1 00:00:01,090 --> 00:00:03,090 In early spring of 1959, 2 00:00:03,090 --> 00:00:06,360 a young American physicist traveled to Copenhagen. 3 00:00:06,360 --> 00:00:08,630 He was just 28 years old. 4 00:00:08,630 --> 00:00:10,633 His name was Hugh Everett. 5 00:00:11,850 --> 00:00:13,590 He was to meet with Niels Bohr, 6 00:00:13,590 --> 00:00:16,560 one of the giants in the world of quantum mechanics. 7 00:00:16,560 --> 00:00:18,140 When Everett was sent to Bohr, 8 00:00:18,140 --> 00:00:20,940 he challenged every part of Bohr's understanding 9 00:00:20,940 --> 00:00:22,980 of quantum theory, his worldview. 10 00:00:22,980 --> 00:00:24,900 What Everett presented to Bohr 11 00:00:24,900 --> 00:00:27,710 was an extraordinary new theory that might be one 12 00:00:27,710 --> 00:00:32,230 of the most important scientific ideas of all time. 13 00:00:32,230 --> 00:00:34,220 Everett was trampling over every 14 00:00:34,220 --> 00:00:35,960 belief that Bohr held dear. 15 00:00:35,960 --> 00:00:37,750 Everett's theory came with strange 16 00:00:37,750 --> 00:00:39,700 and outlandish implications. 17 00:00:39,700 --> 00:00:42,990 It meant that there must be countless parallel universes 18 00:00:42,990 --> 00:00:44,970 and many worlds just like our own, 19 00:00:44,970 --> 00:00:48,020 populated by alternative versions of us. 20 00:00:48,020 --> 00:00:50,750 What's interesting is if quantum theory itself, 21 00:00:50,750 --> 00:00:55,208 taken realistically, tells us there are many worlds, 22 00:00:55,208 --> 00:00:57,610 that's why we take them seriously. 23 00:00:57,610 --> 00:01:01,120 To my view, the many-worlds, 24 00:01:01,120 --> 00:01:03,190 or the Everett interpretation, if you like, 25 00:01:03,190 --> 00:01:05,833 is a reductio ad absurdum. 26 00:01:06,880 --> 00:01:08,040 Bohr rejected the idea 27 00:01:08,040 --> 00:01:11,220 and a disillusioned Everett left academia for good. 28 00:01:11,220 --> 00:01:12,790 But it was a glimpse of a new way 29 00:01:12,790 --> 00:01:14,733 of making sense of quantum theory. 30 00:01:16,480 --> 00:01:18,462 How does a physicist ask what is a physical 31 00:01:18,462 --> 00:01:20,111 interpretation of this quantity, 32 00:01:20,111 --> 00:01:21,590 there is simple-- And the intellectual 33 00:01:21,590 --> 00:01:23,653 battle still rages today. 34 00:01:24,570 --> 00:01:26,680 I don't think that there are two worlds. 35 00:01:26,680 --> 00:01:31,100 I prefer to have a single world 36 00:01:31,100 --> 00:01:33,403 with a number of internal perspectives. 37 00:01:35,000 --> 00:01:37,020 But today there is a new theory emerging 38 00:01:37,020 --> 00:01:40,350 that many believe will finally vindicate Everett. 39 00:01:40,350 --> 00:01:43,050 These results are sensational, 40 00:01:43,050 --> 00:01:46,083 if you can buy the many worlds. 41 00:01:46,083 --> 00:01:48,666 (lively music) 42 00:01:57,364 --> 00:01:59,420 (contemplative music) 43 00:01:59,420 --> 00:02:02,180 Hugh Everett's idea that there are many worlds 44 00:02:02,180 --> 00:02:04,280 may have been dismissed by Niels Bohr, 45 00:02:04,280 --> 00:02:06,460 but it didn't go away entirely. 46 00:02:06,460 --> 00:02:09,210 In Oxford in the '90s, Professor Simon Saunders 47 00:02:09,210 --> 00:02:11,150 was plotting to bring Everett's work back 48 00:02:11,150 --> 00:02:13,620 into the mainstream of philosophy. 49 00:02:13,620 --> 00:02:17,060 The idea of many-worlds was now being taken seriously. 50 00:02:17,060 --> 00:02:22,060 My arrival in Oxford, I had this set of ideas, 51 00:02:22,880 --> 00:02:25,100 some of it written, some of it not, 52 00:02:25,100 --> 00:02:29,830 but what I didn't have were people who got it, 53 00:02:29,830 --> 00:02:32,830 who, as it where, could pick up those ideas and run with it. 54 00:02:33,810 --> 00:02:36,060 Simon is a philosopher and physicist. 55 00:02:36,060 --> 00:02:37,370 Together with Harvey Brown, 56 00:02:37,370 --> 00:02:39,220 they brought together a team of academics 57 00:02:39,220 --> 00:02:41,433 that is now called the Oxford group. 58 00:02:43,350 --> 00:02:46,303 I think there's a certain degree of serendipity. 59 00:02:47,410 --> 00:02:51,253 I think you have to have people who are creative, 60 00:02:52,800 --> 00:02:55,910 think outside the box. (people chattering) 61 00:02:55,910 --> 00:03:00,080 Simon belongs to this Oxford gang of philosophers 62 00:03:00,080 --> 00:03:03,597 who are all enamored by this idea of the many-world, 63 00:03:03,597 --> 00:03:08,320 and are preaching it around. (people chattering) 64 00:03:08,320 --> 00:03:10,130 It was one of the most happy 65 00:03:10,130 --> 00:03:12,020 and fulfilling moments in my life, 66 00:03:12,020 --> 00:03:16,790 to find young people, indeed, even undergraduates, 67 00:03:16,790 --> 00:03:20,023 responding with real talent. 68 00:03:20,860 --> 00:03:22,950 Above all, David Wallace. We know something 69 00:03:22,950 --> 00:03:25,430 about state-dependent solutions. 70 00:03:25,430 --> 00:03:27,890 Simon Saunders nearly single-handedly 71 00:03:27,890 --> 00:03:32,300 brought the Everett interpretation to the fore 72 00:03:32,300 --> 00:03:34,140 in philosophy of physics. 73 00:03:34,140 --> 00:03:37,190 So Simon's work here was both intellectually 74 00:03:37,190 --> 00:03:38,210 very important, and I think actually, 75 00:03:38,210 --> 00:03:40,580 personally very courageous, because that was a time 76 00:03:40,580 --> 00:03:44,180 where in philosophy, if not in physics, 77 00:03:44,180 --> 00:03:46,630 these ideas were not really taken very seriously, 78 00:03:46,630 --> 00:03:48,970 and there was a certain amount of risk, I think, 79 00:03:48,970 --> 00:03:51,140 in working in these areas. 80 00:03:51,140 --> 00:03:53,720 Undeterred, Simon and his fellow philosophers 81 00:03:53,720 --> 00:03:56,483 were brave enough to suggest the unimaginable. 82 00:03:57,640 --> 00:04:00,330 What does it mean to have many worlds? 83 00:04:00,330 --> 00:04:04,110 Many-worlds is difficult, it's fantastical. 