00:00:15.840 Okay, David, you've spent the last six or seven years, I believe, working on this radical
00:00:21.840 proposal for the reformulation of fundamental physics in terms of something that you call
00:00:26.480 constructed theory. So I'd like to know what on earth constructed theory is and how you got us
00:00:32.480 out thinking about it. The way it began was actually significantly different from the way we think
00:00:39.840 about it now. I had been working on the quantum theory of computation and information and at the time
00:00:48.000 I thought of that as like the fundamental theory underlying all physics in a sense because
00:00:53.760 the universal quantum computer can simulate the behavior of any other physical system and therefore
00:01:02.000 in a sense the whole of physics that the behavior of all physical systems is just the study of that
00:01:08.720 is the same as the study of the programs of a universal quantum computer. But then I realized that
00:01:17.520 that was actually inadequate that actually a quantum theory of computation isn't, doesn't underlie
00:01:23.360 everything because when you want to study the behavior of a particular physical system and particular
00:01:31.200 physical laws that that system obeys, you have to know which quantum computer program corresponds
00:01:40.160 to that and that's that theory of which which is which kind of that's what really corresponds
00:01:47.680 to the whole of physics. So I said about trying to extend the quantum theory of computation
00:01:55.680 in a way which would encompass that as well. But more I did that the more I realized that the
00:02:03.280 conventional way of understanding computation and physics was inadequate to this task, the
00:02:13.360 conventional way being that you have initial conditions which for a computer corresponds to that
00:02:19.680 program and you have laws of motion which in a computer corresponds to the elementary
00:02:26.400 operations by which the computer works. And then between those they tell you what happens at all
00:02:32.960 times. But that wasn't adequate to describe simple things like information in more general terms
00:02:43.680 than in the quantum theory of computation. So then I gradually realized that you what what's needed
00:02:51.520 is just a new approach to fundamental physics as well as to fundamental computation.
00:02:57.200 The idea there is that instead of having initial conditions and laws of motion which then
00:03:03.840 tell you everything which everything that happens instead you you you step back a bit and you
00:03:11.680 have a theory of what can and can't happen what's possible and impossible. And what actually
00:03:17.760 happens is then just an emergent property of what can and can't happen. So one of the motivations
00:03:30.240 for constructor theory is that it enables you to bring these things like computation and life
00:03:36.400 and information and thermodynamics into fundamental physics. And I wonder if you could talk a
00:03:42.000 bit more about those areas and how they end up in your picture. Some of these emergent things
00:03:47.600 are are are emergent and in exact in fundamental physics not because they are in nature
00:03:54.080 but because fundamental physics can't deal with them intrinsically can't deal with them exactly
00:03:58.720 and ironically or appropriately depending on which way you look at it. Computation itself is one
00:04:05.360 of those things because to define a universal computer which is at the center of any kind of
00:04:13.360 computation theory. You can't define it by just what it does what it does in fact in reality
00:04:21.360 because universal refers to all the things it could do. And the theory of computation is basically
00:04:28.960 the delineation of the difference between the things it can do could do and the things that it
00:04:34.320 couldn't possibly do the non-computable functions the the the the undecidable problems. And that
00:04:41.360 partition with the benefit of hindsight is is central to the theory of computation as well
00:04:49.440 including quantum computation but it wasn't thought to be central to physics. So that's one of
00:04:54.720 the things that we're taking over from computation to physics. The idea that that what really
00:05:01.680 matters is the distinct for understanding a physical system is understanding what it can't can
00:05:07.200 and can't be made to do not just what it can and can't do what it can and can't be made to do.
00:05:12.960 Right. And what it actually does is kind of contingent that that's not as important. It's an
00:05:19.920 emergent property. It's the other way around from the way physics is normally construed.
00:05:24.400 Okay so if I understand you correctly in theories of computation you work from kind of a few
00:05:36.000 axioms about the kinds of things computers can do through to proofs of what programs are possible
00:05:42.160 or impossible. Yes. And that's really what you're trying to extend to physical reality. That's why
00:05:48.320 constructed theory is a theory of principles from which you derive the rest. Do I have that correct?
