2021-12-01 ToKCast Ep 100 - David Deutsch
Official YouTube Apple Podcasts
Duration: 01:26:04
Transcript
Brett Hall
Welcome to ToKCast and to the long-awaited, or at least often mentioned, episode 100. This here is finally my interview with David in full, unabridged form. It contains all my questions for David and multiple more questions. This is basically an audio-only podcast. There are no visuals today. My friend recently told me that he doesn’t like it when I say to audio-only listeners that the YouTube version is sometimes better because I put lots of pretty visuals in it. He says that he listens while he’s out walking or jogging and can’t look at a screen. Well today, Adam, you don’t need to worry about that. And to all the other Adams out there who I think outnumber the YouTube viewers by about a 10 to 1 ratio, I’ll keep in mind moving forward as we say, that so many of you only listen and don’t watch screens. Perhaps it would be more fair if it was a 10 to 1 ratio of audio-only to visually important podcasts from now on. We’ll see. So this episode is, I have to say upfront, rather an astonishing personal accomplishment for me. Everyone has people they admire growing up. Among my peer group, like any group of school kids I guess, the common thing was looking up to famous sports stars or music idols or movie stars. I know I idolized Arnold Schwarzenegger. This was back in his early days of doing movies, not his more recent political stuff. Back then he was quite the optimist and his personal story was easily as heroic as any of his on-screen characters. Nevertheless, I was never under any illusions I’d ever get to meet Arnold. Much less have a conversation with him. I lived in Australia in the southwestern suburbs of Sydney. It’s not the place to come across movie stars, sports celebrities, singers or accomplished scientists for the most part. But happily for me, some intellectual heroes of mine were accessible by pure chance and circumstance in some cases. I’ve mentioned before on this podcast that Professor Paul Davies, author of The Mind of God and many other books, produced a couple of physics documentary series here in Australia called The Big Questions. He also used to write articles for local news outlets and he appeared on TV here rather regularly when he did live here in Australia for a time. I read, listened to and watched anything and everything Professor Davies ever put out there that I could find and early on he was the guy, my scientific hero so to speak. He seemed to know everything for some reason. So it was an amazing stroke of luck when one day he just happened to turn up unannounced at an event I was helping to host at university, I got his autograph. I actually got his autograph on my philosophy uni notes as it turned out. Not one of his books. The name of the subject in which the uni notes were compiled was called God, Life, the Universe and Everything which was right up his alley and the lecture notes actually had excerpts from one of his books that I proudly showed him.
Brett Hall
I was literally studying his stuff at the time. So I ended up having a brief conversation with Paul Davies, winner of the Templeton Prize. I thought that in my early 20s this was a pretty good accomplishment for an undergrad physics student still living with their parents in an exceedingly typical home on a typical street in a typical suburb, one among hundreds in Greater Sydney Australia. I guess to put it in perspective it would have been rather like an aspiring pop singer just happening to bump into Michael Jackson or Madonna or for younger listeners Justin Bieber. So that was the late 90s by which time I’d already read The Fabric of Reality by David Deutsch multiple times. And the remarkable thing about having read that book was that I just had to talk about it with someone. I think many people who read the book end up feeling that way. It’s kind of unique in that respect. It doesn’t happen often. As much as I loved Paul Davies books I never really felt a need to discuss them with groups of people. But David’s book was different. But sadly there was no one to really talk to about it. At university the people around me even in my physics lectures just didn’t seem to get the significance of it. Well of course they hadn’t read it to begin with. At the same time I was reformulating my own thoughts from decades of schooling and some years of tertiary education in light of what The Fabric of Reality was saying about the nature of knowledge and so on. I was converted by that book to the Many Worlds Interpretation of Quantum Theory. In other words realism. I was converted to Popperian epistemology. In other words I had some insight into what knowledge actually was and how it was created. But whenever I mentioned either of those things, let’s say Many Worlds to a physics tutor or professor or Popper to a philosophy tutor or professor, I barely ever got more than a shrug or maybe a hint that I should go and read about Bohmian mechanics or maybe take more seriously what Feynman had to say about understanding quantum theory. Or in the case of Popper to look more deeply into the Vienna Circle and what Wittgenstein had to say on the matters that Popper was writing about. In other words, in both cases, there were not many sympathetic ears. Now I cannot remember exactly how it happened, and this is in the era before Google, but some search engine or other allowed me to eventually find myself online in the late 90s and the early 2000s able to now and again fire a question to David Deutsch himself via email about The Fabric of Reality. I was astonished that he was the guy, the inventor of quantum computation that seemingly no one much knew about, but seemed to know a heck of a lot more and make considerably more sense than anyone else I’d ever encountered in academia or intellectual circles.
Brett Hall
I was one of the privileged few in retrospect who knew a second book was on the way, though we never knew what it would be called nor when it would arrive. But the anticipation grew over the decade until finally it was published. I think perhaps only Star Wars fans waiting for the next canonical film to drop know exactly that same feeling. Well, even in that case, they don’t really. Films are a transient thing, but a book like The Beginning of Infinity is kind of bottomless in a way that no other consumable content quite is. So I’ve been a fan of David Deutsch for decades now, so it still blows my mind that I could be in a position to actually just have a chat with him as we do here, albeit from the other side of the world. And on that, me being far closer to Antarctica than most of the population of the planet, and most people who are listening to this will be far closer to the Arctic, the finiteness of the speed of light did catch David and I out on more than one occasion, making this episode in some places a little bit disjointed, so editing was required for listenability in some places. But hopefully I’ve done a good enough job that you don’t notice. I’m not a polished interviewer, of course. This seemed to be half conversation, half interview, and I have learned now on putting this together to not speak over the person I’m talking to or try to fill pauses with noises of agreement. I have learned those lessons now, but I did not know them beforehand, which means there are some places where I do seem to forget I am being recorded. As this is episode 100, a sort of announcement is in order, that being that ToKCast will go from strength to strength in a space of podcasts that seems to expand continuously. But I think there is a nice little niche here that needs to be filled. So I’ll continue doing what I do, but with more energy thanks to the support of Patreons and especially of course to Naval Ravikant, whose own worldview and wisdom align so closely with the optimism, view of progress and wealth of David Deutsch and what I am trying to promote here. I’ll continue doing what I do, which is long form podcasts, especially on Deutsch’s The Fabric of Reality, Marletto’s The Science of Can and Can’t and Pinker’s Rationality for Now. But I’ll also be broadening out and refining things, making lots more standalone episodes and really tightening things up on some of the ideas we like to talk about here and making far more brief podcasts as well. And maybe now and again I’ll do the odd recorded conversation once in a while. So look forward to that in the coming weeks and months and years if you’re a fan. Now this introduction is already longer than I hoped for, but I actually haven’t done the job of introduction, which I should do in an episode like this. I need to introduce my guest. But of course David needs no introduction, especially if you’re listening to this as a subscriber of ToKCast, because ToKCast is largely a podcast devoted to the work and worldview of David Deutsch, the inventor of the theory of quantum computation, author of the first quantum algorithm, intellectual successor to the epistemology and philosophy of Karl Popper, author of The Fabric of Reality and The Beginning of Infinity, creator of Constructor Theory.
