1 00:00:08,450 --> 00:00:10,550 MARKUS KLUTE: Welcome again to 8.701. 2 00:00:10,550 --> 00:00:13,880 So this is the fifth section of our introduction. 3 00:00:13,880 --> 00:00:17,510 I'd like to talk about the early history and the people involved 4 00:00:17,510 --> 00:00:19,640 in nuclear and particle physics. 5 00:00:19,640 --> 00:00:23,390 I cover the period from 1820 to the beginning 6 00:00:23,390 --> 00:00:25,140 of the Second World War. 7 00:00:25,140 --> 00:00:27,830 other elements of the later history of the development 8 00:00:27,830 --> 00:00:29,450 of the standard model-- 9 00:00:29,450 --> 00:00:32,189 parity violation, CP violation-- 10 00:00:32,189 --> 00:00:33,890 those aspects will be covered when 11 00:00:33,890 --> 00:00:36,500 we talk about the actual physics involved. 12 00:00:36,500 --> 00:00:40,020 But I'd like to give you some more background. 13 00:00:40,020 --> 00:00:42,380 Especially since we start the discussion with particle 14 00:00:42,380 --> 00:00:45,830 physics, it's good to understand what 15 00:00:45,830 --> 00:00:48,590 was the starting ground, which shoulders did 16 00:00:48,590 --> 00:00:51,860 people stand on at the time. 17 00:00:51,860 --> 00:00:53,870 Important to realize here, at this point, 18 00:00:53,870 --> 00:00:55,670 I'm not an historian. 19 00:00:55,670 --> 00:00:57,650 I like to read about history. 20 00:00:57,650 --> 00:01:00,690 I just finished an interesting book on Einstein. 21 00:01:00,690 --> 00:01:03,680 I like to have a good understanding how 22 00:01:03,680 --> 00:01:07,490 the people of the time, the time itself, and the physics 23 00:01:07,490 --> 00:01:09,320 discovery interacted. 24 00:01:09,320 --> 00:01:16,633 It helps me in understanding the process of being scientific. 25 00:01:16,633 --> 00:01:18,050 When you look at history, you find 26 00:01:18,050 --> 00:01:20,420 a lot of places where progress was 27 00:01:20,420 --> 00:01:27,320 made by curiosity and by doing things which are not 28 00:01:27,320 --> 00:01:29,360 the common way to proceed. 29 00:01:29,360 --> 00:01:32,620 And so one learns this by looking at history. 30 00:01:32,620 --> 00:01:36,850 And I might give you a number of examples here as well. 31 00:01:36,850 --> 00:01:41,440 So diving in one of the questions at the time, 32 00:01:41,440 --> 00:01:48,760 going back, again, almost 200 years is how old was the Earth? 33 00:01:48,760 --> 00:01:50,710 How old is the Earth? 34 00:01:50,710 --> 00:01:56,260 And about 200 years ago, people started to argue whether or not 35 00:01:56,260 --> 00:01:59,410 the 10,000 of years, which was long 36 00:01:59,410 --> 00:02:04,030 thought to be the age of the Earth, are actually correct. 37 00:02:04,030 --> 00:02:09,250 And specifically geologists and biologists argued that this 38 00:02:09,250 --> 00:02:10,419 cannot be true. 39 00:02:10,419 --> 00:02:15,640 They observed how slowly geological and biological 40 00:02:15,640 --> 00:02:19,390 processes such as erosion and evolution occur. 41 00:02:22,590 --> 00:02:29,410 And if you just try to, by observation, put 42 00:02:29,410 --> 00:02:32,860 all of those ducks in a row, if you want, 43 00:02:32,860 --> 00:02:35,650 you find that the Earth must be much, much older 44 00:02:35,650 --> 00:02:38,290 than those 10,000 years. 45 00:02:38,290 --> 00:02:44,950 On the contrary, some physicists argued that the Earth cannot be 46 00:02:44,950 --> 00:02:48,580 as old as several hundreds of millions of years because it 47 00:02:48,580 --> 00:02:52,690 would be, by now, a very cold and dark place. 