00:00:00.000 This is the 12th and final Dennis Sharma lecture, series, and first let me thank the
00:00:25.440 Sharma family, all souls college, and physics department for having sponsored this
00:00:35.280 I thought I'd just read to you the names of the amazingly illustrious speakers have had in
00:00:41.320 the past, Roger Penrose, George Ellis, Stephen Hawking, Julian Barber, John Barrow, Mary
00:00:54.520 Kim Braubowitz, Kipthorn, Martin Reese, Tim Palmer, James Binney, Philip Candles, and today
00:01:06.080 it's a great pleasure to have David Deutsch, and I'm going to invite Philip Candles to introduce
00:01:20.440 So, David Deutsch did his degrees at Cambridge, where he read natural sciences, and then
00:01:32.920 did part three, and then he came to Oxford where he came to work with Dennis Sharma.
00:01:38.680 He wrote a thesis on quantum field theory and curve space time, but I remember it was mostly
00:01:43.840 to do with acceleration in flat space time and also on cosmic energy.
00:01:50.080 And even before finishing his thesis, David went to Austin, to the University of Texas
00:01:55.640 at Austin, on what can best be described as a year's pre-doc, and there he talked with
00:02:02.000 John Wheeler, and this led to a series of visits over a number of years, where each year
00:02:13.040 And during this time, David became interested in foundational issues of quantum mechanics,
00:02:20.720 and I must tell you what must I count as probably my greatest mistake, which is that one
00:02:28.520 day riding the elevator with Steve Meinberg in one of these terms where David wasn't there,
00:02:35.360 Steve turned to me and he said, where's this fellow David Deutsch, and I said, I think
00:02:43.520 he's gone off trying to understand the foundations of quantum mechanics.
00:02:48.400 So, Steve looked at me and we both sighed, and we thought that David had gone down a dark
00:02:55.520 road that others had trodden before and been lost to science.
00:03:00.680 Now, we were. David has since been elected to the Royal Society, and I won't try and improve
00:03:07.680 on the citation. The citation says, David Deutsch laid the foundations of the quantum theory
00:03:13.720 of computation and subsequently made or participated in many of the most important advances
00:03:19.760 in this field, including the discovery of the first quantum algorithms, the theory of quantum
00:03:26.000 logic gates, and quantum computational networks, the first era corrections, the first
00:03:32.080 quantum era corrections scheme, and several quantum universality results. He has set the
00:03:38.200 agenda for worldwide research efforts in this new interdisciplinary field, made progress
00:03:44.480 in understanding the philosophical implications via variant of the many universities interpretation
00:03:50.640 and made it comprehensible to the general public, notably in his book, on the fabric of reality.
00:03:58.000 David has since published a second book, the beginning of infinity, explanations that transformed
00:04:02.880 the world. He was awarded the Dirac Medal of the Institute of Physics in 1998, the
00:04:10.520 International Award on Quantum Communication in 2002, the H computational Science Prize in 2005,
00:04:20.000 David is currently a visiting professor here in Oxford, and he is also working on constructor
00:04:27.040 theory. This is an attempt to generalize the theory of quantum computation to cover not
00:04:32.800 just computation, but all physical processes. This will be part of the subject that
00:04:38.400 is based on this. Thank you. I am also grateful to the organizers and
00:05:03.400 to the Sharma family for giving me this opportunity to honor Dennis Sharma's memory. He was
00:05:11.080 my boss when I was a graduate student and later as a postdoc here in Oxford. He was in charge
00:05:18.320 of theoretical astrophysics and astronomy, which is a branch of physics that is unusually
00:05:25.400 close to all the fundamental laws of physics, general relativity, nuclear and elementary
00:05:32.560 particle physics, thermodynamics, foundations of quantum theory. Of course, fundamental
00:05:40.560 physics in any conception in any branch is about universal laws, but research in astrophysics
00:05:48.400 and cosmology, explaining a single phenomenon can involve several or all the fundamental laws.
