1 00:00:24,400 --> 00:00:29,244 In 1912, in a hot air balloon about three miles above the ground, 2 00:00:29,279 --> 00:00:35,738 an Austrian scientist called Victor Hess made one of the most astonishing discoveries in science. 3 00:00:35,773 --> 00:00:42,198 Up here Hess found that incredibly mysterious rays of energy were pouring in from outer space 4 00:00:42,233 --> 00:00:44,642 and streaming through the Earth. 5 00:00:44,677 --> 00:00:51,676 They were incredibly powerful, yet unlike anything seen before. They were called cosmic rays. 6 00:00:53,996 --> 00:00:57,559 At the same time in laboratories down below, 7 00:00:57,594 --> 00:01:02,554 scientists were studying equally mysterious and powerful energy rays 8 00:01:02,589 --> 00:01:07,478 pouring out from the interior of atoms - known as radioactivity. 9 00:01:07,513 --> 00:01:14,113 Mysterious rays from the vast emptiness of space and mysterious rays from deep within the atom, 10 00:01:14,148 --> 00:01:16,518 the tiniest building block. 11 00:01:16,553 --> 00:01:21,711 No one really understood what they were or if they might be connected. 12 00:01:21,746 --> 00:01:24,315 Then an incredible story unfolded. 13 00:01:24,350 --> 00:01:31,789 Cosmic rays and radioactivity turned out to be connected in a way so shocking that it beggars belief. 14 00:01:31,824 --> 00:01:37,829 The discovery of this connection would force us to rethink the nature of reality itself. 15 00:01:37,864 --> 00:01:43,433 The world we think we know, the solid, reassuring world of our senses, 16 00:01:43,468 --> 00:01:50,867 is just a tiny sliver of an infinitely weirder universe than we could ever conceive of. 17 00:01:50,902 --> 00:01:54,107 Our reality is just an illusion. 18 00:02:09,865 --> 00:02:16,203 In the years up to the mid-1920s, the atom revealed its strange secrets to us 19 00:02:16,238 --> 00:02:22,542 at a prodigious rate, as it produced one scientific revolution after another. 20 00:02:25,062 --> 00:02:31,461 In 1897, Marie Curie studied strange rays pouring out of some rare metals. 21 00:02:31,496 --> 00:02:33,902 She called them radioactivity. 22 00:02:36,381 --> 00:02:43,580 Then, in 1905, Albert Einstein conclusively proved the existence and size of an atom 23 00:02:43,615 --> 00:02:48,700 by studying the way pollen moves in water. 24 00:02:50,258 --> 00:02:56,417 A few years later, the New Zealander Ernest Rutherford performed an experiment in Manchester 25 00:02:56,452 --> 00:03:01,658 that revealed to him the shape of the interior of an atom. 26 00:03:06,656 --> 00:03:12,617 Scientists were shocked to discover that the atom is almost entirely empty space. 27 00:03:14,255 --> 00:03:20,455 The question then became, "How could this empty atom possibly make the solid world around us?" 28 00:03:20,490 --> 00:03:27,494 The answer to that was worked out by a group of revolutionary physicists in Denmark. 29 00:03:29,213 --> 00:03:33,577 They proposed that the world of the atom ran on principles 30 00:03:33,612 --> 00:03:38,418 which were completely different to any mankind had ever seen before. 31 00:03:38,453 --> 00:03:44,891 It meant that the atom, the basic building block of everything in the universe, was unique. 32 00:03:44,926 --> 00:03:49,250 And perhaps outside human comprehension. 33 00:03:50,450 --> 00:03:55,289 Then a scientist explored the nucleus, the tiny heart of the atom. 34 00:03:55,324 --> 00:03:59,294 They found it bursting with powerful energy. 35 00:03:59,329 --> 00:04:05,409 This discovery gave them the potential to bring about the destruction of the Earth, 36 00:04:05,444 --> 00:04:11,408 but in a shocking turnaround, it also gave them a fundamental understanding 37 00:04:11,443 --> 00:04:13,928 of how the universe was created. 38 00:04:15,047 --> 00:04:21,006 And yet, despite this, the journey to understand the strange and capricious atom 39 00:04:21,041 --> 00:04:24,007 had only just started. 40 00:04:31,045 --> 00:04:37,605 In 1927, a young man was studying at the Mathematics Department of Cambridge University. 41 00:04:38,404 --> 00:04:42,443 Shy, awkward, clumsy and frighteningly brilliant, 42 00:04:42,478 --> 00:04:47,203 his name was Paul Adrien Maurice Dirac. 43 00:04:53,002 --> 00:04:57,642 It's probably fair to say that Paul Dirac isn't a household name. 44 00:04:57,677 --> 00:05:02,245 But it should be. He was recently voted, by other physicists, 45 00:05:02,280 --> 00:05:08,044 as the second-greatest English physicist of all time, second only to Newton. 46 00:05:08,079 --> 00:05:14,084 And he deserves the accolade. All the brilliant minds that pioneered atomic physics 47 00:05:14,119 --> 00:05:20,477 were left trailing by Dirac, aghast at the sheer boldness and lateral thinking in his work. 48 00:05:27,396 --> 00:05:31,996 When Einstein read a paper by the then 24-year-old Dirac, he said, 49 00:05:32,031 --> 00:05:34,440 "I have trouble with Dirac. 50 00:05:34,475 --> 00:05:40,234 "This balancing on the dizzying path between genius and madness is awful." 51 00:05:41,594 --> 00:05:45,998 In 1927, for reasons no one has ever really fathomed, 52 00:05:46,033 --> 00:05:50,833 Paul Dirac set himself a task that was monumental in its scope - 53 00:05:50,868 --> 00:05:53,238 to unify science. 54 00:05:53,273 --> 00:05:57,672 To bring its scattered parts into one beautiful entity. 55 00:05:58,592 --> 00:06:03,631 And what this meant, above all, was to unite the two most difficult 56 00:06:03,666 --> 00:06:07,431 and counter-intuitive ideas in history. 57 00:06:08,792 --> 00:06:12,395 Here's what Dirac was trying to reconcile. 58 00:06:12,430 --> 00:06:19,070 First there's quantum mechanics, mathematical equations describing the atom and its component parts. 59 00:06:19,105 --> 00:06:25,668 Then there's Einstein's Special Theory of Relativity, which at first seems unrelated. 60 00:06:25,703 --> 00:06:30,445 It deals with loftier matters like the nature of space and time. 61 00:06:30,480 --> 00:06:35,188 One of its consequences is that objects behave very differently 62 00:06:35,223 --> 00:06:39,146 when they travel close to the speed of light. 63 00:06:45,426 --> 00:06:51,424 The first thing you might ask is why would anyone want to reconcile two such different theories? 64 00:06:51,459 --> 00:06:55,869 Well, by the late 1920s, the equations of quantum mechanics 65 00:06:55,904 --> 00:07:02,384 were consistently getting the wrong answers when describing electrons, one of the constituents of atoms, 66 00:07:02,419 --> 00:07:04,987 as they move at very high speed. 67 00:07:05,022 --> 00:07:09,068 But for Dirac there was a much more esoteric motivation. 68 00:07:09,103 --> 00:07:14,707 He was once quoted as saying, "A physical theory must have mathematical beauty". 69 00:07:14,742 --> 00:07:21,140 So for him, the fact that quantum mechanics and relativity weren't reconciled wasn't just inconvenient, 70 00:07:21,175 --> 00:07:23,740 it was downright ugly. 71 00:07:25,339 --> 00:07:28,984 So around 1925, in Cambridge, 72 00:07:29,019 --> 00:07:36,939 Dirac put his extraordinary mind, a mind that even Einstein had trouble keeping up with, to work. 73 00:07:39,458 --> 00:07:44,217 This is Room A4, New Court. It was Dirac's original study. 74 00:07:45,577 --> 00:07:49,182 The original fireplace has been boarded up, 75 00:07:49,217 --> 00:07:56,335 but it was here that Dirac tried to understand and bring together the two new ideas of physics. 76 00:07:56,370 --> 00:08:01,501 Word is Dirac would sit here in front of his blazing fireplace 77 00:08:01,536 --> 00:08:09,334 and try to understand and bring together these two different theories into one unified picture, 78 00:08:09,369 --> 00:08:11,534 one single equation. 79 00:08:18,373 --> 00:08:23,132 For three frustrating years, he laboured alone on the problem. 80 00:08:25,971 --> 00:08:31,895 Then, one evening in early 1928, he had an amazing revelation. 