84 00:04:04,110 --> 00:04:06,020 In the many-worlds context, you're supposed to be 85 00:04:06,020 --> 00:04:08,360 thinking, oh, here's what's going to happen, 86 00:04:08,360 --> 00:04:10,870 the world's gonna split into two copies. 87 00:04:10,870 --> 00:04:12,990 It does seem to have mind-boggling consequences 88 00:04:12,990 --> 00:04:16,210 that there are many individuals 89 00:04:16,210 --> 00:04:18,290 almost identical to oneself. 90 00:04:18,290 --> 00:04:19,640 There's definitely going to be one 91 00:04:19,640 --> 00:04:21,540 future copy of me that sees A, 92 00:04:21,540 --> 00:04:22,990 and there's definitely going to be one 93 00:04:22,990 --> 00:04:24,867 future copy of me that sees B. 94 00:04:24,867 --> 00:04:29,010 (shears snipping) (playful music) 95 00:04:29,010 --> 00:04:34,010 So quantum mechanics, it begins as a theory 96 00:04:34,070 --> 00:04:37,003 of particles, of atoms, of molecules. 97 00:04:37,900 --> 00:04:39,540 So what many-worlds is saying 98 00:04:39,540 --> 00:04:41,980 is that as Simon decides to prune the roses, 99 00:04:41,980 --> 00:04:43,820 so he splits into two. 100 00:04:43,820 --> 00:04:45,270 In parallel universes, 101 00:04:45,270 --> 00:04:47,470 one Simon goes indoors and reads a book, 102 00:04:47,470 --> 00:04:50,610 whilst the other Simon gets on with the gardening. 103 00:04:50,610 --> 00:04:54,040 So Everett had the basics, he had the basic idea. 104 00:04:54,040 --> 00:04:57,203 The system, very much like a tree, 105 00:04:58,430 --> 00:05:01,030 where innumerable ways of getting 106 00:05:01,030 --> 00:05:03,130 to different paths at the top of the tree. 107 00:05:04,990 --> 00:05:06,780 So we have the branching structure, 108 00:05:06,780 --> 00:05:09,100 where each branch is a world. 109 00:05:09,100 --> 00:05:10,610 On each branch, we have a different 110 00:05:10,610 --> 00:05:13,070 physical states of affairs, just like the one now, 111 00:05:13,070 --> 00:05:14,940 around us. (water hissing) 112 00:05:14,940 --> 00:05:17,220 This does all seem rather bizarre. 113 00:05:17,220 --> 00:05:19,170 We are familiar with the world around us, 114 00:05:19,170 --> 00:05:21,760 where everything behaves as we would expect. 115 00:05:21,760 --> 00:05:24,040 An apple falls from the tree to the ground 116 00:05:24,040 --> 00:05:26,100 according to classical laws. 117 00:05:26,100 --> 00:05:28,000 It all makes sense. 118 00:05:28,000 --> 00:05:31,743 But at the very small, we see classical laws break down. 119 00:05:32,800 --> 00:05:36,110 In the quantum world, particles behave in a weird way. 120 00:05:36,110 --> 00:05:38,750 They can be in different places at the same time, 121 00:05:38,750 --> 00:05:40,910 a cloud of possibilities. 122 00:05:40,910 --> 00:05:43,680 This is the strange world of quantum theory. 123 00:05:43,680 --> 00:05:47,250 Quantum theory has to count as the greatest, 124 00:05:47,250 --> 00:05:50,300 most revolutionary theory ever found in physics, 125 00:05:50,300 --> 00:05:52,290 and the most successful. 126 00:05:52,290 --> 00:05:53,730 Describing the strange, 127 00:05:53,730 --> 00:05:55,840 uncertain behavior of small particles 128 00:05:55,840 --> 00:05:57,293 first seemed impossible. 129 00:05:58,690 --> 00:06:01,970 But in December 1925, an Austrian physicist, 130 00:06:01,970 --> 00:06:04,840 Erwin Schrodinger, retreated to an isolated 131 00:06:04,840 --> 00:06:07,680 mountain cabin to consider the problem. 132 00:06:07,680 --> 00:06:09,500 His great insight was to treat 133 00:06:09,500 --> 00:06:12,880 the electron of a hydrogen atom not as a particle, 134 00:06:12,880 --> 00:06:15,050 but as a kind of wave. 135 00:06:15,050 --> 00:06:17,803 The equation he produced was the Schrodinger equation. 136 00:06:19,290 --> 00:06:21,780 It tells how the state of a quantum system 137 00:06:21,780 --> 00:06:25,640 changes with time, very much like how Newton's second law 138 00:06:25,640 --> 00:06:29,070 describes in the classical world the path of an object 139 00:06:29,070 --> 00:06:32,250 over time. (gentle music) 140 00:06:32,250 --> 00:06:34,940 These changes to a quantum system over time, 141 00:06:34,940 --> 00:06:37,740 he called the wave function. 142 00:06:37,740 --> 00:06:39,880 When we describe an object in quantum mechanics 143 00:06:39,880 --> 00:06:42,580 like an electron or a photon or something like that, 144 00:06:42,580 --> 00:06:45,050 we describe it by a wave function which is something 145 00:06:45,050 --> 00:06:47,420 that's really sort of spread out in space. 146 00:06:47,420 --> 00:06:49,257 It's rather like a field in space. 147 00:06:49,257 --> 00:06:51,597 "Think of the wave function," said Schrodinger, 148 00:06:51,597 --> 00:06:54,297 "as a catalog, a directory of all the future 149 00:06:54,297 --> 00:06:57,270 "possible outcomes for a quantum object." 150 00:06:57,270 --> 00:07:01,140 It was this equation that shaped all of Everett's thinking. 151 00:07:01,140 --> 00:07:04,130 Just take those quantum equations and run with those. 152 00:07:04,130 --> 00:07:06,570 Get everything out of those. 153 00:07:06,570 --> 00:07:09,210 That was Everett's intent. (traffic whizzing) 154 00:07:09,210 --> 00:07:11,310 And that is exactly what he did. 155 00:07:11,310 --> 00:07:14,387 With extraordinary daring, Everett said, "Quantum mechanics 156 00:07:14,387 --> 00:07:17,400 "should be seen as a universal theory." 157 00:07:17,400 --> 00:07:19,310 Just like Newton's law of gravity, 158 00:07:19,310 --> 00:07:20,970 Schrodinger's equation should be 159 00:07:20,970 --> 00:07:23,007 applied to the whole universe. 160 00:07:23,007 --> 00:07:24,777 "Do this," Everett realized, 161 00:07:24,777 --> 00:07:28,920 "and the mathematics perfectly described many-worlds." 162 00:07:28,920 --> 00:07:30,830 Everett didn't put the many-worlds into the theory, 163 00:07:30,830 --> 00:07:33,100 the many-worlds aren't an extra idea 164 00:07:33,100 --> 00:07:35,340 that we bring into quantum mechanics to save it. 165 00:07:35,340 --> 00:07:36,300 The many-worlds are what 166 00:07:36,300 --> 00:07:38,370 quantum mechanics was saying all along. 