00:06:00.480 Perhaps the first problem in physics that human beings ever try to solve out of sheer curiosity
00:06:08.240 namely the appearance of the night sky, why does it look as it does? It is remarkable
00:06:14.560 that even the crudest true explanation of that already requires all the fundamental laws
00:06:23.080 we know today to explain the basic fact that it is black and not white requires general
00:06:30.560 relativity. The colours of the stars, thermodynamics, why they don't go out, nuclear physics
00:06:40.560 and the aurora and thunder and lightning, many other phenomena, electricity and magnetism.
00:06:49.320 Confluence of fundamental laws in astrophysics and cosmology is a hint that there might
00:06:59.440 be a type of unity in nature that is deeper than the mere fact that there are universal laws,
00:07:08.440 namely that there might be a level of explanation of those laws. I first encountered that
00:07:16.440 idea from Dennis, but long before I even met him because when I was in school I'd read
00:07:24.080 his popular book The Unity of the Universe. Here it is, this isn't the original copy
00:07:33.640 I read then, that was a library copy. So, I borrowed its title for this talk.
00:07:44.320 Now, the subtitle, Man's Evolving View of the Cosmos from Ancient Greece to Mount Palomar.
00:07:55.960 Yeah, but I wasn't really interested in the history of cosmology, but I did love the book.
00:08:04.640 And a few years later, still before I'd met Dennis, I read it again and I was amazed
00:08:11.200 to find that it contained, among other things, a powerful advocacy of a false theory, the
00:08:21.760 steady state theory. That's the cosmological theory under which the universe is eternal,
00:08:28.120 has always existed, will always exist in its present state on the cosmological scale.
00:08:37.400 That theory, I knew, had been comprehensively refuted by observations long before I read
00:08:44.720 the book, but I hadn't even noticed that the steady state theory was in the book. In other
00:08:54.560 words, what was in a sense the main thesis of this book had entirely passed me by when
00:09:01.000 I read it. And that was because it wasn't the main thesis. The real theme of this book was
00:09:10.040 what the title says, the unity of the universe. And that unity, as I said, wasn't just
00:09:19.000 this. It wasn't just this. It was this, a unifying principle that would explain something
00:09:36.040 about, not everything, but something about why the laws of nature are as they are. The
00:09:42.920 principle in question in the book was a very natural guess for a cosmologist to make. They
00:09:49.720 called it the perfect cosmological principle. It said simply that the universe on cosmological
00:09:58.360 scales is homogeneous in time. That sounds good because there was already an ordinary cosmological
00:10:08.880 principle that said it's homogeneous in space. And that one is true, as far as we still
00:10:15.880 know. To say a word about principle, physics terminology isn't actually standardized in regard
00:10:27.600 to which laws of nature we call principles and which we just call laws. I'm using the term
00:10:34.240 principle specifically to mean a universal law about universal laws. So the perfect cosmological
00:10:43.240 principle, it placed a constraint on the other laws of physics. It didn't fully explain
00:10:49.200 them, but it would have placed constraints on all the other laws of physics as they were
00:11:02.640 then known. So that, for example, there could be the creation of matter out of nothing,
00:11:08.080 so that the density of the universe could remain constant, even though it's expanding.