Brett Hall
David is a fellow of the Royal Society, winner of the Dirac Prize in 1998 for quote, pioneering work in quantum computation leading to the concept of a quantum computer and for contributing to the understanding of how such devices might be constructed from quantum gates in quantum networks. And he was also the co-winner of the Dirac Medal in 2017 for quote, building the foundations of quantum information science. His books have won prizes and he has won other prizes as well. Go to his website and the About Me section to find out more about all that. But as I record this, I’ve just learned that this week, the final week of November 2021, David has won the Isaac Newton Medal from the Institute of Physics in the United Kingdom for quote, founding the discipline named quantum computation and establishing quantum computation’s fundamental idea now known as the qubit or quantum bit end quote. He has now also been elected a fellow of that Institute of Physics. Prizes, accolades and accomplishments aside, the impact of David Deutsch on the world is yet to be fully realised, but it is going to become perhaps something like that of an Einstein or a Newton or a Darwin in my opinion, which is why we’re so lucky to be able to interrogate him once in a while. Once the first fully functional universal quantum computer is built, the landscape of the world will change. When the average person understands and appreciates that its function can only be explained by recourse to a multiverse, the intellectual world will change. When people look back in retrospect and realise he was right all along in what he said about quantum computation and therefore about explanations and hence Popper, the world will be changed. We can only hope that that change goes deeper still into how the education of young people is handled, how we deal with problems both known and unknown, why progress, optimism and wealth need to motivate us all individually and as a civilisation. The world will be changed and we will see that what I’ve just said about putting David alongside the biggest names in science and philosophy is neither hyperbole nor hubris. It is a serious claim. I just wish it would all happen far sooner. But that’s enough of an introduction. I’ll have a few brief remarks at the end of this. But for now, I present to you my interview, my discussion, my conversation with David Deutsch. Okay, well, thanks very much anyway for doing this, David. It’s a real honour to talk to you and be able to record this conversation. And this book that I’ve spent the last few years exploring now is unlike just about any other to my mind. And I’ve said this a few times, but it’s unlike other popular science books because it’s not really a popular science book. One thing that it contains within it that is different to other science books is what I might say are sort of discoveries in philosophy and physics or at the very least, new ways of refining and explaining material you’ve put out there elsewhere in other forms, perhaps.
Brett Hall
The first time this seems to crop up actually is in chapter one, the Reach of Explanations. And there we get the concept of good explanations, good explanations being actual explanations that account for what is out there in the world and how it works that are hard to vary. These hard to vary explanations. Now I’m convinced and most people who hear about it become convinced over time, but I think there’s a lot of people who think, well, science, isn’t it just about testable theories? Why isn’t testable theories enough? So there was a lot about explanations even in my first book. And some careful readers pointed out that I’d never actually explained what an explanation is in fabric of reality. In fact, I seem to have taken a bit of a cop-out path by just saying about explanations that there isn’t any closed list of attributes that an explanation must have.
David Deutsch
That’s because it seemed to me that, and I think it does seem to many people that the difference between an explanation and a prediction is obvious. But to most people, it was quite opaque. So I thought about it and this is indeed one of the few things that I will acknowledge is kind of a bit of an innovation in The Beginning of Infinity. I thought about what exactly makes the difference. And I thought of some examples such as the explanation of the seasons as opposed to predicting them. And then I think that the example that I like best, although again, not everyone does, is the example of the conjurer. Yes.
Brett Hall
Where you go in and you go in to see this conjuring performance and you see some cups and balls and the conjurer puts the ball under the cup. And after a while, you begin to predict that the cup where he puts the ball under is not going to be the one where it ends up. But that doesn’t mean you can explain what’s going on in the trick.
David Deutsch
Yeah, exactly. So you can predict quite confidently that the ball isn’t there and there you are.
Brett Hall
It’s vindicated. But that’s not what you mean by how is the trick done. What you mean is what has happened to bring about the thing you saw. So you’re not asking for an account of the thing you saw, even a perfect prediction of the thing you saw. You’re asking about the thing you didn’t see.
David Deutsch
Yes. And this comes up so often throughout the book really. And I think it’s a subtle point easily missed. And of course, the other example that you use and that you often go to is this idea of dinosaurs.
Brett Hall
The one thing that you actually can’t observe is indeed the one thing that you’re invoking as actually existing and causing the phenomena that you do observe. And in all these cases, that unobserved thing is the thing that you’re really interested in. It’s the only reason you’re interested in any kind of prediction at all.
David Deutsch
Yes. And now this concept of then hard to vary where all the parts of the explanation have some functional role. And this is really what makes the explanation a good explanation as opposed to any arbitrary account that could be easily varied, mythological accounts and so on, magical accounts.
Brett Hall
Did that come to you? Did you have that in mind during The Fabric of Reality writing it then? Or did it only come later in light of people saying, you know, can you sharpen up what you mean by explanation? It’s the latter. Like I said, while I was writing The Fabric of Reality, I kind of thought that these were, rather foolishly, I thought that these were words that had an uncontroversial meaning. And it didn’t occur to me that most people would not have this meaning in mind. Although many people did. So in that sense, it wasn’t something I invented. It was something that I realized needs to be elaborated, to be written down more clearly. And then I started to think about, well, things that are explanations and things that aren’t explanations and like the ancient myths and so on. And then I thought with the conjurer, there are circumstances where the conjurer did it, is the explanation that the person wants.
David Deutsch
You can imagine a person who believes in magic and is sort of slowly coming out of that state of mind, like James Randi’s family, when his story, when he was a teenager and when he first exposed a fake psychic and was horrified to find that most of the people in the audience didn’t want to know the truth. Right, yeah. That happened more than once.
Brett Hall
There was a famous, I don’t know if you know the famous story that he was on Australian television. He did the same to the spoon bender, Yuri Geller, on one of our daytime lunchtime talkback television shows. And the audience was very upset with him and indeed the host was as well and stormed off the set and did the same thing to Randi. Wow, right. So there you have it. That anger would happen, I suppose, whether or not he revealed how he did it. It was the fact that he was revealing that he did do it that was the thing that angered them. And that was the relevant explanation in context. So that’s why I have to say it’s an explanation is hard to vary. While still solving the problem that it purports to solve. Which might be different for different people as well in the same situation. So you mentioned that piece of prosaic terminology explanation that we all use, but I think that you’ve put a spin on it that is quite helpful. But turning to another word, a word that you in fact don’t use, and it brings me to chapter two. Don’t worry, I’m not going to go through every single chapter chronologically, but there are just a few of them. But chapter two is titled closer to reality. But Karl Popper had this term verisimilitude, which means something like closer to truth. And it seems to me you’ve deliberately avoided that word. But why? Is it because it contains a misconception, it’s misleading, or it’s a needless neologism? What would be the reason to avoid such a word?