167 00:07:38,370 --> 00:07:40,477 Everett just said, "Take the theory seriously. 168 00:07:40,477 --> 00:07:41,830 "Take it literally." 169 00:07:41,830 --> 00:07:43,000 Everett would have been first 170 00:07:43,000 --> 00:07:45,160 introduced to Schrodinger's equation 171 00:07:45,160 --> 00:07:47,680 when he was an undergraduate at Princeton. 172 00:07:47,680 --> 00:07:51,890 It would've been demonstrated by the two-slit experiment. 173 00:07:51,890 --> 00:07:55,320 Every student of quantum mechanics comes across 174 00:07:55,320 --> 00:07:57,640 the two-slit experiment with the single photon. 175 00:07:57,640 --> 00:08:00,237 It is, as Richard Feynman memorably put it, 176 00:08:00,237 --> 00:08:02,760 "The central mystery of quantum mechanics." 177 00:08:02,760 --> 00:08:06,580 So what we're looking at here is the laser source. 178 00:08:06,580 --> 00:08:08,680 Now this is a laser that can be attenuated, 179 00:08:08,680 --> 00:08:11,210 turned down, to such a weak intensity 180 00:08:11,210 --> 00:08:13,430 that a single photon is passing 181 00:08:13,430 --> 00:08:15,000 through this apparatus at a time. 182 00:08:15,000 --> 00:08:17,090 We have the two slits. 183 00:08:17,090 --> 00:08:19,900 Iconic example of the strangeness of quantum mechanics 184 00:08:19,900 --> 00:08:23,870 is a particle that passes through two holes. 185 00:08:23,870 --> 00:08:26,730 The photon, the individual photon goes through one slit, 186 00:08:26,730 --> 00:08:29,470 some interaction here, arrives at the screen, 187 00:08:29,470 --> 00:08:31,230 at the photomultiplier. 188 00:08:31,230 --> 00:08:32,960 It's recorded as a blip. 189 00:08:32,960 --> 00:08:34,430 What would you expect to see, 190 00:08:34,430 --> 00:08:37,060 given only one particle at a time 191 00:08:37,060 --> 00:08:38,580 passing through the apparatus? 192 00:08:38,580 --> 00:08:40,420 Well, if the photons were behaving 193 00:08:40,420 --> 00:08:42,160 like, say, tennis balls, 194 00:08:42,160 --> 00:08:44,500 you would expect that the balls would hit the screen 195 00:08:44,500 --> 00:08:46,913 in just two places behind each slit. 196 00:08:47,950 --> 00:08:50,770 But when single photons are fired at the slits, 197 00:08:50,770 --> 00:08:52,700 something strange happens. 198 00:08:52,700 --> 00:08:54,750 The photons are hitting the screen in a place 199 00:08:54,750 --> 00:08:56,720 that would be impossible if they were traveling 200 00:08:56,720 --> 00:08:58,850 in a straight line through the two slits. 201 00:08:58,850 --> 00:09:02,090 So instead of two clear bands in line with the slits, 202 00:09:02,090 --> 00:09:04,600 the photons create a series of smudges. 203 00:09:04,600 --> 00:09:06,490 It looks as if each single photon 204 00:09:06,490 --> 00:09:09,380 somehow goes through both slits at the same time, 205 00:09:09,380 --> 00:09:11,690 and there is interference between the two 206 00:09:11,690 --> 00:09:13,113 as if it were a wave. 207 00:09:15,310 --> 00:09:18,730 We have to consider that it passes through both holes, 208 00:09:18,730 --> 00:09:22,155 because if we think that it passes through one, 209 00:09:22,155 --> 00:09:24,210 we don't see interference, 210 00:09:24,210 --> 00:09:26,770 we don't see fringes of interference. 211 00:09:26,770 --> 00:09:28,660 Everett was entirely familiar 212 00:09:28,660 --> 00:09:31,730 with this device, this apparatus. 213 00:09:31,730 --> 00:09:35,920 What he made of it is that if, at the very small, 214 00:09:35,920 --> 00:09:38,560 quantum mechanics tells us two contradictory 215 00:09:38,560 --> 00:09:41,210 states of affairs obtains at the same time, 216 00:09:41,210 --> 00:09:44,170 and that is boosted up to the macroscopic level, 217 00:09:44,170 --> 00:09:47,010 then big, as well as small, 218 00:09:47,010 --> 00:09:49,000 can be in two places at the same time. 219 00:09:49,000 --> 00:09:51,020 So we can now do two-slit experiment, 220 00:09:51,020 --> 00:09:54,280 for example, with organic molecules. 221 00:09:54,280 --> 00:09:57,420 So all of the evidence in favor of the microscopic 222 00:09:58,820 --> 00:10:00,990 validity of quantum mechanics seems to me 223 00:10:00,990 --> 00:10:04,290 to spread all the way up to the macroscopic level. 224 00:10:04,290 --> 00:10:06,710 If this sounds a very simple conclusion, 225 00:10:06,710 --> 00:10:08,440 then that is because simplicity 226 00:10:08,440 --> 00:10:11,170 is at the heart of Everett's many-worlds. 227 00:10:11,170 --> 00:10:13,310 Thinking in terms of simplicity, the key element 228 00:10:13,310 --> 00:10:16,530 is just to be very simple-minded about things. 229 00:10:16,530 --> 00:10:19,670 One has a rule, the Schrodinger equation, 230 00:10:19,670 --> 00:10:22,680 which tells you how a something, the wave function, 231 00:10:22,680 --> 00:10:24,053 evolves over time. 232 00:10:25,310 --> 00:10:27,257 The Everettian story is just to say, 233 00:10:27,257 --> 00:10:29,227 "Okay, let me just take that straightforwardly 234 00:10:29,227 --> 00:10:31,460 "as a description of the world." 235 00:10:31,460 --> 00:10:35,410 Things, even people, in two places at once. 236 00:10:35,410 --> 00:10:37,220 This is the central idea that nobody 237 00:10:37,220 --> 00:10:39,120 had understood prior to Everett. 238 00:10:39,120 --> 00:10:40,820 To suggest that quantum mechanics 239 00:10:40,820 --> 00:10:42,300 apply to both big and small 240 00:10:42,300 --> 00:10:45,060 was certainly a revolutionary idea. 241 00:10:45,060 --> 00:10:48,870 Before Everett, physicists had divided the world into two. 242 00:10:48,870 --> 00:10:51,210 The large, familiar classical world, 243 00:10:51,210 --> 00:10:53,647 and the small, weird, quantum world. 244 00:10:53,647 --> 00:10:55,490 And one of the greatest physicists 245 00:10:55,490 --> 00:10:57,970 of them all was Niels Bohr. 246 00:10:57,970 --> 00:10:59,870 When Everett was sent to Bohr, 247 00:10:59,870 --> 00:11:02,000 he challenged Bohr's views. 