1 00:00:01,820 --> 00:00:05,558 So, what we discovered is that the power of the sun requires a source of energy 2 00:00:05,558 --> 00:00:08,823 that we have not discussed yet. It is not electromagnet energy in the 3 00:00:08,823 --> 00:00:11,710 internal structure of an atom. It is not gravitational energy. 4 00:00:11,710 --> 00:00:16,824 Where else can you mine potential energy? This was a puzzle until the twentieth 5 00:00:16,824 --> 00:00:21,020 Century and the answer, of course, is nuclear energy and so we'll have to learn 6 00:00:21,020 --> 00:00:25,325 a little bit about the structure of the nucleus and we talked, when we mentioned 7 00:00:25,325 --> 00:00:29,931 the nucleus and it's contruction out of Protons that are positive and neutrons 8 00:00:29,931 --> 00:00:32,282 that are neutral. That there's a big puzzle. 9 00:00:32,282 --> 00:00:35,834 That's a collection of positive charge without any negative charge. 10 00:00:35,834 --> 00:00:39,988 And there's a very strong electrostatic repulsion between all these charges. 11 00:00:39,988 --> 00:00:43,869 Remember electric forces like gravitational forces decay with distance 12 00:00:43,869 --> 00:00:47,422 like one over R squared. These are the forces at distances of ten 13 00:00:47,422 --> 00:00:50,155 to the minus fifteen meters. The size of a nucleus, 14 00:00:50,155 --> 00:00:52,943 these protons are very close up against each other. 15 00:00:52,943 --> 00:00:57,519 There's a huge repulsive force between them but nuclei I don't fall apart. 16 00:00:57,519 --> 00:01:01,716 Something must hold them together. So it's natural to conjuncture and later 17 00:01:01,716 --> 00:01:06,306 to verify the existence of a very strong attractive force, that binds the nucleons 18 00:01:06,306 --> 00:01:08,992 to each other. And over comes this electrostatic 19 00:01:08,992 --> 00:01:10,895 repulsion. How strong is the force? 20 00:01:10,895 --> 00:01:15,317 Well, strong enough to over come the electrostatic repulsion between protons 21 00:01:15,317 --> 00:01:18,660 and the nucleus. And that's a very strong force indeed. 22 00:01:18,660 --> 00:01:23,630 on the other hand, we know that this force cannot extend beyond the confines 23 00:01:23,630 --> 00:01:28,660 of the nucleus too far because if it did then the the nuclei of two neighboring 24 00:01:28,660 --> 00:01:33,024 atoms would attract each other and collapse on top of each other and so 25 00:01:33,024 --> 00:01:37,630 unlike gravitation and electromagnetic ray interactions that are long range 26 00:01:37,630 --> 00:01:42,055 interactions, volumes interact, every parts of the universe attracts every 27 00:01:42,055 --> 00:01:47,267 other part gravitationally the nuclear force is a short range force that ranges 28 00:01:47,267 --> 00:01:51,450 larger than the size of the nucleus, it effectively decreases to zero. 29 00:01:51,450 --> 00:01:55,089 and so there's no interaction between neighboring nuclei. 30 00:01:55,089 --> 00:01:58,425 We don't have a problem. We have this very strong force. 31 00:01:58,425 --> 00:02:03,339 And this brings up the idea, that maybe just as rearranging electrons lead to the 32 00:02:03,339 --> 00:02:06,068 liberation of electrostatic potential energy. 33 00:02:06,068 --> 00:02:10,436 Maybe rearranging nucleons inside a nucleus can liberate nuclear energy. 34 00:02:10,436 --> 00:02:13,348 And we've learned a lot about nuclear energy. 