1 00:00:13,330 --> 00:00:21,969 This is the 12th and final Danny Schama lecture series. 2 00:00:21,970 --> 00:00:34,870 And first, let me thank the Sharma family, also college and physics department for having sponsored this over the last few years. 3 00:00:35,260 --> 00:00:41,920 I thought I'd just read to you the names of the amazingly illustrious speakers have had in the past. 4 00:00:44,140 --> 00:00:47,680 Roger Penrose. George Ellis. Stephen Hawking. 5 00:00:50,770 --> 00:00:55,030 Julian Barber. John Barrow, American brother. 6 00:00:55,030 --> 00:01:05,080 Vince Kip Thorne, Martin Reese, Tim Palmer, James Finney for the candles. 7 00:01:05,200 --> 00:01:12,400 And today, it's a great pleasure to have David Deutsch. And I'm going to invite Philip countless to introduce David. 8 00:01:23,810 --> 00:01:33,800 Okay. So David Deutsch did his degrees at Cambridge, where he read Natural Sciences, but then did part three. 9 00:01:34,280 --> 00:01:42,080 And then he came to Oxford, where he came to work with Dennis Schroder and he wrote a thesis on quantum field theory in space time. 10 00:01:42,440 --> 00:01:48,950 But I remember it was mostly to do with the acceleration in flat space times and also on cosmic energy. 11 00:01:49,910 --> 00:02:00,040 But even before finishing his thesis, David went to Austin to the University of Texas at Austin on what can best be described as a years pre doc. 12 00:02:00,680 --> 00:02:10,760 And there he talked with John Wheeler and this led to a series of visits over a number of years where each year David would come for a turn. 13 00:02:12,770 --> 00:02:19,400 And during this time, David became interested in foundational issues of quantum mechanics. 14 00:02:20,390 --> 00:02:26,630 And this, I must tell you what must I count as probably my greatest mistake, 15 00:02:27,410 --> 00:02:34,760 which is that one day riding the elevator with Steven Weinberg in one of these terms with David wasn't there. 16 00:02:35,090 --> 00:02:39,620 Steve turned to me and he said, Where's is this fellow David Doyle? 17 00:02:40,550 --> 00:02:46,190 And I said, I think he's gone off trying to understand the foundations. 18 00:02:49,310 --> 00:02:52,510 So Steve looked at me and we both sighed. 19 00:02:52,850 --> 00:02:59,180 And we thought that David had gone down a dark road that others had trodden before and been lost to science. 20 00:03:00,500 --> 00:03:08,750 How wrong we were. David has since been elected to the Royal Society and I won't try and improve on the citation. 21 00:03:09,050 --> 00:03:11,540 The citation, says David Deutsch, 22 00:03:11,540 --> 00:03:20,510 laid the foundations of the quantum theory of computation and subsequently made or participated in many of the most important advances in this field, 23 00:03:21,140 --> 00:03:29,000 including the discovery of the first quantum algorithms, the theory of quantum logic gates and quantum computational networks, 24 00:03:29,840 --> 00:03:36,740 the first error correction, the first quantum error correction scheme, and several quantum universality results. 25 00:03:37,550 --> 00:03:43,010 He has set the agenda for worldwide research efforts in this new into disciplinary field, 26 00:03:43,490 --> 00:03:48,469 made progress in understanding the philosophical implications by a variance of the 27 00:03:48,470 --> 00:03:52,940 many university's interpretation and made it comprehensible to the general public. 28 00:03:53,240 --> 00:03:56,120 Notably in his book On the Fabric of Reality. 29 00:03:57,740 --> 00:04:03,290 David has since published a second book, The Beginning of Infinity Explanations That Transformed the World. 30 00:04:05,330 --> 00:04:09,440 He was awarded the Direct Medal of the Institute of Physics in 1998, 31 00:04:10,220 --> 00:04:18,860 the International Award on Quantum Communication in 2002, the Computational Science Prize in 2005. 32 00:04:19,850 --> 00:04:27,440 David is currently a visiting professor here in Oxford and he is also working on constructor theory. 33 00:04:28,070 --> 00:04:35,810 This is an attempt to generalise the theory of quantum computation to cover not just computation but all physical processes. 34 00:04:36,290 --> 00:04:42,070 This will in part be the subject of today's talks. You. 35 00:05:00,370 --> 00:05:09,730 Well, I'm also grateful to the organisers and and to the Sharma family for giving me this opportunity to honour Dennis Sharma's memory. 36 00:05:10,660 --> 00:05:15,670 He was my boss when I was a graduate student and later as a post-doc here in Oxford. 37 00:05:17,000 --> 00:05:21,680 And he was in charge of theoretical astrophysics and astronomy, 38 00:05:22,280 --> 00:05:30,830 which is a branch of physics that is unusually close to all the fundamental laws of physics, general relativity, 39 00:05:31,370 --> 00:05:39,139 nuclear and elementary, particle physics, thermodynamics, foundations of quantum theory, and, 40 00:05:39,140 --> 00:05:46,130 of course, fundamental physics in any conception, in any branch is about universal laws. 41 00:05:46,970 --> 00:05:57,840 But research in astrophysics and cosmology. Explaining a single phenomenon can involve several or all the fundamental laws. 42 00:06:00,110 --> 00:06:07,340 Perhaps the first problem in physics that human beings ever tried to solve out of sheer curiosity. 43 00:06:08,230 --> 00:06:12,670 Namely the appearance of the night sky. Why does it look as it does? 44 00:06:13,600 --> 00:06:18,940 It's remarkable that even the crudest, true explanation of that. 45 00:06:20,070 --> 00:06:31,320 Already requires all the fundamental laws we know today to to explain the basic fact that it's black and not white requires general relativity. 46 00:06:32,620 --> 00:06:36,370 The colours of the stars. Thermodynamics. 47 00:06:36,730 --> 00:06:41,740 Why they don't go out. Nuclear physics and the aurora. 