00:05:59.120 There's famous quotation by by the physicist Edington who said that the laws of
00:06:04.560 thermodynamics, especially in the second law, are more fundamental than anything else in physics.
00:06:10.800 He said, you know, if your theory violates the law conservation of energy or whatever,
00:06:15.600 then you will say so much the worst for the law of conservation of energy. But if it
00:06:20.000 violates the second law of thermodynamics, then there's nothing left to do for you to
00:06:23.760 job with raw and shame. So something like that. Okay. And therefore in some sense that the
00:06:29.600 thermodynamic laws are deemed to be more fundamental. But in another sense, in the sense of the
00:06:39.680 way that theories in physics have been built up, it's not only is it not fundamental, but it
00:06:45.200 can't belong to fundamental physics at all. It is deemed to be inherently approximative, inherently
00:06:52.080 emergent. And to the extent that the moment you describe a system exactly, in other words, according
00:07:00.000 to the laws of physics according to its laws of motion, you lose the opportunity to describe
00:07:05.040 thermodynamics on that system. Okay. Now, this is an artifact of the initial conditions plus
00:07:13.200 laws of motion conception of physics. And it just goes away in constructed theory. And
00:07:19.840 Kiramaletto has produced a very nice foundational theory of thermodynamics based on an old theory
00:07:32.320 of caratidori, which again, no one knew how to incorporate into fundamental physics. But everyone
00:07:38.720 was saying, oh, yeah, this has got to be the way to go, but they didn't know how. And I think that
00:07:42.560 she has managed to do it. Okay. And it's because of this simple different, apparently simple
00:07:48.080 difference between how construct a theory views the foundations and how the prevailing conception
00:07:55.280 views them. Okay. But I'm always baffled by the motivation for that, because what if it's just the
00:08:03.680 case that thermodynamics is an emergent phenomena? And whatever space do we have that requires moving
00:08:11.520 into fundamental physics? Well, all science is conjectural. It could turn out that thermodynamics isn't
00:08:17.920 fundamental. It could turn out that construct a theory's false. But we have these clues, like I said,
00:08:24.240 did Edington conception and also the fact that there are various clues from different parts of
00:08:32.000 physics that strongly suggest that thermodynamics is a fundamental part of nature. One of them is
00:08:39.680 black hole physics, Stephen Hawking, and before him, Jacob Beckenstein, showed that a black hole
00:08:49.360 has thermodynamic properties, such as entropy, which the black hole entropy, however, is an exact
00:08:57.680 property of the black hole. It's equal to just basically a constant times the mass of the black hole.
00:09:05.120 And this entropy can appear in thermodynamic equations along with normal entropy, and yet the
00:09:12.400 normal entropy is approximative, and the black hole entropy is exact. So it's not, so some people say
00:09:17.760 it's not really entropy is just pseudo-entropy. But the thing is that the thing that has to increase
00:09:23.280 is the sum of the black hole entropy and the normal entropy, not just the normal entropy.
00:09:28.560 Okay. So it really fits into the laws of thermodynamics in an integral way. So that's another
00:09:34.640 clue. I mean, again, this could be an anomaly that's explained in dozens of different ways.
00:09:41.760 Somebody could come up with a different theory. You could say that the black holes don't have
00:09:45.840 real entropy or, you know, in a million different ways. But it's suggestive, and like all
00:09:53.600 motivations for theories, they are not proofs. They're not even proofs that the theory is needed,
00:09:59.600 or that f theory is needed. But they are starting points for a research program.
00:10:05.600 Right. And when you find that there are several different ones of these motivations leading to the
00:10:11.920 same conclusion, and that conclusion is simple. Then, you know, you're led into studying it
00:10:18.640 with the U.L. Fair enough. Fair enough. Another such motivation has to do with the
00:10:31.440 the limitations of what Lee Smolin calls the Newtonian paradigm, are you aware of that phrasing?
