 --> 00:15:01.750 align:middle line:84% And now the idea was that instead 00:15:01.750 --> 00:15:07.720 align:middle line:84% of having a single constant unit strength of gravity 00:15:07.720 --> 00:15:11.530 align:middle line:84% labeled by Newton's constant capital G-- that's the G that's 00:15:11.530 --> 00:15:13.330 align:middle line:90% in like Newton's force law. 00:15:13.330 --> 00:15:15.970 align:middle line:84% F equals G M1 M2 over R squared-- 00:15:15.970 --> 00:15:18.670 align:middle line:90% that unit strength of gravity. 00:15:18.670 --> 00:15:21.070 align:middle line:84% You may remember that in Einstein's general theory 00:15:21.070 --> 00:15:23.560 align:middle line:84% of relativity, there's an exact same constant that 00:15:23.560 --> 00:15:27.200 align:middle line:84% gets carried over by design because Einstein wanted 00:15:27.200 --> 00:15:29.990 align:middle line:84% predictions from his theory to match smoothly 00:15:29.990 --> 00:15:32.660 align:middle line:84% with the Newtonian predictions in the appropriate regime. 00:15:32.660 --> 00:15:36.710 align:middle line:84% So even in Einstein's theory, the unit strength of gravity 00:15:36.710 --> 00:15:41.960 align:middle line:84% is set by some universal constant, the same constant G. 00:15:41.960 --> 00:15:45.350 align:middle line:84% The same here and on Andromeda, the same today 00:15:45.350 --> 00:15:47.160 align:middle line:90% as it was a billion years ago. 00:15:47.160 --> 00:15:49.340 align:middle line:84% So what Brans and Dicke were wondering was, 00:15:49.340 --> 00:15:51.710 align:middle line:84% what if that unit strength of gravity 00:15:51.710 --> 00:15:53.420 align:middle line:90% is actually not a constant? 00:15:53.420 --> 00:15:55.190 align:middle line:84% What if the strength of gravity could 00:15:55.190 --> 00:15:57.570 align:middle line:90% vary across time and space? 00:15:57.570 --> 00:16:00.050 align:middle line:84% So one way to represent that variation 00:16:00.050 --> 00:16:04.100 align:middle line:84% was to say that this unit strength of gravity, Newton's 00:16:04.100 --> 00:16:07.280 align:middle line:84% so-called constant actually could vary because it 00:16:07.280 --> 00:16:09.950 align:middle line:90% was actually a dynamical field. 00:16:09.950 --> 00:16:12.230 align:middle line:84% It was some field extended across space 00:16:12.230 --> 00:16:14.270 align:middle line:90% that could vary over time. 00:16:14.270 --> 00:16:16.970 align:middle line:84% And whatever its value happens to be here and now 00:16:16.970 --> 00:16:19.580 align:middle line:84% is what we interpret as this local strength of gravity. 00:16:19.580 --> 00:16:22.610 align:middle line:84% So it was an attempt to modify Einstein's theory 00:16:22.610 --> 00:16:26.840 align:middle line:84% of general relativity actually in response to this challenge 00:16:26.840 --> 00:16:28.010 align:middle line:90% of Mach's principle. 00:16:28.010 --> 00:16:29.480 align:middle line:90% So what were they doing? 00:16:29.480 --> 00:16:32.030 align:middle line:84% If you look slightly more quantitatively, 00:16:32.030 --> 00:16:34.940 align:middle line:84% in Einstein's version, you can represent 00:16:34.940 --> 00:16:38.150 align:middle line:84% Einstein's general theory of relativity by an action, 00:16:38.150 --> 00:16:40.010 align:middle line:84% in a sense, by writing down a Lagrangian. 00:16:40.010 --> 00:16:43.010 align:middle line:84% And I won't go through all the details, of course, right now. 00:16:43.010 --> 00:16:44.420 align:middle line:90% I'd be glad to chat more. 00:16:44.420 --> 00:16:47.630 align:middle line:84% But what the relevant term in Einstein's equations 00:16:47.630 --> 00:16:48.335 align:middle line:90% looks like this. 00:16:48.335 --> 00:16:51.710 align:middle line:84% This R is one of those geometrical objects 00:16:51.710 --> 00:16:54.980 align:middle line:84% that he had to learn about from his friend 00:16:54.980 --> 00:16:56.420 align:middle line:84% Marcel Grossmann because Einstein 00:16:56.420 --> 00:16:58.500 align:middle line:84% had cut too many of his math courses in school. 00:16:58.500 --> 00:16:59.780 align:middle line:90% We talked about that. 00:16:59.780 --> 00:17:03.110 align:middle line:84% So this is the geometer's tool of quantifying 00:17:03.110 --> 00:17:04.589 align:middle line:90% the warping of space and time. 00:17:04.589 --> 00:17:06.770 align:middle line:84% It's called the Ricci curvature scalar. 00:17:06.770 --> 00:17:09.410 align:middle line:84% And multiplying that is this constant, 00:17:09.410 --> 00:17:13.260 align:middle line:84% this unit strength of gravity in Einstein's theory. 00:17:13.260 --> 00:17:14.849 align:middle line:84% So what Brans and Dicke do is say, 00:17:14.849 --> 00:17:17.819 align:middle line:84% well, instead of this unit constant, 00:17:17.819 --> 00:17:20.150 align:middle line:90% let's replace G by 1 over phi. 00:17:20.150 --> 00:17:23.150 align:middle line:84% So now 1 over G becomes phi in the Lagrangian 00:17:23.150 --> 00:17:24.980 align:middle line:84% or in the action for gravitation, 00:17:24.980 --> 00:17:27.859 align:middle line:84% where in principle phi, the local strength of gravity 00:17:27.859 --> 00:17:29.820 align:middle line:84% could change across time and space. 00:17:29.820 --> 00:17:33.700 align:middle line:84% So if this field phi could vary, if it, in principle, it 00:17:33.700 --> 00:17:37.010 align:middle line:84% could be changing eta over space or over time, 00:17:37.010 --> 00:17:38.510 align:middle line:84% then they also knew they had to take 00:17:38.510 --> 00:17:42.590 align:middle line:84% into account within this total energy budget, the Lagrangian, 00:17:42.590 --> 00:17:45.890 align:middle line:84% they had to take into account the kinetic energy associated 00:17:45.890 --> 00:17:49.100 align:middle line:84% with every time that field changes either gradients 00:17:49.100 --> 00:17:51.810 align:middle line:90% in space or changes over time. 00:17:51.810 --> 00:17:53.840 align:middle line:84% So the second term they had to add in-- 00:17:53.840 --> 00:17:55.670 align:middle line:90% this part here in red-- 00:17:55.670 --> 00:17:58.850 align:middle line:84% is basically the kinetic energy associated with variations 00:17:58.850 --> 00:18:01.910 align:middle line:90% in that new dynamical field. 00:18:01.910 --> 00:18:05.450 align:middle line:84% And they put in very cleverly this extra dimensionless 00:18:05.450 --> 00:18:08.630 align:middle line:84% constant, a fudge factor, that they labeled 00:18:08.630 --> 00:18:10.370 align:middle line:90% by the Greek letter omega. 00:18:10.370 --> 00:18:14.390 align:middle line:84% And this was basically to control how much their version 00:18:14.390 --> 00:18:17.060 align:middle line:84% would depart from the ordinary behavior 00:18:17.060 --> 00:18:18.480 align:middle line:90% from general relativity. 00:18:18.480 --> 00:18:22.310 align:middle line:84% The idea is that if omega is just 00:18:22.310 --> 00:18:25.890 align:middle line:84% a number, just some real number, a positive integer-- 00:18:25.890 --> 00:18:28.540 align:middle line:84% or not integer, positive number-- 00:18:28.540 --> 00:18:32.700 align:middle line:84% if omega is a very small number of order 1 or a fraction of 1, 00:18:32.700 --> 00:18:35.700 align:middle line:84% then, in some sense, it doesn't cost very much. 00:18:35.700 --> 00:18:38.740 align:middle line:84% The kinetic energy is multiplied by a small number. 00:18:38.740 --> 00:18:41.280 align:middle line:84% So the field varying across space and time 00:18:41.280 --> 00:18:44.640 align:middle line:84% wouldn't cost very much of the whole system's energy balance, 00:18:44.640 --> 00:18:47.760 align:middle line:84% so it would not, in principle, be difficult for the field 00:18:47.760 --> 00:18:50.100 align:middle line:84% to actually be really quite wobbly, that it 00:18:50.100 --> 00:18:52.715 align:middle line:84% could vary quite significantly across space and time. 00:18:52.715 --> 00:18:54.090 align:middle line:84% That would be like saying you can 00:18:54.090 --> 00:18:56.670 align:middle line:84% have quite dramatic differences in the local strength 00:18:56.670 --> 00:18:59.160 align:middle line:84% of gravity because this field phi could be wobbling all 00:18:59.160 --> 00:19:01.440 align:middle line:84% over the place because it wouldn't cost much 00:19:01.440 --> 00:19:02.730 align:middle line:90% from the energy balance. 00:19:02.730 --> 00:19:04.800 align:middle line:84% On the other hand, as you tune up 00:19:04.800 --> 00:19:09.060 align:middle line:84% that parameter, the simple number omega, as omega 00:19:09.060 --> 00:19:12.450 align:middle line:84% gets more and more large, then it costs more and more 00:19:12.450 --> 00:19:14.500 align:middle line:90% to have the field vary at all. 00:19:14.500 --> 00:19:18.150 align:middle line:84% And so, in some sense, you're stiffening up the trampoline. 00:19:18.150 --> 00:19:20.580 align:middle line:84% So as omega becomes larger and larger, 00:19:20.580 --> 00:19:25.550 align:middle line:84% the field is much less likely to vary either over space or time. 00:19:25.550 --> 00:19:29.830 align:middle line:84% And in the limit that omega becomes arbitrarily large, then 00:19:29.830 --> 00:19:32.350 align:middle line:84% phi, in a sense, can't afford to vary at all. 00:19:32.350 --> 00:19:34.840 align:middle line:84% The kinetic energy cost is too high. 00:19:34.840 --> 00:19:38.020 align:middle line:84% And so, on average, the field doesn't change at all. 00:19:38.020 --> 00:19:40.850 align:middle line:84% If the field doesn't change at all, it acts like a constant. 00:19:40.850 --> 00:19:43.780 align:middle line:84% So you get back to the Einstein-like limit. 00:19:43.780 --> 00:19:46.630 align:middle line:84% So they had this very clever fudge factor, 00:19:46.630 --> 00:19:49.450 align:middle line:84% a coupling constant, so that, in principle, 00:19:49.450 --> 00:19:52.510 align:middle line:84% the local strength of gravity could be changing all the time. 00:19:52.510 --> 00:19:55.720 align:middle line:84% But the amount of that variation could be controlled by this one 00:19:55.720 --> 00:19:57.280 align:middle line:90% new parameter in the theory. 00:19:57.280 --> 00:19:59.470 align:middle line:84% And as omega becomes large, the behavior 00:19:59.470 --> 00:20:02.440 align:middle line:84% reduces to the kind of constant strength 00:20:02.440 --> 00:20:04.900 align:middle line:84% of gravity of the original Einstein theory. 00:20:04.900 --> 00:20:07.090 align:middle line:84% Now, how does that help with Mach's principle? 00:20:07.090 --> 00:20:10.870 align:middle line:84% The idea is that this new form of matter, this new field phi, 00:20:10.870 --> 00:20:13.570 align:middle line:84% is extended throughout all of the universe 00:20:13.570 --> 00:20:15.700 align:middle line:84% through every nook and cranny of space. 00:20:15.700 --> 00:20:17.800 align:middle line:84% All of matter interacts with phi. 00:20:17.800 --> 00:20:19.930 align:middle line:84% It's almost like a new ether, you might say. 00:20:19.930 --> 00:20:21.670 align:middle line:90% It's everywhere. 00:20:21.670 --> 00:20:25.750 align:middle line:84% And then the fact that phi is extended everywhere 00:20:25.750 --> 00:20:28.210 align:middle line:84% and is interacting universally is 00:20:28.210 --> 00:20:31.510 align:middle line:84% what they thought would take into account Mach's idea 00:20:31.510 --> 00:20:35.020 align:middle line:84% that the local inertial effects, the effects of this strength 00:20:35.020 --> 00:20:37.810 align:middle line:84% of local strength of gravity right here right now, 00:20:37.810 --> 00:20:42.130 align:middle line:84% really is attuned to or sensitive to the broader 00:20:42.130 --> 00:20:44.260 align:middle line:90% cosmic distribution of matter. 00:20:44.260 --> 00:20:46.720 align:middle line:84% So matter in the furthest away galaxies 00:20:46.720 --> 00:20:48.940 align:middle line:84% could be affecting the behavior of phi. 00:20:48.940 --> 00:20:53.570 align:middle line:84% And meanwhile, phi very locally affects local inertial effects. 00:20:53.570 --> 00:20:55.660 align:middle line:84% So this would be a way to incorporate 00:20:55.660 --> 00:20:59.410 align:middle line:84% Mach's principle going beyond even just Einstein's version. 00:20:59.410 --> 00:21:01.780 align:middle line:84% OK, so the idea was there's a problem of mass. 00:21:01.780 --> 00:21:05.260 align:middle line:84% Why do local objects behave with the mass that we observe? 00:21:05.260 --> 00:21:07.960 align:middle line:84% Why do we attribute certain inertial effects 00:21:07.960 --> 00:21:09.250 align:middle line:90% to local objects? 00:21:09.250 --> 00:21:12.970 align:middle line:84% And the suggestion here by, at the time, a very young Carl 00:21:12.970 --> 00:21:16.855 align:middle line:84% Brans and his advisor Robert Dicke was a new form of matter. 00:21:16.855 --> 00:21:19.060 align:middle line:84% They'll label it by the Greek letter phi. 00:21:19.060 --> 00:21:21.190 align:middle line:84% It extends to all of space, and everything else 00:21:21.190 --> 00:21:22.060 align:middle line:90% interacts with it. 00:21:22.060 --> 00:21:26.060 align:middle line:84% OK, that's hypothetical proposal number 1. 00:21:26.060 --> 00:21:32.573 align:middle line:84% Quite separately but right around the same time, 00:21:32.573 --> 00:21:33.990 align:middle line:84% there was a different conversation 00:21:33.990 --> 00:21:36.720 align:middle line:84% going on among people who were specialists 00:21:36.720 --> 00:21:41.070 align:middle line:84% in nuclear and particle physics and, to some degree, 00:21:41.070 --> 00:21:42.540 align:middle line:90% in solid-state physics. 00:21:42.540 --> 00:21:44.490 align:middle line:84% And that was this question about, 00:21:44.490 --> 00:21:47.610 align:middle line:84% why do certain elementary particles have 00:21:47.610 --> 00:21:50.670 align:middle line:84% a short range if we can't accommodate a mass 00:21:50.670 --> 00:21:51.920 align:middle line:90% while keeping this symmetry? 00:21:51.920 --> 00:21:54.045 align:middle line:84% This is now they're back in this question of things 00:21:54.045 --> 00:21:55.620 align:middle line:90% like the nuclear forces. 00:21:55.620 --> 00:21:58.770 align:middle line:84% Could you have nuclear forces mediated 00:21:58.770 --> 00:22:00.928 align:middle line:90% by the exchange of particles? 00:22:00.928 --> 00:22:02.970 align:middle line:84% Could those particles themselves be very massive? 00:22:02.970 --> 00:22:05.820 align:middle line:84% So they don't go very far-- so a short-range force-- 00:22:05.820 --> 00:22:08.730 align:middle line:84% and yet, still be respecting the very symmetries 00:22:08.730 --> 00:22:10.770 align:middle line:84% for which people had invented those particles 00:22:10.770 --> 00:22:11.860 align:middle line:90% in the first place. 00:22:11.860 --> 00:22:16.080 align:middle line:84% So here was, again, a very clever suggestion coming first 00:22:16.080 --> 00:22:17.498 align:middle line:90% actually from Jeffrey Goldstone. 00:22:17.498 --> 00:22:19.290 align:middle line:84% Some of you might know Professor Goldstone. 00:22:19.290 --> 00:22:21.720 align:middle line:84% He's an emeritus professor of physics here at MIT. 00:22:21.720 --> 00:22:23.250 align:middle line:90% He's still very active-- 00:22:23.250 --> 00:22:25.170 align:middle line:84% and then independently a few years later 00:22:25.170 --> 00:22:26.860 align:middle line:84% by people like Peter Higgs and, in fact, 00:22:26.860 --> 00:22:28.800 align:middle line:84% many other theorists along the way. 00:22:28.800 --> 00:22:32.630 align:middle line:84% All this work was bubbling up between 1961 and '64, 00:22:32.630 --> 00:22:35.050 align:middle line:84% again, often being published in that same journal 00:22:35.050 --> 00:22:37.450 align:middle line:84% as the Brans-Dicke work but embedded in quite 00:22:37.450 --> 00:22:38.920 align:middle line:90% a separate conversation. 00:22:38.920 --> 00:22:42.160 align:middle line:84% So their approach was not to tinker with the kinetic energy 00:22:42.160 --> 00:22:45.100 align:middle line:84% like Brans and Dicke were doing, but actually 00:22:45.100 --> 00:22:47.900 align:middle line:84% to study a new form of the potential energy. 00:22:47.900 --> 00:22:51.680 align:middle line:84% So they, again, introduce a new hypothetical form of matter. 00:22:51.680 --> 00:22:53.560 align:middle line:84% They, again, consider a scalar field-- 00:22:53.560 --> 00:22:56.680 align:middle line:84% let's say, a field with zero spin, zero intrinsic angular 00:22:56.680 --> 00:22:57.880 align:middle line:90% momentum. 00:22:57.880 --> 00:23:00.850 align:middle line:84% They again label it by the same Greek letter phi. 00:23:00.850 --> 00:23:03.280 align:middle line:84% But now they focus on the potential energy 00:23:03.280 --> 00:23:06.130 align:middle line:84% stored that might be stored in this hypothetical field. 