David Deutsch
I think all of those things. And I think even Popper, I don’t know the history of this very well, but I think even Popper went off it. Popper in general was very into logic. This is, I think, partly a sign of the times that the people who were doing the prevailing philosophy of science at the time, and positivism and logical positivism, many of them were logicians and mathematical logicians even. Popper wanted a name. Popper originally didn’t want to mention truth at all. And then he got converted by Tarski, and he tells a story about how they were sitting on a park bench in Vienna, and Tarski explained the correspondence theory of truth to him. And he realized that this is in fact a useful concept after all. But I wonder what logical scientific discovery would have looked like if he had in fact written the whole of it without ever mentioning truth. I think it would have been worse in some ways in that it’s difficult to make the case for realism if you don’t have a concept of truth. And Popper was very keen to formulate a realistic philosophy of science. But on the other hand, when you do introduce truth, and especially verisimilitude, then the idea is that somehow we can utter truth, or at least we can utter something that’s 90% true or something like that. And we’d have thereby a method of measuring that 90%. I think verisimilitude was not intended to be something you could measure. But if you can’t measure it, then its only use is a sort of philosophical regulating principle. And I think knowledge and problems are much better concepts. Probably Popper’s fundamental concept, the way it turned out after all these experiments with different conceptual frameworks or whatever, is the idea of a problem. Because that comes up not only in his philosophy of science, but in his political philosophy as well. And more generally, he generalizes it to even the problem situation of a gene. I’m not sure even I would go that far. But it’s a very unifying concept. And on the other hand, I don’t think verisimilitude is useful at all. And as I said, even truth, one has to be very careful to use the concept of truth only as a property of abstractions, never as something you can actually measure or know, have a direct access to. So therefore, would you avoid even saying that scientific explanations, or indeed explanations in any domain that we happen to be interested in, would even be approximately true? Because that would also entail some kind of quantitative claim about how close we are. Or can we still use that word?
Brett Hall
Can we still say, it’s approximately true, or there’s some degree of accuracy in this claim?
David Deutsch
I think we can use it because again, depending on the problem situation, if one is speaking informally, then one can use imprecise terminology. I just caught myself just a moment ago saying probably. I’m generally opposed to the whole concept of probability. I think it’s a scam. But that doesn’t mean that in everyday life, we don’t know what we mean when we’re saying Popper probably meant so and so. I don’t mean that there was a stochastic process where he could have meant something else with the probability 0.15.
Brett Hall
I often find myself catching myself precisely the same way that you said. Generally, it kind of has left my vocabulary in just in day-to-day life using the word probably. But at the same time, I don’t chastise myself too much because in order to communicate, normally people are going to use this word. And they understand, I think, what we mean.
David Deutsch
There is no such thing as a perfectly precise language. So we want to use terms and ways of understanding things that are suitable to the problem.
Brett Hall
That would bring me to my next question. Describing people, for example, as chemical scum might be a very loose way of understanding what people are. But Hawking did it. And you have an excellent refutation of this claim that Stephen Hawking did make, which seems to be the reasonably scientific way of understanding our place in the universe. He said that we are just a chemical scum on the surface of a typical planet, orbiting a typical star and so on. But you say this place is not typical. In what sense isn’t it typical?
David Deutsch
I think the description of humans as a chemical scum on Earth is not accurate in the problem situation in which Hawking used that term. We are certainly made of chemicals, but so is the rest of the Earth. And so is all other matter in the universe. So to single us out as a scum, if you’re going to take these terms in the context of trying to describe something purely in terms, in reductionist terms of their constituents, then it is simply false to add a pejorative term. That pejorative term comes from a different problem situation and a different vocabulary is needed. It would be enough to say we are chemicals like everything else in the universe. And then you see that it would be silly to call the Earth a chemical scum or the Sun or the solar system or the Milky Way galaxy as a chemical scum, though in the sense in which we are, it is too. So we are a special, a very special kind of chemical.
Brett Hall
But then would that entail that the Earth is special by virtue of the fact that it is uniquely suited to ensuring that this set of chemicals called us is able to survive off into some future that we are only sustained by the existence of this planet and will only continue to be sustained by this planet.
David Deutsch
So therefore we better look after the environment or else we’re going to go extinct. Yeah. So you’re putting that in a deliberately exaggerated way that makes it much more wrong.
Brett Hall
You could have said that with one-tenth the emphasis and it would still be wrong. Now, it’s true that the Earth is that living things can exist on Earth only by virtue of the fact that the Earth provides by astronomical standards, a very unusual set of circumstances, which if they were to vary by astronomical standards by only a few percent, then without a lot of technology, we would die and living things in general would die without technological help. But that doesn’t at all mean that the Earth is adapted to human existence or even to life, but especially to human existence very accurately. It would be more true to say that the Earth is barely suitable for life. It provides a relatively stable temperature and sunlight and living things manage to use that even though it’s not very conveniently provided. And that took it hundreds of millions of years to work out. And the rest was provided by life itself. And most of the rest, as far as humans go, was provided by humans.
David Deutsch
There are very, very few places on Earth that humans can live comfortably without technology, that is without ideas provided by them and not provided by the Earth. So I don’t know, maybe some South Sea islands with suitable coconuts or whatever they have there. And even that would be a trap because we are so constituted that if we did live in such an environment for some generations, our numbers would increase to the point where we were living in misery again. So the Earth is, living on Earth, what has been provided is really just the bare necessities for us to live somewhere, originally the great Rift Valley or something, and the rest has to be provided by us. Yes, so the Earth is not adapted to life, but life can adapt itself to the Earth. But the only thing that is going to survive in the long run in terms of regular life, not intelligent life like ourselves, are the genes. The genes are going to want to try and make themselves survive. And the animals won’t care, or the other species don’t have this concept of caring into the infinite future about their own survival. However, we do. So we want to adapt ourselves to the Earth in some way, or if not the Earth in something far larger, we want to adapt ourselves to the universe in which we find ourselves. And the one thing that enables us to do that is of course solving our problems continually to create the knowledge in order to enable us to do so.
Brett Hall
And you have the great dichotomy of saying, problems are inevitable, but problems are soluble. And here I have heard over the years people push back against this, especially the second part that problems are soluble.
David Deutsch
For example, they argue we can’t know everything. And they will invoke that it’s provably the case that given Gödel’s result, there are true statements we cannot prove are true. So there is some, and there are some mathematics that’s not decidable. So some problems are not soluble.
Brett Hall
Isn’t that correct? What’s the argument against that line of reasoning?
