WEBVTT 00:00:00.000 --> 00:00:01.996 align:middle line:90% [SQUEAKING] 00:00:01.996 --> 00:00:04.491 align:middle line:90% [RUSTLING] 00:00:04.491 --> 00:00:05.489 align:middle line:90% [CLICKING] 00:00:05.489 --> 00:00:10.990 align:middle line:90% 00:00:10.990 --> 00:00:13.547 align:middle line:84% DAVID KAISER: Today, we're now sort 00:00:13.547 --> 00:00:17.620 align:middle line:84% of in the middle of our unit of our last main unit 00:00:17.620 --> 00:00:20.680 align:middle line:84% on a kind of quarks to the cosmos, trends 00:00:20.680 --> 00:00:22.810 align:middle line:84% in high energy physics, astrophysics, 00:00:22.810 --> 00:00:24.880 align:middle line:90% and gravitation and cosmology. 00:00:24.880 --> 00:00:26.740 align:middle line:84% And so today, we'll be picking up 00:00:26.740 --> 00:00:29.230 align:middle line:84% part of the kind of threads that we were looking 00:00:29.230 --> 00:00:31.600 align:middle line:84% at before the Thanksgiving break, 00:00:31.600 --> 00:00:34.330 align:middle line:84% and we'll be looking in particular how was it 00:00:34.330 --> 00:00:36.580 align:middle line:84% and over what kind of time scale was it 00:00:36.580 --> 00:00:40.690 align:middle line:84% that most physicists came to be convinced 00:00:40.690 --> 00:00:44.860 align:middle line:84% that many, many types of matter are actually formed of quarks, 00:00:44.860 --> 00:00:49.270 align:middle line:84% that there's a constituent elementary unit 00:00:49.270 --> 00:00:50.680 align:middle line:90% within many kinds of matter. 00:00:50.680 --> 00:00:53.830 align:middle line:84% How did people come to be convinced of that 00:00:53.830 --> 00:00:56.110 align:middle line:84% and over what kind of time scale? 00:00:56.110 --> 00:00:58.400 align:middle line:84% So that's what we'll look at for part of today. 00:00:58.400 --> 00:01:01.840 align:middle line:84% So the three parts-- we'll revisit some of the trends 00:01:01.840 --> 00:01:03.940 align:middle line:84% that we already began looking at briefly, again, 00:01:03.940 --> 00:01:06.790 align:middle line:84% in the class sessions before the Thanksgiving break, 00:01:06.790 --> 00:01:10.730 align:middle line:84% in particular this what seemed to be a really unexpected 00:01:10.730 --> 00:01:15.140 align:middle line:84% and at times almost overwhelming proliferation of the number 00:01:15.140 --> 00:01:17.210 align:middle line:84% of particles that seem to come out practically 00:01:17.210 --> 00:01:19.250 align:middle line:84% every time that the physicists turned 00:01:19.250 --> 00:01:22.220 align:middle line:84% on their often brand new particle accelerators. 00:01:22.220 --> 00:01:25.640 align:middle line:84% We saw there was a huge spate, a kind of building boom coming 00:01:25.640 --> 00:01:28.880 align:middle line:84% out of the Second World War of making larger and larger more 00:01:28.880 --> 00:01:30.590 align:middle line:90% and more powerful accelerators. 00:01:30.590 --> 00:01:34.130 align:middle line:84% MIT had our own synchrotron in operation for many years. 00:01:34.130 --> 00:01:36.480 align:middle line:84% Many, many individual universities had them, 00:01:36.480 --> 00:01:39.830 align:middle line:84% and of course, there were larger national laboratory facilities 00:01:39.830 --> 00:01:43.790 align:middle line:84% as well, all a kind of carryover into the post-war period 00:01:43.790 --> 00:01:46.100 align:middle line:84% of the wartime Manhattan Project. 00:01:46.100 --> 00:01:48.560 align:middle line:84% These were Atomic Energy Commission facilities 00:01:48.560 --> 00:01:50.090 align:middle line:90% by and large. 00:01:50.090 --> 00:01:52.790 align:middle line:84% So what were some of the responses to this proliferation 00:01:52.790 --> 00:01:54.650 align:middle line:84% of new kinds of particles that seem to be 00:01:54.650 --> 00:01:56.000 align:middle line:90% popping up all over the place? 00:01:56.000 --> 00:01:58.850 align:middle line:84% We'll see that that was really on many people's minds 00:01:58.850 --> 00:02:02.000 align:middle line:84% when they began thinking about what we now call quarks. 00:02:02.000 --> 00:02:05.060 align:middle line:84% The idea about quarks itself has a really, I 00:02:05.060 --> 00:02:08.030 align:middle line:84% think, fascinating kind of up-and-down history 00:02:08.030 --> 00:02:11.240 align:middle line:84% where not everyone was on board at the same time 00:02:11.240 --> 00:02:13.030 align:middle line:90% for the same reasons. 00:02:13.030 --> 00:02:15.880 align:middle line:84% There was a kind of long ambivalence or confusion 00:02:15.880 --> 00:02:16.707 align:middle line:90% or-- what are they? 00:02:16.707 --> 00:02:17.290 align:middle line:90% Are they real? 00:02:17.290 --> 00:02:18.072 align:middle line:90% Are they not? 00:02:18.072 --> 00:02:20.530 align:middle line:84% And so we'll look at that for a good chunk of today's class 00:02:20.530 --> 00:02:21.280 align:middle line:90% session. 00:02:21.280 --> 00:02:24.910 align:middle line:84% And the last part will be a kind of what 00:02:24.910 --> 00:02:28.780 align:middle line:84% has become called the so-called standard model. 00:02:28.780 --> 00:02:31.420 align:middle line:84% We'll look at a key part of that called quantum 00:02:31.420 --> 00:02:33.740 align:middle line:84% chromodynamics, or QCD, which you've probably heard of, 00:02:33.740 --> 00:02:34.240 align:middle line:90% at least. 00:02:34.240 --> 00:02:35.775 align:middle line:90% You probably recognize the name. 00:02:35.775 --> 00:02:37.900 align:middle line:84% We'll talk a little bit about where that comes from 00:02:37.900 --> 00:02:41.530 align:middle line:84% and how that fits into the kind of larger picture today, 00:02:41.530 --> 00:02:45.940 align:middle line:84% and it's drawing on this many decades coming before it 00:02:45.940 --> 00:02:47.240 align:middle line:90% that we'll look at for today. 00:02:47.240 --> 00:02:48.573 align:middle line:90% So that's what we're heading to. 00:02:48.573 --> 00:02:51.610 align:middle line:84% Quick reminder-- as I just mentioned, after the war, 00:02:51.610 --> 00:02:54.170 align:middle line:84% pictures like this were becoming more and more common. 00:02:54.170 --> 00:02:59.380 align:middle line:84% So by 1955, Berkeley's Bevatron, a billion-electron-volt 00:02:59.380 --> 00:03:02.890 align:middle line:84% accelerator, was filling kind of factory room floors. 00:03:02.890 --> 00:03:05.350 align:middle line:84% Again, here's a human operator to give you 00:03:05.350 --> 00:03:07.450 align:middle line:84% a sense of the scale-- absolutely enormous, 00:03:07.450 --> 00:03:09.070 align:middle line:90% certainly for its time. 00:03:09.070 --> 00:03:12.860 align:middle line:84% And as I mentioned, that was driving this exponential rise 00:03:12.860 --> 00:03:16.160 align:middle line:84% in the numbers of seemingly new, seemingly elementary 00:03:16.160 --> 00:03:17.780 align:middle line:90% nuclear particles. 00:03:17.780 --> 00:03:20.630 align:middle line:84% When physicists could achieve higher and higher interaction 00:03:20.630 --> 00:03:23.630 align:middle line:84% rates, they found not just higher energy stuff flying out 00:03:23.630 --> 00:03:25.760 align:middle line:84% but new kinds of stuff flying out, 00:03:25.760 --> 00:03:28.710 align:middle line:84% new kinds of particles among the decay chains and all that. 00:03:28.710 --> 00:03:31.460 align:middle line:84% So that was often called the particle zoo. 00:03:31.460 --> 00:03:33.590 align:middle line:84% We saw in the previous class session 00:03:33.590 --> 00:03:38.570 align:middle line:84% one of the first challenges, kind of intellectual challenges 00:03:38.570 --> 00:03:41.030 align:middle line:84% to try to make sense of this very large number 00:03:41.030 --> 00:03:44.480 align:middle line:84% of nuclear particles is that they interacted strongly 00:03:44.480 --> 00:03:45.270 align:middle line:90% with each other. 00:03:45.270 --> 00:03:48.410 align:middle line:84% In fact, we now call this the strong nuclear force. 00:03:48.410 --> 00:03:52.310 align:middle line:84% It's called strong because the analogy 00:03:52.310 --> 00:03:56.390 align:middle line:84% to the electric charge, which we often abbreviate just 00:03:56.390 --> 00:04:00.300 align:middle line:84% by the letter G, the coupling constant, is very big. 00:04:00.300 --> 00:04:01.850 align:middle line:84% So whereas the electric charge is 00:04:01.850 --> 00:04:06.980 align:middle line:84% small in kind of appropriate units, it's roughly 1/137, 00:04:06.980 --> 00:04:09.440 align:middle line:84% so you could do a perturbation expansion, kind 00:04:09.440 --> 00:04:14.329 align:middle line:84% of Taylor expand, and more and more complicated contributions, 00:04:14.329 --> 00:04:16.579 align:middle line:84% just countless because they have more and more factors 00:04:16.579 --> 00:04:18.380 align:middle line:90% of a small number in front. 00:04:18.380 --> 00:04:21.829 align:middle line:84% The opposite seemed to happen for these strongly interacting 00:04:21.829 --> 00:04:25.250 align:middle line:84% nuclear particles, where an arbitrarily complicated 00:04:25.250 --> 00:04:28.460 align:middle line:84% interaction, a Feynman diagram with many, many vertices, 00:04:28.460 --> 00:04:31.580 align:middle line:84% would actually weigh more in your final answer 00:04:31.580 --> 00:04:33.980 align:middle line:84% than the simple ones, and there was, in principle, 00:04:33.980 --> 00:04:35.715 align:middle line:84% an infinite number of complicated ones. 00:04:35.715 --> 00:04:37.340 align:middle line:84% It really didn't make any sense for how 00:04:37.340 --> 00:04:41.390 align:middle line:84% to calculate in this strong nuclear force regime. 00:04:41.390 --> 00:04:43.280 align:middle line:84% There was a second challenge, which was maybe 00:04:43.280 --> 00:04:46.430 align:middle line:84% a bit more meta, which was really this question of, 00:04:46.430 --> 00:04:50.030 align:middle line:84% could all these hundred or soon 200 or almost 300 00:04:50.030 --> 00:04:54.260 align:middle line:84% seemingly elementary particles-- are they all really elementary? 00:04:54.260 --> 00:04:58.400 align:middle line:84% Are they all kind of to be understood on the same footing? 00:04:58.400 --> 00:05:01.707 align:middle line:84% Is there any pattern or any underlying order 00:05:01.707 --> 00:05:04.040 align:middle line:84% that one might be able to bring to what otherwise looked 00:05:04.040 --> 00:05:08.630 align:middle line:84% like really just a kind of scattershot display of soon 00:05:08.630 --> 00:05:12.220 align:middle line:84% hundreds, many hundreds, of seemingly elementary particles? 00:05:12.220 --> 00:05:15.190 align:middle line:84% How do you make sense of all that stuff? 00:05:15.190 --> 00:05:19.090 align:middle line:84% So we saw, just a brief recap for last time, one response 00:05:19.090 --> 00:05:21.760 align:middle line:84% to this, very creative response, which was really dominant 00:05:21.760 --> 00:05:24.760 align:middle line:84% for a better part of 15 years or so, 00:05:24.760 --> 00:05:27.958 align:middle line:84% was associated with the particle physicist Geoffrey Chew, 00:05:27.958 --> 00:05:30.500 align:middle line:84% and we talked a bit about him and read about some of his work 00:05:30.500 --> 00:05:31.900 align:middle line:90% last time. 00:05:31.900 --> 00:05:34.540 align:middle line:84% He introduced the idea of nuclear democracy and then, 00:05:34.540 --> 00:05:37.210 align:middle line:84% with some of his students, the idea of the bootstrap, 00:05:37.210 --> 00:05:38.920 align:middle line:84% and they were really being very creative 00:05:38.920 --> 00:05:41.860 align:middle line:84% with these simple Feynman diagrams, where 00:05:41.860 --> 00:05:44.230 align:middle line:84% if you simply rotate the diagram, 00:05:44.230 --> 00:05:47.180 align:middle line:84% turn it on its head by 90 degrees, 00:05:47.180 --> 00:05:50.080 align:middle line:84% then the roles that these particles seem to play 00:05:50.080 --> 00:05:51.100 align:middle line:90% could swap. 00:05:51.100 --> 00:05:54.730 align:middle line:84% And so maybe, Chew and his very active group-- 00:05:54.730 --> 00:05:57.610 align:middle line:84% maybe, they wondered, that all these many hundreds 00:05:57.610 --> 00:06:00.550 align:middle line:84% of particles are actually, in a sense, equally 00:06:00.550 --> 00:06:03.130 align:middle line:84% elementary because they're all bound states of each other. 00:06:03.130 --> 00:06:06.130 align:middle line:84% They're equally composite and equally elementary 00:06:06.130 --> 00:06:08.140 align:middle line:84% and that the labels we apply to them 00:06:08.140 --> 00:06:10.750 align:middle line:84% really depend on which orientation 00:06:10.750 --> 00:06:12.970 align:middle line:84% of a kind of scattering diagram or Feynman 00:06:12.970 --> 00:06:14.990 align:middle line:90% diagram we happen to consider. 00:06:14.990 --> 00:06:16.360 align:middle line:90% So it was all about-- 00:06:16.360 --> 00:06:19.420 align:middle line:84% Geoffrey Chew's program was all about dynamics. 00:06:19.420 --> 00:06:24.190 align:middle line:84% Is there one self-consistent set of forces with which they could 00:06:24.190 --> 00:06:27.310 align:middle line:84% make sense of all these many types of particles 00:06:27.310 --> 00:06:29.060 align:middle line:84% and their scatterings and interactions? 00:06:29.060 --> 00:06:33.550 align:middle line:84% So one response to the strongly coupled particle zoo 00:06:33.550 --> 00:06:37.600 align:middle line:84% was to look for a single, kind of self-consistent bootstrap 00:06:37.600 --> 00:06:40.300 align:middle line:84% that might encompass more and more examples 00:06:40.300 --> 00:06:42.008 align:middle line:90% of this nuclear domain. 00:06:42.008 --> 00:06:44.050 align:middle line:84% So what we'll mostly talk about today is actually 00:06:44.050 --> 00:06:47.500 align:middle line:84% a distinct approach, a kind of not even rival 00:06:47.500 --> 00:06:49.960 align:middle line:84% but a complementary approach that 00:06:49.960 --> 00:06:53.080 align:middle line:84% started to gather more and more attention around the same time 00:06:53.080 --> 00:06:56.200 align:middle line:84% period, during the 1950s and 1960s, 00:06:56.200 --> 00:07:00.010 align:middle line:84% and that was to basically ignore or kind of bracket for the time 00:07:00.010 --> 00:07:03.250 align:middle line:84% being questions of forces or dynamics and just 00:07:03.250 --> 00:07:06.670 align:middle line:84% say that's beyond us right now but focus instead 00:07:06.670 --> 00:07:08.170 align:middle line:90% on classification. 00:07:08.170 --> 00:07:10.690 align:middle line:84% Were there patterns among those dozens 00:07:10.690 --> 00:07:12.890 align:middle line:84% and soon hundreds of nuclear particles 00:07:12.890 --> 00:07:14.990 align:middle line:84% that came out of the accelerators? 00:07:14.990 --> 00:07:17.938 align:middle line:84% Were some more like others, more like each other 00:07:17.938 --> 00:07:18.980 align:middle line:90% and distinct from others? 00:07:18.980 --> 00:07:20.690 align:middle line:90% Are there family groups? 00:07:20.690 --> 00:07:21.980 align:middle line:90% Are there symmetries? 00:07:21.980 --> 00:07:26.690 align:middle line:84% Are there ways to group this plethora in a kind of 00:07:26.690 --> 00:07:29.450 align:middle line:90% ordered fashion, so to classify? 00:07:29.450 --> 00:07:32.320 align:middle line:84% So in some sense, that approach was really also not very new. 00:07:32.320 --> 00:07:35.290 align:middle line:84% As we'll see, it actually reached back to some ideas 00:07:35.290 --> 00:07:37.840 align:middle line:84% from the earliest days of nuclear physics. 00:07:37.840 --> 00:07:42.160 align:middle line:84% In the early 1930s, one idea in particular I'll talk about now 00:07:42.160 --> 00:07:47.080 align:middle line:84% was introduced by Werner Heisenberg as early as 1932. 00:07:47.080 --> 00:07:50.260 align:middle line:84% Heisenberg-- it was a series of articles under the title 00:07:50.260 --> 00:07:53.440 align:middle line:84% "On the structure of atomic nuclei," the first of which 00:07:53.440 --> 00:07:56.590 align:middle line:84% was received in June of 1932, you may remember, 00:07:56.590 --> 00:07:58.570 align:middle line:90% though it was several weeks ago. 00:07:58.570 --> 00:08:01.120 align:middle line:84% James Chadwick presented the first really kind 00:08:01.120 --> 00:08:03.100 align:middle line:84% of compelling evidence that the neutron 00:08:03.100 --> 00:08:05.980 align:middle line:84% exists, that atomic nuclei might have 00:08:05.980 --> 00:08:07.810 align:middle line:84% more than one kind of particle within them, 00:08:07.810 --> 00:08:09.670 align:middle line:90% both protons and neutrons. 00:08:09.670 --> 00:08:11.560 align:middle line:84% He presented that evidence very early 00:08:11.560 --> 00:08:15.970 align:middle line:84% in 1932, January, Februaryish, pretty early in the year. 00:08:15.970 --> 00:08:18.670 align:middle line:84% Within not even half a year later, Heisenberg 00:08:18.670 --> 00:08:21.820 align:middle line:84% had really picked up that idea and run with it 00:08:21.820 --> 00:08:24.250 align:middle line:84% and introduced this idea called isospin. 00:08:24.250 --> 00:08:26.410 align:middle line:84% So what was the idea that Heisenberg had? 00:08:26.410 --> 00:08:29.600 align:middle line:84% The early experiments with both protons and neutrons 00:08:29.600 --> 00:08:32.120 align:middle line:84% in these early months after Chadwick 00:08:32.120 --> 00:08:35.659 align:middle line:84% had kind of isolated evidence for the neutron itself. 00:08:35.659 --> 00:08:38.840 align:middle line:84% The early follow-on experiments suggested 00:08:38.840 --> 00:08:42.020 align:middle line:84% that protons and neutrons were kind of interchangeable 00:08:42.020 --> 00:08:44.300 align:middle line:84% when it came to nuclear reactions, 00:08:44.300 --> 00:08:47.030 align:middle line:84% that they seem to have, for example, the same scattering 00:08:47.030 --> 00:08:48.950 align:middle line:84% strength, that what would later be called 00:08:48.950 --> 00:08:52.970 align:middle line:84% the coupling constant g, the analog to the electric charge. 00:08:52.970 --> 00:08:57.170 align:middle line:84% It looked to be the same, so you get very similar patterns, 00:08:57.170 --> 00:09:00.830 align:middle line:84% reaction rates, and so on whether you considering 00:09:00.830 --> 00:09:04.250 align:middle line:84% proton-proton scattering, proton-neutron scattering, 00:09:04.250 --> 00:09:05.630 align:middle line:90% or neutron-neutron scattering. 