00:23:06.130 --> 00:23:08.320 align:middle line:84% And as they showed-- in fact, Goldstone 00:23:08.320 --> 00:23:10.360 align:middle line:84% was really among the very first to show this-- 00:23:10.360 --> 00:23:15.040 align:middle line:84% if you adopt a certain kind of characteristic shape 00:23:15.040 --> 00:23:17.030 align:middle line:84% for that potential energy function, 00:23:17.030 --> 00:23:19.240 align:middle line:84% then you could accomplish what they came to call 00:23:19.240 --> 00:23:21.885 align:middle line:90% spontaneous symmetry breaking. 00:23:21.885 --> 00:23:23.260 align:middle line:84% So this picture is actually taken 00:23:23.260 --> 00:23:26.280 align:middle line:84% from Goldstone's very first article in 1961. 00:23:26.280 --> 00:23:27.430 align:middle line:90% It doesn't look so special. 00:23:27.430 --> 00:23:29.800 align:middle line:84% You've probably seen similar things yourselves. 00:23:29.800 --> 00:23:32.020 align:middle line:84% It looks like basically a double-well potential. 00:23:32.020 --> 00:23:34.980 align:middle line:84% So the idea was, at the level of the governing 00:23:34.980 --> 00:23:36.810 align:middle line:84% equations for these nuclear forces, 00:23:36.810 --> 00:23:39.750 align:middle line:84% you would continue to assume that 00:23:39.750 --> 00:23:43.840 align:middle line:84% these force-carrying particles really were genuinely massless. 00:23:43.840 --> 00:23:45.780 align:middle line:84% They had zero mass, just like the photon 00:23:45.780 --> 00:23:47.380 align:middle line:90% from electromagnetism. 00:23:47.380 --> 00:23:49.530 align:middle line:84% So that would preserve all the symmetries. 00:23:49.530 --> 00:23:52.650 align:middle line:84% The equations for the governing nuclear forces 00:23:52.650 --> 00:23:54.840 align:middle line:84% could retain all the fancy symmetries 00:23:54.840 --> 00:23:57.120 align:middle line:84% for which these new particles were introduced 00:23:57.120 --> 00:24:01.200 align:middle line:84% to reinforce that symmetry at each point in space and time. 00:24:01.200 --> 00:24:03.780 align:middle line:84% And you wouldn't put in any symmetry-breaking terms 00:24:03.780 --> 00:24:04.560 align:middle line:90% by hand. 00:24:04.560 --> 00:24:06.330 align:middle line:84% You'd leave those particles massless. 00:24:06.330 --> 00:24:10.440 align:middle line:84% However, you'd add in a new additional form of matter 00:24:10.440 --> 00:24:12.600 align:middle line:84% even beyond those force-carrying particles. 00:24:12.600 --> 00:24:14.580 align:middle line:84% It's what's now called the Higgs field. 00:24:14.580 --> 00:24:18.450 align:middle line:84% Though, really, many people were working on similar ideas. 00:24:18.450 --> 00:24:20.610 align:middle line:84% That separate field isn't responsible 00:24:20.610 --> 00:24:22.140 align:middle line:90% for the nuclear forces. 00:24:22.140 --> 00:24:23.850 align:middle line:84% It's responsible for giving everything 00:24:23.850 --> 00:24:26.130 align:middle line:84% else the masses that we measure, including 00:24:26.130 --> 00:24:27.880 align:middle line:90% those force-carrying particles. 00:24:27.880 --> 00:24:30.450 align:middle line:90% So the idea is this. 00:24:30.450 --> 00:24:33.220 align:middle line:84% Here's an example of that double-well potential. 00:24:33.220 --> 00:24:35.740 align:middle line:84% The equations governing the full system-- 00:24:35.740 --> 00:24:40.180 align:middle line:84% the nuclear forces plus this coupled scalar field phi-- 00:24:40.180 --> 00:24:43.690 align:middle line:84% retain all the symmetries that seem 00:24:43.690 --> 00:24:45.640 align:middle line:84% to be so important for the nuclear forces. 00:24:45.640 --> 00:24:47.200 align:middle line:84% In this case, as a toy model, it'd 00:24:47.200 --> 00:24:49.520 align:middle line:90% be like a left-right symmetry. 00:24:49.520 --> 00:24:52.870 align:middle line:84% So the governing equations don't care 00:24:52.870 --> 00:24:54.970 align:middle line:84% whether the field ultimately lands up 00:24:54.970 --> 00:24:57.610 align:middle line:84% in this local minimum of the potential 00:24:57.610 --> 00:24:59.110 align:middle line:90% or this local minimum. 00:24:59.110 --> 00:25:01.930 align:middle line:84% The equations are perfectly symmetrical, in this case, 00:25:01.930 --> 00:25:03.530 align:middle line:90% by a simple left-right symmetry. 00:25:03.530 --> 00:25:06.730 align:middle line:84% You can make that more fancy with a more involved symmetries 00:25:06.730 --> 00:25:08.200 align:middle line:90% of the nuclear forces. 00:25:08.200 --> 00:25:10.510 align:middle line:84% So the equations respect the symmetries. 00:25:10.510 --> 00:25:12.700 align:middle line:84% However, this is a dynamical field. 00:25:12.700 --> 00:25:15.820 align:middle line:84% At some point, it will settle into a local minimum. 00:25:15.820 --> 00:25:18.130 align:middle line:84% The system will minimize its energy. 00:25:18.130 --> 00:25:20.810 align:middle line:84% That's how it will seek its equilibrium. 00:25:20.810 --> 00:25:24.730 align:middle line:84% And so the system will settle in either to this local minimum 00:25:24.730 --> 00:25:28.390 align:middle line:84% on the right-hand side or this equivalent local minimum 00:25:28.390 --> 00:25:30.230 align:middle line:84% on the left-hand side-- same energy, 00:25:30.230 --> 00:25:36.210 align:middle line:84% so neither one is intrinsically preferable. 00:25:36.210 --> 00:25:38.400 align:middle line:90% And there's a 50/50 chance. 00:25:38.400 --> 00:25:40.110 align:middle line:84% So the idea is that the solutions 00:25:40.110 --> 00:25:42.180 align:middle line:84% to this dynamical system will break 00:25:42.180 --> 00:25:44.740 align:middle line:84% the symmetries that govern the equations themselves. 00:25:44.740 --> 00:25:48.030 align:middle line:84% That's why this became called spontaneous symmetry breaking. 00:25:48.030 --> 00:25:51.720 align:middle line:84% The governing equations maintain the symmetry, 00:25:51.720 --> 00:25:54.180 align:middle line:84% but the symmetry is broken spontaneously 00:25:54.180 --> 00:25:57.510 align:middle line:84% when the system relaxes to some lowest energy state. 00:25:57.510 --> 00:25:59.760 align:middle line:90% Now, why would that help? 00:25:59.760 --> 00:26:02.220 align:middle line:84% Because I assume, much like Brans and Dicke 00:26:02.220 --> 00:26:05.310 align:middle line:84% assumed, that this new hypothetical state of matter, 00:26:05.310 --> 00:26:08.610 align:middle line:84% this new field phi, stretches through all of space, 00:26:08.610 --> 00:26:10.770 align:middle line:84% through every nook and cranny of universe. 00:26:10.770 --> 00:26:13.810 align:middle line:84% And all the rest of matter interacts with it. 00:26:13.810 --> 00:26:16.740 align:middle line:84% So now once this scalar field, once this what we now 00:26:16.740 --> 00:26:19.380 align:middle line:84% call Higgs field gets anchored or gets 00:26:19.380 --> 00:26:21.330 align:middle line:84% stuck in one of its local minima, 00:26:21.330 --> 00:26:24.390 align:middle line:84% it now acts like it has a molasses. 00:26:24.390 --> 00:26:28.050 align:middle line:84% It actually is stuck at a nonzero value of its field. 00:26:28.050 --> 00:26:29.730 align:middle line:84% Instead of being stuck at the origin, 00:26:29.730 --> 00:26:32.020 align:middle line:84% it's stuck at some nonzero value. 00:26:32.020 --> 00:26:34.570 align:middle line:84% If these other nuclear force-carrying particles, 00:26:34.570 --> 00:26:37.670 align:middle line:84% for example, interact with phi, then, all of a sudden, 00:26:37.670 --> 00:26:39.880 align:middle line:84% they have drag this field around. 00:26:39.880 --> 00:26:43.870 align:middle line:84% They start acting as if they have a large mass. 00:26:43.870 --> 00:26:47.050 align:middle line:84% At the level of the equations alone, they have zero mass. 00:26:47.050 --> 00:26:50.500 align:middle line:84% They should be exactly as massless as the photon. 00:26:50.500 --> 00:26:53.620 align:middle line:84% But unlike the photon, these newer force-carrying 00:26:53.620 --> 00:26:57.100 align:middle line:84% particles interact with this new scalar field. 00:26:57.100 --> 00:26:58.930 align:middle line:84% When the scalar field gets anchored 00:26:58.930 --> 00:27:01.690 align:middle line:84% to some energy-minimizing value, that 00:27:01.690 --> 00:27:04.930 align:middle line:84% changes the interactions of the effective dynamics of all 00:27:04.930 --> 00:27:06.340 align:middle line:90% the fields that coupled to it. 00:27:06.340 --> 00:27:08.980 align:middle line:84% And now they number across space as if they 00:27:08.980 --> 00:27:10.180 align:middle line:90% have a very large mass. 00:27:10.180 --> 00:27:13.630 align:middle line:84% It's an induced mass coming from this spontaneous symmetry 00:27:13.630 --> 00:27:14.270 align:middle line:90% breaking. 00:27:14.270 --> 00:27:16.062 align:middle line:84% So now, again, you can have your symmetries 00:27:16.062 --> 00:27:17.140 align:middle line:90% and your short range. 00:27:17.140 --> 00:27:20.980 align:middle line:84% You can have your cake and eat it too-- very fascinating, 00:27:20.980 --> 00:27:23.290 align:middle line:84% a lovely idea being introduced right 00:27:23.290 --> 00:27:26.290 align:middle line:84% around the same time as Brans-Dicke also as one way 00:27:26.290 --> 00:27:29.890 align:middle line:84% to try to get to this question of why do objects have mass. 00:27:29.890 --> 00:27:32.900 align:middle line:84% So these two communities saw very different things 00:27:32.900 --> 00:27:35.180 align:middle line:84% even though they use the same Greek letter phi. 00:27:35.180 --> 00:27:38.180 align:middle line:84% They were embedded in different kinds of conversations. 00:27:38.180 --> 00:27:41.060 align:middle line:84% To people like Brans and Dicke and their colleagues 00:27:41.060 --> 00:27:44.060 align:middle line:84% in gravitation and cosmology, this new field, 00:27:44.060 --> 00:27:47.270 align:middle line:84% the Brans-Dicke field phi, seemed very exciting. 00:27:47.270 --> 00:27:49.100 align:middle line:84% It got a lot of attention, as I'll 00:27:49.100 --> 00:27:51.620 align:middle line:84% say more about in a moment, because it offered 00:27:51.620 --> 00:27:55.310 align:middle line:84% the first really concrete quantitative alternative 00:27:55.310 --> 00:27:57.470 align:middle line:84% to Einstein's general theory of relativity 00:27:57.470 --> 00:28:01.860 align:middle line:90% in nearly half a century. 00:28:01.860 --> 00:28:04.910 align:middle line:84% And so that helped to spur high-precision tests. 00:28:04.910 --> 00:28:08.900 align:middle line:84% Now there was not just Einstein versus Newton, which had really 00:28:08.900 --> 00:28:11.750 align:middle line:84% been put to bed already by earlier in the century, 00:28:11.750 --> 00:28:14.390 align:middle line:84% but is Einstein's the only possible 00:28:14.390 --> 00:28:16.910 align:middle line:84% relativistic generalization of Newton's gravity? 00:28:16.910 --> 00:28:19.910 align:middle line:84% Now it seemed very clear it was not the only possible one. 00:28:19.910 --> 00:28:23.540 align:middle line:84% And in fact, depending on the value of that parameter omega, 00:28:23.540 --> 00:28:26.630 align:middle line:84% there might be really measurable astrophysical effects 00:28:26.630 --> 00:28:29.270 align:middle line:84% between whether it's really what Einstein said 00:28:29.270 --> 00:28:33.110 align:middle line:84% or this other seemingly self-consistent alternative. 00:28:33.110 --> 00:28:35.360 align:middle line:84% So this seemed to be really exciting to learn more 00:28:35.360 --> 00:28:38.870 align:middle line:84% about the behavior of gravity on a fine scale experimentally. 00:28:38.870 --> 00:28:41.420 align:middle line:84% So lots of people paid attention to Brans-Dicke gravity 00:28:41.420 --> 00:28:43.320 align:middle line:90% very quickly. 00:28:43.320 --> 00:28:46.040 align:middle line:84% On the other side of these discussions, 00:28:46.040 --> 00:28:49.670 align:middle line:84% two theorists focusing on nuclear and particle physics-- 00:28:49.670 --> 00:28:53.510 align:middle line:84% what we now call the Higgs field, the separate field phi-- 00:28:53.510 --> 00:28:55.910 align:middle line:84% was tremendously exciting for the reasons 00:28:55.910 --> 00:28:57.530 align:middle line:90% I was emphasizing a moment ago. 00:28:57.530 --> 00:29:00.230 align:middle line:84% It finally seemed to offer a way forward 00:29:00.230 --> 00:29:04.010 align:middle line:84% to be able to maintain these very fancy, very abstract 00:29:04.010 --> 00:29:06.350 align:middle line:84% mathematical symmetries of the nuclear forces 00:29:06.350 --> 00:29:09.320 align:middle line:84% and keep them short-range, that the force-carrying particles 00:29:09.320 --> 00:29:12.110 align:middle line:84% could become massive due to their interactions 00:29:12.110 --> 00:29:14.510 align:middle line:84% with this all-pervasive field even though they had 00:29:14.510 --> 00:29:16.760 align:middle line:90% no intrinsic mass on their own. 00:29:16.760 --> 00:29:20.450 align:middle line:84% So that was answering a separate set of puzzles and quandaries. 00:29:20.450 --> 00:29:23.420 align:middle line:84% No one suggested, at least in print, 00:29:23.420 --> 00:29:26.390 align:middle line:84% that these two scalar fields with the same Greek letter 00:29:26.390 --> 00:29:29.030 align:middle line:84% might be similar or even worth considering side 00:29:29.030 --> 00:29:34.610 align:middle line:84% by side for nearly 20 years, until the later 1970s. 00:29:34.610 --> 00:29:38.120 align:middle line:84% The sets of ideas were published to very wide acclaim 00:29:38.120 --> 00:29:41.630 align:middle line:84% in their own separate fields as early as 1961. 00:29:41.630 --> 00:29:45.500 align:middle line:84% But it took more than 15 years-- nearly 20 years-- until people 00:29:45.500 --> 00:29:49.160 align:middle line:84% began to say, hey, these two scalar fields are 00:29:49.160 --> 00:29:50.720 align:middle line:90% meant to pervade all of nature. 00:29:50.720 --> 00:29:52.720 align:middle line:84% All of other forms of matter interact with them. 00:29:52.720 --> 00:29:54.095 align:middle line:84% They give rise to what we measure 00:29:54.095 --> 00:29:55.250 align:middle line:90% as local inertial effects. 00:29:55.250 --> 00:29:57.208 align:middle line:84% What if they're actually similar to each other? 00:29:57.208 --> 00:30:01.190 align:middle line:84% That question simply wasn't asked for a long, long time. 00:30:01.190 --> 00:30:02.900 align:middle line:84% And as I mentioned a few times it's 00:30:02.900 --> 00:30:05.420 align:middle line:84% not because these were obscure papers. 00:30:05.420 --> 00:30:08.750 align:middle line:84% So I mentioned some time ago that particle physicists, 00:30:08.750 --> 00:30:11.000 align:middle line:84% in particular, love to count citations. 00:30:11.000 --> 00:30:12.507 align:middle line:84% We love to count our own citations. 00:30:12.507 --> 00:30:14.090 align:middle line:84% We love to lord it over our colleagues 00:30:14.090 --> 00:30:15.980 align:middle line:84% when we have more citations than them. 00:30:15.980 --> 00:30:20.780 align:middle line:84% It's the way we bully each other or assess inherent worth, 00:30:20.780 --> 00:30:22.280 align:middle line:90% either which is inappropriate. 00:30:22.280 --> 00:30:24.800 align:middle line:84% Anyway, the idea is we have these very strict categories 00:30:24.800 --> 00:30:27.350 align:middle line:90% for how we sort citations. 00:30:27.350 --> 00:30:29.720 align:middle line:84% And the highest category we could think of 00:30:29.720 --> 00:30:32.300 align:middle line:90% is called renowned. 00:30:32.300 --> 00:30:34.240 align:middle line:84% It's not quite Beyonce, but that's 00:30:34.240 --> 00:30:35.420 align:middle line:90% the best we can reach for. 00:30:35.420 --> 00:30:38.350 align:middle line:84% And so a paper is considered technically renowned 00:30:38.350 --> 00:30:41.410 align:middle line:84% if it accumulates at least 500 citations 00:30:41.410 --> 00:30:43.960 align:middle line:90% in the scientific literature. 00:30:43.960 --> 00:30:45.910 align:middle line:84% And that's the highest category we invented. 00:30:45.910 --> 00:30:49.378 align:middle line:84% So there's famous, very well-known, but renowned 00:30:49.378 --> 00:30:50.170 align:middle line:90% is the highest one. 00:30:50.170 --> 00:30:51.610 align:middle line:90% So each of these papers-- 00:30:51.610 --> 00:30:53.980 align:middle line:84% the Brans-Dicke paper and the Higgs papers-- 00:30:53.980 --> 00:30:58.090 align:middle line:84% became technically renowned within fewer than 20 years. 00:30:58.090 --> 00:31:00.320 align:middle line:84% These were getting a lot of attention very quickly. 00:31:00.320 --> 00:31:02.710 align:middle line:84% So on the right-hand-- excuse me-- on the left-hand side 00:31:02.710 --> 00:31:05.440 align:middle line:84% in red are the worldwide citations 00:31:05.440 --> 00:31:07.540 align:middle line:90% to the 1961 Brans-Dicke paper. 00:31:07.540 --> 00:31:09.220 align:middle line:90% That was the gravity-type paper. 00:31:09.220 --> 00:31:12.460 align:middle line:84% It crosses 500 accumulated citations 00:31:12.460 --> 00:31:14.230 align:middle line:90% in fewer than 20 years. 