48 00:02:52,690 --> 00:02:58,330 One of the opposers of evolution was Lord Kelvin, 49 00:02:58,330 --> 00:02:59,770 or William Thompson. 50 00:02:59,770 --> 00:03:05,710 And he argued with classical thermodynamics calculations 51 00:03:05,710 --> 00:03:11,090 that the Earth cannot be as old as those 300 million years 52 00:03:11,090 --> 00:03:14,860 as Darwin writes in initial printing of The Origin 53 00:03:14,860 --> 00:03:17,480 of Species. 54 00:03:17,480 --> 00:03:21,680 Herman Helmholtz, a few years later, 55 00:03:21,680 --> 00:03:27,570 tried to use energy compensation principles to calculate 56 00:03:27,570 --> 00:03:34,040 how much heat from the sun would radiate if the energy comes 57 00:03:34,040 --> 00:03:35,930 from slow contraction. 58 00:03:35,930 --> 00:03:41,890 And he, by converting gravitational potential energy 59 00:03:41,890 --> 00:03:46,580 to heat, calculated age cannot be more than 18 million years. 60 00:03:46,580 --> 00:03:49,810 So putting those together, you find, on the one side, 61 00:03:49,810 --> 00:03:53,770 the physicists, theology might be 62 00:03:53,770 --> 00:03:55,610 a different dimension to this discussion, 63 00:03:55,610 --> 00:03:58,060 and then geologists and biologists. 64 00:03:58,060 --> 00:04:01,358 And a complicated question. 65 00:04:01,358 --> 00:04:03,400 I mean, really there was something to be learned. 66 00:04:03,400 --> 00:04:05,668 Something was not quite understood. 67 00:04:05,668 --> 00:04:07,210 And so we come back to this question. 68 00:04:12,600 --> 00:04:14,700 Next slide. 69 00:04:14,700 --> 00:04:20,279 But then progress was made in the understanding of physics. 70 00:04:20,279 --> 00:04:25,140 And here to be named are Henri Becquerel, for example, 71 00:04:25,140 --> 00:04:29,400 for the discovery of radiation from uranium 72 00:04:29,400 --> 00:04:32,040 and Ernest Rutherford for the discovery, 73 00:04:32,040 --> 00:04:34,680 by studying this radiation, that there 74 00:04:34,680 --> 00:04:38,400 must be at least two different sources of radiation. 75 00:04:38,400 --> 00:04:41,100 And he called them, simply by following the Greek alphabet, 76 00:04:41,100 --> 00:04:43,830 alpha and beta rays. 77 00:04:43,830 --> 00:04:46,080 In the same year, J. J. Thompson discovered 78 00:04:46,080 --> 00:04:50,010 a particle, the electron charged particle, or the electron. 79 00:04:53,480 --> 00:04:55,490 Becquerel's story is quite interesting, 80 00:04:55,490 --> 00:05:02,750 as he was trying to understand the material Roentgen studied. 81 00:05:02,750 --> 00:05:07,090 And he was interested in figuring out what 82 00:05:07,090 --> 00:05:09,200 fluorescence material can do. 83 00:05:09,200 --> 00:05:12,530 And again, this is one of the examples where he, by accident, 84 00:05:12,530 --> 00:05:16,280 discovered that it is actually not the fact 85 00:05:16,280 --> 00:05:19,970 that you have a material, you expose this to sunlight, 86 00:05:19,970 --> 00:05:22,950 you wait a little while, and it still radiates the light. 87 00:05:22,950 --> 00:05:26,030 So this has delayed the fluorescence of the material. 88 00:05:26,030 --> 00:05:31,080 It is not quite the full story to some fluorescent materials. 