2025-10-06 Theories of Everything Einstein Would Fail Modern Grant Applications
Duration: 02:12:52
Transcript
Curt Jaimungal
Why would Einstein fail a modern grant application?
David Deutsch
Fortunately, I’m not very familiar with the way that grant applications are dealt with. I only know the gross features, in both senses of the word “gross.” That is where I noticed that grants that should have been awarded aren’t, fairly reliably, when it comes to fundamental research, which is what I’m mainly interested in.
So judging by my experience, and I certainly don’t know what it was like over a hundred years ago in Germany, but judging by my experience today in Britain and in America, he wouldn’t have stood a very good chance. He wouldn’t have been able to say very clearly what he was trying to do, because there was no one versed in relativity on the panel that judged physics applications. None of them would have known what a manifold is, what the Riemann tensor is, so his application would have had to explain that in very elementary terms.
They would have had in front of them a pile of applications which, from their point of view, had much more merit because they could see that these were open research problems that needed to be solved. Only in the bigger picture were they incremental, whereas Einstein’s was fundamental. So I think he would have had difficulty getting a grant, and I think in historical fact he did have such difficulty. It was only when Max Planck saw that there was something interesting about him that he got him a job.
Again, that isn’t possible nowadays because there’d be anti-nepotism rules. There’d be rules about the procedure, and every procedural rule is an impediment to any new kind of thing being tried. Anti-nepotism sounds positive. What’s the negative side to it? Well, so nepotism, literally, if it means not giving a job to your nephew, then perhaps that has merit. But what it means in practice is that if you know someone personally, you can’t take part in the selection process for that person and you certainly can’t get the university to let that person in. So it all has to be at arm’s length in order to ensure fairness. Now the amount of unfairness that can be caused by not having that rule is very tiny compared with the enormous unfairness of having that rule, because having that rule means that only people who know nothing about the candidate can rule on whether the candidate is accepted.
Curt Jaimungal
So the natural question that arises in someone’s mind is: why is it that we need grants anyhow? Professors of the past, and I’m speaking about fundamental physics, not just a generic professor, but say Einstein or Feynman or Everett, which I know you have some personal stories on, and I’d like to get to those at some point, I’d never heard them complain about the grant system. However, I hear complaints, especially off-air, from contemporary professors about it frequently. So why does one require grants if they’re already paid a salary? What are the grants for? What if you don’t get a grant? Are you just sitting around twiddling your thumbs? Are you then seen as a net cost to the university and they just make you lecture?
David Deutsch
Yeah, that’s what it is. I repeat: I’m not very familiar with the system, and that’s not an accident. I have intentionally distanced myself from any knowledge of the system because it is a very unpleasant thing to be connected with. But yes, professors can do full-time teaching. Most of them don’t want to, and that’s not because they don’t like teaching, I think in many cases, but because they have to teach to a regimented curriculum and syllabus. They can’t exercise their creativity and reveal to students why they are passionate about the subject. They have to get through a lot of it because the students are all competing with each other to get the exam results, which are a very inaccurate measure of how suitable they are for future research. I mean, it’s not even intended for that. It’s intended for displaying their qualifications, usually to do something else which is by two orders of remove away from suitability for research.
Here in Britain, I don’t know what it’s like in the US, the professor has to apply for a grant to do research, and some of that money goes to the university so he doesn’t have to do as much teaching. But the student or graduate student has to separately apply for a grant. The mere fact that the professor wants to include him on his research team is not enough to provide for subsistence for the student, and again the university takes a cut, notionally because of desk space or lab space or whatever it is.
So I think this whole superstructure is contrary to what is needed for fundamental research, and actually for all the functions of a university, but particularly for fundamental research. What it should be is that the grant-giving authority, or the entity that pays for somebody to do fundamental research, whether it’s a private charity, a private individual, the government, various branches of the government, the military, whichever it is, should be awarding the grant to one person. Sometimes that one person alone, like it would have been with Einstein, is the research group. But in a more general case, that person would then spend part of his grant on hiring postdocs and graduate students and undergraduate students. I mean, this whole hierarchy is counterproductive.