35 00:02:13,348 --> 00:02:18,427 the way, they have parameterized the amount of potential energy liberated at 36 00:02:18,427 --> 00:02:23,102 creating a nucleus similar to the way we computed the gravitational energy 37 00:02:23,102 --> 00:02:26,991 liberated in creating the song. Is to say imagine starting with a bunch 38 00:02:26,991 --> 00:02:30,546 of nucleons, put them together. Of course, you'll have to input a lot of 39 00:02:30,546 --> 00:02:34,100 energy to get them close to each other because at large distances the 40 00:02:34,100 --> 00:02:38,112 electrostatic repulsion will dominate. But once they get close there's a strong 41 00:02:38,112 --> 00:02:41,108 attractive force. They'll crash into each other and 42 00:02:41,108 --> 00:02:43,495 liberate a lot of energy producing the nucleus. 43 00:02:43,495 --> 00:02:47,507 The question we want to ask is if we have a whole bunch of nucleons, what's the 44 00:02:47,507 --> 00:02:51,214 most efficient way to pack them together. To do that, we measure the total 45 00:02:51,214 --> 00:02:53,788 liberated energy. nuclear minus electrostatic. 46 00:02:53,788 --> 00:02:58,297 The total difference, the total energy gained by constructing nuclei out of nu, 47 00:02:58,297 --> 00:03:02,638 a bunch of nucleons, and that's, we want to do that energy per nucleon, so that if 48 00:03:02,638 --> 00:03:07,202 you have a, some fixed number of nucleons what's the most efficient way to pack 49 00:03:07,202 --> 00:03:09,763 them. And one would think that because we have 50 00:03:09,763 --> 00:03:14,216 this very strong attractive interaction, that certainly a hydrogen atom is not a 51 00:03:14,216 --> 00:03:18,390 good way, to produce efficient packing. A hydrogen atom is a proton it's not 52 00:03:18,390 --> 00:03:22,064 interacting with anybody it hasn't, liberated any potential energy. 53 00:03:22,064 --> 00:03:25,460 but one would think that very heavy elements like uranium. 54 00:03:25,460 --> 00:03:30,941 Big nuclei, where there is a lot of nuclear interactions would be the ones 55 00:03:30,941 --> 00:03:35,923 that liberate the most energy and what this plot shows us, we have here atomic 56 00:03:35,923 --> 00:03:40,905 number or number of nucleons running mass number running to the right and the 57 00:03:40,905 --> 00:03:44,034 average energy liberated per nucleon going up. 58 00:03:44,034 --> 00:03:48,633 And we see that the most stable configuration is not a bunch of uranium 59 00:03:48,633 --> 00:03:53,679 nuclei, but a bunch as many as you have nucleons for of iron nuclei, the sort of 60 00:03:53,679 --> 00:03:58,470 intermediate range, intermediate size nucleus is the most efficient packing. 61 00:03:58,470 --> 00:04:03,346 And the explanation for this is an important one, the explanation for this 62 00:04:03,346 --> 00:04:08,413 is very close to the discussion we had of the cross over between short range 63 00:04:08,413 --> 00:04:13,606 chemical forces that are short range but are very strong and enact on the surface 64 00:04:13,606 --> 00:04:19,116 and ling range gravitational forces at about a size of kilometer in the solar 65 00:04:19,116 --> 00:04:21,800 system. A similar thing happens with nuclei. 66 00:04:21,800 --> 00:04:26,778 What goes on is that if you have a great big uranium nucleus, then a proton that 67 00:04:26,778 --> 00:04:31,756 is located on this side of the nucleus and a proton on that side of the nucleus 68 00:04:31,756 --> 00:04:36,423 basically are too far apart to really interact through the strong force, so 69 00:04:36,423 --> 00:04:40,530 they do not attract each other. Remember, the attraction falls to O. 70 00:04:40,530 --> 00:04:47,483 in some sense each of these protons on the edge of a very large uranium nucleus. 