2018-12-08 Joe Boswell Constructor Theory
Duration: 00:20:36
Transcript
Joe Boswell
Okay, David, you’ve spent the last six or seven years, I believe, working on this radical proposal for the reformulation of fundamental physics in terms of something that you call Constructor Theory. So I’d like to know what on Earth constructor theory is and how you got started thinking about it.
David Deutsch
The way it began was actually significantly different from the way we think about it now. Okay. I had been working on the quantum theory of computation and information and at the time I thought of that as like the fundamental theory underlying all physics in the sense because the universal quantum computer can simulate the behavior of any other physical system and therefore in a sense the whole of physics, the behavior of all physical systems, is just the study of that is the same as the study of the programs of a universal quantum computer. But then I realized that that was actually inadequate, that actually quantum computation doesn’t underlie everything because when you want to study the behavior of a particular physical system and particular physical laws that that system obeys, you have to know which quantum computer program corresponds to that. And that’s a theory of which is which, that’s what really corresponds to the whole of physics. Okay. So I set about trying to extend the quantum theory of computation in a way which would encompass that as well. But the more I did that, the more I realized that the conventional way of understanding computation and physics was inadequate to this task. The conventional way being that you have initial conditions which for a computer corresponds to that program and you have laws of motion which in a computer corresponds to the elementary operations by which the computer works and then between those they tell you what happens at all times. But that wasn’t adequate to describe simple things like information in more general terms than in the quantum theory of computation. So then I gradually realized that what’s needed is just a new approach to fundamental physics as well as to fundamental computation. And the idea there is that instead of having initial conditions and laws of motion which then tell you everything that happens, instead you step back a bit and you have a theory of what can and can’t happen, what’s possible and impossible. And what actually happens is then just an emergent property of what can and can’t happen. And that’s constructor theory.
Joe Boswell
Okay so one of the motivations for constructor theory is that it enables you to bring these things like computation and life and information and thermodynamics into fundamental physics. And I wonder if you could talk a bit more about those areas and how they end up in your picture.
David Deutsch
Some of these emergent things are emergent and inexact in fundamental physics, not because they are in nature but because fundamental physics can’t deal with them intrinsically, can’t deal with them exactly. Ironically or appropriately depending on which way you look at it, computation itself is one of those things. Because to define a universal computer which is at the center of any kind of computation theory, you can’t define it by just what it does, what it in fact in reality, because universal refers to all the things it could do. And the theory of computation is basically the delineation of the difference between the things it can do, could do, and the things that it couldn’t possibly do, the non-computable functions, the undecidable problems. And that partition with the benefit of hindsight is central to the theory of computation as well, including quantum computation. But it wasn’t thought to be central to physics. So that’s one of the things that we’re taking over from computation to physics. The idea that what really matters is the distinction for understanding a physical system is understanding what it can 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. And what it actually does is kind of contingent. That’s not as important. It’s an emergent property. It’s the other way around from the way physics is normally construed.
Joe Boswell
Okay. So if I understand you correctly, in theories of computation, you work from kind of a few axioms about the kinds of things computers can do through to proofs of what problems are possible or impossible.
David Deutsch
Yes.
Joe Boswell
And that’s really what you’re trying to extend to physical reality.
David Deutsch
That’s why constructor theory is a theory of principles from which you derive the rest. That’s exactly right.
Joe Boswell
Okay, great. There’s a famous quotation by the physicist Eddington who said that the laws of thermodynamics, especially the second law, are more fundamental than anything else in physics. He said, you know. if your theory violates the law of conservation of energy or whatever, then you will say so much the worst for the law of conservation of energy. But if it violates the second law of thermodynamics, then there’s nothing left to do but for you to withdraw in shame, something like that.
David Deutsch
Okay. And therefore, in some sense, the thermodynamic laws are deemed to be more fundamental. But in another sense, in the sense of the way that theories in physics have been built up, it’s not only is it not fundamental, but it can’t belong to fundamental physics at all. It is deemed to be inherently approximative, inherently emergent. And to the extent that the moment you describe a system exactly, in other words, according to the laws of physics, according to its laws of motion, you lose the opportunity to describe thermodynamics on that system. Okay. Now, this is an artifact of the initial conditions plus laws of motion conception of physics. And it just goes away in constructor theory. And Chiara Marletto has produced a very nice foundational theory of thermodynamics based on an old theory of Carathéodory, which again, no one knew how to incorporate into fundamental physics. But everyone was saying, oh, yeah, this has got to be the way to go. But they didn’t know how. And I think that she has managed to do it. And it’s because of this simple difference, apparently simple difference between how constructor theory views the foundations and how the prevailing conception views them.