81 00:08:31,930 --> 00:08:37,929 The only way I can explain what happened is to say that the equations of quantum mechanics 82 00:08:37,964 --> 00:08:41,974 and special relativity coalesced inside Dirac's mind. 83 00:08:42,009 --> 00:08:48,408 Einstein's description of space and time somehow stretched and squeezed the existing equations of the atom. 84 00:08:48,443 --> 00:08:53,252 They bent and twisted them into new weird and wonderful shapes. 85 00:08:53,287 --> 00:08:58,892 Then, guided by his unshakeable belief that nature's laws must be beautiful, 86 00:08:58,927 --> 00:09:07,125 Dirac homed in on one equation, an entirely new description of what goes on inside the atom. 87 00:09:15,925 --> 00:09:20,884 Dirac knew it was right because it had mathematical beauty. 88 00:09:30,601 --> 00:09:33,203 Here it is, the Dirac equation. 89 00:09:39,442 --> 00:09:43,486 Don't try to understand it. Just look at it and marvel. 90 00:09:43,521 --> 00:09:50,919 As human achievements go, it's up there with King Lear, Beethoven's Fifth or The Origin of the Species. 91 00:09:50,954 --> 00:09:58,358 Hidden in these symbols is the perfect description of how reality works at a fundamental level. 92 00:09:58,393 --> 00:10:02,558 It's the key to nature's secret code. 93 00:10:04,318 --> 00:10:07,362 With perfect mathematical elegance, 94 00:10:07,397 --> 00:10:13,356 Dirac's equation describes an atomic particle travelling at any speed, 95 00:10:13,391 --> 00:10:15,916 right up to the speed of light. 96 00:10:17,397 --> 00:10:23,995 That much Dirac was expecting to achieve, but when he looked at his own equation more carefully, 97 00:10:24,030 --> 00:10:28,633 he noticed something breathtakingly revolutionary about it. 98 00:10:28,668 --> 00:10:32,679 He later said his equation knew more than he did. 99 00:10:32,714 --> 00:10:39,233 In essence, Dirac's equation was telling him there's another universe we've never noticed before. 100 00:10:45,552 --> 00:10:49,551 That's because instead of his equation having one answer, 101 00:10:49,586 --> 00:10:51,551 it has two. 102 00:10:53,150 --> 00:10:58,791 The first describes the universe we know, made of the atoms we're familiar with. 103 00:10:58,826 --> 00:11:02,967 The second describes a mirror image to our universe, 104 00:11:03,002 --> 00:11:07,073 made of atoms whose properties are somehow reversed. 105 00:11:07,108 --> 00:11:11,908 Science fiction fans will know what's coming. As well as matter, 106 00:11:11,943 --> 00:11:16,468 Dirac's equation predicts the existence of antimatter. 107 00:11:18,308 --> 00:11:22,992 Dirac's theory seemed to say that for everything in our known world, 108 00:11:23,027 --> 00:11:29,545 for every part of an atom, every particle, there can exist a corresponding anti-particle 109 00:11:29,580 --> 00:11:34,549 with the same mass, but exactly opposite in every other way. 110 00:11:34,584 --> 00:11:40,904 And just like a world in a mirror, the universe made of antimatter atoms would look and work 111 00:11:40,939 --> 00:11:42,910 just like ours. 112 00:11:42,945 --> 00:11:47,982 It would be perfectly possible for me to be made out of antimatter. 113 00:11:48,017 --> 00:11:52,787 Anti-me would look and behave exactly the same as original me. 114 00:11:52,822 --> 00:11:57,861 And it's possible that out there in the vast expanses of the cosmos, 115 00:11:57,896 --> 00:12:02,901 there are stars and planets and even living beings made of antimatter. 116 00:12:02,936 --> 00:12:06,904 There's one final prediction of the Dirac equation. 117 00:12:06,939 --> 00:12:12,540 It states that matter and antimatter must never come into contact. If they do, 118 00:12:12,575 --> 00:12:18,223 they will annihilate each other in a conflagration of pure energy. 119 00:12:18,258 --> 00:12:24,257 The combined mass of matter and antimatter would convert completely into energy, 120 00:12:24,292 --> 00:12:28,223 according to Einstein's famous equation, E=MC2. 121 00:12:28,258 --> 00:12:32,702 So if I ever do meet my doppelganger, we would explode 122 00:12:32,737 --> 00:12:38,496 with an energy equivalent to a million Hiroshima-sized atom bombs. 123 00:12:45,535 --> 00:12:53,173 All this sounds like science fiction and the idea of antimatter has inspired huge swathes of it. 124 00:12:53,208 --> 00:12:59,253 But the truth is antimatter, particularly antimatter electrons, called positrons 125 00:12:59,288 --> 00:13:03,018 are made routinely now in laboratories. 126 00:13:03,053 --> 00:13:08,252 Positrons are used in sophisticated medical imaging devices called PET scanners 127 00:13:08,287 --> 00:13:14,131 that can see through our skulls and accurately map pictures of our brains. 128 00:13:16,530 --> 00:13:23,249 But back in the 1920s, the initial reaction to Dirac's equation among physicists was deeply sceptical. 129 00:13:24,329 --> 00:13:28,129 Even Dirac had trouble believing his own results. 130 00:13:28,164 --> 00:13:31,893 Antimatter seemed such a preposterous concept. 131 00:13:31,928 --> 00:13:37,927 Then came resounding confirmation of the Dirac equation and all its paradoxical implications 132 00:13:37,962 --> 00:13:42,847 and it came from the most unexpected place - outer space. 133 00:13:43,886 --> 00:13:50,365 In 1932, physicist Carl Anderson was working here at Caltech in Los Angeles 134 00:13:50,400 --> 00:13:53,162 when he made an amazing discovery. 135 00:13:53,197 --> 00:13:55,889 He'd been studying cosmic rays. 136 00:13:55,924 --> 00:14:02,364 These are high-energy subatomic particles that continuously bombard the Earth from outer space. 137 00:14:02,399 --> 00:14:08,009 To do this, he used a device called a cloud chamber. 138 00:14:08,044 --> 00:14:12,728 This is basically a vessel filled with a fine mist of water vapour. 139 00:14:12,763 --> 00:14:18,281 This shows up the tracks of the particles as they stream down through the vapour. 140 00:14:18,316 --> 00:14:23,765 Placed inside a magnetic field, these tracks are deflected one way or the other, 141 00:14:23,800 --> 00:14:30,080 depending on the electric charge of the particle. Positive tracks go one way, negative the other. 142 00:14:30,115 --> 00:14:34,719 Anderson found evidence of particles that look exactly like electrons, 143 00:14:34,754 --> 00:14:38,764 but which are deflected in the opposite direction. 144 00:14:38,800 --> 00:14:44,757 He had discovered Dirac's anti-electrons, particles of antimatter. 145 00:14:48,317 --> 00:14:52,362 The Dirac equation is an impressive achievement. 146 00:14:52,397 --> 00:14:58,395 Its prediction of the existence of antimatter, using abstract mathematics alone, would be enough 147 00:14:58,430 --> 00:15:03,440 to make it a significant milestone in the history of human thought. 148 00:15:03,475 --> 00:15:09,554 But within just a few years of publication, first Dirac and then others sensed 149 00:15:09,589 --> 00:15:13,692 that his new equation was telling them something profound, 150 00:15:13,727 --> 00:15:17,758 something completely new about nature. And they were right. 151 00:15:17,793 --> 00:15:24,552 But the revelation hidden within Dirac's equation would take the best efforts of the greatest minds 152 00:15:24,587 --> 00:15:26,593 30 years to uncover. 153 00:15:27,632 --> 00:15:33,556 The problem with Dirac's equation was this - 154 00:15:33,591 --> 00:15:38,035 although it was incredibly powerful and led to the discovery of antimatter, 155 00:15:38,070 --> 00:15:44,149 ultimately it could only describe a single electron. It fails completely to explain what happens 156 00:15:44,184 --> 00:15:50,148 when there is more than one electron present. What was needed was a new theory to explain 157 00:15:50,183 --> 00:15:54,108 how electrons interact with each other. 