89 00:05:31,080 --> 00:05:32,820 So he discovered this by accident, 90 00:05:32,820 --> 00:05:35,210 by putting the mineral in a drawer, together 91 00:05:35,210 --> 00:05:40,370 with a photo plate, and found that there 92 00:05:40,370 --> 00:05:47,450 was only a very short, limited evidence from that photo plate 93 00:05:47,450 --> 00:05:49,370 to be radiated by the sun. 94 00:05:49,370 --> 00:05:52,610 But it was basically foggy from being in the same drawer 95 00:05:52,610 --> 00:05:53,600 as a mineral. 96 00:05:53,600 --> 00:05:57,380 But this was a rather accidental discovery. 97 00:06:01,080 --> 00:06:06,450 Marie and Pierre Curie proposed the new term, radioactivity, 98 00:06:06,450 --> 00:06:08,820 for materials which generally emit light. 99 00:06:08,820 --> 00:06:11,700 And they discovered additional materials 100 00:06:11,700 --> 00:06:17,670 to the uranium which was discovered by Becquerel. 101 00:06:17,670 --> 00:06:19,730 So they discovered thorium, for example, 102 00:06:19,730 --> 00:06:22,215 and later also the elements of polonium and radium. 103 00:06:22,215 --> 00:06:24,900 And they discovered that those elements 104 00:06:24,900 --> 00:06:28,050 radiate a lot of radioactivity. 105 00:06:28,050 --> 00:06:30,780 So Marie Curie was able to measure the energy being 106 00:06:30,780 --> 00:06:33,720 radiated, and found that a gram of radium 107 00:06:33,720 --> 00:06:38,840 can emit up to 140 calories per hour. 108 00:06:38,840 --> 00:06:42,840 And so you find that a gram of radium 109 00:06:42,840 --> 00:06:45,510 is able to power, basically, the energy you need in order 110 00:06:45,510 --> 00:06:47,070 to survive-- 111 00:06:47,070 --> 00:06:48,000 provide the energy. 112 00:06:50,720 --> 00:06:53,840 So moving a little forward, so then uranium 113 00:06:53,840 --> 00:06:57,230 specifically, but other radioactive materials, 114 00:06:57,230 --> 00:06:58,340 were studied. 115 00:06:58,340 --> 00:07:02,620 And Paul Villard discovered that there 116 00:07:02,620 --> 00:07:05,230 must be a third component of radiation which behaves 117 00:07:05,230 --> 00:07:07,000 different from the other two. 118 00:07:07,000 --> 00:07:09,610 And he called these gamma rays, again, 119 00:07:09,610 --> 00:07:14,140 simply following the alphabet and moving along 120 00:07:14,140 --> 00:07:17,110 to the third letter. 121 00:07:17,110 --> 00:07:21,820 Rutherford then connects these findings first 122 00:07:21,820 --> 00:07:24,670 to the question of the age of the Earth. 123 00:07:24,670 --> 00:07:28,960 And he simply suggests that it's those radioactive elements 124 00:07:28,960 --> 00:07:33,070 which are in ores in the core of the Earth which provide 125 00:07:33,070 --> 00:07:36,400 additional source of feed sufficient, 126 00:07:36,400 --> 00:07:39,790 because of the connectivity of the Earth, 127 00:07:39,790 --> 00:07:43,570 to keep the Earth geologically active. 128 00:07:43,570 --> 00:07:45,820 And he comes to the conclusion that the Earth 129 00:07:45,820 --> 00:07:48,310 might be as well a few billion years old, 130 00:07:48,310 --> 00:07:51,730 as we know now it is. 131 00:07:51,730 --> 00:07:55,000 So putting this in context, at the very same time 132 00:07:55,000 --> 00:08:02,320 in Bern, Switzerland, a clerk named Albert Einstein 133 00:08:02,320 --> 00:08:04,060 has a fantastic year. 134 00:08:04,060 --> 00:08:07,420 He, in one year, comes up with a sequence 135 00:08:07,420 --> 00:08:09,220 of theoretical discoveries. 136 00:08:09,220 --> 00:08:11,710 One is special relativity. 