# 2025-07-03 - IAI - There Is Only One Interpretation of Quantum Mechanics - Full Interview [YouTube](https://www.youtube.com/watch?v=ilEjjnkgTfQ) Duration: 00:19:47 ## Transcript ### David Deutsch 00:00:00 - 00:00:09 However long we last, we can spend that time improving our knowledge, or we can spend it not doing so, we can spend it destroying our knowledge. We have free will. ### Charlie Barnett 00:00:15 - 00:00:30 I'm delighted to be joined by Professor David Deutsch, a physicist at the University of Oxford, often described as the father of quantum computing. He's the author of several books, including The Beginning of Infinity and The Fabric of Reality. David Deutsch, welcome to IAI. ### David Deutsch 00:00:30 - 00:00:32 Hi, thanks for having me. ### Charlie Barnett 00:00:32 - 00:00:49 So you've said in a previous interview, specifically one with Alex O'Connor, regarding the multiverse interpretation of quantum mechanics, that, quote, to do proper science you have to be a realist, unquote. Could you outline what you mean by this specifically? ### David Deutsch 00:00:49 - 00:02:07 Yes, traditionally science was just straightforwardly understood, and I think this is the only reasonable thing to do, as being a project of understanding the world. Understanding what is there, how it behaves, crucially what is there that's not visible, not perceptible, but still affects the things that are visible and perceptible. But in the 20th century, for various bad philosophical reasons, people started to question that, and they said things like, they tried to champion ideas like that the world doesn't really exist, or that, I mean the physical world, or that if it does exist we can't have any knowledge of it, or that we are only playing with words, or that there isn't a difference between a truer theory and a less true theory, and so on. And those are all dead ends. They all are promoting an agenda of not understanding the world, not of understanding it better by realizing that we're only talking about words and that kind of thing, but an agenda of not understanding anything, which isn't worth having. ### Charlie Barnett 00:02:07 - 00:02:20 And you use the phrase bad philosophical reasons there. When you say that, are you saying more than just people being wrong-headed? Are you saying that there was some malign motivation behind that? And what would you say that is? ### David Deutsch 00:02:20 - 00:03:35 Not malign. I'm drawing a distinction between philosophy or philosophical theories that are false, like that there's no such thing as infinity, or that sort of thing, and philosophies that are bad, which are philosophies that intend to sabotage the debate or to prevent progress about a particular thing. So I call that bad philosophy. And the 20th century was increasingly filled with the bad philosophy, as well as false philosophy. But false is not pejorative. All human knowledge is false. We're always, as Popper said, we're all alike in our infinite ignorance. So there's no shame in being wrong, but there is some sort of shame, even if you do it unintentionally, in trying to short circuit arguments on principle and settling into a fixed, unchanging worldview. ### Charlie Barnett 00:03:35 - 00:03:46 Why would people be wanting to short circuit arguments in principle? Are they trying to achieve certain ends? Is it something that they are unknowingly doing? ### David Deutsch 00:03:46 - 00:05:49 So it all comes down to the theory of knowledge, epistemology, that historically people adopted a rather naive theory of knowledge, which is workable. It required fine-tuning at the edges and so on. But gradually, in the 18th and especially the 19th century, people started taking seriously that, for example, knowledge comes from our senses, but our senses are fallible. Therefore, there's no such thing as knowledge. When that was first proposed, people didn't take it seriously. It was just like, oh yeah, we might all be blown up any moment or whatever. And then they just carried on with solving their problems as best they could. But then in the 20th century, people started taking that seriously. There were philosophies like Wittgenstein and so on, who took seriously that there's no such thing as knowledge, including philosophical knowledge. Therefore, this very theory about philosophical knowledge isn't knowledge and should be discarded like the ladder. I don't think Wittgenstein actually says where we're sitting when we've thrown away the ladder. We're supposed to be not sitting on anything. Then Karl Popper came along and made a philosophical breakthrough to the effect that we don't need a firm foundation. We don't need certified true knowledge. We only ever have conjecture, but by criticism and experiment, we can remove some of the errors from our conjectures and therefore get closer to the truth. ### Charlie Barnett 00:05:49 - 00:06:06 It sounds there quite strongly that you're arguing that reality is independent of human perception. But how does this align with the idea that our scientific theories are ultimately human constructions or are they not ultimately human constructions? ### David Deutsch 00:06:06 - 00:07:24 You see, this term ultimately gives away the origin of that critique. Yes, they are human