In the research group I believe in flat hierarchies, so ideally it should be the boss and everyone else, and the boss and everyone else should be equal as well. No one’s commanding anything. They’re all there on a joint project, which they believe passionately in.
That reminds me of another incident with my former supervisor Dennis Sciama. By the way, I’ve been very lucky. I was very lucky with my supervisors. He was once told by the higher-ups that they would be instituting a clocking-in system where the students and postdocs, in the morning, would sign whatever it was, the register, that they had to be in by 9 a.m. And so Dennis Sciama objected to this, saying if any of my people is in the department at 9 a.m.
David Deutsch
It’s because they’ve been up all night. Right. There’s a story about Schwinger, who was told, can you lecture at 10 a.m.? And then he paused and thought, I don’t know if I could stay up that late. That is the way.
When physics was a small enterprise, and there weren’t that many physicists in the world, they were all rather eccentric and they were all supported by various other means other than a system. They all had their quirks, and the thing, whatever it was, that sustained them approved of those quirks. So like in the Institute for Advanced Study as well, the first thing someone was told when they were given their ten-year or 15-year grant, or whatever, is: do whatever you like. That’s what the grant holder should be told, and that’s what the grant holder, in my opinion, should tell the postdocs and the graduate students and whoever else is working with him.
You might think, well, if they’re told that, why shouldn’t they just spend all their time drinking? Well, it’s because they’ve been hired for the purpose. They’ve been hired because they are passionate about something. When you want to do a joint project, when you’re interviewing someone, you want to see whether they are passionate about that project, not whether they have some standardized qualification for it. There is no such thing if you’re working on something new. Yeah, enthusiasm is therefore everything.
Curt Jaimungal
Peter Higgs said, I believe this was a decade ago now, that he wouldn’t be able to get an academic job in today’s environment. I think he said it’s “as simple as that,” that he wouldn’t be productive, or something akin to that. Yes, nor would I. Could quantum computing get invented today?
David Deutsch
I think what would happen, probably yes, but the way it would happen is that which is happening a lot already: people who are really passionate about some new fundamental thing apply for a grant to do something else, to do something incremental. They do the incremental thing to the minimum level required to sort of pass the various tests. They publish, they publish again, they publish again. Meanwhile, their passionate thing they don’t necessarily publish, because they’re grappling with very difficult problems that don’t admit of successive papers doing it better and better. They’re waiting for a breakthrough, so they do that in their spare time. That is highly unsatisfactory.
Curt Jaimungal
Imagine the retort would be: look, there’s plenty of great work occurring. It would be foolish to paint all of academia with a brush of stagnation, although that word hasn’t come up. I would like to talk about that, as you had a whole conversation with the Conjecture Institute. I’ll place a link on screen to that. It was fantastic about that subject.
Anyhow, we don’t want to paint all of academia with a brush. Some would say, even to say physics is stagnant, they would quip, well, I wish the field was stagnant; I wouldn’t have so much to do. I could catch up on the arXiv, for instance. So what do you say to those who argue: look, there’s been innovation. There are gravitational waves, black hole imagery, topological insulators and phases, time crystals, exoplanets have been discovered, quantum advantage.
David Deutsch
So I don’t want to appear to be trashing incremental research. Also, in that list of things you just mentioned, I don’t want to classify all of those as incremental research. Some of them are indeed fundamental.
What I want to say is that the research landscape taken as a whole is heavily biased against fundamental discoveries. Everything we’ve talked about, the criterion for getting a grant, the structure of careers, the structure of university departments, all of them are heavily biased against fundamental research, such that it is much more done in people’s spare time than it is done in pursuance of the grant that they’re getting.