137 00:08:11,710 --> 00:08:14,770 And he uses the findings of special relativity 138 00:08:14,770 --> 00:08:17,560 to derive that there's an equivalence between energy 139 00:08:17,560 --> 00:08:18,580 and mass. 140 00:08:18,580 --> 00:08:20,530 And this equivalence, as we will see later, 141 00:08:20,530 --> 00:08:23,080 when we discussed nuclear physics specifically, 142 00:08:23,080 --> 00:08:28,480 is very important to understand nuclear decay, nuclear fission, 143 00:08:28,480 --> 00:08:33,070 and nuclear fusion, and figuring out why, 144 00:08:33,070 --> 00:08:35,320 if you have a component state that 145 00:08:35,320 --> 00:08:38,559 seems to be lighter than the sum of the individual components 146 00:08:38,559 --> 00:08:40,830 making up this particle. 147 00:08:40,830 --> 00:08:45,610 Rutherford was a pioneer with collider experiments 148 00:08:45,610 --> 00:08:48,880 in the sense that he used other particles a lot 149 00:08:48,880 --> 00:08:52,790 to bombard all kinds of materials. 150 00:08:52,790 --> 00:08:57,520 So what he found first is that, if alpha particles, 151 00:08:57,520 --> 00:08:59,780 when stopped, turn into helium. 152 00:08:59,780 --> 00:09:01,600 So the alpha particle itself grabs 153 00:09:01,600 --> 00:09:04,840 on to the electrons from the material it collides in. 154 00:09:09,570 --> 00:09:11,003 There was a technical problem. 155 00:09:11,003 --> 00:09:11,795 I'll just continue. 156 00:09:15,040 --> 00:09:17,020 And that turns into helium. 157 00:09:20,830 --> 00:09:22,420 His students, Marsden and Geiger, 158 00:09:22,420 --> 00:09:25,180 then perform a very famous gold foil experiment. 159 00:09:25,180 --> 00:09:27,580 So you all probably have heard about this, 160 00:09:27,580 --> 00:09:30,940 taking an alpha particle source and you shine it 161 00:09:30,940 --> 00:09:35,080 on a foil of gold. 162 00:09:35,080 --> 00:09:38,350 And then you look at the angular distribution of the particles, 163 00:09:38,350 --> 00:09:40,565 which go through or which have been 164 00:09:40,565 --> 00:09:42,490 backscattered from this foil. 165 00:09:42,490 --> 00:09:45,580 And Rutherford then takes those measurements 166 00:09:45,580 --> 00:09:49,270 and turns them into a solar-system-like model 167 00:09:49,270 --> 00:09:52,150 of atoms which are essentially made out 168 00:09:52,150 --> 00:09:56,630 of empty space and a very small, intense nuclei. 169 00:09:56,630 --> 00:09:59,410 So Rutherford then continues with this experiment 170 00:09:59,410 --> 00:10:06,130 and it's able to produce, by bombarding nitrogen 171 00:10:06,130 --> 00:10:08,990 with other particles, protons and oxygen. 172 00:10:08,990 --> 00:10:12,070 And this is, in fact, the first human-engineered nuclear 173 00:10:12,070 --> 00:10:12,740 reaction. 174 00:10:12,740 --> 00:10:17,050 So now we are in the year 1919, just after the First World 175 00:10:17,050 --> 00:10:19,390 War ended. 176 00:10:19,390 --> 00:10:21,250 On the theoretical side, this is the time 177 00:10:21,250 --> 00:10:24,490 that quantum mechanics is developed. 178 00:10:24,490 --> 00:10:26,530 And Dirac then combines relativity 179 00:10:26,530 --> 00:10:30,010 with quantum mechanics, which then 180 00:10:30,010 --> 00:10:32,020 leads to the so-called Dirac equation, which 181 00:10:32,020 --> 00:10:37,540 we're going to look at very shortly in this class as well. 182 00:10:37,540 --> 00:10:39,160 This equation is quite interesting 183 00:10:39,160 --> 00:10:41,740 because it predicts the existence of negative energy 184 00:10:41,740 --> 00:10:43,010 states. 