158 00:15:55,147 --> 00:16:01,187 And that turned out to be the most difficult question of the mid-20th Century, 159 00:16:01,222 --> 00:16:07,227 but when an answer came, it was to bring with it an unexpected revelation. 160 00:16:12,345 --> 00:16:18,664 This office in Caltech used to belong to the great Richard Feynman. 161 00:16:19,664 --> 00:16:24,550 In our story of so many geniuses of science, 162 00:16:24,585 --> 00:16:30,703 Feynman stands, in my view, second only to Einstein in the list of greatest 20th Century physicists. 163 00:16:31,863 --> 00:16:35,906 Feynman wasn't just a common or garden genius. 164 00:16:35,941 --> 00:16:41,581 Many referred to him as a magician, he was so smart, such an innovative thinker. 165 00:16:41,616 --> 00:16:46,837 Like Einstein, he became this mythical figure, a household name. 166 00:16:46,872 --> 00:16:52,060 Feynman was a larger-than-life character with a huge personality. 167 00:16:52,095 --> 00:16:55,345 He loved cultivating and telling anecdotes about himself. 168 00:16:55,380 --> 00:17:01,778 He used to frequent strip clubs, he had affairs with his students and was rumoured to go to orgies, 169 00:17:01,813 --> 00:17:05,819 but his greatest contribution to physics was the part he played 170 00:17:05,854 --> 00:17:09,499 in developing the next phase of quantum mechanics. 171 00:17:12,697 --> 00:17:19,236 Feynman and his contemporaries were attempting to pick up the atomic torch from Paul Dirac 172 00:17:19,271 --> 00:17:25,775 and develop a theory that took our understanding of the atom literally a quantum leap further. 173 00:17:25,810 --> 00:17:32,775 Like Dirac's antimatter equation before, the intention of the new theory was unification. 174 00:17:32,810 --> 00:17:37,419 They wanted to understand how electrons affect each other. 175 00:17:37,454 --> 00:17:44,413 In other words, it aimed to explain how everything works together through the electromagnetic field. 176 00:17:44,448 --> 00:17:50,772 They called their unification project quantum electrodynamics or QED. 177 00:17:51,811 --> 00:17:58,015 The project was a formidable challenge, but the end result was magnificent - 178 00:17:58,050 --> 00:18:03,770 nothing less than the most far-reaching and accurate scientific theory ever conceived. 179 00:18:05,009 --> 00:18:10,529 For instance, it predicts a certain property of the electron 180 00:18:10,564 --> 00:18:13,368 called its magnetic moment to have a value of... 181 00:18:19,487 --> 00:18:25,527 Experiments measure precisely the same number. That's an agreement between theory and experiment 182 00:18:25,562 --> 00:18:30,932 to one part in ten billion. It's an unprecedented level of agreement. 183 00:18:30,967 --> 00:18:37,606 It's like measuring the distance between London and New York to within the thickness of a hair. 184 00:18:37,641 --> 00:18:41,649 The phenomenal accuracy of quantum electrodynamics 185 00:18:41,684 --> 00:18:47,684 shows it to underpin almost everything we experience in the physical world. 186 00:18:47,719 --> 00:18:52,083 It's as close to a theory of everything as we have ever come. 187 00:18:53,083 --> 00:18:57,088 It defies the laws of nature - the atomic scale. 188 00:18:57,123 --> 00:19:04,381 It explains shape, colour, texture and the way almost everything interacts and fits together. 189 00:19:04,416 --> 00:19:11,640 It encompasses everything from the biochemistry of life to why we don't fall through the floor. 190 00:19:13,079 --> 00:19:16,284 So what does QED actually say? 191 00:19:16,319 --> 00:19:19,923 Well, this is where the going gets very tough. 192 00:19:19,958 --> 00:19:24,004 It may be a wonderful scientific description of nature, 193 00:19:24,039 --> 00:19:31,037 but trying to understand what Feynman was doing with his theory is almost impossible. 194 00:19:31,072 --> 00:19:34,842 This is what he himself said when he introduced his theory: 195 00:19:34,877 --> 00:19:40,881 "It is my task to convince you not to turn away because you don't understand it. 196 00:19:40,916 --> 00:19:47,276 "My physics students don't understand it. That's because I don't understand it. Nobody does." 197 00:19:49,355 --> 00:19:55,794 If the inventor of the theory doesn't understand, what possible hope is there for the rest of us? 198 00:19:55,829 --> 00:20:00,358 With that disclaimer, I'm going to try to explain anyway. 199 00:20:00,393 --> 00:20:05,392 First, you have to abandon your most basic intuition about nature. 200 00:20:05,427 --> 00:20:10,031 You have to give up the notion that empty space is empty. 201 00:20:10,066 --> 00:20:12,636 Let me try to explain. 202 00:20:12,671 --> 00:20:16,517 If I were to suck out all the air from this jar, 203 00:20:16,552 --> 00:20:22,111 you'd quite rightly say that having removed all the atoms, I'm left with a vacuum, 204 00:20:22,146 --> 00:20:25,475 a volume of pure emptiness. 205 00:20:25,510 --> 00:20:29,354 Quantum electrodynamics flies in the face of this idea 206 00:20:29,389 --> 00:20:36,388 by saying that the vacuum is NOT, I repeat not, a place where nothing exists and nothing happens. 207 00:20:36,423 --> 00:20:39,166 Instead, it's full of stuff. 208 00:20:39,201 --> 00:20:41,908 And it's heaving with activity. 209 00:20:43,308 --> 00:20:45,867 How can this possibly be true? 210 00:20:54,505 --> 00:20:57,669 Well, let's imagine one tiny point in the emptiness. 211 00:20:57,704 --> 00:21:04,030 Common sense tells us that there's nothing there, but quantum physics tells us 212 00:21:04,065 --> 00:21:11,703 there's only nothing there ON AVERAGE. This forces us to rethink our understanding of reality. 213 00:21:11,738 --> 00:21:17,303 Think of empty space like a bank account, which on average has nothing in it. 214 00:21:17,338 --> 00:21:19,947 This is a concept I'm familiar with! 215 00:21:19,982 --> 00:21:24,942 Some days it might be £100 in credit, others £100 overdrawn. 216 00:21:24,977 --> 00:21:28,545 But on average it has a zero balance. 217 00:21:28,580 --> 00:21:32,661 Empty space turns out to have similar accounting skills, 218 00:21:32,696 --> 00:21:36,224 but it can borrow energy rather than money 219 00:21:36,259 --> 00:21:42,023 and this is literally borrowed from the future, provided it's paid back very quickly. 220 00:21:42,058 --> 00:21:49,298 In practice this means the borrowed energy can be used to create a particle and an anti-particle, 221 00:21:49,333 --> 00:21:55,338 which are spontaneously formed from the void, provided that a fraction of a second later 222 00:21:55,373 --> 00:21:59,296 they annihilate each other and disappear. 223 00:22:00,697 --> 00:22:03,781 Energy is borrowed out of nowhere. 224 00:22:03,816 --> 00:22:09,734 It's turned into matter. The matter then self-destructs back into energy. 225 00:22:10,855 --> 00:22:15,459 And this happens in an instant all over the void. 226 00:22:15,494 --> 00:22:19,898 In fact, in a stunning confirmation of Dirac's antimatter theory, 227 00:22:19,933 --> 00:22:25,054 the vacuum seethes with huge numbers of matter and antimatter particles, 228 00:22:25,089 --> 00:22:28,612 continually being created and annihilated. 229 00:22:37,851 --> 00:22:43,731 Down at the smallest scale, space is a constant storm of creation and destruction. 230 00:22:43,766 --> 00:22:47,689 Physicists call it the quantum foam. 231 00:22:48,690 --> 00:22:55,448 The particles in the quantum foam come and go so quickly, we're completely unaware of them. 232 00:22:55,483 --> 00:23:02,172 We refer to them as virtual particles, but if we could slow time down almost to a standstill, 233 00:23:02,207 --> 00:23:08,767 we'd be able to see this seething activity, this constant creation and annihilation of matter and energy 234 00:23:08,802 --> 00:23:11,367 that's the fabric of reality itself. 