00:31:14.230 --> 00:31:17.170 align:middle line:84% And in blue are citations just to Peter Higgs's papers, 00:31:17.170 --> 00:31:20.470 align:middle line:84% not even counting the comparable number 00:31:20.470 --> 00:31:22.760 align:middle line:84% that went to Geoffrey Goldstone's early work. 00:31:22.760 --> 00:31:25.210 align:middle line:84% So in blue, I'm plotting just cumulative citations 00:31:25.210 --> 00:31:26.680 align:middle line:84% to Peter Higgs's work specifically 00:31:26.680 --> 00:31:28.760 align:middle line:84% on what we now call the Higgs mechanism. 00:31:28.760 --> 00:31:32.470 align:middle line:84% And again, they cross 500 within fewer than 20 years. 00:31:32.470 --> 00:31:35.710 align:middle line:84% These papers were setting their own fields on fire. 00:31:35.710 --> 00:31:40.210 align:middle line:84% These were very highly prominent contributions. 00:31:40.210 --> 00:31:42.155 align:middle line:84% And yet, there's almost no overlap. 00:31:42.155 --> 00:31:44.530 align:middle line:84% So if you do more than just count and start saying, well, 00:31:44.530 --> 00:31:49.570 align:middle line:84% who wrote those papers that are adding up to these kind of dots 00:31:49.570 --> 00:31:52.300 align:middle line:84% on each of these histograms, you see it really 00:31:52.300 --> 00:31:53.350 align:middle line:90% is like oil and water. 00:31:53.350 --> 00:31:55.840 align:middle line:84% These two communities were quite separated 00:31:55.840 --> 00:31:59.110 align:middle line:90% during this 20-year span. 00:31:59.110 --> 00:32:01.480 align:middle line:84% So you can see more than 500 each. 00:32:01.480 --> 00:32:04.360 align:middle line:84% In fact, it's 1,083 distinct papers 00:32:04.360 --> 00:32:07.060 align:middle line:84% doing the citing if you add up all the ones between these two 00:32:07.060 --> 00:32:08.290 align:middle line:90% plots. 00:32:08.290 --> 00:32:10.690 align:middle line:90% And yet, only six of those-- 00:32:10.690 --> 00:32:12.430 align:middle line:90% so less than 1%-- 00:32:12.430 --> 00:32:15.640 align:middle line:84% cited both the Brans-Dicke paper and the Higgs papers 00:32:15.640 --> 00:32:18.500 align:middle line:84% in the same article during this whole 20-year period. 00:32:18.500 --> 00:32:20.500 align:middle line:84% So both articles are getting lots of attention, 00:32:20.500 --> 00:32:23.290 align:middle line:84% but within really quite separated communities. 00:32:23.290 --> 00:32:25.600 align:middle line:84% The earliest paper that co-cited-- 00:32:25.600 --> 00:32:28.630 align:middle line:84% the earliest article that cited both Brans-Dicke and Higgs 00:32:28.630 --> 00:32:32.600 align:middle line:84% in the same article came fully 11 years later in , say, 00:32:32.600 --> 00:32:35.990 align:middle line:84% 1972 after their original publications. 00:32:35.990 --> 00:32:37.910 align:middle line:84% And then most of the rest of them 00:32:37.910 --> 00:32:42.210 align:middle line:84% out of only these six came after 1975. 00:32:42.210 --> 00:32:46.280 align:middle line:84% So it was really in this late period when a very small 00:32:46.280 --> 00:32:48.170 align:middle line:90% handful-- still less than 1%-- 00:32:48.170 --> 00:32:51.210 align:middle line:84% started citing the papers together. 00:32:51.210 --> 00:32:54.290 align:middle line:84% And so that's by article reference lists. 00:32:54.290 --> 00:32:56.900 align:middle line:84% You can do play the same game and look at authorship. 00:32:56.900 --> 00:33:00.080 align:middle line:84% So there are 990 distinct authors 00:33:00.080 --> 00:33:03.440 align:middle line:84% represented by all these papers doing all the citing, roughly 00:33:03.440 --> 00:33:05.810 align:middle line:84% equal numbers between the gravitation and the particle 00:33:05.810 --> 00:33:07.010 align:middle line:90% theory side. 00:33:07.010 --> 00:33:11.720 align:middle line:84% And yet, only 21 of them-- so a little over 2% of the author 00:33:11.720 --> 00:33:12.350 align:middle line:90% pool-- 00:33:12.350 --> 00:33:15.950 align:middle line:84% cited both Brans-Dicke and Higgs usually in separate papers, 00:33:15.950 --> 00:33:18.170 align:middle line:84% but actually cited them in any of their work, 00:33:18.170 --> 00:33:20.330 align:middle line:84% again, over that first 20-year period. 00:33:20.330 --> 00:33:24.680 align:middle line:84% These are just ways to say that these two subfields really 00:33:24.680 --> 00:33:26.870 align:middle line:90% were not strongly interacting. 00:33:26.870 --> 00:33:32.740 align:middle line:84% You can have very attention-grabbing research, 00:33:32.740 --> 00:33:35.020 align:middle line:84% literally renowned research, bubbling 00:33:35.020 --> 00:33:37.450 align:middle line:84% up from the two separate communities at the same time, 00:33:37.450 --> 00:33:39.370 align:middle line:84% often in the same journals, asking 00:33:39.370 --> 00:33:41.890 align:middle line:84% similar kinds of questions introducing 00:33:41.890 --> 00:33:45.220 align:middle line:84% similar kinds of responses and new field labeled finds on 00:33:45.220 --> 00:33:48.520 align:middle line:84% and, yet, have almost no kind of crosstalk between them. 00:33:48.520 --> 00:33:51.350 align:middle line:84% That's how separate of those fields were. 00:33:51.350 --> 00:33:53.690 align:middle line:84% So why was there such a sharp divide? 00:33:53.690 --> 00:33:55.188 align:middle line:90% Why was there no overlap? 00:33:55.188 --> 00:33:57.230 align:middle line:84% Were these fields just different from each other? 00:33:57.230 --> 00:33:59.070 align:middle line:84% Am I just asking a silly question? 00:33:59.070 --> 00:34:00.890 align:middle line:84% Why would anyone have ever thought 00:34:00.890 --> 00:34:02.600 align:middle line:84% that the Brans-Dicke field had anything 00:34:02.600 --> 00:34:03.980 align:middle line:90% to do with the Higgs field? 00:34:03.980 --> 00:34:04.790 align:middle line:90% Well, no. 00:34:04.790 --> 00:34:09.800 align:middle line:84% Actually, as we'll see in the third part for today, in 1979, 00:34:09.800 --> 00:34:13.050 align:middle line:84% two separate theorists working independently of each other 00:34:13.050 --> 00:34:14.750 align:middle line:84% actually suggested that the two fields 00:34:14.750 --> 00:34:18.020 align:middle line:84% might be literally the same, not just comparable or worth 00:34:18.020 --> 00:34:19.489 align:middle line:90% considering side by side. 00:34:19.489 --> 00:34:22.250 align:middle line:84% But they developed a very cool model 00:34:22.250 --> 00:34:24.409 align:middle line:84% which depended on these two fields 00:34:24.409 --> 00:34:26.870 align:middle line:84% being literally the same field-- only one new field 00:34:26.870 --> 00:34:27.920 align:middle line:90% of nature, not two. 00:34:27.920 --> 00:34:31.010 align:middle line:84% So it's not that they're somehow intrinsically totally 00:34:31.010 --> 00:34:32.820 align:middle line:84% separate or different from each other. 00:34:32.820 --> 00:34:35.840 align:middle line:84% So instead, their status, really, is historical. 00:34:35.840 --> 00:34:39.320 align:middle line:84% How people assess them or what they thought they were good for 00:34:39.320 --> 00:34:41.159 align:middle line:90% was changing over time. 00:34:41.159 --> 00:34:42.290 align:middle line:90% And so what had changed? 00:34:42.290 --> 00:34:44.082 align:middle line:84% That's what we'll pick up in the next part. 00:34:44.082 --> 00:34:46.080 align:middle line:84% Let me pause here and ask for any questions. 00:34:46.080 --> 00:34:49.010 align:middle line:84% I see something come up in the chat. 00:34:49.010 --> 00:34:52.219 align:middle line:84% So Alex asks, was it an accident that both parties chose phi? 00:34:52.219 --> 00:34:52.862 align:middle line:90% Not really. 00:34:52.862 --> 00:34:54.320 align:middle line:84% I make a lot of that because I just 00:34:54.320 --> 00:34:56.237 align:middle line:84% think it's visually so striking they literally 00:34:56.237 --> 00:34:57.740 align:middle line:90% chose the same Greek letter. 00:34:57.740 --> 00:35:01.520 align:middle line:84% It wasn't too surprising, to be honest. 00:35:01.520 --> 00:35:06.050 align:middle line:84% It was not a rule but a pretty widespread convention 00:35:06.050 --> 00:35:10.710 align:middle line:84% by that point that a field that had no spin-- 00:35:10.710 --> 00:35:12.200 align:middle line:90% so a scalar field-- 00:35:12.200 --> 00:35:16.340 align:middle line:84% would often be labeled by the Greek letter phi. 00:35:16.340 --> 00:35:19.130 align:middle line:84% The Greek letter psi was often reserved by this point 00:35:19.130 --> 00:35:21.390 align:middle line:90% for spin 1/2 particles. 00:35:21.390 --> 00:35:23.960 align:middle line:84% So an electron field would often be written with a psi 00:35:23.960 --> 00:35:25.400 align:middle line:90% then as now-- 00:35:25.400 --> 00:35:28.880 align:middle line:84% or eventually quark fields, spin 1/2 fields. 00:35:28.880 --> 00:35:32.150 align:middle line:84% And fields that have one whole unit of spin-- 00:35:32.150 --> 00:35:34.610 align:middle line:84% vector fields would often be labeled 00:35:34.610 --> 00:35:38.060 align:middle line:84% with a Latin letter, like capital A or capital B. 00:35:38.060 --> 00:35:40.640 align:middle line:84% So it wasn't super, super shocking 00:35:40.640 --> 00:35:43.443 align:middle line:84% that both groups reached for the same Greek letter. 00:35:43.443 --> 00:35:45.860 align:middle line:84% Nonetheless, it is amazing to flip through Physical Review 00:35:45.860 --> 00:35:49.260 align:middle line:84% and see the same thing popping up and yet being 00:35:49.260 --> 00:35:51.300 align:middle line:90% seemingly in mutual oblivion. 00:35:51.300 --> 00:35:53.760 align:middle line:84% So the notation isn't super surprising 00:35:53.760 --> 00:35:54.900 align:middle line:90% that it was so similar. 00:35:54.900 --> 00:35:58.583 align:middle line:84% But the rest of it that is a new hypothetical state of matter, 00:35:58.583 --> 00:36:00.750 align:middle line:84% it pervades all of nature, everything else interacts 00:36:00.750 --> 00:36:02.458 align:middle line:84% with it, that's what gives rise to mass-- 00:36:02.458 --> 00:36:04.620 align:middle line:84% it was more than just the letter that they chose. 00:36:04.620 --> 00:36:09.540 align:middle line:84% There was a lot of what we might have considered similarities. 00:36:09.540 --> 00:36:11.790 align:middle line:84% And yet, the two sets of ideas really 00:36:11.790 --> 00:36:13.850 align:middle line:90% were treated so separately. 00:36:13.850 --> 00:36:14.920 align:middle line:90% This is a good question. 00:36:14.920 --> 00:36:15.920 align:middle line:90% Other questions on that? 00:36:15.920 --> 00:36:23.130 align:middle line:90% 00:36:23.130 --> 00:36:25.110 align:middle line:90% OK, I will charge on. 00:36:25.110 --> 00:36:26.740 align:middle line:84% But as usual, please don't be shy. 00:36:26.740 --> 00:36:30.060 align:middle line:84% Feel free to jump in with questions anytime. 00:36:30.060 --> 00:36:34.920 align:middle line:84% Let's look at what might have changed to make it possible, 00:36:34.920 --> 00:36:39.210 align:middle line:84% let alone feasible or likely for those theorists 00:36:39.210 --> 00:36:42.690 align:middle line:84% in the later '70s to consider these two scalar 00:36:42.690 --> 00:36:44.260 align:middle line:90% fields in a new light. 00:36:44.260 --> 00:36:46.530 align:middle line:84% So we might wonder, well, was it changes in data? 00:36:46.530 --> 00:36:48.810 align:middle line:84% Did experiments force a new evaluation? 00:36:48.810 --> 00:36:51.060 align:middle line:90% No, not really. 00:36:51.060 --> 00:36:52.860 align:middle line:84% Let's look at the gravity side first. 00:36:52.860 --> 00:36:56.230 align:middle line:84% So I mentioned that part of what got the gravitation community 00:36:56.230 --> 00:36:59.400 align:middle line:84% so excited about Brans-Dicke gravity was it now gave them 00:36:59.400 --> 00:37:02.610 align:middle line:84% something very tangible, very specific to try to test 00:37:02.610 --> 00:37:05.658 align:middle line:84% for to look for what could be subtle deviations from 00:37:05.658 --> 00:37:07.200 align:middle line:84% the predictions of Einstein's theory, 00:37:07.200 --> 00:37:11.580 align:middle line:84% not the larger-scale differences between Newton's universal 00:37:11.580 --> 00:37:13.430 align:middle line:90% gravity and Einstein's. 00:37:13.430 --> 00:37:18.590 align:middle line:84% And especially, in the so-called space age, by the later '60s 00:37:18.590 --> 00:37:21.080 align:middle line:84% and throughout the '70s, astrophysicists 00:37:21.080 --> 00:37:23.330 align:middle line:84% had all kinds of new tools with which 00:37:23.330 --> 00:37:25.410 align:middle line:84% to try to look for these effects, 00:37:25.410 --> 00:37:28.640 align:middle line:84% including things like human-built artificial 00:37:28.640 --> 00:37:31.090 align:middle line:84% satellites that can be sent throughout the solar system. 00:37:31.090 --> 00:37:33.140 align:middle line:84% Now you can do very precision monitoring 00:37:33.140 --> 00:37:35.900 align:middle line:84% of very weak or subtle gravitational effects, 00:37:35.900 --> 00:37:38.090 align:middle line:84% not just in the space of one laboratory, 00:37:38.090 --> 00:37:42.620 align:middle line:84% but literally on a solar system scale. 00:37:42.620 --> 00:37:45.650 align:middle line:84% So these things were both, as I say, predictions 00:37:45.650 --> 00:37:48.300 align:middle line:84% both from Einstein's general theory of relativity 00:37:48.300 --> 00:37:50.720 align:middle line:84% and from Brans-Dicke gravity, we're 00:37:50.720 --> 00:37:53.480 align:middle line:84% subjected to some very, very clever and increasingly 00:37:53.480 --> 00:37:55.610 align:middle line:84% high-precision experimental tests 00:37:55.610 --> 00:37:58.010 align:middle line:84% starting, really, in the mid 1960s, some of them 00:37:58.010 --> 00:38:00.050 align:middle line:84% invented by Robert Dicke himself. 00:38:00.050 --> 00:38:01.970 align:middle line:84% Dicke's quite astonishing, I think. 00:38:01.970 --> 00:38:03.890 align:middle line:84% He was one of the, I think, maybe one 00:38:03.890 --> 00:38:08.310 align:middle line:84% of the last physicists of the 20th century who was really, 00:38:08.310 --> 00:38:11.270 align:middle line:84% really active and gifted both in theory and experiment. 00:38:11.270 --> 00:38:12.960 align:middle line:90% Enrico Fermi was like that. 00:38:12.960 --> 00:38:14.610 align:middle line:84% But as we've seen throughout this term, 00:38:14.610 --> 00:38:17.400 align:middle line:84% there had been a real professional division 00:38:17.400 --> 00:38:20.730 align:middle line:84% going back even to the late 19th century between theory 00:38:20.730 --> 00:38:21.450 align:middle line:90% and experiment. 00:38:21.450 --> 00:38:24.900 align:middle line:84% And Robert Dicke was one of these really quite 00:38:24.900 --> 00:38:28.620 align:middle line:84% nimble physicists who was quite accomplished in both. 00:38:28.620 --> 00:38:31.740 align:middle line:84% So he actually began testing his own theory 00:38:31.740 --> 00:38:34.710 align:middle line:84% with his group, his students, to do things 00:38:34.710 --> 00:38:38.490 align:middle line:84% like very sensitive measurements of the shape of the Sun, 00:38:38.490 --> 00:38:40.170 align:middle line:84% what was called solar oblateness. 00:38:40.170 --> 00:38:42.690 align:middle line:84% If the Sun is measurably different 00:38:42.690 --> 00:38:46.290 align:middle line:84% from an actual sphere, then that could actually 00:38:46.290 --> 00:38:49.530 align:middle line:84% have an impact on things like the orbit of the planet 00:38:49.530 --> 00:38:50.640 align:middle line:90% Mercury. 00:38:50.640 --> 00:38:54.360 align:middle line:84% If you have a more of an oblate spheroid from the Sun, 00:38:54.360 --> 00:38:57.030 align:middle line:84% then some effects that had been attributed 00:38:57.030 --> 00:38:59.730 align:middle line:84% to Einstein's general relativity might actually be different 00:38:59.730 --> 00:39:03.690 align:middle line:84% than assumed if you don't assume the Sun is perfectly spherical 00:39:03.690 --> 00:39:07.860 align:middle line:84% and, in fact, might be more consistent with this modified 00:39:07.860 --> 00:39:08.970 align:middle line:90% gravity Brans-Dicke. 00:39:08.970 --> 00:39:11.040 align:middle line:84% So Robert Dicke and his group started 00:39:11.040 --> 00:39:14.300 align:middle line:84% conducting high-precision visual measures 00:39:14.300 --> 00:39:17.323 align:middle line:84% of the shape of the Sun with the implication being 00:39:17.323 --> 00:39:19.490 align:middle line:84% that if that same mass had a different distribution, 00:39:19.490 --> 00:39:20.910 align:middle line:90% could you test between them? 00:39:20.910 --> 00:39:23.240 align:middle line:84% And again, I'd say, to Dicke's credit, 00:39:23.240 --> 00:39:25.790 align:middle line:84% even though he was testing his own beloved theory, 00:39:25.790 --> 00:39:27.950 align:middle line:84% his group found basically no evidence 00:39:27.950 --> 00:39:30.560 align:middle line:84% for a significant departure from sphericity. 