By the way, you mentioned what do I mean by grants. Somebody pays for research which is blue-sky research. It used to be that aristocrats did it; they funded their own research. So that’s one way to go. But in regard to who funds it, the main thing that is wrong with the existing setup is that there are too few sources of funding. Because the government has entered the field, not only have they sort of crowded out other means of funding and also prevented it in various ways, but say the private charity, for example, who funds research, they’re going to use the same criteria. What they do is they have a committee whom they assign the task of sifting through all the applications and picking the best ones, and they don’t know how to do that either. They can’t possibly know, and they’re forbidden from using one of the few ways that they could, namely to ask their colleagues: do you know someone who is worthy of this grant? They’re not allowed to ask that.
So in other words, broadly speaking, the government has now contributed, it sounds like a positive, to physics, to fundamental research. They’ve given plenty of money. However, with that money comes some poor practices. These poor practices are then adopted by the individuals who previously used to donate without these poor practices. Yes, and the result is not stagnation. I mean, there is a kind of stagnation, but that’s a different story. It’s that there is, or rather indirectly it is, de-emphasis of the fundamental in favor of the incremental, not that there’s anything bad about incremental research, as I said.
Curt Jaimungal
Yes, yes, and I would like to get to this definition of fundamental research. But just to pause here about government funding, I see public funding, which is a synonym for government funding, as a net good. Am I incorrect in that, or do I have to delineate between different types of public funding in my mind?
David Deutsch
The thing when the government funds something, it’s very rarely doing harm. I mean, that does happen as well, but on the whole the things it funds are worth doing. They’re not always worth the money, especially when there are other things that could be done which are prevented by the system by which the funds are allocated.
Curt Jaimungal
Now on the arXiv, for those people who are listening who aren’t researchers, there’s something called arXiv, pronounced “archive,” where researchers post, and also where researchers look on a weekly to daily basis for new research. There’s HEP, so high-energy physics, and then there’s quantum physics. Is there a specific subcategory of the arXiv for this “fundamental research” that you speak of, or does it just get pulled into one of these two?
David Deutsch
It’s done by subject. There is no category for fundamental, and of course there is no category either there or in grant application forms for things that haven’t been invented yet. So when I applied for a grant to do research in quantum computation, of course one of the things you had to do was check the checkboxes for what kind of physics you’re doing: solid-state physics, astrophysics, and none of them were quantum computing, because computing wasn’t considered a branch of physics in the first place, and especially not quantum computing.
Now there is a checkbox for quantum computing, and consequently now you can get a grant to do incremental research in quantum computing. But you can’t get a grant for inventing a new thing that would go on that list. I’m not saying there should be a thing on that list. It’s impossible to put something like that on the list, and therefore it’s impossible to classify applications according to what they’re trying to do like that. And if you had another box for fundamental, none of the above, for example, the people on the committee wouldn’t know how to judge that.
The only way to judge that is, for example with quantum computing, there would have been some people like Wheeler and Feynman who were aware that there was something to be learned in the physics of computation or the physics of information that hadn’t yet been incorporated into physics, and they might have been able to point to young researchers who would be deserving of getting a grant. But they weren’t on the committee and the committee couldn’t consult them.
Curt Jaimungal
So, okay. Do you think that there should be a new box? Maybe this is not the solution, but let me just posit it. Do you think there should be a new box that is for new boxes?
David Deutsch
As I said, if you had such an application, suppose I was on the committee and I had an application in front of me for a new propulsion system for spacecraft. Let’s say now I know nothing about that and I know no way of judging. I mean, I could probably tell if it’s a crank or crackpot, sure, but if it’s something which is viable but is not a modification of something in existence already, obviously I can’t give you an example of that right now because I’m giving you an example of something that I don’t know about.
