1 00:00:00,880 --> 00:00:05,653 So that's a very nice story. I gave you a whole rigmarole and I claimed that this 2 00:00:05,653 --> 00:00:10,078 is how the sun produces it's energy. How would you go about verifying such a 3 00:00:10,078 --> 00:00:12,640 thing? You can't get inside the suns, there's 4 00:00:12,640 --> 00:00:17,239 hundred of thousands of kilometers of sun between you and the core, where these 5 00:00:17,239 --> 00:00:20,848 reactions are going on. Which is very good because it prevents 6 00:00:20,848 --> 00:00:25,737 all the gamma rays from escaping. And the basic reason we knew how stars 7 00:00:25,737 --> 00:00:31,520 worked in the 30's was because Eddington had the theoretical idea and later 8 00:00:31,520 --> 00:00:35,630 figured out how to get through the weak decay process. 9 00:00:35,630 --> 00:00:40,509 But it turns out that we can in fact look right inside the sun. With a beautiful 10 00:00:40,509 --> 00:00:44,901 experiment, the idea is due to an experimentalist called Ray Davis and a 11 00:00:44,901 --> 00:00:49,719 theorist called John Bahcall who in the 60's, come up with the idea that, wait a 12 00:00:49,719 --> 00:00:52,891 minute. One of the properties one of the products 13 00:00:52,891 --> 00:00:56,490 of this PP process that we discussed, is electron neutrinos. 14 00:00:56,490 --> 00:01:01,318 Two electron neutrinos for every alpha particle, for every helium nucleus form. 15 00:01:01,318 --> 00:01:05,909 And we know how many helium nuclei are formed per second, because we know the 16 00:01:05,909 --> 00:01:09,188 total luminosity. So we know how many neutrinos the Sun 17 00:01:09,188 --> 00:01:13,159 produces every second. These neutrinos are produced in the core, 18 00:01:13,159 --> 00:01:17,246 but, being neutrinos, they sail right through hundreds of thousands of 19 00:01:17,246 --> 00:01:22,046 kilometers of sun, with essentially no attenuation, maybe three interact, it 20 00:01:22,046 --> 00:01:25,870 doesn't matter. This is great because this gives us a 21 00:01:25,870 --> 00:01:30,402 telescope that essentially eliminates all of the distraction of the sun and lets us 22 00:01:30,402 --> 00:01:33,802 look right into the core. The disadvantage, of course, is [LAUGH] 23 00:01:33,802 --> 00:01:37,902 that they also sail through the earth and through whatever your detector is. 24 00:01:37,902 --> 00:01:42,057 And most of the neutrinos will cause absolutely no interaction to give you a 25 00:01:42,057 --> 00:01:46,212 sense, as I said, ten to the eleven solar neutrinos hit every meter squared of 26 00:01:46,212 --> 00:01:49,449 earth every second. So you are being bombarded with millions 27 00:01:49,449 --> 00:01:53,297 of electrons, of neutrinos as we speak. Obviously nothing happens. 28 00:01:53,297 --> 00:01:57,653 So, how do you build a detector such that something will happen. 29 00:01:57,653 --> 00:02:03,391 Well in the 60's these two guys put together this brilliant experiment, where 30 00:02:03,391 --> 00:02:07,901 essentially they take a huge tank of cleaning solvent, bleach, which 31 00:02:07,901 --> 00:02:13,106 containing's chlorine. They stick it underground in a gold mine and the other 32 00:02:13,106 --> 00:02:17,920 reason they stick underground is to prevent cosmic ray and to insulate it 33 00:02:17,920 --> 00:02:22,409 from all kinds of noise. and once in a very long while an electron 34 00:02:22,409 --> 00:02:28,184 neutrino, penetrating this tank of chlorine will interact with a neutron and 35 00:02:28,184 --> 00:02:34,223 a chlorine atom and convert that neutron, through the reaction that we talked 36 00:02:34,223 --> 00:02:40,184 about, where an electron neutrino plus a neutron can become a proton plus an 37 00:02:40,184 --> 00:02:44,326 electron. through that reaction this occurs inside 38 00:02:44,326 --> 00:02:49,957 the nucleus that converts one of the neutrons here to a proton that converts 39 00:02:49,957 --> 00:02:54,930 chlorine to an radioactive, unstable isotope of argon, a noble gas. 40 00:02:54,930 --> 00:03:00,390 Unfortunately, the actual neutrinos from the pp process that I showed, the way it 41 00:03:00,390 --> 00:03:04,076 works would not have sufficient energy to lead to this reaction. 42 00:03:04,076 --> 00:03:08,147 But there are some other modes of interaction that do produce high-energy 43 00:03:08,147 --> 00:03:11,007 neutrinos. And so one could imagine detecting the 44 00:03:11,007 --> 00:03:13,978 neutrinos from those. And if you understand your model 45 00:03:13,978 --> 00:03:18,323 correctly, then you can relate how many pp processes go on to how many of these 46 00:03:18,323 --> 00:03:21,899 other processes are going on because you understand the rates. 