248 00:11:02,000 --> 00:11:04,840 He challenged every part of Bohr's understanding 249 00:11:04,840 --> 00:11:06,900 of quantum theory, his worldview. 250 00:11:06,900 --> 00:11:09,490 For Bohr, quantum theory 251 00:11:09,490 --> 00:11:11,550 only kicked in the microscopic level, 252 00:11:11,550 --> 00:11:13,700 and only in a well-defined way, 253 00:11:13,700 --> 00:11:16,653 given an explicit experimental context. 254 00:11:18,500 --> 00:11:19,650 Bohr's philosophy was called 255 00:11:19,650 --> 00:11:23,320 the Copenhagen interpretation of quantum mechanics. 256 00:11:23,320 --> 00:11:25,330 The interpretation was developed by Bohr 257 00:11:25,330 --> 00:11:29,490 with help from his colleague Werner Heisenberg in 1927. 258 00:11:29,490 --> 00:11:33,290 It was named after the institute he directed in Copenhagen. 259 00:11:33,290 --> 00:11:35,840 However, Bohr's theory may have divided the world 260 00:11:35,840 --> 00:11:38,370 into big classical things like tables and chairs, 261 00:11:38,370 --> 00:11:41,160 and small quantum particles such as atoms, 262 00:11:41,160 --> 00:11:43,730 but he also said something else. 263 00:11:43,730 --> 00:11:45,750 He said that when you look at a particle 264 00:11:45,750 --> 00:11:47,660 in its fuzzy, quantum world, 265 00:11:47,660 --> 00:11:50,050 it stops having an infinite number of different 266 00:11:50,050 --> 00:11:52,690 possible positions and immediately appears 267 00:11:52,690 --> 00:11:56,110 as a single particle in one particular place. 268 00:11:56,110 --> 00:11:57,830 So in the case of the two-slit experiment, 269 00:11:57,830 --> 00:12:00,650 for example, the electron goes through the slit 270 00:12:00,650 --> 00:12:02,290 or the photon goes through the double slit, 271 00:12:02,290 --> 00:12:04,910 it spreads out, but when we look at the screen, 272 00:12:04,910 --> 00:12:07,280 we always find the electron or the photon 273 00:12:07,280 --> 00:12:08,853 in one definite place. 274 00:12:10,990 --> 00:12:12,610 Bohr's Copenhagen interpretation 275 00:12:12,610 --> 00:12:15,920 provided an explanation for this strange behavior 276 00:12:15,920 --> 00:12:18,840 and it was all to do with the particle's wavefunction, 277 00:12:18,840 --> 00:12:21,453 its catalog of quantum possibilities. 278 00:12:22,570 --> 00:12:24,720 Bohr said that it was the act of measurement, 279 00:12:24,720 --> 00:12:26,110 simply looking at something, 280 00:12:26,110 --> 00:12:27,640 that collapsed the wavefunction, 281 00:12:27,640 --> 00:12:29,300 with all its many possibilities, 282 00:12:29,300 --> 00:12:32,010 into just the one single outcome, 283 00:12:32,010 --> 00:12:34,713 the reality around us that we actually see. 284 00:12:36,330 --> 00:12:39,660 But to Everett, this made no sense, why? 285 00:12:39,660 --> 00:12:40,820 For something to exist, 286 00:12:40,820 --> 00:12:43,450 does someone need to be looking at it? 287 00:12:43,450 --> 00:12:45,853 And Everett was not alone in this thought. 288 00:12:47,110 --> 00:12:49,607 So Einstein famously asked, 289 00:12:49,607 --> 00:12:51,680 "Is the moon there when nobody looks?" 290 00:12:51,680 --> 00:12:53,650 And of course, he meant it rhetorically. 291 00:12:53,650 --> 00:12:55,140 He was giving example of something 292 00:12:55,140 --> 00:13:00,140 where any fool would concede that the moon is simply there. 293 00:13:00,330 --> 00:13:01,530 I think that the moon is there. 294 00:13:01,530 --> 00:13:02,820 There's no doubt that the moon is there 295 00:13:02,820 --> 00:13:04,350 when we're not looking at it, right? 296 00:13:04,350 --> 00:13:08,800 We're not a special object in the universe. 297 00:13:08,800 --> 00:13:12,150 We're just natural creatures in a natural world. 298 00:13:12,150 --> 00:13:13,580 We're just a piece of the universe, 299 00:13:13,580 --> 00:13:15,480 a piece of physics like everything else. 300 00:13:15,480 --> 00:13:17,030 And like everything else, 301 00:13:17,030 --> 00:13:19,160 we interact with the rest of the world, 302 00:13:19,160 --> 00:13:24,160 so things get a reality for us when they interact with us. 303 00:13:26,444 --> 00:13:28,630 Schrodinger, in particular, 304 00:13:28,630 --> 00:13:31,560 Schrodinger did not agree with Bohr. 305 00:13:31,560 --> 00:13:34,070 The idea that on performing a measurement, 306 00:13:34,070 --> 00:13:36,739 something radical and dramatic 307 00:13:36,739 --> 00:13:41,739 and discontinuous and indeterministic takes place 308 00:13:44,420 --> 00:13:47,370 on measurement, he found absurd. 309 00:13:47,370 --> 00:13:49,295 What counts as a measurement? 310 00:13:49,295 --> 00:13:52,270 (upbeat music) 311 00:13:52,270 --> 00:13:55,300 To bring out this absurdity, 312 00:13:55,300 --> 00:13:59,380 he gave his experiment involving a cat. 313 00:13:59,380 --> 00:14:01,090 What Schrodinger did to the poor cat 314 00:14:01,090 --> 00:14:05,700 was use it to sort of make clear how crazy sounding 315 00:14:05,700 --> 00:14:07,860 the way people were talking about quantum mechanics is, 316 00:14:07,860 --> 00:14:11,100 because the cat is just a very vivid way 317 00:14:11,100 --> 00:14:12,778 of measuring a system. 318 00:14:12,778 --> 00:14:14,306 (cat meows) A cat is locked in a box. 319 00:14:14,306 --> 00:14:15,139 (cat purrs) In the box 320 00:14:15,139 --> 00:14:18,650 is a radioactive substance, a flask of poisonous gas, 321 00:14:18,650 --> 00:14:20,680 and hammer triggered by radiation 322 00:14:20,680 --> 00:14:22,650 from the decaying uranium. 323 00:14:22,650 --> 00:14:24,130 So Schrodinger said, "Right". 324 00:14:24,130 --> 00:14:25,207 You couldn't do this these days. 325 00:14:25,207 --> 00:14:28,687 "Wire up the possibly decayed atom 326 00:14:28,687 --> 00:14:30,247 "to some poor cat 327 00:14:30,247 --> 00:14:32,607 "and to some gadget that kills cats 328 00:14:32,607 --> 00:14:34,627 "and so if the atom decays, 329 00:14:34,627 --> 00:14:36,117 "the cat dies, (cat meows) 330 00:14:36,117 --> 00:14:38,420 "if the atom doesn't decay, the cat lives." 