48 00:06:41,740 --> 00:06:46,690 And Thunder. And Lightning. Many other phenomena. Electricity and magnetism. 49 00:06:48,980 --> 00:07:03,290 This confluence of fundamental laws in astrophysics and cosmology is a hint that there might be a type of unity in nature. 50 00:07:03,830 --> 00:07:14,090 This deeper than the mere fact that there are universal laws, namely that there might be a level of explanation of those laws. 51 00:07:15,190 --> 00:07:19,840 I first encountered that idea from Dennis, but long before I even met him, 52 00:07:20,560 --> 00:07:27,310 because when I was in school, I'd read his popular book, The Unity of the Universe. 53 00:07:31,740 --> 00:07:35,330 It is. This isn't the original copy I read then. That was a library copy. 54 00:07:38,520 --> 00:07:42,020 So. And I borrowed its title for this talk. 55 00:07:44,120 --> 00:07:47,980 Yeah. Yeah. 56 00:07:48,080 --> 00:07:55,270 Now the subtitle Man's Evolving View of the Cosmos From Ancient Greece to Mount Palomar. 57 00:07:56,620 --> 00:08:03,340 Yeah, but I wasn't really interested in the history of cosmology, but I did love the book. 58 00:08:04,390 --> 00:08:08,860 And a few years later, still before I'd met Denis, I read it again. 59 00:08:10,060 --> 00:08:22,720 And I was amazed to find that it contained, among other things, a powerful advocacy of a false theory, the steady state theory. 60 00:08:22,990 --> 00:08:27,430 That's the cosmological theory under which the universe is eternal, 61 00:08:27,910 --> 00:08:35,560 has always existed, will always exist in its present state on the cosmological scale. 62 00:08:37,160 --> 00:08:45,140 And that theory, I knew, had been comprehensively refuted by observations long before I read the book. 63 00:08:47,200 --> 00:08:52,370 But. I hadn't even noticed that the steady state theory was in the book. 64 00:08:54,170 --> 00:09:01,610 In other words, what was in a sense, the main thesis of this book had entirely passed me by when I read it. 65 00:09:02,940 --> 00:09:06,360 And that was because it wasn't the main thesis. 66 00:09:07,200 --> 00:09:13,650 The real theme of this book was what the title says The unity of the universe. 67 00:09:14,220 --> 00:09:19,560 And that unity, as I said, wasn't just this. 68 00:09:23,450 --> 00:09:29,840 So his theory wasn't just this. It was this. 69 00:09:31,640 --> 00:09:41,480 A unifying principle that would explain something about not everything, but something about why the laws of nature are as they are. 70 00:09:42,710 --> 00:09:48,830 The principle in question in the book was a very natural guess for a cosmologist to make. 71 00:09:49,580 --> 00:09:52,580 They called it the perfect cosmological principle. 72 00:09:54,820 --> 00:10:00,850 It said simply that the universe on on cosmological scales is homogeneous in time. 73 00:10:01,580 --> 00:10:11,800 That just that sounded good because there was already an ordinary cosmological principle that said it's homogeneous in space. 74 00:10:12,790 --> 00:10:16,180 And that one is true as far as far as we still know. 75 00:10:20,020 --> 00:10:25,450 You say a word about principle? The physics terminology isn't actually standardised. 76 00:10:27,010 --> 00:10:31,900 In regard to which laws of nature we call principals and which we just call laws. 77 00:10:32,710 --> 00:10:40,300 I'm using the term principles specifically to mean a universal law about universal laws. 78 00:10:41,430 --> 00:10:48,180 So the perfect cosmological principle, it placed the constraint on the other laws of physics. 79 00:10:48,180 --> 00:11:03,260 He didn't fully explain them. But. It would have placed constraints on all the other laws of physics as they were then known, 80 00:11:03,710 --> 00:11:07,970 so that, for example, there could be the creation of matter out of nothing, 81 00:11:07,970 --> 00:11:10,700 so that the density of the universe could remain constant, 82 00:11:10,850 --> 00:11:19,520 even though its expanding and the total entropy could remain constant, even though stars were burning their fuel and so on. 83 00:11:21,270 --> 00:11:25,140 Now despite being totally false. 84 00:11:26,140 --> 00:11:35,200 This principle has some desirable features that make the steady state theory a good theory intrinsically. 85 00:11:36,230 --> 00:11:42,860 And the first of these, of course, most famously, is that this principle made this theory highly falsifiable. 86 00:11:43,950 --> 00:11:53,790 To conform to it. The laws of motion and the various parameters constants of nature had to be just so to make it. 87 00:11:55,080 --> 00:12:02,760 I. Thank. 88 00:12:16,350 --> 00:12:25,140 Thank you. If you don't write so the all the constants of nature would have to be exactly right. 89 00:12:25,500 --> 00:12:35,040 To make it happen that things like galaxies swirling through the through the intergalactic gas would cause 90 00:12:35,040 --> 00:12:42,359 density variations which would just result in the form in the later in the formation of fresh galaxies, 91 00:12:42,360 --> 00:12:52,870 of just the right size and type, containing stars of just the right composition and so on to reproduce maybe a billion years later or something. 92 00:12:52,890 --> 00:12:58,140 The the all the statistics that have existed forever. 93 00:12:59,770 --> 00:13:03,670 Which makes the principle itself hard to vary. 94 00:13:05,040 --> 00:13:08,230 Which makes it an intrinsically good explanation. 95 00:13:08,250 --> 00:13:13,200 And one consequence of that was that it was strongly falsifiable by observation. 96 00:13:13,470 --> 00:13:17,700 And indeed, it was duly falsified. For instance. 97 00:13:20,020 --> 00:13:28,060 By cosmological standards, light actually travels very slowly so that when we look out at something very far away, 98 00:13:28,780 --> 00:13:36,790 we're actually seeing how it was in the distant past. So if the universe is homogeneous in space and time. 99 00:13:38,040 --> 00:13:44,610 Then very distant vistas on the universe should look very much like the universe looks here. 100 00:13:46,150 --> 00:13:49,860 Or nearby here. And so astronomers looked. 