235 00:23:14,446 --> 00:23:18,450 From this comes the most jaw-dropping idea of all. 236 00:23:18,485 --> 00:23:25,684 Quantum electrodynamics says that the matter we think of as the stuff that makes up the everyday world, 237 00:23:25,719 --> 00:23:28,208 the world that we see and feel, 238 00:23:28,243 --> 00:23:32,684 is basically just a kind of leftover from all the feverish activity 239 00:23:32,719 --> 00:23:36,961 that virtual particles get up to in the void. 240 00:23:36,996 --> 00:23:41,166 So you, me, the Earth, the stars, everything, 241 00:23:41,201 --> 00:23:48,005 is basically just a part of a deeper, infinitely more complex reality than we ever imagined. 242 00:23:48,040 --> 00:23:55,239 Of course, when Feynman first started to develop his revolutionary ideas in Caltech in the mid '40s, 243 00:23:55,274 --> 00:23:57,924 his contemporaries were horrified 244 00:23:57,959 --> 00:24:04,359 because at that time the general opinion was that the quantum electrodynamics project 245 00:24:04,394 --> 00:24:06,996 was an unmitigated disaster. 246 00:24:07,031 --> 00:24:09,564 The theory couldn't be solved. 247 00:24:09,599 --> 00:24:16,003 The equations had no sensible solutions. The mathematics had spiralled out of control. 248 00:24:16,038 --> 00:24:22,596 But Feynman believed that he could see a way through the mathematical complexity to a new truth. 249 00:24:25,235 --> 00:24:30,835 What Feynman did, with all the arrogance and confidence of youth, 250 00:24:30,870 --> 00:24:34,920 was slash through the insanely complicated maths. 251 00:24:34,955 --> 00:24:40,914 Feynman developed a new series or revolutionary, but almost childlike, diagrams 252 00:24:40,949 --> 00:24:43,359 to explain his new ideas. 253 00:24:43,394 --> 00:24:49,712 Their elegant simplicity flew in the face of the complex maths of traditional quantum mechanics. 254 00:24:52,192 --> 00:24:54,792 Conflict seemed inevitable. 255 00:24:56,911 --> 00:25:04,471 Then, in 1948, at the age of 30, Richard Feynman decided to unveil his controversial version 256 00:25:04,506 --> 00:25:10,887 of quantum electrodynamics with his idiosyncratic diagrams to the physics world. 257 00:25:10,922 --> 00:25:17,234 And he chose the most important science conference of the American calendar. 258 00:25:17,269 --> 00:25:23,707 Set on the coast of Pennsylvania, the Shelter Island Conference was a physics celebrity circus. 259 00:25:23,742 --> 00:25:28,512 Present were Niels Bohr, so-called "father of atomic physics", 260 00:25:28,547 --> 00:25:34,547 the discoverer of antimatter Paul Dirac and the man behind America's atom bomb, 261 00:25:34,582 --> 00:25:36,550 Robert Oppenheimer. 262 00:25:36,585 --> 00:25:40,625 The atmosphere at the start of the conference was grim. 263 00:25:40,660 --> 00:25:44,665 Confidence in quantum electrodynamics was at rock bottom. 264 00:25:44,700 --> 00:25:48,229 It seemed a hopeless mess. 265 00:25:48,264 --> 00:25:56,144 One after another, the physicists stood up and droned on despairingly about failing to find a solution. 266 00:25:56,179 --> 00:25:59,508 Then it was the turn of Richard Feynman. 267 00:25:59,543 --> 00:26:05,582 Barely 30 years old, he stood up and took his place in front of the world's most illustrious scientists 268 00:26:05,617 --> 00:26:09,547 and started to unveil his new diagrams and equations. 269 00:26:09,582 --> 00:26:13,021 What happened next was astonishing. A row broke out, not over Feynman's weird description of reality - 270 00:26:13,181 --> 00:26:16,980 What happened next was astonishing. A row broke out, not over Feynman's weird description of reality - 271 00:26:17,015 --> 00:26:19,826 physicists were used to weird - 272 00:26:19,861 --> 00:26:23,700 but because he dared to visualise what was going on. 273 00:26:23,735 --> 00:26:27,505 Instead of using arcane, complicated mathematics, 274 00:26:27,540 --> 00:26:33,899 Feynman was describing what all his virtual particles were up to, using his simple pictures. 275 00:26:35,577 --> 00:26:41,981 There was uproar. Niels Bohr, the father of quantum mechanics, leapt from his chair in disgust. 276 00:26:42,016 --> 00:26:49,095 He hated Feynman's diagrams because they went completely against everything he'd devoted his life to. 277 00:26:49,130 --> 00:26:55,534 He believed that atomic particles could not be visualised under any circumstances. 278 00:26:55,569 --> 00:26:58,140 Feynman defended his new theory, 279 00:26:58,175 --> 00:27:05,012 trying to explain that the diagrams were simply a tool to help visualise his new equations. 280 00:27:05,047 --> 00:27:09,470 But the rest of the scientists, including Dirac, wouldn't hear it, 281 00:27:09,505 --> 00:27:13,893 calling him an idiot who understood nothing about quantum mechanics. 282 00:27:14,932 --> 00:27:19,011 Feynman ended his lecture bruised, but unrepentant. 283 00:27:19,046 --> 00:27:23,090 He knew that his diagrams and equations were correct. 284 00:27:23,125 --> 00:27:26,011 If only he could convince the others. 285 00:27:32,849 --> 00:27:38,855 That evening, Feynman met another young physicist called Julian Schwinger. 286 00:27:38,890 --> 00:27:44,853 He was the same age as Feynman and had been identified as a child prodigy at the age of 12. 287 00:27:44,888 --> 00:27:52,128 Although he and Feynman had been working independently and approached the problem very differently, 288 00:27:52,163 --> 00:27:55,731 they'd reached identical conclusions. 289 00:27:55,766 --> 00:28:01,205 With their new equations, they could solve quantum electrodynamics 290 00:28:01,240 --> 00:28:06,610 and with Feynman's diagrams they produced a theory of awesome power. 291 00:28:06,645 --> 00:28:12,645 Together now as allies, they planned a full-frontal attack on Niels Bohr and the conservatives. 292 00:28:12,680 --> 00:28:15,048 By the end of the conference, 293 00:28:15,083 --> 00:28:21,083 the mood in the Pennsylvanian roadhouse had changed from one of frustrated hopelessness 294 00:28:21,118 --> 00:28:23,727 to one of excitement and idealism. 295 00:28:23,762 --> 00:28:28,207 Over the next few years, their theory was fleshed out 296 00:28:28,242 --> 00:28:35,080 and rapidly became the most accurate and powerful theory mankind had ever had. 297 00:28:40,799 --> 00:28:45,444 Despite finally being tamed, quantum electrodynamics' talk 298 00:28:45,479 --> 00:28:52,078 of empty space seething with energy we can't feel and virtual particles we can't see 299 00:28:52,113 --> 00:28:56,723 does make many people, including physicists, a little suspicious. 300 00:28:56,758 --> 00:29:02,716 And many sceptics might say these ghostly objects that allegedly fill the vacuum 301 00:29:02,751 --> 00:29:05,162 aren't actually real. 302 00:29:05,197 --> 00:29:10,202 Yes, the complicated mathematical equations seem to require them, 303 00:29:10,237 --> 00:29:16,236 but that doesn't itself mean they exist. They might just be mathematical fantasies 304 00:29:16,271 --> 00:29:18,834 with no basis in reality. 305 00:29:21,193 --> 00:29:24,638 Well, I have bad news for the sceptics. 306 00:29:24,673 --> 00:29:31,438 Since the late 1950s, direct evidence that empty space isn't in the slightest bit empty 307 00:29:31,473 --> 00:29:37,513 but is, in fact, seething with activity has been observed time and time again in laboratories. 308 00:29:37,548 --> 00:29:42,351 And what's wonderful about the proof that emptiness isn't empty 309 00:29:42,386 --> 00:29:47,231 is that the first clue came from a jar of mayonnaise. 310 00:29:48,271 --> 00:29:55,470 In 1948, a physicist called Hendrik Casimir was working at the Philips Research Laboratories in Holland 311 00:29:55,505 --> 00:29:59,148 on the seemingly obscure problem of colloidal solutions. 312 00:29:59,183 --> 00:30:03,749 This is just a fancy name for substances like paint 313 00:30:03,784 --> 00:30:05,793 and mayonnaise 314 00:30:05,828 --> 00:30:10,787 which consist of tiny solid particles suspended in a liquid. 