So who should get such a grant? Well, that person who deserves such a grant will have been talking to somebody. With luck they will have been talking to somebody who already has a reputation for making progress somewhere in physics, and that person should be listened to. There should be a mechanism for that person to cause somebody to be funded to do some fundamental research. I have several times tried to recommend such people, people that don’t fit into the standard categories, and without success. Only private entities have funded them. But even that was very difficult because, as I say, they use a very similar system and very similar criteria. But at least there’s diversity; at least there’s more than one place to do that. At least there’s more than one place you can apply to. There ought to be dozens of places you can apply to.
Okay, so about the quantum computing checkbox, I imagine, and I know that you mentioned that you’re not as familiar with the grant system as one could be, or maybe you do not want to be, but I imagine that it’s not as simple as the checkbox for quantum computing. I imagine there are sub-checkboxes like quantum hardware, cryptography, fault tolerance, algorithms, or what have you, in the quantum computing space. Now there would be, yes, but when I was doing it there were none of those.
Curt Jaimungal
Right. Okay for the creation of these new checkboxes. Are you saying that they would be done inadvertently? You can’t predict ahead of time. So what you should do is you should fund people with potential.
David Deutsch
Yes, fund people. That’s how it should be. That’s how it should be in incremental research as well. The whole of scientific research should be like that. I thought you were about to say you’d be allowed to make a new box and say what should be in it and all the sub-boxes. I wouldn’t have known what the sub-boxes are. They too were only invented later, and not by me.
So the communities that I also traffic in, other than physics, are philosophy and math, and the grant situation there doesn’t seem to be as dire. There are no expectations of grants as a prerequisite for tenure, for instance; just the hope and the promoting of people who have strong publications.
Curt Jaimungal
So a math department meeting may say something like, congratulations on your Annals paper. But I imagine that a physics department meeting would include, like: your grant expires next year, what’s your renewal plan? So what’s the difference here between physics and math? And I’m speaking about fundamental physics, because you can always say, well, if it’s experimental physics, it’s quite clear you do need plenty of money to fund your machines and your computers and your servers and your students and so on. But fundamental physics?
David Deutsch
Yes. So for the structure that I advocated earlier, the research group leader should indeed be judged not on his past papers, not always, but on his previous success in advancing the subject. It should be a well-known figure in the field who has a track record of making progress, and now he wants to make progress in a way that can engage several other people. Maybe, if he’s an experimentalist, he wants to build this with a machine, or if he’s a theoretician, then the field has broadened enough for him to see that there is potential there that he doesn’t yet know what it is. But he knows who he wants. That’s the thing. Ten years earlier he knew what he wanted to do. Now he knows what kind of person he wants to work with, and he knows that he wants to hire five of them or ten of them, but not five hundred of them.
So he is funded not because he can say what his next paper is going to be about. He’s funded because he says he’s very interested in stuff and he’s going to do research on it, and somebody who funds him will be saying, I think that this guy is good. Or gal. Yeah, well, I don’t want to use gender-neutral language because I think it’s silly. Obviously when I say he, I mean he or she, and if I said mister or Mrs., I might also mean His Majesty or Master so-and-so.
It’s completely natural in this whole scheme of things that I’m advocating that everything is tuned to doing the research, creating the new knowledge. Everything is subordinate to that. If somebody is going to care whether the graduate student is male or female, then they’re not the funder that I want. They need to be obsessed with the thing itself, and so should be the people they hire.
Curt Jaimungal
Suppose right now there’s a wealthy patron, or multiple, and you could speak directly to them. These people who are watching care about fundamental physics, maybe foundational research in computer science as well, just foundational in general, which we can get to distinguish in between fundamental and foundational. To me I see them as quite close. I can’t distinguish them. Maybe you can. But anyhow, what is your message to them? What are they to do? They have this money. What are they to do? They want to help.
David Deutsch
Yeah, so each one of them is different. Each one of them has interests. Each one of them has reasons for wanting to promote fundamental physics, and each one of them has a different conception of what fundamental physics is. They might also have a conception that it’s being slowed down by various sociological facts and so on, so they want to get around that.