00:09:05.630 --> 00:09:09.020 align:middle line:84% It looked like it was just one kind of particle, not two very 00:09:09.020 --> 00:09:10.880 align:middle line:90% different-seeming particles. 00:09:10.880 --> 00:09:14.660 align:middle line:84% There was a kind of symmetry in the way these particles were 00:09:14.660 --> 00:09:16.830 align:middle line:90% experiencing the nuclear force. 00:09:16.830 --> 00:09:20.900 align:middle line:84% So Heisenberg suggested in this series of papers back in 1932 00:09:20.900 --> 00:09:24.020 align:middle line:84% that maybe it's actually not two separate particles after all. 00:09:24.020 --> 00:09:26.990 align:middle line:84% Maybe it's not protons and separately neutrons. 00:09:26.990 --> 00:09:29.180 align:middle line:84% Maybe there's one kind of nuclear particle 00:09:29.180 --> 00:09:33.230 align:middle line:84% inside the atomic nuclei, but it had different internal states. 00:09:33.230 --> 00:09:34.730 align:middle line:84% And so it would be one particle that 00:09:34.730 --> 00:09:38.510 align:middle line:84% could be in one kind of condition or another 00:09:38.510 --> 00:09:42.110 align:middle line:84% but not actually different kinds of particles altogether 00:09:42.110 --> 00:09:46.310 align:middle line:84% and that the symmetry, one particle that could show up 00:09:46.310 --> 00:09:47.930 align:middle line:90% as either proton or neutron-- 00:09:47.930 --> 00:09:49.740 align:middle line:90% that symmetry would be broken. 00:09:49.740 --> 00:09:51.380 align:middle line:84% We would notice the difference when 00:09:51.380 --> 00:09:54.350 align:middle line:84% we put those particles in external electromagnetic 00:09:54.350 --> 00:09:55.130 align:middle line:90% fields. 00:09:55.130 --> 00:09:57.470 align:middle line:84% Then we'd see quite obviously, for example, 00:09:57.470 --> 00:10:01.160 align:middle line:84% that the proton has one unit of positive electric charge, 00:10:01.160 --> 00:10:03.510 align:middle line:84% whereas the neutron is electrically neutral. 00:10:03.510 --> 00:10:05.960 align:middle line:84% But we only notice that if we're measuring 00:10:05.960 --> 00:10:08.600 align:middle line:84% electromagnetic effects, if we're putting these particles 00:10:08.600 --> 00:10:09.925 align:middle line:90% in an external field. 00:10:09.925 --> 00:10:11.300 align:middle line:84% So Heisenberg said, that could be 00:10:11.300 --> 00:10:13.670 align:middle line:84% just like an electron, which could 00:10:13.670 --> 00:10:15.620 align:middle line:90% be either spin up or spin down. 00:10:15.620 --> 00:10:18.770 align:middle line:84% Its internal angular momentum could be spin up or spin down 00:10:18.770 --> 00:10:20.990 align:middle line:84% along some direction of space, but we'd only 00:10:20.990 --> 00:10:23.630 align:middle line:84% notice that if we put that electron 00:10:23.630 --> 00:10:28.910 align:middle line:84% in some external magnetic field like a Stern-Gerlach device. 00:10:28.910 --> 00:10:31.340 align:middle line:84% So we don't say there's two separate electrons. 00:10:31.340 --> 00:10:33.020 align:middle line:84% We say there's one electron that could 00:10:33.020 --> 00:10:35.180 align:middle line:90% be one of two internal states. 00:10:35.180 --> 00:10:37.550 align:middle line:84% Heisenberg was saying maybe these nuclear particles 00:10:37.550 --> 00:10:39.210 align:middle line:90% were similar. 00:10:39.210 --> 00:10:42.440 align:middle line:84% So maybe it's not protons versus neutrons. 00:10:42.440 --> 00:10:44.330 align:middle line:84% Maybe there's one kind of particle. 00:10:44.330 --> 00:10:46.940 align:middle line:84% After the war, it became common to call 00:10:46.940 --> 00:10:51.320 align:middle line:84% that single type of particle the nucleon, which could manifest 00:10:51.320 --> 00:10:54.230 align:middle line:84% as either a proton or a neutron, depending 00:10:54.230 --> 00:10:57.950 align:middle line:84% on its internal state, but one could imagine them 00:10:57.950 --> 00:11:01.080 align:middle line:84% as not being literally separate or distinct particles. 00:11:01.080 --> 00:11:03.830 align:middle line:84% So this is a kind of internal symmetry 00:11:03.830 --> 00:11:06.740 align:middle line:84% that would be distinguished by a new quantum number, 00:11:06.740 --> 00:11:08.750 align:middle line:84% not the same set that people had been thinking 00:11:08.750 --> 00:11:11.540 align:middle line:84% about through the '20s, an additional way 00:11:11.540 --> 00:11:14.390 align:middle line:84% to characterize that state called isospin. 00:11:14.390 --> 00:11:17.180 align:middle line:84% And the proton we would assign to, say, 00:11:17.180 --> 00:11:22.910 align:middle line:84% plus 1 unit of isospin, and the neutron would be isospin down. 00:11:22.910 --> 00:11:24.500 align:middle line:84% But maybe they're just internal states 00:11:24.500 --> 00:11:26.080 align:middle line:90% of a single nuclear particle. 00:11:26.080 --> 00:11:28.370 align:middle line:84% So that was an idea that Heisenberg suggested 00:11:28.370 --> 00:11:32.210 align:middle line:84% very creatively in the earliest days of nuclear physics, 00:11:32.210 --> 00:11:36.200 align:middle line:84% and after the Second World War, a number of theorists 00:11:36.200 --> 00:11:39.020 align:middle line:84% kind of went back to that and said, let's look at that again. 00:11:39.020 --> 00:11:41.450 align:middle line:84% Let's maybe take that a bit more seriously 00:11:41.450 --> 00:11:43.160 align:middle line:84% because there were now many, many 00:11:43.160 --> 00:11:46.430 align:middle line:84% more particles to try to sift through and make sense of. 00:11:46.430 --> 00:11:49.850 align:middle line:84% So one of the first patterns that a few physicists began 00:11:49.850 --> 00:11:53.420 align:middle line:84% to notice soon after the Second World War 00:11:53.420 --> 00:11:56.480 align:middle line:84% was that certain of these new particles-- they were often 00:11:56.480 --> 00:12:00.020 align:middle line:84% dubbed "strange" because they were unexpected or unfamiliar. 00:12:00.020 --> 00:12:03.380 align:middle line:84% These strange particles didn't show up one at a time. 00:12:03.380 --> 00:12:05.270 align:middle line:90% They seemed to come in pairs. 00:12:05.270 --> 00:12:06.920 align:middle line:90% They seemed to come together. 00:12:06.920 --> 00:12:09.590 align:middle line:84% This became known as associated production. 00:12:09.590 --> 00:12:12.740 align:middle line:84% They were produced in particle scatterings in association 00:12:12.740 --> 00:12:15.665 align:middle line:84% with each other, so fancy way of saying you didn't get only one 00:12:15.665 --> 00:12:16.790 align:middle line:90% of these strange particles. 00:12:16.790 --> 00:12:20.180 align:middle line:84% They seemed to come often in pairs. 00:12:20.180 --> 00:12:26.240 align:middle line:84% And so the idea then was could one assign to these, again, yet 00:12:26.240 --> 00:12:29.780 align:middle line:84% another internal quantum number, not isospin 00:12:29.780 --> 00:12:34.400 align:middle line:84% but a whole new kind of charge or quantum quantity? 00:12:34.400 --> 00:12:37.070 align:middle line:84% And so independently, Murray Gell-Mann, 00:12:37.070 --> 00:12:39.620 align:middle line:84% who was a very young particle theorist at the time, 00:12:39.620 --> 00:12:41.443 align:middle line:84% and Abraham Pais, a little bit older, 00:12:41.443 --> 00:12:42.860 align:middle line:84% still pretty young in his career-- 00:12:42.860 --> 00:12:44.570 align:middle line:84% they separately came up with the idea 00:12:44.570 --> 00:12:46.550 align:middle line:84% that maybe to make sense of these patterns 00:12:46.550 --> 00:12:50.660 align:middle line:84% that these kaons and hyperon particles, for example, were 00:12:50.660 --> 00:12:53.970 align:middle line:84% always produced together or, in a similar way, 00:12:53.970 --> 00:12:57.290 align:middle line:84% the sigma particles were produced with kaons 00:12:57.290 --> 00:13:00.950 align:middle line:84% but not separate from these kind of other strange particles. 00:13:00.950 --> 00:13:04.850 align:middle line:84% Maybe there's a kind of charge called strangeness, 00:13:04.850 --> 00:13:08.420 align:middle line:84% and some particles were strangeness neutral, 00:13:08.420 --> 00:13:11.180 align:middle line:84% like the familiar proton, neutron, and pion, 00:13:11.180 --> 00:13:14.215 align:middle line:84% whereas other particles carried 1 or minus 1 units 00:13:14.215 --> 00:13:14.840 align:middle line:90% of strangeness. 00:13:14.840 --> 00:13:17.630 align:middle line:84% And strangeness had to be conserved. 00:13:17.630 --> 00:13:21.380 align:middle line:84% And so it was out of the way to make sense of these patterns. 00:13:21.380 --> 00:13:24.070 align:middle line:84% And so a few years after that, the better part of a decade 00:13:24.070 --> 00:13:28.720 align:middle line:84% after that, Gell-Mann and then, again, separately in this case, 00:13:28.720 --> 00:13:32.110 align:middle line:84% a theorist based in Israel named Yuval Ne'eman-- 00:13:32.110 --> 00:13:35.230 align:middle line:84% in 1960, they separately introduced a new way 00:13:35.230 --> 00:13:37.460 align:middle line:84% to try to account for these kind of patterns. 00:13:37.460 --> 00:13:40.330 align:middle line:84% Remember that no one yet is talking about forces or decay 00:13:40.330 --> 00:13:40.930 align:middle line:90% rates. 00:13:40.930 --> 00:13:43.472 align:middle line:84% They're really just trying to say, is there a kind of pattern 00:13:43.472 --> 00:13:46.750 align:middle line:84% to this very large number of new particles? 00:13:46.750 --> 00:13:50.650 align:middle line:84% Are there new ways to make sense of why some particles seem 00:13:50.650 --> 00:13:55.275 align:middle line:84% to appear with others in these large experiments, for example? 00:13:55.275 --> 00:13:56.650 align:middle line:84% So they introduced something that 00:13:56.650 --> 00:13:58.750 align:middle line:90% came to be called hypercharge. 00:13:58.750 --> 00:14:01.840 align:middle line:84% So take a couple of these as yet entirely 00:14:01.840 --> 00:14:04.270 align:middle line:84% hypothetical internal quantum numbers. 00:14:04.270 --> 00:14:06.880 align:middle line:84% They just keep inventing new ones that help 00:14:06.880 --> 00:14:09.130 align:middle line:90% us sort what goes with what. 00:14:09.130 --> 00:14:11.620 align:middle line:84% One of them they called baryon number, 00:14:11.620 --> 00:14:14.020 align:middle line:84% and that applied to many familiar particles 00:14:14.020 --> 00:14:16.660 align:middle line:90% like protons and neutrons. 00:14:16.660 --> 00:14:19.890 align:middle line:84% It would also apply to many of these new strange particles. 00:14:19.890 --> 00:14:24.290 align:middle line:84% So one kind of charge would be baryon number or baryon charge. 00:14:24.290 --> 00:14:26.240 align:middle line:84% Some of those particles would also 00:14:26.240 --> 00:14:28.640 align:middle line:84% carry this so-called strangeness charge. 00:14:28.640 --> 00:14:31.850 align:middle line:84% So the hyperon particles or the sigma particles 00:14:31.850 --> 00:14:34.640 align:middle line:84% would have one unit of baryon charge 00:14:34.640 --> 00:14:36.800 align:middle line:84% and one unit of strangeness charge, 00:14:36.800 --> 00:14:41.060 align:middle line:84% whereas the more familiar, the not strange baryons 00:14:41.060 --> 00:14:42.620 align:middle line:84% would have 1 unit of baryon charge 00:14:42.620 --> 00:14:44.520 align:middle line:84% but 0 units of strange, that kind of thing. 00:14:44.520 --> 00:14:48.590 align:middle line:84% So it's a new way to try to out this large kind of Sudoku 00:14:48.590 --> 00:14:51.260 align:middle line:90% puzzle of new particles. 00:14:51.260 --> 00:14:53.270 align:middle line:84% And then, again, Gell-Mann and Ne'eman 00:14:53.270 --> 00:15:00.050 align:middle line:84% showed in 1960 if you pick up on Heisenberg's notion of isospin, 00:15:00.050 --> 00:15:03.080 align:middle line:84% this internal state that could distinguish, for example, 00:15:03.080 --> 00:15:05.510 align:middle line:84% a proton from a neutron, and then 00:15:05.510 --> 00:15:08.330 align:middle line:84% add in 1/2 of the hypercharge, this new one they had just 00:15:08.330 --> 00:15:10.520 align:middle line:84% basically made up, then you could, again, 00:15:10.520 --> 00:15:13.370 align:middle line:84% start making kind of sense of these patterns 00:15:13.370 --> 00:15:14.280 align:middle line:90% of these particles. 00:15:14.280 --> 00:15:17.930 align:middle line:84% So if you go back to the familiar nucleons, 00:15:17.930 --> 00:15:19.880 align:middle line:84% the proton, according to Heisenberg, 00:15:19.880 --> 00:15:22.550 align:middle line:84% we could assign as this isospin-up state, 00:15:22.550 --> 00:15:24.950 align:middle line:90% so its isospin is plus 1/2. 00:15:24.950 --> 00:15:28.280 align:middle line:84% It has one unit of baryon charge but is not strange, 00:15:28.280 --> 00:15:30.590 align:middle line:90% so its hypercharge is plus 1. 00:15:30.590 --> 00:15:32.480 align:middle line:84% Plug in over here-- you see, well, then you'd 00:15:32.480 --> 00:15:34.670 align:middle line:90% expect any appropriate units. 00:15:34.670 --> 00:15:38.030 align:middle line:84% It should have 1 positive unit of electric charge. 00:15:38.030 --> 00:15:42.260 align:middle line:84% Conversely, the other state of that nucleon, the neutron, 00:15:42.260 --> 00:15:45.360 align:middle line:90% would be an isospin-down state. 00:15:45.360 --> 00:15:49.040 align:middle line:84% It also has 1 unit of baryon charge but 0 strangeness, 00:15:49.040 --> 00:15:51.290 align:middle line:84% and so you would expect it to be electrically neutral. 00:15:51.290 --> 00:15:54.530 align:middle line:84% And then you can play that game dozens and dozens of more times 00:15:54.530 --> 00:15:57.890 align:middle line:84% to see, is there a kind of single, self-consistent pattern 00:15:57.890 --> 00:15:59.510 align:middle line:84% with which you could start grouping 00:15:59.510 --> 00:16:02.660 align:middle line:84% like with like among the even less-familiar particles, not 00:16:02.660 --> 00:16:04.760 align:middle line:90% just protons and neutrons? 00:16:04.760 --> 00:16:09.140 align:middle line:84% And so in 1960, Gell-Mann presented these groupings, 00:16:09.140 --> 00:16:12.320 align:middle line:84% these group theory structures, that 00:16:12.320 --> 00:16:15.680 align:middle line:84% seem to make sense of families of particles, 00:16:15.680 --> 00:16:18.980 align:middle line:84% and he could array them by mapping them 00:16:18.980 --> 00:16:22.700 align:middle line:84% along two axes of hypercharge, this new internal quantum 00:16:22.700 --> 00:16:25.220 align:middle line:84% number he'd invented, he and Ne'eman invented, 00:16:25.220 --> 00:16:29.060 align:middle line:84% so hypercharge versus isospin as opposed to other ways one 00:16:29.060 --> 00:16:31.730 align:middle line:84% might try to group particles, electric charge, and mass 00:16:31.730 --> 00:16:32.510 align:middle line:90% and like that. 00:16:32.510 --> 00:16:35.510 align:middle line:84% He was finding order by placing them 00:16:35.510 --> 00:16:38.180 align:middle line:84% in these abstract kind of mathematical spaces, 00:16:38.180 --> 00:16:42.200 align:middle line:84% grouping them based on their hypercharge and isospin, 00:16:42.200 --> 00:16:44.780 align:middle line:84% and he found these very distinct patterns. 00:16:44.780 --> 00:16:47.000 align:middle line:84% Some of them were eightfold patterns. 00:16:47.000 --> 00:16:49.460 align:middle line:84% Others were tenfold or decuplets. 00:16:49.460 --> 00:16:52.160 align:middle line:84% So he labeled this first one the eightfold way. 00:16:52.160 --> 00:16:53.410 align:middle line:90% He was being very playful. 00:16:53.410 --> 00:16:55.160 align:middle line:84% Actually, he was just kind of showing off. 00:16:55.160 --> 00:16:58.130 align:middle line:84% Gell-Mann loved to the very end of his life-- 00:16:58.130 --> 00:16:59.600 align:middle line:90% he only died a few years ago. 00:16:59.600 --> 00:17:01.590 align:middle line:90% He lived quite a long life. 00:17:01.590 --> 00:17:04.220 align:middle line:84% He loved showing off his knowledge 00:17:04.220 --> 00:17:06.768 align:middle line:84% of many languages, many literatures and cultures 00:17:06.768 --> 00:17:07.310 align:middle line:90% of the world. 00:17:07.310 --> 00:17:09.240 align:middle line:84% We'll see more examples of that even today. 00:17:09.240 --> 00:17:11.480 align:middle line:84% So he very playfully borrowed the term 00:17:11.480 --> 00:17:14.900 align:middle line:84% for this from the Buddhists' eight-step path 00:17:14.900 --> 00:17:17.569 align:middle line:84% to achieving Nirvana, which, for a long, long time, 00:17:17.569 --> 00:17:19.369 align:middle line:84% had been known in English translation 00:17:19.369 --> 00:17:22.000 align:middle line:90% at least as the eightfold way. 00:17:22.000 --> 00:17:24.640 align:middle line:84% Gell-Mann said that's just as important or just 00:17:24.640 --> 00:17:30.190 align:middle line:84% as evocative a term for these eightfold particle 00:17:30.190 --> 00:17:33.910 align:middle line:84% groupings he was finding when he mapped 00:17:33.910 --> 00:17:38.210 align:middle line:84% some of these nuclear particles in hypercharge isospin space. 00:17:38.210 --> 00:17:40.300 align:middle line:84% So here are the familiar neutron and proton. 00:17:40.300 --> 00:17:41.652 align:middle line:90% They have 0 strangeness charge. 00:17:41.652 --> 00:17:44.110 align:middle line:84% They're not strange because they've been known for a while. 00:17:44.110 --> 00:17:45.670 align:middle line:90% Here are the sigma particles. 00:17:45.670 --> 00:17:47.620 align:middle line:90% Here's the neutral hyperon. 00:17:47.620 --> 00:17:51.490 align:middle line:84% Here are other particles with 2 units of strangeness charge 00:17:51.490 --> 00:17:53.090 align:middle line:90% instead of 1 and so on. 00:17:53.090 --> 00:17:55.630 align:middle line:84% So we call that the eightfold way. 00:17:55.630 --> 00:17:57.850 align:middle line:84% He went on to show there are other groupings, 00:17:57.850 --> 00:18:01.990 align:middle line:84% other very specific group theoretic structures, again, 00:18:01.990 --> 00:18:05.980 align:middle line:84% in this abstract hypercharge isospin plane, which 00:18:05.980 --> 00:18:07.720 align:middle line:84% no one in their right mind otherwise 00:18:07.720 --> 00:18:09.160 align:middle line:90% would have thought to use. 00:18:09.160 --> 00:18:11.590 align:middle line:84% But there were some times not just eightfold patterns 00:18:11.590 --> 00:18:15.340 align:middle line:84% but tenfold, decuplets, again, of particles 00:18:15.340 --> 00:18:19.000 align:middle line:84% that seemed to be kind of associated with each other 00:18:19.000 --> 00:18:22.340 align:middle line:84% based on their varying hypercharge and isospin. 