1 00:00:00,820 --> 00:00:04,885 Now, if all this particle physics sounds a little bit like magic trick 2 00:00:04,885 --> 00:00:09,241 mumbo-jumbo,, we'll become a little bit more comfortable with it, hopefully, as 3 00:00:09,241 --> 00:00:13,016 we go along and use it. the point to be taken away, is that, you 4 00:00:13,016 --> 00:00:15,340 don't just invent, oh, maybe this happens. 5 00:00:15,340 --> 00:00:19,791 The good thing is that protons and neutrons are things that exist on earth 6 00:00:19,791 --> 00:00:24,479 and upon which we can do experiments and people did and they could measure the 7 00:00:24,479 --> 00:00:27,981 energies involved. The energies involved in nuclear physics 8 00:00:27,981 --> 00:00:32,610 are higher and we understand that exactly why the energies involved in atomic 9 00:00:32,610 --> 00:00:36,884 physics are essentially the electric attraction between a proton and an 10 00:00:36,884 --> 00:00:41,335 electron at a distance of ten to the minus eight centimeters or ten to the 11 00:00:41,335 --> 00:00:46,131 minus ten meter the size of an atom. The nuclear forces are approximately the 12 00:00:46,131 --> 00:00:48,794 right strength, to hold together two protons. 13 00:00:48,794 --> 00:00:53,757 The charge is the same as the charge of the electron and the proton only that the 14 00:00:53,757 --> 00:00:57,026 signs different. And a distance a 100,000 times closer. 15 00:00:57,026 --> 00:01:01,868 Remember the force decays like one over R squared, so this is a factor of ten to 16 00:01:01,868 --> 00:01:06,286 the ten, in the magnitude of the force. The nuclear force, and the energies 17 00:01:06,286 --> 00:01:08,707 involved, are therefore, much, much larger. 18 00:01:08,707 --> 00:01:12,400 And this is, the source, of solar energy that became clear, and. 19 00:01:12,400 --> 00:01:17,856 as I said before, E=mc squared, is really tangential to the entire exercise. 20 00:01:17,856 --> 00:01:21,470 We'll deal with that, when that becomes important. 21 00:01:21,470 --> 00:01:25,889 And then it is in the early decades of the twentieth century that physicists 22 00:01:25,889 --> 00:01:30,251 finally figure out the mechanism that allows the sun to produce such copius 23 00:01:30,251 --> 00:01:32,605 amounts of energy and, by extension, stars. 24 00:01:32,605 --> 00:01:36,967 And one can imagine, the sense, in fact, I think it was [INAUDIBLE] who writes 25 00:01:36,967 --> 00:01:40,066 about it. of looking up at the stars and being able 26 00:01:40,066 --> 00:01:44,888 to say, and I understand why they shine. And that's an emotional high that we want 27 00:01:44,888 --> 00:01:47,700 to share. So, the following animation will help us 28 00:01:47,700 --> 00:01:51,030 follow the process by which the sun generates it's energy. 29 00:01:51,030 --> 00:01:55,498 The process by which the sun generates its energy is called the pp chain, and 30 00:01:55,498 --> 00:01:59,735 we'll animate it here in a minute. The end result is that one starts with 31 00:01:59,735 --> 00:02:02,887 four protons, and through a sequence of collisions ends 32 00:02:02,887 --> 00:02:06,597 up with a helium nucleus. The helium nucleus has a lot of binding 33 00:02:06,597 --> 00:02:08,253 energy. This liberates energy. 34 00:02:08,253 --> 00:02:12,248 That is what lights the sun. And understanding the process again, by 35 00:02:12,248 --> 00:02:14,965 doing experiments on Earth. It took a long time. 36 00:02:14,965 --> 00:02:18,140 We're going to follow through the stages for a minute. 37 00:02:18,140 --> 00:02:21,727 So here we have up here at the top, two protons collliding. 38 00:02:21,727 --> 00:02:26,489 most of the time when protons collide electrostatic repulsion just pushes them 39 00:02:26,489 --> 00:02:29,076 apart. If they're moving fast enough and aim 40 00:02:29,076 --> 00:02:33,074 directly enough right at each other then they crash into each other, 41 00:02:33,074 --> 00:02:37,072 and they sometimes form this unstable bound state of two protons, 42 00:02:37,072 --> 00:02:41,591 which is called helium 2. Now helium 2, as I said, is very 43 00:02:41,591 --> 00:02:44,433 unstable. It decays very rapidly. 