Joe Boswell
Okay. But I’m always baffled by the motivation for that. Because what if it’s just the case that thermodynamics is an emergent phenomenon? And what evidence base do we have that requires moving into fundamental physics?
David Deutsch
Well, all science is conjectural. It could turn out that thermodynamics isn’t fundamental. It could turn out that constructor theory is false. But we have these clues, like I said, that Eddington conception and also the fact that there are various clues from different parts of physics that strongly suggest that thermodynamics is a fundamental part of nature. One of them is black hole physics. Stephen Hawking and before him, Jacob Bekenstein showed that a black hole has thermodynamic properties such as entropy, which the black hole entropy, however, is an exact property of the black hole. It’s equal to just basically a constant times the mass of the black hole. And this entropy can appear in thermodynamic equations along with normal entropy. And yet the normal entropy is approximative, and the black hole entropy is exact. So some people say it’s not really entropy, it’s just pseudo entropy. But the thing is that the thing that has to increase is the sum of the black hole entropy and the normal entropy, not just the normal entropy. Okay, so it really fits into the laws of thermodynamics in an integral way. So that’s another clue. I mean, again, this could be an anomaly that’s explained in dozens of different ways. Somebody could come up with a different theory could say that the black holes don’t have real entropy or, you know, a million different ways. But it’s suggestive. And like all motivations for theories, they are not proofs, they’re not even proofs that the theory is needed or that a theory is needed. But they are starting points for a research program. Right. And when you find that there are several different ones of these motivations leading to the same conclusion, and that conclusion is simple, then you know, you’re led into studying it, whether you like it or not.
Joe Boswell
Fair enough. Fair enough. I’ve got another such motivation that has to do with the limitations of what Lee Smolin calls the Newtonian paradigm. Are you aware of that phrasing?
David Deutsch
No.
Joe Boswell
He says that, and you say too, I think that when you analyze the universe in terms of initial conditions and laws of motion, that rules out explaining what the initial conditions were, because you can’t explain the initial conditions in terms of a model, which includes the necessity of initial conditions. And constructor theory might be able to do that, you believe?
David Deutsch
Oh, yes. Well, if constructor theory works, it will have to do that. Okay. Because when I said that what actually happens will emerge as an emergent property, that includes the initial conditions. So constructor theory is based on the dichotomy between tasks that are possible and tasks that are impossible, such as the possible ones might be, it might include the building of a universal computer. And the impossible ones would include the building of a perpetual motion machine of the first kind or the second kind. Okay. But now if we focus on the possible ones, a task being possible means that building a constructor to perform the task is possible, which means that there must be raw materials suitable for building such a constructor, which means that the initial conditions of the universe must be such as to provide such raw materials, not just once, but generically, because possible and impossible have to be universal concepts. So generically, we have hydrogen or protons and so on. And it must be possible to get from that to whatever the theory says is possible. Okay. So therefore, if constructor theory is to be self consistent as a universal theory of physics, it has to explain initial conditions, albeit only emergently, like it will describe laws of motion, you know.
Joe Boswell
Huh, okay, I think I get it. I’ve gotten quite comfortable thinking about emergent phenomena, not being part of fundamental physics. So there’s a part of me that kind of resists this picture you’re painting. And one of your motivations for constructor theory is bringing something like causation or possibility into fundamental physics. And the way I tend to think about causation, and the reason that causation tends to just appear everywhere in the special sciences, but not in the fundamental picture necessarily, is that human beings are kind of extended objects in space time with memories, and we get to watch the way the universe behaves over time. And we develop a concept of counterfactuals, right? Because what we see, well, when the bird is, it’s not flapping its wings, it’s on the floor when it is flapping its wings, it flies. And I suspect that this is just a function of human psychology. The reason that the causes don’t appear in fundamental physics is that you just have the initial conditions and the laws of motion, and you generate the block universe by cranking the algorithm forward, or backwards, or backwards, or in both ways from this present moment. Right, right, which is Bertrand Russell’s worry about why causation was a goner as far as physics and philosophy was concerned. So have I made my point? Do you sort of see what I’m grasping at? That causation might just be visible to humans because of their vantage point as kind of temporally extended beings with memories who see things going one way or another and therefore develop an idea of the counterfactual, and it might not be necessary to put it in the fundamental picture?