00:18:22.340 --> 00:18:26.300 align:middle line:84% And this involves, again, some of the both unstrange and 00:18:26.300 --> 00:18:27.630 align:middle line:90% strange particles. 00:18:27.630 --> 00:18:30.260 align:middle line:84% So the delta particles seem to have 0 strangeness. 00:18:30.260 --> 00:18:33.170 align:middle line:84% That meant they weren't subject to that associated production. 00:18:33.170 --> 00:18:35.340 align:middle line:84% You didn't have to make a separate strange particle 00:18:35.340 --> 00:18:35.840 align:middle line:90% with them. 00:18:35.840 --> 00:18:39.350 align:middle line:84% So they seemed to not carry conserved strangeness charge. 00:18:39.350 --> 00:18:41.960 align:middle line:84% Here are the sigmas, the cascades, and so on. 00:18:41.960 --> 00:18:44.090 align:middle line:84% What he pointed out this time was 00:18:44.090 --> 00:18:46.850 align:middle line:84% that there seemed to be such a pristine geometrical pattern 00:18:46.850 --> 00:18:48.110 align:middle line:90% and yet a gap. 00:18:48.110 --> 00:18:52.340 align:middle line:84% There was a missing, an as yet unknown particle 00:18:52.340 --> 00:18:56.660 align:middle line:84% that Gell-Mann suggested maybe really is out there after all. 00:18:56.660 --> 00:18:59.180 align:middle line:84% To fill out this very clear pattern, 00:18:59.180 --> 00:19:03.230 align:middle line:84% he thought maybe there should be a particle with minus 2 units 00:19:03.230 --> 00:19:06.410 align:middle line:84% of hypercharge but 0 units of isospin, 00:19:06.410 --> 00:19:09.770 align:middle line:84% that red circle here, the kind of gap 00:19:09.770 --> 00:19:12.140 align:middle line:84% in his otherwise very clear mapping. 00:19:12.140 --> 00:19:15.290 align:middle line:84% Moreover, for this decuplet, he went further 00:19:15.290 --> 00:19:17.720 align:middle line:84% and found that there was a kind of pattern 00:19:17.720 --> 00:19:21.360 align:middle line:84% in the masses between these rows as you went down 00:19:21.360 --> 00:19:23.130 align:middle line:90% in hypercharge. 00:19:23.130 --> 00:19:27.150 align:middle line:84% So the delta particles all had a mass of roughly 1,200 00:19:27.150 --> 00:19:30.960 align:middle line:84% million electron volts, whereas the sigma particles had 00:19:30.960 --> 00:19:34.740 align:middle line:84% around 1,380, a gap of roughly 150 MeV. 00:19:34.740 --> 00:19:41.280 align:middle line:84% The cascade particles were another 150 MeV heavier still. 00:19:41.280 --> 00:19:44.370 align:middle line:84% So Gell-Mann went even further, and in fact, very dramatically, 00:19:44.370 --> 00:19:46.470 align:middle line:84% in the middle of a conference with a number 00:19:46.470 --> 00:19:48.900 align:middle line:84% of experimental physicists in 1962, 00:19:48.900 --> 00:19:51.240 align:middle line:84% he basically stood up and challenged them. 00:19:51.240 --> 00:19:53.440 align:middle line:84% He said, I bet if you look hard enough, 00:19:53.440 --> 00:19:56.130 align:middle line:84% you'll find a particle with exactly these properties, 00:19:56.130 --> 00:19:59.340 align:middle line:84% with hypercharge minus 2, isospin 0. 00:19:59.340 --> 00:20:03.150 align:middle line:84% And he could then work out it should have 1 unit of minus 00:20:03.150 --> 00:20:04.110 align:middle line:90% electric charge. 00:20:04.110 --> 00:20:06.150 align:middle line:84% And he could even give a mass estimate. 00:20:06.150 --> 00:20:09.570 align:middle line:84% He said it would probably be about 150 MeV heavier 00:20:09.570 --> 00:20:11.790 align:middle line:84% than the cascades, and sure enough, 00:20:11.790 --> 00:20:15.023 align:middle line:84% about not quite two years later, experimentalists 00:20:15.023 --> 00:20:17.190 align:middle line:84% came back and announced that they had actually found 00:20:17.190 --> 00:20:19.950 align:middle line:84% evidence for exactly that particle, almost exactly 00:20:19.950 --> 00:20:21.630 align:middle line:84% with the properties that Gell-Mann had 00:20:21.630 --> 00:20:23.370 align:middle line:90% very dramatically predicted. 00:20:23.370 --> 00:20:26.400 align:middle line:84% The mass was extraordinarily close to Gell-Mann's kind 00:20:26.400 --> 00:20:27.900 align:middle line:84% of back-of-the-envelope estimate. 00:20:27.900 --> 00:20:30.660 align:middle line:84% It had indeed 1 unit of negative electric charge. 00:20:30.660 --> 00:20:33.720 align:middle line:84% It seemed to be consistent with a strangeness of minus 3 00:20:33.720 --> 00:20:36.720 align:middle line:84% and so on, the so-called omega minus particle. 00:20:36.720 --> 00:20:38.290 align:middle line:90% That was a big, big deal. 00:20:38.290 --> 00:20:40.470 align:middle line:84% In fact, it was such a big deal that Gell-Mann 00:20:40.470 --> 00:20:44.400 align:middle line:84% was awarded the Nobel Prize only five years later for all 00:20:44.400 --> 00:20:48.090 align:middle line:84% of these efforts to bring a kind of classification order 00:20:48.090 --> 00:20:50.220 align:middle line:84% to what it seemed like an orderless, just 00:20:50.220 --> 00:20:53.010 align:middle line:84% this kind of sludge of new particles. 00:20:53.010 --> 00:20:55.800 align:middle line:84% This kind of development impressed 00:20:55.800 --> 00:20:59.244 align:middle line:84% many of his colleagues very, very quickly in real time. 00:20:59.244 --> 00:21:00.744 align:middle line:84% So let me pause there and ask if you 00:21:00.744 --> 00:21:03.130 align:middle line:84% have any questions on that classification approach. 00:21:03.130 --> 00:21:12.260 align:middle line:90% 00:21:12.260 --> 00:21:13.940 align:middle line:90% I really find it astonishing. 00:21:13.940 --> 00:21:16.700 align:middle line:84% I don't know if any fans of puzzles like Sudoku 00:21:16.700 --> 00:21:19.820 align:middle line:84% or other highly constrained, "you can go here but not there" 00:21:19.820 --> 00:21:21.860 align:middle line:84% kinds of pencil and paper puzzles 00:21:21.860 --> 00:21:25.010 align:middle line:84% like a crossword puzzle, but he's doing that now 00:21:25.010 --> 00:21:29.450 align:middle line:84% with 300 seemingly elementary nuclear particles and finding, 00:21:29.450 --> 00:21:32.600 align:middle line:84% I think, these very abstract or at least not the most obvious 00:21:32.600 --> 00:21:33.710 align:middle line:90% properties to focus on. 00:21:33.710 --> 00:21:36.510 align:middle line:84% But he just sort of rearranged things 00:21:36.510 --> 00:21:39.910 align:middle line:84% then began finding order where few had found it before, 00:21:39.910 --> 00:21:42.690 align:middle line:90% so it's pretty cool. 00:21:42.690 --> 00:21:46.300 align:middle line:84% I should say Gell-Mann did his PhD at MIT, 00:21:46.300 --> 00:21:49.170 align:middle line:84% so we should be proud of him for that, I guess, 00:21:49.170 --> 00:21:51.000 align:middle line:90% so a local fella. 00:21:51.000 --> 00:21:52.630 align:middle line:90% He had been in graduate school-- 00:21:52.630 --> 00:21:54.630 align:middle line:84% I think he finished in 1951, so this 00:21:54.630 --> 00:21:57.000 align:middle line:84% is some work he was doing pretty soon after his PhD. 00:21:57.000 --> 00:21:59.777 align:middle line:90% 00:21:59.777 --> 00:22:00.860 align:middle line:90% Any other questions on it? 00:22:00.860 --> 00:22:03.847 align:middle line:90% If not, I'm happy to press on. 00:22:03.847 --> 00:22:05.305 align:middle line:84% Oh, here's a question from Tiffany. 00:22:05.305 --> 00:22:08.100 align:middle line:90% 00:22:08.100 --> 00:22:08.880 align:middle line:90% Oh, yes, right. 00:22:08.880 --> 00:22:09.720 align:middle line:90% So that's true. 00:22:09.720 --> 00:22:11.732 align:middle line:84% Some of you might know Sabine Hossenfelder 00:22:11.732 --> 00:22:12.690 align:middle line:90% is a colleague of mine. 00:22:12.690 --> 00:22:13.920 align:middle line:90% I know Sabine pretty well. 00:22:13.920 --> 00:22:16.253 align:middle line:84% She wrote a really interesting popular book about a year 00:22:16.253 --> 00:22:20.700 align:middle line:84% or two ago, not too long ago, called Lost in Math, 00:22:20.700 --> 00:22:24.120 align:middle line:84% and she's become a really very trenchant and outspoken critic 00:22:24.120 --> 00:22:28.650 align:middle line:84% of developments like string theory in high-energy physics. 00:22:28.650 --> 00:22:32.880 align:middle line:84% And so her argument in brief is that sometimes, 00:22:32.880 --> 00:22:35.820 align:middle line:84% in more recent years, physicists have maybe become too enamored 00:22:35.820 --> 00:22:37.860 align:middle line:84% of these kinds of symmetry arguments, 00:22:37.860 --> 00:22:40.380 align:middle line:84% that just because there's a pattern that nature must have 00:22:40.380 --> 00:22:41.940 align:middle line:84% taken advantage of what we consider 00:22:41.940 --> 00:22:44.100 align:middle line:84% a beautiful symmetrical pattern, and could 00:22:44.100 --> 00:22:45.770 align:middle line:84% we have a little more input than that? 00:22:45.770 --> 00:22:48.570 align:middle line:84% And I should say, back in Gell-Mann's day, the early part 00:22:48.570 --> 00:22:52.080 align:middle line:84% of his career, he could make an announcement in 1962 00:22:52.080 --> 00:22:54.780 align:middle line:84% with real confidence that his colleagues could actually 00:22:54.780 --> 00:22:56.340 align:middle line:84% try to find empirical evidence that 00:22:56.340 --> 00:23:00.320 align:middle line:84% was consistent with it within a short while, let alone 00:23:00.320 --> 00:23:01.620 align:middle line:90% 50 years later. 00:23:01.620 --> 00:23:04.400 align:middle line:84% And so the kind of symmetry argument 00:23:04.400 --> 00:23:06.440 align:middle line:84% was getting a lot of feedback, let's say, 00:23:06.440 --> 00:23:10.280 align:middle line:84% positive reinforcement in the '60s because these experiments 00:23:10.280 --> 00:23:12.350 align:middle line:84% were feasible, and many, many places could do it. 00:23:12.350 --> 00:23:17.090 align:middle line:84% And it actually wasn't based only 00:23:17.090 --> 00:23:21.200 align:middle line:84% on mathematical beauty but a kind of pattern 00:23:21.200 --> 00:23:24.410 align:middle line:84% that maybe one could then have a kind of empirical dialogue 00:23:24.410 --> 00:23:25.070 align:middle line:90% about. 00:23:25.070 --> 00:23:27.830 align:middle line:84% And I think Sabine's point, quite well taken, 00:23:27.830 --> 00:23:31.040 align:middle line:84% is that, more recently, the energy scales involved 00:23:31.040 --> 00:23:32.690 align:middle line:84% in the hypothetical string theories 00:23:32.690 --> 00:23:36.140 align:middle line:84% and so on are so far removed from earthbound experiments 00:23:36.140 --> 00:23:39.800 align:middle line:84% that we've lost that kind of back-and-forth kind 00:23:39.800 --> 00:23:42.290 align:middle line:84% of conversation, so to speak, between theory 00:23:42.290 --> 00:23:43.950 align:middle line:84% and experiment in a lot of these ideas 00:23:43.950 --> 00:23:46.340 align:middle line:84% so that what seems to some people 00:23:46.340 --> 00:23:48.980 align:middle line:84% like beautiful mathematics seems to others 00:23:48.980 --> 00:23:51.320 align:middle line:90% like groundless speculation. 00:23:51.320 --> 00:23:54.810 align:middle line:84% And that seems to be a different situation than in the mid '60s. 00:23:54.810 --> 00:23:57.080 align:middle line:84% So she's a very good writer, and I 00:23:57.080 --> 00:24:00.050 align:middle line:84% think she's a great kind of popular science writer anyway, 00:24:00.050 --> 00:24:02.550 align:middle line:84% let alone that she has, I think, really interesting insights 00:24:02.550 --> 00:24:06.600 align:middle line:84% into some of the sociology of more recent high-energy theory. 00:24:06.600 --> 00:24:07.680 align:middle line:90% Yeah. 00:24:07.680 --> 00:24:08.820 align:middle line:90% Good. 00:24:08.820 --> 00:24:10.300 align:middle line:90% Any other questions or comments? 00:24:10.300 --> 00:24:13.890 align:middle line:84% If not, I'll jump in to the next part. 00:24:13.890 --> 00:24:14.760 align:middle line:90% OK. 00:24:14.760 --> 00:24:18.360 align:middle line:84% Well, let's see what people do with all this crazy symmetry 00:24:18.360 --> 00:24:20.870 align:middle line:90% stuff. 00:24:20.870 --> 00:24:23.810 align:middle line:84% So I mentioned that in 1962, Gell-Mann 00:24:23.810 --> 00:24:27.442 align:middle line:84% made this kind of very dramatic prediction at a conference, 00:24:27.442 --> 00:24:29.150 align:middle line:84% saying, go look for the omega minus right 00:24:29.150 --> 00:24:31.460 align:middle line:84% where I told you to look for it, essentially. 00:24:31.460 --> 00:24:34.700 align:middle line:84% Not long after that, very early in calendar 00:24:34.700 --> 00:24:39.090 align:middle line:84% year 1964, Gell-Mann and then, again, independently, 00:24:39.090 --> 00:24:42.080 align:middle line:84% another much younger theorist, George Zweig, 00:24:42.080 --> 00:24:47.090 align:middle line:84% separately suggested, proposed a new hypothetical way 00:24:47.090 --> 00:24:51.707 align:middle line:84% to bring order to these kind of hypercharged isospin groupings 00:24:51.707 --> 00:24:54.290 align:middle line:84% that Gell-Mann by this point had been doing for quite a while. 00:24:54.290 --> 00:24:57.050 align:middle line:84% So Gell-Mann's article was received at the journal, 00:24:57.050 --> 00:25:00.860 align:middle line:84% you can see here, received on the 4th of January 1964. 00:25:00.860 --> 00:25:03.680 align:middle line:84% It was published three and a half weeks later. 00:25:03.680 --> 00:25:06.260 align:middle line:84% It was pretty quick to get through peer review. 00:25:06.260 --> 00:25:09.110 align:middle line:84% George Zweig's preprint from CERN-- he was at that point 00:25:09.110 --> 00:25:11.270 align:middle line:90% a very new postdoc at CERN. 00:25:11.270 --> 00:25:14.578 align:middle line:84% His preprint was dated the 17th of January 1964. 00:25:14.578 --> 00:25:17.120 align:middle line:84% So they were working separately, and not even two whole weeks 00:25:17.120 --> 00:25:20.580 align:middle line:84% elapsed between each of them committing this to paper. 00:25:20.580 --> 00:25:24.840 align:middle line:84% And they came up with a remarkably similar set of ideas 00:25:24.840 --> 00:25:28.920 align:middle line:84% that maybe actually the order that Gell-Mann in Yuval 00:25:28.920 --> 00:25:31.560 align:middle line:84% Ne'eman and others have been finding for a couple of years, 00:25:31.560 --> 00:25:35.220 align:middle line:84% eightfold way and the decuplets and so on-- 00:25:35.220 --> 00:25:39.900 align:middle line:84% maybe that was due to an even more deeper underlying symmetry 00:25:39.900 --> 00:25:42.000 align:middle line:84% involving actually a very small number 00:25:42.000 --> 00:25:45.420 align:middle line:84% of truly elementary particles or constituents. 00:25:45.420 --> 00:25:47.400 align:middle line:84% Gell-Mann very famously called them quarks. 00:25:47.400 --> 00:25:48.900 align:middle line:84% You see them even in quotation marks 00:25:48.900 --> 00:25:52.980 align:middle line:84% here on the first page of his-- it was only a two-page article. 00:25:52.980 --> 00:25:54.780 align:middle line:90% Zweig had called them aces. 00:25:54.780 --> 00:25:57.060 align:middle line:84% These were really very, very similar ideas, 00:25:57.060 --> 00:26:01.260 align:middle line:84% and it is that all the patterns on these hypercharged isospin 00:26:01.260 --> 00:26:05.430 align:middle line:84% plots among the then-known nuclear particles 00:26:05.430 --> 00:26:09.960 align:middle line:84% could be reproduced under a very simple-sounding assumption 00:26:09.960 --> 00:26:12.270 align:middle line:84% that there existed only three types 00:26:12.270 --> 00:26:15.330 align:middle line:84% of elementary constituents, quarks or aces. 00:26:15.330 --> 00:26:18.690 align:middle line:84% And if you were very careful in assigning them very specific 00:26:18.690 --> 00:26:22.770 align:middle line:84% values for hypercharge, isospin, strangeness, and all the rest 00:26:22.770 --> 00:26:24.840 align:middle line:84% that you could reproduce those group theory 00:26:24.840 --> 00:26:28.210 align:middle line:84% patterns of eightfold patterns and tenfold patterns. 00:26:28.210 --> 00:26:29.790 align:middle line:84% So Gell-Mann, as I mentioned earlier, 00:26:29.790 --> 00:26:32.657 align:middle line:84% loved to show off his wide-ranging knowledge 00:26:32.657 --> 00:26:34.240 align:middle line:84% of languages and literature and so on. 00:26:34.240 --> 00:26:37.080 align:middle line:84% So among this very short list-- this is the entire reference 00:26:37.080 --> 00:26:40.740 align:middle line:84% list, eight references only for this entire two-page article, 00:26:40.740 --> 00:26:43.800 align:middle line:84% number six of which is to James Joyce's very famous 00:26:43.800 --> 00:26:46.800 align:middle line:84% and famously obtuse novel Finnegans Wake, 00:26:46.800 --> 00:26:49.470 align:middle line:84% where this line many of you, by now, might recognize-- 00:26:49.470 --> 00:26:52.047 align:middle line:90% "Three quarks for Muster Mark!" 00:26:52.047 --> 00:26:53.880 align:middle line:84% If that doesn't make any sense to you, good. 00:26:53.880 --> 00:26:55.650 align:middle line:90% It's all nonsensical. 00:26:55.650 --> 00:26:59.160 align:middle line:84% The Irish novelist James Joyce loved basically making up 00:26:59.160 --> 00:27:03.630 align:middle line:84% his own words and not bothering to, for example, define them, 00:27:03.630 --> 00:27:05.885 align:middle line:84% and "quarks" was one of these kind of made-up names. 00:27:05.885 --> 00:27:07.260 align:middle line:84% He kind of liked the sound of it. 00:27:07.260 --> 00:27:09.100 align:middle line:84% Well, Gell-Mann liked the sound of that, 00:27:09.100 --> 00:27:14.310 align:middle line:84% so he borrowed it to label this new hypothetical constituent. 00:27:14.310 --> 00:27:16.140 align:middle line:84% Zweig stuck with "aces," which may 00:27:16.140 --> 00:27:18.330 align:middle line:84% be a little more familiar but still kind 00:27:18.330 --> 00:27:21.520 align:middle line:90% of a nonsensical word. 00:27:21.520 --> 00:27:23.890 align:middle line:84% So they could go through this exercise 00:27:23.890 --> 00:27:26.790 align:middle line:84% and assign baryon charge, strangeness charge, 00:27:26.790 --> 00:27:28.680 align:middle line:84% and therefore, those two together 00:27:28.680 --> 00:27:30.060 align:middle line:90% would give you hypercharge. 