David Deutsch
I think that this line of reasoning is reaching for a standard of knowing that is impossible for anything. It’s not possible to know even provable things by the standard that they’re asking to know unprovable things. When we say that something is provable, we mean that it’s provable from certain axioms. And Gödel’s theorem is about provability, which means provability from axioms. Now, if you have an undecidable statement in some formal system, and if it was true, you could always add that as another axiom, and then it would be provable. So you might ask, yeah, but how do we know that that statement you’re adding is true, if we can’t prove it? Well, unfortunately, that objection applies to any set of axioms, and it applies to the original set of axioms as well. And this is a mistake that has been made by many mathematicians, even after Gödel. I think it was made by virtually all mathematicians before Gödel. But even after, mathematicians tend to think that there is a class of axioms that don’t need proof or something, that they are self-proving or self-evident, or they come from mathematical intuition, which is infallible or something like that. But in fact, the axioms, the rules of inference in logic, that for example, if the proposition A and B is true, then it is necessarily the case that B is also true. Now, it seems self-evident, but it is not provable. So, what are you going to do? Are you going to say that this is a problem that is insoluble, and therefore it’s not true that problems are soluble? No, people don’t bother saying it with axioms because they have this misconception that axioms are somehow enshrined by God or something, we can know for sure that they are true. But that’s not what knowledge is about. We are trying to solve problems. We’re not trying to establish truths. And Gödel’s theorem is not about establishing truths either. It’s about deriving things from other things.
Brett Hall
Yes, yes. And these, one line of discussion I often pursue here, and correct me if I’m wrong, but this vast class of undecidable or unprovable statements, which must vastly outnumber the number that are provable statements, they’re in a sense uninteresting because that’s not what the claims of science, physics, for example, consist of. And so they don’t have, and again, I would appeal to your better knowledge on this, they don’t have any, they can’t possibly have any effects on our world. They’re not going to help us to solve problems, as you would say. We know they exist, but beyond that, are they interesting in any way?
David Deutsch
Well, they’re certainly interesting because mathematics is interesting. And we’re interested in abstractions as well as in physical problems. And the reason that it seems implausible at the moment that undecidable propositions might be relevant to some practical problem, like how to feed the world or how to eliminate viruses and so on, is that the undecidable propositions are all about infinite sets, infinite sets of things, like the set of all numbers and that kind of thing. And physics, as we currently conceive of it, the infinite sets that appear in physics are very tame and don’t have these problems in them. But that’s like, I can conceive that maybe one day somebody invents a different physical theory where the undecidability of certain propositions about the real numbers would tell you something about how to do and how not to do certain things that you want. I think Roger Penrose once pointed out that certain theories of quantum gravity do depend on Turing undecidable propositions. Basically, it’s like if you’re working out a Feynman diagram to work out what the probability is that a certain gravitational wave will interact with another gravitational wave, this is very far from ordinary experience, then if you want to work that out accurately, you have to do a sum, you have to sum the amplitudes for all possible processes over all geometries and for all possible topologies. And to enumerate all possible topologies in the relevant sense, I mean, I’m not a mathematician, I don’t know about this stuff, but apparently that’s an undecidable task and therefore you couldn’t work out this probability amplitude for the gravitational wave doing something or other because you would have to have a mathematical expression for all topologies in a given sense. So that’s an example of how in principle laws of physics might have undecidable consequences.
Brett Hall
Yes, and therefore that the undecidable consequences are actually interesting.
David Deutsch
Well, it would be physically interesting, yes, in principle, something that you wanted to do might depend on working out this thing. But I don’t see any particular reason for making a distinction, so long as we have the necessities of life and comfort, I don’t see any reason for making a distinction between problems in engineering and in physics and in pure mathematics. I mean, they’re all problems, they’re all conflicts between theories that interest people. People want to know what the resolution is. One thing I say in the book about pure mathematical problems is that sometimes discovering that something is undecidable is the solution to the problem in the sense that it’s what you wanted to know because you have in mind some prima facie argument that certain proposition is true and another prima facie argument that it’s false. And then you discover that it’s actually undecidable. And that explains why you could have these two conflicting intuitions about it. And then you can go and criticize your intuitions in other ways, because you know that you’re not going to be able to resolve it by proving one of them true or false.
Brett Hall
Now you mentioned in that answer there the concept of abstractions and you actually invoked abstractions in talking about it of course. Pure mathematics is about abstractions and I don’t know what might be the most controversial chapter in the book, but I think among intellectual types to at least some extent it would be chapter five if it’s not going to be the why flowers are beautiful chapter. It’s going to be the one that claims there is a reality to abstractions because to some people this seems weirdly enough, you know not to me, I think not to most people listening to this, but it can sound like an appeal to the supernatural or some sort of woo because some people are physicalists. They think that everything just has to come down to the behavior of atoms. But I wanted to ask you if you distinguish between kinds of abstractions rather than defending the thesis that abstractions are real. I know this is not necessarily a well-defined area, but for example something like numbers, they’re abstractions, are they different in kind to let’s say knowledge because after all there may be infinitely, well there are infinitely many prime numbers and not all of those prime numbers ever need to be instantiated in a physical substrate anywhere. Nevertheless they still exist, but knowledge to be knowledge has to be instantiated in a physical substrate. It’s also abstract, but well I might pull the brakes there. Can you explain if there are these different species of abstractions and if so how do we understand the differences in their existence?
David Deutsch
Yes, there are different species of abstractions. I mean maybe it’s better to say there is a classification of abstractions and some classifications are quite useful because abstractions within the same category of classification have similar properties and can be understood with similar explanations or the same explanation. But then with the different problem situations you might want to classify things differently. Just like in biology we may wish to sometimes we want to classify things as mammals and birds and reptiles and so on. And on other occasions we may want to classify them as aquatic creatures and terrestrial creatures and aerial creatures and so on. So classifications are useful relative to the problems that they are that you want to talk about. And it’s the same with abstractions. It’s definitely true that some abstractions as you say like knowledge and information more generally don’t exist unless they’re instantiated. So they have to have this extra element of being instantiated. But that’s also true of there are more classifications than that even among pure abstractions. There are abstractions that it seems to be important in mathematics to like I said to distinguish between abstractions that involve infinite sets and abstractions that don’t. And then the whole idea of a set is not universally applicable. So the set of all sets it famously turned out even before Gödel it turned out that this from Russell that it turned out that the set of all sets isn’t a set. So then you have to invoke a class. It’s just a class of things. Or the set of all sets that don’t contain itself. So that’s right. But it doesn’t help because it doesn’t help with everything because there’s no such thing as the class of all classes either. Right. So yes, there are different kinds of things. And another thing which I think is still slightly mysterious is what kind of an abstraction the laws of physics are. Some people think precisely by arguments such as you’ve mentioned that there really are no laws of physics. All there are are physical events and physical processes. And laws are just a kind of way of summarizing physical events and processes. They just are. So because I see that looking out of the window, I can see that the sky is cloudless at the moment. That is some kind of emergent consequence of the laws of physics. But if it’s true, then people who deny that there are laws of physics will say, well, that’s just a fact about the universe. And you could summarize it and say that on three of the days of this week, the sky was clear. And that’s not a law of nature. So why should there be anything special about the statement that nothing can exceed the speed of light? Well, it’s because the statement that nothing exceeds the speed of light is not just a prediction.