David Deutsch
Yeah, I think you’re worrying unnecessarily. First of all, it’s not the project of constructor theory to bring emergent quantities into fundamental physics. And in any case, it doesn’t seem to be on the horizon that all of them will be. It’s just certain ones. And those certain ones, in most cases, have already been problematic within fundamental physics. So for example, you said we’re extended objects, we involve vast numbers of atoms, and therefore we were interested in large scale regularities, which are approximate. Well, yes, but a computer is also one of those. And the fact that, say the Turing principle, that a single machine can simulate every other machine is a drastic limitation on what the laws of physics can be. Or to put that another way, it is a principle of physics. And yet, it talks about computers, which cannot be fundamental objects. They are always extended objects. So unless we’re going to throw away this insight about which unifies the whole of physics, we’ve got to allow the Turing principle to be fundamental. If it’s fundamental, it should be integrated with the other laws of physics. And at the moment, it can’t be.
Joe Boswell
Good. I’m glad you said that because it reminds me of something else I wanted to ask, which is that why are only some emergent macro phenomena insights into fundamental physics and why not others? I want to know why our having this conversation in this room isn’t something by which we would have to kind of constrain what the initial conditions of the universe are. Or like, why is my scratching my nose not a clue into fundamental physics in the way that the existence of the computer is?
David Deutsch
The simple answer is that in those cases, scratching the nose sort of case, our basic explanations say that it’s not happening in other branches of the universal wave function. Whereas the universal quantum computer is a thing that describes the whole of the universal wave function. So it’s inescapable in that sense. So that’s one thing. I mean, there is still such a thing as contingent facts and necessary facts. The necessary facts are the ones that figure in universal explanations. We don’t expect universal explanations to explain everything. Not even everything at an atomic level and certainly not everything at an emergent level.
Joe Boswell
I gather you are hopeful, maybe you still are, that constructor theory will provide a key to understanding something like free will. Do you still believe that? And if so, how would it do that?
David Deutsch
There’s a whole cluster of emergent properties or even philosophical properties of matter and of people and of the way things are that kind of go together. Free will, consciousness, knowledge. Now, constructor theory has got a strong handle on some of those and looks as though it might extend to the others. So in counterfactuals, of course, constructor theory is completely based on counterfactuals. So counterfactuals become almost the foundations of everything. And so explaining that, explaining counterfactuals itself would be down to the next theory, which will explain constructor theory. However, those other things, since we have such a powerful theory of information now, for example, within constructor theory, it seems, and thermodynamics and so on, it seems that knowledge could be the next thing. And if knowledge, then why not free will? If knowledge and free will, why not consciousness? And if those, why not artificial general intelligence?
Joe Boswell
Right, right, right. So one can hope. That’s fantastic. One of the things I talked to Chiara about a couple of years ago was Dan Dennett’s take on free will. Are you aware of his positions?
David Deutsch
Yes, I think he’s tied down by the prevailing conception of fundamental physics, right? Initial conditions plus laws of motion that leaves not much room for these philosophical and emergent things. See, those things have their intrinsic difficulty of explaining what they are, how they work and that kind of thing. But there’s additional difficulty caused by fundamental physics. And if you insist on that, at least we know that they’re going to have to conform to this fundamental physics conception, then you’re drastically limited in what kind of a theory of those things you can form. And in particular, I think it really limits you to reductionist theories of those things. Whereas if constructor theory is the fundamental theory, you’re not limited in that way. And you don’t have to arrive at any of these conclusions like that, you know, that the self doesn’t exist or, you know, that kind of thing may not may or may not exist. It may be one of these emergent things that isn’t fundamental. Or it may be an illusion or not exist. But on the other hand, there’s no reason to if other aspects of the problem seem to indicate that the free will is connected with counterfactuals and so on, then there’s no problem, fundamental problem with accommodating that within fundamental physics, if that’s constructor theory.
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