00:11:13.600 And the total entropy could remain constant, even though stars were burning their fuel and
00:11:19.120 so on. Now, despite being totally false, this principle has some desirable features that
00:11:30.960 make the steady state theory a good theory intrinsically. And the first of these, of course,
00:11:37.760 most famously, is that this principle made the theory highly falsifiable. To conform to it,
00:11:46.560 the laws of motion and the various parameters, constants of nature, had to be just so to
00:11:53.280 make it, done right. So all the constants of nature were
00:12:23.240 going to have to be exactly right to make it happen that things like galaxies swirling
00:12:29.960 through the intergalactic gas would cause density variations, which would just result
00:12:38.720 in the form in the later in the formation of fresh galaxies of just the right size and
00:12:43.920 type containing stars of just the right composition and so on to reproduce, maybe a billion years
00:12:51.800 later of something, the all the statistics that have existed forever, which makes the
00:13:00.760 principle itself hard to vary, which makes it an intrinsically good explanation. And one
00:13:09.240 consequence of that was that it was strongly falsifiable by observation. And indeed, it was
00:13:15.080 duly falsified, for instance, by cosmological standards, light actually travels very slowly,
00:13:24.600 so that when we look out at something very far away, we're actually seeing how it was in a distant
00:13:30.680 past. So if the universe is homogeneous in space and time, then very distant vistas on the universe
00:13:42.040 should look very much like the universe looks here, or nearby here, and so astronomers looked
00:13:51.000 and it didn't look the same. And then there was the famous discovery of microwaves,
00:13:56.680 pervading space, and microwaves don't last in an expanding universe. They get red or in red
00:14:02.760 up rather like the Doppler effect. And so to maintain a steady state, they'd have to be replenished
00:14:10.200 and to fix that up required ad hoc modifications so nasty that it made the theory a bad explanation
00:14:19.960 after all, especially as its rival, the Big Bang Theory, did have an elegant and hard to vary
00:14:28.120 explanation for the microwaves. Now, I should say I'm drawing a distinction here between
00:14:35.720 a true explanation, which means objectively corresponding to reality, and being an intrinsically
00:14:47.080 good explanation, which is a transient property depending on the state of other knowledge at the
00:14:53.880 time, it's the property, as I said, of being hard to vary while still accounting for the
00:15:02.120 things it purports to explain. The Big Bang Theory in the steady state theory,
00:15:20.280 another mute button. The Big Bang Theory in the steady state theory were both very good
00:15:28.840 explanations because they were both severely constrained by other good explanations and by evidence.
00:15:37.880 One of the two was false, which is why Dennis went to work on the other. But that idea,
00:15:49.080 this idea, that there are universal principles which at least partially explain
00:15:55.640 the universal laws, which in turn explain the phenomena, was not overturned by the observations.
00:16:04.600 Only the particular principle that had as it were auditioned for that role, the perfect cosmological
00:16:11.640 principle, that had been overturned. Now, the second nice thing about the steady state theory
00:16:19.480 was the way in which it dealt with the initial conditions of the universe. Now, this may seem
00:16:27.960 like a technicality, but it isn't. It's quite fundamental conceptually. You see,
00:16:36.120 ever since the time of Galileo and Newton, the prevailing conception of how theories are supposed
00:16:43.960 to explain the world is that they provide laws of motion, which, given the state of the world
00:16:52.200 at any one time, predict or contradict it at any other time, or its probability at any other time,
00:16:59.480 but never mind that. The awkward fact is that while we have superb theories about what the laws
00:17:08.200 of motion are, we have never had a successful theory specifying the initial conditions.
00:17:16.120 And it's awkward because in the prevailing conception, the state of the universe,
00:17:22.120 what actually happens at all times and at any time, is the very thing that science sets out
00:17:29.080 to explain. So it's at least as fundamental as the laws of motion. We would like to explain it. In
00:17:38.280 the classic Big Bang theory, the initial conditions were that the state of the universe was spatially
00:17:43.880 homogeneous across an initial singularity that was causally extended, even though it was zero
00:17:52.760 in size. But that couldn't be exactly right, because if that were the exact initial state,
00:18:00.360 then nothing would ever happen. What starts exactly homogeneous stays exactly homogeneous under
00:18:07.000 the laws of motion. So there were various ideas, maybe quantum fluctuations spontaneously break
00:18:15.080 this imagery. But there was never actually a viable theory that predicted the details of the
00:18:22.920 in-homogeneity such as would lead to galaxy formation under things we observe. Roger Penrose had
00:18:33.160 the elegant idea that the vile curvature is zero at the beginning of the universe and maximum at
00:18:38.920 the end. But that doesn't seem to have been fruitful either. And today's prevailing theory,
00:18:46.680 which is called inflationary cosmology, is actually worse in that respect because it doesn't even
00:18:52.440 address initial conditions. That doesn't mean that inflation didn't happen. It just means that
00:18:59.160 it doesn't by itself solve the explanatory problem about the initial conditions that it was intended,
00:19:06.040 I think, to solve. You could always fudge these type of initial condition questions by resorting
00:19:17.640 to the anthropic principle, namely that there are lots of universes with all possible initial
00:19:23.560 conditions and that we're in one of the few in which astrophysicists exist to ask what the initial
00:19:30.920 conditions were. But if that were the only thing explaining the initial conditions, it would predict
00:19:38.520 that it's overwhelmingly likely that we're living in a bubble of order,
00:19:45.880 which is going to be snuffed out nanoseconds from now. And so it's refuted.