331 00:14:38,420 --> 00:14:41,020 (cat purrs) 332 00:14:41,020 --> 00:14:43,260 If you take Schrodinger's cat, 333 00:14:43,260 --> 00:14:46,170 that would be something where you initially have a cat 334 00:14:46,170 --> 00:14:48,440 and then you do an experiment 335 00:14:48,440 --> 00:14:52,240 which according to the evolution of Schrodinger's equation, 336 00:14:52,240 --> 00:14:55,840 would provide what we call a superposition 337 00:14:55,840 --> 00:14:58,440 of a cat being alive and dead at the same time 338 00:14:58,440 --> 00:15:01,680 and that's what the Schrodinger equation evolves to, 339 00:15:01,680 --> 00:15:03,450 but then you say, you do a measurement on the cat, 340 00:15:03,450 --> 00:15:04,700 or look at it, 341 00:15:04,700 --> 00:15:08,350 and what you see is either a dead cat or a live cat. 342 00:15:08,350 --> 00:15:09,910 The idea of superposition 343 00:15:09,910 --> 00:15:12,730 is at the heart of the Schrodinger equation. 344 00:15:12,730 --> 00:15:14,090 The wavefunciton, remember, 345 00:15:14,090 --> 00:15:17,360 describes each of the states of the superposition. 346 00:15:17,360 --> 00:15:19,730 It's what makes it like a wave equation. 347 00:15:19,730 --> 00:15:22,870 States superpose rather in the way that waves of water 348 00:15:22,870 --> 00:15:24,383 can be added to one another. 349 00:15:25,320 --> 00:15:27,810 Now, quite why looking in the box suddenly makes 350 00:15:27,810 --> 00:15:30,010 just one of the states of the superposition, 351 00:15:30,010 --> 00:15:31,670 say, a live cat, appear, 352 00:15:31,670 --> 00:15:32,990 whilst another, the dead cat, 353 00:15:32,990 --> 00:15:34,280 just blinks out of existence, 354 00:15:34,280 --> 00:15:36,210 even Bohr could not explain. 355 00:15:36,210 --> 00:15:38,110 It did seem to be all rather mystical. 356 00:15:40,070 --> 00:15:43,664 Neils Bohr and his Copenhagen interpretation 357 00:15:43,664 --> 00:15:45,880 sort of admitted it was a cheat in a sense 358 00:15:45,880 --> 00:15:49,097 by saying, "You've got to treat the measuring apparatus 359 00:15:49,097 --> 00:15:50,837 "as a classical device, 360 00:15:50,837 --> 00:15:53,460 "so you don't treat it according to quantum mechanics." 361 00:15:53,460 --> 00:15:55,820 If you did treat it according to quantum mechanics, 362 00:15:55,820 --> 00:15:57,820 then the measurement would say 363 00:15:58,660 --> 00:16:01,693 cat dead and cat alive at the same time, 364 00:16:02,760 --> 00:16:04,830 and the thing is that the Schrodinger equation 365 00:16:04,830 --> 00:16:07,960 preserves the superposition forever. 366 00:16:07,960 --> 00:16:08,960 And so to Everett, 367 00:16:08,960 --> 00:16:11,470 it was clear that there was no need to cheat. 368 00:16:11,470 --> 00:16:13,757 He said "Just treat the meausrment apparatus 369 00:16:13,757 --> 00:16:16,240 "as part of the quantum system." 370 00:16:16,240 --> 00:16:19,320 This was Everett's big central idea. 371 00:16:19,320 --> 00:16:22,760 Simply let the Schrodinger equation apply to everything, 372 00:16:22,760 --> 00:16:26,820 the whole universe, including the experimental apparatus. 373 00:16:26,820 --> 00:16:29,600 For Everett, quantum theory was to be taken 374 00:16:29,600 --> 00:16:32,029 as a universal theory, a fundamental theory 375 00:16:32,029 --> 00:16:35,020 and applies to everything, big and small, 376 00:16:35,020 --> 00:16:37,550 so apply it to the experimental apparatus as well, 377 00:16:37,550 --> 00:16:40,080 apply it to the experimentalist as well. 378 00:16:40,080 --> 00:16:42,440 These ideas were anathema to Bohr. 379 00:16:42,440 --> 00:16:43,670 To many physicists, 380 00:16:43,670 --> 00:16:47,110 Everett's ideas were also profoundly disturbing. 381 00:16:47,110 --> 00:16:49,310 If the wavefunction didn't really collapse, 382 00:16:49,310 --> 00:16:51,680 then this could only mean one thing. 383 00:16:51,680 --> 00:16:53,860 Every possible state of the superposition 384 00:16:53,860 --> 00:16:56,410 of the cat would be preserved. 385 00:16:56,410 --> 00:16:58,410 The Everett interpretation had arrived 386 00:16:58,410 --> 00:17:01,700 at the fantastical idea of many worlds, 387 00:17:01,700 --> 00:17:05,693 two cats, each cat in its own separate parallel world. 388 00:17:07,050 --> 00:17:11,600 And imagine people, also involved in this experiment, 389 00:17:11,600 --> 00:17:14,460 we have the superposition of people seeing the cat alive, 390 00:17:14,460 --> 00:17:17,580 the superposition with people seeing the cat dead. 391 00:17:17,580 --> 00:17:19,690 So instead of just being cat alive 392 00:17:19,690 --> 00:17:20,560 and dead at the same time, 393 00:17:20,560 --> 00:17:23,290 there's now cat alive, David sees living cat, 394 00:17:23,290 --> 00:17:27,280 and cat dead, David sees dead cat at the same time. 395 00:17:27,280 --> 00:17:30,363 It's the inevitable consequence of the equations. 396 00:17:31,460 --> 00:17:33,330 Everett was prepared to run with that. 397 00:17:33,330 --> 00:17:37,662 Nobody else before him had the courage to suggest this. 398 00:17:37,662 --> 00:17:40,640 But the measurement paradox afflicts the recovery 399 00:17:40,640 --> 00:17:42,207 of anything like the world that we see 400 00:17:42,207 --> 00:17:43,757 and the mathematical formulism. 401 00:17:44,640 --> 00:17:46,480 But for Simon and the Oxford group, 402 00:17:46,480 --> 00:17:48,367 their support of the many-worlds interpretation 403 00:17:48,367 --> 00:17:51,700 means facing the criticism of leading physicists 404 00:17:51,700 --> 00:17:54,670 who simply do not buy Everett's ideas. 405 00:17:54,670 --> 00:17:55,620 For Roger Penrose, 406 00:17:55,620 --> 00:17:57,590 the problem with the Everett interpretation 407 00:17:57,590 --> 00:17:59,580 is Schrodinger's equation itself. 408 00:17:59,580 --> 00:18:02,700 If you believe that the Schrodinger equation, 409 00:18:02,700 --> 00:18:04,899 or unitary evolution as it's sometimes 410 00:18:04,899 --> 00:18:07,070 more correctly called if you like, 411 00:18:07,070 --> 00:18:10,970 that unitary evolution is an exact law of nature, 412 00:18:10,970 --> 00:18:13,470 then you are lead to universes 413 00:18:13,470 --> 00:18:14,410 in which all of these 414 00:18:14,410 --> 00:18:17,563 different alternatives somehow coexist. 