00:10:36.160 No. What he says that, as you say too, I think, that when you analyze the universe in terms of
00:10:44.960 initial conditions and laws of motion, that rules out explaining what the initial conditions
00:10:51.520 were, because you can't explain the initial conditions in terms of a model which includes
00:10:55.600 the necessity of initial conditions. Yes. And construction theory might be able to do that,
00:11:00.560 you believe. Oh, yes. Well, if constructive theory works, it will have to do that.
00:11:05.280 Okay. Because when I said that what actually happens will emerge as an emergent property,
00:11:13.440 that includes the initial conditions. So, constructive theory is based on the dichotomy between
00:11:22.160 tasks that are possible and tasks that are impossible. Such as, the possible ones might be,
00:11:29.440 might include the building of a universal computer. Okay. And the impossible ones would include
00:11:37.040 the building of a perpetual motion machine of the first kind or the second kind. Okay. Okay.
00:11:41.600 But now, if we focus on possible, on the possible ones, a task being possible means that the
00:11:48.560 building constructor to perform the task is possible, which means that there must be raw materials
00:11:55.040 suitable for building such a constructor, which means that the initial conditions of the universe
00:12:00.080 must be such as to provide such raw materials, not just once, but generically, because possible
00:12:07.520 inputs will have to be universal concepts. So, generically, we have hydrogen on protons and so on,
00:12:14.960 and it must be possible to get from that to whatever the theory says is possible. Okay.
00:12:22.640 So, therefore, if constructive theory is to be self-consistent as a universal theory of physics,
00:12:28.240 it has to explain initial conditions or be it only,
00:12:32.960 emergently, like it will discover laws of motion, emergently. Hi. Okay. I think I get it.
00:12:41.680 I've gotten quite comfortable thinking about an emergent phenomena
00:12:47.280 not being positive fundamental physics. So, there's a part of me that kind of resists this picture
00:12:50.960 you're painting. And one of your motivations for constructive theory is bringing something like
00:12:55.760 causation or possibility into fundamental physics. And the way I tend to think about causation,
00:13:01.440 and the reason that causation tends to just appear everywhere in the special sciences,
00:13:05.760 but not in fundamental picture necessarily, is that human beings are kind of extended objects
00:13:13.680 in space-time with memories, and we get to watch the way the universe behaves over time,
00:13:19.520 and we develop a concept of counterfactuals, right, because what we see well when the bird is
00:13:25.360 it's not plapping its wings, it's on the floor, when it is plapping its wings, it flies. And I suspect
00:13:34.560 that this is just a function of human psychology. The reason that the causes don't appear in
00:13:40.640 fundamental physics is that you just have the initial conditions and the laws of motion,
00:13:45.840 and you generate the blocking of verse by cranking the algorithm forward. Or backwards.
00:13:50.640 Or both ways from this present moment. Right, right, which is version Russell's worry about
00:13:56.240 why causation was a goner as far as physics and philosophy was concerned.
00:14:02.880 So have I made my point, do you sort of see what I'm grasping at, that causation might
00:14:07.440 just be visible to humans because of their vantage point as kind of temporarily extended
00:14:11.200 scenes with memories, who see things going one way or another, and therefore develop an idea of
00:14:15.920 the counterfactual, and it might not be necessary to put it in the fundamental picture.
00:14:23.040 I think you're worrying unnecessarily. First of all, it's not the project of constructive theory
00:14:31.200 to bring emergent quantities into fundamental physics. And in any case, it doesn't seem to be
00:14:37.440 on the horizon that all of them will be. It's just certain ones. And those certain ones,
00:14:43.920 in most cases, have already been problematic within fundamental physics.
00:14:48.960 Okay. So for example, you said we extended objects, we've all vast numbers of atoms,
00:14:56.480 and therefore we were interested in large-scale regularities, which are approximate.
00:15:04.640 Well, yes, but a computer is also one of those. And the fact that, say the Turing principle,
00:15:11.520 that a single machine can simulate every other machine is a drastic limitation on what the
00:15:20.640 laws of physics can be, or to put them another way, it is a principle of physics. And yet,
00:15:26.640 it talks about computers, which cannot be fundamental objects. They are always extended objects.