47 00:03:21,899 --> 00:03:26,371 You do the calculation, you figure out how many high energy neutrinos the sun 48 00:03:26,371 --> 00:03:30,401 produces, it's a copious number. How many of them hit your detector? 49 00:03:30,401 --> 00:03:33,149 It's a huge number. How many of them interact? 50 00:03:33,149 --> 00:03:37,668 One argon atom should be formed in this big tank of bleach every six days. 51 00:03:37,668 --> 00:03:40,660 So Mr. Davis' job is to detect one atom of argon 52 00:03:40,660 --> 00:03:45,484 every six days, he flushes the detector and looks for the radioactive signature 53 00:03:45,484 --> 00:03:48,964 of the argon, which would be in the noble gas bubbles out. 54 00:03:48,964 --> 00:03:54,032 And he's trying to find an atom every six days, he empties the tank every couple of 55 00:03:54,032 --> 00:03:58,872 months and looks for fifteen atoms. And he finds some argon in the tank but 56 00:03:58,872 --> 00:04:02,859 less than predicted. And this is a result that comes out in 57 00:04:02,859 --> 00:04:08,399 the late 60s and over 30 or 40 years 30 years I guess, Davis is making these 58 00:04:08,399 --> 00:04:14,075 measurements and continually obtaining results that are too small that do not 59 00:04:14,075 --> 00:04:17,994 match predictions. Remember we know how many neutrinos the 60 00:04:17,994 --> 00:04:23,264 sun forms, because we form precisely two neutrinos every time we form an alpha 61 00:04:23,264 --> 00:04:26,170 particle. And that's how we make our energy. 62 00:04:26,170 --> 00:04:28,879 So maybe people say the solar model is wrong. 63 00:04:28,879 --> 00:04:33,335 maybe our understanding of course, we know how many alpha particles are 64 00:04:33,335 --> 00:04:36,346 produced. But maybe the details of the rate, ratios 65 00:04:36,346 --> 00:04:40,863 of the rates of the high energy and low energy neutrinos are somehow wrong. 66 00:04:40,863 --> 00:04:45,740 people, this, this is a great incentive for people like Bacall to refine the 67 00:04:45,740 --> 00:04:48,270 solar model to exquisitely high precision. 68 00:04:48,270 --> 00:04:52,504 maybe our understanding of the processes in the detector are wrong. 69 00:04:52,504 --> 00:04:57,161 in fact it turns out in that, that, that the answers given in 2001, by 70 00:04:57,161 --> 00:05:01,758 measurements using the Sudbury Neutrino Observatory this is a completely 71 00:05:01,758 --> 00:05:05,509 different technology. It's sensitive to all different kinds of 72 00:05:05,509 --> 00:05:08,231 neutrinos, including the low-energy neutrinos. 73 00:05:08,231 --> 00:05:12,949 And if you detect the problem with the flux in those then you'd discover that 74 00:05:12,949 --> 00:05:17,103 the Sun doesn't shine. And so the Sudbury Observatory in 2001 75 00:05:17,103 --> 00:05:22,414 realizes, resolves the issue, and what we realize is that what was wrong was our 76 00:05:22,414 --> 00:05:27,011 understanding of particle physics. Particle physics again? 77 00:05:27,011 --> 00:05:29,220 Yeah, I'm sorry. Here it comes. 78 00:05:29,220 --> 00:05:31,912 This is a going to be a fun story, though. 79 00:05:31,912 --> 00:05:35,030 So, we need to extend our table of particles. 80 00:05:35,030 --> 00:05:38,508 Here I'm not bothering with the antiparticles because we understand that 81 00:05:38,508 --> 00:05:42,505 everything comes in doubles. But here is the list excluding the photon 82 00:05:42,505 --> 00:05:46,413 that I didn't bother to list, of all the particles we've seen so far. 83 00:05:46,413 --> 00:05:51,124 I've added their masses in these units we like to use, that we'll discuss later. 84 00:05:51,124 --> 00:05:55,434 And you'll see a neutron is a little bit heavier than a proton as promised. 85 00:05:55,434 --> 00:05:58,250 And the mass of the neutrino is effectively zero. 86 00:05:58,250 --> 00:06:03,208 It turns out that along with electrons, there are other negatively charged 87 00:06:03,208 --> 00:06:06,878 elementary particles much more massive than the electron. 88 00:06:06,878 --> 00:06:12,416 First 200 and then about 3,500 times the mass of the electron and these are the 89 00:06:12,416 --> 00:06:16,795 Mu-Meson and the Tau-Meson. otherwise they're for all intents and 90 00:06:16,795 --> 00:06:19,500 purposes electrons except heavy electrons. 