415 00:18:18,510 --> 00:18:21,420 Now the question is why don't we see that? 416 00:18:21,420 --> 00:18:22,810 And this is where the problem 417 00:18:22,810 --> 00:18:26,230 for this Everett-type interpretation comes in 418 00:18:26,230 --> 00:18:28,095 because we don't see that kind of a world. 419 00:18:28,095 --> 00:18:29,340 (people chatting) 420 00:18:29,340 --> 00:18:34,340 Simon is following the steps of Schrodinger 421 00:18:35,740 --> 00:18:39,793 and so what exists is this wavefunction. 422 00:18:41,540 --> 00:18:43,140 This is a realistic position. 423 00:18:43,140 --> 00:18:44,830 That's the thing that is out there. 424 00:18:44,830 --> 00:18:47,700 It creates a number of problems 425 00:18:47,700 --> 00:18:50,230 like how do you go from this wavefunction 426 00:18:50,230 --> 00:18:52,660 to the actual things of the world we see, 427 00:18:52,660 --> 00:18:55,373 which are chairs, shoes, trousers, 428 00:18:55,373 --> 00:18:58,110 bee, air, particles, stuff. 429 00:18:58,110 --> 00:19:01,640 To my view, the many-worlds, 430 00:19:01,640 --> 00:19:03,690 or the Everett interpretation if you like, 431 00:19:03,690 --> 00:19:06,600 is a reducto ad absurdum. 432 00:19:06,600 --> 00:19:09,910 It's telling you what, if you simply believe 433 00:19:09,910 --> 00:19:11,190 that the Schrodinger equation, 434 00:19:11,190 --> 00:19:15,100 or unitary evolution, is exactly true of the world, 435 00:19:15,100 --> 00:19:16,600 you get nonsense. 436 00:19:16,600 --> 00:19:20,440 When we use a metaphor, we are used to the fact 437 00:19:20,440 --> 00:19:25,170 that each of us has a different perspective on the world. 438 00:19:25,170 --> 00:19:29,860 I see this room from a different perspective. 439 00:19:29,860 --> 00:19:32,150 I see a chair from here. 440 00:19:32,150 --> 00:19:34,440 You see a chair from there. 441 00:19:34,440 --> 00:19:37,040 Another person see a chair from there, 442 00:19:37,040 --> 00:19:39,220 so the complexity from reality 443 00:19:39,220 --> 00:19:42,830 can be seen as an intersection of perspective 444 00:19:42,830 --> 00:19:45,320 that match or don't match. 445 00:19:45,320 --> 00:19:47,400 Simon want to break it up 446 00:19:47,400 --> 00:19:50,380 into a multiplicity of worlds. 447 00:19:50,380 --> 00:19:54,800 I prefer to have a single world 448 00:19:54,800 --> 00:19:57,073 with a number of internal perspectives. 449 00:19:58,055 --> 00:19:58,990 'Cause Carlo is not prepared 450 00:19:58,990 --> 00:20:01,373 to apply quantum theory to everything. 451 00:20:02,400 --> 00:20:03,864 I've talked about big and small. 452 00:20:03,864 --> 00:20:06,690 Even more important is that it's everything. 453 00:20:06,690 --> 00:20:09,760 For Carlo, there's always the observer 454 00:20:09,760 --> 00:20:12,009 who is outside the system. 455 00:20:12,009 --> 00:20:14,600 (upbeat music) 456 00:20:14,600 --> 00:20:15,700 That's the difference. 457 00:20:16,768 --> 00:20:19,101 (sad music) 458 00:20:20,630 --> 00:20:22,076 Many-worlds is difficult. 459 00:20:22,076 --> 00:20:26,513 It's fantastical and in a way, 460 00:20:27,950 --> 00:20:32,723 there's concern that the ideas can't be taken seriously. 461 00:20:34,530 --> 00:20:37,580 This has been extremely difficult for me 462 00:20:37,580 --> 00:20:39,830 in those early days. 463 00:20:39,830 --> 00:20:42,100 The trouble was Simon knew all too well 464 00:20:42,100 --> 00:20:45,230 that Everett's theory was not without its problems. 465 00:20:45,230 --> 00:20:47,040 Everett may have described the development 466 00:20:47,040 --> 00:20:49,050 of the superposition of worlds, 467 00:20:49,050 --> 00:20:51,970 but he could only do so through experiment. 468 00:20:51,970 --> 00:20:53,250 But there was the problem. 469 00:20:53,250 --> 00:20:56,040 The only way that he had of getting out 470 00:20:56,040 --> 00:20:58,290 these macro superpositions 471 00:20:58,290 --> 00:21:01,503 was in the context of an experiment. 472 00:21:02,470 --> 00:21:04,280 It seemed it was no different from Bohr. 473 00:21:04,280 --> 00:21:05,560 You needed the experiment. 474 00:21:05,560 --> 00:21:07,230 You needed actually the dynamics 475 00:21:07,230 --> 00:21:09,450 to be cooked up for the experiment 476 00:21:09,450 --> 00:21:12,030 to give you the superposition of worlds. 477 00:21:12,030 --> 00:21:14,000 That leads you to the many-worlds theory, which is my. 478 00:21:14,000 --> 00:21:16,050 Everett's insight, it didn't really take 479 00:21:16,050 --> 00:21:17,640 measurement itself out of the story, 480 00:21:17,640 --> 00:21:20,760 so it didn't fully fulfill Einstein's hope 481 00:21:20,760 --> 00:21:22,430 of a physics with no reference 482 00:21:22,430 --> 00:21:25,060 to measurement or observation. 483 00:21:25,060 --> 00:21:26,250 I mean, what about a world, 484 00:21:26,250 --> 00:21:29,230 a universe without any people, without any experiments? 485 00:21:29,230 --> 00:21:32,570 Would there not be many-worlds in that universe? 486 00:21:32,570 --> 00:21:34,560 Is it just somehow with human beings 487 00:21:34,560 --> 00:21:36,593 that we create these many worlds? 488 00:21:37,720 --> 00:21:42,720 This made nonsense of the theory for many people. 489 00:21:43,040 --> 00:21:44,910 To try to reformulate quantum mechanics 490 00:21:44,910 --> 00:21:48,480 to avoid troublesome mention of measurement 491 00:21:48,480 --> 00:21:50,470 or observation. But Simon had arrived 492 00:21:50,470 --> 00:21:52,010 at Oxford with a set of ideas 493 00:21:52,010 --> 00:21:54,000 that he believed would solve this problem 494 00:21:54,000 --> 00:21:55,700 and vindicate Everett. 495 00:21:55,700 --> 00:21:59,240 The 1980s had seen the emergence of a new mathematical tool 496 00:21:59,240 --> 00:22:01,110 not available to Everett. 