00:27:30.060 --> 00:27:35.190 align:middle line:84% Likewise, just kind of intuit or posit that the two of those 00:27:35.190 --> 00:27:39.020 align:middle line:84% quarks would be an isospin doublet. 00:27:39.020 --> 00:27:40.020 align:middle line:90% So one would be spin up. 00:27:40.020 --> 00:27:41.610 align:middle line:90% One would be isospin down. 00:27:41.610 --> 00:27:43.800 align:middle line:84% The third one would be what's called an isospin 00:27:43.800 --> 00:27:46.230 align:middle line:84% singlet, which means it has 0 isospin, 00:27:46.230 --> 00:27:47.870 align:middle line:84% and if you do that, then again, you 00:27:47.870 --> 00:27:50.220 align:middle line:84% can reproduce the electric charge assignments. 00:27:50.220 --> 00:27:53.010 align:middle line:84% So these became known as the up quark, the down quark, 00:27:53.010 --> 00:27:54.540 align:middle line:90% and the strange quark. 00:27:54.540 --> 00:27:57.510 align:middle line:84% And the idea was that any of those so-called strange nuclear 00:27:57.510 --> 00:28:01.380 align:middle line:84% particles, the kaons, the hyperons, the sigmas, and all 00:28:01.380 --> 00:28:05.340 align:middle line:84% that, must include at least one constituent strange quark. 00:28:05.340 --> 00:28:07.860 align:middle line:84% That's what gave it its strangeness charge. 00:28:07.860 --> 00:28:10.650 align:middle line:84% So it has strangeness charge minus 1, 00:28:10.650 --> 00:28:14.280 align:middle line:84% whereas particles that were made only from up and down quarks 00:28:14.280 --> 00:28:17.560 align:middle line:84% would have 0 strangeness charge like protons, neutrons, 00:28:17.560 --> 00:28:18.628 align:middle line:90% and pions and so on. 00:28:18.628 --> 00:28:20.170 align:middle line:84% So with those assignments, they could 00:28:20.170 --> 00:28:22.150 align:middle line:84% go through the known particles, even 00:28:22.150 --> 00:28:25.210 align:middle line:84% the more exotic or more recently found particles 00:28:25.210 --> 00:28:28.270 align:middle line:84% like the omega minus, and make these self-consistent 00:28:28.270 --> 00:28:31.090 align:middle line:84% assignments always on either triplets 00:28:31.090 --> 00:28:34.150 align:middle line:84% of these constituent quarks if they were baryons, 00:28:34.150 --> 00:28:35.830 align:middle line:84% like protons, neutrons, or the omega 00:28:35.830 --> 00:28:38.860 align:middle line:84% minus, or a quark-antiquark pair, 00:28:38.860 --> 00:28:42.250 align:middle line:84% bound states of only two quarks, if they were mesons, 00:28:42.250 --> 00:28:44.390 align:middle line:90% like pions or kaons or others. 00:28:44.390 --> 00:28:47.800 align:middle line:84% So it was, again, even more a kind of-- 00:28:47.800 --> 00:28:50.140 align:middle line:84% it went from 2D Sudoku to 3D Sudoku, 00:28:50.140 --> 00:28:54.070 align:middle line:84% basically, even more tightly constrained assignments 00:28:54.070 --> 00:28:57.790 align:middle line:84% for these three constituent hypothetical entities, 00:28:57.790 --> 00:29:00.940 align:middle line:84% and they could reproduce all the kind of grouping structure 00:29:00.940 --> 00:29:04.600 align:middle line:84% in these abstract spaces like hypercharge, isospin. 00:29:04.600 --> 00:29:07.180 align:middle line:84% And there seemed to be a unique assignment for every one 00:29:07.180 --> 00:29:09.770 align:middle line:90% of those particles. 00:29:09.770 --> 00:29:13.870 align:middle line:84% And so this looked like a remarkable kind of efficiency. 00:29:13.870 --> 00:29:18.490 align:middle line:84% Instead of having 100, let alone 300, seemingly elementary 00:29:18.490 --> 00:29:21.310 align:middle line:84% nuclear particles, you maybe only had to worry about three 00:29:21.310 --> 00:29:23.350 align:middle line:84% of them and then all these combinations. 00:29:23.350 --> 00:29:25.540 align:middle line:84% It looked more like the periodic table 00:29:25.540 --> 00:29:28.660 align:middle line:84% from chemistry where there are on the order of 100 chemical 00:29:28.660 --> 00:29:31.150 align:middle line:84% elements, but based on developments 00:29:31.150 --> 00:29:35.020 align:middle line:84% from the early 20th century, it looked like only a small number 00:29:35.020 --> 00:29:37.810 align:middle line:84% of elementary constituents whose combinations 00:29:37.810 --> 00:29:41.350 align:middle line:84% would yield 100 or more distinct chemical elements. 00:29:41.350 --> 00:29:45.500 align:middle line:84% It was that kind of move in the classify and simplify. 00:29:45.500 --> 00:29:50.650 align:middle line:84% So with these two papers written early in 1964, 00:29:50.650 --> 00:29:53.620 align:middle line:84% the idea was to put forward these very specific assignments 00:29:53.620 --> 00:29:57.700 align:middle line:84% for three constituent pieces within those nuclear particles. 00:29:57.700 --> 00:30:00.760 align:middle line:84% Now, this raised new questions, and it was not 00:30:00.760 --> 00:30:03.800 align:middle line:84% unfamiliar even to Gell-Mann and Zweig themselves. 00:30:03.800 --> 00:30:05.650 align:middle line:84% The first one and maybe the most obvious 00:30:05.650 --> 00:30:09.040 align:middle line:84% is over here in this orange column, which maybe 00:30:09.040 --> 00:30:10.640 align:middle line:90% gave some of you pause already. 00:30:10.640 --> 00:30:13.090 align:middle line:84% If not, I'll draw your attention to it. 00:30:13.090 --> 00:30:17.290 align:middle line:84% The suggestion seemed to be that these new particles of nature, 00:30:17.290 --> 00:30:19.180 align:middle line:84% the up, down, and strange quarks, 00:30:19.180 --> 00:30:21.940 align:middle line:84% would carry fractional electric charge. 00:30:21.940 --> 00:30:24.370 align:middle line:84% The up quark would have plus 2/3 unit 00:30:24.370 --> 00:30:26.950 align:middle line:84% of charge in the unit of, say, the electron. 00:30:26.950 --> 00:30:29.050 align:middle line:84% Both the down and the strange quarks 00:30:29.050 --> 00:30:32.050 align:middle line:84% would have minus 1/3 units of electric charge, 00:30:32.050 --> 00:30:34.840 align:middle line:84% and that rightly should give people pause. 00:30:34.840 --> 00:30:37.900 align:middle line:84% It certainly gave the authors of these ideas pause. 00:30:37.900 --> 00:30:41.050 align:middle line:84% There had been no compelling experimental evidence 00:30:41.050 --> 00:30:43.660 align:middle line:84% for half a century at that point for 00:30:43.660 --> 00:30:45.400 align:middle line:90% fractional electric charges. 00:30:45.400 --> 00:30:48.670 align:middle line:84% There have been early, kind of ambiguous evidence 00:30:48.670 --> 00:30:51.820 align:middle line:84% in the early years of the 1900s and 1910s. 00:30:51.820 --> 00:30:55.600 align:middle line:84% It triggered, actually, some famously bitter fights 00:30:55.600 --> 00:30:59.290 align:middle line:84% among physicists in the early 1910s about 00:30:59.290 --> 00:31:02.980 align:middle line:84% whether labs had found evidence for fractional electric charges 00:31:02.980 --> 00:31:03.550 align:middle line:90% or not. 00:31:03.550 --> 00:31:07.090 align:middle line:84% By the 1960s, that had seemed to be very well settled, 00:31:07.090 --> 00:31:08.330 align:middle line:90% and the answer was no. 00:31:08.330 --> 00:31:09.950 align:middle line:84% There was no compelling evidence, 00:31:09.950 --> 00:31:13.550 align:middle line:84% despite half a century of earnest experimental searching, 00:31:13.550 --> 00:31:15.470 align:middle line:84% no evidence for fractional charges, 00:31:15.470 --> 00:31:20.120 align:middle line:84% and now they were saying that all of nature, all 00:31:20.120 --> 00:31:23.330 align:middle line:84% these nuclear particles, every constituent inside even 00:31:23.330 --> 00:31:26.090 align:middle line:84% very humdrum atoms like hydrogen and helium, 00:31:26.090 --> 00:31:28.050 align:middle line:84% is somehow teeming with particles 00:31:28.050 --> 00:31:29.300 align:middle line:90% of fractional electric charge. 00:31:29.300 --> 00:31:31.280 align:middle line:84% That seems hard to square with the evidence. 00:31:31.280 --> 00:31:32.030 align:middle line:90% So they knew that. 00:31:32.030 --> 00:31:35.060 align:middle line:84% They wrote about that, as I'll mention more in a moment. 00:31:35.060 --> 00:31:37.130 align:middle line:84% There's a little more subtlety if you 00:31:37.130 --> 00:31:40.250 align:middle line:84% look at this great triumph of Gell-Mann's earlier 00:31:40.250 --> 00:31:43.130 align:middle line:84% symmetry-driven approach, the omega minus particle. 00:31:43.130 --> 00:31:45.560 align:middle line:84% That was consistent on this new scheme 00:31:45.560 --> 00:31:48.590 align:middle line:84% with being a bound state of three strange quarks, 00:31:48.590 --> 00:31:49.760 align:middle line:90% s, s, and s. 00:31:49.760 --> 00:31:52.520 align:middle line:84% But that now raises a little more subtle question 00:31:52.520 --> 00:31:56.270 align:middle line:84% for quantum theory based on the Pauli exclusion principle, 00:31:56.270 --> 00:31:59.420 align:middle line:84% which we looked at together in this class a few weeks ago. 00:31:59.420 --> 00:32:02.360 align:middle line:84% The idea is that if you have spin-1/2 particles-- 00:32:02.360 --> 00:32:05.180 align:middle line:84% and that these quarks were assumed to be spin 1/2 00:32:05.180 --> 00:32:08.030 align:middle line:84% in the angular momentum kind of spin-- 00:32:08.030 --> 00:32:13.640 align:middle line:84% then Pauli's exclusion principle forbids having any two spin-1/2 00:32:13.640 --> 00:32:17.040 align:middle line:84% particles in the identical quantum state at the same time. 00:32:17.040 --> 00:32:20.780 align:middle line:84% So if you're assigning all these internal quantum numbers 00:32:20.780 --> 00:32:22.760 align:middle line:84% to these particles, how could you possibly 00:32:22.760 --> 00:32:25.520 align:middle line:84% have three strange quarks bound together? 00:32:25.520 --> 00:32:28.790 align:middle line:84% That means at least two of them must have all these assignments 00:32:28.790 --> 00:32:31.580 align:middle line:84% plus also either be two of them spin up or two of them 00:32:31.580 --> 00:32:32.600 align:middle line:90% spin down. 00:32:32.600 --> 00:32:35.030 align:middle line:84% The last charge you have to assign 00:32:35.030 --> 00:32:37.100 align:middle line:84% would be the actual angular momentum spin. 00:32:37.100 --> 00:32:41.240 align:middle line:84% That could also either be only plus 1/2 or minus 1/2. 00:32:41.240 --> 00:32:43.880 align:middle line:84% If you have three of them bound together, two of them 00:32:43.880 --> 00:32:45.260 align:middle line:90% are going to overlap with spin. 00:32:45.260 --> 00:32:47.540 align:middle line:84% That seems to violate the exclusion principle. 00:32:47.540 --> 00:32:49.850 align:middle line:84% That's a bit more abstract than no evidence 00:32:49.850 --> 00:32:52.640 align:middle line:84% for fractional charge, but the exclusion principle 00:32:52.640 --> 00:32:56.138 align:middle line:84% had been awfully well tested by the 1960s. 00:32:56.138 --> 00:32:57.680 align:middle line:84% That seemed like a pretty significant 00:32:57.680 --> 00:32:59.750 align:middle line:90% conceptual challenge. 00:32:59.750 --> 00:33:04.550 align:middle line:84% And then third, as I mentioned earlier, in neither Gell-Mann's 00:33:04.550 --> 00:33:08.240 align:middle line:84% nor Zweig's scheme was there any discussion of forces 00:33:08.240 --> 00:33:09.110 align:middle line:90% of dynamics. 00:33:09.110 --> 00:33:11.120 align:middle line:84% Why do these objects interact with each other? 00:33:11.120 --> 00:33:13.640 align:middle line:84% Why do some have very large reaction rates, 00:33:13.640 --> 00:33:16.770 align:middle line:84% others have very small reaction rates, and so on? 00:33:16.770 --> 00:33:18.620 align:middle line:84% So there's still no idea of the kind 00:33:18.620 --> 00:33:21.920 align:middle line:84% of forces that might interact either 00:33:21.920 --> 00:33:24.900 align:middle line:84% between these constituent quarks or as a result of them. 00:33:24.900 --> 00:33:28.560 align:middle line:84% And so as a result of that, Gell-Mann, who was no dummy, 00:33:28.560 --> 00:33:29.300 align:middle line:90% he hedged. 00:33:29.300 --> 00:33:32.630 align:middle line:84% You might have noticed on the very title of his article 00:33:32.630 --> 00:33:35.030 align:middle line:84% that I showed in the previous set of slides 00:33:35.030 --> 00:33:38.930 align:middle line:84% was called "A schematic model of baryons and mesons." 00:33:38.930 --> 00:33:41.480 align:middle line:84% You may remember back to very early this term 00:33:41.480 --> 00:33:43.550 align:middle line:84% we talked about when Einstein introduced 00:33:43.550 --> 00:33:47.600 align:middle line:84% the idea of light quanta, he called it a heuristic 00:33:47.600 --> 00:33:49.400 align:middle line:84% suggestion, and that's very much like what 00:33:49.400 --> 00:33:50.600 align:middle line:90% Gell-Mann is doing here. 00:33:50.600 --> 00:33:54.170 align:middle line:84% It's a schematic model, he announces even in the title. 00:33:54.170 --> 00:33:56.140 align:middle line:84% He goes on to say-- near the very end, 00:33:56.140 --> 00:33:59.630 align:middle line:84% the last main paragraph of this brief article, 00:33:59.630 --> 00:34:03.260 align:middle line:84% Gell-Mann writes, "It's fun to speculate about the way quarks 00:34:03.260 --> 00:34:06.050 align:middle line:84% would behave if they were physical particles instead 00:34:06.050 --> 00:34:08.120 align:middle line:84% of purely mathematical entities." 00:34:08.120 --> 00:34:10.090 align:middle line:90% That's my italics. 00:34:10.090 --> 00:34:13.540 align:middle line:84% A search for stable quarks at the highest energy accelerators 00:34:13.540 --> 00:34:16.239 align:middle line:84% would help to reassure us that they don't exist at all, 00:34:16.239 --> 00:34:19.179 align:middle line:84% reassure us of the nonexistence of real quarks, 00:34:19.179 --> 00:34:21.489 align:middle line:84% precisely because of things like no one's 00:34:21.489 --> 00:34:25.260 align:middle line:84% ever found fractional electric charges, these subtleties 00:34:25.260 --> 00:34:26.510 align:middle line:90% about the exclusion principle. 00:34:26.510 --> 00:34:30.550 align:middle line:84% And so Gell-Mann, in his very first article from 1964, 00:34:30.550 --> 00:34:33.219 align:middle line:84% was not saying, eureka, I found them. 00:34:33.219 --> 00:34:38.330 align:middle line:84% He was saying, it's helpful to think about as if protons, 00:34:38.330 --> 00:34:40.810 align:middle line:84% neutrons, and omega minuses and everything else 00:34:40.810 --> 00:34:42.790 align:middle line:84% were made up of these constituent parts, 00:34:42.790 --> 00:34:45.610 align:middle line:84% but it's really just a kind of mathematical shell game. 00:34:45.610 --> 00:34:48.850 align:middle line:84% These are mathematical classifiers rather than 00:34:48.850 --> 00:34:49.780 align:middle line:90% parts of nature. 00:34:49.780 --> 00:34:52.540 align:middle line:84% That's, I think, a fair reading of what Gell-Mann 00:34:52.540 --> 00:34:54.670 align:middle line:90% is saying in this 1964 article. 00:34:54.670 --> 00:34:57.910 align:middle line:84% Meanwhile, George Zweig, a couple years younger 00:34:57.910 --> 00:35:00.790 align:middle line:84% than Gell-Mann, didn't even get the benefit of the doubt. 00:35:00.790 --> 00:35:03.790 align:middle line:84% So Gell-Mann's paper was rushed into print after three weeks. 00:35:03.790 --> 00:35:05.440 align:middle line:84% Gell-Mann and he wrote two papers 00:35:05.440 --> 00:35:08.920 align:middle line:84% within a few days of each other in January of 1964, 00:35:08.920 --> 00:35:11.168 align:middle line:84% both of which were rejected for publication. 00:35:11.168 --> 00:35:13.210 align:middle line:84% Neither of them even made it through peer review. 00:35:13.210 --> 00:35:16.300 align:middle line:84% He was a very young postdoc at that point, and they said, 00:35:16.300 --> 00:35:18.850 align:middle line:84% this doesn't make any sense at all, no fractional charge, 00:35:18.850 --> 00:35:20.380 align:middle line:90% exclusion principle, and so on. 00:35:20.380 --> 00:35:23.800 align:middle line:84% Plus, everyone seemed to have confidence by this point-- 00:35:23.800 --> 00:35:25.270 align:middle line:90% many people did-- 00:35:25.270 --> 00:35:27.220 align:middle line:84% that actually Geoffrey Chew's approach, 00:35:27.220 --> 00:35:29.710 align:middle line:84% the kind of single self-consistent bootstrap, 00:35:29.710 --> 00:35:31.510 align:middle line:90% was going to save the day. 00:35:31.510 --> 00:35:34.240 align:middle line:84% That had made enormous progress experimentally 00:35:34.240 --> 00:35:36.110 align:middle line:84% by the late '50s, and this seemed 00:35:36.110 --> 00:35:39.910 align:middle line:84% to go exactly counter to that single self-consistent 00:35:39.910 --> 00:35:40.960 align:middle line:90% bootstrap idea. 00:35:40.960 --> 00:35:44.470 align:middle line:84% This was reintroducing or seemed to be reintroducing 00:35:44.470 --> 00:35:48.340 align:middle line:84% the idea of a small set of special particles breaking 00:35:48.340 --> 00:35:50.962 align:middle line:84% that so-called nuclear democracy that 00:35:50.962 --> 00:35:53.170 align:middle line:84% had been so central to Geoffrey Chew's otherwise very 00:35:53.170 --> 00:35:54.230 align:middle line:90% successful program. 00:35:54.230 --> 00:35:56.140 align:middle line:84% So the young postdoc George Zweig 00:35:56.140 --> 00:35:57.790 align:middle line:84% can't even get his papers published. 00:35:57.790 --> 00:36:01.840 align:middle line:84% Gell-Mann is maybe a little wiser, a little bit more long 00:36:01.840 --> 00:36:04.420 align:middle line:84% in his career, and he knows to bracket this 00:36:04.420 --> 00:36:06.665 align:middle line:84% as kind of schematic hypothetical mathematical 00:36:06.665 --> 00:36:09.220 align:middle line:90% [INAUDIBLE]. 00:36:09.220 --> 00:36:12.370 align:middle line:90% So this is now in early 1964. 00:36:12.370 --> 00:36:18.640 align:middle line:84% A few years later, some very dramatic new experiments 00:36:18.640 --> 00:36:20.805 align:middle line:90% were conducted. 