185 00:10:43,010 --> 00:10:46,180 And so then that just comes out of the equations. 186 00:10:46,180 --> 00:10:49,100 And then you ask yourself, what's happening here? 187 00:10:49,100 --> 00:10:52,990 You can have an interpretation that, for an electron, 188 00:10:52,990 --> 00:10:55,660 of the electron which traveled backwards in time, 189 00:10:55,660 --> 00:10:59,380 or you interpret them as electrons 190 00:10:59,380 --> 00:11:01,100 with negative energies. 191 00:11:01,100 --> 00:11:04,600 And so this then leads to the prediction of antimatter. 192 00:11:08,410 --> 00:11:14,020 Pauli and Fermi, they're puzzled by a problem 193 00:11:14,020 --> 00:11:17,720 of energy conservation in the second case. 194 00:11:17,720 --> 00:11:22,810 And so this is something which is rather weird 195 00:11:22,810 --> 00:11:25,630 and is a big challenge to the physics of the time. 196 00:11:25,630 --> 00:11:27,310 And they've solved this challenge 197 00:11:27,310 --> 00:11:30,790 by proposing a new particle which is rather light 198 00:11:30,790 --> 00:11:32,650 and doesn't interact with the detectors they 199 00:11:32,650 --> 00:11:33,890 had available at the time. 200 00:11:33,890 --> 00:11:36,520 So it just escapes undetected. 201 00:11:36,520 --> 00:11:38,890 They call this particle the neutrino. 202 00:11:38,890 --> 00:11:41,410 A year later, neutrons are directly 203 00:11:41,410 --> 00:11:44,260 detected in experiments by Chadwick 204 00:11:44,260 --> 00:11:47,110 with beryllium and other particles again. 205 00:11:47,110 --> 00:11:51,100 And then the predicted anti-electrons, the positrons, 206 00:11:51,100 --> 00:11:55,840 were discovered by Anderson in tracts of photographic plates 207 00:11:55,840 --> 00:11:57,340 which looked like electrons but they 208 00:11:57,340 --> 00:11:58,880 curve in the wrong direction. 209 00:11:58,880 --> 00:12:01,420 So either they have the opposite charge 210 00:12:01,420 --> 00:12:03,410 or they travel backwards in time. 211 00:12:03,410 --> 00:12:06,356 They didn't have quite the time resolution to [INAUDIBLE].. 212 00:12:09,780 --> 00:12:15,020 All right, also on the theoretical side, 213 00:12:15,020 --> 00:12:19,040 it needed to be understood how neutrons and protons actually 214 00:12:19,040 --> 00:12:21,840 bind together in nuclei. 215 00:12:21,840 --> 00:12:28,580 And so Hideki Yukawa proposes the existence of a strong force 216 00:12:28,580 --> 00:12:31,100 which is really, really strong and binds those nuclear 217 00:12:31,100 --> 00:12:35,570 together to a degree that you cannot easily break them apart. 218 00:12:35,570 --> 00:12:40,460 And then Bethe calculates how nuclear fusion, rather than 219 00:12:40,460 --> 00:12:44,180 the fission process, can be used in order to power the sun. 220 00:12:44,180 --> 00:12:48,500 So for this, he proposes a three-step process, 221 00:12:48,500 --> 00:12:50,610 the so-called proton-proton chain, 222 00:12:50,610 --> 00:12:52,820 which I will not discuss here but we will certainly 223 00:12:52,820 --> 00:12:56,040 discuss later in this class. 224 00:12:56,040 --> 00:12:57,620 And then there's more developments 225 00:12:57,620 --> 00:13:03,470 in the area of nuclear physics. 226 00:13:03,470 --> 00:13:06,780 And this progress is made by, again, 227 00:13:06,780 --> 00:13:11,462 using all kinds of materials and bombarding with each other. 