They need to find somebody that they think is good. Usually this will be somebody who has already done some of the thing that they want done, and then they should approach that person and say, could you use some money? Often the answer will be no, but often it will be yes, because with money they can do a lot of things in parallel that they would otherwise have to do in series, by themselves or with a smaller group.
Now there is a thing that’s just started up, the Conjecture Institute, and I don’t know how they make their choices, but what I’ve seen seems to be following the pattern that I advocate quite closely. They fund the person, not the research project, and they seem to fund people who are interested in foundations. I don’t know whether that’s because their thing is to fund foundations, or whether their thing is to fund things which aren’t normally funded. I don’t know which of those it is. But either of those would do, and lots of variations on that would also do. Like I said, I would like there to be dozens of such entities, all with a different ethos, all with a different theory of what foundations are or what they’re for, and all with a different theory of what’s wrong with the present thing, why somebody hasn’t already funded the thing that they want to fund, that sort of thing.
So firstly, what is the difference between fundamental research and foundational research? I don’t make much difference between those things, but foundational suggests to me that you have a field and you’re drilling into its foundations. So you want to understand it more deeply than it has been before. Fundamental means to do with the fundamental knowledge, that is, knowledge that is needed for all sorts of different areas. For example, quantum computation I think is fundamental, or was, because it has to do with mathematics and epistemology, as well as physics and computation and computer engineering. So there’s a whole bunch of things that it might unite if it works. But it’s fundamental in its conception. It’s not like working at existing foundations of anything.
Curt Jaimungal
Okay, so someone who’s listening who doesn’t care about fundamental or foundational research, they hear you keep bringing it up. Why is it so important to you? And of course you are not saying that the incremental, confirmational research that’s done on existing theories is not important, but that the fundamental, foundational has been somewhat excluded or not incentivized properly. But that implies: why should we even care that it’s incentivized properly? What is it about foundational and fundamental research that’s so vital to your conception of knowledge in the world?
David Deutsch
The thing that unites them is that the growth of knowledge can’t be regimented. You know, Henry Ford said something like, if I asked people what they want, they would have said a better horse. The essential thing to intellectual progress of all kinds, whether incremental, fundamental, whatever, is interest, that somebody is interested in doing this. If they weren’t paid, they’d still do it. They’d get a job doing something else and do it in their spare time, like Van Gogh with his painting. You know, nobody ever bought a painting from him in his lifetime, even though his brother owned an art gallery. I mean, I don’t know the story of that, but you know, it’s obviously not the standard story of slotting into an existing structure. So some …
Curt Jaimungal
But …
David Deutsch
Yeah, sorry, I’ve gone off the subject slightly. What unifies fundamental and incremental research is that someone’s interested in it, and it’s that interest that drives all progress. It’s true that fundamental research eventually, typically eventually, drives something useful as well, but not always. You could ask, well, if the general theory of relativity hadn’t been invented for another 60 years, let’s say after Einstein, nothing practical would have been affected then. It was needed for the GPS system then. Now it’s being needed for other things. But perhaps if you were interested in purely utilitarian outputs you would have delayed Einstein.
But then if you take that kind of utilitarian attitude to Einstein, you would have taken the utilitarian attitude to everything, and you would never have had antibiotics and rocketry and satellites and that sort of thing. The reason that it’s all connected is not so much that progress in the whole of science and engineering comes from fundamental research as a sort of wellspring, that also happens, but the main thing is that the whole of progress in human ideas is a single thing, an indivisible thing, which is all powered by interest, by curiosity, by dissatisfaction with the way things are currently.
Curt Jaimungal
Okay, so it’s not an argument to pursue foundational research because in your mind maybe a decade from now, maybe 200 years from now, it will prove to be useful.
David Deutsch
No, it’s not that. I thought what you’re going to say.