00:39:30.560 --> 00:39:33.330 align:middle line:84% So this was not a reason in favor of Brans-Dicke. 00:39:33.330 --> 00:39:35.420 align:middle line:84% So Dicke himself found some compelling evidence 00:39:35.420 --> 00:39:38.780 align:middle line:84% against Brans-Dicke theory, again, I'd say, to his credit. 00:39:38.780 --> 00:39:41.060 align:middle line:84% Likewise, by the later '70s, there 00:39:41.060 --> 00:39:45.080 align:middle line:84% were these very cool efforts to do long range distance 00:39:45.080 --> 00:39:47.660 align:middle line:84% measurements between not just reflectors on the moon, 00:39:47.660 --> 00:39:51.290 align:middle line:84% but even to moving objects like the Viking Mars spacecraft 00:39:51.290 --> 00:39:52.200 align:middle line:90% and so on. 00:39:52.200 --> 00:39:55.460 align:middle line:84% So these were being done throughout the '60s and '70s. 00:39:55.460 --> 00:39:58.460 align:middle line:84% By the end of the '70s, things did not 00:39:58.460 --> 00:40:02.030 align:middle line:84% look very good for Brans-Dicke gravity experimentally. 00:40:02.030 --> 00:40:04.460 align:middle line:84% That is all the tests were easily 00:40:04.460 --> 00:40:07.010 align:middle line:84% consistent with the predictions from Einstein's theory 00:40:07.010 --> 00:40:09.020 align:middle line:90% within experimental errors. 00:40:09.020 --> 00:40:11.450 align:middle line:84% And yet, to make the Brans-Dicke, 00:40:11.450 --> 00:40:13.385 align:middle line:84% the modified gravity version consistent, 00:40:13.385 --> 00:40:17.160 align:middle line:84% you had to crank up that one free parameter, that 00:40:17.160 --> 00:40:20.262 align:middle line:84% dimensionless number omega, to be very large. 00:40:20.262 --> 00:40:21.720 align:middle line:84% Remember, that's the parameter that 00:40:21.720 --> 00:40:25.350 align:middle line:84% lets-- that would tell you how easy or difficult 00:40:25.350 --> 00:40:28.030 align:middle line:84% it would be for that new field phi to vary. 00:40:28.030 --> 00:40:29.460 align:middle line:90% It's almost like its stiffness. 00:40:29.460 --> 00:40:31.830 align:middle line:84% You had to tune that new field to be very stiff, 00:40:31.830 --> 00:40:34.290 align:middle line:84% so it would behave practically like a constant, which 00:40:34.290 --> 00:40:37.660 align:middle line:84% is what brings you back to the Einstein-like behavior. 00:40:37.660 --> 00:40:40.800 align:middle line:84% So to match all of these increasingly precise 00:40:40.800 --> 00:40:44.250 align:middle line:84% new experimental measurements, Brans-Dicke theory 00:40:44.250 --> 00:40:45.510 align:middle line:90% was not highly favored. 00:40:45.510 --> 00:40:48.070 align:middle line:84% In fact, Einstein's looked really good. 00:40:48.070 --> 00:40:50.250 align:middle line:84% So it was not that somehow experiments 00:40:50.250 --> 00:40:52.150 align:middle line:84% were in favor of Brans-Dicke gravity, 00:40:52.150 --> 00:40:54.820 align:middle line:84% and that's why everyone else started paying attention. 00:40:54.820 --> 00:40:56.800 align:middle line:90% So that's on the gravity side. 00:40:56.800 --> 00:40:59.290 align:middle line:84% Meanwhile, as most of you probably know, 00:40:59.290 --> 00:41:01.800 align:middle line:84% there was literally zero experimental evidence, 00:41:01.800 --> 00:41:06.120 align:middle line:84% a big fat goose egg nothing in favor of the Higgs-Boson until 00:41:06.120 --> 00:41:09.600 align:middle line:84% July of 2012, or if you're extra generous, 00:41:09.600 --> 00:41:13.950 align:middle line:84% maybe December of 2011, the first hints experimentally-- 00:41:13.950 --> 00:41:15.720 align:middle line:84% well past the period we're talking about. 00:41:15.720 --> 00:41:18.030 align:middle line:84% Meaning it was not that experiments 00:41:18.030 --> 00:41:20.430 align:middle line:84% found conclusive evidence of the Higgs-Boson, 00:41:20.430 --> 00:41:22.080 align:middle line:84% and that's what got the gravitation 00:41:22.080 --> 00:41:23.280 align:middle line:90% experts to pay attention. 00:41:23.280 --> 00:41:25.470 align:middle line:84% So each of these sets of ideas were 00:41:25.470 --> 00:41:28.710 align:middle line:84% inspiring intense efforts to do new experiments, 00:41:28.710 --> 00:41:30.540 align:middle line:84% but it wasn't that new experimental results 00:41:30.540 --> 00:41:33.270 align:middle line:84% had changed people's minds on the time scale 00:41:33.270 --> 00:41:35.580 align:middle line:84% that we're talking about for people very 00:41:35.580 --> 00:41:38.640 align:middle line:84% eventually to start taking these two sets of ideas 00:41:38.640 --> 00:41:40.560 align:middle line:90% consider them side by side. 00:41:40.560 --> 00:41:42.630 align:middle line:90% So it wasn't a new experiment. 00:41:42.630 --> 00:41:44.640 align:middle line:84% The main story that's mostly given-- 00:41:44.640 --> 00:41:47.070 align:middle line:84% I alluded to this in the very beginning of today's class-- 00:41:47.070 --> 00:41:50.580 align:middle line:84% is actually hearkens to changes in ideas and, in particular, 00:41:50.580 --> 00:41:53.010 align:middle line:84% on the particle physics or particle theory side. 00:41:53.010 --> 00:41:55.750 align:middle line:84% And these are brilliant and beautiful ideas. 00:41:55.750 --> 00:41:58.530 align:middle line:84% These ideas are well worth appreciating. 00:41:58.530 --> 00:42:00.630 align:middle line:84% I just don't think of the whole story. 00:42:00.630 --> 00:42:02.640 align:middle line:84% And two of these sets of ideas, in particular, 00:42:02.640 --> 00:42:05.730 align:middle line:84% are usually pointed to-- and they came in rapid fire 00:42:05.730 --> 00:42:08.490 align:middle line:90% in 1973 and 1974. 00:42:08.490 --> 00:42:11.830 align:middle line:84% The first of them is called asymptotic freedom. 00:42:11.830 --> 00:42:14.820 align:middle line:84% And actually, it's the reason why our friend and colleague 00:42:14.820 --> 00:42:17.880 align:middle line:84% here at MIT, Frank Wilczek, received the Nobel Prize. 00:42:17.880 --> 00:42:21.570 align:middle line:84% So he wasn't at MIT at the time, but he's now at MIT. 00:42:21.570 --> 00:42:25.680 align:middle line:84% So this was work introduced by Frank and his then advisor 00:42:25.680 --> 00:42:29.040 align:middle line:84% David Gross, and independently by a different very young 00:42:29.040 --> 00:42:31.207 align:middle line:84% grad student at the time, David Politzer. 00:42:31.207 --> 00:42:32.790 align:middle line:84% His grad students doing work for which 00:42:32.790 --> 00:42:34.080 align:middle line:90% they won the Nobel prizes-- 00:42:34.080 --> 00:42:35.400 align:middle line:90% amazing. 00:42:35.400 --> 00:42:38.160 align:middle line:84% And what they found was that the strength 00:42:38.160 --> 00:42:41.400 align:middle line:84% of the strong nuclear force, that QCD force that we talked 00:42:41.400 --> 00:42:43.900 align:middle line:84% about quite a bit at the end of last class 00:42:43.900 --> 00:42:48.250 align:middle line:84% session, the interaction between quarks and gluons, 00:42:48.250 --> 00:42:50.320 align:middle line:84% that the strength of that force actually 00:42:50.320 --> 00:42:52.750 align:middle line:90% decreases with the energy scale. 00:42:52.750 --> 00:42:55.090 align:middle line:84% If you rev up quarks to more and more energy, 00:42:55.090 --> 00:42:57.250 align:middle line:84% they actually interact less strongly 00:42:57.250 --> 00:42:59.230 align:middle line:84% with each other rather than more strongly. 00:42:59.230 --> 00:43:01.210 align:middle line:84% And that was the opposite from how 00:43:01.210 --> 00:43:06.100 align:middle line:84% the other known forces behave, both the strength 00:43:06.100 --> 00:43:08.390 align:middle line:90% of the electric force-- 00:43:08.390 --> 00:43:10.090 align:middle line:84% if you think about the unit strength 00:43:10.090 --> 00:43:12.370 align:middle line:84% the unit charge of the electron-- and also 00:43:12.370 --> 00:43:15.610 align:middle line:84% this other weak nuclear force, the radioactive decay force. 00:43:15.610 --> 00:43:18.470 align:middle line:84% Those forces that literally the coupling constants, 00:43:18.470 --> 00:43:21.550 align:middle line:84% the effective charge, they get stronger 00:43:21.550 --> 00:43:23.830 align:middle line:84% as you go to higher and higher energies, which 00:43:23.830 --> 00:43:26.650 align:middle line:84% is like saying probing shorter and shorter distance scales. 00:43:26.650 --> 00:43:29.650 align:middle line:84% And yet, the strong nuclear force, the quark gluon force 00:43:29.650 --> 00:43:31.040 align:middle line:90% had the opposite behavior. 00:43:31.040 --> 00:43:33.070 align:middle line:84% So this is showing the interaction strength 00:43:33.070 --> 00:43:36.220 align:middle line:84% as you change the average energy of the particles involved 00:43:36.220 --> 00:43:39.320 align:middle line:84% in any given scattering situation. 00:43:39.320 --> 00:43:44.950 align:middle line:84% And so the flow of the average charge for the strong force 00:43:44.950 --> 00:43:48.130 align:middle line:84% has the opposite sign to what was then known 00:43:48.130 --> 00:43:49.930 align:middle line:90% and was assumed to be universal. 00:43:49.930 --> 00:43:52.120 align:middle line:84% So this was called asymptotic freedom. 00:43:52.120 --> 00:43:55.030 align:middle line:84% As you go to asymptotically large energies, 00:43:55.030 --> 00:43:56.980 align:middle line:84% the quarks would become ultimately free. 00:43:56.980 --> 00:43:58.630 align:middle line:90% They would feel no force at all. 00:43:58.630 --> 00:44:00.487 align:middle line:84% The effective force would go to 0. 00:44:00.487 --> 00:44:02.320 align:middle line:84% That's why it was called asymptotic freedom. 00:44:02.320 --> 00:44:03.910 align:middle line:84% They would be free from these forces 00:44:03.910 --> 00:44:06.250 align:middle line:84% in the arbitrarily high-energy limit 00:44:06.250 --> 00:44:10.720 align:middle line:84% or arbitrarily short distance limit-- same limit. 00:44:10.720 --> 00:44:13.420 align:middle line:90% So that was introduced in 1973. 00:44:13.420 --> 00:44:17.560 align:middle line:84% That suggests that the behavior of quarks and gluons 00:44:17.560 --> 00:44:20.470 align:middle line:84% might be really different at very high energies 00:44:20.470 --> 00:44:25.270 align:middle line:84% compared to energies that might be probed even in particle 00:44:25.270 --> 00:44:26.500 align:middle line:90% accelerators on Earth. 00:44:26.500 --> 00:44:27.910 align:middle line:84% You can see the energy scale here 00:44:27.910 --> 00:44:29.077 align:middle line:90% to give it a sense for that. 00:44:29.077 --> 00:44:32.920 align:middle line:84% This is measured in units of billion electron volts, or GeV. 00:44:32.920 --> 00:44:37.480 align:middle line:84% To give you a sense of scale, the present-day experiments 00:44:37.480 --> 00:44:40.280 align:middle line:84% at the Large Hadron Collider are here. 00:44:40.280 --> 00:44:43.780 align:middle line:84% So the highest energy particle accelerator on the planet, 00:44:43.780 --> 00:44:50.860 align:middle line:84% the Large Hadron Collider, is at about 1,000 GeV, or maybe now 00:44:50.860 --> 00:44:51.970 align:middle line:90% getting close to 10 TeV. 00:44:51.970 --> 00:44:53.080 align:middle line:90% So maybe it's up here. 00:44:53.080 --> 00:44:55.455 align:middle line:84% I'll give them one more notch because they've tried hard. 00:44:55.455 --> 00:44:56.410 align:middle line:90% They're here. 00:44:56.410 --> 00:44:59.590 align:middle line:84% The effects that Frank Wilczek, and David Gross, 00:44:59.590 --> 00:45:01.420 align:middle line:84% and David Politzer were talking about 00:45:01.420 --> 00:45:05.600 align:middle line:84% would be noticeable exponentially higher energies, 00:45:05.600 --> 00:45:06.100 align:middle line:90% right? 00:45:06.100 --> 00:45:08.960 align:middle line:84% Well beyond anything that can be achieved even today, 00:45:08.960 --> 00:45:10.930 align:middle line:90% let alone in the 1970s. 00:45:10.930 --> 00:45:14.200 align:middle line:84% Slack was basically around here at the time. 00:45:14.200 --> 00:45:16.870 align:middle line:84% So it's suggested that if one could ever 00:45:16.870 --> 00:45:19.150 align:middle line:84% get to really super crazy high energies, 00:45:19.150 --> 00:45:21.130 align:middle line:84% you might see some very qualitatively 00:45:21.130 --> 00:45:23.440 align:middle line:84% different behavior among nuclear particles-- 00:45:23.440 --> 00:45:26.680 align:middle line:84% really cool, a very interesting set of ideas. 00:45:26.680 --> 00:45:29.650 align:middle line:84% Right on the heels of that, there 00:45:29.650 --> 00:45:31.570 align:middle line:84% was the introduction of what were called GUTs, 00:45:31.570 --> 00:45:33.880 align:middle line:90% Grand Unified theories. 00:45:33.880 --> 00:45:36.250 align:middle line:84% My friend Alan Guth likes to call these 00:45:36.250 --> 00:45:37.960 align:middle line:84% actually GUTHs because theory should 00:45:37.960 --> 00:45:40.060 align:middle line:84% be spelled TH, not-- coincidentally that's 00:45:40.060 --> 00:45:41.350 align:middle line:90% his last name. 00:45:41.350 --> 00:45:43.920 align:middle line:90% Anyway, we won't give him that. 00:45:43.920 --> 00:45:44.920 align:middle line:90% He's not here right now. 00:45:44.920 --> 00:45:45.850 align:middle line:90% We'll call them GUTs. 00:45:45.850 --> 00:45:49.030 align:middle line:84% So grand unified theories sprung from this idea 00:45:49.030 --> 00:45:50.440 align:middle line:90% from asymptotic freedom. 00:45:50.440 --> 00:45:52.720 align:middle line:84% Going back to this chart here, if you 00:45:52.720 --> 00:45:55.000 align:middle line:84% look at the average strength of the three 00:45:55.000 --> 00:45:58.180 align:middle line:84% main forces of nature, setting gravity aside, 00:45:58.180 --> 00:46:02.170 align:middle line:84% this first one is basically the strength of electromagnetism, 00:46:02.170 --> 00:46:05.920 align:middle line:84% the kind of QED force, photon scattering off electrons. 00:46:05.920 --> 00:46:09.520 align:middle line:84% This one is basically the weak nuclear force, 00:46:09.520 --> 00:46:12.970 align:middle line:84% radioactive decay, the W Boson stuff. 00:46:12.970 --> 00:46:15.040 align:middle line:84% And then this is the quark gluon force. 00:46:15.040 --> 00:46:18.250 align:middle line:84% The force strengths look like they might converge. 00:46:18.250 --> 00:46:21.078 align:middle line:84% It's not just that these two get stronger with high energy, 00:46:21.078 --> 00:46:22.120 align:middle line:90% and this one gets weaker. 00:46:22.120 --> 00:46:24.790 align:middle line:84% They actually might overlap at a single value. 00:46:24.790 --> 00:46:27.940 align:middle line:84% If you squint and take these error bars somewhat generously, 00:46:27.940 --> 00:46:31.720 align:middle line:84% it looks like that at around 10 to the 16, give or take, 00:46:31.720 --> 00:46:35.320 align:middle line:84% roughly 10 to the 16 billion electron volts, 00:46:35.320 --> 00:46:38.740 align:middle line:84% if you could scatter particles with that average energy, maybe 00:46:38.740 --> 00:46:42.040 align:middle line:84% all these three separate forces would have the same unit 00:46:42.040 --> 00:46:42.850 align:middle line:90% strength. 00:46:42.850 --> 00:46:44.350 align:middle line:84% If they have the same unit strength, 00:46:44.350 --> 00:46:46.250 align:middle line:90% maybe they're the same force. 00:46:46.250 --> 00:46:49.240 align:middle line:84% So the idea was that maybe all of these highly symmetry 00:46:49.240 --> 00:46:53.125 align:middle line:84% mediated forces that we see as very different at these low 00:46:53.125 --> 00:46:55.000 align:middle line:84% energies-- they have very different behaviors 00:46:55.000 --> 00:46:57.640 align:middle line:90% and characteristic strengths-- 00:46:57.640 --> 00:47:01.180 align:middle line:84% maybe they're actually all signs of a single force-- 00:47:01.180 --> 00:47:04.960 align:middle line:84% the grand unified theory, which would unite these three 00:47:04.960 --> 00:47:06.790 align:middle line:84% forces of nature into a single one 00:47:06.790 --> 00:47:10.090 align:middle line:84% modulated by a single force strength, 00:47:10.090 --> 00:47:12.610 align:middle line:90% a single effective charge. 00:47:12.610 --> 00:47:13.485 align:middle line:90% That was called GUTs. 00:47:13.485 --> 00:47:15.027 align:middle line:84% And you can see that only makes sense 00:47:15.027 --> 00:47:16.450 align:middle line:84% in the light of asymptotic freedom 00:47:16.450 --> 00:47:18.910 align:middle line:84% to get this strength to come down so that it 00:47:18.910 --> 00:47:23.110 align:middle line:84% can meet the very gently rising strengths of the two forces. 