David Deutsch
It is an explanation. It’s an explanation of how the universe is. That the universe is a four-dimensional pseudo-Riemannian manifold with a metric and so on and so on. And the fact that the speed of light is finite emerges from that explanation. And you could write down all the predictions that follow from that in a sort of infinitely long list of things like this dog can’t run faster than light and this car can’t travel faster than light. And once you’ve made that infinitely long list, or maybe just very long, very, very long list, you would have written down everything that can be deduced about what happens in nature. But you wouldn’t have ever listed that law. And you wouldn’t have ever mentioned manifolds and four-dimensional or anything like that.
Brett Hall
Well, you’d have some significant explaining to do, wouldn’t you?
David Deutsch
Well, if you wanted explanations, you’d have a lot of explaining to do. You would in fact have all the explaining to do at the end of that huge process that you had at the beginning. You wouldn’t have explained anything.
Brett Hall
We were just talking about how the nature of physical laws and whether or not, and to what extent they might be pure abstractions or a kind of abstraction of some kind. And I was just thinking to myself that for anyone who denies the existence of physical laws in their own right, laws of physics, and to just say, well, there are just facts of the matter about the universe. For example, the orbits of planets going around stars are approximately circular or elliptical. Then those facts of the matter, those regularities in nature seem to be crying out for an explanation. So to deny that there would be really existing physical laws behind those regularities in nature seems to me to be going to a lot of work to avoid what might otherwise be called Occam’s razor. Well, isn’t the simple explanation there are physical laws out there governing these things?
David Deutsch
Yes. Of course, if somebody insists on the non-existence of explanations, like exactly like insistence on the non-existence of anything, you can’t be proved wrong. But I think the motivation for denying the existence of laws is the old mistake of empiricism. It is the assumption that raw facts or raw sensory impressions have a privileged status in that we can access them directly. And people contrast this with things like laws and explanations in general, which they say we can’t access directly. So they have some kind of a lesser reality and we can in principle not insist on their being real. One can insist on their not being real and it doesn’t make any difference. The trouble with that is that exactly the same is true of sense impressions as well. And so this argument that abstractions don’t really exist or that laws don’t really exist and so on are hidden in there is the assumption that sense impressions or that kind of thing do really exist in some sense of really, which is itself a mistake that all knowledge is conjectural, all observations are theory laden. There is nothing that is an authoritative source of knowledge. Everything is conjecture. And so once you’ve realized that everything is conjecture, but that knowledge can still exist, then the reason for making a distinction between different kinds of existence, well, no, there is a distinction between different kinds of existence, but the justification, the motivation for denying that certain things exist, even if we need them in our explanations, goes away. And this, once one becomes at least somewhat familiar with the worldview that you’ve presented in your books and with Popper’s worldview, it’s very difficult to try and reimagine what people mean by directly observe.
Brett Hall
I struggle now to try and conceive of what one really means when they say, well, there are certain things you can just observe, you can just see, but on explaining to them, well, let’s think about what that whole process of seeing actually consists of photons being absorbed and then re-emitted back towards the eye and then being converted into electrical signals, which then go into your brain. This whole concept of direct observation actually itself vanishes into a cloud of explanation, not so much a cloud, but a kind of way of understanding the world that denies the possibility of direct observation. I don’t even know what someone really intends by this terminology, this way of denying the reality of things, which we don’t have direct access to. When we don’t have direct access to anything, even I would tentatively argue that even the contents of our own minds. I agree. In a way, the contents of our own minds are quite a highly sophisticated kind of knowledge that we only know through an extensive chain of interpretations, some of which are notoriously unreliable. Even in everyday language, we know what we mean when we say you’re just fooling yourself or the content of those ideas is incompatible with empiricism, with the idea that knowledge, reliable knowledge comes from the senses. I think people regard that as just one of those things. It’s one of the mysteries of nature. It’s the problem of induction or something like that. You say it’s hard to imagine what people mean when they talk about direct observation. It’s interesting that that concept of direct observation also had to be invented at one time. In fact, it was quite a liberation when it was first invented, like in various stages. Empiricism in the John Locke sense is relatively recent, but the idea that we can gain knowledge by looking at the world, which is false, was these concepts of knowledge and looking and reliable and that kind of thing, they’re not built into our genes. Somebody invented those concepts and at the time when they invented them, they were an improvement on something that was more vague and more false than that.
David Deutsch
Trust the authority or something similar. For example, yes, but there was a time when you didn’t even have to say trust the authorities. Trusting the authorities was just the way that the world was and nobody bothered to put that into words.
Brett Hall
That reminds me of the divine right of kings, which is a concept that was invented only after the authority of kings was questioned.
David Deutsch
Yes. Well, this all raises the fact that there is this creativity going on, this creation of concepts in order to try and come to better understand the world. That is the uniquely creative capacity of the human mind and mind itself.
Brett Hall
How do you understand the nature of mind? I sometimes think of it as this abstraction. I think you’ve said this to me. It’s a funny kind of abstraction. It’s an abstraction that itself needs to be instantiated in matter, but also needs to be running. It can’t just be like any other piece of knowledge which you can transmit from one place to another in different physical forms. That’s one way of understanding what knowledge is. It allows itself to continue to persist over time and to get itself copied, to get itself replicated. But our mind, a human mind, this thing that is enabling the creation of this knowledge, it itself is an unusual kind of abstraction. How do you think of it at the moment?
David Deutsch
Yes. It’s an unusual kind of knowledge. We have yet another classification. A mind is a kind of knowledge, you might say, but it’s a kind of knowledge that doesn’t only have to be instantiated, but as you have just said, it has to be running. Now, what does that mean? Running how fast? Running in what? Because running in the wrong kind of computer would make it gibberish, or in a kind of computer that has the wrong mapping to reality. We don’t know how it works. We don’t know how the mind is characterized by creativity. I think we can go that far. At least the human mind is characterized by creativity, but we don’t know what creativity is. We don’t know what the distinction is between a computer program that is running creatively and one that is not running creatively. One day we will know, but we don’t know yet. I think also there are, and again, minds, that minds exist is common sense, even though we don’t know how they work or what specifically they are. So more recent ideas that maybe minds don’t exist, that had to be invented too. It is not at all common sense, and it doesn’t at all follow from any good philosophy, but we don’t know. One of the things I think that is a misconception that dates from a very long time ago is that a mind is mostly consistent, that it consists of the ideas in the mind are mostly consistent, and when they’re inconsistent, that’s a kind of emergency, and we have to fix that because that means that it’s not a proper mind really. It contains inconsistencies, and therefore that’s no different from a load of propositions written down on a piece of paper.