constructions, but that doesn't mean that they are false or meaningless just because they're human and humans are fallible. The point is, we only ever have fallible knowledge. We only ever trust propositions that are strictly false even though they may contain knowledge. I may take the bus to town and use Newton's laws to, if I wanted to do such a thing, maybe it's a bad example, but check that it won't fall through the floor. Newton's laws are good enough even though we know they are false. So they contain a vast amount of knowledge as witnessed the fact that they are the result of correcting errors in previous theories, which themselves contain knowledge and had corrected previous errors and so on. And sometimes we even go down blind alleys and we have to backtrack and so on. There's never any guarantee, but we can proceed without absolute guarantees. ### Charlie Barnett 00:07:24 - 00:07:40 It seems to be that you're implying that scientific progress is infinite in some respect. Does this imply that we'll never arrive at a true and final understanding of the universe? If so, how does that align to the commitment of scientific realism? ### David Deutsch 00:07:40 - 00:08:22 Yes, well, we definitely can't expect to reach a final truth. And again, as Karl Popper said, if we did find it, we wouldn't know because, say, a true scientific theory has infinitely many consequences and we can't explore them all. We can't even explore a finite fraction of them. So, yes, if we last long enough, then however long we last, we can spend that time improving our knowledge or we can spend it not doing so. We can spend it destroying our knowledge. We have free will. ### Charlie Barnett 00:08:22 - 00:08:54 I'd just like to put forward or put to you Carlo Rovelli's, what he's written about the nature of reality, wrote this in reality is not what it seems in subsequent follow-ups. So he said that, quote, the external point of view is a point of view that doesn't exist and every description of the world is from inside it. So therefore, the externally observed world does not exist. What exists are only internal perspectives on the world which are partial and reflect one another. What would you say is wrong with that? ### David Deutsch 00:08:54 - 00:10:32 Again, by the way, physicists during the 20th century again, but still to this day, are inclined to take this instrumentalist view of what knowledge is or what theories are. So we can't be sure what is outside the world that we have access to, but we can theorize about it and know that if that's the criterion of having access, then we don't have access to the interior world either. We don't have access to our own thoughts with that degree of reliability. Dan Dennett points out many cases of experiments on the brain where by performing certain experiments and attaching electrodes to the place in the brain that receives tactile signals, we can not only make people experience things that aren't happening, we can make them remember their own thoughts of a past that never happened. That's because the brain makes up the differences in timings of nerve signals by confabulating what it itself thought. And all memory is like that ultimately. So we have no access to our internal state either. But as I keep saying, we don't need it. We have conjectures and they can be improved, as Dennett's example exactly shows. ### Charlie Barnett 00:10:33 - 00:11:12 Your examples about the mind bring me on to the next topic that I wanted to ask you about, which is consciousness and the computability of it. It sounds like, or at least in the past, that you've implied that consciousness is computable. Some like Roger Penrose have argued the opposite and he's argued that consciousness is non-computational and he uses Gödel's incompleteness theorems to argue that the mind can see truths that a purely algorithmic system can't derive. And therefore the brain must be using some kind of non-computable process when it comes to consciousness, something beyond what machines can do. What would you say to a view like that? ### David Deutsch 00:11:12 - 00:13:23 Yet again, it is using an impossible conception of what knowledge is. So Penrose thinks that when we see a proof of a mathematical theorem, we are touching certainty. We are god-like entities when we're mathematicians. But that's not true. Our mathematical knowledge is conjectural, just like our knowledge of physics. It's even more removed from our senses because it's not true that the interior of our brains and the interior of our thoughts is more accessible to us than the world we perceive through our senses or the world that we perceive through our theories, like the center of the sun. We know lots about the center of the sun, even though no one has ever perceived it and perhaps no one ever will. So mathematical truths are based on conjecture. What Gödel showed is that there is no firm ground underneath mathematical theories either. There's no way of proving that the standards of proof that we currently use are perfectly rigorous. And there have been cases in history where they have shown not to be rigorous. I think Peano, who was the first to axiomatize the principles of the natural numbers, his first attempt at that was wrong. And it's