228 00:13:11,462 --> 00:13:17,660 So for example, by colliding neutrons with uranium, 229 00:13:17,660 --> 00:13:20,150 one discovers a process of nuclear fission. 230 00:13:20,150 --> 00:13:23,750 This was done by Lise Meitner and Otto Hahn 231 00:13:23,750 --> 00:13:27,980 in the late 1930s. 232 00:13:27,980 --> 00:13:33,790 So from there on, there's interesting further 233 00:13:33,790 --> 00:13:36,010 developments going on in the sense 234 00:13:36,010 --> 00:13:44,075 that many physicists at the time in Europe 235 00:13:44,075 --> 00:13:45,880 are rather concerned by the developments 236 00:13:45,880 --> 00:13:48,760 of the Nazi Party in Germany. 237 00:13:48,760 --> 00:13:52,690 In the '40s already after the start of the Second World War, 238 00:13:52,690 --> 00:13:56,980 Albert Einstein wrote a letter to Theodore Roosevelt pointing 239 00:13:56,980 --> 00:14:00,740 out that there is a real threat that the Nazis are 240 00:14:00,740 --> 00:14:06,870 going to develop a bomb based on nuclear processes. 241 00:14:06,870 --> 00:14:09,620 And so this then led to the Manhattan Program 242 00:14:09,620 --> 00:14:13,280 in the US and the development of the first nuclear bombs or atom 243 00:14:13,280 --> 00:14:13,820 bombs. 244 00:14:13,820 --> 00:14:18,530 And in August 1945, the first two bombs 245 00:14:18,530 --> 00:14:22,070 were dropped on Japan, which led then 246 00:14:22,070 --> 00:14:24,830 to the surrender of the Japanese empire 247 00:14:24,830 --> 00:14:27,860 and the end of the Second World War. 248 00:14:27,860 --> 00:14:32,690 With that, I stop the discussion of those early developments. 249 00:14:32,690 --> 00:14:35,330 I hope you got a first glimpse and use 250 00:14:35,330 --> 00:14:38,960 this as a starting point to read further. 251 00:14:38,960 --> 00:14:41,090 Those characters, Lise Meitner, for example-- 252 00:14:41,090 --> 00:14:43,820 I'm looking at her picture right now-- 253 00:14:43,820 --> 00:14:47,930 very, very interesting to see how those people were 254 00:14:47,930 --> 00:14:51,710 connected, how those people communicated, 255 00:14:51,710 --> 00:14:54,020 and in which environment they had to work. 256 00:14:54,020 --> 00:14:56,750 Lise Meitner, for example, was Jewish. 257 00:14:56,750 --> 00:15:01,220 And she left Europe, had to flee from the Nazis in the '30s, 258 00:15:01,220 --> 00:15:05,090 while making these kind of discoveries. 259 00:15:05,090 --> 00:15:07,880 Also interesting is maybe the historical introduction 260 00:15:07,880 --> 00:15:09,290 to elementary particles. 261 00:15:09,290 --> 00:15:13,880 I have this here in David Griffiths' book. 262 00:15:13,880 --> 00:15:16,700 This starts with this kind of classical era 263 00:15:16,700 --> 00:15:19,220 and then goes beyond the Second World War 264 00:15:19,220 --> 00:15:24,270 and introduces the findings in particle physics beyond what I 265 00:15:24,270 --> 00:15:25,910 explained to this point. 266 00:15:25,910 --> 00:15:29,020 So I hope you enjoyed this. 267 00:15:29,020 --> 00:15:32,090 This is basically the last of these introductory lectures 268 00:15:32,090 --> 00:15:34,610 which doesn't come with a set of problems, 269 00:15:34,610 --> 00:15:37,740 with a set of things you should be interacting with. 270 00:15:37,740 --> 00:15:40,673 So the next one will already do that. 271 00:15:40,673 --> 00:15:44,990 And we will use this in the Thursday 272 00:15:44,990 --> 00:15:52,600 recitation of the first week to have discussion.