Curt Jaimungal
It’s an in-and-of-itself argument, but it doesn’t sound like that. It sounds like: pursue it because this is part of a larger knowledge-creation process.
David Deutsch
Exactly. It’s needed for that, and if you suppress the impulse to create, the impulse to improve anywhere, you’re going to affect everywhere, or you may affect everywhere. I mean, you could be lucky and not affect theory of evolution or whatever, but in practice you usually do.
Curt Jaimungal
Interesting. Okay, sorry to interrupt you. So it sounds like you’re saying that, look, a child has a natural curiosity. As you get older, your curiosity morphs into various subjects. One of those subjects could be foundational research in physics, but that is an example of foundational-research curiosity, and yes, it is important.
David Deutsch
Yes, and therefore if you’re talking about your hypothetical rich person, a hypothetical rich person who has that interest, or who wishes they could have pursued that interest when they didn’t have time to do it when they were younger, or that kind of thing, somebody who for reasons of their own thinks that that is important and they’re curious as to where that will go and they want to get the answer before they die, that is the thing that this hypothetical rich person should be funding.
Curt Jaimungal
Yes, and I imagine this hypothetical rich person was not able to pursue foundational research because you hypothetically don’t get rich by pursuing fundamental research.
David Deutsch
That’s the whole point of our conversation for the past 30 minutes. Yes, presumably something else interested them, and that involved making money. I don’t think, by the way, that there’s hardly anybody who’s interested in making money per se. They make money because that is what they need to do the thing that they’re interested in.
Curt Jaimungal
Right. Should physicists study philosophy?
David Deutsch
Well, if it’s relevant to their research. Now in the case of quantum computing, it’s rather paradoxical, because I think philosophy is extremely important in the foundations of quantum computing, but the state of the art in academic philosophy is terrible. People who study that and internalize it become less proficient at the kind of philosophy that’s needed to make progress in physics. Now there are exceptions to that, and I won’t name them because then the people I don’t name will be offended. But there are certainly philosophers who take the right attitude to philosophy. The overwhelming majority do not. So physicists should know philosophy, provided they find the right philosophy.
Curt Jaimungal
Speaking of naming, can you name a physics department that is doing extremely well in your eyes now? I said department, but it could also be an institute, like the Perimeter Institute, for instance.
David Deutsch
Again, I’d rather not, for the same reason. I’m a theorist. I prefer to talk theory rather than practice. If the things that I’m saying are true, or even half true, people will recognize it. They’ll recognize that they’ve seen this happening. When I speak to people about this, I’ve very rarely had anyone contradict what I’m saying. They usually agree, but they say, yes, but what can I do about it?
What was it, about five years ago? I was trying to get the rules changed about how foreign postdocs are treated in the British visa system. That may seem to be a rather esoteric thing, but it was important to me at the time because, well, never mind. So I thought, who can I go to? The head of the physics department? Well, the head of the physics department said to me, I’ve got no power over that. That’s my superiors. The superiors said, we have no power over that. So I thought, well, I’ll go to the vice chancellor of the university. No, the vice chancellor doesn’t deal with such things at all.
Then, as it happened, there came into my email box a Royal Society document saying the structure of research funding. This is one of the reasons why I avoid this whole field, by the way, the structure of fundamental research funding in Britain. So I downloaded it. It’s like, I don’t know, a big fat thing, and I looked at it. I found that if I had pursued this line of who should I ask to change this rule, there is no one, basically. The structure of decision-making goes right up to the minister, the minister for science and education, but the minister is required to consult various committees before making any decision. So there is nobody I can go to to make the change I wanted to. And that’s why I gave up on that.
Curt Jaimungal
Well, this sounds hopeless, so there must be some hope here. Professors of physics who watch this channel are listening and they’re thinking: look, I don’t like this “system” that I’m in, and I would like to change it in various specific ways that are important to me. David, what is your advice?