interesting that he did not say, well, I've axiomatized them, therefore there's nothing to them other than my axioms. No, he said, oh dear, my axioms don't correctly represent the natural numbers, so I have to change them. So he was grasping, conjecturing for a reality, an abstract reality, just like scientists try to grasp physical reality. So the same epistemology applies to mathematics as it does to science. ### Charlie Barnett 00:13:23 - 00:13:51 I just wanted to ask you how this conception of conjectural knowledge versus reality applies to the multiverse interpretation of quantum mechanics, as in, is the particle that doesn't collapse or the collapse of the wave function that doesn't take place, is that now real in separate universes? Can you explain simply to a layman what the reality of that looks like? ### David Deutsch 00:13:51 - 00:16:08 Yes, well, I'm always slightly provoked when someone calls that an interpretation of quantum mechanics, because no one says that about the theory of evolution or other theories in physics. We say we have a theory, we're well aware that it might not be perfect, but it is our claim as to what reality is like, it's our best claim as to what reality is like. In quantum mechanics things have gone wrong, thanks to that bad philosophy that I spoke of earlier, and the fact is that there are experiments where one can show that individual quantum particles are strongly affected by something that is never seen. This is quite familiar in physics, even Newton postulated invisible fields of force, not fields, invisible forces that operated on the planets, even though one can never see a force, never perceive a force. And then in astronomy in the early 20th century, we realized that, well, before that we realized that the stars are things like the sun, they're not dots like they seem in the sky, they are spheres million kilometers across of incandescent gas and so on. And then we found that our galaxy isn't the only one in the universe, and in quantum mechanics we discovered that the world we see around us, the everyday world we see around us or in the laboratory, the photon that we see is only one of many that are accompanying it, and they're all imperceptible to us, except in very specially contrived situations where they affect the photon that we do see. So that's Everettian quantum mechanics, I'd rather say, rather than an interpretation. There is only one interpretation in my view of quantum mechanics. ### Charlie Barnett 00:16:08 - 00:16:21 In the interpretation, or the Everettian quantum mechanics, could you just explain, so by what virtue can we talk about the branches, the various branches being different? Do the laws of physics apply differently within those branches? ### David Deutsch 00:16:21 - 00:18:29 The laws of physics are the same and these laws also talk about interactions between the universes, which in everyday experience are too slight to notice, but in careful experiments one can notice them. And the most amazing one you can do at home with a laser pointer and a card with holes in it, make pinholes in it, you can see that when you pass light through a bunch of holes in the card, it makes a weird pattern, not the same pattern as the holes, a completely different pattern. And there are places on the screen on the wall where you're shining this, where if you open an additional hole, that place goes dark. And so that's called interference, the photons that would have arrived there have been interfered with and then interfered with by what? Well, you can also do this not at home, but you'll have to believe me about this part. If you do it with individual photons, going through pinholes and then not projected onto a screen, but captured by photomultipliers, which are sensitive enough to detect individual photons, you will see that exactly the same is true for single photons. So what has interfered with them? Well, it doesn't matter what we call it, but whatever it is, obeys the same equations as photons, you can reflect it off mirrors, you can slow it down with glass and it behaves exactly like a photon that we do see. And what's more, the equations describing it say that that's what it will do. And so that's the prototype argument by which we build up into saying that actually the whole world is like that. ### Charlie Barnett 00:18:29 - 00:18:44 I just want to ask you one final question, which is that you're taking part in a panel with Lee Smolin and Amanda Gefter on the scientific and philosophical nature of nothing. What is the main point that you'd like to get across? ### David Deutsch 00:18:44 - 00:19:34 Oh, I just want to say that this kind of issue, you know, when I'm speaking about nothing, am I really speaking about anything, that sort of question, is unfair when the question has arisen honestly. So when the question arises honestly, not just as a philosophical plaything, which is then a word game and then it's criticized for being a word game, but people really do want to know things like if the universe began at the big bang, what happened before? Was there anything before? And if there was nothing before, how do we understand that? That's a real question. ### Charlie Barnett 00:19:34 - 00:19:40 David Deutsch, thank you very much.