71 00:04:47,483 --> 00:04:53,629 Interact only with his neighbors what that means on the other hand the electric 72 00:04:53,629 --> 00:04:59,464 forces between these protons decay only as the distance squared those are long 73 00:04:59,464 --> 00:05:05,587 range forces so these guys definitely do repel each other through electro static 74 00:05:05,587 --> 00:05:08,828 forces. And therefore the sort of efficient way 75 00:05:08,828 --> 00:05:14,807 to arrange the energy just as we saw this is really analogous to the discussion 76 00:05:14,807 --> 00:05:19,895 what holds a mountain together is to cleave this nucleus in two, perhaps. 77 00:05:19,895 --> 00:05:26,373 we now have, lost all the nuclear energy that was involved in the bonds along the 78 00:05:26,373 --> 00:05:29,995 place we cut. But when these two positive sub-nuclei 79 00:05:29,995 --> 00:05:33,617 are created, they are two positive charges very near 80 00:05:33,617 --> 00:05:36,994 each other. They repel with the great electrostatic 81 00:05:36,994 --> 00:05:40,047 repulsion. That produces a lot of energy as they 82 00:05:40,047 --> 00:05:43,546 accelerate away. And the net result is a gain of energy 83 00:05:43,546 --> 00:05:48,634 and so what we see indeed is that the most efficient way to pack is iron and 84 00:05:48,634 --> 00:05:51,274 nuclei. Heavier than iron are not efficient 85 00:05:51,274 --> 00:05:56,089 packing, and by rearranging their nucleons, they can re-liberate energy 86 00:05:56,089 --> 00:06:00,843 well known case is the case of fussion. This is the case exactly, where this 87 00:06:00,843 --> 00:06:05,232 nucleus of californium breaks into cadmium and tin and some debris, some 88 00:06:05,232 --> 00:06:09,776 extra neutrons flying around. And in the process, liberates essentially 89 00:06:09,776 --> 00:06:14,572 the electrostatic repulsion that, once these two nuclei are free, they are 90 00:06:14,572 --> 00:06:18,737 repelled and proceed at great energy to move away from each other. 91 00:06:18,737 --> 00:06:23,596 Another popular mode of breaking up, if you're a heavy nucleus, is, we see here, 92 00:06:23,596 --> 00:06:26,372 rutherfordium, emitting, an alpha particle. 93 00:06:26,372 --> 00:06:31,357 Remember, an alpha particle is a nucleus of helium, two protons and two neutrons, 94 00:06:31,357 --> 00:06:34,260 and the rutherfordium decays to seaborgium. 95 00:06:34,260 --> 00:06:37,724 so both of those modes occur in heavy nuclei. 96 00:06:37,724 --> 00:06:42,817 They liberate a lot of energy. This was the radioactive energy that 97 00:06:42,817 --> 00:06:46,237 powred heated up the, the interior of Earth. 98 00:06:46,237 --> 00:06:51,985 Remember that was the 89 watts per liter squared or 87 watts per liter squared 99 00:06:51,985 --> 00:06:56,860 that were bubbling up from nuclear reactions in the core but 100 00:06:57,960 --> 00:07:04,901 it's not going to be enough to power the sun, because in the sun we don't have 101 00:07:04,901 --> 00:07:11,500 rutherfordium or californium or uranium. We have hydrogen and hydrogen remember is 102 00:07:11,500 --> 00:07:16,349 way on the other side of the divide. a hydrogen atom nucleus is just a proton. 103 00:07:16,349 --> 00:07:20,410 It's not bound to anything. Of course if we could make bound states 104 00:07:20,410 --> 00:07:24,774 of protons, maybe we could gain. But there are no bound states of protons 105 00:07:24,774 --> 00:07:27,381 only. helium 2 is not a stable nucleus. 106 00:07:27,381 --> 00:07:31,684 If you put two protons together, they may bind for a short time but the 107 00:07:31,684 --> 00:07:35,745 electrostatic repulsion will break that nucleus apart very quickly. 108 00:07:35,745 --> 00:07:41,240 And so the first really stable nucleus and it's a very stable nucleus indeed, is 109 00:07:41,240 --> 00:07:46,044 helium 4, that alpha particle. If you can make a helium nucleus out of 110 00:07:46,044 --> 00:07:49,680 the hydrogen, you will have gained all this energy. 