00:19:52.920 But there's another niggling problem, or I could say extremely fundamental problem,
00:20:02.840 depending on your outlook, with the initial states being one of the basic explanatory ideas
00:20:11.000 from which other explanations ought to be derived. There's no reason for anything else that we
00:20:20.920 know about physics that singles out the initial conditions as being preferred.
00:20:28.120 And all incidents are also predictably equal. In fact, the idea that the initial conditions
00:20:37.960 are special in the scheme of things has uncomfortable echoes of a pre-scientific
00:20:44.920 conception of what the physical world even is. See, there's a moment of creation
00:20:52.920 before which the physical universe didn't exist. Then the initial conditions are set by something.
00:21:04.600 And then, beautiful laws come into operation from which everything that subsequently happens
00:21:10.360 emerges. No wonder some people took the big bang theory as vindicating creationism
00:21:21.080 while other people, for the same reason, didn't want the big bang theory to be true.
00:21:29.240 Well, the steady state theory would have elegantly solved all those problems at once,
00:21:35.000 though it doesn't contradict the prevailing conception. It does radically augment it.
00:21:43.560 The state of the universe would now be deduced, at least in principle,
00:21:49.880 not from conditions at any preferred time, but from the perfect cosmological principle itself.
00:21:56.680 Since they'd also be the conditions that every other time no instant would be preferred,
00:22:05.240 and symmetry would not be broken, and even the size and character of the deviations from
00:22:12.680 homogeneity would have been determined by the principle. Nice, isn't it? But not true as it turned out,
00:22:23.400 which brings me to the third inherently nice thing about the steady state theory.
00:22:32.920 The perfect cosmological principle introduces a new mode of explanation into physics,
00:22:40.680 which supplements the prevailing conception. It doesn't only relegate the initial conditions
00:22:55.000 to being a mere consequence rather than fundamental principles. It also requires
00:23:03.320 that the laws of physics be fine-tuned to make a particular thing happen,
00:23:11.240 namely the steadiness of the steady state. In fact, that makes it much more fine-tuned than you might
00:23:17.560 think because, well, the steady state people were aware that their theory wouldn't work
00:23:26.680 if the process that reproduced the state over time were not also stable, because if it were unstable,
00:23:36.680 say if a small deviation from ideal steadiness produced a larger deviation, let's say a million
00:23:44.840 billion years later, or a trillion years later, or 10 to the 100 years later, then after a certain
00:24:00.200 number of cycles, the state would no longer have the steadiness property, and so it had to be
00:24:08.520 that a small deviation would be reversed in due course, that stability. They work hard to construct
00:24:16.840 their cosmological model to have that property, but stability is not enough if you want to make
00:24:23.720 the universe eternal. However stable the state is to small changes, a large nudge will eventually
00:24:32.360 happen given the normal assumptions of statistical mechanics, and even quantum mechanical tunneling
00:24:40.360 would eventually have the same effect, and so the steady state would degenerate, degenerate,
00:24:49.000 into the far larger realm of states that evolve with time into other states, thus violating the
00:24:58.280 principle. So the perfect cosmological principle would have required the quantum state of the
00:25:04.440 universe to be exquisitely pruned to eliminate what Bryce Dewitt called the maverick universes
00:25:14.840 that would evolve to violate the principle at any time in the future. The specialness of that state
00:25:24.200 in the view of the opponents of the steady state theory was just creationism. The entire
00:25:33.000 eternal universe created all at once in a state finally crafted to give the appearance of an
00:25:40.680 evolving textured mixture of structure and randomness, but actually all along wreaked to conform
00:25:51.160 to a certain ideal throughout its infinite extent. So here we had the proponents of two rival
00:25:59.480 theories, each accusing the others of in effect creationism, while other people were delighted
00:26:08.680 that their favorite theory brought meaning as they saw it back into physics, into the universe.