David Deutsch
I suppose what they would need to do is get together and form a proposal to take to government, because it’s pointless taking it to the minister. The minister doesn’t have that power. It’s the government that has to change the rules under which the minister makes these decisions, and then that can go right down through the hierarchy. Somehow these people would have to present it in such a way that it bubbles up to the top of what the government wants to do. I don’t know how to do that. I don’t know how to politically campaign. But perhaps there are such people, perhaps they’re watching.
Curt Jaimungal
What do you think the reason is that more physicists aren’t actively critiquing the academic organization that they’re a part of? I hear plenty of critiques off-air from professors that I speak to, but on air they’re much more reluctant. One reason that occurs to people who are listening could be: well, the academic positions are precarious, especially without tenure, so when you speak it’s like you’re biting the hand that feeds you. So maybe you have the incentive to do the opposite, to say, no, no, I love everything about where I work and everything is copacetic.
David Deutsch
No, I think very few people say that …
Curt Jaimungal
Okay, so you’re one of maybe ten people that I know who are currently in the academic institution, who are willing to say there’s something rotten at the core of the institution. The difficulty here is when most people say something is rotten at the core of an institution, they get labeled as a conspiracy theorist. The mental image people have of what you think is, okay, at some point people sat around with cigars thinking, how can we make this less efficient and more beneficial to myself? And there were distributions of notes that said burn after reading.
David Deutsch
Yeah, nothing like that. This is nobody’s fault. Nobody is to blame. That is part of why it’s hard to change.
Curt Jaimungal
So why is it that you’re a part of a small handful of people who are willing to publicly talk about this …
David Deutsch
Well, again, I can’t psychologize. Sorry, I’m being…
Curt Jaimungal
That’s fine. Then you can feel free to disagree with the premise. You can also say, Curt, no, no, I think that’s false; I can list 20 people. I think it’s true that few people want to criticize it, especially in public.
David Deutsch
But I can’t speculate on why. Is it because of their career, like you said? Is it because they consider their position precarious? A whole load of other considerations come in once you have tenure, because you’re not a free agent when you have tenure. It’s supposed to free you from peer pressure, or whatever you call it, or public pressure, but in practice that doesn’t really happen. Very many people who get permanent jobs just slot into the system, and I don’t know why. Some sociological reason, perhaps. Again, you’re making me speculate about things I don’t know about, so I’m extremely vexed with you.
Curt Jaimungal
Good. Yes, that’s worth being. It’s rare that there’s a physicist who’s made significant contributions even to their own field. That’s something to note. To make contributions to the philosophy of physics is something else. And then with your book, The Beginning of Infinity, which I’ll place a link on screen and in the description, you’ve made contributions to philosophy proper. So that’s vexing. What is it about you…
David Deutsch
I’ve been very lucky. As I said earlier, when I was a graduate student and postdoc, I was very lucky to have supervisors who did exactly what I advocate. They said, when I came in on day one, or rather even in my interview before I was even accepted, they said…
Curt Jaimungal
What is it you want to work on?
David Deutsch
And I didn’t say anything specific, because I didn’t know anything specific at the time. So I just described what kind of thing I wanted to work on. I remember saying, for example, to Dennis Sciama in my first interview that it seems to me that the most urgent problem in physics is quantum gravity. So I’d like to work on that. I know a bit of quantum field theory, but I don’t know enough relativity yet.
Now it turned out that that was a bad idea, and I decided to turn from quantum gravity, which I thought was too difficult to work on at my stage yet. I was getting interested in other things, which eventually led to physics of information and to quantum computation and so on. Dennis somehow saw that I had the thing he wants in his students, and so he hired me. When I wanted to change and to study something completely different, not only did he not object in any way, he was interested. He was interested in what I wanted to do and why and so on, but he never tried to direct my research because he assumed that I wanted what he wanted.
Curt Jaimungal
You mentioned that quantum gravity was too difficult. What do you mean?