111 00:07:49,680 --> 00:07:53,317 that's very good. The problem is that to make a helium 112 00:07:53,317 --> 00:07:55,997 nucleus you need two protons, two neutrons. 113 00:07:55,997 --> 00:08:01,895 The sun is full of protons but there's no neutrons essential where do you get the 114 00:08:01,895 --> 00:08:08,502 neutrons we can't using the srong force, convert photons to neutrons but we know 115 00:08:08,502 --> 00:08:12,445 that somebody can. And the reason we know that somebody can 116 00:08:12,445 --> 00:08:18,394 is because we talked about beta decay of nuclei and we see here some of the decay 117 00:08:18,394 --> 00:08:22,472 of oxygen to fluorine. And what's going on here is, since this 118 00:08:22,472 --> 00:08:28,020 is a positive beta decay, you can look at the atomic number and see that in fact, 119 00:08:28,020 --> 00:08:32,899 one of the protons in the fluorine is being converted to a neutron. 120 00:08:32,899 --> 00:08:38,507 So, fluorine is decaying to oxygen with the emission of a positron and there's 121 00:08:38,507 --> 00:08:44,366 also the inverse reaction, in which here carbon-14 converts one of its neutrons to 122 00:08:44,366 --> 00:08:48,761 a proton becomes nitrogen-14. And emits an electron and so we know 123 00:08:48,761 --> 00:08:52,489 about these decays. In fact, the free neutron decays to a 124 00:08:52,489 --> 00:08:55,618 proton within about fifteen or sixteen minutes. 125 00:08:55,618 --> 00:09:00,479 And so there is this thing called the weak nuclear force, probably better 126 00:09:00,479 --> 00:09:04,207 called the weak interaction, which mediates these decays. 127 00:09:04,207 --> 00:09:08,868 It is possible for one elementary particle to be converted to another 128 00:09:08,868 --> 00:09:12,082 elementary particle. And this is a very confusing thing. 129 00:09:12,082 --> 00:09:15,792 So we have to sort of stop. We're not going to discuss the physics of 130 00:09:15,792 --> 00:09:18,858 the weak interactions, fascinating though it is, in detail. 131 00:09:18,858 --> 00:09:21,116 But we need to have, set some ground rules. 132 00:09:21,116 --> 00:09:25,052 So what is this kind of force? Magically convert one particle into each 133 00:09:25,052 --> 00:09:27,601 other. We understand that a force can cause a 134 00:09:27,601 --> 00:09:30,319 rearrangement of the constituance of a particle. 135 00:09:30,319 --> 00:09:35,077 So one could image maybe that a neutron is really just a very, very tiny hydrogen 136 00:09:35,077 --> 00:09:37,343 atom. There already were a proton and an 137 00:09:37,343 --> 00:09:40,628 electron inside the neutron and then they just fell apart. 138 00:09:40,628 --> 00:09:45,102 The answer is no, it's a very bad idea to think of a neutron as a hydrogen atom. 139 00:09:45,102 --> 00:09:49,803 there was no proton inside the neutron. The neutron honestly decayed to a proton 140 00:09:49,803 --> 00:09:53,030 and an electron. and yes the weaker interactions can 141 00:09:53,030 --> 00:09:57,551 change one particle to each, to another. But, there are going to be some rules 142 00:09:57,551 --> 00:10:01,537 that we need to understand. And then there was this third object 143 00:10:01,537 --> 00:10:04,334 there. it's called an anti electron neutrino. 144 00:10:04,334 --> 00:10:06,892 So the Greek letter nu stands for neutrino. 145 00:10:06,892 --> 00:10:10,937 The bar says anti, and the subscript E makes it an electron neutrino. 146 00:10:10,937 --> 00:10:13,853 And what all that means, we'll have to understand. 