00:36:20.805 --> 00:36:22.180 align:middle line:84% Depending on who you ask, they're 00:36:22.180 --> 00:36:24.970 align:middle line:84% either called the SLAC-MIT experiments 00:36:24.970 --> 00:36:26.410 align:middle line:90% or the MIT-SLAC experiments. 00:36:26.410 --> 00:36:29.170 align:middle line:84% I'll let you guess which coast prefers which version. 00:36:29.170 --> 00:36:32.440 align:middle line:84% These were some of the first experiments conducted at what 00:36:32.440 --> 00:36:34.210 align:middle line:84% was then a brand new accelerator-- 00:36:34.210 --> 00:36:36.010 align:middle line:84% you can see it above ground here-- 00:36:36.010 --> 00:36:38.350 align:middle line:90% that now we simply call SLAC. 00:36:38.350 --> 00:36:42.010 align:middle line:84% That stands for the Stanford Linear Accelerator Laboratory. 00:36:42.010 --> 00:36:43.225 align:middle line:90% It is a linear accelerator. 00:36:43.225 --> 00:36:44.350 align:middle line:90% You see that straight line. 00:36:44.350 --> 00:36:47.930 align:middle line:84% It's 3.2 kilometers, basically 2 miles long, 00:36:47.930 --> 00:36:52.300 align:middle line:84% and it's a series of electrostatic voltage 00:36:52.300 --> 00:36:55.360 align:middle line:84% gaps along which you can accelerate electrons 00:36:55.360 --> 00:36:57.160 align:middle line:84% to very, very high energies, basically 00:36:57.160 --> 00:36:59.710 align:middle line:84% up to a significant fraction of the speed of light. 00:36:59.710 --> 00:37:02.140 align:middle line:84% You just keep shoving electrons with very clever 00:37:02.140 --> 00:37:04.510 align:middle line:84% electric fields down a 2-mile track 00:37:04.510 --> 00:37:07.000 align:middle line:84% and smash them into stationary targets. 00:37:07.000 --> 00:37:09.010 align:middle line:84% And so one of the first applications 00:37:09.010 --> 00:37:11.470 align:middle line:84% of this new device built for the same 00:37:11.470 --> 00:37:13.060 align:middle line:84% reasons we've talked about before as 00:37:13.060 --> 00:37:15.190 align:middle line:84% part of the Atomic Energy Commission effort 00:37:15.190 --> 00:37:17.690 align:middle line:84% to get lots and lots of physicists kind 00:37:17.690 --> 00:37:19.400 align:middle line:90% of trained and at the ready-- 00:37:19.400 --> 00:37:22.670 align:middle line:84% not that this would help make weapons for defense, 00:37:22.670 --> 00:37:24.980 align:middle line:84% but it would make people well trained 00:37:24.980 --> 00:37:28.020 align:middle line:84% who could be mobilized in a new Manhattan Project if needed. 00:37:28.020 --> 00:37:32.720 align:middle line:84% This is a huge version of that post-war policy 00:37:32.720 --> 00:37:34.910 align:middle line:84% of making very, very expensive machinery 00:37:34.910 --> 00:37:38.660 align:middle line:84% available for nonmilitary purposes 00:37:38.660 --> 00:37:41.030 align:middle line:84% to keep communities well trained, 00:37:41.030 --> 00:37:44.090 align:middle line:84% even though this device itself is strictly 00:37:44.090 --> 00:37:47.150 align:middle line:90% for kind of peacetime questions. 00:37:47.150 --> 00:37:51.250 align:middle line:84% So SLAC came online in 1966, and one of the very first sets 00:37:51.250 --> 00:37:53.000 align:middle line:84% of experiments to go in there was actually 00:37:53.000 --> 00:37:56.900 align:middle line:84% directed and designed by two members of the MIT physics 00:37:56.900 --> 00:37:59.300 align:middle line:84% department, one of whom is still with us. 00:37:59.300 --> 00:38:01.100 align:middle line:84% He's emeritus Professor Jerry Friedman. 00:38:01.100 --> 00:38:03.230 align:middle line:84% You might have met Professor Friedman. 00:38:03.230 --> 00:38:05.190 align:middle line:84% He comes to colloquia and so on-- 00:38:05.190 --> 00:38:07.890 align:middle line:84% and then Henry Kendall, who passed away some years ago. 00:38:07.890 --> 00:38:10.220 align:middle line:84% The two of them were partners in these experiments. 00:38:10.220 --> 00:38:14.420 align:middle line:84% The idea was to accelerate these electrons to very high energies 00:38:14.420 --> 00:38:17.270 align:middle line:84% and smash them into very proton-rich targets, 00:38:17.270 --> 00:38:19.550 align:middle line:84% so you could actually study very high-energy 00:38:19.550 --> 00:38:21.410 align:middle line:90% electron-proton scattering. 00:38:21.410 --> 00:38:24.860 align:middle line:84% In the detector bay at the end of that line, 00:38:24.860 --> 00:38:26.673 align:middle line:84% you have this unbelievable equipment-- 00:38:26.673 --> 00:38:28.340 align:middle line:84% you can see this is like train tracking, 00:38:28.340 --> 00:38:30.620 align:middle line:84% and here's a person for a sense of scale-- 00:38:30.620 --> 00:38:33.920 align:middle line:84% where you could actually change the angle at which you measure 00:38:33.920 --> 00:38:36.170 align:middle line:84% the detritus that comes out of these very high-energy 00:38:36.170 --> 00:38:37.130 align:middle line:90% scatterings. 00:38:37.130 --> 00:38:40.910 align:middle line:84% This is basically a redo of Rutherford scattering 00:38:40.910 --> 00:38:45.560 align:middle line:84% but now across miles as opposed to across a single desktop. 00:38:45.560 --> 00:38:48.980 align:middle line:84% You may remember back from in the early-- 00:38:48.980 --> 00:38:51.920 align:middle line:84% several weeks ago in our class, we were looking at Rutherford 00:38:51.920 --> 00:38:57.170 align:middle line:84% scattering from around 1909, 1911, firing 00:38:57.170 --> 00:39:00.170 align:middle line:84% alpha particles at very thin gold foil 00:39:00.170 --> 00:39:03.240 align:middle line:84% and then surrounding it with scintillating screen, 00:39:03.240 --> 00:39:06.170 align:middle line:84% so you could get information for the number of scatterings 00:39:06.170 --> 00:39:11.210 align:middle line:84% per angle as a function of scattering angle. 00:39:11.210 --> 00:39:13.370 align:middle line:84% And Rutherford was so shocked when 00:39:13.370 --> 00:39:16.610 align:middle line:84% he found a significant number of backscatter events, 00:39:16.610 --> 00:39:21.050 align:middle line:84% where the incoming projectile scattered almost 00:39:21.050 --> 00:39:23.000 align:middle line:84% all the way backwards, which was consistent 00:39:23.000 --> 00:39:26.600 align:middle line:84% only with a very, very small, very massive inner core 00:39:26.600 --> 00:39:27.290 align:middle line:90% to the atom. 00:39:27.290 --> 00:39:29.580 align:middle line:84% That became known as the atomic nucleus. 00:39:29.580 --> 00:39:34.460 align:middle line:84% Now they're doing the same thing but not with a thin metal foil 00:39:34.460 --> 00:39:38.030 align:middle line:84% but literally moving these enormous, enormous detectors 00:39:38.030 --> 00:39:42.200 align:middle line:84% around in a semicircle on these kind of inbuilt train tracks, 00:39:42.200 --> 00:39:44.420 align:middle line:84% so they could get things like scattering rates 00:39:44.420 --> 00:39:45.560 align:middle line:90% as a function of angle. 00:39:45.560 --> 00:39:49.160 align:middle line:84% I just find that astonishing, that in that case, well, 00:39:49.160 --> 00:39:54.860 align:middle line:84% roughly 50 years or 60 years, maybe, 00:39:54.860 --> 00:39:57.140 align:middle line:84% they'd gone from the same basic concept 00:39:57.140 --> 00:39:59.690 align:middle line:84% but now taking miles and train tracks as opposed 00:39:59.690 --> 00:40:01.580 align:middle line:84% to a grad student sitting in the dark. 00:40:01.580 --> 00:40:04.700 align:middle line:84% Anyway, what they found, the SLAC-MIT experiments, 00:40:04.700 --> 00:40:08.120 align:middle line:84% was again very similar pattern to what Rutherford and his team 00:40:08.120 --> 00:40:10.250 align:middle line:90% had found about atoms. 00:40:10.250 --> 00:40:13.310 align:middle line:84% When they were scattering off of protons, 00:40:13.310 --> 00:40:16.280 align:middle line:84% they found evidence for an internal structure 00:40:16.280 --> 00:40:17.150 align:middle line:90% within protons. 00:40:17.150 --> 00:40:20.920 align:middle line:84% Protons seem to have internal hard scattering sites 00:40:20.920 --> 00:40:23.560 align:middle line:84% just like atoms in that gold foil 00:40:23.560 --> 00:40:26.410 align:middle line:84% seem to have internal hard scattering sites that we now 00:40:26.410 --> 00:40:28.180 align:middle line:90% call the nucleus. 00:40:28.180 --> 00:40:29.770 align:middle line:84% The particle physicist by this point 00:40:29.770 --> 00:40:33.250 align:middle line:84% were using a bit funny variables. 00:40:33.250 --> 00:40:35.710 align:middle line:84% It wasn't just scattering angle, but this angle q 00:40:35.710 --> 00:40:39.460 align:middle line:84% depends both on the energy but also on the scattering angle. 00:40:39.460 --> 00:40:41.710 align:middle line:90% It's a larger q here. 00:40:41.710 --> 00:40:45.110 align:middle line:84% q squared corresponds to larger scattering angle. 00:40:45.110 --> 00:40:48.520 align:middle line:84% And so if there were just kind of equally diffuse smush 00:40:48.520 --> 00:40:52.060 align:middle line:84% inside a proton, you'd expect a very rapid falloff with angle. 00:40:52.060 --> 00:40:55.420 align:middle line:84% You'd expect very few large backscatter events, 00:40:55.420 --> 00:40:58.300 align:middle line:84% but instead, they were finding on this logarithmic scale 00:40:58.300 --> 00:41:01.240 align:middle line:84% quite substantial numbers of large-angle scatter. 00:41:01.240 --> 00:41:05.440 align:middle line:84% So again, the argument was really identical conceptually 00:41:05.440 --> 00:41:07.850 align:middle line:90% to the Rutherford scattering. 00:41:07.850 --> 00:41:10.423 align:middle line:84% So how do you make sense of the quantitative details, 00:41:10.423 --> 00:41:11.840 align:middle line:84% not just the fact that there seems 00:41:11.840 --> 00:41:14.630 align:middle line:84% to be a structure inside these proton targets? 00:41:14.630 --> 00:41:17.090 align:middle line:84% But could you actually reproduce those curves 00:41:17.090 --> 00:41:21.140 align:middle line:84% with the real specific structure inside the proton? 00:41:21.140 --> 00:41:24.320 align:middle line:84% So a number of theorists who were following the SLAC-MIT 00:41:24.320 --> 00:41:25.670 align:middle line:90% experiments closely-- 00:41:25.670 --> 00:41:27.920 align:middle line:84% they jumped in, one of whom is Richard Feynman, some 00:41:27.920 --> 00:41:30.590 align:middle line:84% of whose work we already looked at together in this class. 00:41:30.590 --> 00:41:32.840 align:middle line:84% The other was, at the time, a much younger theoretical 00:41:32.840 --> 00:41:34.940 align:middle line:90% physicist named James Bjorken. 00:41:34.940 --> 00:41:38.150 align:middle line:84% He often just goes by BJ for the start of his last name. 00:41:38.150 --> 00:41:42.320 align:middle line:84% They were able to make sense of the scattering data 00:41:42.320 --> 00:41:45.410 align:middle line:84% quantitatively, not just kind of qualitatively. 00:41:45.410 --> 00:41:46.790 align:middle line:90% There's a scattering site. 00:41:46.790 --> 00:41:49.250 align:middle line:84% But they could actually reproduce these very specific 00:41:49.250 --> 00:41:52.610 align:middle line:84% curves, those scaling laws, by introducing 00:41:52.610 --> 00:41:53.870 align:middle line:90% what they called partons. 00:41:53.870 --> 00:41:56.180 align:middle line:90% This was Feynman's kind of joke. 00:41:56.180 --> 00:42:00.677 align:middle line:84% So he was absolutely clear not to call these quarks, partly 00:42:00.677 --> 00:42:03.260 align:middle line:84% because, as we'll see, they were conceptually quite different, 00:42:03.260 --> 00:42:07.320 align:middle line:84% partly because, by this point, both Feynman and Gell-Mann were 00:42:07.320 --> 00:42:09.900 align:middle line:84% together at Caltech in the same department of physics, 00:42:09.900 --> 00:42:11.563 align:middle line:84% and they were kind of friendly rivals. 00:42:11.563 --> 00:42:13.230 align:middle line:84% Feynman had a lot of these relationships 00:42:13.230 --> 00:42:14.380 align:middle line:90% throughout his life. 00:42:14.380 --> 00:42:16.740 align:middle line:84% So by this point, he traded Julian Schwinger 00:42:16.740 --> 00:42:19.890 align:middle line:84% as his main rival for Murray Gell-Mann. 00:42:19.890 --> 00:42:21.900 align:middle line:84% Their offices were just down the hall. 00:42:21.900 --> 00:42:25.200 align:middle line:84% And so Feynman was as much tweaking his nose at Gell-Mann 00:42:25.200 --> 00:42:28.770 align:middle line:84% as being kind of maybe appropriately skeptical by very 00:42:28.770 --> 00:42:32.520 align:middle line:84% intentionally not calling his new theoretical particles 00:42:32.520 --> 00:42:33.090 align:middle line:90% quarks. 00:42:33.090 --> 00:42:34.798 align:middle line:84% They were called partons, meaning they're 00:42:34.798 --> 00:42:36.630 align:middle line:90% some parts inside of protons. 00:42:36.630 --> 00:42:38.280 align:middle line:90% I thought it was a cute name. 00:42:38.280 --> 00:42:40.350 align:middle line:90% These are parts inside protons. 00:42:40.350 --> 00:42:42.210 align:middle line:84% So the idea was, why would this help? 00:42:42.210 --> 00:42:45.270 align:middle line:84% How would you make theoretical sense of these experiments? 00:42:45.270 --> 00:42:48.000 align:middle line:84% And this, again, I think was kind of classic Feynman 00:42:48.000 --> 00:42:50.640 align:middle line:90% in his use of intuition. 00:42:50.640 --> 00:42:54.060 align:middle line:84% At low energies inside a nucleus, 00:42:54.060 --> 00:42:56.460 align:middle line:84% a proton would actually be a big mess. 00:42:56.460 --> 00:42:59.250 align:middle line:84% If it has internal structure, whether they're 00:42:59.250 --> 00:43:03.120 align:middle line:84% quarks or anything else, if they're parts within a proton, 00:43:03.120 --> 00:43:06.570 align:middle line:84% then they're going to be subject to very strong internal forces. 00:43:06.570 --> 00:43:08.490 align:middle line:84% After all, protons don't fall apart, 00:43:08.490 --> 00:43:10.110 align:middle line:84% so whatever internal parts it might 00:43:10.110 --> 00:43:13.200 align:middle line:84% have must be stuck together very, very tightly, 00:43:13.200 --> 00:43:15.450 align:middle line:90% very strong nuclear forces. 00:43:15.450 --> 00:43:17.580 align:middle line:84% So it would be some jumble of moving parts 00:43:17.580 --> 00:43:19.320 align:middle line:90% with a big dynamical mess. 00:43:19.320 --> 00:43:21.510 align:middle line:84% And people had no idea how to calculate that. 00:43:21.510 --> 00:43:23.950 align:middle line:84% Remember, perturbation theory failed and all that. 00:43:23.950 --> 00:43:25.920 align:middle line:84% So at low energies, a proton would basically 00:43:25.920 --> 00:43:30.360 align:middle line:84% be extremely difficult to characterize quantitatively. 00:43:30.360 --> 00:43:34.440 align:middle line:84% Luckily for Feynman and Bjorken and their colleagues, 00:43:34.440 --> 00:43:36.840 align:middle line:84% that was not the situation at hand. 00:43:36.840 --> 00:43:39.360 align:middle line:84% To make sense of the SLAC-MIT experiments, 00:43:39.360 --> 00:43:42.210 align:middle line:84% they were going to, in a sense, the opposite regime. 00:43:42.210 --> 00:43:44.460 align:middle line:84% The protons were stationary targets, 00:43:44.460 --> 00:43:48.390 align:middle line:84% but what mattered was the view from the speeding electron. 00:43:48.390 --> 00:43:51.150 align:middle line:84% And the electron had just as much license 00:43:51.150 --> 00:43:54.570 align:middle line:84% as the person riding on Einstein's imaginary trains 00:43:54.570 --> 00:43:57.240 align:middle line:84% to pretend that the electron was fully at rest, 00:43:57.240 --> 00:44:01.120 align:middle line:84% and it saw the target racing toward it at very high speed. 00:44:01.120 --> 00:44:03.150 align:middle line:84% So if you imagine sitting on the electron, 00:44:03.150 --> 00:44:06.630 align:middle line:84% you could say you're sitting at rest while the proton target 00:44:06.630 --> 00:44:09.700 align:middle line:84% races toward you at a very high fraction of the speed of light. 00:44:09.700 --> 00:44:12.060 align:middle line:84% So from your point of view as the electron, 00:44:12.060 --> 00:44:14.850 align:middle line:84% you see the proton undergo length contraction, 00:44:14.850 --> 00:44:18.000 align:middle line:84% become a pancake along its direction of motion. 00:44:18.000 --> 00:44:20.340 align:middle line:84% And its internal clocks will be slowed. 00:44:20.340 --> 00:44:22.350 align:middle line:84% It will undergo time dilation, at least 00:44:22.350 --> 00:44:23.550 align:middle line:90% as far as you're concerned. 00:44:23.550 --> 00:44:27.480 align:middle line:84% So all of the time scales for these very big, complicated 00:44:27.480 --> 00:44:30.270 align:middle line:84% nuclear forces will be slowed down as if they 00:44:30.270 --> 00:44:31.660 align:middle line:90% were kind of frozen in time. 00:44:31.660 --> 00:44:36.630 align:middle line:84% What the electron would see in this target of the proton 00:44:36.630 --> 00:44:39.300 align:middle line:84% would basically be a kind of frozen set 00:44:39.300 --> 00:44:43.140 align:middle line:84% of free, non-interacting stationary targets. 00:44:43.140 --> 00:44:46.230 align:middle line:84% All the stuff that people had no possibility to calculate, 00:44:46.230 --> 00:44:49.230 align:middle line:84% the strong, fast changing nuclear forces 00:44:49.230 --> 00:44:53.670 align:middle line:84% become irrelevant in the limits of very high energies as 00:44:53.670 --> 00:44:55.920 align:middle line:84% relevant for these SLAC-MIT experiments. 00:44:55.920 --> 00:44:59.430 align:middle line:84% So you can go for these internal parts called partons 00:44:59.430 --> 00:45:02.340 align:middle line:84% and analyze all this very specific scattering 00:45:02.340 --> 00:45:07.530 align:middle line:84% data based on free partons because in the energy 00:45:07.530 --> 00:45:10.290 align:middle line:84% regimes of interest, the dynamical forces should 00:45:10.290 --> 00:45:13.380 align:middle line:84% basically be irrelevant or subdominant. 00:45:13.380 --> 00:45:15.570 align:middle line:84% So you might say, was this a victory for quarks? 00:45:15.570 --> 00:45:16.470 align:middle line:90% Well, not right away. 00:45:16.470 --> 00:45:18.780 align:middle line:84% As I've mentioned, Feynman went out of his way not 00:45:18.780 --> 00:45:21.300 align:middle line:90% to call these partons quarks. 