00:47:23.110 --> 00:47:24.400 align:middle line:90% Those are awesome ideas. 00:47:24.400 --> 00:47:25.960 align:middle line:90% They're very cool. 00:47:25.960 --> 00:47:29.290 align:middle line:84% You can probably get a sense for where this is heading. 00:47:29.290 --> 00:47:31.870 align:middle line:84% It starts to become a natural question 00:47:31.870 --> 00:47:35.050 align:middle line:84% to ask about conditions when particles could have interacted 00:47:35.050 --> 00:47:38.710 align:middle line:84% with literally astronomically high energies, cosmologically 00:47:38.710 --> 00:47:39.610 align:middle line:90% high energies. 00:47:39.610 --> 00:47:42.580 align:middle line:84% In particular-- and we'll talk more about this in the next 00:47:42.580 --> 00:47:44.080 align:middle line:90% class session-- 00:47:44.080 --> 00:47:47.170 align:middle line:84% if one takes the Big Bang model seriously, 00:47:47.170 --> 00:47:48.940 align:middle line:84% cosmologically, if the whole universe 00:47:48.940 --> 00:47:53.620 align:middle line:84% began in a very, very hot, dense state, an unbelievably 00:47:53.620 --> 00:47:56.890 align:middle line:84% high-energy state, then at very early moments 00:47:56.890 --> 00:47:59.590 align:middle line:84% in cosmic history, the average energy 00:47:59.590 --> 00:48:02.770 align:middle line:84% of anything in equilibrium would have been really, really high, 00:48:02.770 --> 00:48:05.230 align:middle line:84% that you could have maybe gotten to a time 00:48:05.230 --> 00:48:08.748 align:middle line:84% in early cosmic history when the average energy of, say, 00:48:08.748 --> 00:48:11.290 align:middle line:84% quarks scattering off each other would have been more like 10 00:48:11.290 --> 00:48:16.170 align:middle line:84% to the 16 GeV, rather than 10 to the 3 GeV. 00:48:16.170 --> 00:48:18.600 align:middle line:84% So this became a natural reason-- 00:48:18.600 --> 00:48:20.130 align:middle line:90% this is the main argument-- 00:48:20.130 --> 00:48:22.320 align:middle line:84% for particle theorists to ask about things 00:48:22.320 --> 00:48:25.170 align:middle line:84% like a very high-energy cosmology, very 00:48:25.170 --> 00:48:26.430 align:middle line:90% high-energy early universe. 00:48:26.430 --> 00:48:27.870 align:middle line:84% And the phrase that was often used 00:48:27.870 --> 00:48:30.690 align:middle line:84% at the time, a gendered term, was a cosmology 00:48:30.690 --> 00:48:33.150 align:middle line:84% would provide the so-called poor man's accelerator, 00:48:33.150 --> 00:48:35.250 align:middle line:84% the poor person's accelerator, that it's 00:48:35.250 --> 00:48:39.570 align:middle line:84% really expensive to build Slack, and Fermilab, and the LHC. 00:48:39.570 --> 00:48:43.650 align:middle line:84% And yet, all they're doing is probing down here 00:48:43.650 --> 00:48:45.970 align:middle line:84% based on technological limitations. 00:48:45.970 --> 00:48:49.290 align:middle line:84% So instead of spending another billion plus dollars, 00:48:49.290 --> 00:48:50.400 align:middle line:90% turn to cosmology. 00:48:50.400 --> 00:48:52.650 align:middle line:84% Let's use astrophysics and cosmology 00:48:52.650 --> 00:48:56.380 align:middle line:84% to probe this energy regime of interest. 00:48:56.380 --> 00:48:59.670 align:middle line:84% And so this becomes the main reason that's usually given, 00:48:59.670 --> 00:49:03.870 align:middle line:84% the main kind of cause for why these two previously quite 00:49:03.870 --> 00:49:06.600 align:middle line:84% separated fields of study-- gravitation and particle 00:49:06.600 --> 00:49:07.110 align:middle line:90% theory-- 00:49:07.110 --> 00:49:10.470 align:middle line:84% were somehow merged into this new field of particle cosmology 00:49:10.470 --> 00:49:13.380 align:middle line:90% as an ideas-driven hybrid. 00:49:13.380 --> 00:49:15.330 align:middle line:90% And that has a lot going for it. 00:49:15.330 --> 00:49:17.260 align:middle line:90% It's not a dumb explanation. 00:49:17.260 --> 00:49:19.720 align:middle line:84% I just think it's a radically incomplete one. 00:49:19.720 --> 00:49:23.080 align:middle line:84% OK, so this is a rhetorical question. 00:49:23.080 --> 00:49:24.040 align:middle line:90% Is it the whole story? 00:49:24.040 --> 00:49:27.190 align:middle line:84% Of course, you know by now I think it's not the whole story. 00:49:27.190 --> 00:49:29.180 align:middle line:84% It's a large part of ingredients, 00:49:29.180 --> 00:49:30.370 align:middle line:90% but it doesn't quite add up. 00:49:30.370 --> 00:49:35.920 align:middle line:84% If we go back to things that we can investigate empirically, 00:49:35.920 --> 00:49:38.117 align:middle line:84% it doesn't quite make sense of the time scales 00:49:38.117 --> 00:49:39.700 align:middle line:84% or some of the other phenomena that we 00:49:39.700 --> 00:49:44.390 align:middle line:84% can try to ask about physics work at that time. 00:49:44.390 --> 00:49:47.590 align:middle line:84% So here, I'm plotting just the publications on cosmology 00:49:47.590 --> 00:49:50.890 align:middle line:84% worldwide, as indexed by these kind of worldwide physics 00:49:50.890 --> 00:49:53.120 align:middle line:84% literature searches, physics abstracts, 00:49:53.120 --> 00:49:54.400 align:middle line:90% and that kind of thing. 00:49:54.400 --> 00:49:56.590 align:middle line:84% You can see that there was a very steep rise, 00:49:56.590 --> 00:50:01.840 align:middle line:84% a decided shift, a inflection in the publication rate well 00:50:01.840 --> 00:50:05.025 align:middle line:84% before 1973, well before asymptotic freedom, 00:50:05.025 --> 00:50:07.150 align:middle line:84% and GUTs, and this so-called reason, 00:50:07.150 --> 00:50:08.800 align:middle line:84% this whole theory-driven argument 00:50:08.800 --> 00:50:11.530 align:middle line:84% to ask about the particle physics of the early universe. 00:50:11.530 --> 00:50:15.130 align:middle line:84% And in fact, the rate goes from roughly six or six 00:50:15.130 --> 00:50:18.320 align:middle line:84% and one half papers per year worldwide to around 21 00:50:18.320 --> 00:50:18.820 align:middle line:90% on average. 00:50:18.820 --> 00:50:22.030 align:middle line:84% And not only that, you can see that the inflection point 00:50:22.030 --> 00:50:24.280 align:middle line:84% really comes quite a few years earlier 00:50:24.280 --> 00:50:28.520 align:middle line:84% than this purported cause from asymptotic freedom. 00:50:28.520 --> 00:50:30.970 align:middle line:84% And GUTs-- I think even more important 00:50:30.970 --> 00:50:35.240 align:middle line:84% is that GUTs became really, really hot years later. 00:50:35.240 --> 00:50:37.210 align:middle line:84% So many particle theorists got very 00:50:37.210 --> 00:50:39.670 align:middle line:84% excited about that notion of a Grand Unified Theory, 00:50:39.670 --> 00:50:43.570 align:middle line:84% but not in 1974, more like in 1980. 00:50:43.570 --> 00:50:46.540 align:middle line:84% So if we want to account for changes in 1974-- 00:50:46.540 --> 00:50:49.120 align:middle line:84% and even the particle theorists weren't paying much attention 00:50:49.120 --> 00:50:50.560 align:middle line:90% till a half a decade later. 00:50:50.560 --> 00:50:52.900 align:middle line:84% Again, the timing doesn't quite make sense. 00:50:52.900 --> 00:50:55.490 align:middle line:84% The ideas were published in 1974. 00:50:55.490 --> 00:50:56.230 align:middle line:90% That's true. 00:50:56.230 --> 00:50:58.490 align:middle line:84% But as we've seen a few times in this course, 00:50:58.490 --> 00:51:00.113 align:middle line:84% just publishing an article doesn't 00:51:00.113 --> 00:51:02.530 align:middle line:84% guarantee that people will pay attention to it right away. 00:51:02.530 --> 00:51:04.660 align:middle line:84% And that was certainly the case with that work 00:51:04.660 --> 00:51:05.930 align:middle line:90% in particle theory. 00:51:05.930 --> 00:51:09.910 align:middle line:84% So even though it offered a very interesting reason 00:51:09.910 --> 00:51:12.500 align:middle line:84% to ask new questions, empirically speaking, 00:51:12.500 --> 00:51:15.310 align:middle line:84% most people weren't asking those questions at that time. 00:51:15.310 --> 00:51:18.000 align:middle line:84% That certainly doesn't seem to be the main driver. 00:51:18.000 --> 00:51:19.300 align:middle line:90% So what else was going on? 00:51:19.300 --> 00:51:21.170 align:middle line:84% And we've seen this plot a number of times. 00:51:21.170 --> 00:51:24.450 align:middle line:84% This is now the familiar plot of a number of PhDs 00:51:24.450 --> 00:51:27.210 align:middle line:84% in physics granted in the US over time. 00:51:27.210 --> 00:51:29.580 align:middle line:84% We spent quite a while looking at both the reason why 00:51:29.580 --> 00:51:32.940 align:middle line:84% it grew so rapidly, both after the Second World War 00:51:32.940 --> 00:51:35.940 align:middle line:84% and after the launch of Sputnik, but also this really quite 00:51:35.940 --> 00:51:37.530 align:middle line:90% precipitous crash. 00:51:37.530 --> 00:51:40.710 align:middle line:84% And a whole jumble of reasons combine, Some. 00:51:40.710 --> 00:51:45.240 align:middle line:84% Of them economic, a bunch of them more about policy 00:51:45.240 --> 00:51:49.260 align:middle line:84% shifts in geopolitics even that really combined-- 00:51:49.260 --> 00:51:52.980 align:middle line:84% that lined up in time that was a perfect storm. 00:51:52.980 --> 00:51:56.070 align:middle line:84% The result of which was a very, very sudden collapse 00:51:56.070 --> 00:51:59.790 align:middle line:84% in the funding and job prospects and enrollments 00:51:59.790 --> 00:52:02.280 align:middle line:84% for physics, which fell, as we've seen, 00:52:02.280 --> 00:52:05.940 align:middle line:84% a few times faster than any other field in the Academy. 00:52:05.940 --> 00:52:09.990 align:middle line:84% And so physicists, starting the late '60s 00:52:09.990 --> 00:52:12.750 align:middle line:84% and really accelerating in the early '70s, 00:52:12.750 --> 00:52:17.430 align:middle line:84% saw a really dramatic change in the kind of infrastructure 00:52:17.430 --> 00:52:18.940 align:middle line:90% for the discipline. 00:52:18.940 --> 00:52:21.200 align:middle line:84% It turns out, again, we can ask a bit more-- 00:52:21.200 --> 00:52:22.390 align:middle line:90% that's very coarse-grained. 00:52:22.390 --> 00:52:24.460 align:middle line:84% That's looking at the whole field at once. 00:52:24.460 --> 00:52:28.810 align:middle line:84% We can dig in a bit more and look at subfield by subfield. 00:52:28.810 --> 00:52:31.900 align:middle line:84% And by these kinds of measures, the single hardest hit 00:52:31.900 --> 00:52:34.330 align:middle line:84% subfield, the specialty within physics 00:52:34.330 --> 00:52:37.630 align:middle line:84% that was affected most dramatically even as all fields 00:52:37.630 --> 00:52:39.910 align:middle line:90% felt it, was particle physics. 00:52:39.910 --> 00:52:42.460 align:middle line:84% The field that had, so to speak, the most to lose 00:52:42.460 --> 00:52:45.040 align:middle line:84% or that lost the most during this reversal of fortune 00:52:45.040 --> 00:52:47.560 align:middle line:84% was actually high-energy particle physics. 00:52:47.560 --> 00:52:50.830 align:middle line:84% The US budget for that subfield fell in half 00:52:50.830 --> 00:52:52.480 align:middle line:90% in just four years. 00:52:52.480 --> 00:52:54.550 align:middle line:90% It wasn't a 20% reduction. 00:52:54.550 --> 00:52:57.400 align:middle line:84% It literally fell by 50% in four years. 00:52:57.400 --> 00:52:59.380 align:middle line:90% That's a very sudden drop. 00:52:59.380 --> 00:53:02.290 align:middle line:84% And that was combined with the drop in the job market 00:53:02.290 --> 00:53:03.440 align:middle line:90% demand and all the rest. 00:53:03.440 --> 00:53:04.998 align:middle line:84% So even as the whole field starts 00:53:04.998 --> 00:53:06.790 align:middle line:84% going through some pretty dramatic changes, 00:53:06.790 --> 00:53:12.460 align:middle line:84% particle physics feels the brunt of it in the most extreme way. 00:53:12.460 --> 00:53:16.570 align:middle line:84% So that leads to an interesting set of internal migrations 00:53:16.570 --> 00:53:18.405 align:middle line:90% within the discipline. 00:53:18.405 --> 00:53:21.140 align:middle line:90% 00:53:21.140 --> 00:53:25.250 align:middle line:84% This plot comes from a series of studies 00:53:25.250 --> 00:53:27.680 align:middle line:84% that were produced very soon after the crash 00:53:27.680 --> 00:53:29.813 align:middle line:90% by the physics community. 00:53:29.813 --> 00:53:31.730 align:middle line:84% This one, I think, was from a National Academy 00:53:31.730 --> 00:53:33.390 align:middle line:90% of Sciences study. 00:53:33.390 --> 00:53:37.580 align:middle line:84% And so what you see is this is now 00:53:37.580 --> 00:53:40.880 align:middle line:84% looking only at the years between 1968 and 1970. 00:53:40.880 --> 00:53:43.820 align:middle line:84% The plot was made around 1972 or '73 00:53:43.820 --> 00:53:48.200 align:middle line:84% and is charting inflows and outflows among the recognized 00:53:48.200 --> 00:53:51.680 align:middle line:84% subfields of the discipline during the time when, 00:53:51.680 --> 00:53:53.090 align:middle line:90% really, the bottom falls out. 00:53:53.090 --> 00:53:54.740 align:middle line:84% So when particle physics is getting 00:53:54.740 --> 00:53:56.660 align:middle line:84% hit harder than any other field, you 00:53:56.660 --> 00:53:59.000 align:middle line:84% have twice as many people leaving that field 00:53:59.000 --> 00:54:00.110 align:middle line:90% than joining it. 00:54:00.110 --> 00:54:03.830 align:middle line:84% And that's where the people who stay within physics at all 00:54:03.830 --> 00:54:07.730 align:middle line:84% that you have a net outflow by a factor of 2 00:54:07.730 --> 00:54:10.873 align:middle line:84% just within the field of people fleeing particle physics, 00:54:10.873 --> 00:54:13.040 align:middle line:84% partly because its budget was cut most dramatically. 00:54:13.040 --> 00:54:14.900 align:middle line:84% And the job scene was most hard hit, 00:54:14.900 --> 00:54:17.990 align:middle line:84% and experiments looked like they'd be on hold and so on. 00:54:17.990 --> 00:54:20.000 align:middle line:84% And so in some of these reports that 00:54:20.000 --> 00:54:22.320 align:middle line:84% tried to make sense of the crash, 00:54:22.320 --> 00:54:25.530 align:middle line:84% like this 1972 report commissioned by the National 00:54:25.530 --> 00:54:26.760 align:middle line:90% Academy of Sciences. 00:54:26.760 --> 00:54:30.870 align:middle line:84% It was actually a huge three or four-volume thick government 00:54:30.870 --> 00:54:31.380 align:middle line:90% report-- 00:54:31.380 --> 00:54:37.020 align:middle line:84% 2,500 pages full of tables and charts, lots of fun 00:54:37.020 --> 00:54:38.730 align:middle line:90% stuff to geek out on. 00:54:38.730 --> 00:54:42.182 align:middle line:84% And yet, this blue-ribbon panel with several leading MIT 00:54:42.182 --> 00:54:43.890 align:middle line:84% colleagues who served on it, plus members 00:54:43.890 --> 00:54:45.630 align:middle line:84% from all across the country, were 00:54:45.630 --> 00:54:48.600 align:middle line:84% trying to survey the US-based physics profession and what 00:54:48.600 --> 00:54:50.940 align:middle line:84% had gone wrong during this period of very 00:54:50.940 --> 00:54:52.680 align:middle line:90% rapid transition. 00:54:52.680 --> 00:54:57.390 align:middle line:84% And they single out particle theory, in particular, 00:54:57.390 --> 00:55:00.330 align:middle line:84% for special kind of concern or critique. 00:55:00.330 --> 00:55:03.660 align:middle line:84% They say it's not coincidental that 00:55:03.660 --> 00:55:06.840 align:middle line:84% these young theoretical physicists in particle theory 00:55:06.840 --> 00:55:09.780 align:middle line:84% had the hardest time when the trouble came 00:55:09.780 --> 00:55:12.120 align:middle line:84% because this report claimed, at least, 00:55:12.120 --> 00:55:13.830 align:middle line:90% they'd been poorly trained. 00:55:13.830 --> 00:55:16.170 align:middle line:84% It's not the young student's fault, per se. 00:55:16.170 --> 00:55:19.980 align:middle line:84% It's because the programs that were producing 00:55:19.980 --> 00:55:23.750 align:middle line:84% many, many, many highly trained particle theorists 00:55:23.750 --> 00:55:25.880 align:middle line:84% were training them much too narrowly. 00:55:25.880 --> 00:55:28.070 align:middle line:84% They were too narrowly specialized 00:55:28.070 --> 00:55:30.440 align:middle line:84% only in the esoteric of these symmetry 00:55:30.440 --> 00:55:32.510 align:middle line:90% arguments about nuclear forces. 00:55:32.510 --> 00:55:35.870 align:middle line:84% They weren't being exposed to or responsible 00:55:35.870 --> 00:55:38.340 align:middle line:84% to the many, many other fields of physics. 00:55:38.340 --> 00:55:41.840 align:middle line:84% And so when trouble came, they were least adaptable. 