315 00:30:12,826 --> 00:30:19,225 You see, no one knew why mayonnaise wasn't runny. Why doesn't it behave like a normal liquid? 316 00:30:19,260 --> 00:30:25,625 It's as if some strange force holds the molecules of mayonnaise together, making it viscous. 317 00:30:25,660 --> 00:30:28,022 And that got Casimir thinking. 318 00:30:28,057 --> 00:30:30,350 In an astonishing insight, 319 00:30:30,385 --> 00:30:36,424 Casimir realised that the mysterious force that attracts molecules of mayonnaise together 320 00:30:36,459 --> 00:30:41,629 is related to the mysterious virtual particles in empty space. 321 00:30:41,664 --> 00:30:48,502 And even better, he came up with an experiment that would reveal these particles for all to see. 322 00:30:48,537 --> 00:30:54,701 It took another ten years of tinkering in labs to carry out Casimir's experiment, 323 00:30:54,736 --> 00:30:57,220 but in essence it's quite simple. 324 00:31:03,379 --> 00:31:08,385 You suspend two metal plates very close to each other in a vacuum. 325 00:31:08,420 --> 00:31:14,419 These plates aren't magnetic or electrically charged, so you'd expect them to sit there immobile, 326 00:31:14,454 --> 00:31:16,982 unaffected by each other. 327 00:31:17,017 --> 00:31:21,858 In fact, over time, they start to move towards each other 328 00:31:21,893 --> 00:31:25,862 due to a tiny force that pushes them together. 329 00:31:25,897 --> 00:31:30,376 And this force doing the pushing, Casimir showed, was caused 330 00:31:30,411 --> 00:31:34,380 by the virtual particles that fill the vacuum. 331 00:31:34,415 --> 00:31:41,774 Like wind pushing the sail of a boat at sea, the stuff that emptiness is made of pushes the plates together. 332 00:31:41,809 --> 00:31:49,214 The fact that nothingness, pure emptiness, could exert a small, but real mechanical force 333 00:31:49,249 --> 00:31:53,217 is surely one of nature's greatest magic tricks. 334 00:31:53,253 --> 00:31:59,251 In their more fanciful moments, physicists speculate that this so-called vacuum energy 335 00:31:59,286 --> 00:32:01,696 might one day be harnessed. 336 00:32:01,731 --> 00:32:07,770 They imagine it powering intergalactic spaceships carrying humans across the cosmos. 337 00:32:07,805 --> 00:32:13,811 Who knows if this will ever come to pass, but that mayonnaise might lead to space travel 338 00:32:13,846 --> 00:32:17,489 is a connection Douglas Adams would be proud of. 339 00:32:19,368 --> 00:32:23,368 Quantum electrodynamics is, by any measure, 340 00:32:23,403 --> 00:32:25,973 a truly magnificent discovery. 341 00:32:26,008 --> 00:32:33,027 It's one great pinnacle of our story, a glorious conclusion to five amazing decades of science. 342 00:32:33,062 --> 00:32:40,046 In quantum electrodynamics, the atom had given us a theory that explains much of our universe 343 00:32:40,081 --> 00:32:42,650 with stunning accuracy. 344 00:32:42,685 --> 00:32:47,166 But since quantum electrodynamics' triumphant arrival in the late '40s, 345 00:32:47,201 --> 00:32:51,489 our story becomes rather messy and awkward. 346 00:32:51,524 --> 00:32:58,623 As a result of quantum electrodynamics, scientists were convinced that the vast majority 347 00:32:58,658 --> 00:33:05,722 of everything in the universe consisted of essentially just two things - atoms and light. 348 00:33:06,762 --> 00:33:10,726 Light was made out of tiny particles called photons. 349 00:33:10,761 --> 00:33:16,801 And atoms were made out of three components - the electron, the proton and the neutron. 350 00:33:16,836 --> 00:33:22,639 And because of antimatter, there were anti-protons, anti-neutrons and positrons - 351 00:33:22,674 --> 00:33:27,284 a bit strange, but pleasingly symmetrical. 352 00:33:27,319 --> 00:33:33,359 Everything in the physics garden was rosy thanks to the rules of quantum electrodynamics, 353 00:33:33,394 --> 00:33:39,163 but then, much to the profound irritation of every working physicist, 354 00:33:39,198 --> 00:33:45,562 a load of new and exotic particles suddenly appeared like party gatecrashers to spoil the fun. 355 00:33:45,597 --> 00:33:52,036 Exotic entities that didn't fit in to any known theories were appearing in physics labs with such frequency 356 00:33:52,071 --> 00:33:56,081 that scientists couldn't keep up with naming them all. 357 00:33:56,116 --> 00:34:02,114 The neutrino, the positive pion, the negative pion, the kaon, the lambda, the delta... 358 00:34:02,149 --> 00:34:06,120 And each of these had their antimatter counterparts. 359 00:34:06,155 --> 00:34:13,034 When one new particle, the muon, was discovered, a physicist quipped, "Who ordered that?" 360 00:34:13,069 --> 00:34:19,913 The whole thing was a mess and physicists despairingly refer to it as the particle zoo. 361 00:34:21,352 --> 00:34:27,391 It began to seem as though every time scientists solved one of nature's mysteries, 362 00:34:27,426 --> 00:34:31,796 the atom would present them with something even more weird. 363 00:34:31,831 --> 00:34:37,830 Within just a few years, atomic physics had gone from a position of quiet confidence 364 00:34:37,865 --> 00:34:39,915 to total chaos. 365 00:34:39,950 --> 00:34:45,908 And, of course, to make some sense of this new mystery would require - yes, you've guessed it - 366 00:34:45,943 --> 00:34:48,508 another scientific revolution. 367 00:34:49,947 --> 00:34:54,352 The third genius in our story is Murray Gell-Mann. 368 00:34:54,387 --> 00:35:00,392 Gell-Mann was a child prodigy. By 15, he'd already started at Yale to study Physics 369 00:35:00,427 --> 00:35:06,765 and finished his PhD by his early 20s. His incredible intelligence terrified those around him. 370 00:35:06,800 --> 00:35:13,104 He spoke many languages and seemed to have a deep knowledge of any subject you threw at him. 371 00:35:13,139 --> 00:35:17,745 Like Richard Feynman, whom he joined here at Caltech in the early '60s, 372 00:35:17,780 --> 00:35:22,543 he seemed to have this ability to see beyond the mathematics 373 00:35:22,578 --> 00:35:25,987 to the underlying secrets of nature below. 374 00:35:26,022 --> 00:35:30,382 Together, Gell-Mann and Feynman made an awesome duo. 375 00:35:30,417 --> 00:35:34,706 This office, Number 456, used to belong to Feynman. 376 00:35:34,741 --> 00:35:40,780 What's great is that just two doors along the corridor was the office of Murray Gell-Mann. 377 00:35:40,815 --> 00:35:45,779 There was an intense academic rivalry between these two giants, 378 00:35:45,814 --> 00:35:50,457 but they fed off the creativity. They were very different. 379 00:35:50,492 --> 00:35:55,064 Feynman played the buffoon, Gell-Mann the cultured elitist. 380 00:35:55,099 --> 00:36:01,137 Gell-Mann used to get upset by Feynman's loud voice. Feynman enjoyed winding him up. 381 00:36:01,172 --> 00:36:06,737 But during the 1960s and '70s, these two geniuses here at Caltech 382 00:36:06,772 --> 00:36:11,137 dominated the world of particle physics. 383 00:36:14,136 --> 00:36:20,175 Their bitter rivalry pushed them both to the very limits of their imaginations 384 00:36:20,210 --> 00:36:25,775 and Gell-Mann especially was desperate to prove himself over Feynman 385 00:36:25,810 --> 00:36:28,860 by bringing order to the particle zoo. 386 00:36:28,895 --> 00:36:33,333 Within the feverishly intellectual atmosphere of Caltech, 387 00:36:33,368 --> 00:36:36,938 Gell-Mann's mind did something very strange. 388 00:36:36,973 --> 00:36:41,031 He started working with a different kind of mathematics 389 00:36:41,066 --> 00:36:45,056 to deal with the preponderance of subatomic particles. 390 00:36:45,091 --> 00:36:51,710 He used an obscure form of maths called group theory. As its name suggests, this is a theory 391 00:36:51,745 --> 00:36:58,329 that analyses groups of numbers and symbols and tries to organise them into simple patterns. 392 00:36:58,364 --> 00:37:02,375 It's like working with an abstract form of origami. 