00:45:21.300 --> 00:45:23.610 align:middle line:84% The two sets of ideas really were answering 00:45:23.610 --> 00:45:25.230 align:middle line:90% different kinds of questions. 00:45:25.230 --> 00:45:28.080 align:middle line:84% So Gell-Mann's or even more importantly George 00:45:28.080 --> 00:45:31.650 align:middle line:84% Zweig's notion of quarks from early 1964-- 00:45:31.650 --> 00:45:33.960 align:middle line:84% that was really a kind of constituent model. 00:45:33.960 --> 00:45:35.460 align:middle line:84% What are the little pieces that are 00:45:35.460 --> 00:45:39.780 align:middle line:84% bound tightly inside protons, neutrons, hyperons, 00:45:39.780 --> 00:45:40.800 align:middle line:90% and all the rest? 00:45:40.800 --> 00:45:45.570 align:middle line:84% It's about the kind of internal parts and eventually the forces 00:45:45.570 --> 00:45:47.220 align:middle line:84% that must keep them bound together so 00:45:47.220 --> 00:45:50.070 align:middle line:84% that we don't see things like free fractional charge. 00:45:50.070 --> 00:45:51.990 align:middle line:84% And Feynman's partons were really 00:45:51.990 --> 00:45:54.330 align:middle line:84% answering just an entirely separate set of questions, 00:45:54.330 --> 00:45:56.370 align:middle line:90% very high energy behavior. 00:45:56.370 --> 00:45:58.740 align:middle line:84% When you basically ignore the internal forces, 00:45:58.740 --> 00:46:01.320 align:middle line:84% you have effectively free scatterers as opposed 00:46:01.320 --> 00:46:04.600 align:middle line:84% to bound strongly interacting particles. 00:46:04.600 --> 00:46:07.090 align:middle line:84% And again, you don't just take my word for it 00:46:07.090 --> 00:46:10.600 align:middle line:84% that this wasn't seen in its day as direct evidence for quarks. 00:46:10.600 --> 00:46:13.120 align:middle line:84% Gell-Mann himself, three years later, 00:46:13.120 --> 00:46:16.240 align:middle line:84% three and four years later, mentioned 00:46:16.240 --> 00:46:20.830 align:middle line:84% at a very well-attended conference 00:46:20.830 --> 00:46:23.470 align:middle line:84% for high-energy physics that quarks were still 00:46:23.470 --> 00:46:26.740 align:middle line:84% fictitious, his word, and that his own favored theoretical 00:46:26.740 --> 00:46:30.460 align:middle line:84% approach was actually equivalent to the ongoing work 00:46:30.460 --> 00:46:34.120 align:middle line:84% from that Berkeley tradition on the bootstrap, which 00:46:34.120 --> 00:46:38.950 align:middle line:84% disavows a set of literally elementary particles and talks 00:46:38.950 --> 00:46:42.430 align:middle line:84% about the kind of self-consistent composites one 00:46:42.430 --> 00:46:44.450 align:middle line:84% can make from the nuclear particles. 00:46:44.450 --> 00:46:47.590 align:middle line:84% So Gell-Mann himself didn't leap on the SLAC-MIT results 00:46:47.590 --> 00:46:50.620 align:middle line:84% and say, now we found quarks, let alone Feynman. 00:46:50.620 --> 00:46:53.663 align:middle line:84% So this was, again, a kind of slow drift. 00:46:53.663 --> 00:46:55.246 align:middle line:84% So let me pause there and see if there 00:46:55.246 --> 00:46:58.300 align:middle line:84% are any questions on SLAC-MIT or any of that kind of stuff. 00:46:58.300 --> 00:47:05.180 align:middle line:90% 00:47:05.180 --> 00:47:08.720 align:middle line:84% I know I've said it before, but I just love that contrast from 00:47:08.720 --> 00:47:11.360 align:middle line:84% the Rutherford scattering-- and the whole apparatus was maybe 00:47:11.360 --> 00:47:13.290 align:middle line:90% a meter across-- 00:47:13.290 --> 00:47:17.030 align:middle line:84% to 2 miles with that train track to change scattering angle 00:47:17.030 --> 00:47:17.780 align:middle line:90% of your detectors. 00:47:17.780 --> 00:47:19.782 align:middle line:84% I just think that's pretty-- that 00:47:19.782 --> 00:47:21.740 align:middle line:84% right there is the story of high-energy physics 00:47:21.740 --> 00:47:23.660 align:middle line:84% in the 20th century, that transition. 00:47:23.660 --> 00:47:25.730 align:middle line:84% You're asking basically the same question, 00:47:25.730 --> 00:47:27.460 align:middle line:84% getting remarkably consistent results, 00:47:27.460 --> 00:47:29.210 align:middle line:84% but you had to do a lot of work in between 00:47:29.210 --> 00:47:31.400 align:middle line:84% and convince a lot more people to pay the money 00:47:31.400 --> 00:47:33.783 align:middle line:90% to let you do it. 00:47:33.783 --> 00:47:34.700 align:middle line:90% Any questions on that? 00:47:34.700 --> 00:47:37.780 align:middle line:90% 00:47:37.780 --> 00:47:41.160 align:middle line:90% OK, I will press on. 00:47:41.160 --> 00:47:44.410 align:middle line:84% If any questions come up, of course, please chime in. 00:47:44.410 --> 00:47:47.170 align:middle line:84% Let's go to that last part for today and talk 00:47:47.170 --> 00:47:51.820 align:middle line:90% about quantum chromodynamics. 00:47:51.820 --> 00:47:54.160 align:middle line:84% So right around the time when Gell-Mann stood up 00:47:54.160 --> 00:47:57.170 align:middle line:84% at the conference in Florida and said, quarks, as we all know, 00:47:57.170 --> 00:47:59.320 align:middle line:84% are fictitious, he was actually working 00:47:59.320 --> 00:48:02.290 align:middle line:84% on a whole new theoretical approach, 00:48:02.290 --> 00:48:05.750 align:middle line:84% sometimes with coauthors and others working independently. 00:48:05.750 --> 00:48:08.020 align:middle line:84% Another key architect of this was a younger theorist 00:48:08.020 --> 00:48:10.030 align:middle line:90% named Harald Fritzsch. 00:48:10.030 --> 00:48:12.850 align:middle line:84% They were developing what came to be called 00:48:12.850 --> 00:48:15.820 align:middle line:90% quantum chromodynamics, or QCD. 00:48:15.820 --> 00:48:19.750 align:middle line:84% The idea now was, in some sense, to finally ask this question 00:48:19.750 --> 00:48:22.090 align:middle line:84% about dynamics, about forces, which 00:48:22.090 --> 00:48:25.540 align:middle line:84% Gell-Mann himself had very successfully kind of bracketed 00:48:25.540 --> 00:48:28.330 align:middle line:84% or left aside from the '50s and '60s 00:48:28.330 --> 00:48:30.370 align:middle line:84% with his focus on classification. 00:48:30.370 --> 00:48:34.810 align:middle line:84% The idea now is to try to look more squarely at forces 00:48:34.810 --> 00:48:38.110 align:middle line:84% and try to find a quantitative way of making sense 00:48:38.110 --> 00:48:42.350 align:middle line:90% of these nuclear constituents. 00:48:42.350 --> 00:48:44.690 align:middle line:84% So they're working in very explicit analogy 00:48:44.690 --> 00:48:46.850 align:middle line:84% to QED, to quantum electrodynamics, 00:48:46.850 --> 00:48:50.570 align:middle line:84% the work that we saw in previous class sessions that really had 00:48:50.570 --> 00:48:52.820 align:middle line:84% kind of come together soon after the Second World 00:48:52.820 --> 00:48:55.430 align:middle line:84% War in the late 1940s, though the roots of it 00:48:55.430 --> 00:48:57.900 align:middle line:84% went back to the early days of quantum theory. 00:48:57.900 --> 00:49:00.590 align:middle line:84% So there really was a kind of step-by-step analogy. 00:49:00.590 --> 00:49:04.970 align:middle line:84% In the new work, in place of electrons and positrons, 00:49:04.970 --> 00:49:06.950 align:middle line:84% the new work would focus on quarks. 00:49:06.950 --> 00:49:09.170 align:middle line:84% Those were the kind of elementary particles 00:49:09.170 --> 00:49:11.670 align:middle line:84% that would be subjecting each other to forces. 00:49:11.670 --> 00:49:13.850 align:middle line:84% So just like electrons could repel electrons, 00:49:13.850 --> 00:49:16.400 align:middle line:84% quarks could repel or attract other quarks, 00:49:16.400 --> 00:49:19.790 align:middle line:84% and the quarks would interact in the new scheme 00:49:19.790 --> 00:49:22.670 align:middle line:84% by exchanging a new kind of force-carrying particle called 00:49:22.670 --> 00:49:23.450 align:middle line:90% the gluon. 00:49:23.450 --> 00:49:25.970 align:middle line:84% This would be the glue, the nuclear glue, 00:49:25.970 --> 00:49:29.210 align:middle line:84% that would keep, say, an up-down and down quark bound 00:49:29.210 --> 00:49:32.790 align:middle line:84% within a proton or within, in this case, a neutron. 00:49:32.790 --> 00:49:35.750 align:middle line:84% So the gluons, which were maybe not the most creative name-- 00:49:35.750 --> 00:49:37.460 align:middle line:84% they were meant to be nuclear glue. 00:49:37.460 --> 00:49:40.550 align:middle line:84% They would play the analogous role to the photons. 00:49:40.550 --> 00:49:44.300 align:middle line:84% Remember, we saw electrons on the idea from quantum field 00:49:44.300 --> 00:49:48.170 align:middle line:84% theory would repel each other by firing force-carrying 00:49:48.170 --> 00:49:51.200 align:middle line:84% photons at each other back and forth, virtual photons, 00:49:51.200 --> 00:49:53.060 align:middle line:84% and now you have a similar structure, 00:49:53.060 --> 00:49:56.030 align:middle line:84% at least hypothetically, about quarks interacting 00:49:56.030 --> 00:49:58.322 align:middle line:90% by the exchange of gluons. 00:49:58.322 --> 00:49:59.780 align:middle line:84% So there are some new ideas, again, 00:49:59.780 --> 00:50:03.740 align:middle line:84% to try to make sense of all the newer classifications 00:50:03.740 --> 00:50:05.510 align:middle line:84% and symmetries and internal charges 00:50:05.510 --> 00:50:07.490 align:middle line:84% and stuff that have been introduced 00:50:07.490 --> 00:50:10.790 align:middle line:84% for the nuclear particles over the previous 20 years. 00:50:10.790 --> 00:50:13.670 align:middle line:84% The main idea the main new ingredient conceptually 00:50:13.670 --> 00:50:17.600 align:middle line:84% for this newer QCD was to introduce yet 00:50:17.600 --> 00:50:21.080 align:middle line:84% another internal quantum number, or charge, again, strictly 00:50:21.080 --> 00:50:24.290 align:middle line:84% hypothetical at this point, and they called it color. 00:50:24.290 --> 00:50:26.750 align:middle line:84% The idea was that each quark carries yet 00:50:26.750 --> 00:50:28.970 align:middle line:90% another internal quantum number. 00:50:28.970 --> 00:50:31.270 align:middle line:84% This one could have one of three values. 00:50:31.270 --> 00:50:34.220 align:middle line:84% So instead of spin or isospin, which were usually 00:50:34.220 --> 00:50:36.950 align:middle line:84% one of two values, spin up or spin down, 00:50:36.950 --> 00:50:40.760 align:middle line:84% the color charge could come in one of three values, 00:50:40.760 --> 00:50:42.920 align:middle line:84% and rather than call it minus 1, 0, or plus 1, 00:50:42.920 --> 00:50:44.960 align:middle line:84% they called them-- they gave them names 00:50:44.960 --> 00:50:48.840 align:middle line:84% like colors, the primary colors, red, green, and blue. 00:50:48.840 --> 00:50:52.430 align:middle line:84% And they did that because they posited several features 00:50:52.430 --> 00:50:56.240 align:middle line:84% of this color charge, that the color charge overall is 00:50:56.240 --> 00:50:59.150 align:middle line:84% conserved-- you can neither create nor destroy, say, 00:50:59.150 --> 00:51:01.280 align:middle line:90% red quark charge-- 00:51:01.280 --> 00:51:03.740 align:middle line:84% that the free particles you could ever measure 00:51:03.740 --> 00:51:07.100 align:middle line:84% or experience in nature-- bound protons, neutrons, pions, 00:51:07.100 --> 00:51:07.970 align:middle line:90% or anything else-- 00:51:07.970 --> 00:51:11.430 align:middle line:84% have to have an exact balance among the color charges. 00:51:11.430 --> 00:51:14.570 align:middle line:84% So if you have one unit of red color charge, 00:51:14.570 --> 00:51:18.300 align:middle line:84% you must have exactly one unit of both green and blue, 00:51:18.300 --> 00:51:19.800 align:middle line:90% and vice versa. 00:51:19.800 --> 00:51:22.070 align:middle line:84% And that's why they appealed to the primary colors. 00:51:22.070 --> 00:51:24.980 align:middle line:84% If you mix red light, green light, and blue light 00:51:24.980 --> 00:51:28.010 align:middle line:84% in equal intensity, together, they will make white light, 00:51:28.010 --> 00:51:30.610 align:middle line:84% or all the color will vanish, basically. 00:51:30.610 --> 00:51:33.170 align:middle line:84% You'll have perfect color balance. 00:51:33.170 --> 00:51:36.110 align:middle line:84% So that's what they were positing among the color 00:51:36.110 --> 00:51:40.280 align:middle line:84% charges on these quarks, that baryon, a bound state of three 00:51:40.280 --> 00:51:43.580 align:middle line:84% quarks, must have exactly one of each color to balance out. 00:51:43.580 --> 00:51:46.850 align:middle line:84% The mesons, which were like a pion, which 00:51:46.850 --> 00:51:49.160 align:middle line:84% is one quark and a bound antiquark, 00:51:49.160 --> 00:51:52.220 align:middle line:84% would have to have one red and one anti-red quark, 00:51:52.220 --> 00:51:54.440 align:middle line:84% so together, the color charge would exactly balance, 00:51:54.440 --> 00:51:56.610 align:middle line:84% or one blue and anti-blue and so on. 00:51:56.610 --> 00:51:58.790 align:middle line:84% So that's the first two assumptions. 00:51:58.790 --> 00:52:00.320 align:middle line:84% Color charge is conserved, and you 00:52:00.320 --> 00:52:02.510 align:middle line:90% have to have an exact balance. 00:52:02.510 --> 00:52:04.880 align:middle line:84% And the last one is that this new force 00:52:04.880 --> 00:52:07.700 align:middle line:84% law has to be symmetric with respect 00:52:07.700 --> 00:52:11.150 align:middle line:84% to random permutations of the color charge, 00:52:11.150 --> 00:52:14.960 align:middle line:84% and that's, again, a lot like the symmetries of isospin, 00:52:14.960 --> 00:52:17.900 align:middle line:84% that when you consider a lot of these nuclear interactions 00:52:17.900 --> 00:52:20.930 align:middle line:84% about, say, proton-proton scattering or proton-neutron 00:52:20.930 --> 00:52:23.390 align:middle line:84% or neutron-neutron, it didn't matter. 00:52:23.390 --> 00:52:25.970 align:middle line:84% Any random permutation gave the same results, 00:52:25.970 --> 00:52:28.730 align:middle line:84% any random permutation of isospin. 00:52:28.730 --> 00:52:32.330 align:middle line:84% So the nuclear reactions were isospin symmetric, 00:52:32.330 --> 00:52:35.000 align:middle line:84% and the idea was that maybe that same kind of symmetry 00:52:35.000 --> 00:52:38.150 align:middle line:84% held among this as yet unobserved, maybe even 00:52:38.150 --> 00:52:42.380 align:middle line:84% unobservable charge on these elementary constituents, 00:52:42.380 --> 00:52:43.130 align:middle line:90% the quarks. 00:52:43.130 --> 00:52:45.190 align:middle line:84% And they jumbled up in any old way 00:52:45.190 --> 00:52:47.230 align:middle line:84% as long as the total balance remained. 00:52:47.230 --> 00:52:50.680 align:middle line:84% So you'd have no free color charge leaking out 00:52:50.680 --> 00:52:52.310 align:middle line:84% because you'd have an exact balance. 00:52:52.310 --> 00:52:54.370 align:middle line:90% So those were the assumptions. 00:52:54.370 --> 00:52:57.760 align:middle line:84% Then you could go back and resolve some of the puzzles 00:52:57.760 --> 00:52:59.470 align:middle line:90% like the omega minus. 00:52:59.470 --> 00:53:02.950 align:middle line:84% So now maybe the omega minus would not violate the Pauli 00:53:02.950 --> 00:53:06.130 align:middle line:84% exclusion principle because it was not three literally 00:53:06.130 --> 00:53:08.770 align:middle line:84% identical spin-1/2 particles in a balanced state. 00:53:08.770 --> 00:53:12.820 align:middle line:84% They differed on one more set of internal quantum numbers 00:53:12.820 --> 00:53:15.100 align:middle line:84% that had not yet been taken into account. 00:53:15.100 --> 00:53:19.510 align:middle line:84% The omega minus would have to be a bound state of one red, one 00:53:19.510 --> 00:53:23.440 align:middle line:84% green, and one blue strange quark, so no two of them 00:53:23.440 --> 00:53:25.570 align:middle line:84% would be in the same quantum state, 00:53:25.570 --> 00:53:28.180 align:middle line:84% even if they had the same values for all the other quantum 00:53:28.180 --> 00:53:29.510 align:middle line:90% numbers, including spin. 00:53:29.510 --> 00:53:32.350 align:middle line:84% So now you can have bound states like the omega minus. 00:53:32.350 --> 00:53:34.480 align:middle line:84% This also suggested at least why it 00:53:34.480 --> 00:53:37.180 align:middle line:84% might be possible to avoid having 00:53:37.180 --> 00:53:39.010 align:middle line:90% fractional electric charge. 00:53:39.010 --> 00:53:41.590 align:middle line:84% If you always have to have this exact balancing, 00:53:41.590 --> 00:53:45.320 align:middle line:84% maybe the electric charge always has to come in integer units 00:53:45.320 --> 00:53:49.610 align:middle line:84% and never see, say, a plus 2/3 charge that 00:53:49.610 --> 00:53:53.420 align:middle line:84% wasn't appropriately balanced by the color charge-preserving 00:53:53.420 --> 00:53:56.990 align:middle line:84% quarks that, as a consequence, would also get you back 00:53:56.990 --> 00:53:59.150 align:middle line:84% to only integer values for electric charge. 00:53:59.150 --> 00:54:02.000 align:middle line:90% That was at least a hypothesis. 00:54:02.000 --> 00:54:05.450 align:middle line:84% So as I mentioned even just in that brief discussion 00:54:05.450 --> 00:54:08.360 align:middle line:84% a moment ago, really central to the idea of quantum 00:54:08.360 --> 00:54:11.720 align:middle line:84% chromodynamics, or QCD, is the idea of symmetry, 00:54:11.720 --> 00:54:16.040 align:middle line:84% that some property of the system remains invariant or unchanged 00:54:16.040 --> 00:54:19.640 align:middle line:84% even when certain parts are changed to undergo 00:54:19.640 --> 00:54:21.080 align:middle line:90% what we call transformations. 00:54:21.080 --> 00:54:24.530 align:middle line:84% We saw how important this was in the study of relativity. 00:54:24.530 --> 00:54:27.110 align:middle line:84% It plays an increasing role in quantum theory, 00:54:27.110 --> 00:54:31.220 align:middle line:84% and by this point, by the early and mid-1970s, the theorists 00:54:31.220 --> 00:54:33.890 align:middle line:84% like Gell-Mann and Fritzsch and their colleagues were elevating 00:54:33.890 --> 00:54:36.230 align:middle line:84% this to a kind of guiding principle 00:54:36.230 --> 00:54:40.310 align:middle line:84% to be driven by what has to remain unchanged even 00:54:40.310 --> 00:54:42.180 align:middle line:90% among symmetry transformations. 