Brett Hall
I don’t think that’s true at all. I think a better way to think of the mind is as a set of problems, and a problem consists of conflicting theories. So, or rather, a problem is more like some conflicting theories that we have noticed because we don’t really think of something as a problem unless it’s giving rise to some thought. But there must be ideas that are conflicting with each other before we have noticed it. So the mind, I think of it as sort of mass of tangled ideas which are largely inconsistent. The parts of it which we have sorted out into a kind of consistency are a minority that are currently not giving rise to any thought. They might be invoked in some thought that’s about some other inconsistency, but the part of the mind that is thinking is full of inconsistency. And so, therefore, a model of the mind that thinks of the mind as a set of consistent propositions with occasional inconsistencies that we then fix is, I think, very far from the truth. It’s not a set of consistent ideas, and it’s not a set of propositions because propositions have a definite truth value.
David Deutsch
Each proposition is either true or false, and it has a perfectly definite meaning if it has a meaning, whereas the ideas in minds don’t have a definite meaning, and they’re not precisely true or false. We can make them have as sharp a meaning as our current problem situation requires. That’s what we should be aiming for, and we can make them as consistent as the current problem situation requires. And we’re striving, you talked earlier about truth, we’re striving to make them as true as the current problem situation requires.
Brett Hall
Yeah, so it sounds that whatever the nature of the mind is in a computational sense has to be radically different to any other kind of program that we’ve hitherto encountered, because all those other programs are written in such a way that it’s proposition after proposition, perhaps one following from another. It’s an algorithm of a kind, and these statements need to be consistent, or the computer program itself isn’t going to run, but you’re saying that it’s possibly the case that underlying this uniquely creative capacity of the human mind is something that is quite the mirror image of that in at least some respects. And maybe this is what is needed in order to go some way to explaining what’s going on here with creativity. Or another way that you’ve put that in The Beginning of Infinity is, of course, explanatory universality. This idea that even if there were other kinds of creativity prior to us, what we have is a kind of creativity which allows us to create knowledge, which is going to be able to solve problems in principle about anything that exists in the physical universe. Which brings me to one of my little hobby horses, which is this concept that people sometimes raise that, well, there might be Snalians out there, the super advanced aliens who are going to have some successor to the successor of the unification of quantum theory and general relativity. And even if they could bring it to us, we wouldn’t understand it. It would be written in a mathematics that’s too complicated for us or consists of concepts that are beyond our puny human minds to understand either that or the super intelligence is going to come along at some point in the future. And it too is going to recognise that it’s so much smarter than us and we probably all deserve to be exterminated, something like that. What’s wrong with these, I think, quite commonly held views about other kinds of minds, super advanced alien minds or super intelligent artificial intelligences? Again, of course, we can’t prove that such things don’t exist. There’s a very famous one called God, which there have been disputes about for many centuries. They have?
David Deutsch
Yes. I mean, even the Bible speaks of unbelievers, so they must have existed. And to be a believer in that kind of entity means that you’re believing in an entity whose mind, whose mental processes are, first of all, not embedded in physics, not instantiated in physics in the same way that ours are. And also that they can reach further. They can understand things that things embedded in mere physics can never understand. And you can’t prove that’s false, but it is a bad explanation, as I point out in The Beginning of Infinity. And so one thing is that there are many such theories, and there are many theories about such entities, whether they be Snalians or gods or devils or demons or artificial general intelligences. Those theories, since they’re bad explanations, they have the property that if two people, such believers in different ones, come to persuade me of their view, then there’s nothing I can do to choose between them. They are invoking some non-understandable thing, and the two of them are invoking some two non-understandable things with a non-understandable difference between them. So I would like to put both those people into a sack and tie it shut, and then only have to debate with the survivor for efficiency. Very good. So that’s related to this idea that again, problems are soluble. And what I would suggest is that a kind of a special case of that almost is that the ‘all evils are due to lack of knowledge’ principle of optimism that you have.
Brett Hall
But here it’s really interesting. The principle of optimism, this claim that all evils are due to a lack of knowledge, seems to be linking quite strongly epistemology, stuff about knowledge, to evil, stuff about morality. Isn’t that a big leap? Surely people who do evil things are doing evil things because of their ideas, which is to say what they know. It’s not a lack of knowledge, is it?
David Deutsch
It is the presence of some knowledge. Oh, well, that’s the same thing. They are doing it because their knowledge is inadequate, is false in some relevant way. And even the good guys, their knowledge is better than the bad guys, but it’s still not true. It’s not perfect. It can be improved. In fact, the good guys are characterized to a large extent by the fact that they are open to correcting their existing knowledge and to adopting better ideas that they previously didn’t know. Yes. I think it’s the natural thing. It should be one’s first guess that moral knowledge is included in the same epistemology as physical or mathematical knowledge, because as I’ve said, the arguments that there are different kinds of knowledge, depending on some kind of privileged access to one or the other, is just a mistake. It’s empiricism or something of the kind that is just not understanding the conjectural nature of all ideas. And that must include moral ones and mathematical ones and physical ones and so on. And uniformly, the way to get better ideas is to try to get better explanations in the light of the problems that one has with those ideas.
Brett Hall
There might be…
David Deutsch
I think there’s even another way that The Beginning of Infinity does create a strong link in one’s mind between epistemology and morality. And that would be in the moral injunction, do not destroy the means of error correction. So there we have error correction, that’s epistemology, and do not do this thing. So there’s a moral claim. Now, I wanted to focus on this for a minute, because this might raise an issue within constructor theory, and that this is your new theory of physics along with Chiara Marletto, about the physics of the possible and the impossible, or the possible and impossible transformations, I think is one way it might be described. And among many things to be excited about with this approach is the link, it seems to me, between physics and epistemology. And we already know some of this story.
Brett Hall
We know that what can be proven in mathematics depends on what we know about what the laws of physics are, and how the laws of physics operate. And because we know that proofs in mathematics are a kind of computation, computation requires a computer, the computer has to be made out of something, they’re made out of matter, the matter obeys laws of physics. So we can only prove things that the laws of physics say it is possible to prove. But constructor theory seems to be generalizing this further. I’m not sure if that’d be fair to say. So we’re going beyond mathematics now, and perhaps into realms where we are saying, look, some things are not possible to know, and some things are possible to know, even outside of mathematics, but there may be things impossible to know in history, for example, or in science, because there’ll be no transformation allowing us to actually construct the knowledge that solves the problem there. But this, if I’ve just gone way off the rails in my reasoning here, would seem to contradict the problems are soluble claim in The Beginning of Infinity. The one example I want to call on, I think you might’ve mentioned this in The Fabric of Reality, I can’t remember, but it’s something to the effect of, let’s say you wanted to know what Augustus Caesar ate on his 13th birthday. That’s a reasonable scientific question, but it might also be unknowable. But it’s not going to be a problem for our understanding, because it’s going to be inherently uninteresting. It’s not going to solve any actual historical problems. But is this way of thinking about how constructor theory comes to bear on morality and epistemology at all valid?