497 00:22:01,110 --> 00:22:03,780 It was called decoherence theory. 498 00:22:03,780 --> 00:22:06,090 The Oxford group believe that decoherence theory 499 00:22:06,090 --> 00:22:08,461 could describe how the branching of worlds occurs 500 00:22:08,461 --> 00:22:11,820 regardless of whether measurements are performed 501 00:22:11,820 --> 00:22:14,500 and why it is that the world in which we exist 502 00:22:14,500 --> 00:22:17,200 never recombines with all the other branching worlds 503 00:22:18,160 --> 00:22:21,720 and why each world is oblivious to all others. 504 00:22:21,720 --> 00:22:23,980 What decoherence theory taught us 505 00:22:23,980 --> 00:22:27,120 was that this constant branching 506 00:22:27,120 --> 00:22:28,960 is something that's happening naturally, 507 00:22:28,960 --> 00:22:33,000 just as a result of physical processes all the time. 508 00:22:33,000 --> 00:22:34,220 The University of Oxford, 509 00:22:34,220 --> 00:22:36,500 today at the forefront of quantum computing, 510 00:22:36,500 --> 00:22:39,110 is no stranger to decoherence theory. 511 00:22:39,110 --> 00:22:41,090 Quantum computers work by maintaining 512 00:22:41,090 --> 00:22:44,910 the quantum state of a system in a superposition of states, 513 00:22:44,910 --> 00:22:48,330 each computation delicately interacting with each other. 514 00:22:48,330 --> 00:22:50,170 If there's any contact between the system 515 00:22:50,170 --> 00:22:51,890 and the surrounding environment, 516 00:22:51,890 --> 00:22:53,710 then just as the ripples from a stone 517 00:22:53,710 --> 00:22:56,000 dropped in a lake disappear into the environment, 518 00:22:56,000 --> 00:22:57,860 so the interference effects, 519 00:22:57,860 --> 00:23:00,890 so typical of the quantum, are washed out. 520 00:23:00,890 --> 00:23:03,420 This is decoherence, and when it happens, 521 00:23:03,420 --> 00:23:05,323 all quantum behavior is lost. 522 00:23:07,350 --> 00:23:10,620 I took over these methods developed by others, 523 00:23:10,620 --> 00:23:13,460 decoherence theory and very specifically, 524 00:23:13,460 --> 00:23:15,740 the decoherent histories formulism 525 00:23:15,740 --> 00:23:17,470 because that is the one that really 526 00:23:17,470 --> 00:23:20,980 makes best sense of Everett's ideas 527 00:23:20,980 --> 00:23:24,670 and I shared how the standard problems 528 00:23:24,670 --> 00:23:27,700 that beset the theory could be solved. 529 00:23:27,700 --> 00:23:29,720 Decoherence theory turns out to be the answer 530 00:23:29,720 --> 00:23:33,360 of why it is that one actually gets 531 00:23:33,360 --> 00:23:36,640 a plurality of macroscopic determinate worlds 532 00:23:36,640 --> 00:23:40,330 out of a fundamentally, if you will, 533 00:23:40,330 --> 00:23:43,810 formless, evolving wavefunction. 534 00:23:43,810 --> 00:23:45,940 The whole language of worlds and branches 535 00:23:45,940 --> 00:23:48,430 becomes okay when those interference effects 536 00:23:48,430 --> 00:23:49,723 get washed out 537 00:23:49,723 --> 00:23:51,190 and that's what decoherence does. 538 00:23:51,190 --> 00:23:53,100 First and foremost, you no longer 539 00:23:53,100 --> 00:23:55,040 have to make reference to experiments. 540 00:23:55,040 --> 00:23:56,930 That was the most important of all. 541 00:23:56,930 --> 00:24:01,930 These ideas work for any kind of macroscopic process. 542 00:24:02,450 --> 00:24:04,800 There doesn't have to be a measurement process. 543 00:24:06,750 --> 00:24:09,860 In fact, all it needs is a universe itself. 544 00:24:09,860 --> 00:24:12,138 It is the leaking of the quantum interference effects 545 00:24:12,138 --> 00:24:14,620 into the environment that makes it appear 546 00:24:14,620 --> 00:24:16,563 there is wavefunction collapse. 547 00:24:17,830 --> 00:24:21,260 Interference, which is the quantum phenomenon 548 00:24:21,260 --> 00:24:23,280 that leads to all the paradoxical results 549 00:24:23,280 --> 00:24:25,910 of the two-slit experiments and the like in quantum theory, 550 00:24:25,910 --> 00:24:28,010 interference is a delicate flower 551 00:24:28,010 --> 00:24:30,330 and when you're trying to create 552 00:24:30,330 --> 00:24:34,630 quantum mechanical phenomena on large scales, 553 00:24:34,630 --> 00:24:36,615 then very quickly that interference disappears, 554 00:24:36,615 --> 00:24:38,240 and that disappearance of interference 555 00:24:38,240 --> 00:24:40,260 is what we mean when we say that the worlds, 556 00:24:40,260 --> 00:24:42,320 the branches in the quantum wavefunction 557 00:24:42,320 --> 00:24:44,020 stop interacting with each other. 558 00:24:44,020 --> 00:24:45,710 When decoherence happens, 559 00:24:45,710 --> 00:24:47,710 that's when somebody in one branch 560 00:24:47,710 --> 00:24:51,150 can no longer interact with the goings on in another branch. 561 00:24:51,150 --> 00:24:52,717 It all sounds rather fantastical 562 00:24:52,717 --> 00:24:55,830 that we are surrounded by numerous other universes 563 00:24:55,830 --> 00:24:57,740 that don't interfere with each other 564 00:24:57,740 --> 00:25:00,970 and that we can only see the one we ourselves are in, 565 00:25:00,970 --> 00:25:03,721 but is it really so strange? 566 00:25:03,721 --> 00:25:06,120 ♪ Close down, make decisions now ♪ 567 00:25:06,120 --> 00:25:07,530 The collapse of the wavefunction 568 00:25:07,530 --> 00:25:10,810 is this idea of signals in radio waves 569 00:25:10,810 --> 00:25:12,210 being made at the same time. 570 00:25:13,860 --> 00:25:15,920 Each of them is oblivious to all of the others 571 00:25:15,920 --> 00:25:16,933 that are being made, 572 00:25:18,140 --> 00:25:22,783 but we think, hang on, there can only be one signal, 573 00:25:24,320 --> 00:25:27,115 so all of the others somehow have to go away. 574 00:25:27,115 --> 00:25:28,520 ♪ On the gulf stream ♪ 575 00:25:28,520 --> 00:25:31,060 Why do the others have to go away? 576 00:25:31,060 --> 00:25:33,580 If they don't interfere with one another, 577 00:25:33,580 --> 00:25:36,233 why couldn't they all be present at the same time? 578 00:25:37,750 --> 00:25:39,140 And of course, they are. 579 00:25:39,140 --> 00:25:41,440 We tune into different radio stations, 580 00:25:41,440 --> 00:25:43,440 we only hear one at a time. 581 00:25:43,440 --> 00:25:47,060 Each frequency has decohered from all the others. 