44 00:02:44,433 --> 00:02:51,624 we don't find helium 2 anywhere, but once in a long while and this is the insight 45 00:02:51,624 --> 00:02:57,341 of Hans Betten that really told us how the sun produces it's energy. 46 00:02:57,341 --> 00:03:04,068 once in a long while, right here, while this ephemeral helium 2 nucleus 47 00:03:04,068 --> 00:03:08,145 exists, one of the protons is able to beta decay 48 00:03:08,145 --> 00:03:14,347 or positive beta decay, becoming a neutron and emitting a positron and an 49 00:03:14,347 --> 00:03:19,105 electron neutrino. So, the proton is undergoing the decay 50 00:03:19,105 --> 00:03:22,478 proton, goes to neutron plus positron, conserving 51 00:03:22,478 --> 00:03:27,560 electric charge, and since this has negative electron number, it produces an 52 00:03:27,560 --> 00:03:31,084 electron neutrino. This is an important decay for us. 53 00:03:31,084 --> 00:03:36,777 Now this process cannot happen with just a free proton flying around, because the 54 00:03:36,777 --> 00:03:42,198 neutron is more massive than the proton. That's why the decay of a neutron works 55 00:03:42,198 --> 00:03:46,874 but the decay of a proton doesn't. The free proton, a hydrogen atom, is 56 00:03:46,874 --> 00:03:52,063 stable, but in the context of a nucleus, this helium 2 nucleus, this process is 57 00:03:52,063 --> 00:03:55,135 possible. The extra energy needed to create the 58 00:03:55,135 --> 00:03:58,859 neutron is borrowed from the binding energy of deuterium, 59 00:03:58,859 --> 00:04:02,258 hydrogen 2. So this is, this converts helium 2 to 60 00:04:02,258 --> 00:04:07,159 hydrogen 2, because one of the protons was converted to a neutron, this is 61 00:04:07,159 --> 00:04:09,184 deuterium. That's a bound state. 62 00:04:09,184 --> 00:04:14,020 And so there's nuclear binding energy, and that provides the excess energy. 63 00:04:14,020 --> 00:04:16,700 Now this positron goes off and, of course. 64 00:04:16,700 --> 00:04:20,922 There's a sea of electrons floating around, finds an electron, annihilates, 65 00:04:20,922 --> 00:04:25,788 and we get some extra energy. Now we have this nucleus of deuterium 66 00:04:25,788 --> 00:04:31,619 flying around, and we need another hydrogen nucleus, another proton, to 67 00:04:31,619 --> 00:04:36,907 crash into that, and that, if the conditions are just right, will form 68 00:04:36,907 --> 00:04:42,498 helium three, another isotope of Helium. And we need that process to happen again, 69 00:04:42,498 --> 00:04:46,179 the whole process, with all its stages, to happen again elsewhere. 70 00:04:46,179 --> 00:04:50,671 And then we need these two helium 3 nuclei to crash into each other just 71 00:04:50,671 --> 00:04:53,729 right. And if the collision is just right, then, 72 00:04:53,729 --> 00:04:57,722 they merge, they form some unstable beryllium isotope or something. 73 00:04:57,722 --> 00:05:03,683 And the net result is that they emit two protons and form a helium matter, or an 74 00:05:03,683 --> 00:05:08,004 alpha particle. in the process we've had two separate 75 00:05:08,004 --> 00:05:10,960 weak decays. And remember weak decays are rare. 76 00:05:10,960 --> 00:05:15,137 So the stakes are wild. every, an average proton in the sun has 77 00:05:15,137 --> 00:05:19,250 to wait 10 billion years before it's, it's turn to weakly decay. 78 00:05:19,250 --> 00:05:22,141 And we've liberated energy at several stages. 79 00:05:22,141 --> 00:05:25,290 these protons come off with kinetic energy. 80 00:05:25,290 --> 00:05:30,045 These positrons go off and annihilate. There's some gamma rays emitted that 81 00:05:30,045 --> 00:05:32,873 carry energy and this heats the surrounding. 82 00:05:32,873 --> 00:05:37,500 This is how the sun produces energy. and it was the insight into this. 83 00:05:37,500 --> 00:05:41,741 The understanding that this process could happen, that explain to us how stars 84 00:05:41,741 --> 00:05:44,030 work. So this is 85 00:05:44,030 --> 00:05:49,313 the way the sun produces energy. This reviews the exact same process we 86 00:05:49,313 --> 00:05:52,884 just saw. The net result is that you, we had six 87 00:05:52,884 --> 00:05:55,811 protons involved, but two were re-emitted. 