David Deutsch
I think that one can address that issue without using constructor theory directly. If constructor theory works, then there will be a theory of knowledge within constructor theory, just like there will be a theory of quantum mechanics within constructor theory.
Brett Hall
We don’t know how these subsidiary theories, whether these subsidiary theories will be changed, or if they have to be changed to what extent they have to be. Our theory of knowledge is nowhere near as precise and accurate as our theories about physics.
David Deutsch
I don’t quite understand your question, because suppose we had a problem about Caesar’s, what, Caesar ate on his 13th birthday. I think the example I usually use is Julius Caesar’s last meal. Never mind. I mean, Augustus Caesar’s 13th birthday meal is just as good. If some problem depended on knowing that as a solution, then it’s not accurate to cast this problem in terms of having to have evidence of what it was. This is not necessarily a question of which observables can be observed in physics. We know that most of the observables, the detailed observables of the past, have been randomized to such an extent that it would require an infeasible amount of collection of knowledge, an infeasible amount of measurement, shall we say, to reconstitute it. That might well be true. I mean, if it isn’t true of these Caesar meal events, it’ll be true of lots of events. It’ll be true of most events at the time. Assume that it is infeasible because of the laws of physics, and this could be expressed precisely in constructor theory to measure Julius Caesar’s last meal. That doesn’t mean that we can’t solve the problem that gave rise to this question because it can’t be a question about physics. From the point of view of physics, this is a very uninteresting question. So it would be a thing. It would be a question about history or about sociology or about some subject that hasn’t been invented yet in which there would be explanations that rival explanations, both of which had reasonable arguments in their favor and which somebody decided could be resolved by making this measurement. And then it turns out this measurement is impossible. Well, that doesn’t mean that it can’t be resolved in other ways. Yes. I mean, we know that that last meal was not a cheeseburger. And that is not because we have measurements to that effect. It’s because we have high level theories about the development of human culture and the problem about what he ate, etc. would have come out of that kind of field. And the solution may well come out of that kind of field as well.
Brett Hall
Yeah, I can only imagine that the question would arise if there was a problem about how and why he died. So if it was Julius Caesar and there was some concern that maybe he’d been poisoned just before he died, well, then there would presumably be theories about existing historical documents about how various other people living with him at the time didn’t particularly like him. And then presumably, although we might not be able to tell what his final meal was, some very smart forensic scientists might be able to detect some kind of toxin in where his stomach used to be. So I imagine that even a problem like this, it wouldn’t be framed in exactly those words, what was his last meal, but it might be some other kind of historical question. Exactly. It would arise out of some problem that wasn’t a physics problem for sure. By the way, there is this problem with Napoleon. Did he die of wallpaper?
David Deutsch
Because it’s controversial how Napoleon died. He wasn’t that old. And he had some symptoms which are consistent with arsenic poisoning. And there were people who would have preferred to see him dead. And then other people have said, well, yes, it was arsenic poisoning, but it was because of the wallpaper. The color of the wallpaper was caused by arsenic paint. And other people have said, yes, so there was arsenic in the wallpaper, but not enough to kill him, and so on and so on. And this problem is eminently addressable, not by going and finding video evidence of the poisoner. Okay. So following Popper and like myself and others in that tradition, we don’t place any real currency on the source of an idea, because any idea can be evaluated utterly independent of its source or its author. And I can take all that on board.
Brett Hall
I think you go even further in this, that you’re not even interested much in the origin of ideas or the history of ideas. And this is actually encapsulated in what Naval has been at pains to point out in his podcast with me anyway, but he’s not much interested in names, who said what and so on. It’s basically about the ideas. And I think that’s quite right. Nevertheless, personally, and I guess as a point of indulgence for anything else, I am interested in the history of ideas to some extent. So, I like to know the fact that Einstein really was the prime originator of general relativity, but it’s interesting to know that Marcel Grossmann, for example, helped him and I think one or more of his wives might have helped him as well with the mathematics on general relativity. So it’s interesting historically about how these great theories kind of came to us, what the lineage is. Now that brings me to this question about quantum computation. I’m interested in this because you were inspired obviously by Turing and your paper as of, I think it was last week anyway, it’s now been cited 6,122 times, which is no small feat, that paper being titled quantum theory, the Church-Turing principle and the universal quantum computer. What I wanted to ask you about that is as you wrote it, because I think when Turing wrote his paper on the Turing machine, he was thinking, well, it’s an abstract mathematical object. This isn’t going to have much. I don’t know. Did he think it was going to have much in the way of a huge industry of engineering these things was going to follow?
David Deutsch
I don’t know if that was in his mind, but did you have any sense in writing the paper about quantum computation that it would have spawned an industry, a multi-billion dollar at the moment, industry of racing to build these things?
Brett Hall
Did that enter your mind at the time?
David Deutsch
No. I was entirely concerned with the laws of physics and that this was a hitherto unsuspected facet of quantum theory, that it had this intimate relationship with computation. So I had in the back of my mind that maybe this could be built, a quantum computer could be built for the purpose of demonstrating or testing properties of the laws of physics of quantum theory. But I wasn’t concerned with practical uses of it at all and not even other laboratory uses of it. And by the way, I think I’m not a historian of science. I don’t know much about the history of ideas in general, only certain very narrow facets. But I think that history in general is an extremely important worthwhile subject and the history of ideas in particular is, I think, the most important aspect of history. It’s just that I happen to have studied other things mostly. But I think that Turing was also not interested at the time when he wrote that paper in making anything useful. Babbage, interestingly, was. Again, I’m going by TV documentaries rather than any real research of my own, but it seems that that he was initially concerned with a very practical application of computers, namely the automation of the printing of mathematical tables, which had to do with navigation and so on. Even when he conceived of the universal version of this machine, which was then called the analytical engine, he still wasn’t… Then what he saw was this was a universal machine for arithmetic and for algebra, but not even for algebra, I’m not sure, for arithmetic at least. And it was Lovelace who saw that this machine would be universal in the sense that we mean today.
Brett Hall
Yes, yes. And so then that leads me to the question that I don’t want it to be too uncomfortable, but there is this question about, was it Church, was it Turing?
David Deutsch
And now, of course, you bring in Babbage as well. And Lovelace, you say, is the first to figure out that these machines could be universal. Well, to what extent do you think Feynman understood the universality of quantum computation? Did he have a conception of it at all around the time when you were writing this paper, or was it only in light of you telling him about the proof that he then sort of got what you were going on about?