00:15:33.120 So unless we're going to throw away this insight about which unifies the whole of physics,
00:15:40.160 we've got to allow the Turing principle to be fundamental.
00:15:44.640 Okay. If it's fundamental, it should be integrated with the other laws of physics,
00:15:50.240 and of the moment it can't be. Good, I'm glad you said that, because it reminds me of something
00:15:53.680 else I wanted to ask, which is that, why are only some emergent macro phenomena
00:16:01.360 insights into fundamental physics and why not others? So if I want to know why
00:16:05.040 our having this conversation in this room isn't something by which we would have to kind of
00:16:10.800 constrain what the initial conditions of the universe are, or like, why is my scratching
00:16:16.560 my nose not a clue into fundamental physics in the way that the existence of a computer is?
00:16:21.760 The simple answer is that, in those cases scratching the nose sort of case,
00:16:25.280 our basic explanation, say that it's not happening in other branches of the universal wave
00:16:31.840 function, whereas the universal quantum computer is a thing that describes the whole of the
00:16:39.440 universal wave function. So it's inescapable in that sense. So that's one thing. I mean,
00:16:46.640 there is still such a thing as contingent facts and necessary facts. The necessary facts are
00:16:54.000 being the ones that figure in universal explanations. We don't expect universal explanations
00:17:01.360 to explain everything, not even everything at an atomic level, and certainly not everything
00:17:08.800 at an emergent level. I gather you a hopeful maybe still art that's
00:17:15.840 constructed theory will provide a key to understanding something like free will. Do you still
00:17:22.080 believe that? And if so, how would it do that? There's a whole cluster of emergent properties
00:17:29.040 or even philosophical properties of matter and of people and of the way things are that kind of
00:17:40.400 go together, free will, consciousness, knowledge, and now constructed theory has got a strong
00:17:49.840 handle on some of those. Right. And looks as though it might extend to the others.
00:17:56.480 So in that, counterfactuals of course, constructed theory is completely based on counterfactuals.
00:18:03.120 Right. So counterfactuals become almost the foundations of everything. And so explaining that
00:18:11.440 explained counterfactuals itself would be down to the next theory which will explain
00:18:15.520 constructive theory. However, those other things, since we have such a powerful theory of
00:18:20.960 information now, for example, within constructed theory, it seems and terminus and so on, it
00:18:29.920 seems that knowledge could be the next thing. And if knowledge then why not free will? If
00:18:36.320 knowledge and free will, why not consciousness? And if those why not artificial general intelligence?
00:18:42.400 Right. Right. So one can hope. That's fantastic. One of the things I've talked to
00:18:50.640 Kiara about a couple of years ago was done, then it's taken free. Well, are you aware of his
00:18:56.880 position? Yes. I think he's tied down by the prevailing conception of fundamental physics.
00:19:03.520 Right. Initial conditions plus laws of motion. That leaves not much room for these
00:19:11.200 philosophical and emergent things. See, those things have their intrinsic difficulty
00:19:17.440 of explaining what they are, how they work, and that kind of thing. But there's additional
00:19:21.920 difficulty caused by fundamental physics. And if you insist on, at least we know that they're
00:19:29.200 going to have to conform to this fundamental physics conception, then you're drastically
00:19:34.560 limited in what kind of a theory of those things you can form. Yes, yes. And in particular, I think
00:19:40.400 it really limits you to reductionist theories of those things. Whereas if construct a theory is
00:19:49.280 the fundamental theory, you're not limited in that way. And you don't have to arrive at any of
00:19:54.800 these conclusions like that, you know, that the self doesn't exist or, you know, that right thing.
00:19:59.760 I mean, it may or may not exist. It may be one of these emergent things that isn't fundamental
00:20:05.600 or it may be an illusion or not exist. But on the other hand, there's no reason to, if other
00:20:13.360 aspects of the problem seem to indicate that three words connected with counterfactuals and so on,
00:20:19.360 then there's no problem fundamental problem with accommodating that within fundamental physics