101 00:13:51,020 --> 00:14:01,840 And it didn't look the same. And then there was the famous discovery of microwaves pervading space and microwaves don't last in an expanding universe, 102 00:14:01,850 --> 00:14:05,300 they get redder and redder, rather like the Doppler effect. 103 00:14:05,600 --> 00:14:10,370 And so to maintain a steady state, they'd have to be replenished. 104 00:14:11,030 --> 00:14:20,059 And to fix that up, required ad hoc modifications so nasty that it made the theory a bad explanation. 105 00:14:20,060 --> 00:14:30,290 After all, especially as its rival, the Big Bang Theory, did have an elegant and hard to vary explanation for the microwaves. 106 00:14:31,770 --> 00:14:35,940 Now, I should say I'm drawing a distinction here between. 107 00:14:38,240 --> 00:14:49,040 A true explanation, which means objectively corresponding to reality and being an intrinsically good explanation, 108 00:14:49,940 --> 00:14:54,260 which is a transient property depending on the state of other knowledge at the time. 109 00:14:54,440 --> 00:15:03,710 It's the property, as I said, of being hard to vary while still accounting for the for the things it purports to explain. 110 00:15:06,060 --> 00:15:29,830 The Big Bang Theory and the steady state theory. But having reviewed the Big Bang Theory and the steady state theory were both very good explanations 111 00:15:30,610 --> 00:15:37,210 because they were both severely constrained by either good explanations and by evidence. 112 00:15:37,930 --> 00:15:43,960 One of the two was false, which is why Denis went to work on the other. 113 00:15:45,850 --> 00:15:49,950 But. That idea. This idea. 114 00:15:51,680 --> 00:16:00,470 That there are universal principles which at least partially explain the universal laws, which in turn explain the phenomena. 115 00:16:01,720 --> 00:16:09,430 Was not overturned by the observations. Only the particular principle that had, as it were, auditioned for that role. 116 00:16:10,610 --> 00:16:13,400 The perfect cosmological principle that had been overturned. 117 00:16:16,630 --> 00:16:24,490 Now the second nice thing about the steady state theory was the way in which it dealt with the initial conditions. 118 00:16:26,180 --> 00:16:29,690 Of the universe. Now, this may seem like a technicality, but it isn't. 119 00:16:29,720 --> 00:16:33,680 It's quite fundamental conceptually. You see. 120 00:16:36,140 --> 00:16:38,660 Ever since the time of Galileo and Newton. 121 00:16:39,530 --> 00:16:50,299 The prevailing conception of how theories are supposed to explain the world is that they provide laws of motion which, 122 00:16:50,300 --> 00:16:59,590 given the state of the world at any one time, predict or retro dictate at any other time, or its probability at any other time. 123 00:16:59,600 --> 00:17:09,080 But never mind that the awkward fact is that while we have superb theories about what the laws of motion are. 124 00:17:10,380 --> 00:17:14,760 We have never had a successful theory specifying the initial conditions. 125 00:17:16,170 --> 00:17:21,660 And it's awkward because in the prevailing conception, the state of the universe, 126 00:17:22,170 --> 00:17:29,670 what actually happens at all times and at any time is the very thing that science sets out to explain. 127 00:17:29,680 --> 00:17:33,030 So it's at least as fundamental as the laws of motion. 128 00:17:34,620 --> 00:17:39,860 We would like to explain it. In the classic Big Bang Theory, 129 00:17:40,520 --> 00:17:50,780 the initial conditions were that the state of the universe was spatially homogeneous across an initial singularity that was causally extended, 130 00:17:51,680 --> 00:18:02,050 even though it was zero in size. That couldn't be exactly right, because if that was the exact initial state, then nothing would ever happen. 131 00:18:03,160 --> 00:18:07,960 What starts exactly homogeneous stays exactly homogeneous under the laws of motion. 132 00:18:08,800 --> 00:18:15,850 So there were various ideas. Maybe quantum fluctuations spontaneously break the symmetry. 133 00:18:17,070 --> 00:18:24,000 But there was never actually a viable theory that predicted the details of the homage in homogeneity, 134 00:18:24,480 --> 00:18:28,680 such as would lead to galaxy formation and the things we observe. 135 00:18:31,290 --> 00:18:39,540 Roger Penrose had the elegant idea that the vile curvature is zero at the beginning of the universe and maximum at the end. 136 00:18:41,380 --> 00:18:48,970 But that doesn't seem to have been fruitful either. And today's prevailing theory, which is called inflationary cosmology, 137 00:18:49,690 --> 00:18:54,070 is actually worse in that respect because it doesn't even address initial conditions. 138 00:18:56,530 --> 00:18:59,740 That doesn't mean that inflation didn't happen. It just means that it doesn't. 139 00:19:00,930 --> 00:19:06,870 That by itself solved the explanatory problem about the initial conditions that it was intended, I think, to solve. 140 00:19:11,230 --> 00:19:19,000 You could always fudge these type of initial condition questions by resorting to the anthropic principle, 141 00:19:19,570 --> 00:19:24,310 namely that there are lots of universes with all possible initial conditions, 142 00:19:24,460 --> 00:19:31,750 and that we were in one of the few in which astrophysicists exist to ask what the initial conditions were. 143 00:19:33,140 --> 00:19:37,520 But if that was the only thing explaining the initial conditions, it would predict. 144 00:19:38,560 --> 00:19:41,860 That it's overwhelmingly likely that we're living in a bubble. 