00:54:42.180 --> 00:54:44.390 align:middle line:84% So again, we're all familiar with symmetries. 00:54:44.390 --> 00:54:47.340 align:middle line:84% You can imagine, for example, a sphere. 00:54:47.340 --> 00:54:49.280 align:middle line:90% It will appear unchanged. 00:54:49.280 --> 00:54:51.260 align:middle line:84% Its appearance will remain invariant 00:54:51.260 --> 00:54:53.630 align:middle line:84% even if you rotate the sphere by an arbitrary 00:54:53.630 --> 00:54:55.610 align:middle line:90% angle around an arbitrary axis. 00:54:55.610 --> 00:54:58.310 align:middle line:84% That's an example of a continuous symmetry. 00:54:58.310 --> 00:55:01.700 align:middle line:84% The transformations can be by any angle, 00:55:01.700 --> 00:55:07.790 align:middle line:84% 7.1 degrees, 7.1003 degrees, any continuously variable 00:55:07.790 --> 00:55:08.780 align:middle line:90% transformation. 00:55:08.780 --> 00:55:11.960 align:middle line:84% Likewise, you could rotate the axis itself by any amount. 00:55:11.960 --> 00:55:14.390 align:middle line:84% We're also familiar with things like discrete symmetries. 00:55:14.390 --> 00:55:18.410 align:middle line:84% Imagine a square, a simple two-dimensional square. 00:55:18.410 --> 00:55:20.870 align:middle line:84% Its appearance will remain invariant 00:55:20.870 --> 00:55:24.980 align:middle line:84% if you rotate it by any integer number of times 90 degrees. 00:55:24.980 --> 00:55:28.190 align:middle line:84% Rotated by 90 degrees, you can't tell the difference by 180, 00:55:28.190 --> 00:55:29.330 align:middle line:90% by 270. 00:55:29.330 --> 00:55:31.733 align:middle line:84% If you rotate a square by 37 degrees, 00:55:31.733 --> 00:55:32.900 align:middle line:90% you can tell the difference. 00:55:32.900 --> 00:55:35.310 align:middle line:90% It no longer looks unchanged. 00:55:35.310 --> 00:55:38.210 align:middle line:84% So that's an example of an object that remains invariant 00:55:38.210 --> 00:55:39.920 align:middle line:84% under discrete symmetries, a more 00:55:39.920 --> 00:55:42.890 align:middle line:84% kind of constrained set of transformations 00:55:42.890 --> 00:55:45.130 align:middle line:84% that will nonetheless leave the appearance unchanged. 00:55:45.130 --> 00:55:47.380 align:middle line:84% So we're familiar with these kind of everyday examples 00:55:47.380 --> 00:55:48.935 align:middle line:90% from geometry. 00:55:48.935 --> 00:55:50.560 align:middle line:84% Then we're going to start applying this 00:55:50.560 --> 00:55:53.830 align:middle line:84% to these more abstract, internal mathematical spaces 00:55:53.830 --> 00:55:55.900 align:middle line:84% of these things like color charge. 00:55:55.900 --> 00:55:58.690 align:middle line:84% But what if there were symmetrical rotations, 00:55:58.690 --> 00:56:01.360 align:middle line:84% in this case discrete ones, because only three values 00:56:01.360 --> 00:56:03.800 align:middle line:84% of the color charge, red, green, or blue? 00:56:03.800 --> 00:56:05.260 align:middle line:90% So that's more like the square. 00:56:05.260 --> 00:56:09.970 align:middle line:84% You can rotate a red quark by kind of 1 unit in color space 00:56:09.970 --> 00:56:12.940 align:middle line:84% or 2 units or 3, but not by 1.2 units. 00:56:12.940 --> 00:56:15.760 align:middle line:84% That would be like the square being rotated by something 00:56:15.760 --> 00:56:17.090 align:middle line:90% other than 90 degrees. 00:56:17.090 --> 00:56:20.632 align:middle line:84% So it's a discrete symmetry, and not that you're literally 00:56:20.632 --> 00:56:22.090 align:middle line:84% picking up a quark and rotating it, 00:56:22.090 --> 00:56:24.580 align:middle line:84% but you're changing its description 00:56:24.580 --> 00:56:28.060 align:middle line:84% in this abstract mathematical space of color space. 00:56:28.060 --> 00:56:31.270 align:middle line:84% So you could perform rotations on your quantum field 00:56:31.270 --> 00:56:33.400 align:middle line:90% that represents a given quark. 00:56:33.400 --> 00:56:36.010 align:middle line:84% You could transform it by rotating it 00:56:36.010 --> 00:56:38.290 align:middle line:90% by some angle in color space. 00:56:38.290 --> 00:56:39.910 align:middle line:90% The angle has some constraints. 00:56:39.910 --> 00:56:41.980 align:middle line:84% Like I mentioned, it's a discrete symmetry. 00:56:41.980 --> 00:56:43.900 align:middle line:84% Moreover, in this case, you could 00:56:43.900 --> 00:56:46.340 align:middle line:84% be changing the angle of rotation 00:56:46.340 --> 00:56:47.960 align:middle line:90% at different times and places. 00:56:47.960 --> 00:56:50.730 align:middle line:84% This is what's called a local transformation. 00:56:50.730 --> 00:56:53.360 align:middle line:84% So it's not just that you take a red color quark 00:56:53.360 --> 00:56:56.030 align:middle line:84% and you rotate it once and you're done. 00:56:56.030 --> 00:56:58.790 align:middle line:84% The excitation of the quanta of your quark field 00:56:58.790 --> 00:57:01.790 align:middle line:84% could be rotated by 2 clicks of the turnover 00:57:01.790 --> 00:57:03.830 align:middle line:84% here but by minus 1 click over here, 00:57:03.830 --> 00:57:05.730 align:middle line:84% and you could change your mind over time. 00:57:05.730 --> 00:57:08.120 align:middle line:84% So these are called local transformations, 00:57:08.120 --> 00:57:12.655 align:middle line:84% and the idea, which was really just a hypothesis that 00:57:12.655 --> 00:57:14.030 align:middle line:84% was driving people like Gell-Mann 00:57:14.030 --> 00:57:15.988 align:middle line:84% and Fritzsch and others, was that what would it 00:57:15.988 --> 00:57:19.550 align:middle line:84% take to leave any observable features of such a theory 00:57:19.550 --> 00:57:22.520 align:middle line:84% invariant even under that set of quite 00:57:22.520 --> 00:57:25.080 align:middle line:90% wide-spanning transformations. 00:57:25.080 --> 00:57:27.770 align:middle line:84% So one thing you could do is make sure 00:57:27.770 --> 00:57:29.660 align:middle line:84% that the observable features of the system 00:57:29.660 --> 00:57:33.050 align:middle line:84% only depend on the kind of length of that quantum field. 00:57:33.050 --> 00:57:34.670 align:middle line:84% That would be like a radial symmetry, 00:57:34.670 --> 00:57:36.440 align:middle line:90% if you think back to the sphere. 00:57:36.440 --> 00:57:40.640 align:middle line:84% If you rotate your quantum field by some fancy phase 00:57:40.640 --> 00:57:44.600 align:middle line:84% here, if the observable only depend on the absolute square, 00:57:44.600 --> 00:57:47.390 align:middle line:84% then the rotations will cancel out. 00:57:47.390 --> 00:57:51.960 align:middle line:84% Any local transformation would fall out. 00:57:51.960 --> 00:57:54.350 align:middle line:84% But remember, they're now trying to handle dynamics. 00:57:54.350 --> 00:57:56.660 align:middle line:84% They're trying to explain change over time 00:57:56.660 --> 00:57:58.910 align:middle line:84% and not just a static configuration, 00:57:58.910 --> 00:58:00.600 align:middle line:84% and that's when it gets much trickier. 00:58:00.600 --> 00:58:02.990 align:middle line:84% And this is really where the real work had to come in. 00:58:02.990 --> 00:58:07.000 align:middle line:90% 00:58:07.000 --> 00:58:10.890 align:middle line:84% If you try to describe the behavior of that quantum quark 00:58:10.890 --> 00:58:15.162 align:middle line:84% field over time quantitatively, it obeys an equation kind 00:58:15.162 --> 00:58:17.370 align:middle line:84% of like the Schrodinger equation, a little different, 00:58:17.370 --> 00:58:19.380 align:middle line:90% but it has time dependence. 00:58:19.380 --> 00:58:20.850 align:middle line:90% It has spatial gradients. 00:58:20.850 --> 00:58:22.230 align:middle line:90% It has derivatives. 00:58:22.230 --> 00:58:25.770 align:middle line:84% Well, now if you perform these local transformations where 00:58:25.770 --> 00:58:29.280 align:middle line:84% your rotation angle itself can, in principle, depend 00:58:29.280 --> 00:58:32.310 align:middle line:84% on both space and time, you get in trouble 00:58:32.310 --> 00:58:34.770 align:middle line:84% with the kinetic energy, with the change 00:58:34.770 --> 00:58:36.210 align:middle line:90% of that quantum field. 00:58:36.210 --> 00:58:38.760 align:middle line:84% If the field itself undergoes a rotation-- 00:58:38.760 --> 00:58:41.130 align:middle line:90% so psi goes to psi prime-- 00:58:41.130 --> 00:58:44.130 align:middle line:84% now we're taking the time change of this slightly more 00:58:44.130 --> 00:58:46.710 align:middle line:84% complicated object, and we'll have a chain rule. 00:58:46.710 --> 00:58:51.240 align:middle line:84% There'll be psi times the derivative of theta plus theta 00:58:51.240 --> 00:58:52.620 align:middle line:90% times the derivative of psi. 00:58:52.620 --> 00:58:55.950 align:middle line:84% It will not be such that the phase terms will just 00:58:55.950 --> 00:58:56.740 align:middle line:90% cancel out. 00:58:56.740 --> 00:58:59.910 align:middle line:84% If we go back to this term for the kinetic energy, 00:58:59.910 --> 00:59:05.255 align:middle line:84% if the derivative of the rotated quark field had just a phase 00:59:05.255 --> 00:59:06.880 align:middle line:84% pulled out front, then it would cancel. 00:59:06.880 --> 00:59:09.850 align:middle line:84% We'd have one term times its complex conjugate. 00:59:09.850 --> 00:59:12.340 align:middle line:84% It would be just like the radial symmetry here, 00:59:12.340 --> 00:59:16.220 align:middle line:84% but we don't have that anymore because of the chain rule. 00:59:16.220 --> 00:59:20.440 align:middle line:84% We have to involve a more complicated set of dynamics 00:59:20.440 --> 00:59:23.870 align:middle line:84% to ensure that overall symmetry is preserved. 00:59:23.870 --> 00:59:26.110 align:middle line:84% And so that's what then Gell-Mann, Fritzsch, 00:59:26.110 --> 00:59:27.580 align:middle line:90% and their colleagues do next. 00:59:27.580 --> 00:59:30.820 align:middle line:84% To maintain that symmetry under local rotations 00:59:30.820 --> 00:59:32.020 align:middle line:90% in that color space-- 00:59:32.020 --> 00:59:35.050 align:middle line:84% red, green, and blue quark color-- 00:59:35.050 --> 00:59:37.870 align:middle line:84% they add in these force-carrying particles 00:59:37.870 --> 00:59:40.870 align:middle line:84% with very specific properties that the gluons 00:59:40.870 --> 00:59:42.710 align:middle line:84% have to behave a certain way as well. 00:59:42.710 --> 00:59:44.900 align:middle line:84% In fact, the gluons in some sense, 00:59:44.900 --> 00:59:49.160 align:middle line:84% are added for the sole purpose of enforcing that symmetry. 00:59:49.160 --> 00:59:52.480 align:middle line:84% So the quark field could undergo these very fancy rotations, 00:59:52.480 --> 00:59:54.520 align:middle line:84% all the permutations of color charge, 00:59:54.520 --> 00:59:56.440 align:middle line:84% and to leave all the observables, including 00:59:56.440 --> 00:59:59.170 align:middle line:84% the dynamics of forces over time. 00:59:59.170 --> 01:00:02.800 align:middle line:84% To leave all those unchanged, these other kinds of matter, 01:00:02.800 --> 01:00:06.190 align:middle line:84% the gluon fields, have to have very specific properties. 01:00:06.190 --> 01:00:07.720 align:middle line:84% And this goes is a little technical. 01:00:07.720 --> 01:00:09.920 align:middle line:84% Don't worry if this goes by too fast. 01:00:09.920 --> 01:00:10.803 align:middle line:90% There's no quiz. 01:00:10.803 --> 01:00:12.220 align:middle line:84% But what they do is they construct 01:00:12.220 --> 01:00:17.080 align:middle line:84% what's called a covariant derivative, which combines 01:00:17.080 --> 01:00:18.550 align:middle line:90% the change in space and time. 01:00:18.550 --> 01:00:21.190 align:middle line:84% This is just shorthand for taking the time derivative 01:00:21.190 --> 01:00:22.433 align:middle line:90% and the spatial gradient. 01:00:22.433 --> 01:00:24.100 align:middle line:84% This is just the derivative of the quark 01:00:24.100 --> 01:00:25.930 align:middle line:90% field in time and space. 01:00:25.930 --> 01:00:28.390 align:middle line:84% But instead of only worrying about its change that way, 01:00:28.390 --> 01:00:29.800 align:middle line:90% you say it also coupled. 01:00:29.800 --> 01:00:33.130 align:middle line:84% It also interacts with these gluon fields. 01:00:33.130 --> 01:00:36.070 align:middle line:84% And if the gluon field itself has 01:00:36.070 --> 01:00:40.300 align:middle line:84% a very specific transformation-- so if you rotate a quark color, 01:00:40.300 --> 01:00:42.610 align:middle line:84% you also have to do a transformation of the gluon 01:00:42.610 --> 01:00:44.140 align:middle line:90% field at the same time. 01:00:44.140 --> 01:00:47.590 align:middle line:84% If you choose that transformation very cleverly, 01:00:47.590 --> 01:00:51.940 align:middle line:84% then your covariant derivative, this combination of changes 01:00:51.940 --> 01:00:54.910 align:middle line:84% in space and time plus the very particular way 01:00:54.910 --> 01:00:57.550 align:middle line:84% that the quark field interacts with the gluon field, 01:00:57.550 --> 01:00:59.870 align:middle line:84% that will leave this souped-up derivative, 01:00:59.870 --> 01:01:02.470 align:middle line:84% the covariant derivative, such that you actually 01:01:02.470 --> 01:01:05.150 align:middle line:84% get this overall phase factor out front, 01:01:05.150 --> 01:01:08.060 align:middle line:84% and now the kinetic energy remains unchanged. 01:01:08.060 --> 01:01:11.020 align:middle line:84% Now your e to the plus i theta is exactly 01:01:11.020 --> 01:01:12.980 align:middle line:84% canceled by its complex conjugate, 01:01:12.980 --> 01:01:16.840 align:middle line:84% so now your souped-up kinetic term 01:01:16.840 --> 01:01:19.360 align:middle line:84% remains totally invariant even under these more 01:01:19.360 --> 01:01:22.570 align:middle line:84% elaborate color charge symmetries. 01:01:22.570 --> 01:01:25.150 align:middle line:84% So you can have quarks that run around the world 01:01:25.150 --> 01:01:26.320 align:middle line:90% with different colors-- 01:01:26.320 --> 01:01:27.580 align:middle line:90% red, green, blue. 01:01:27.580 --> 01:01:30.160 align:middle line:84% The color assignment to a given quark 01:01:30.160 --> 01:01:32.680 align:middle line:84% could change over time and across space, 01:01:32.680 --> 01:01:35.830 align:middle line:84% and as long as you have an additional kind of matter 01:01:35.830 --> 01:01:38.620 align:middle line:84% running around with them, these gluon fields, 01:01:38.620 --> 01:01:41.800 align:middle line:84% and if you specify very quantitatively how 01:01:41.800 --> 01:01:44.260 align:middle line:84% the gluons have to behave much like the quarks, 01:01:44.260 --> 01:01:47.327 align:middle line:84% together, the whole assembly will remain symmetric 01:01:47.327 --> 01:01:48.160 align:middle line:90% under that symmetry. 01:01:48.160 --> 01:01:50.600 align:middle line:84% That's, again, a bit more abstract, 01:01:50.600 --> 01:01:52.870 align:middle line:84% but the idea conceptually is, I think, 01:01:52.870 --> 01:01:55.150 align:middle line:90% really pretty astonishing. 01:01:55.150 --> 01:01:57.940 align:middle line:84% Basically, Gell-Mann and Fritzsch and their colleagues 01:01:57.940 --> 01:02:00.070 align:middle line:84% have to dream up, have to hypothesize 01:02:00.070 --> 01:02:04.780 align:middle line:84% a whole new kind of matter, the gluon fields, strictly 01:02:04.780 --> 01:02:06.970 align:middle line:84% to enforce a symmetry which was just 01:02:06.970 --> 01:02:09.080 align:middle line:84% of their own thinking in the first place. 01:02:09.080 --> 01:02:12.250 align:middle line:84% So because they're so enamored by the symmetry properties that 01:02:12.250 --> 01:02:14.147 align:middle line:84% made sense of all the kind of classifications 01:02:14.147 --> 01:02:16.480 align:middle line:84% of the nuclear particles-- hypercharge, isospin, and all 01:02:16.480 --> 01:02:19.810 align:middle line:84% those eightfold ways and tenfold ways and so on, 01:02:19.810 --> 01:02:23.865 align:middle line:84% they're really smitten with this idea of an underlying symmetry. 01:02:23.865 --> 01:02:25.240 align:middle line:84% But when they really, really want 01:02:25.240 --> 01:02:29.980 align:middle line:84% to describe the forces that would obey that symmetry, 01:02:29.980 --> 01:02:31.720 align:middle line:84% remain consistent with that symmetry, 01:02:31.720 --> 01:02:33.730 align:middle line:84% they can't do it with quarks alone. 01:02:33.730 --> 01:02:35.440 align:middle line:84% They have to have quarks interacting 01:02:35.440 --> 01:02:38.800 align:middle line:84% in a very specific way within a whole separate family 01:02:38.800 --> 01:02:41.410 align:middle line:84% of matter, the gluon field, and in this kind 01:02:41.410 --> 01:02:43.780 align:middle line:84% of coordinated dance, and then the whole collection, 01:02:43.780 --> 01:02:48.100 align:middle line:84% this kind of rickety collection would protect or enforce 01:02:48.100 --> 01:02:48.700 align:middle line:90% that symmetry. 01:02:48.700 --> 01:02:52.170 align:middle line:90% I find that pretty astonishing. 01:02:52.170 --> 01:02:54.690 align:middle line:84% So the symmetry of quantum chromodynamics 01:02:54.690 --> 01:02:58.440 align:middle line:84% is more complicated than that of other examples of these kinds 01:02:58.440 --> 01:03:00.450 align:middle line:84% of models, like quantum electrodynamics, 01:03:00.450 --> 01:03:02.400 align:middle line:84% and therefore, the properties of the gluons 01:03:02.400 --> 01:03:04.692 align:middle line:84% are actually more complicated than those of the photon. 01:03:04.692 --> 01:03:09.300 align:middle line:84% In particular, the color charge is a discrete symmetry, 01:03:09.300 --> 01:03:12.690 align:middle line:84% whereas the phase freedom for the electric charge 01:03:12.690 --> 01:03:13.930 align:middle line:90% is a continuous symmetry. 01:03:13.930 --> 01:03:15.720 align:middle line:84% So the gluons really have much more kind 01:03:15.720 --> 01:03:18.750 align:middle line:84% of articulated mathematical structure, 01:03:18.750 --> 01:03:21.630 align:middle line:84% and one consequence of that is that the gluons can actually 01:03:21.630 --> 01:03:23.220 align:middle line:90% interact with other gluons. 