00:55:41.840 --> 00:55:44.160 align:middle line:84% Now, I don't know if that's really the case or not, 00:55:44.160 --> 00:55:46.310 align:middle line:84% but I think it's quite telling that was 00:55:46.310 --> 00:55:48.830 align:middle line:84% the kind of explanation that this blue-ribbon panel 00:55:48.830 --> 00:55:51.680 align:middle line:84% of leading educators, including many leading particle 00:55:51.680 --> 00:55:57.740 align:middle line:84% physicists themselves-- that was their explanation for why 00:55:57.740 --> 00:56:01.310 align:middle line:84% that one subfield fared so especially poorly, even 00:56:01.310 --> 00:56:04.280 align:middle line:90% as the whole field had trouble. 00:56:04.280 --> 00:56:05.720 align:middle line:84% And so they make recommendations. 00:56:05.720 --> 00:56:07.700 align:middle line:84% Their job, after all, is to say, how do you 00:56:07.700 --> 00:56:08.790 align:middle line:90% avoid this in the future? 00:56:08.790 --> 00:56:12.350 align:middle line:84% It was a very official high-profile blue-ribbon 00:56:12.350 --> 00:56:13.010 align:middle line:90% committee. 00:56:13.010 --> 00:56:15.560 align:middle line:84% So they make all these recommendations, some of which 00:56:15.560 --> 00:56:18.050 align:middle line:84% actually get taken up pretty quickly by departments 00:56:18.050 --> 00:56:19.260 align:middle line:90% across the country. 00:56:19.260 --> 00:56:20.810 align:middle line:84% And one of the recommendations is 00:56:20.810 --> 00:56:25.310 align:middle line:84% to forcibly broaden the training of young physicists in particle 00:56:25.310 --> 00:56:25.893 align:middle line:90% theory. 00:56:25.893 --> 00:56:28.310 align:middle line:84% They have recommendations for other fields too, of course. 00:56:28.310 --> 00:56:31.100 align:middle line:84% But partly because they singled out particle theory, they say, 00:56:31.100 --> 00:56:34.190 align:middle line:84% we have to start changing how we train PhD students 00:56:34.190 --> 00:56:36.200 align:middle line:90% in this particular subfield. 00:56:36.200 --> 00:56:39.740 align:middle line:84% And so the idea is to actually formalize their exposure 00:56:39.740 --> 00:56:41.790 align:middle line:84% to more and more parts of the discipline, 00:56:41.790 --> 00:56:46.910 align:middle line:84% including explicitly more focus on gravitation and cosmology. 00:56:46.910 --> 00:56:49.400 align:middle line:84% So that means that more and more departments, including 00:56:49.400 --> 00:56:53.300 align:middle line:84% very elite trend-setting departments across the country, 00:56:53.300 --> 00:56:55.127 align:middle line:84% start rushing to offer new graduate 00:56:55.127 --> 00:56:56.960 align:middle line:84% courses in general relativity, many of which 00:56:56.960 --> 00:56:59.390 align:middle line:84% had offered zero courses in the field until then. 00:56:59.390 --> 00:57:01.340 align:middle line:90% Or it was only an elective. 00:57:01.340 --> 00:57:04.760 align:middle line:84% Now became a requirement or was only offered in astronomy. 00:57:04.760 --> 00:57:06.340 align:middle line:84% And that was also offered in physics. 00:57:06.340 --> 00:57:08.715 align:middle line:84% They're rushing to get more and more coursework on things 00:57:08.715 --> 00:57:10.500 align:middle line:90% like gravitation and cosmology. 00:57:10.500 --> 00:57:13.590 align:middle line:84% And likewise, questions on that field, 00:57:13.590 --> 00:57:15.990 align:middle line:84% really for the first time in the United States, 00:57:15.990 --> 00:57:19.050 align:middle line:84% start showing up regularly on the general exams 00:57:19.050 --> 00:57:21.510 align:middle line:84% for physicists across all fields, 00:57:21.510 --> 00:57:25.740 align:middle line:84% all specialties, not just those who wanted to study relativity, 00:57:25.740 --> 00:57:27.660 align:middle line:90% and gravitation, and cosmology. 00:57:27.660 --> 00:57:29.820 align:middle line:84% So now you have more and more students 00:57:29.820 --> 00:57:33.240 align:middle line:84% in their PhDs responsible to have learned something 00:57:33.240 --> 00:57:35.700 align:middle line:84% in neighboring subfields, whereas before then, 00:57:35.700 --> 00:57:38.730 align:middle line:84% the general exams had not emphasized or required that. 00:57:38.730 --> 00:57:41.220 align:middle line:84% And you see a market response as well. 00:57:41.220 --> 00:57:44.430 align:middle line:84% You see a flood of new graduate-level textbooks 00:57:44.430 --> 00:57:48.000 align:middle line:84% on general relativity on gravitation and cosmology-- 00:57:48.000 --> 00:57:51.570 align:middle line:84% twice as many published in the 1970s versus the 1960s. 00:57:51.570 --> 00:57:53.940 align:middle line:84% And in fact, even of those 1970s books, 00:57:53.940 --> 00:57:57.390 align:middle line:84% the vast majority of those came really in the later '70s, 00:57:57.390 --> 00:58:00.660 align:middle line:84% in the wake of these pedagogical reforms. 00:58:00.660 --> 00:58:03.780 align:middle line:84% So remember that big report comes out in 1972. 00:58:03.780 --> 00:58:07.260 align:middle line:84% You start seeing curricular changes as early as '73, '74. 00:58:07.260 --> 00:58:10.470 align:middle line:84% And by '75, '76, '77, you start seeing, in some sense, 00:58:10.470 --> 00:58:14.650 align:middle line:84% the market respond with many textbooks being really rushed 00:58:14.650 --> 00:58:15.150 align:middle line:90% into print. 00:58:15.150 --> 00:58:16.980 align:middle line:84% Some of these textbooks were basically 00:58:16.980 --> 00:58:18.300 align:middle line:90% mimeographed lecture notes. 00:58:18.300 --> 00:58:21.450 align:middle line:84% It was seen as such a rush, such a demand 00:58:21.450 --> 00:58:24.480 align:middle line:84% to get new pedagogical materials because more and more places 00:58:24.480 --> 00:58:27.330 align:middle line:84% wanted to offer courses on this where they hadn't before, 00:58:27.330 --> 00:58:31.560 align:middle line:84% that you had some informal kind of lecture note transmutes 00:58:31.560 --> 00:58:33.240 align:middle line:84% the transmutations into textbooks. 00:58:33.240 --> 00:58:35.730 align:middle line:84% And now there's very, very fancy books published 00:58:35.730 --> 00:58:36.990 align:middle line:90% in a more typical way. 00:58:36.990 --> 00:58:39.450 align:middle line:84% You see really a rush to get more books 00:58:39.450 --> 00:58:42.340 align:middle line:90% on that subfield in particular. 00:58:42.340 --> 00:58:44.173 align:middle line:90% So let me pause there. 00:58:44.173 --> 00:58:45.090 align:middle line:90% Any questions on that? 00:58:45.090 --> 00:58:48.750 align:middle line:84% I see, again, some things in the chat. 00:58:48.750 --> 00:58:51.140 align:middle line:84% Yeah, so Alex says, quite rightly, 00:58:51.140 --> 00:58:55.010 align:middle line:84% if we can convince Alan Guth to clean his office, 00:58:55.010 --> 00:58:57.210 align:middle line:84% then we'll work on nomenclature change. 00:58:57.210 --> 00:59:00.678 align:middle line:84% Some of you might know that Alan-- 00:59:00.678 --> 00:59:01.970 align:middle line:90% I can't talk about Alan enough. 00:59:01.970 --> 00:59:04.160 align:middle line:84% He was my PhD advisor, so I've been 00:59:04.160 --> 00:59:08.120 align:middle line:84% teasing Alan for more than half of my life, 00:59:08.120 --> 00:59:09.800 align:middle line:90% to his great chagrin. 00:59:09.800 --> 00:59:13.040 align:middle line:84% So you might know that some years ago, 00:59:13.040 --> 00:59:15.170 align:middle line:84% before there was a construction to build 00:59:15.170 --> 00:59:17.630 align:middle line:84% a new Center for Theoretical Physics, 00:59:17.630 --> 00:59:19.700 align:middle line:84% we all had to move out of our old offices. 00:59:19.700 --> 00:59:22.940 align:middle line:84% And it was around that time that The Boston Globe ran a contest 00:59:22.940 --> 00:59:26.900 align:middle line:84% for the entire Boston metro area for the area's messiest 00:59:26.900 --> 00:59:27.920 align:middle line:90% office-- 00:59:27.920 --> 00:59:31.160 align:middle line:84% not messiest academic's office-- the messiest office 00:59:31.160 --> 00:59:32.720 align:middle line:90% in the Boston metro area. 00:59:32.720 --> 00:59:34.200 align:middle line:90% And I give it away. 00:59:34.200 --> 00:59:35.360 align:middle line:90% Guess who won. 00:59:35.360 --> 00:59:37.190 align:middle line:84% Single-- I mean, he's won a number 00:59:37.190 --> 00:59:41.360 align:middle line:84% of awards, gold prizes and that big award-winning fella. 00:59:41.360 --> 00:59:43.910 align:middle line:84% But the award that I take most pleasure in 00:59:43.910 --> 00:59:46.040 align:middle line:84% is the fact that he won Boston's Messiest Office. 00:59:46.040 --> 00:59:48.870 align:middle line:90% So yes, we'll work on that. 00:59:48.870 --> 00:59:49.370 align:middle line:90% Let's see. 00:59:49.370 --> 00:59:51.078 align:middle line:84% Fisher says, on the chart of interactions 00:59:51.078 --> 00:59:52.870 align:middle line:84% with the field strengths, are those groups? 00:59:52.870 --> 00:59:53.390 align:middle line:90% Yes. 00:59:53.390 --> 00:59:53.930 align:middle line:90% Fisher, thank you. 00:59:53.930 --> 00:59:54.472 align:middle line:90% That's right. 00:59:54.472 --> 00:59:58.310 align:middle line:84% So I was avoiding the nomenclature, 00:59:58.310 --> 00:59:59.330 align:middle line:90% but you're quite right. 00:59:59.330 --> 01:00:03.470 align:middle line:84% On that chart that's associated with asymptotic freedom, what 01:00:03.470 --> 01:00:06.830 align:middle line:84% I was referring to as the strength of electromagnetism-- 01:00:06.830 --> 01:00:08.900 align:middle line:90% it was labeled on the chart U1. 01:00:08.900 --> 01:00:10.910 align:middle line:84% And that is the fancy way of labeling 01:00:10.910 --> 01:00:15.920 align:middle line:84% the symmetry group that is associated 01:00:15.920 --> 01:00:16.910 align:middle line:90% with electromagnetism. 01:00:16.910 --> 01:00:19.490 align:middle line:84% It's a continuous unitary symmetry, 01:00:19.490 --> 01:00:22.700 align:middle line:84% which is like saying you could rotate the electron 01:00:22.700 --> 01:00:28.190 align:middle line:84% field by any continuum amount, and the equations 01:00:28.190 --> 01:00:29.940 align:middle line:90% remain unchanged. 01:00:29.940 --> 01:00:32.630 align:middle line:84% And so that calls for certain properties 01:00:32.630 --> 01:00:34.430 align:middle line:90% of the force-carrying field. 01:00:34.430 --> 01:00:39.560 align:middle line:84% The photon only has to mop up a relatively simple symmetry, 01:00:39.560 --> 01:00:41.240 align:middle line:90% the U1 gauge symmetry. 01:00:41.240 --> 01:00:43.670 align:middle line:84% Whereas SU2 was what I was pointing 01:00:43.670 --> 01:00:47.132 align:middle line:84% to when I was referring to the weak nuclear force. 01:00:47.132 --> 01:00:47.840 align:middle line:90% And you're right. 01:00:47.840 --> 01:00:49.500 align:middle line:90% That's a discrete symmetry. 01:00:49.500 --> 01:00:53.670 align:middle line:84% It's a more complicated symmetry structure. 01:00:53.670 --> 01:00:57.480 align:middle line:84% And that's the symmetry group that these force-carrying 01:00:57.480 --> 01:00:59.250 align:middle line:84% particles-- the W and the Z particles-- 01:00:59.250 --> 01:01:01.620 align:middle line:90% are invented to enforce. 01:01:01.620 --> 01:01:03.480 align:middle line:84% So that has different implications 01:01:03.480 --> 01:01:05.440 align:middle line:84% for their properties, like we saw last time. 01:01:05.440 --> 01:01:09.630 align:middle line:84% And then SU3, that refers to three different color charges 01:01:09.630 --> 01:01:11.010 align:middle line:90% of quantum chromodynamics. 01:01:11.010 --> 01:01:13.758 align:middle line:84% You have a discrete three-way permutation. 01:01:13.758 --> 01:01:16.050 align:middle line:84% And so therefore, the gluons have still a different set 01:01:16.050 --> 01:01:17.342 align:middle line:90% of properties, so that's right. 01:01:17.342 --> 01:01:21.330 align:middle line:84% So that chart was labeling the symmetry groups 01:01:21.330 --> 01:01:23.790 align:middle line:84% associated with the different kinds of forces. 01:01:23.790 --> 01:01:27.510 align:middle line:84% And so what's really at the heart of GUTs, or GUTHs-- 01:01:27.510 --> 01:01:30.120 align:middle line:84% I'll throw him a bone-- is that maybe you can play that game 01:01:30.120 --> 01:01:33.780 align:middle line:84% one more time and say, each of those three symmetry groups are 01:01:33.780 --> 01:01:35.010 align:middle line:90% actually sub-- 01:01:35.010 --> 01:01:35.970 align:middle line:90% you think about it. 01:01:35.970 --> 01:01:38.010 align:middle line:84% You can represent any of them as matrices. 01:01:38.010 --> 01:01:43.080 align:middle line:84% Is there one single larger set of matrices 01:01:43.080 --> 01:01:47.940 align:middle line:84% that would include the U1, SU2, and SU3 as submatrices? 01:01:47.940 --> 01:01:50.780 align:middle line:84% And there is the smallest group that 01:01:50.780 --> 01:01:53.690 align:middle line:84% includes those three as subgroups is an SU5. 01:01:53.690 --> 01:01:55.700 align:middle line:84% So what if there were five-fold symmetry 01:01:55.700 --> 01:01:59.060 align:middle line:84% with still different kinds of requirements on its force 01:01:59.060 --> 01:02:00.210 align:middle line:90% carrying particles? 01:02:00.210 --> 01:02:04.640 align:middle line:84% So the first GUT was actually an SU5 model put forward 01:02:04.640 --> 01:02:08.210 align:middle line:84% by a then very young Howard Georgi and Sheldon Glashow. 01:02:08.210 --> 01:02:10.460 align:middle line:84% They were very young theorists at Harvard at the time. 01:02:10.460 --> 01:02:12.050 align:middle line:84% And it was to do exactly that, to try 01:02:12.050 --> 01:02:13.580 align:middle line:84% to build on that symmetry structure 01:02:13.580 --> 01:02:15.890 align:middle line:84% and find the next simplest, most minimalist 01:02:15.890 --> 01:02:18.770 align:middle line:84% symmetry structure-- basically a matrix representation-- 01:02:18.770 --> 01:02:21.380 align:middle line:84% into which you could fit those three known symmetry 01:02:21.380 --> 01:02:23.480 align:middle line:84% groups and just a little bit else. 01:02:23.480 --> 01:02:24.770 align:middle line:90% It's a little bit bigger. 01:02:24.770 --> 01:02:26.377 align:middle line:84% So that led to new phenomena that 01:02:26.377 --> 01:02:28.460 align:middle line:84% were predicted that turned out not to be measured, 01:02:28.460 --> 01:02:29.930 align:middle line:90% but that was the idea at least. 01:02:29.930 --> 01:02:33.560 align:middle line:84% And that maybe the photon, the W, Z, and the gluons 01:02:33.560 --> 01:02:38.480 align:middle line:84% actually are all instances of this one unified force carrier. 01:02:38.480 --> 01:02:41.660 align:middle line:84% And at very high energies, they would all be indistinguishable. 01:02:41.660 --> 01:02:44.150 align:middle line:84% But as the symmetry gets broken at lower energies, 01:02:44.150 --> 01:02:49.380 align:middle line:90% they take on different features. 01:02:49.380 --> 01:02:51.953 align:middle line:84% It's another example of spontaneous symmetry breaks. 01:02:51.953 --> 01:02:53.370 align:middle line:84% Their low-energy features would be 01:02:53.370 --> 01:02:55.380 align:middle line:84% different than their high-energy features. 01:02:55.380 --> 01:02:56.670 align:middle line:90% That's right. 01:02:56.670 --> 01:02:58.763 align:middle line:84% And that's super cool and fun and lots more 01:02:58.763 --> 01:02:59.680 align:middle line:90% to be said about that. 01:02:59.680 --> 01:03:01.770 align:middle line:84% But that is, indeed, where that nomenclature came from. 01:03:01.770 --> 01:03:02.160 align:middle line:90% Great. 01:03:02.160 --> 01:03:03.327 align:middle line:90% Any other questions on that? 01:03:03.327 --> 01:03:05.880 align:middle line:90% 01:03:05.880 --> 01:03:06.900 align:middle line:90% OK. 01:03:06.900 --> 01:03:09.570 align:middle line:84% Anyone else want to share stories of Alan's messy office? 01:03:09.570 --> 01:03:10.920 align:middle line:90% No, going once? 01:03:10.920 --> 01:03:12.928 align:middle line:90% OK. 01:03:12.928 --> 01:03:14.970 align:middle line:84% Then let me press on for the last-- the last part 01:03:14.970 --> 01:03:16.410 align:middle line:84% is pretty short, so the last little part, 01:03:16.410 --> 01:03:17.520 align:middle line:84% and we'll have time for more questions 01:03:17.520 --> 01:03:18.603 align:middle line:90% and discussion after that. 01:03:18.603 --> 01:03:25.090 align:middle line:90% 01:03:25.090 --> 01:03:29.625 align:middle line:84% OK, so I mentioned very briefly that in 1979, 01:03:29.625 --> 01:03:31.000 align:middle line:84% two separate theorists-- in fact, 01:03:31.000 --> 01:03:33.070 align:middle line:84% they were working independently at the time-- 01:03:33.070 --> 01:03:35.920 align:middle line:84% Anthony Zee and Lee Smolin separately 01:03:35.920 --> 01:03:39.670 align:middle line:84% introduced a whole new model where they didn't only 01:03:39.670 --> 01:03:42.460 align:middle line:84% cite Brans-Dicke and Higgs, they didn't only 01:03:42.460 --> 01:03:44.845 align:middle line:84% say these two Greek letter phis might be similar, 01:03:44.845 --> 01:03:45.970 align:middle line:90% they literally united them. 01:03:45.970 --> 01:03:47.980 align:middle line:84% They proposed-- it was a hypothesis. 