00:26:16.920 But this was all misconceived. Everyone was simply assuming that all fundamental explanations had
00:26:27.320 to be in the form prescribed by the prevailing conception, possibly with the addendum of the
00:26:36.680 perfect cosmological principle. Now it is possible to eject to the whole idea of principles in
00:26:44.440 nature, in that sense. Laws about laws. Can't we confine ourselves to laws about phenomena?
00:26:53.000 Is it possible to restrict science to those laws and reject laws about laws?
00:27:02.680 Well, here's an object lesson. The principle of the conservation of energy started out as a
00:27:11.960 mere law about phenomena. In fact, less than the law, it was merely a mathematical theorem
00:27:18.680 of Newtonian mechanics, initially that just for a system of particles moving in space without
00:27:25.960 friction and with elastic collisions, the quantity half mv squared summed over all the particles
00:27:33.480 is a constant. We now call that the kinetic energy of the system. But the theorem
00:27:44.280 was known for centuries before the concept of energy was even conceived and it wasn't necessary
00:27:51.080 at that time. In the meantime, people realized then that if you add that quantity to what we now call
00:27:58.200 the gravitational potential energy, which is minus gm1m2 over r squared, then the result is a
00:28:09.640 constant even if they're gravitating particles. But it still won't be true if there's friction,
00:28:14.680 for example. But now that's a theorem of Newton's law's emotion and gravity.
00:28:21.400 It still has strictly no more content than those laws themselves. In fact, less.
00:28:29.320 And it applies only when those laws are the full explanation of what is happening.
00:28:36.760 Every prediction of Newton's theories can be made without any reference to energy,
00:28:42.680 without even knowing of the existence of energy or of its conservation. And in particular,
00:28:48.760 those theorems predict nothing about the content of undiscovered laws of physics.
00:28:58.200 So that, and they didn't call it energy yet, but if they had, their energy would not have been
00:29:03.160 the energy that we know, nor is its constancy under those theorems, the conservation law that we know.
00:29:11.320 But then, in the 19th century, after Kant-Rumford's experiments on canon
00:29:24.840 that got hot when they were being drilled out, people guessed that if you add a further term to
00:29:32.200 that Newtonian scheme of summing half m v squared and gm1 m2 over r minus that, you can add
00:29:43.560 you can also add the Newtonian work done. And if you add to that an expression for the heat
00:29:52.200 normalized with suitable units, then the total will now be conserved even if there is friction.
00:29:59.640 And now, you have something that isn't a theorem. It's not deduced from laws of motion or
00:30:10.120 indeed anything. It's a law of physics in its own right, the law of the constellation of energy.
00:30:18.280 And indeed, it couldn't have been deduced because the laws of motion that underlie frictional
00:30:25.000 processes were still unknown at the time. In fact, I'm not sure they're known today,
00:30:32.360 but if they are, they're quantum mechanical. And at that point, people tweaked,
00:30:40.280 they realized that this new law now, to make sense, had to be respected by even by
00:30:50.360 as yet unknown forces and unknown substances. It was a law about laws, the principle of the
00:30:59.720 conservation of energy. And that is exactly when the term energy was invented.
00:31:10.360 So the key, that's the law's conservation of energy expressed with this in mind.
00:31:15.960 And the key word there is not energy. It's every. And when the theory of electromagnetism
00:31:26.600 was later invented, it did indeed conform to this principle even though electromagnetism hadn't,
00:31:35.240 theory of electromagnetism also was not known at the time when the principle was invented.