147 00:10:13,853 --> 00:10:17,660 so lets set the rules. What can, interactions do, what can they 148 00:10:17,660 --> 00:10:20,100 not do, they cannot do anything they want. 149 00:10:20,100 --> 00:10:24,224 The rules, basically it turns out, are restricted to a good old fashion 150 00:10:24,224 --> 00:10:28,643 conservation loss, that's why we made such a big deal about it, when we were 151 00:10:28,643 --> 00:10:29,410 learning. Now, 152 00:10:29,410 --> 00:10:33,873 there's a tricky business here, we're getting into the, stepping into the 153 00:10:33,873 --> 00:10:37,681 beginnings of relativity, where you allow the particles to decay. 154 00:10:37,681 --> 00:10:42,085 We will spend almost an entire week understanding relativity, later in the 155 00:10:42,085 --> 00:10:45,120 class. for now, I'll just say the words we had 156 00:10:45,120 --> 00:10:49,583 in Utonian physics of conservation of energy, and of course, conservation of 157 00:10:49,583 --> 00:10:52,368 mass in relativistic theories, those are 158 00:10:52,368 --> 00:10:55,952 combined in the conservation of the sum of energy and mass. 159 00:10:55,952 --> 00:10:59,840 Mass is a form of energy. There's the famous conversion ratio 160 00:10:59,840 --> 00:11:03,060 square of the speed of light. We will talk about this. 161 00:11:03,060 --> 00:11:07,371 it is not that relevant to the processes that take place in the sun despite what 162 00:11:07,371 --> 00:11:10,400 you may have heard. And so when it's important we will talk 163 00:11:10,400 --> 00:11:13,531 about relativistic physics. But mass and energy are conserved. 164 00:11:13,531 --> 00:11:15,944 momentum is conserved just as it always was. 165 00:11:15,944 --> 00:11:20,153 angular momentum is conserved if you are dealing with elementary particles you 166 00:11:20,153 --> 00:11:23,746 need to remember that elementary particles carry intrinsic angular 167 00:11:23,746 --> 00:11:27,083 momentum it's called spin. You could imagine that a proton always 168 00:11:27,083 --> 00:11:29,990 spins and there's a. This, the rotation. 169 00:11:29,990 --> 00:11:33,562 Is at a constant rate and points in some particular direction. 170 00:11:33,562 --> 00:11:38,056 And, then there are the other kinds of conserved quantities, that are not to do 171 00:11:38,056 --> 00:11:40,591 with motion. We talked about electric charge. 172 00:11:40,591 --> 00:11:44,410 Electric charges conserve. No matter what processes go on 173 00:11:44,410 --> 00:11:49,617 the total electric charge in any region of the universe doesn't change unless a 174 00:11:49,617 --> 00:11:53,631 charge is flowing in our out. And then, there's a new one that we 175 00:11:53,631 --> 00:11:56,830 discover as we start to develop particle mechanics. 176 00:11:56,830 --> 00:11:59,707 And that is, Property called electron number. 177 00:11:59,707 --> 00:12:03,693 An electron carries electron number. A proton obviously has zero electron 178 00:12:03,693 --> 00:12:06,806 number, as does a neutron. An electron number will see its 179 00:12:06,806 --> 00:12:10,683 conserved, and this restricts what can happen in interactions involving 180 00:12:10,683 --> 00:12:13,250 electrons. And the interactions we are going to 181 00:12:13,250 --> 00:12:14,561 study. Weak interactions. 182 00:12:14,561 --> 00:12:17,564 What make them weak is that they're not really a force. 183 00:12:17,564 --> 00:12:21,005 They are things that happen. the strength of the interaction 184 00:12:21,005 --> 00:12:24,227 essentially measures, how frequently a reaction will happen. 185 00:12:24,227 --> 00:12:27,138 Weak interactions. Occur very slowly, they are rare. 186 00:12:27,138 --> 00:12:30,635 One out of many, many attempts will generate a reaction. 187 00:12:30,635 --> 00:12:35,457 The example is the neutron that survives as a free particle for fifteen minutes. 188 00:12:35,457 --> 00:12:38,230 By particle standards, that's a very long time. 189 00:12:38,230 --> 00:12:43,181 So, who are the players? Well, here's a list, we're into particle 190 00:12:43,181 --> 00:12:48,850 physics now, of all of the particles we have or should be discussing. 