145 00:19:43,490 --> 00:19:49,500 Of order. Which is going to be snuffed out nanoseconds from now. 146 00:19:51,330 --> 00:20:02,910 And so it's refuted. But there's another niggling problem or I could say extremely fundamental problem, 147 00:20:02,920 --> 00:20:14,260 depending on your outlook with the initial states being one of the one of the basic explanatory ideas from which other explanations are to be derived. 148 00:20:17,020 --> 00:20:26,350 There's no reason for anything in anything else that we know about physics that singles out the initial conditions as being as being preferred. 149 00:20:28,170 --> 00:20:31,560 And all incidents are also predictably equal. 150 00:20:33,500 --> 00:20:41,639 In fact. The idea that the initial conditions are special in the scheme of things has 151 00:20:41,640 --> 00:20:47,580 uncomfortable echoes of a pre scientific conception of what the physical world even is. 152 00:20:49,310 --> 00:20:55,750 See, there's there's a moment of creation. Before which the physical universe didn't exist. 153 00:20:56,850 --> 00:21:01,500 Then the initial conditions are set by something. 154 00:21:04,610 --> 00:21:12,050 And then beautiful laws come into operation from which everything that subsequently happens emerges. 155 00:21:13,890 --> 00:21:19,080 No wonder some people took the Big Bang Theory as vindicating creationism. 156 00:21:21,160 --> 00:21:26,140 While other people, for the same reason, didn't want the Big Bang Theory to be true. 157 00:21:29,240 --> 00:21:35,210 Well, the steady state theory would have elegantly solved all those problems at once. 158 00:21:36,400 --> 00:21:39,580 Though it doesn't contradict the prevailing conception. 159 00:21:40,670 --> 00:21:49,240 It does radically augment it. The state of the universe would now be deduced, at least in principle, 160 00:21:49,870 --> 00:21:56,770 not from conditions at any preferred time, but from the perfect cosmological principle itself. 161 00:21:59,760 --> 00:22:04,230 Since they'd also be the conditions at every other time. No instant would be preferred. 162 00:22:05,380 --> 00:22:16,660 And symmetry would not be broken, and even the size and character of the deviations from homogeneity would have been determined by the principle. 163 00:22:19,270 --> 00:22:23,450 Nice, isn't it? But not true, as it turned out. 164 00:22:24,540 --> 00:22:30,370 Which. Brings me to the third inherently nice thing about the steady state theory. 165 00:22:32,980 --> 00:22:47,240 The perfect cosmological principle. Introduces a new mode of explanation into physics, which supplements the the prevailing conception. 166 00:22:51,990 --> 00:23:02,120 It doesn't only relegate the initial conditions to being a a mere consequence rather than fundamental, but fundamental principles. 167 00:23:02,130 --> 00:23:09,840 It also requires that the laws of physics be fine tuned to make a particular thing happen. 168 00:23:11,240 --> 00:23:19,250 Namely the steadiness of this steady state. And in fact, that makes it much more fine tuned than you might think because. 169 00:23:21,000 --> 00:23:34,320 Well, the steady state. People were aware that their theory wouldn't work if the process that reproduced the state over time were not also stable. 170 00:23:35,040 --> 00:23:40,530 Because if it were unstable. So if a small deviation from ideal steadiness. 171 00:23:41,770 --> 00:23:50,140 Produced a larger deviation, let's say 1000000 to 1000000000 years later or a trillion years later, or 10 to 10 years later. 172 00:23:53,270 --> 00:23:57,320 10 to 100 years later. Sorry. Then. 173 00:23:59,140 --> 00:24:04,960 After after a certain number of cycles, the state would no longer have the steadiness property. 174 00:24:05,770 --> 00:24:11,920 And so it had to be that a small deviation would be reversed in due course. 175 00:24:12,490 --> 00:24:19,340 That stability. And they work hard to construct the cosmological model to have that property. 176 00:24:19,970 --> 00:24:25,160 But stability is not enough. If you want to make the universe eternal. 177 00:24:26,240 --> 00:24:30,200 However stable the state is too small changes. 178 00:24:30,590 --> 00:24:32,810 A large nudge will eventually happen. 179 00:24:34,480 --> 00:24:42,820 Given the normal assumptions of statistical mechanics and even quantum mechanical tunnelling would eventually have the same effect. 180 00:24:43,660 --> 00:24:45,670 And so the steady state. 181 00:24:46,750 --> 00:24:58,900 Would degenerate, degenerate into the far larger realm of states that evolve with time into other states, thus violating the principle. 182 00:24:59,910 --> 00:25:07,920 So the perfect cosmological principle would have required the quantum state of the universe to be exquisitely pruned. 183 00:25:08,980 --> 00:25:20,230 To eliminate. What Bryce DeWitt called the maverick universes that would evolve to violate the principle at any time in the future. 184 00:25:22,410 --> 00:25:31,320 The specialness of that state. In the view of the opponents of the city, state theory was just creationism. 185 00:25:32,270 --> 00:25:41,599 The entire eternal universe created all at once in a state finely crafted to give the appearance of an evolving, 186 00:25:41,600 --> 00:25:45,080 textured mixture of structure and randomness. 187 00:25:45,800 --> 00:25:55,270 But actually all along. Rigged to conform to a certain ideal throughout its infinite extent. 188 00:25:57,310 --> 00:26:05,680 So here we had the proponents of two rival theories, each accusing the other's of, in effect, creationism, 189 00:26:06,940 --> 00:26:17,050 while other people were delighted that their favoured theory brought meaning as they saw it back into physics, into the universe. 190 00:26:18,650 --> 00:26:21,050 But this was all misconceived. 191 00:26:22,410 --> 00:26:31,110 Everyone was simply assuming that all fundamental explanations had to be in the form prescribed by the prevailing conception. 