582 00:25:47,060 --> 00:25:49,950 Just as we are surrounded by different radio channels, 583 00:25:49,950 --> 00:25:53,250 so we are surrounded by entire universes, 584 00:25:53,250 --> 00:25:55,900 all possible versions of the superposition 585 00:25:55,900 --> 00:25:58,377 all independent from each other. 586 00:25:58,377 --> 00:25:59,510 (radio crackling) 587 00:25:59,510 --> 00:26:01,670 The process of decohrence provided 588 00:26:02,580 --> 00:26:06,020 the basis of a unique way of looking 589 00:26:06,020 --> 00:26:08,020 at the superposition principle. 590 00:26:08,020 --> 00:26:12,150 In other words it provided a unique account 591 00:26:12,150 --> 00:26:14,820 of how these branches were described with in the theory. 592 00:26:14,820 --> 00:26:16,600 If decoherence has happened, 593 00:26:16,600 --> 00:26:18,260 then that's when the many-worlds interpretation 594 00:26:18,260 --> 00:26:20,020 will say branching has occurred 595 00:26:20,020 --> 00:26:21,610 and then its description of the situation 596 00:26:21,610 --> 00:26:23,860 will be no, there are really two cats here, 597 00:26:23,860 --> 00:26:25,160 one on one branch of the universe 598 00:26:25,160 --> 00:26:26,367 and one on the other branch of the universe. 599 00:26:26,367 --> 00:26:28,060 One of them's alive, one of them's dead. 600 00:26:28,060 --> 00:26:30,060 It's all quite classical. 601 00:26:30,060 --> 00:26:32,560 And it's how this classical world emerges 602 00:26:32,560 --> 00:26:34,114 out of the wavefunction, 603 00:26:34,114 --> 00:26:38,050 where decoherence finally answers Everett's critics. 604 00:26:38,050 --> 00:26:39,440 Just like radio stations, 605 00:26:39,440 --> 00:26:41,480 decoherence theory has a unique way 606 00:26:41,480 --> 00:26:44,190 of showing the superposition of worlds. 607 00:26:44,190 --> 00:26:45,900 The Schrodinger equation dictates 608 00:26:45,900 --> 00:26:49,000 how the sequence of superpositions unfold, 609 00:26:49,000 --> 00:26:51,940 but track an individual term in the superposition 610 00:26:51,940 --> 00:26:53,710 from one time to the next, 611 00:26:53,710 --> 00:26:56,780 and what emerges is a classical world. 612 00:26:56,780 --> 00:26:58,300 Do the same for each term 613 00:26:58,300 --> 00:27:01,950 and you have, like radio stations, many parallel worlds, 614 00:27:01,950 --> 00:27:05,150 each obeying approximately classical equations, 615 00:27:05,150 --> 00:27:08,060 each story slightly different from each other, 616 00:27:08,060 --> 00:27:09,950 and it's not just Newtonian laws 617 00:27:09,950 --> 00:27:10,910 that reveal themselves, 618 00:27:10,910 --> 00:27:13,350 but also equations for gases, for fluids, 619 00:27:13,350 --> 00:27:15,740 equations for crystals, for chemical reactions, 620 00:27:15,740 --> 00:27:17,590 for air particles, for materials 621 00:27:17,590 --> 00:27:20,690 like chairs, tables and books are made of 622 00:27:20,690 --> 00:27:22,360 and even cats. (cat meows) 623 00:27:22,360 --> 00:27:24,630 Decoherence theory gave us 624 00:27:26,670 --> 00:27:30,200 macroscopic superpositions without measurements. 625 00:27:30,200 --> 00:27:31,880 They're happening all the time. 626 00:27:31,880 --> 00:27:36,330 It gave us the full vivdness and richness and variety 627 00:27:36,330 --> 00:27:37,820 of macroscopic world 628 00:27:37,820 --> 00:27:39,823 and matter in all its forms. 629 00:27:40,960 --> 00:27:42,840 It took those branches 630 00:27:42,840 --> 00:27:46,600 and it showed why branching structure is there, 631 00:27:46,600 --> 00:27:49,316 why you don't get recombination of branches. 632 00:27:49,316 --> 00:27:50,340 (woman laughing) 633 00:27:50,340 --> 00:27:52,540 The contribution of Simon and his colleagues 634 00:27:52,540 --> 00:27:55,540 to the idea of many-worlds is nearly done. 635 00:27:55,540 --> 00:27:58,970 Bohr relied on the idea of classical experiments, 636 00:27:58,970 --> 00:28:01,970 Everett replaced them by quantum experiments, 637 00:28:01,970 --> 00:28:03,180 the Oxford group got rid 638 00:28:03,180 --> 00:28:06,170 of the dependence on experiments altogether, 639 00:28:06,170 --> 00:28:09,290 and they showed how the classical reality of our own world 640 00:28:09,290 --> 00:28:11,687 arises out of the wavefunction. 641 00:28:12,590 --> 00:28:15,260 Whilst the debate about many-worlds will continue on, 642 00:28:15,260 --> 00:28:17,560 the Oxford group is now breaking up. 643 00:28:17,560 --> 00:28:20,600 After two decades, they're now going their separate ways, 644 00:28:20,600 --> 00:28:22,520 some to new posts in America, 645 00:28:22,520 --> 00:28:24,290 others to retirement, 646 00:28:24,290 --> 00:28:26,913 but their legacy is clear. 647 00:28:26,913 --> 00:28:28,146 Cheers. 648 00:28:28,146 --> 00:28:30,392 (upbeat music) 649 00:28:30,392 --> 00:28:32,150 So you basically only need 650 00:28:32,150 --> 00:28:34,000 things like the principle of superposition 651 00:28:34,000 --> 00:28:36,630 and the basic mechanisms of decoherence 652 00:28:36,630 --> 00:28:39,070 for the many-worlds story to emerge. 653 00:28:39,070 --> 00:28:41,300 These results are sensational 654 00:28:41,300 --> 00:28:44,450 from the point of view of realism, 655 00:28:44,450 --> 00:28:46,913 if you can buy the many-worlds. 656 00:29:10,259 --> 00:29:12,970 (audience applauding) 657 00:29:12,970 --> 00:29:15,070 What I believe is that the only way 658 00:29:15,070 --> 00:29:18,240 to interpret quantum mechanics realistically 659 00:29:18,240 --> 00:29:20,150 is in terms of parallel universes. 660 00:29:20,150 --> 00:29:21,220 There are parallel universes 661 00:29:21,220 --> 00:29:23,470 in so far as quantum mechanics is true 662 00:29:23,470 --> 00:29:25,770 and I don't know if quantum mechanics is true.