191 00:12:48,850 --> 00:12:53,083 and they include well, I'll start at the bottom. 192 00:12:53,083 --> 00:12:57,126 We have our friend Photon denoted by gamma, its gamma rays 193 00:12:57,126 --> 00:13:01,640 and I have written the quantities of their conserved charges, of course their 194 00:13:01,640 --> 00:13:06,272 momenta and energy depends which way they are moving but a photon is a neutral 195 00:13:06,272 --> 00:13:10,610 particle, it carries no electric charge, it is not an electron carrier, zero 196 00:13:10,610 --> 00:13:15,359 electron number and then here we have our proton, neutron and electron with their 197 00:13:15,359 --> 00:13:20,049 known electric charges and with the fact that the electron carries a charge not 198 00:13:20,049 --> 00:13:23,140 carried by any of the others. And then. 199 00:13:23,140 --> 00:13:27,752 There is this other interesting object called the neutrino. 200 00:13:27,752 --> 00:13:32,600 And the neutrino was discovered because when neutrons decayed. 201 00:13:32,600 --> 00:13:37,213 Studying the conservation of energy mass it was discovered there was missing 202 00:13:37,213 --> 00:13:40,096 energy. it was Enrico Fermi's idea, I believe, to 203 00:13:40,096 --> 00:13:42,749 think that maybe there was a neutral particle. 204 00:13:42,749 --> 00:13:46,613 It doesn't interact with anything. It is not detected by any of our 205 00:13:46,613 --> 00:13:49,150 detectors. Sails right through the detectors. 206 00:13:49,150 --> 00:13:52,956 And produces and carries off our remaining energy since it was 207 00:13:52,956 --> 00:13:57,570 electrically neutral and didn't interact with anything, it was called a neutrino. 208 00:13:57,570 --> 00:14:01,434 We now know that these particles exist and indeed are part of this. 209 00:14:01,434 --> 00:14:05,990 And, in fact an electron neutrino carries a electron charge, electron number one. 210 00:14:05,990 --> 00:14:11,107 Travis will become important in what follows, because the weak interactions of 211 00:14:11,107 --> 00:14:14,388 electrons and protons conserve electron number. 212 00:14:14,388 --> 00:14:19,834 What that means, in particular, is that when a reaction occurs we need to balance 213 00:14:19,834 --> 00:14:24,362 all of these quantum numbers. They're all conserved so, for example, 214 00:14:24,362 --> 00:14:29,955 the decay of a neutron into a proton plus an electron conserves electric charge. 215 00:14:29,955 --> 00:14:35,181 Because the neutron is neutral. And the total electric charge of the 216 00:14:35,181 --> 00:14:39,450 products is zero. But it does not conserve electron number. 217 00:14:39,450 --> 00:14:45,707 Electron number was magically created. And indeed this means that if you have a 218 00:14:45,707 --> 00:14:50,687 neutrino and an electron. And they react, that can produce a proton 219 00:14:50,687 --> 00:14:54,013 and an electron. Now, I was about to write something on 220 00:14:54,013 --> 00:14:58,296 the right hand side, because of course a free neutron decays without anything 221 00:14:58,296 --> 00:15:01,410 hitting it. this brings up the fact that there's this 222 00:15:01,410 --> 00:15:05,415 whole bottom line of the table. And it turns out to be another important 223 00:15:05,415 --> 00:15:09,308 principal of physics, that for every particle there is an antiparticle. 224 00:15:09,308 --> 00:15:13,591 An antiparticle is not something mystical or mysterious or a particle moving 225 00:15:13,591 --> 00:15:16,150 backward. It's simply another kind of particle. 226 00:15:16,150 --> 00:15:20,712 Which has the same mass as the original, and all the opposite charges. 