00:31:40.440 Furthermore, thermodynamics was born with several further principles about heat and work
00:31:49.160 and temperature and a new quantity entropy. And none of those principles were deduced from
00:31:55.160 laws of motion. In fact, many attempts have been made in the century and a half or so
00:32:01.720 since the inauguration of the thermodynamics to establish a connection between
00:32:08.360 between those laws and laws of motion or somehow to express thermodynamics within the prevailing
00:32:16.200 conception. And none have been satisfactory. They all involve fudges such as coarse graining
00:32:26.360 and infinite ensembles. And even the exact distinction between work and heat remains elusive to
00:32:36.680 this day. Then of course, still later, the 20th century in the early study of radioactive beta decay
00:32:46.360 when physicists added up the kinetic energies and the MC squared energies in radioactive decays
00:32:53.240 and finding that they didn't add up to a constant, powerly and fermi could guess that there was
00:33:02.520 just a hitherto unknown particle, the neutrino, for whose existence at first, the only evidence was
00:33:12.680 that principle. And again, that version of the conservation of energy couldn't possibly
00:33:19.400 regard it as a theorem whose premises were the neutrinos laws of motion and interaction
00:33:26.360 because those laws were not yet known. It was just predicted that once they were known
00:33:33.960 they would be found to obey the principles of quantum mechanics which by now included the principle
00:33:40.360 of the conservation of energy. And so they did. And that prediction was an indispensable guide
00:33:51.720 to discovering those laws at all. So the rule of restricting science to laws about phenomena
00:34:04.760 and rejecting laws about laws is untenable. Note that that rule is itself a principle,
00:34:15.320 the anti-principled principle. And as we've just seen, it's false.
00:34:27.320 More generally, I think the whole purpose of theoretical science is to explain the world,
00:34:34.920 the physical world, and therefore the sole criterion by which theories ought to be judged
00:34:41.240 is their explanatory power. This rules out having preferences between modes of explanation,
00:34:51.240 preferences that if those preferences are independent of how good the explanations are,
00:34:56.440 that should be what counts, the only thing that counts. So just as a scientific theory about
00:35:05.160 phenomena is much more than just an instrument of prediction of those phenomena,
00:35:11.880 much more than just a compressed summary of them, but is an explanation of them.
00:35:20.280 So a principle of nature is not just a statement of shared properties among theories,
00:35:26.280 it is an explanation of those properties. Now the prevailing conception is a principle too, isn't it?
00:35:40.600 And I believe it's just as false as I'll explain in a moment.
00:35:47.960 So what other principles of nature might be true aside from those of thermodynamics that I've
00:35:57.080 mentioned? Well, both quantum theory and relativity are partly principles that in addition to
00:36:04.680 making direct predictions about phenomena, they also assert that all other laws of nature,
00:36:11.320 including each other, conform to certain principles such as the principle that laws are formulated
00:36:21.480 in terms of geometrical objects in the case of general relativity, and in quantum theory the
00:36:27.400 principle of unitarity. And as those two examples illustrate, we shouldn't expect there to be
00:36:37.400 a rigid hierarchy of principles with ordinary laws being subordinate to principles,
00:36:44.040 but we should expect that the immense explanatory power of some of our theories, of our best theories,
00:36:53.720 implies that if they are true of some physical systems, they must be true of all of them.
00:37:01.240 I think it was Feynman who called this, in the case of quantum theory, the totalitarian property
00:37:07.800 of quantum theory, which, I think, Bryce DeWitt proved the same thing, and I think independently,
00:37:23.240 that if any system in the universe is governed by quantum theory,
00:37:29.000 then no system that could interact with that one could obey classical laws of motion.
00:37:36.520 Relativity isn't quite as totalitarian as quantum theory, but its principles do seem to be
00:37:42.200 inconsistent with those of quantum theory, so presumably one or both of them must be superseded.
00:37:49.400 Something that we couldn't know unless we regarded those two theories as principles.
00:37:55.000 Now, for example, they might just apply to different phenomena.
00:38:05.480 Now, the way in which DeWitt, in particular, proved the totalitarian property,
00:38:12.600 I'm not sure how Feynman did it, is quite significant from my present perspective.
00:38:17.800 He used the so-called uncertainty principle of quantum mechanics, horribly misnamed,
00:38:25.480 and he assumed for the sake of argument that it did only apply to quantum systems,
00:38:32.920 and that classical objects could exist in nature too, and could interact with some quantum system.
00:38:40.360 And then he showed by an ingenious set of arguments that by making certain measurements,
00:38:48.040 one could violate the uncertainty principle, not just for the combined system,
00:38:56.920 So loosely speaking, quantum theory either applies to everything or to nothing.