393 00:37:02,410 --> 00:37:08,849 Using this technique, Gell-Mann started working all known particles into an organised system, 394 00:37:08,884 --> 00:37:13,652 which he called the Eightfold Way, after a Buddhist poem. 395 00:37:13,687 --> 00:37:20,806 But then he had his most awesome revelation. Gell-Mann realised that his group theory pointed 396 00:37:20,841 --> 00:37:27,926 to a deeper underlying mathematical truth, with the potential to rewrite the atomic rule book. 397 00:37:31,724 --> 00:37:38,644 What Gell-Mann's mathematics revealed to him was that in order to make coherent patterns 398 00:37:38,679 --> 00:37:42,248 of all the new particles in his Eightfold Way, 399 00:37:42,283 --> 00:37:46,889 he had to acknowledge a deeper, underlying, fundamental reality. 400 00:37:46,924 --> 00:37:53,442 Once again, it turned out that things were not at all as they seemed. 401 00:37:54,522 --> 00:38:01,162 Physicists had been comfortable with the notion that atoms have three different kinds of particles - 402 00:38:01,197 --> 00:38:07,166 electrons orbiting around the outside of a nucleus made up of proton and neutrons. 403 00:38:07,201 --> 00:38:13,239 Gell-Mann had the temerity to suggest that protons and neutrons were themselves composed 404 00:38:13,274 --> 00:38:17,998 of more elementary particles, particles that he called quarks. 405 00:38:24,117 --> 00:38:30,361 Murray Gell-Mann was cultured and arrogant, but at heart lacked confidence. 406 00:38:30,396 --> 00:38:36,356 He knew that for his colleagues, even those used to the strangeness of the atom, 407 00:38:36,391 --> 00:38:38,921 quarks were a step too far. 408 00:38:38,956 --> 00:38:44,675 And, in any case, there'd been no evidence of anything remotely like a quark. 409 00:38:44,710 --> 00:38:50,394 He was convinced his new theory would be declared outlandish or just wrong, 410 00:38:50,429 --> 00:38:52,839 so Gell-Mann sat on his revelation 411 00:38:52,874 --> 00:38:57,673 and one of the greatest ideas in science was almost lost forever. 412 00:39:00,112 --> 00:39:06,672 Then something extraordinary turned up, just a few hundred miles north of his office. 413 00:39:16,710 --> 00:39:21,515 This is the Stanford Linear Accelerator, south of San Francisco. 414 00:39:21,550 --> 00:39:26,389 What you can see is one end of what is basically a giant electron gun. 415 00:39:26,424 --> 00:39:31,594 A beam of high-energy electrons is fired through a tunnel 416 00:39:31,629 --> 00:39:38,387 that starts off over two miles away in the hills, travels under the freeway and comes out here 417 00:39:38,422 --> 00:39:43,986 where it enters the experimental area. The grey building is End Station A, 418 00:39:44,021 --> 00:39:48,243 where one of the most important discoveries in physics was made. 419 00:39:48,278 --> 00:39:52,465 It was built during the 1960s, when it was - and still is today - 420 00:39:52,500 --> 00:39:56,869 the longest single building on Earth. Although 40 years old, 421 00:39:56,904 --> 00:40:03,343 there's construction work going on, and it's still being used for fundamental research today. 422 00:40:08,742 --> 00:40:13,582 I'm now inside the two-mile-long linear accelerator building. 423 00:40:13,617 --> 00:40:17,588 The red objects on your right are called klystrons 424 00:40:17,623 --> 00:40:23,225 and they provide the power that boosts the electron beam 20 feet beneath us. 425 00:40:23,260 --> 00:40:29,299 Such is the acceleration, these electrons will, within the first few metres, have reached 426 00:40:29,334 --> 00:40:33,344 99% the speed of light. Let me put it another way. 427 00:40:33,379 --> 00:40:39,419 If these electrons were to start off their journey at the same time as you fire a bullet from a gun, 428 00:40:39,454 --> 00:40:46,138 they would have covered the full two-mile distance before the bullet has left the barrel. 429 00:40:47,417 --> 00:40:54,016 The electron beam now travelling at almost the speed of light would have arrived at the target area. 430 00:40:54,051 --> 00:41:00,021 There would have been, in 1968, where I'm standing now, a large tank of hydrogen - 431 00:41:00,056 --> 00:41:06,055 basically, protons. The electrons would smash into the protons and scatter off through tubes 432 00:41:06,090 --> 00:41:10,855 to be picked up by huge detectors that filled the hall outside. 433 00:41:20,652 --> 00:41:25,552 And as they did this, physicists got their biggest ever confirmation 434 00:41:25,587 --> 00:41:30,451 that there might be a deeper set of rules underpinning the particle zoo. 435 00:41:34,091 --> 00:41:40,130 What they had discovered from the way the electrons scattered with their extremely high energy 436 00:41:40,165 --> 00:41:44,534 was conclusive proof that protons had internal structure. 437 00:41:44,569 --> 00:41:49,769 In other words, protons were made of more elementary particles. 438 00:41:49,804 --> 00:41:52,528 Here were Gell-Mann's quarks. 439 00:41:53,527 --> 00:41:57,093 This was an astonishing moment. 440 00:41:57,128 --> 00:42:03,726 For decades, people were confident that the components of the atomic nucleus - the proton and neutron - 441 00:42:03,761 --> 00:42:06,331 were absolutely fundamental. 442 00:42:06,366 --> 00:42:11,165 And now, for the first time, there was evidence of something deeper. 443 00:42:17,044 --> 00:42:21,088 The quark is a tricky and elusive beast. 444 00:42:21,123 --> 00:42:25,563 There are six different kinds or flavours of quark - 445 00:42:25,598 --> 00:42:29,569 up, down, strange, charm, top and bottom. 446 00:42:29,604 --> 00:42:35,442 Also, quarks never exist in isolation, only in combination with other quarks. 447 00:42:35,477 --> 00:42:41,245 This makes them impossible to see directly. We can only infer their presence. 448 00:42:41,280 --> 00:42:47,280 Despite these caveats, the quark brought some semblance of order to the particle zoo. 449 00:42:47,315 --> 00:42:50,924 In recent years, it's allowed us to concoct 450 00:42:50,959 --> 00:42:55,360 a simple, yet powerful description of how the universe is built up. 451 00:42:55,395 --> 00:43:00,116 Basically, everything in the universe made of atoms 452 00:43:00,151 --> 00:43:04,838 is built up from just quarks and electrons. That's it. 453 00:43:04,873 --> 00:43:08,757 This now brings us pretty well up to date. 454 00:43:08,792 --> 00:43:11,881 The discovery of the quark in 1967 455 00:43:11,916 --> 00:43:18,680 was the last significant experimental discovery of a new type of fundamental particle. 456 00:43:18,715 --> 00:43:25,115 Some say we may yet discover the quark is made of something even stranger. And it's possible. 457 00:43:25,150 --> 00:43:27,955 But for now it's as good as it gets. 458 00:43:30,234 --> 00:43:36,314 Our journey from Einstein's proof of the existence of atoms in 1905 until now 459 00:43:36,349 --> 00:43:39,278 has been extraordinary. 460 00:43:39,313 --> 00:43:45,351 We've learnt so much about the atomic world, from the size and shape of the atom 461 00:43:45,386 --> 00:43:49,396 to how its centre holds the secret of the universe itself. 462 00:43:49,431 --> 00:43:55,352 From how it reveals an unknown world of antimatter to how empty space is far from empty. 463 00:43:58,111 --> 00:44:02,790 From what we thought was a basic building block of the universe 464 00:44:02,825 --> 00:44:07,470 to the discovery of something even more fundamental inside it. 465 00:44:09,708 --> 00:44:14,108 And yet, despite all the powerful science which we've uncovered, 466 00:44:14,143 --> 00:44:16,512 something doesn't quite add up. 467 00:44:16,547 --> 00:44:20,592 There are two startling and worrying anomalies. 