227 00:15:20,712 --> 00:15:26,068 So, for example, there is an antiproton, which has the same mass as a proton but 228 00:15:26,068 --> 00:15:30,102 is negatively charged. And of course, carries the same zero 229 00:15:30,102 --> 00:15:33,408 electron charge. There's an anti-neutrino, which is 230 00:15:33,408 --> 00:15:36,582 neutral. There are anti electrons, they're called 231 00:15:36,582 --> 00:15:39,756 positrons. That was what was emitted in positive 232 00:15:39,756 --> 00:15:43,327 beta decay. They carry the same mass as an electron. 233 00:15:43,327 --> 00:15:48,508 But electric charge positive and negative electron number charge. 234 00:15:48,508 --> 00:15:51,889 And the neutrino has its own anti-particle. 235 00:15:51,889 --> 00:15:58,493 And the anti-electron neutrino is neutral like the electron neutrino but has the 236 00:15:58,493 --> 00:16:04,316 opposite sign of electron charge. And so, the decay can proceed if an empty 237 00:16:04,316 --> 00:16:08,073 electron neutrino is produced over here. This conserves all the charges. 238 00:16:08,073 --> 00:16:10,560 Now these neutrini are very hard to understand. 239 00:16:10,560 --> 00:16:14,529 They interact only through this weak interaction which as I said, makes for 240 00:16:14,529 --> 00:16:17,280 rare events. So mostly, mostly, they don't interact at 241 00:16:17,280 --> 00:16:19,661 all. In the time I've been talking about this 242 00:16:19,661 --> 00:16:23,471 slide, some millions of neutrinos have penetrated each and every of your 243 00:16:23,471 --> 00:16:26,329 fingernails. You have not felt them because they don't 244 00:16:26,329 --> 00:16:29,662 interact with anything. In fact, the great majority of them will 245 00:16:29,662 --> 00:16:34,055 sail right through the Earth and come out the other end, and keep going through the 246 00:16:34,055 --> 00:16:36,778 universe. they carry very little mass if any. 247 00:16:36,778 --> 00:16:40,179 In fact for a long time they were thought to be massless. 248 00:16:40,179 --> 00:16:44,416 And they don't interact which makes detecting them a very difficult 249 00:16:44,416 --> 00:16:48,295 proposition as we will see. But they're important because they 250 00:16:48,295 --> 00:16:53,009 balance the charges in weak decays. And as we'll see they have some other 251 00:16:53,009 --> 00:16:57,049 important role in our story. I didn't bring them up just to tell you 252 00:16:57,049 --> 00:17:01,841 about particle decay and there's a the property that if you take a particle and 253 00:17:01,841 --> 00:17:05,190 an antiparticle so you have an electron and a positatron. 254 00:17:05,190 --> 00:17:10,237 That combination has total charge zero with respect to all the concerned charges 255 00:17:10,237 --> 00:17:13,915 except for energy momentum. And so if you bring an electron or 256 00:17:13,915 --> 00:17:17,736 positron near each other, and they're addressed, there's just energy. 257 00:17:17,736 --> 00:17:22,070 That's in their mass, and that can indeed be converted to straight radiation. 258 00:17:22,070 --> 00:17:26,689 The electron and positron annihilate, and a burst of gamma radiation flies off in 259 00:17:26,689 --> 00:17:31,137 both directions and the energy is essentially the mass of the disappearing 260 00:17:31,137 --> 00:17:34,274 electrons. This is a process that does occur and you 261 00:17:34,274 --> 00:17:39,064 will have to understand how that happens, as I said, when we talk about relativity 262 00:17:39,064 --> 00:17:43,740 for now, we recognize that if you have an electron and a positron, you can liberate 263 00:17:43,740 --> 00:17:46,984 energy. We'll take all this information, and see 264 00:17:46,984 --> 00:17:49,900 what it tells us about, how the sun works.