01:03:47.980 --> 01:03:50.110 align:middle line:84% It was a new model in which these two 01:03:50.110 --> 01:03:51.760 align:middle line:90% fields were literally the same. 01:03:51.760 --> 01:03:53.420 align:middle line:90% They glued them together. 01:03:53.420 --> 01:03:55.530 align:middle line:84% And I found that really compelling to say 01:03:55.530 --> 01:03:58.630 align:middle line:84% it wasn't that the fields were in principle separable. 01:03:58.630 --> 01:04:00.070 align:middle line:84% Here were some clever folks coming 01:04:00.070 --> 01:04:02.950 align:middle line:84% along almost 20 years later to say, 01:04:02.950 --> 01:04:04.300 align:middle line:90% maybe they're actually the same. 01:04:04.300 --> 01:04:07.090 align:middle line:84% Maybe there's one field phi pervading 01:04:07.090 --> 01:04:10.060 align:middle line:84% all of nature, all of space, with which 01:04:10.060 --> 01:04:12.350 align:middle line:90% other matter interacts. 01:04:12.350 --> 01:04:16.840 align:middle line:84% So again, if you look at the key parts of the Lagrangian 01:04:16.840 --> 01:04:19.030 align:middle line:90% that they put forward-- 01:04:19.030 --> 01:04:21.610 align:middle line:84% you can think of it as like the energy balance-- 01:04:21.610 --> 01:04:23.860 align:middle line:84% they combine the features from Brans-Dicke 01:04:23.860 --> 01:04:25.050 align:middle line:90% for the gravity side. 01:04:25.050 --> 01:04:28.940 align:middle line:84% So there's a direct coupling between this new hypothetical 01:04:28.940 --> 01:04:33.260 align:middle line:84% field and this geometrical structure, the local curvature 01:04:33.260 --> 01:04:34.920 align:middle line:90% of space and time. 01:04:34.920 --> 01:04:40.250 align:middle line:84% And so this is really, again, playing the role of the varying 01:04:40.250 --> 01:04:41.510 align:middle line:90% unit strength of gravity. 01:04:41.510 --> 01:04:44.150 align:middle line:84% That's in place of Newton's constant G. 01:04:44.150 --> 01:04:46.820 align:middle line:84% If that field can vary, then you better 01:04:46.820 --> 01:04:50.810 align:middle line:84% include its kinetic energy with, again, some fudge factor. 01:04:50.810 --> 01:04:52.430 align:middle line:84% That's all the Brans-Dicke stuff. 01:04:52.430 --> 01:04:55.280 align:middle line:84% And give that field its own potential energy 01:04:55.280 --> 01:04:59.720 align:middle line:84% with that very specific shape, that symmetric double-well type 01:04:59.720 --> 01:05:03.410 align:middle line:84% shape from Goldstone, Higgs, and all the rest. 01:05:03.410 --> 01:05:05.010 align:middle line:90% Why would you do that? 01:05:05.010 --> 01:05:11.060 align:middle line:84% They wanted to ask why gravity appears to be so weak compared 01:05:11.060 --> 01:05:12.450 align:middle line:90% to all the other forces. 01:05:12.450 --> 01:05:17.000 align:middle line:84% By this point, they knew about things like the strong force. 01:05:17.000 --> 01:05:18.830 align:middle line:84% Quarks and gluons interact very strongly. 01:05:18.830 --> 01:05:22.430 align:middle line:84% Even the electromagnetic force is exponentially stronger 01:05:22.430 --> 01:05:23.750 align:middle line:90% than the gravitational force. 01:05:23.750 --> 01:05:27.020 align:middle line:84% The force between an electron and a positron when they're 01:05:27.020 --> 01:05:32.090 align:middle line:84% close together is exponentially higher due to their Coulomb 01:05:32.090 --> 01:05:35.000 align:middle line:84% attraction compared to their gravitational attraction, even 01:05:35.000 --> 01:05:35.910 align:middle line:90% just classically. 01:05:35.910 --> 01:05:37.760 align:middle line:84% So why is there such a strange hierarchy? 01:05:37.760 --> 01:05:42.290 align:middle line:84% Why such a huge divide in the average strength of gravity 01:05:42.290 --> 01:05:45.600 align:middle line:84% compared to these other seemingly elementary forces? 01:05:45.600 --> 01:05:47.780 align:middle line:84% So the idea was that this local strength of gravity, 01:05:47.780 --> 01:05:53.270 align:middle line:84% Newton's constant, which goes 1 over this phi squared, 01:05:53.270 --> 01:05:57.710 align:middle line:84% would get anchored to a very small value when phi gets stuck 01:05:57.710 --> 01:05:59.490 align:middle line:90% at a relatively large value. 01:05:59.490 --> 01:06:02.898 align:middle line:84% So when this dynamical field, this field phi-- 01:06:02.898 --> 01:06:04.440 align:middle line:84% which, they also use the Greek letter 01:06:04.440 --> 01:06:06.960 align:middle line:84% phi-- which skitters around the universe, 01:06:06.960 --> 01:06:10.260 align:middle line:84% it eventually reaches some kind of lowest equilibrium state 01:06:10.260 --> 01:06:13.470 align:middle line:84% at the bottom of one of these symmetric minima 01:06:13.470 --> 01:06:15.430 align:middle line:90% of that potential. 01:06:15.430 --> 01:06:18.750 align:middle line:84% And so now it gets anchored at some large nonzero 01:06:18.750 --> 01:06:24.180 align:middle line:84% value instead of being at a local unstable equilibrium 01:06:24.180 --> 01:06:26.250 align:middle line:90% point of phi, roughly 0. 01:06:26.250 --> 01:06:29.580 align:middle line:84% So if phi becomes stuck at some large nonzero 01:06:29.580 --> 01:06:32.190 align:middle line:84% value, either plus or minus, then the square of that 01:06:32.190 --> 01:06:34.900 align:middle line:84% will be some large number, some large positive number. 01:06:34.900 --> 01:06:38.580 align:middle line:84% And so that could be setting the inverse gravitational field 01:06:38.580 --> 01:06:39.220 align:middle line:90% strength. 01:06:39.220 --> 01:06:40.950 align:middle line:90% So why is gravity so weak? 01:06:40.950 --> 01:06:43.770 align:middle line:84% They suggested maybe it's because it's arising 01:06:43.770 --> 01:06:45.780 align:middle line:90% from some broken symmetry. 01:06:45.780 --> 01:06:49.590 align:middle line:84% Much like the Higgs-Goldstone mechanism, the field 01:06:49.590 --> 01:06:51.870 align:middle line:84% is dynamical, but it's getting stuck. 01:06:51.870 --> 01:06:54.750 align:middle line:84% And only in the broken-symmetry phase 01:06:54.750 --> 01:06:58.380 align:middle line:84% do we experience a phenomena that we are used to. 01:06:58.380 --> 01:07:02.340 align:middle line:84% So gravity gets stuck being weak because its local strength is 01:07:02.340 --> 01:07:05.460 align:middle line:84% arising through the Brans-Dicke field getting 01:07:05.460 --> 01:07:07.380 align:middle line:84% accurate in a symmetry-breaking potential. 01:07:07.380 --> 01:07:08.070 align:middle line:90% It's lovely. 01:07:08.070 --> 01:07:10.840 align:middle line:90% It's a very cool idea. 01:07:10.840 --> 01:07:12.960 align:middle line:84% So how did these two individuals come to that? 01:07:12.960 --> 01:07:16.170 align:middle line:84% Here's a photo of Tony Zee a few years ago. 01:07:16.170 --> 01:07:19.680 align:middle line:84% He wandered into this field really accidentally. 01:07:19.680 --> 01:07:22.380 align:middle line:84% He finished his PhD at Harvard in 1970. 01:07:22.380 --> 01:07:24.120 align:middle line:84% He was a grad student in the late '60s. 01:07:24.120 --> 01:07:27.120 align:middle line:84% His work was squarely in particle physics, 01:07:27.120 --> 01:07:30.580 align:middle line:84% the fancy new symmetries and new nuclear forces. 01:07:30.580 --> 01:07:32.310 align:middle line:84% That's what he studied for his thesis. 01:07:32.310 --> 01:07:35.130 align:middle line:90% He finished his PhD in 1970. 01:07:35.130 --> 01:07:40.380 align:middle line:84% He then had a sabbatical early on in his faculty career, 01:07:40.380 --> 01:07:42.060 align:middle line:90% and he happened to go to Paris. 01:07:42.060 --> 01:07:44.850 align:middle line:84% And he swapped apartments with a Parisian physicist 01:07:44.850 --> 01:07:46.740 align:middle line:84% who was about to come to the United States. 01:07:46.740 --> 01:07:49.560 align:middle line:84% So they basically swapped apartments for several months. 01:07:49.560 --> 01:07:51.960 align:middle line:84% And as Tony recalled, the physicist 01:07:51.960 --> 01:07:53.670 align:middle line:84% with whom he again kind of accidentally 01:07:53.670 --> 01:07:55.680 align:middle line:84% wound up swapping apartments happens 01:07:55.680 --> 01:07:58.050 align:middle line:84% to have been immersed in gravitation and cosmology, 01:07:58.050 --> 01:08:00.690 align:middle line:84% more so than in Tony's main field. 01:08:00.690 --> 01:08:04.078 align:middle line:84% And as Tony recalls, he found these stacks of preprints 01:08:04.078 --> 01:08:06.120 align:middle line:84% all around the apartment that looked interesting. 01:08:06.120 --> 01:08:09.360 align:middle line:84% He was basically in a stranger's apartment, 01:08:09.360 --> 01:08:13.470 align:middle line:84% reading the mail, just reading papers on the coffee table. 01:08:13.470 --> 01:08:14.970 align:middle line:84% And these looked really interesting. 01:08:14.970 --> 01:08:16.140 align:middle line:90% And in fact, they sparked. 01:08:16.140 --> 01:08:17.850 align:middle line:84% He had a little extra preparation. 01:08:17.850 --> 01:08:20.910 align:middle line:84% He'd been an undergraduate at Princeton, Tony had. 01:08:20.910 --> 01:08:23.399 align:middle line:84% And there, he studied a little bit of gravity 01:08:23.399 --> 01:08:26.189 align:middle line:84% with John Wheeler for his undergraduate thesis. 01:08:26.189 --> 01:08:28.710 align:middle line:84% So he knew a little bit about Einstein's theory 01:08:28.710 --> 01:08:31.020 align:middle line:84% of gravitation, had worked with a renowned expert 01:08:31.020 --> 01:08:32.700 align:middle line:84% in the field as an undergraduate, 01:08:32.700 --> 01:08:35.010 align:middle line:84% but really focused on quite different topics 01:08:35.010 --> 01:08:38.189 align:middle line:84% throughout his own PhD and post-PhD training. 01:08:38.189 --> 01:08:42.660 align:middle line:84% So he accidentally stumbles back into the topic 01:08:42.660 --> 01:08:44.399 align:middle line:90% of gravitation and cosmology. 01:08:44.399 --> 01:08:45.987 align:middle line:90% And that rekindles an interest. 01:08:45.987 --> 01:08:48.029 align:middle line:84% He actually gets back in touch with John Wheeler. 01:08:48.029 --> 01:08:51.160 align:middle line:84% He's not doing this on his own after his sabbatical is over. 01:08:51.160 --> 01:08:54.149 align:middle line:84% So by the mid to late '70s, he's now asking questions 01:08:54.149 --> 01:08:57.569 align:middle line:84% at this interface between his formal training 01:08:57.569 --> 01:09:01.350 align:middle line:84% in high-energy particle theory and this new hobby interest 01:09:01.350 --> 01:09:02.910 align:middle line:90% in gravitation and cosmology. 01:09:02.910 --> 01:09:05.399 align:middle line:84% And it's in the midst of that new set of studies 01:09:05.399 --> 01:09:08.609 align:middle line:84% that he writes his version of his broken-symmetric gravity 01:09:08.609 --> 01:09:12.210 align:middle line:84% to suggest very creatively but tentatively 01:09:12.210 --> 01:09:14.340 align:middle line:84% that the Brans-Dicke field and the Higgs field 01:09:14.340 --> 01:09:15.540 align:middle line:90% might be identical. 01:09:15.540 --> 01:09:18.510 align:middle line:84% OK, that's his route in-- a kind of accidental. 01:09:18.510 --> 01:09:21.420 align:middle line:84% For Lee Smolin, it was really not very accidental at all. 01:09:21.420 --> 01:09:23.189 align:middle line:84% He was the other person who independently 01:09:23.189 --> 01:09:27.120 align:middle line:84% introduced that broken-symmetric theory in 1979-- same year 01:09:27.120 --> 01:09:28.710 align:middle line:90% as Tony. 01:09:28.710 --> 01:09:32.138 align:middle line:84% So Lee actually was still in graduate school 01:09:32.138 --> 01:09:33.180 align:middle line:90% when he wrote that paper. 01:09:33.180 --> 01:09:34.859 align:middle line:90% He entered grad school in '75. 01:09:34.859 --> 01:09:37.830 align:middle line:84% So he entered roughly 10 years after Zee had. 01:09:37.830 --> 01:09:39.300 align:middle line:90% He was roughly 10 years younger. 01:09:39.300 --> 01:09:41.729 align:middle line:84% Both at Harvard, as it turns out, both at the same school, 01:09:41.729 --> 01:09:44.520 align:middle line:84% but 10 years apart, in the midst of which 01:09:44.520 --> 01:09:46.020 align:middle line:84% some pretty significant curricular 01:09:46.020 --> 01:09:49.479 align:middle line:84% changes had begun to take place there and elsewhere. 01:09:49.479 --> 01:09:53.310 align:middle line:84% So unlike Zee, who focused pretty exclusively on particle 01:09:53.310 --> 01:09:56.190 align:middle line:84% theory as a graduate student, Smolin 01:09:56.190 --> 01:09:59.310 align:middle line:84% was actually, from the start, combining the two fields, 01:09:59.310 --> 01:10:01.170 align:middle line:84% both in the courses he took and eventually 01:10:01.170 --> 01:10:03.300 align:middle line:84% with his advising team for his thesis 01:10:03.300 --> 01:10:05.200 align:middle line:90% and for his dissertation itself. 01:10:05.200 --> 01:10:07.560 align:middle line:84% So from his first semester as he was taking courses 01:10:07.560 --> 01:10:10.590 align:middle line:84% with experts in gravitation and cosmology, 01:10:10.590 --> 01:10:13.230 align:middle line:84% like Stanley Deser and Steven Weinberg, 01:10:13.230 --> 01:10:16.860 align:middle line:84% as well as experts in particle theory and nuclear forces, 01:10:16.860 --> 01:10:19.500 align:middle line:84% like Sidney Coleman, Howard Georgi, 01:10:19.500 --> 01:10:21.370 align:middle line:84% I who introduced that GUT I mentioned, 01:10:21.370 --> 01:10:24.030 align:middle line:84% and a visiting professor at the time, Gerard Hooft. 01:10:24.030 --> 01:10:27.240 align:middle line:84% So he was really being schooled, formally and informally, 01:10:27.240 --> 01:10:30.060 align:middle line:84% from the start to work at that boundary. 01:10:30.060 --> 01:10:34.680 align:middle line:84% And his paper that introduced this unification of the two 01:10:34.680 --> 01:10:35.940 align:middle line:90% scalar fields-- 01:10:35.940 --> 01:10:37.830 align:middle line:84% also published in '79-- that was part 01:10:37.830 --> 01:10:39.450 align:middle line:90% of his formal graduate training. 01:10:39.450 --> 01:10:41.563 align:middle line:90% That was part of his thesis. 01:10:41.563 --> 01:10:43.230 align:middle line:84% It was actually that second version that 01:10:43.230 --> 01:10:45.070 align:middle line:90% becomes more and more common. 01:10:45.070 --> 01:10:48.450 align:middle line:84% So unlike this accident of trading apartments in Paris 01:10:48.450 --> 01:10:51.090 align:middle line:84% and reading a few preprints, more and more members 01:10:51.090 --> 01:10:54.420 align:middle line:84% of Lee Smolin's generation were going through a training 01:10:54.420 --> 01:10:56.970 align:middle line:84% more and more like his, partly by design, 01:10:56.970 --> 01:10:59.970 align:middle line:84% in the wake of the National Academies report 01:10:59.970 --> 01:11:01.830 align:middle line:90% and similar curricular reforms. 01:11:01.830 --> 01:11:04.360 align:middle line:84% So Lee was hardly alone from this generation. 01:11:04.360 --> 01:11:07.360 align:middle line:84% People like Michael Turner Edward-- he goes by Rocky-- 01:11:07.360 --> 01:11:09.490 align:middle line:84% Kolb or Paul Steinhardt-- all of whom 01:11:09.490 --> 01:11:11.870 align:middle line:84% finished their PhDs around the same time as Smolin 01:11:11.870 --> 01:11:13.780 align:middle line:90% in different universities. 01:11:13.780 --> 01:11:16.000 align:middle line:84% They were trained in very similar ways 01:11:16.000 --> 01:11:19.220 align:middle line:84% that they likely had entered with an interest in particle 01:11:19.220 --> 01:11:19.720 align:middle line:90% theory. 01:11:19.720 --> 01:11:21.520 align:middle line:84% They learned a lot of particle theory, 01:11:21.520 --> 01:11:23.710 align:middle line:84% but both formally and informally, 01:11:23.710 --> 01:11:26.800 align:middle line:84% coursework general exams and advising for theses, 01:11:26.800 --> 01:11:31.270 align:middle line:84% they were working from the start with a combined set of ideas 01:11:31.270 --> 01:11:33.400 align:middle line:84% and advisors, quite different from what 01:11:33.400 --> 01:11:36.490 align:middle line:84% had been typical for say Tony Zee's 01:11:36.490 --> 01:11:39.220 align:middle line:84% generation, or, for that matter, Alan Gates. 