192 00:26:33,610 --> 00:26:38,290 Possibly with the addendum of the perfect cosmological principle. 193 00:26:40,530 --> 00:26:45,040 Now it is possible to eject to the whole idea of principles in nature. 194 00:26:45,060 --> 00:26:48,970 In that sense. Laws about laws come. 195 00:26:48,990 --> 00:27:00,570 We confine ourselves to laws about phenomena. Is it possible to restrict science to those laws and reject laws about laws? 196 00:27:02,670 --> 00:27:06,180 Well, here's an object lesson. 197 00:27:07,560 --> 00:27:14,010 The principle of the conservation of energy started out as a mere law about phenomenon. 198 00:27:14,580 --> 00:27:20,040 In fact, less than the law, it was merely a mathematical theorem of Newtonian mechanics. 199 00:27:21,610 --> 00:27:30,730 Initially that that just for a system of particles moving in space without friction and with elastic collisions, the quantity half and v squared. 200 00:27:31,870 --> 00:27:39,450 Summed over all the particles is a constant. We now call that the kinetic energy of the system. 201 00:27:42,170 --> 00:27:49,380 But the theorem. Was known for centuries before the concept of energy was even conceived. 202 00:27:50,040 --> 00:27:51,720 And it wasn't necessary at that time. 203 00:27:52,700 --> 00:28:00,170 In the meantime, people realised then that if you add that quantity to what we now call the gravitational potential energy. 204 00:28:04,500 --> 00:28:07,560 Which is minus GM one and two over R squared. 205 00:28:08,580 --> 00:28:12,719 Then the result is a constant, even if gravitating particles. 206 00:28:12,720 --> 00:28:21,460 But it still won't be true if there's friction, for example. But now that's the theorem of Newton's laws of motion and gravity. 207 00:28:22,820 --> 00:28:27,700 It still has strictly no more content than those laws themselves. 208 00:28:27,710 --> 00:28:35,780 In fact, less. And it applies only when they are when those laws are the full explanation of what is happening. 209 00:28:36,770 --> 00:28:42,110 Every prediction of Newton's theories can be made without any reference to energy, 210 00:28:42,740 --> 00:28:46,970 without even knowing of the existence of energy or of its conservation. 211 00:28:47,780 --> 00:28:55,960 And in particular. Those theorems predict nothing about the content of undiscovered laws of physics. 212 00:28:58,270 --> 00:29:04,420 So that. And they didn't call it energy yet. But if they had, that energy would not have been the energy that we know. 213 00:29:05,440 --> 00:29:11,380 Nor is its constancy under those theorems, the conservation law that we know. 214 00:29:13,620 --> 00:29:16,920 But then. In the 19th century. 215 00:29:17,850 --> 00:29:20,850 After Count Rumford experiments. 216 00:29:22,730 --> 00:29:26,850 On Cannon. That got hot when they were being drilled down. 217 00:29:28,140 --> 00:29:30,420 People guessed that. 218 00:29:30,690 --> 00:29:44,010 If you add a further term to that Newtonian scheme of summing half and B squared and G.M. two overall minus that you can add any and you can. 219 00:29:45,530 --> 00:29:51,170 You can also add the Newtonian work done, and if you add to that, an expression for the heat. 220 00:29:52,220 --> 00:29:59,750 Normalised with suitable units, then the total will now be conserved even if there is friction. 221 00:30:01,290 --> 00:30:05,550 And now you have something that isn't a serum. 222 00:30:06,690 --> 00:30:10,950 It's not deduced from laws, emotion or indeed anything. 223 00:30:11,700 --> 00:30:17,370 It's a law physics in its own right. The law of the conservation of energy. 224 00:30:18,240 --> 00:30:28,200 And indeed it couldn't have been deduced because the laws of motion that underlie frictional processes were still unknown at the time. 225 00:30:29,260 --> 00:30:34,210 In fact, I'm not sure they're known today, but if they are, they're quantum mechanical. 226 00:30:35,940 --> 00:30:39,090 And at that point, people tweaked. 227 00:30:40,350 --> 00:30:43,920 They realised that this new law now. 228 00:30:45,100 --> 00:30:54,380 To make sense. Had to be respected by even by as yet unknown forces and unknown substances. 229 00:30:55,750 --> 00:31:04,890 It was a law about laws. The principle of the conservation of energy, and that is exactly when the term energy was invented. 230 00:31:10,490 --> 00:31:16,120 So the key that's the rule of conservation, of energy. Expressed with this in mind. 231 00:31:16,780 --> 00:31:21,130 And the key word there is not energy. It's every. 232 00:31:23,290 --> 00:31:31,600 And when the theory of electromagnetism was later invented, it did indeed conform to this principle. 233 00:31:32,350 --> 00:31:40,540 Even though electromagnetism hadn't, the theory of electromagnetism and also was not known at the time when the principle was invented. 234 00:31:42,100 --> 00:31:52,150 Furthermore, thermodynamics was born with several further principles about heat and work and temperature and a new quantity entropy. 235 00:31:52,960 --> 00:31:55,990 And none of those principles were deduced from laws of motion. 236 00:31:56,020 --> 00:32:04,480 In fact, many attempts have been made in the century and a half or so since the inauguration of the 237 00:32:05,050 --> 00:32:11,320 summit in OMICS to establish a connection between between those laws and laws of motion, 238 00:32:11,320 --> 00:32:16,900 or somehow to express thermodynamics within the prevailing conception. 239 00:32:17,740 --> 00:32:28,750 And none have been satisfactory. They all involve fudges, such as coarse graining and infinite ensembles. 240 00:32:29,760 --> 00:32:37,230 And. Even the exact distinction between work and hate remains elusive to this day. 241 00:32:39,330 --> 00:32:46,190 Then, of course, still later, the 20th century in the early study of radioactive beta decay, 242 00:32:46,410 --> 00:32:55,290 when physicists added up the kinetic energies and the M.C. squared energies in radioactive decays and finding that they didn't add up. 243 00:32:56,570 --> 00:33:06,180 To a constant. Pauli and Fermi could guess that there was just a hitherto unknown particle. 