01:03:23.220 --> 01:03:25.860 align:middle line:84% Or at least, they're all still hypothetical at this stage, 01:03:25.860 --> 01:03:28.650 align:middle line:84% but in order for all that symmetry preservation stuff 01:03:28.650 --> 01:03:30.870 align:middle line:84% to work, for the math to work out, 01:03:30.870 --> 01:03:34.050 align:middle line:84% the gluons have to be able to scatter off of other gluons 01:03:34.050 --> 01:03:36.840 align:middle line:84% or attract other gluons, whereas photons don't do that. 01:03:36.840 --> 01:03:39.360 align:middle line:84% Photons don't have electric charge, 01:03:39.360 --> 01:03:42.990 align:middle line:84% so photons only scatter off objects with electric charge, 01:03:42.990 --> 01:03:45.450 align:middle line:90% at least according to QED. 01:03:45.450 --> 01:03:47.940 align:middle line:84% So photons will scatter off of electrons 01:03:47.940 --> 01:03:49.440 align:middle line:90% or positrons or protons. 01:03:49.440 --> 01:03:52.180 align:middle line:84% Photons won't scatter off of neutrons, at least not 01:03:52.180 --> 01:03:54.340 align:middle line:84% directly, and they certainly won't scatter off 01:03:54.340 --> 01:03:57.280 align:middle line:84% each other, whereas gluons can scatter off each other 01:03:57.280 --> 01:03:59.830 align:middle line:90% as well as off of the quarks. 01:03:59.830 --> 01:04:03.100 align:middle line:84% Another way to say that a bit more succinctly 01:04:03.100 --> 01:04:07.690 align:middle line:84% is that gluons also carry color charge much like quarks do, 01:04:07.690 --> 01:04:10.420 align:middle line:84% whereas photons do not carry electric charge. 01:04:10.420 --> 01:04:13.660 align:middle line:84% So the fact that gluons can attract each other actually 01:04:13.660 --> 01:04:16.510 align:middle line:84% changes the kind of force law between quarks. 01:04:16.510 --> 01:04:20.537 align:middle line:84% The force between quarks will actually grow with distance 01:04:20.537 --> 01:04:22.870 align:middle line:84% because the distance between them is filled with gluons, 01:04:22.870 --> 01:04:25.400 align:middle line:84% and the gluons themselves can interact with each other. 01:04:25.400 --> 01:04:27.160 align:middle line:84% So it starts looking a bit actually 01:04:27.160 --> 01:04:28.780 align:middle line:84% like a kind of stretched rubber band. 01:04:28.780 --> 01:04:31.570 align:middle line:84% There's a kind of elastic restoring force 01:04:31.570 --> 01:04:34.990 align:middle line:84% at least as predicted by quantum chromodynamics which 01:04:34.990 --> 01:04:38.590 align:middle line:84% has no analog in, say, the force between an electron 01:04:38.590 --> 01:04:39.970 align:middle line:90% and a positron. 01:04:39.970 --> 01:04:42.370 align:middle line:84% Remember, the attractive force between 01:04:42.370 --> 01:04:45.170 align:middle line:84% opposite electric charges falls off with the distance. 01:04:45.170 --> 01:04:46.420 align:middle line:90% That's a famous Coulomb's law. 01:04:46.420 --> 01:04:48.253 align:middle line:84% In fact, the force falls off like the square 01:04:48.253 --> 01:04:49.070 align:middle line:90% of the distance. 01:04:49.070 --> 01:04:51.890 align:middle line:84% So they could still exert an attraction across space, 01:04:51.890 --> 01:04:54.000 align:middle line:84% but if you double the distance between them, 01:04:54.000 --> 01:04:58.660 align:middle line:84% the strength of the attraction falls by a factor of 4. 01:04:58.660 --> 01:05:01.120 align:middle line:84% With quarks, attracting each other 01:05:01.120 --> 01:05:04.510 align:middle line:84% by this highly symmetric, strong nuclear force, 01:05:04.510 --> 01:05:07.150 align:middle line:84% it seems to go the other way, that the force actually 01:05:07.150 --> 01:05:09.040 align:middle line:84% seems to get stronger with distance, 01:05:09.040 --> 01:05:11.290 align:middle line:84% and the explanation of why it's so different 01:05:11.290 --> 01:05:13.840 align:middle line:84% is because the gluons that are mediating 01:05:13.840 --> 01:05:16.850 align:middle line:84% that force can actually kind of glue on to each other, 01:05:16.850 --> 01:05:17.350 align:middle line:90% so to speak. 01:05:17.350 --> 01:05:18.530 align:middle line:90% They can attract each other. 01:05:18.530 --> 01:05:20.260 align:middle line:84% So if you imagine trying to stretch 01:05:20.260 --> 01:05:22.792 align:middle line:84% to a bound state of a quark and an antiquark 01:05:22.792 --> 01:05:24.250 align:middle line:84% apart-- this could be, for example, 01:05:24.250 --> 01:05:27.400 align:middle line:84% a pion, some sort of meson, and you 01:05:27.400 --> 01:05:32.930 align:middle line:84% could instill external energy by pulling on these two particles 01:05:32.930 --> 01:05:35.150 align:middle line:84% to try to pry them apart by adding external energy 01:05:35.150 --> 01:05:36.430 align:middle line:90% to the system. 01:05:36.430 --> 01:05:39.370 align:middle line:84% At some point, you will have to add so much external energy, 01:05:39.370 --> 01:05:42.850 align:middle line:84% you'll be tugging so hard, adding so much external energy, 01:05:42.850 --> 01:05:47.080 align:middle line:84% that you'll actually create a whole new quark-antiquark pair. 01:05:47.080 --> 01:05:49.030 align:middle line:84% In a sense, these were already around. 01:05:49.030 --> 01:05:51.310 align:middle line:84% They were virtual quark-antiquark particles, 01:05:51.310 --> 01:05:53.170 align:middle line:84% but they can now borrow the energy 01:05:53.170 --> 01:05:56.320 align:middle line:84% you've given to that system to become real particles. 01:05:56.320 --> 01:05:58.763 align:middle line:84% They now can pay off their debt to the vacuum. 01:05:58.763 --> 01:06:00.430 align:middle line:84% If they were virtual particles, they now 01:06:00.430 --> 01:06:03.250 align:middle line:84% can pay off the energy they borrowed as soon 01:06:03.250 --> 01:06:08.110 align:middle line:84% as you give to the system enough energy to cover their masses. 01:06:08.110 --> 01:06:10.930 align:middle line:84% So in fact, when you try to pull a quark 01:06:10.930 --> 01:06:13.030 align:middle line:84% and an antiquark apart within this scheme, 01:06:13.030 --> 01:06:14.980 align:middle line:84% at least hypothetically, you'll never 01:06:14.980 --> 01:06:18.190 align:middle line:90% get a free quark isolated. 01:06:18.190 --> 01:06:20.240 align:middle line:84% You'll never see a fractional charge. 01:06:20.240 --> 01:06:22.330 align:middle line:84% In fact, what you do is you create 01:06:22.330 --> 01:06:25.150 align:middle line:84% two bound states of quark-antiquark pairs 01:06:25.150 --> 01:06:27.020 align:middle line:90% where before you only had one. 01:06:27.020 --> 01:06:31.510 align:middle line:84% And so that, again, comes because the gluons can 01:06:31.510 --> 01:06:33.320 align:middle line:90% interact with gluons directly. 01:06:33.320 --> 01:06:35.680 align:middle line:84% So that was a prediction from QCD 01:06:35.680 --> 01:06:38.200 align:middle line:84% by the later 1970s that was worked out 01:06:38.200 --> 01:06:41.560 align:middle line:84% a few years after this symmetry-preserving scheme, 01:06:41.560 --> 01:06:43.630 align:middle line:84% and that, again, gave experimental colleagues 01:06:43.630 --> 01:06:46.030 align:middle line:84% something to look for, some specific kind of thing 01:06:46.030 --> 01:06:48.550 align:middle line:84% to look for in the high-energy scatterings 01:06:48.550 --> 01:06:50.020 align:middle line:90% of these nuclear particles. 01:06:50.020 --> 01:06:54.640 align:middle line:84% And the first evidence in concert with that idea 01:06:54.640 --> 01:06:57.580 align:middle line:84% was presented by an experimental team in Europe. 01:06:57.580 --> 01:07:01.240 align:middle line:84% I think this was a group in Germany in the early 1980s. 01:07:01.240 --> 01:07:03.220 align:middle line:90% These came to be called jets. 01:07:03.220 --> 01:07:06.340 align:middle line:84% The idea would have these jets streaming off 01:07:06.340 --> 01:07:08.680 align:middle line:84% where the original beam might have gone in one direction 01:07:08.680 --> 01:07:14.830 align:middle line:84% and is not quite torn-apart but rapidly reassembled pair 01:07:14.830 --> 01:07:20.230 align:middle line:84% of mesons would jet off in opposite directions or nearly 01:07:20.230 --> 01:07:21.200 align:middle line:90% opposite directions. 01:07:21.200 --> 01:07:23.500 align:middle line:84% So you can have a very specific pattern 01:07:23.500 --> 01:07:27.220 align:middle line:84% of the debris coming out of the very high-energy collisions 01:07:27.220 --> 01:07:29.350 align:middle line:84% of these particles because in the effort 01:07:29.350 --> 01:07:32.650 align:middle line:84% to crack apart a quark and an antiquark pair, for example, 01:07:32.650 --> 01:07:35.890 align:middle line:84% you wind up making more kinds of particles that will jet off 01:07:35.890 --> 01:07:37.250 align:middle line:90% away from each other. 01:07:37.250 --> 01:07:40.960 align:middle line:84% So these QCD or quark-gluon jets were actually 01:07:40.960 --> 01:07:43.150 align:middle line:84% a prediction of this new theory based, again, 01:07:43.150 --> 01:07:45.070 align:middle line:84% on the idea of preserving symmetry and having 01:07:45.070 --> 01:07:47.830 align:middle line:84% a certain way in which the force-carrying particles 01:07:47.830 --> 01:07:49.660 align:middle line:84% interact with the quarks, and that 01:07:49.660 --> 01:07:52.480 align:middle line:84% was at least assistant with some of the new experimental data 01:07:52.480 --> 01:07:54.040 align:middle line:90% within just a few years. 01:07:54.040 --> 01:07:57.610 align:middle line:84% It was at that point by the late '70s, early 80s, 01:07:57.610 --> 01:08:02.560 align:middle line:84% not in 1964 or 1967 or any other steps along the way-- 01:08:02.560 --> 01:08:04.720 align:middle line:84% it was really at that point that the community 01:08:04.720 --> 01:08:06.460 align:middle line:84% began to coalesce around the idea 01:08:06.460 --> 01:08:09.280 align:middle line:84% that quarks might actually be real particles in nature 01:08:09.280 --> 01:08:11.260 align:middle line:84% and not just a mathematical abstraction. 01:08:11.260 --> 01:08:14.260 align:middle line:90% So let me wrap up here. 01:08:14.260 --> 01:08:17.260 align:middle line:84% One of the responses to this particle 01:08:17.260 --> 01:08:20.170 align:middle line:84% zoo after the end of the Second World War, as we saw, 01:08:20.170 --> 01:08:27.910 align:middle line:84% was to double down on dynamics, like Geoffrey Chew, 01:08:27.910 --> 01:08:30.870 align:middle line:84% to say all these particles are, in some deep sense, 01:08:30.870 --> 01:08:32.520 align:middle line:84% equally elementary because they're 01:08:32.520 --> 01:08:34.800 align:middle line:90% all composites of each other. 01:08:34.800 --> 01:08:37.680 align:middle line:84% But as we've been tracking in today's class, 01:08:37.680 --> 01:08:40.979 align:middle line:84% there's a kind of parallel complementary approach 01:08:40.979 --> 01:08:44.087 align:middle line:84% really kind of led by Murray Gell-Mann, 01:08:44.087 --> 01:08:46.170 align:middle line:84% though many other people were making contributions 01:08:46.170 --> 01:08:49.560 align:middle line:84% along the way, which was to bracket the question of forces 01:08:49.560 --> 01:08:53.520 align:middle line:84% or dynamics for a while, really, for 20 years, 01:08:53.520 --> 01:08:56.040 align:middle line:84% and just to try to figure out, are there 01:08:56.040 --> 01:08:58.500 align:middle line:84% patterns in the kind of detritus that comes out 01:08:58.500 --> 01:09:01.890 align:middle line:84% of those new accelerators to emphasize classification 01:09:01.890 --> 01:09:05.250 align:middle line:84% and internal abstract symmetries? 01:09:05.250 --> 01:09:08.550 align:middle line:84% So Gell-Mann and others-- but Gell-Mann, again, 01:09:08.550 --> 01:09:10.080 align:middle line:90% really took the lead at this-- 01:09:10.080 --> 01:09:12.540 align:middle line:84% began grouping these particles into families 01:09:12.540 --> 01:09:16.859 align:middle line:84% based on these kind of invented new quantities that 01:09:16.859 --> 01:09:21.600 align:middle line:84% did not have an obvious relation to measurable properties. 01:09:21.600 --> 01:09:23.100 align:middle line:90% So he invents hypercharge. 01:09:23.100 --> 01:09:26.340 align:middle line:84% He makes use of Heisenberg's older notion of isospin, 01:09:26.340 --> 01:09:28.170 align:middle line:84% and he starts grouping these particles 01:09:28.170 --> 01:09:30.689 align:middle line:84% in these very abstract, mathematical, highly 01:09:30.689 --> 01:09:33.660 align:middle line:84% symmetric spaces of hypercharge versus isospin. 01:09:33.660 --> 01:09:37.200 align:middle line:84% That enables him to find kind of gaps 01:09:37.200 --> 01:09:39.510 align:middle line:84% where he predicts very boldly that there should 01:09:39.510 --> 01:09:41.130 align:middle line:90% be actual particles out there. 01:09:41.130 --> 01:09:43.830 align:middle line:84% Famously, this works for the omega minus, 01:09:43.830 --> 01:09:49.260 align:middle line:84% and then he suggests very kind of schematically, heuristically 01:09:49.260 --> 01:09:52.620 align:middle line:84% early in 1964 that maybe those patterns were 01:09:52.620 --> 01:09:55.770 align:middle line:84% consistent with a small set of something more fundamental 01:09:55.770 --> 01:09:58.260 align:middle line:84% that might not actually be real bits of matter. 01:09:58.260 --> 01:10:00.390 align:middle line:84% So the quarks or what Zweig calls 01:10:00.390 --> 01:10:02.580 align:middle line:84% aces are very kind of tentatively 01:10:02.580 --> 01:10:06.227 align:middle line:84% suggested as a kind of way of accounting for symmetries 01:10:06.227 --> 01:10:08.310 align:middle line:84% rather than saying these things are out there part 01:10:08.310 --> 01:10:10.470 align:middle line:84% of the world, in part because there 01:10:10.470 --> 01:10:13.230 align:middle line:84% was such steep conceptual challenges to thinking 01:10:13.230 --> 01:10:14.520 align:middle line:90% of them as part of the world-- 01:10:14.520 --> 01:10:17.070 align:middle line:84% fractional electric charge, exclusion principle, and all 01:10:17.070 --> 01:10:18.820 align:middle line:90% the rest. 01:10:18.820 --> 01:10:21.370 align:middle line:84% The SLAC-MIT experiments or, I should say, 01:10:21.370 --> 01:10:24.850 align:middle line:84% the MIT-SLAC experiments from the later '60s produce 01:10:24.850 --> 01:10:28.420 align:middle line:84% suggestive compelling evidence that protons and similar 01:10:28.420 --> 01:10:31.450 align:middle line:84% nuclear particles had internal structure, 01:10:31.450 --> 01:10:33.790 align:middle line:84% but it was not at all clear to the community, 01:10:33.790 --> 01:10:36.640 align:middle line:84% even to people like Gell-Mann, let alone to friendly rivals 01:10:36.640 --> 01:10:37.840 align:middle line:90% like Richard Feynman-- 01:10:37.840 --> 01:10:39.970 align:middle line:84% it was not at all clear right away 01:10:39.970 --> 01:10:42.010 align:middle line:84% that those internal scattering sites 01:10:42.010 --> 01:10:43.780 align:middle line:90% were the same as these quarks. 01:10:43.780 --> 01:10:46.480 align:middle line:84% In fact, even years later, Gell-Mann 01:10:46.480 --> 01:10:48.550 align:middle line:84% would say that quarks are fictitious. 01:10:48.550 --> 01:10:50.980 align:middle line:84% There might be parton structure within protons, 01:10:50.980 --> 01:10:52.660 align:middle line:84% but that need not be the same as quarks. 01:10:52.660 --> 01:10:55.570 align:middle line:84% We saw that was in hindsight taken 01:10:55.570 --> 01:11:00.130 align:middle line:84% to be very, very compelling evidence that quarks exist. 01:11:00.130 --> 01:11:03.010 align:middle line:84% Jerry Friedman won the Nobel Prize 01:11:03.010 --> 01:11:04.780 align:middle line:90% for leading these experiments. 01:11:04.780 --> 01:11:07.900 align:middle line:84% And now we say this is in hindsight one of the really 01:11:07.900 --> 01:11:09.550 align:middle line:84% important touchstones for why we think 01:11:09.550 --> 01:11:11.710 align:middle line:84% quarks are physical particles, but again, it's 01:11:11.710 --> 01:11:14.650 align:middle line:84% important to see this didn't tie everything up with a bow 01:11:14.650 --> 01:11:15.920 align:middle line:90% right away. 01:11:15.920 --> 01:11:19.550 align:middle line:84% It was really the combination of that plus follow-on experiments 01:11:19.550 --> 01:11:23.840 align:middle line:84% like the jets from 15 years later plus a much more 01:11:23.840 --> 01:11:26.210 align:middle line:84% articulated theoretical structure 01:11:26.210 --> 01:11:28.580 align:middle line:84% which reintroduces quantum fields, 01:11:28.580 --> 01:11:31.610 align:middle line:84% doubles down on these internal symmetries that becomes known 01:11:31.610 --> 01:11:32.840 align:middle line:90% as quantum chromodynamics. 01:11:32.840 --> 01:11:36.530 align:middle line:84% This combination of theoretical and experimental ideas 01:11:36.530 --> 01:11:40.940 align:middle line:84% and contributions come together over a kind of 20-year period, 01:11:40.940 --> 01:11:43.670 align:middle line:84% by the end of which most physicists then would say, 01:11:43.670 --> 01:11:45.080 align:middle line:90% the world is made of quarks. 01:11:45.080 --> 01:11:48.450 align:middle line:84% They obey a very specific kind of force, 01:11:48.450 --> 01:11:50.840 align:middle line:84% and even though quarks themselves 01:11:50.840 --> 01:11:53.330 align:middle line:84% are physical particles with fractional charge, 01:11:53.330 --> 01:11:56.660 align:middle line:84% the particles we ever can measure in our experiments 01:11:56.660 --> 01:11:58.610 align:middle line:84% only have integer units of charge. 01:11:58.610 --> 01:11:59.550 align:middle line:90% So I'll stop there. 01:11:59.550 --> 01:12:01.633 align:middle line:84% We have time for some more questions or discussion 01:12:01.633 --> 01:12:02.600 align:middle line:90% if people would like. 01:12:02.600 --> 01:12:03.510 align:middle line:90% Stay well. 01:12:03.510 --> 01:12:04.940 align:middle line:90% I'll see you on Wednesday. 01:12:04.940 --> 01:12:07.180 align:middle line:90% Take care, everyone. 01:12:07.180 --> 01:12:20.000 align:middle line:90%