01:11:39.220 --> 01:11:42.070 align:middle line:84% And each of them, not just Lee Smolin, each of the folks 01:11:42.070 --> 01:11:44.770 align:middle line:84% here like, Turner Kolb Steinhardt and many 01:11:44.770 --> 01:11:49.990 align:middle line:84% of their colleagues, also took up this interesting idea 01:11:49.990 --> 01:11:52.180 align:middle line:84% and, in the years soon after 1979 01:11:52.180 --> 01:11:54.310 align:middle line:84% throughout the early mid '80s, began 01:11:54.310 --> 01:11:56.530 align:middle line:84% pursuing other studies of their own 01:11:56.530 --> 01:12:00.010 align:middle line:84% in which they physically united the Brans-Dicke field 01:12:00.010 --> 01:12:02.620 align:middle line:84% and the Higgs field in new kinds of models to try 01:12:02.620 --> 01:12:04.300 align:middle line:90% to understand exotic phenomena. 01:12:04.300 --> 01:12:07.900 align:middle line:84% So that becomes the norm instead of the exception. 01:12:07.900 --> 01:12:10.540 align:middle line:84% So I find this really interesting 01:12:10.540 --> 01:12:14.260 align:middle line:84% because there's a tradition for trying to think about how 01:12:14.260 --> 01:12:15.760 align:middle line:90% physics changes over time. 01:12:15.760 --> 01:12:17.800 align:middle line:84% We, often think about whole-scale theories, 01:12:17.800 --> 01:12:19.850 align:middle line:90% and one theory replaces another. 01:12:19.850 --> 01:12:21.640 align:middle line:84% And I think there's just that misses lot 01:12:21.640 --> 01:12:23.510 align:middle line:90% of what people do all the time. 01:12:23.510 --> 01:12:26.080 align:middle line:84% It certainly misses what people like Lee Smolin, 01:12:26.080 --> 01:12:29.510 align:middle line:84% or Tony Zee, or Mike Turner were doing in these instances. 01:12:29.510 --> 01:12:32.050 align:middle line:84% So very few people, very few physicists today, 01:12:32.050 --> 01:12:36.310 align:middle line:84% even including Carl Brans, think that Brans-Dicke theory 01:12:36.310 --> 01:12:38.950 align:middle line:90% of gravity best describes our. 01:12:38.950 --> 01:12:40.090 align:middle line:90% Universe 01:12:40.090 --> 01:12:42.730 align:middle line:84% It's highly constrained by solar system tests. 01:12:42.730 --> 01:12:44.830 align:middle line:84% Any deviations would be pretty modest compared 01:12:44.830 --> 01:12:46.570 align:middle line:90% to pure Einstein gravity. 01:12:46.570 --> 01:12:50.050 align:middle line:84% And yet, this theoretical object that theorists still 01:12:50.050 --> 01:12:52.480 align:middle line:84% think about, the Brans-Dicke scalar field, 01:12:52.480 --> 01:12:54.910 align:middle line:84% hardly died off by his experiments. 01:12:54.910 --> 01:12:57.160 align:middle line:84% In fact, arguably, interest in the field 01:12:57.160 --> 01:12:59.710 align:middle line:84% grew even as it was getting experimentally 01:12:59.710 --> 01:13:01.810 align:middle line:90% less and less favored. 01:13:01.810 --> 01:13:03.730 align:middle line:84% So this new generation of theorists-- 01:13:03.730 --> 01:13:07.030 align:middle line:84% people like Lee Smolin, and Rocky Kolb, and Mike Turner, 01:13:07.030 --> 01:13:09.500 align:middle line:84% and Paul Steinhardt, and others-- 01:13:09.500 --> 01:13:11.380 align:middle line:84% they were trained to work at this interface. 01:13:11.380 --> 01:13:14.140 align:middle line:84% And they found that this Brans-Dicke-like feature kept 01:13:14.140 --> 01:13:16.210 align:middle line:90% popping up all over the place. 01:13:16.210 --> 01:13:19.210 align:middle line:84% When you try to study the self-consistent quantum 01:13:19.210 --> 01:13:23.830 align:middle line:84% mechanics of these scalar fields in a warping space time, 01:13:23.830 --> 01:13:25.990 align:middle line:84% you are actually forced into these kinds 01:13:25.990 --> 01:13:27.610 align:middle line:90% of Brans-Dicke couplings. 01:13:27.610 --> 01:13:29.500 align:middle line:84% That wasn't Brans' and Dicke's motivation, 01:13:29.500 --> 01:13:31.360 align:middle line:84% but through this new set of questions, 01:13:31.360 --> 01:13:33.550 align:middle line:90% more particle theory inspired. 01:13:33.550 --> 01:13:36.700 align:middle line:84% Then these terms come up unavoidably. 01:13:36.700 --> 01:13:39.490 align:middle line:84% They became more and more common in other efforts 01:13:39.490 --> 01:13:42.580 align:middle line:84% to unify the fundamental forces, like in string theory. 01:13:42.580 --> 01:13:45.910 align:middle line:84% They had all kinds of potential roles to play in new work 01:13:45.910 --> 01:13:48.860 align:middle line:84% that we'll look at together next time on inflation. 01:13:48.860 --> 01:13:50.920 align:middle line:84% So even though experimental evidence 01:13:50.920 --> 01:13:52.960 align:middle line:84% for or against Brans-Dicke gravity 01:13:52.960 --> 01:13:56.320 align:middle line:84% was tilting more and more against, at that same moment, 01:13:56.320 --> 01:13:58.570 align:middle line:84% people were finding more and more creative reasons 01:13:58.570 --> 01:14:00.760 align:middle line:84% to pay attention to the scalar field, 01:14:00.760 --> 01:14:04.240 align:middle line:84% even though the full-blown theory was falling out 01:14:04.240 --> 01:14:05.020 align:middle line:90% of favor. 01:14:05.020 --> 01:14:08.380 align:middle line:84% And you see that, again, in this countables. 01:14:08.380 --> 01:14:12.200 align:middle line:84% If you reset the clock to zero and start counting citations 01:14:12.200 --> 01:14:15.560 align:middle line:84% starting in 1981 just to the Brans-Dicke paper, 01:14:15.560 --> 01:14:17.360 align:middle line:84% it becomes renowned all over again. 01:14:17.360 --> 01:14:21.860 align:middle line:84% It gets another 500, in face, almost 700 citations just 01:14:21.860 --> 01:14:24.890 align:middle line:84% in the 15 or 16 years after 1981, 01:14:24.890 --> 01:14:27.560 align:middle line:84% when it was already most disfavored 01:14:27.560 --> 01:14:29.970 align:middle line:84% by the astrophysical observations. 01:14:29.970 --> 01:14:32.860 align:middle line:84% So the idea that we're picking single theories 01:14:32.860 --> 01:14:34.610 align:middle line:84% and that they replace each other, I think, 01:14:34.610 --> 01:14:38.210 align:middle line:84% just misses this fine structure, that while Brans-Dicke, 01:14:38.210 --> 01:14:41.180 align:middle line:84% as a description of gravity in our local solar system 01:14:41.180 --> 01:14:43.850 align:middle line:84% was under most pressure, was when people were actually 01:14:43.850 --> 01:14:47.870 align:middle line:84% doing the most things with the actual scalar field Brans-Dicke 01:14:47.870 --> 01:14:49.053 align:middle line:90% phi. 01:14:49.053 --> 01:14:51.470 align:middle line:84% I find that really curious, and it's still very much alive 01:14:51.470 --> 01:14:51.900 align:middle line:90% today. 01:14:51.900 --> 01:14:53.510 align:middle line:84% In fact, one of the leading contenders 01:14:53.510 --> 01:14:55.700 align:middle line:84% for our understanding of the very early universe 01:14:55.700 --> 01:14:59.180 align:middle line:84% does exactly the kinds of things that Smolin and Zee 01:14:59.180 --> 01:15:02.180 align:middle line:84% had been doing, trying to unify the Brans-Dicke and Higgs 01:15:02.180 --> 01:15:02.880 align:middle line:90% field. 01:15:02.880 --> 01:15:06.950 align:middle line:84% So I want to emphasize that neither kind of new experiments 01:15:06.950 --> 01:15:09.470 align:middle line:84% nor new theories alone are really 01:15:09.470 --> 01:15:12.632 align:middle line:84% going to help us account for either the specific instance 01:15:12.632 --> 01:15:14.840 align:middle line:84% of people doing creative things with these two scalar 01:15:14.840 --> 01:15:18.500 align:middle line:84% fields or the broader instance of working at this new hybrid 01:15:18.500 --> 01:15:21.492 align:middle line:84% or boundary area of particle theory. 01:15:21.492 --> 01:15:23.700 align:middle line:84% And in fact, there were a bunch of concrete changes-- 01:15:23.700 --> 01:15:28.010 align:middle line:84% some of them geopolitical, the ramping up the Vietnam 01:15:28.010 --> 01:15:35.030 align:middle line:84% War, worldwide economic crisis, huge changes in policy 01:15:35.030 --> 01:15:36.560 align:middle line:84% priorities within the United States, 01:15:36.560 --> 01:15:38.227 align:middle line:84% shifts with the University departments-- 01:15:38.227 --> 01:15:40.610 align:middle line:84% all these things on a huge range of scales, 01:15:40.610 --> 01:15:44.390 align:middle line:84% from individual departments up to Cold War geopolitics, 01:15:44.390 --> 01:15:48.860 align:middle line:84% these things are helping to mold what's 01:15:48.860 --> 01:15:52.010 align:middle line:84% going to seem natural or, quote/unquote natural, 01:15:52.010 --> 01:15:54.680 align:middle line:84% for younger people to consider doing because it all helps 01:15:54.680 --> 01:15:56.780 align:middle line:84% to shape what counts as their formal 01:15:56.780 --> 01:15:58.700 align:middle line:84% and even their informal training. 01:15:58.700 --> 01:16:01.130 align:middle line:84% And so the training of people like Lee Smolin 01:16:01.130 --> 01:16:03.650 align:middle line:84% and his generation just was importantly 01:16:03.650 --> 01:16:06.290 align:middle line:84% different from the very excellent training 01:16:06.290 --> 01:16:08.960 align:middle line:84% that people like Tony Zee had had even just a mere 10 01:16:08.960 --> 01:16:10.160 align:middle line:90% years earlier. 01:16:10.160 --> 01:16:12.560 align:middle line:84% And in turn, these new folks, especially people 01:16:12.560 --> 01:16:15.470 align:middle line:84% like Mike Turner and Rocky Kolb, went on 01:16:15.470 --> 01:16:18.390 align:middle line:84% to become real institution builders in their own right. 01:16:18.390 --> 01:16:20.780 align:middle line:84% So in fact, they were accelerates. 01:16:20.780 --> 01:16:22.882 align:middle line:84% Not only had they been trained to think carefully 01:16:22.882 --> 01:16:24.590 align:middle line:84% at this new interface, they helped really 01:16:24.590 --> 01:16:26.210 align:middle line:90% accelerate the trend. 01:16:26.210 --> 01:16:27.770 align:middle line:84% Turner and Kolb became the directors 01:16:27.770 --> 01:16:30.110 align:middle line:84% of the very first institutional center 01:16:30.110 --> 01:16:32.060 align:middle line:90% devoted to particle cosmology. 01:16:32.060 --> 01:16:34.490 align:middle line:84% It was called the Center for Particle Astrophysics 01:16:34.490 --> 01:16:36.920 align:middle line:84% at Fermilab when they were still relatively 01:16:36.920 --> 01:16:38.450 align:middle line:90% young in their careers. 01:16:38.450 --> 01:16:40.310 align:middle line:84% And then they wrote the very first textbook 01:16:40.310 --> 01:16:42.950 align:middle line:84% on the field first published in 1990. 01:16:42.950 --> 01:16:44.670 align:middle line:84% I actually own three copies of that book. 01:16:44.670 --> 01:16:45.690 align:middle line:90% I love that book. 01:16:45.690 --> 01:16:47.420 align:middle line:84% And so they were actively working 01:16:47.420 --> 01:16:51.980 align:middle line:84% to perpetuate this new hybrid area, not only because 01:16:51.980 --> 01:16:55.560 align:middle line:84% of the force of lovely ideas like asymptotic freedom 01:16:55.560 --> 01:16:56.360 align:middle line:90% and GUTs. 01:16:56.360 --> 01:16:57.950 align:middle line:90% So I'm going to pause there. 01:16:57.950 --> 01:16:59.130 align:middle line:90% We have time for-- 01:16:59.130 --> 01:16:59.630 align:middle line:90% oh, OK. 01:16:59.630 --> 01:17:02.240 align:middle line:90% Sorry, last little bit here. 01:17:02.240 --> 01:17:05.850 align:middle line:84% This is really a setup, then, for the next class session. 01:17:05.850 --> 01:17:08.240 align:middle line:84% We'll look more squarely at inflation in cosmology. 01:17:08.240 --> 01:17:11.220 align:middle line:84% We'll have more opportunities to make fun of Alan Guth, which 01:17:11.220 --> 01:17:13.050 align:middle line:90% is what's in it for me. 01:17:13.050 --> 01:17:15.810 align:middle line:84% But we'll look at an instantiation of work 01:17:15.810 --> 01:17:21.390 align:middle line:84% at this new hybrid area, really of what becomes the poster 01:17:21.390 --> 01:17:23.370 align:middle line:90% example of particle cosmology. 01:17:23.370 --> 01:17:25.600 align:middle line:84% And we'll look squarely at that in the next time. 01:17:25.600 --> 01:17:28.410 align:middle line:84% And as I say, within that work, including my own students 01:17:28.410 --> 01:17:30.420 align:middle line:84% and many people now around the world, 01:17:30.420 --> 01:17:36.060 align:middle line:84% it's now just totally bizarre not 01:17:36.060 --> 01:17:37.978 align:middle line:84% to consider the Brans-Dicke field 01:17:37.978 --> 01:17:40.020 align:middle line:84% and the Higgs field is somehow relatable or maybe 01:17:40.020 --> 01:17:41.020 align:middle line:90% even identical. 01:17:41.020 --> 01:17:43.470 align:middle line:84% So it's gone from really, really just 01:17:43.470 --> 01:17:48.000 align:middle line:84% never done for nearly 20 years to now remarkable 01:17:48.000 --> 01:17:49.680 align:middle line:90% if people even question it. 01:17:49.680 --> 01:17:51.330 align:middle line:84% So it goes from who even thought of it 01:17:51.330 --> 01:17:52.710 align:middle line:90% to who wouldn't even try that? 01:17:52.710 --> 01:17:57.017 align:middle line:84% What counts as natural can shift in a pretty short time scale. 01:17:57.017 --> 01:17:59.100 align:middle line:84% And as we've been seeing throughout the whole term 01:17:59.100 --> 01:18:01.530 align:middle line:84% and including today, those shifts 01:18:01.530 --> 01:18:06.000 align:middle line:84% can be driven as much by things well outside of the physicist's 01:18:06.000 --> 01:18:09.390 align:middle line:84% control, geopolitics, and national scale budgets, 01:18:09.390 --> 01:18:12.990 align:middle line:84% and blue-ribbon committees as by the force of new ideas 01:18:12.990 --> 01:18:13.980 align:middle line:90% and experiments. 01:18:13.980 --> 01:18:15.630 align:middle line:84% So that's where I'll actually pause. 01:18:15.630 --> 01:18:18.510 align:middle line:84% And we have time for a few more questions or comments. 01:18:18.510 --> 01:18:19.980 align:middle line:90% Any other thoughts on that? 01:18:19.980 --> 01:18:28.460 align:middle line:90% 01:18:28.460 --> 01:18:30.333 align:middle line:84% With my last moment here, I'll say, 01:18:30.333 --> 01:18:32.750 align:middle line:84% before the construction for the new Center for Theoretical 01:18:32.750 --> 01:18:35.930 align:middle line:84% Physics, I had an office just down the hall from Alan's. 01:18:35.930 --> 01:18:38.240 align:middle line:84% And by a quirk of the old building 6, 01:18:38.240 --> 01:18:39.710 align:middle line:90% we had the same key. 01:18:39.710 --> 01:18:41.480 align:middle line:84% A single key would open the whole hallway. 01:18:41.480 --> 01:18:42.950 align:middle line:84% I guess, we all trusted each other then. 01:18:42.950 --> 01:18:44.075 align:middle line:90% Couldn't get rid of it now. 01:18:44.075 --> 01:18:45.740 align:middle line:84% So my key would open Alan's office. 01:18:45.740 --> 01:18:48.470 align:middle line:84% And one time, my parents were visiting. 01:18:48.470 --> 01:18:50.570 align:middle line:84% And I basically broke into Alan's office. 01:18:50.570 --> 01:18:52.820 align:middle line:84% They couldn't believe me when I described 01:18:52.820 --> 01:18:55.822 align:middle line:84% what it was like to try to work with this person. 01:18:55.822 --> 01:18:57.280 align:middle line:84% So I actually broke into his office 01:18:57.280 --> 01:19:03.040 align:middle line:84% to show them the safety violation, fire code violation, 01:19:03.040 --> 01:19:07.330 align:middle line:84% horror show that was the den of entropy known as Alan's office. 01:19:07.330 --> 01:19:08.398 align:middle line:90% So that's a true story. 01:19:08.398 --> 01:19:09.940 align:middle line:84% I no longer have a key to his office, 01:19:09.940 --> 01:19:12.570 align:middle line:90% so his mess is safe from me. 01:19:12.570 --> 01:19:16.040 align:middle line:84% Any questions about messy offices, or group theory, 01:19:16.040 --> 01:19:20.590 align:middle line:84% or geopolitics, or anything else? 01:19:20.590 --> 01:19:24.040 align:middle line:84% If not, I will say good luck with paper 3. 01:19:24.040 --> 01:19:25.710 align:middle line:90% Please, please get back to it. 01:19:25.710 --> 01:19:26.960 align:middle line:90% Don't leave it to last minute. 01:19:26.960 --> 01:19:29.830 align:middle line:84% Please get in touch with your teaching assistant. 01:19:29.830 --> 01:19:31.750 align:middle line:84% Think about the optional paper 1 rewrite 01:19:31.750 --> 01:19:33.820 align:middle line:84% if you have time and inclination for that. 01:19:33.820 --> 01:19:35.530 align:middle line:84% And stay well, and I look forward 01:19:35.530 --> 01:19:37.130 align:middle line:90% to seeing you on Monday. 01:19:37.130 --> 01:19:39.240 align:middle line:90% See you soon, everyone. 01:19:39.240 --> 01:19:51.000 align:middle line:90%