244 00:33:06,540 --> 00:33:13,650 The neutrino. For whose existence? At first, the only evidence was that principal. 245 00:33:15,040 --> 00:33:24,499 And again, that version of the conservation of energy couldn't possibly regard it as a theorem whose premises were the neutrinos, 246 00:33:24,500 --> 00:33:28,840 the laws of motion and interaction, because those laws were not yet known, 247 00:33:29,800 --> 00:33:37,270 which just predicted that once they were known, they would be found to obey the principles of quantum mechanics, 248 00:33:37,450 --> 00:33:41,680 which by now included the principle of the conservation of energy. 249 00:33:42,810 --> 00:33:46,920 And so they did. And. 250 00:33:48,420 --> 00:33:54,270 That prediction was an indispensable guide to discovering those laws at all. 251 00:33:57,440 --> 00:34:08,750 So the rule of restricting science to laws about phenomena and rejecting laws about laws is untenable. 252 00:34:10,340 --> 00:34:15,460 Note that that rule. Is itself a principal. 253 00:34:17,910 --> 00:34:23,190 The anti principal principle. And as we've just seen, it's. 254 00:34:24,570 --> 00:34:34,230 False. More generally, I think the whole purpose of theoretical science is to explain the world, 255 00:34:35,010 --> 00:34:41,370 the physical world, and therefore the sole criterion by which theories ought to be judged. 256 00:34:42,470 --> 00:34:50,660 Is there explanatory power? This rules out having preferences between modes of explanation, 257 00:34:51,260 --> 00:34:57,980 preferences that if those preferences are independent of how good the explanations are, that should be what counts. 258 00:34:58,550 --> 00:35:11,240 The only thing that counts. So just as a scientific theory about phenomena is much more than just an instrument of prediction of those phenomena, 259 00:35:11,870 --> 00:35:16,040 much more than just a compressed summary of them. 260 00:35:17,000 --> 00:35:26,440 But is an explanation of them. So a principle of nature is not just a statement of shared properties among theories. 261 00:35:27,130 --> 00:35:30,490 It is an explanation of those properties. 262 00:35:32,810 --> 00:35:36,980 Now the prevailing conception is a principle too, isn't it? 263 00:35:40,660 --> 00:35:44,770 And I believe it's just as false. 264 00:35:46,550 --> 00:35:49,990 As I'll explain in a moment. So. 265 00:35:51,220 --> 00:35:57,580 What other principles of nature might be true, aside from those of thermodynamics that I've mentioned? 266 00:35:57,940 --> 00:36:03,760 Well, both quantum theory and relativity are partly principles. 267 00:36:03,910 --> 00:36:07,330 In addition to making direct predictions about phenomena, 268 00:36:07,840 --> 00:36:16,090 they also assert that all other laws of nature, including each other, conform to certain principles. 269 00:36:16,390 --> 00:36:23,200 Certain principles such as the principle that laws are formulated in terms of geometrical objects. 270 00:36:23,440 --> 00:36:28,840 In the case of general relativity and in quantum theory, the principle of unitary t. 271 00:36:31,220 --> 00:36:31,640 And. 272 00:36:33,240 --> 00:36:43,440 As those two examples illustrate, we shouldn't expect there to be a rigid hierarchy of principles with ordinary laws being subordinate to principles. 273 00:36:44,130 --> 00:36:53,010 But we should expect that the immense explanatory power of some of our theories, of our best theories, 274 00:36:53,640 --> 00:37:00,930 implies that if they are true of some physical systems, they must be true of all of them. 275 00:37:01,260 --> 00:37:09,090 I think it was Feynman who called this in the case of quantum theory, the totalitarian property of quantum theory. 276 00:37:12,690 --> 00:37:21,490 Which. I think Bryce DeWitt proved the same thing. 277 00:37:21,610 --> 00:37:31,870 And I think independently that if any system in the universe is governed by quantum theory, then no system that could interact with that. 278 00:37:31,870 --> 00:37:34,990 One could obey classical laws of motion. 279 00:37:36,600 --> 00:37:44,520 Relativity isn't quite as totalitarian as quantum theory, but its principles do seem to be inconsistent with those of quantum theory. 280 00:37:44,520 --> 00:37:48,360 So presumably one or both of them must be superseded. 281 00:37:49,410 --> 00:37:55,110 Something that we couldn't know unless we regarded those two theories as principles. 282 00:37:57,990 --> 00:38:02,970 No. For example, they might just apply two different phenomena. 283 00:38:05,510 --> 00:38:11,600 Now the way in which the wit in particular proved the totalitarian property. 284 00:38:12,230 --> 00:38:17,960 I'm not sure how FINEMAN did it. It's quite significant from my present perspective. 285 00:38:19,630 --> 00:38:24,760 He used the so called uncertainty principle of quantum mechanics, horribly misnamed, 286 00:38:25,510 --> 00:38:36,549 and he assumed for the sake of argument that it did only apply to quantum systems and that classical objects could exist in nature too, 287 00:38:36,550 --> 00:38:40,430 and could interact. With some quantum system. 288 00:38:41,270 --> 00:38:48,110 And then he showed by an ingenious set of arguments that by making certain measurements, 289 00:38:48,110 --> 00:38:55,940 one could violate the uncertainty principle not just for the combined system, but even within the quantum system. 290 00:38:56,950 --> 00:39:02,140 So loosely speaking, quantum theory either applies to everything or to nothing. 291 00:39:04,230 --> 00:39:09,240 And the reason that that mode of proof is significant to me now. 292 00:39:10,690 --> 00:39:15,040 Is that it uses the uncertainty principle in the form. 293 00:39:16,130 --> 00:39:20,120 Such and such a class of tasks is impossible. 