00:39:02.040 And the reason that that mode of proof is significant to me now
00:39:10.760 is that it uses the uncertainty principle in the form such and such a class of tasks is impossible.
00:39:21.400 And if a certain physical object is possible, the classical object,
00:39:26.760 then a further process would be possible that would lead to a contradiction.
00:39:32.920 And therefore, if the principle is true, the classical object isn't possible.
00:39:39.720 He expressed the proof in the prevailing conception, but it's barely used as such.
00:39:45.640 You see, saying that a given task is impossible in the sense that the uncertainty principle does
00:39:56.760 means that it, not just that it doesn't happen, but that it can't be caused to happen by anything.
00:40:04.040 Can't be caused with the help of anything else.
00:40:07.320 Even things not explicitly referred to, things not yet known,
00:40:18.680 So while ago, I proposed a new mode of explanation that Philip preferred to construct a theory
00:40:26.120 or rather a theory called construct a theory, which I hope will incorporate this new mode of
00:40:32.120 explanation, which is intended eventually to supersede the prevailing conception,
00:40:39.400 though the two are inter-translatable in many cases.
00:40:45.320 The first principle of construct a theory is this.
00:40:52.360 The laws of physics are expressible entirely in terms of statements about which
00:40:58.120 physical transformations it's possible to cause to happen and which are impossible to cause
00:41:05.320 and why. So this is about transformations that are caused by something,
00:41:15.080 some agent which is itself not specified, that any agent
00:41:22.760 except that this agent must itself be possible.
00:41:26.360 And there's a condition that the agent retained its ability to cause the transformation again.
00:41:31.240 Otherwise, it's only partly an agent and partly a patient.
00:41:36.840 Chemical catalyst is an example of such an agent.
00:41:41.160 It causes chemical reactions but does not participate in them.
00:41:46.440 By the way, we're told in elementary chemistry classes that it doesn't cause
00:41:51.160 chemical reactions, it just changes their speed, but that is not the case.
00:42:01.720 It converts things to have different temperatures and so on, but it itself stays the same.
00:42:07.640 So as a computer, we call these agents generically constructors.
00:42:15.080 By possible to cause, we mean possible with arbitrary accuracy.
00:42:20.920 That is, you give me an epsilon and if a task is possible, you give me an epsilon and someone
00:42:27.080 could design a constructor which causes that task to happen with accuracy, epsilon, or better.
00:42:35.320 And impossible means that the laws of physics exclude the possibility that anyone could ever produce
00:42:43.640 such a design or the laws of physics rule out the existence of such an agent, such a constructor.
00:42:52.360 So there are no probabilities in the constructor theoretic conception of the world.
00:42:58.360 Task that look probabilistic like building a fair rule at wheel,
00:43:03.320 I expressed in terms of preparing it in a specified quantum mechanical state with a given
00:43:11.240 So while the prevailing conception seeks to distinguish at a fundamental level what happens
00:43:21.720 from what doesn't happen, in which case possible and impossible are just a manner of speaking
00:43:29.400 about certain approximations or about our ignorance, but in constructor theory, it is the other
00:43:36.760 way around. The laws of nature are about what's possible and impossible, in the sense I've
00:43:42.200 just described. And what actually happens is in general an emergent consequence of that. Sometimes
00:43:49.480 it can be calculated, in which case the constructor theory and the prevailing conception are
00:44:00.440 equivalent, but sometimes it can't be calculated either because it's intractable or for some more
00:44:07.160 profound reason. And in those cases, constructor theory can express exact laws that are inaccessible
00:44:21.000 One important case of the latter are initial conditions of the universe.
00:44:27.160 They are in constructor theory, they are supposed to be incalculable consequences of laws about
00:44:44.680 As I said, you wouldn't expect there to be a fundamental law specifying the state at any other
00:44:52.600 time than the initial time, such as today, including all the locations of all the cows in
00:45:00.360 Oxfordshire that were auctioned today, you wouldn't expect the state of those cows to have
00:45:05.800 fundamental significance. Why expected of the initial state, especially as it,
00:45:14.280 this violates symmetries that exist everywhere else in physics.