Brett Hall
Right.
David Deutsch
I don’t know. I only met him once, and we had a long conversation, only part of which was about quantum computation. And I was not at all focused on finding out what he knows. Right, okay, yes. So I don’t know, but I know that in his published work, there is no real hint of the thing that is now called universal quantum computer, or actually it’s now more often called the universal quantum Turing machine, which is a very bad terminology. But although he, again, he was aware that quantum computation could be more powerful than classical computation, and he was aware of a certain level of universality. But his idea for how to make one of these was itself to put on some kind of circuit board, a quantum system that had specially tuned Hamiltonians. Whereas in the real quantum computer, the special tuning is itself a quantum program. There’s a single machine, you don’t have to change the hardware to compute a new quantum computation. There is a single machine that can be programmed to enact any quantum computation. And I don’t think there’s any hint of that in Feynman’s published work. Although the speed with which he cottoned on to the algorithm that I explained to him suggests that he was on the verge of that, either before I spoke to him or just after.
Brett Hall
Okay, two final, just purely for fun questions. But as you said, who cares?
David Deutsch
This is true. Except for historians of science. I’m sure in the years and decades to come, this will be certainly a point of interest. In the future, they’ll analyse the wallpaper and find out things.
Brett Hall
Okay, this question is just purely for fun. Do you have any opinion on tic tacs? That’s a big thing going on at the moment. And do you know what I’m talking about?
David Deutsch
No, I don’t know what it is.
Brett Hall
Okay. So this is the American military, specifically, I think the Navy, who’ve released these videos of what they say are unidentified objects of some kind.
David Deutsch
And I know this is ridiculous. They say things like they’re violating laws of physics, or at least they are evidence of technology, which is not explicable given the current state of technology. You haven’t heard about these things, seen anything about these things, nor have any opinion on these things? Now that you mention it, I’ve seen things on Twitter, which are now explicable in the light of is it the American military or something having said something about UFOs? I see. Okay, well, the opportunities for error are enormous. And to perform a scientific experiment, that is a crucial test of sophisticated scientific theories, is very, very difficult and universally, almost universally underestimated. It’s sort of taken for granted that Einstein did a very difficult thing, but it’s not so much understood that Stern and Gerlach, who did experiments on quantum superpositions of particles in different positions, they did an amazing thing as well. Yes. Good experiments are rare, and they require usually a great deal of skill and creativity. And it’s not the kind of thing that military pilots or policemen are typically cognizant of. Yes. So I think when people like that report a thing that, quote, violates the laws of physics, I would consider it unreasonable to go for any explanation of that other than human error.
Brett Hall
Yes, yes. And of course, when you do look at the footage, there is no apparent violation of laws of physics, even if you grant that this thing is moving at a high velocity beyond anything that we might be capable of. That’s still sub-light speed, so it hasn’t violated relativity on the one hand.
David Deutsch
And by the way, even if you don’t have an answer for what’s going on there, well, that’s kind of where you might have to stop. You don’t immediately leap to its aliens from the other side of the galaxy.
Brett Hall
My last question is from my father, and I’ll just ask you this before I go.
David Deutsch
It’s about dark energy. And we know the universe is accelerating in its expansion. It’s behaving as if there is negative pressure on the outside of the observable universe. Isn’t this evidence that the observable universe is actually inside of a much larger region itself of lower density, but magnificently greater size, perhaps infinite in size, and perhaps it’s actually got zero density, perhaps this void beyond our universe, is the thing into which the universe is necessarily expanding because we’ve got positive pressure inside of our universe and outside of our universe that’s negative pressure.
Brett Hall
Could this be a solution? So first of all, dark energy is just the name given to this anomalous expansion, which we haven’t explained. So I think the reason that dark energy was chosen as a name is because of dark matter, where all the reasonable theories of it so far have postulated that it’s a kind of matter and it’s dark because it doesn’t interact with photons and so on. With dark energy, we don’t have such a thing. We don’t know that there is a source of pressure or that these observations are caused by something pushing on the universe and so on. But with the prevailing theories of what dark energy does, never mind what it is, but what it does, the universe is not expanding into anything. The length scales in the universe are increasing intrinsically. And this is the same as was the case with the prevailing theories before dark energy. The universe initially had, well, at any rate, very near to the Big Bang. It was very small. There was a time when it was only the size of an atom. There was a time when it was only the size of a neutron and so on. And at that time, and now it’s much bigger, the difference between then and now is not that it has expanded to fill a void. It has just expanded intrinsically. Some of the present theories say that it is in fact infinitely large and that it is infinitely large and homogeneous so that the total amount of matter in it is infinite.
David Deutsch
I don’t think that there are any theories. I mean, one could easily write down a theory in which the universe was inhomogeneous and we are in the only place in it that has matter. But that wouldn’t help in any way with any of the existing theories. And more generally, by the way, theories of inhomogeneity in the universe, as far as I know, I’m not an expert on this, but I think they’ve only been invoked by people who want to say that there is no dark energy, that it’s just a coincidence caused by inhomogeneities. Excellent. Okay. I think that absolutely answered the question. I think that will do for now, but I hope not for the last time. I’m making my way through The Fabric of Reality after all now at the moment. So perhaps at the end of that, we can speak again, if not sooner. So thank you very much for writing the books. I think there’ve been, well, the explanations that transform the world. I think they’ve been life-transforming for many people. So it’s been wonderful to talk to you. Very gratifying today to have this conversation. So have a wonderful remainder of the day yourself. Same to you. It’s always fun chatting.
Brett Hall
So there we go. I hope you enjoyed that. I certainly did. As I say, I’m working on what might be called interview technique. It’s a steep learning curve. The content of Deutsch and Popper’s oeuvre is deep and subtle enough, but adding to that, how to run a podcast and being surrounded by new upgraded podcast hardware and software that I’m struggling to learn to use is a whole other thing, but hopefully will improve rapidly over the coming weeks and months. If you’re a new listener, I’d encourage you to go out and to listen to all of the David Deutsch content that is out there. Hear his TED Talks and other interviews. Of course, buy his books. Subscribe to my podcast here, wherever you’re listening. There is a YouTube version, as I say, if you are on audio only, which sometimes has those pretty visuals. Subscribe to the Naval podcast, where we are exploring all the major themes in The Fabric of Reality and The Beginning of Infinity, and Naval is broadening all that content out into the realms of wealth, wisdom and life. And finally, if you’re so inclined, go to www.bretthall.org and follow the links there to support this podcast. There are lots of great podcasts out there, all about science and philosophy and even thinking, but I think here we have a small community uniquely devoted to a particularly special kind of clear thinking about life, informed by the best that science and philosophy can teach us. We’d like to grow so the ideas can reach into places where the errors need correcting, so I’ll keep doing my bit. Until next time, let me steal a line from Naval and say, stay optimistic. Bye bye.
Markdown