468 00:44:20,627 --> 00:44:26,227 The first of these is now at the forefront of theoretical physics across the world 469 00:44:26,262 --> 00:44:30,145 and it concerns one of the oldest scientific principles there is. 470 00:44:30,180 --> 00:44:34,843 Gravity. It's been thoroughly understood since Einstein, 471 00:44:34,878 --> 00:44:39,471 but never really been part of atomic theory, until now. 472 00:44:39,506 --> 00:44:45,509 Suddenly there's a glimmer of hope from ideas that sound almost too crazy to be possible. 473 00:44:45,544 --> 00:44:53,742 Some of these are called string theories, that regard all subatomic particles as tiny vibrating strings 474 00:44:53,777 --> 00:44:58,426 that have higher dimensions of space trapped inside them. 475 00:44:58,461 --> 00:45:04,461 Some, called brain theories, suggest that our entire space and time 476 00:45:04,496 --> 00:45:08,465 is just a membrane floating through the multiverse. 477 00:45:08,500 --> 00:45:14,699 Another, called quantum loop gravity, suggests that nothing really exists at all 478 00:45:14,734 --> 00:45:20,899 and everything is ultimately made up of tiny loops in space and time themselves. 479 00:45:22,058 --> 00:45:26,463 But despite gravity's unwillingness to fit in with quantum theory, 480 00:45:26,498 --> 00:45:33,058 I believe there's something worse lurking in the quantum shadows, something truly nightmarish. 481 00:45:35,096 --> 00:45:39,501 Late into the night at physics conferences all over the world, 482 00:45:39,536 --> 00:45:46,775 over drinks at the bar when we huddle together to debate and discuss our strangest ideas, 483 00:45:46,810 --> 00:45:50,780 there are still things that really, really bother us. 484 00:45:50,815 --> 00:45:55,419 Chief among these are the quantum mechanical laws that atoms obey. 485 00:45:55,454 --> 00:46:00,858 In particular, one aspect of them. Something called the measurement problem. 486 00:46:00,893 --> 00:46:07,458 If you want to see fear in a quantum physicist's eyes, just say "the measurement problem". 487 00:46:07,493 --> 00:46:14,532 The measurement problem is this - an atom only appears in a particular place if you measure it. 488 00:46:14,567 --> 00:46:18,971 In other words, an atom is spread out all over the place 489 00:46:19,006 --> 00:46:23,015 until a conscious observer decides to look at it. 490 00:46:23,050 --> 00:46:29,168 So the act of measurement, or observation, creates the entire universe. 491 00:46:29,203 --> 00:46:31,854 Just to show how mad this idea is, 492 00:46:31,888 --> 00:46:38,209 I'm going to explain one of the most famous hypothetical experiments in the whole of science. 493 00:46:45,048 --> 00:46:48,651 It's called the Schrodinger's Cat Experiment. 494 00:46:48,686 --> 00:46:52,691 Erwin Schrodinger was a founding father of atomic theory. 495 00:46:52,726 --> 00:46:59,404 In the mid-1930s he devised a thought experiment to highlight the absurdity of quantum mechanics. 496 00:46:59,439 --> 00:47:06,244 He suggested you take a box in which you place an unopened container of cyanide, 497 00:47:06,279 --> 00:47:10,603 connected to a radiation detector and some radioactive material. 498 00:47:10,638 --> 00:47:14,608 If an atom in the material emits a particle, 499 00:47:14,643 --> 00:47:19,487 this is picked up by the detector, which releases the cyanide. 500 00:47:19,522 --> 00:47:27,161 Next you take Schrodinger's cat, which in this case is a lovely Norwegian forest cat called Dawkins. 501 00:47:27,196 --> 00:47:31,166 I should point out that this isn't real cyanide. 502 00:47:31,201 --> 00:47:36,720 You place the cat in the box, you close the lid...and wait. 503 00:47:36,755 --> 00:47:39,645 Here's the conundrum - 504 00:47:39,680 --> 00:47:45,719 according to traditional quantum mechanics, known as the Copenhagen Interpretation, 505 00:47:45,754 --> 00:47:50,763 all the time the box is closed, the radioactive atom inside 506 00:47:50,798 --> 00:47:56,802 has yet to make up its mind whether it has decayed and spat out a particle. 507 00:47:56,837 --> 00:48:03,201 So we have to describe it as having both decayed and not decayed at the same time. 508 00:48:03,236 --> 00:48:09,236 Think about what this means. Since the radioactive particle triggers the release of the poison, 509 00:48:09,271 --> 00:48:13,240 the cat is both poisoned AND not poisoned. 510 00:48:13,275 --> 00:48:19,279 So until we open the lid to check on the fate of the cat, what's called making a measurement, 511 00:48:19,314 --> 00:48:26,873 it's not just that we don't know, but that the cat is literally both dead and alive at the same time. 512 00:48:26,908 --> 00:48:29,513 This is clearly a paradox. 513 00:48:29,548 --> 00:48:31,512 Or is it? 514 00:48:35,912 --> 00:48:42,631 The paradox of Schrodinger's cat and the contradictory nature of the measurement problem 515 00:48:42,666 --> 00:48:49,350 really does force us to accept that tiny objects down at the atomic scale obey their own set 516 00:48:49,385 --> 00:48:51,873 of profoundly strange rules. 517 00:48:51,908 --> 00:48:57,909 But at larger scales, those of everyday experience, those rules vanish 518 00:48:57,944 --> 00:49:01,987 and an utterly new set of nice, intuitive rules take over. 519 00:49:02,022 --> 00:49:04,593 How can this be? 520 00:49:04,628 --> 00:49:08,666 Some argue that, in fact, the quantum weirdness of the atom 521 00:49:08,701 --> 00:49:12,630 might actually be writ large across the cosmos, 522 00:49:12,665 --> 00:49:17,225 that we may have to rethink everything known about the universe. 523 00:49:17,260 --> 00:49:21,743 Welcome to the many worlds interpretation of quantum mechanics 524 00:49:21,778 --> 00:49:26,189 and its chief adherent, David Deutsch, of Oxford University. 525 00:49:26,224 --> 00:49:31,269 Deutsch proposes that reality itself is profoundly misunderstood. 526 00:49:31,304 --> 00:49:37,343 He says that what quantum mechanics actually describes is not one universe, 527 00:49:37,378 --> 00:49:41,342 but an infinite number of parallel universes. 528 00:49:41,377 --> 00:49:43,786 He calls it a multiverse, 529 00:49:43,821 --> 00:49:50,220 in which every possible quantum mechanical outcome for each and every atom in the universe 530 00:49:50,255 --> 00:49:53,385 exists somewhere. 531 00:49:53,420 --> 00:49:58,259 So an atom and its electron are multiversal objects. 532 00:49:58,294 --> 00:50:00,585 And that multiversal object is 533 00:50:00,620 --> 00:50:03,383 what quantum mechanics is describing. 534 00:50:03,418 --> 00:50:10,417 Now that means that the parallel universe aspect of reality as described by quantum theory 535 00:50:10,452 --> 00:50:15,857 must apply to objects of all sizes - humans, stars, galaxies, everything. 536 00:50:15,892 --> 00:50:20,737 And that's why we call it the parallel universe theory 537 00:50:20,772 --> 00:50:25,073 rather than just parallel electrons theory. 538 00:50:25,108 --> 00:50:29,341 Because we are made of atoms. That's right. 539 00:50:29,376 --> 00:50:35,380 The same theory that says the atoms exist in more than one place 540 00:50:35,415 --> 00:50:40,453 says that we humans also exist in more than one place in different universes. 541 00:50:40,488 --> 00:50:45,018 And there are some universes where you and I don't exist at all. 542 00:50:45,053 --> 00:50:51,456 'The highly-respected author and physicist, Paul Davies, has an even more bizarre idea. 543 00:50:51,491 --> 00:50:59,211 'He suggests that the strangeness of the measurement problem explains how the universe came into being.' 544 00:51:00,212 --> 00:51:04,214 The experimenter today in the lab can make a measurement 545 00:51:04,249 --> 00:51:09,575 that affects the nature of reality as it was five billion years ago. 546 00:51:09,610 --> 00:51:16,248 There's a sort of feedback loop between the existence of living organisms and observers 547 00:51:16,283 --> 00:51:21,333 and the laws and conditions which have given rise to them. 548 00:51:21,368 --> 00:51:27,367 Otherwise it just seems a bit miraculous that the universe happens to have started out 549 00:51:27,402 --> 00:51:32,607 with the right laws and conditions that lead to observers like ourselves 550 00:51:32,642 --> 00:51:37,291 who can make measurements and make sense of it all. 551 00:51:37,326 --> 00:51:41,325 But quantum mechanics provides just such a feedback through time. 552 00:51:41,360 --> 00:51:45,330 It allows this backwards in time effect. Not causation. 