00:45:18.840 And with that constructive theoretic perspective, we can begin to notice that there
00:45:26.760 are already other principles of nature that are already known, but are not usually acknowledged
00:45:33.560 as such, nor even acknowledged as being part of physics at all, simply because they don't
00:45:43.240 There's the principles of the theory of computation, for example.
00:45:49.160 The distinction between computable and non-computable functions doesn't refer to what the
00:45:55.000 computers made of. We expect it to be the same for any make or model or technology of general
00:46:02.520 purpose computer, even ones using laws of physics or materials not yet discovered.
00:46:11.160 So it's a principle, difficult or impossible to express in the prevailing conception.
00:46:20.520 But in some work that my colleague Kiara Maleto and I have done,
00:46:26.360 we have shown that there is a beautiful expression of this principle in constructor theory.
00:46:33.800 And this is in the context of a full constructor theoretic information theory
00:46:39.800 in which processes like computation and quantities like information are characterized
00:46:48.040 in elegant exact terms. That is, in constructive theoretic terms, in terms of what
00:46:59.080 classes of physical transformation it's possible or impossible to cause.
00:47:03.720 This new theory of information, which I commend to you all, unlike Shannon's existing theory,
00:47:14.440 naturally includes quantum information and predicts all its strange and distinctive properties,
00:47:21.880 such as the impossibility of cloning, a quantum, the information, the quantum state,
00:47:26.600 and the famous unpredictability of quantum measurement despite its deterministic law of motion.
00:47:36.520 And Maleto is also used to construct a theory in a biological application
00:47:41.640 to characterize what precisely it is about the laws of physics that permits the origin and
00:47:49.320 evolution of life. Among other things, the apparently non-physics concept of the appearance of
00:47:58.440 design, which was coined I think by Richard Dawkins, has an exact definition in constructor
00:48:05.320 theoretic physics. The result is that regardless of the so-called fine-tuning coincidences
00:48:14.120 in the constants of physics, the laws of physics do not in fact have and do not in fact have
00:48:29.000 The laws of thermodynamics which I've mentioned already have some
00:48:33.640 existing constructive theoretic formulations, like you can't build a perpetual motion machine
00:48:39.560 of the first kind or of the second kind and so on. But these are considered vague and hand-waving
00:48:47.800 in the prevailing conception. Another example you can't convert heat entirely into work without
00:48:56.040 side effects. But if this further worked by Maleto pans out, it would revolutionize the
00:49:06.120 foundations of thermodynamics because with slightly different versions of the first and second
00:49:11.560 laws from ones we know, it would express those known hand-waving formulations exactly and would
00:49:21.800 provide an exact characterization of the distribution between work and heat and hence of entropy
00:49:28.200 without coarse graining, without distinction between macro states and micro states, without
00:49:34.120 ensembles, just constructive theory. The basic reason that constructive theory can work
00:49:45.480 that sort of magic is that it abstracts away the constructor. Like the theory of catalysis
00:49:54.520 in chemistry, which is another as I said, another example of an existing constructive theoretic
00:50:00.040 theory. It's not this process that is declared to be possible or impossible in constructive
00:50:10.600 theory. It's this, just that. So the constructor, which is the usually the macroscopic
00:50:26.840 part of the process, is abstracted away and this is what makes constructive theory a natural
00:50:34.360 vehicle for expressing scale invariant laws and substrate invariant laws about quantities like
00:50:42.040 information and heat and work exactly. Like the perfect cosmological principle,
00:50:53.640 which had to be developed into the sophisticated steady state theory, constructive theory will
00:51:02.600 have to be developed quite a bit more before we derive testable predictions from it.
00:51:09.480 But unlike steady state theory, constructive theory has already provided a significant
00:51:15.720 unification and illumination of fundamental matters in diverse areas of physics and beyond, as I
00:51:25.000 said. And I think this already makes it a substantial step towards the unity that Dennis was looking
00:51:56.920 And so to mark the closing of this wonderful series of lectures, I've been asked to present