294 00:39:21,350 --> 00:39:29,480 And if a certain physical object is possible, the classical object, then a further process would be possible. 295 00:39:29,840 --> 00:39:37,540 That would lead to a contradiction. And therefore, if the principle is true, the classical object isn't possible. 296 00:39:39,800 --> 00:39:45,770 He expressed the proof in the prevailing conception, but it's barely used as such. 297 00:39:46,490 --> 00:39:55,240 You see. Saying that a given task is impossible in the sense that the uncertainty principle does. 298 00:39:56,750 --> 00:40:01,940 Means that it not just that it doesn't happen, but that it can't be caused to happen by anything. 299 00:40:04,100 --> 00:40:10,520 Can't be closed with the help of anything else. Even things not explicitly referred to. 300 00:40:11,240 --> 00:40:16,440 Things not yet known. And that's the core of the totalitarian proof. 301 00:40:18,780 --> 00:40:29,100 So a while ago, I proposed a new mode of explanation that Philip referred to constructive theory, or rather a theory called constructor theory, 302 00:40:29,460 --> 00:40:39,540 which I hope will incorporate this new mode of explanation which is intended eventually to supersede the prevailing conception. 303 00:40:40,320 --> 00:40:49,530 Though the two into translatable in many cases, the first principle of constructor theory is this. 304 00:40:52,450 --> 00:40:58,689 The laws of physics are expressed above entirely in terms of statements about which physical 305 00:40:58,690 --> 00:41:05,470 transformations it's possible to cause to happen and which are impossible to cause. 306 00:41:06,250 --> 00:41:11,090 And why? So this is about transformations. 307 00:41:12,200 --> 00:41:20,040 That are caused by something. Some agent, which is itself not specified that any agent. 308 00:41:22,760 --> 00:41:31,040 Except that this agent must itself be possible. And there is a condition that the agent retain its ability to cause the transformation again. 309 00:41:31,280 --> 00:41:35,450 Otherwise, it's only partly an agent and partly a patient. 310 00:41:36,750 --> 00:41:39,920 A chemical catalyst is an example of such an agent. 311 00:41:41,220 --> 00:41:45,300 It causes chemical reactions but does not participate in them. 312 00:41:46,470 --> 00:41:53,400 By the way, we're told in elementary chemistry classes that it doesn't cause chemical reactions, it just changes the speed. 313 00:41:53,400 --> 00:42:00,840 But that is not the case. So the heat engine is also such an agent. 314 00:42:01,750 --> 00:42:05,229 It converts things to air, to have different temperatures and so on. 315 00:42:05,230 --> 00:42:13,570 But it's it itself stays the same. So as a computer, we call these agents generically constructors. 316 00:42:15,120 --> 00:42:21,040 And by possible to cause we mean possible with arbitrary accuracy. 317 00:42:21,040 --> 00:42:28,479 That is, you could be in if silent and if a task is possible you give me and if silent and someone could design a 318 00:42:28,480 --> 00:42:37,780 constructor which causes that task to happen with accuracy if silent or better and impossible means that. 319 00:42:38,770 --> 00:42:44,409 The laws of physics exclude the possibility that anyone could ever produce such a design, 320 00:42:44,410 --> 00:42:48,069 or the laws of physics rule out the existence of such an agent. 321 00:42:48,070 --> 00:42:58,250 Central constructor. So there are no probabilities in the constructor theoretic conception of the world task. 322 00:42:58,320 --> 00:43:02,300 Tasks that look probabilistic like building a fair roulette wheel. 323 00:43:03,340 --> 00:43:11,350 I expressed in terms of preparing it in a specified quantum mechanical state with a given density matrix, for example. 324 00:43:13,430 --> 00:43:23,240 So while the prevailing conception seeks to distinguish at a fundamental level what happens from what doesn't happen. 325 00:43:24,460 --> 00:43:27,720 In which case. Possible and impossible. 326 00:43:27,870 --> 00:43:33,060 Just a manner of speaking about certain approximations or about our ignorance. 327 00:43:34,840 --> 00:43:43,000 But in constructor theory, it's the other way round. The laws of nature are about what's possible and impossible in the sense I've just described. 328 00:43:43,300 --> 00:43:48,760 And what actually happens is, in general, an emergent consequence of that. 329 00:43:49,060 --> 00:43:55,980 Sometimes it can be calculated. In which case. 330 00:43:57,000 --> 00:44:01,230 The constructive theory and the prevailing conception are equivalent. 331 00:44:02,150 --> 00:44:08,240 But sometimes it can't be calculated either because it's intractable or for some more profound reason. 332 00:44:08,750 --> 00:44:17,630 And in those cases, constructive theory can express exact laws that are inaccessible in the prevailing conception. 333 00:44:21,000 --> 00:44:24,360 One important case of the latter are. 334 00:44:25,530 --> 00:44:31,050 Initial conditions of the universe. They are in constructive theory. 335 00:44:31,500 --> 00:44:38,790 They are supposed to be incalculable consequences of laws about what's possible and what's impossible. 336 00:44:41,020 --> 00:44:53,010 And that is so reasonable. As I said, you wouldn't expect there to be a fundamental law specifying the state at any other time. 337 00:44:53,940 --> 00:45:02,310 Then the initial time, such as today, including all the locations of all the cows in Oxfordshire that were auctioned today. 338 00:45:03,320 --> 00:45:09,260 You wouldn't expect the state of those cows to have fundamental significance y expected of the initial state. 339 00:45:12,550 --> 00:45:18,940 Especially as it this violates symmetries that exist everywhere else in physics. 340 00:45:20,590 --> 00:45:23,350 And with that constructed theoretic perspective. 