88 00:05:55,811 --> 00:06:00,667 So you started with four protons, you produced an alpha particle, two 89 00:06:00,667 --> 00:06:06,450 positrons, two electron neutrinos, and in the process liberated nuclear binding 90 00:06:06,450 --> 00:06:10,949 energy to the tune of 4.3 times ten to the minus twelve joules. 91 00:06:10,949 --> 00:06:15,947 Note this is a factor of ten to the seventh, ten million more than was 92 00:06:15,947 --> 00:06:18,682 available in Chemical interactions. 93 00:06:18,682 --> 00:06:24,473 And so, chemical interactions would power the sun for 10,000 years at a power of 94 00:06:24,473 --> 00:06:28,526 10,000,000. And you realize that if the sun converted 95 00:06:28,526 --> 00:06:32,870 all of its hydrogen to helium, it would be able to let 96 00:06:32,870 --> 00:06:36,900 produce energy at its current luminosity for 100 billion years. 97 00:06:36,900 --> 00:06:41,569 Of course, as we'll see, the sun will never convert all of its hydrogen to 98 00:06:41,569 --> 00:06:44,896 helium. But this at least is consistent with that 99 00:06:44,896 --> 00:06:48,990 being the energy source of the sun and, indeed, that's what it is. 100 00:06:48,990 --> 00:06:52,508 Now, why won't the sun convert all its hydrogen to helium? 101 00:06:52,508 --> 00:06:55,163 Well, it's not that easy to generate fusion. 102 00:06:55,163 --> 00:06:59,237 There are two crucial steps. One is, I need pairs of protons to get 103 00:06:59,237 --> 00:07:03,250 close enough to each other to actually experience nuclear forces. 104 00:07:03,250 --> 00:07:07,448 Remember, protons are positive. They don't like to come close to each 105 00:07:07,448 --> 00:07:10,287 other. What that means is that they need to be 106 00:07:10,287 --> 00:07:15,102 moving fast enough that their kinetic energy carries them close to each other 107 00:07:15,102 --> 00:07:19,547 despite the electric repulsion. This requires temperatures of at least a 108 00:07:19,547 --> 00:07:23,899 million, probably ten million kelvin. Only in the core of the sun are 109 00:07:23,899 --> 00:07:27,908 temperatures this high. as we will see the sun is cooler out 110 00:07:27,908 --> 00:07:33,424 there, so that the outer envelope of the sun does not produce, Helium in, in large 111 00:07:33,424 --> 00:07:36,647 quantities. There's no fusion going on except in the 112 00:07:36,647 --> 00:07:39,870 interior of the sun where the temperatures are high. 113 00:07:39,870 --> 00:07:44,675 Moreover, we need these protons to crash into each other many times because, on 114 00:07:44,675 --> 00:07:49,006 average, most of the time when they crash into each other nothing happens. 115 00:07:49,006 --> 00:07:53,753 And so we need very high density, again, the density obtaining in the core of the 116 00:07:53,753 --> 00:07:56,363 sun. We'll talk about the density, when we 117 00:07:56,363 --> 00:07:59,626 review our models. But only in the core of the sun, only 118 00:07:59,626 --> 00:08:03,483 about 10% of the sun, is available for converting, to helium. 119 00:08:03,483 --> 00:08:07,992 That reduces our estimate from 100 billion years to 10 billion years, which 120 00:08:07,992 --> 00:08:11,730 is roughly the lifetime of the sun. So maybe we found something. 121 00:08:11,730 --> 00:08:16,974 And then, the other bottleneck is that the rate of fusion is restricted by the 122 00:08:16,974 --> 00:08:22,554 fact that 2 in fact, protons must beta decay before helium 2 breaks up and this 123 00:08:22,554 --> 00:08:27,125 slows down the process. And that is what prevents the sun from 124 00:08:27,125 --> 00:08:29,949 essentially blowing itself to smithereens. 125 00:08:29,949 --> 00:08:33,243 The reaction rates inside the sun are controlled, 126 00:08:33,243 --> 00:08:36,740 and we'll see how it maintains a stable equilibrium.