553 00:51:45,365 --> 00:51:50,043 It's not that we here now can change the past to fix it so that we exist, 554 00:51:50,078 --> 00:51:54,723 but we have an influence on the past through quantum measurements we make. 555 00:51:54,758 --> 00:51:58,647 But there's a pragmatic side to the debate, too. 556 00:51:58,682 --> 00:52:04,763 Other scientists are worried that these bizarre and metaphysical speculations 557 00:52:04,798 --> 00:52:09,201 leave the world of measurement and laboratory experiment far behind. 558 00:52:10,240 --> 00:52:14,485 Professor Andrew Jackson of the Niels Bohr Institute in Copenhagen 559 00:52:14,520 --> 00:52:21,120 says that ultimately we shouldn't worry about the interpretation or the measurement problem or the cat. 560 00:52:21,155 --> 00:52:26,719 He says we shouldn't be concerned about the so-called true nature of reality. 561 00:52:26,754 --> 00:52:29,317 It's enough that the theory works. 562 00:52:31,159 --> 00:52:37,157 All of the things we can measure give us questions we can answer from quantum mechanics. 563 00:52:37,193 --> 00:52:39,641 So the quantum mechanics itself, 564 00:52:39,676 --> 00:52:45,677 without the need for interpretation, provides us with answers or predictions regarding the result 565 00:52:45,712 --> 00:52:48,241 of every experiment we can do. 566 00:52:48,276 --> 00:52:52,841 So I don't know... That's enough for me. Yeah. 567 00:52:52,875 --> 00:52:58,915 So you don't think quantum mechanics needs a unique interpretation because it doesn't add anything? 568 00:52:58,950 --> 00:53:05,479 It doesn't add anything and I don't think it will lead us to the next step. 569 00:53:05,514 --> 00:53:11,517 An interpretation doesn't change the results or the rules and that's why it's not testable. 570 00:53:11,552 --> 00:53:17,592 The whole purpose of the last 200 years in physics, this incredible leap forward that we've made, 571 00:53:17,627 --> 00:53:23,630 has come because experiment confronted theory and led to new theory when theory broke down 572 00:53:23,665 --> 00:53:28,670 and back and forth and back and forth. Interpretations don't do that. 573 00:53:28,705 --> 00:53:33,275 Interpretations only give us some kind of a way of believing 574 00:53:33,310 --> 00:53:40,148 we understand what quantum mechanics tells us, but that's a fixed point. There's no new content to it. 575 00:53:40,183 --> 00:53:45,672 Quantum mechanics is counterintuitive and goes against common sense. 576 00:53:45,707 --> 00:53:51,346 What do you say to people who insist on wanting to know what an atom is doing 577 00:53:51,381 --> 00:53:56,951 when you're not looking at it? I'm not sure I'm quoting, Feynman or Dirac, 578 00:53:56,986 --> 00:54:03,865 but the answer is, "Shut up and calculate". So shut up and use the maths. Right. 579 00:54:03,900 --> 00:54:08,144 "Shut up and calculate"? Is this really scientific pragmatism 580 00:54:08,179 --> 00:54:11,588 or just dogmatic fundamentalism? 581 00:54:11,623 --> 00:54:16,668 The reason that this is unacceptable, philosophically, 582 00:54:16,703 --> 00:54:22,702 can I think be best understood by comparing it with an earlier episode in the history of physics, 583 00:54:22,737 --> 00:54:27,507 namely the Inquisition's attitude several hundred years ago 584 00:54:27,542 --> 00:54:33,200 to the idea that the Earth goes round the Sun, not the Sun round the Earth. 585 00:54:33,235 --> 00:54:38,859 They wanted to promote a compromise with Galileo where they would admit 586 00:54:38,894 --> 00:54:43,260 that the positions of the stars and planets and Sun in the sky 587 00:54:43,295 --> 00:54:47,596 are exactly as predicted by that theory, 588 00:54:47,631 --> 00:54:51,864 but that it was presumptuous of humans 589 00:54:51,899 --> 00:54:56,737 to purport to be able to describe the underlying reality - 590 00:54:56,772 --> 00:55:00,022 why the stars appeared there. 591 00:55:00,057 --> 00:55:04,062 The same thing happened with quantum mechanics. 592 00:55:04,097 --> 00:55:09,500 A group of people who didn't like the implications of the theory about reality 593 00:55:09,535 --> 00:55:16,295 realised you could use it in practice by just using its predictions. That is a move you can always make 594 00:55:16,330 --> 00:55:20,980 with any scientific theory. You can always deny it describes reality. 595 00:55:21,015 --> 00:55:29,293 You can't be proved wrong by experiment, but as a philosophical position it's a dead end. Sterile. 596 00:55:33,331 --> 00:55:37,536 I think it's fair to say that most physicists use quantum mechanics 597 00:55:37,571 --> 00:55:43,611 to describe the subatomic world without worrying too much about the interpretation. 598 00:55:43,646 --> 00:55:47,014 Personally, I'm not in favour of this view. 599 00:55:47,050 --> 00:55:53,449 I don't have a preferred interpretation, but believe nature must behave in a particular way. 600 00:55:53,484 --> 00:55:57,453 So only one of the interpretations can be correct 601 00:55:57,488 --> 00:56:02,129 and, to be quite honest, we probably haven't found the final answer yet, 602 00:56:02,164 --> 00:56:05,573 but I think it's only a matter of time. 603 00:56:05,608 --> 00:56:09,928 I certainly don't subscribe to "Shut up and calculate". 604 00:56:09,963 --> 00:56:14,004 I prefer the "Shut up WHILE you calculate" view. 605 00:56:14,039 --> 00:56:18,010 I'm happy to do my calculations to study atoms, 606 00:56:18,045 --> 00:56:23,605 but when I'm away from my work, I still worry about what it all means. 607 00:56:35,123 --> 00:56:41,047 In the last 100 years we have peered deep inside the atom, 608 00:56:41,082 --> 00:56:48,841 the basic building block of the universe, and inside this tiny object we found a strange, new world 609 00:56:48,876 --> 00:56:54,480 governed by exotic laws that at times seemed to defy reason. 610 00:56:54,515 --> 00:56:58,925 Atoms present us with dizzying contradictions. 611 00:56:58,960 --> 00:57:05,398 They can behave both as particles or waves, they appear to be in more than one place, 612 00:57:05,433 --> 00:57:11,404 they force us to rethink what we mean by past and future, by cause and effect, 613 00:57:11,438 --> 00:57:17,237 and they tell us strange things about where the universe came from and where it's going. 614 00:57:17,272 --> 00:57:22,117 Pretty amazing stuff for something a millionth of a millimetre across. 615 00:57:22,152 --> 00:57:27,154 That's why Niels Bohr, the father of atomic physics, once said 616 00:57:27,189 --> 00:57:32,155 that when it comes to atoms language can only be used as poetry. 617 00:57:33,035 --> 00:57:39,675 What's fascinating to me is that although we've learnt an incredible amount about atoms, 618 00:57:39,710 --> 00:57:43,679 our scientific journey has only just begun. 619 00:57:43,714 --> 00:57:48,359 Although we know how a single atom or just a few atoms behave, 620 00:57:48,394 --> 00:57:54,393 the way trillions of them come together in concert to create the world around us 621 00:57:54,428 --> 00:57:58,997 is still largely a mystery. To give you one dramatic example - 622 00:57:59,032 --> 00:58:05,072 the atoms that make up my body are identical to the atoms in the rocks, the trees, the air, 623 00:58:05,107 --> 00:58:10,310 even the stars. And yet they come together to create a conscious being 624 00:58:10,345 --> 00:58:14,315 who can ask the question, "What is an atom?" 625 00:58:14,350 --> 00:58:19,548 Explaining all that is surely the next great challenge in science.