341 00:45:24,360 --> 00:45:34,650 We can begin to notice that there are already other principles of nature that are already known but are not usually acknowledged as such, 342 00:45:35,220 --> 00:45:43,320 nor even acknowledged as being part of physics at all, simply because they don't conform to the prevailing conception. 343 00:45:44,830 --> 00:45:47,770 There's the principles of the theory of computation, for example. 344 00:45:49,280 --> 00:45:56,330 The distinction between computable and non computable functions doesn't refer to what the computer is made of. 345 00:45:56,990 --> 00:46:03,590 We expect it to be the same for any make or model or technology of general purpose computer. 346 00:46:04,570 --> 00:46:11,260 Even ones using laws of physics or materials not yet discovered. 347 00:46:12,500 --> 00:46:19,010 So it's a principle. Difficult or impossible to express in the prevailing conception. 348 00:46:20,550 --> 00:46:25,530 But. In some work that my colleague Chiara Molto and I have done. 349 00:46:26,420 --> 00:46:32,450 We have shown that there is a beautiful expression of this principle in constructive theory. 350 00:46:33,820 --> 00:46:36,910 And this is in the context of a full construct, 351 00:46:36,910 --> 00:46:51,550 a theoretic information theory in which processes like computation and quantities like information are characterised in elegant, exact terms. 352 00:46:53,420 --> 00:47:03,840 That is. In constructive theoretic terms in in terms of what classes of physical transformation it's possible or impossible to cause. 353 00:47:04,880 --> 00:47:10,490 This new theory of information. Which I commend to you. 354 00:47:11,960 --> 00:47:20,600 Unlike Shannon's existing theory, naturally includes quantum information and predicts all its strange and distinctive properties. 355 00:47:21,940 --> 00:47:30,640 Such as the impossibility of cloning a quantum information, the quantum state, and the famous unpredictability of quantum measurement. 356 00:47:31,630 --> 00:47:34,930 Despite its deterministic law of motion. 357 00:47:36,630 --> 00:47:42,870 And my letter is also used to construct a theory in a biological application to characterise 358 00:47:43,410 --> 00:47:50,550 what precisely it is about the laws of physics that permits the origin and evolution of life. 359 00:47:51,610 --> 00:48:01,970 Among other things. The apparently non physics concept of the appearance of design, which is coined I think by Richard Dawkins. 360 00:48:02,930 --> 00:48:06,680 Has an exact definition in constructor theoretic physics. 361 00:48:07,920 --> 00:48:16,260 The result is that regardless of the so-called fine tuning coincidences in the constants of physics, 362 00:48:16,710 --> 00:48:25,170 the laws of physics do not in fact have to have and do not, in fact, have the appearance of design for causing life. 363 00:48:28,990 --> 00:48:36,850 The laws of thermodynamics, which I've mentioned already, have some existing constructor theoretic formulations. 364 00:48:37,540 --> 00:48:43,000 Like you can't build a perpetual motion machine of the first kind or of the second kind, and so on. 365 00:48:43,480 --> 00:48:49,420 But these are considered vague and hand-waving in the prevailing conception. 366 00:48:51,370 --> 00:48:56,890 And another example, you can't convert heat entirely into work without side effects. 367 00:48:58,900 --> 00:49:03,850 But if this further work by my letter pans out, 368 00:49:04,660 --> 00:49:13,629 it would revolutionise the foundations of thermodynamics because with slightly different versions of the first and second laws for looks and ones, 369 00:49:13,630 --> 00:49:19,840 we know, it would express those known hand-waving formulations. 370 00:49:19,840 --> 00:49:27,550 Exactly, and would provide an exact characterisation of the distinction between work and heat, and hence of entropy, 371 00:49:28,270 --> 00:49:35,050 without coarse graining, without distinction between macro states and micro states, without ensembles. 372 00:49:35,680 --> 00:49:41,230 Just construct a theory. The basic reason. 373 00:49:42,740 --> 00:49:46,610 The constructor theory can work that sort of magic. 374 00:49:47,950 --> 00:49:51,220 Is that it abstracts away the constructor. 375 00:49:52,920 --> 00:50:00,480 Like The Theory of catalysis in chemistry, which is another, as I said, another example of a of an existing constructor theoretic theory. 376 00:50:00,930 --> 00:50:10,600 It's not this. Process that is declared to be possible or impossible in constructed. 377 00:50:11,350 --> 00:50:16,070 It's this. Just that. 378 00:50:17,680 --> 00:50:22,690 So the constructor which is the the usually the. 379 00:50:25,790 --> 00:50:30,610 The macroscopic. Part of the process is abstracted away. 380 00:50:30,910 --> 00:50:37,719 And this is what makes constructor theory a natural vehicle for expressing scale invariant 381 00:50:37,720 --> 00:50:45,850 laws and substrate invariant laws about quantities like information and heat and work. 382 00:50:46,870 --> 00:50:53,800 Exactly. Like the perfect cosmological principle. 383 00:50:56,310 --> 00:51:02,100 Which had to be developed into the sophisticated, steady state theory constructor. 384 00:51:02,100 --> 00:51:08,310 Theory will have to be developed quite a bit more before we derive testable predictions from it. 385 00:51:09,620 --> 00:51:11,450 But unlike steady state theory, 386 00:51:12,620 --> 00:51:22,940 constructor theory has already provided a significant unification and illumination of fundamental matters in in diverse areas of physics. 387 00:51:23,790 --> 00:51:35,700 And beyond, as I said. And I think this already makes it a substantial step towards the unity that Dennis was looking for. 388 00:51:36,990 --> 00:52:05,330 Okay. Thanks. And so to mark the closing of this wonderful series of lectures, I've been asked to present these two things I thought.