91 00:06:19,500 --> 00:06:23,965 and each of them it turns out, is accompanied by a neutrino. 92 00:06:23,965 --> 00:06:28,111 So just as an electron has an electron neutrino, there's a muon and a muon 93 00:06:28,111 --> 00:06:32,650 neutrino, a tau meson and a tau neutrino. All of the mesons are negatively charged. 94 00:06:32,650 --> 00:06:36,180 All the neutrinos are neutral and don't interact with anything. 95 00:06:36,180 --> 00:06:42,183 except for the fact that the electron neutrino carriers electron number and we 96 00:06:42,183 --> 00:06:48,846 have two analogous conserved numbers for new number carried only by the muon and 97 00:06:48,846 --> 00:06:54,776 his electron and tau number carried only by the tau meson and his electron. 98 00:06:54,776 --> 00:06:59,930 And, so, for example the favorite decay mode, of one or the decay 99 00:06:59,930 --> 00:07:05,750 modes, of a mu meson is miu which is negatively charged decays to an electron 100 00:07:05,750 --> 00:07:10,739 which is negatively charged. plus, well electron number has been 101 00:07:10,739 --> 00:07:13,977 violated. But not if I produce an empty electron 102 00:07:13,977 --> 00:07:18,697 neutrino, and now muon number has disappeared but not if I have a new mu, 103 00:07:18,697 --> 00:07:23,877 and this is indeed one of the allowed and existing decay modes of the negative 104 00:07:23,877 --> 00:07:26,696 muon.. So we have all of these particles and 105 00:07:26,696 --> 00:07:31,876 they're interactions of electrons, the weak interactions of electrons and muons 106 00:07:31,876 --> 00:07:36,990 and so on conserve not only electron number but also mu number and tau number. 107 00:07:36,990 --> 00:07:42,400 And so, for example, the decay in the sun that converts a proton to a neutron 108 00:07:42,400 --> 00:07:46,099 produces, not a muon neutrino, but an electron neutrino. 109 00:07:46,099 --> 00:07:51,509 Similarly, the process by which chlorine changes to argon, can only occur in the 110 00:07:51,509 --> 00:07:57,236 presence of an electron neutrino. And it turns out that neutrinos oscillate 111 00:07:57,236 --> 00:08:02,242 from one kind of neutrino into the other. In other words, electron muon and tau 112 00:08:02,242 --> 00:08:07,132 number are not in fact conserved. Not by the interactions but by the 113 00:08:07,132 --> 00:08:11,504 neutrino propagation itself, neutrinos spontaneously change from one flavor to 114 00:08:11,504 --> 00:08:15,989 the other, other en route, this is called neutrino oscillation it's the discovery 115 00:08:15,989 --> 00:08:20,194 at 2001, I think it was the 2002 Nobel prize including Ray Davis for his 116 00:08:20,194 --> 00:08:22,885 prediction. the PP process as I said produces 117 00:08:22,885 --> 00:08:27,202 electron neutrinos, by the time they get here they've all mixed up and only a 118 00:08:27,202 --> 00:08:31,799 third of them are electron neutrinos, the other two thirds are mu or tau neutrinos 119 00:08:31,799 --> 00:08:36,060 which do not interact in the way that Davis was looking for in his detector. 120 00:08:36,060 --> 00:08:39,679 And in particular this implies that despite the fact that we thought 121 00:08:39,679 --> 00:08:43,560 neutrinos are massless, the question marks on their masses in the previous 122 00:08:43,560 --> 00:08:47,808 slide are because neutrino oscillations and there have been other discoveries on 123 00:08:47,808 --> 00:08:52,109 neutrino oscillations since show us that neutrinos cannot possibly be massless, 124 00:08:52,109 --> 00:08:54,312 and massless particle could not oscillate. 125 00:08:54,312 --> 00:08:57,934 And so we made by us trying to understand the structure of the sun. 126 00:08:57,934 --> 00:09:01,948 Well we got very well refined solar models we also discovered a whole new 127 00:09:01,948 --> 00:09:05,907 property of fundamental particle physics, and I find that really exciting. 128 00:09:05,907 --> 00:09:10,030 We'll meet these neutrinos again, they turn out to be more useful than you'd 129 00:09:10,030 --> 00:09:12,687 imagine. But I thought you would enjoy this great 130 00:09:12,687 --> 00:09:17,515 story where after decades, and I remember sitting in conferences where the particle 131 00:09:17,515 --> 00:09:21,203 physicists would argue with the astro physicists over who was wrong. 132 00:09:21,203 --> 00:09:25,434 And the net result at the end of the day was that we were wrong and they were 133 00:09:25,434 --> 00:09:25,760 right.