David Deutsch
So at the time I didn’t know what was so difficult about it. I think I’d absorbed the standard view that what we’re trying to do is cure the infinities and cure the nonlinearities and that kind of thing. The answer would be an equation which had the desired properties. I realized as I got into the subject that those are trivial problems. It wouldn’t matter if we didn’t solve that, but the chances are that we will solve that once we’ve got the deeper incompatibility between the two theories sorted out.
The fundamental, I keep saying fundamental, but I don’t always mean fundamental in the same sense, we have to unpick this. At root, all the existing field theories are theories of fields on spacetime, whereas general relativity is a theory of spacetime itself. It’s not a field. You can think of it as a fixed spacetime with the field on top of it, so it’s the sort of static part and the varying part which people then try to quantize. But that’s very alien to general relativity. General relativity is the theory of spacetime as a dynamical thing itself. People have tried everything in quantum gravity, and in my view everything has failed.
In every other part of physics, these fields evolve in time, and what you’re looking for is a dynamical equation, an equation of motion that says how they evolve in time. But in general relativity, viewed in that sense, there is no time. Either it’s a four-dimensional thing, or in the quantum sense it’s a manifold where every point is a three-geometry, and evolution in time is just a strip of things with a higher wave function than the rest. But how that turns into time, there are various proposals. Anyway, what can I say? It’s conceptually very incompatible. The way we conceive of quantum fields, which have their own problems by the way, and the way we conceive of spacetime, is fundamentally incompatible.
There are also problems with quantum field theory itself. Now I think general relativity in itself would be a viable theory. Okay, there’s the big bang and black holes and we’re not sure how to deal with singularities, but basically it’s a viable theory and there’s no kind of contradiction in it. Whereas quantum field theory is full of contradictions in its own right, let alone before you try to unify it with gravity.
Look, my favorite one at the moment, I like being baffled, and this is one of my favorite problems with quantum field theory because it’s so baffling. One of the basic axioms of quantum field theory is that field quantities at spacelike separated points, that is, at points at the same time, should commute with each other. That is, if there’s a quantity A and B, then . At a later time they don’t commute, and that’s the whole reason why quantum fields evolve in time. It’s because the later thing doesn’t commute with the earlier thing. So now if two things, let’s say in the same space, there is a quantity here and a quantity there that don’t commute.
David Deutsch
It means they must be described by separate algebras. And no matter how close they are, they must still be described by separate algebras, which means that these separate algebras all commute. So no matter how close together these algebras are, they still commute, and yet when they coincide they don’t commute, because field quantities at the same point don’t commute, because that’s what drives the whole thing forwards.
So this axiom of commutativity at spacelike separations is disastrous. One way of looking at the infinities of quantum field theory is that they are precisely caused by that axiom. Interesting. If you try to remove that axiom, which I have tried to do, then you run into problems of causality and other problems, and problems of interpretation, what the meaning of the different quantities are. It’s like you’re not in Kansas anymore. Just that tiny change in quantum theory leads to a theory that can’t even be interpreted in the normal way as being things having values at different points. So it becomes something else. You’re not in Kansas anymore.
So that’s one of the problems, and I like thinking about it. You’ll have to stop me talking about it.
Curt Jaimungal
Hmm. Oh, I don’t want to stop you talking about it. I want to know: what were some of your attempted solutions?
David Deutsch
So I tried to set up a quantum theory where the fields at each point don’t range over the real numbers for their possible values, but they just range over plus and minus one. So they’re qubits. I called this qubit field theory. You have a field of qubits, and there’s a qubit at each point in space. They don’t have to commute with each other at different points. You just assume that somehow, dynamically, when they’re far enough apart, they’ll approximately commute. But they won’t commute, and as you choose two points closer and closer together, you’ll find that their algebras become more and more the same, until when they coincide there’s no blowing up or anything. It’s just a perfectly well-behaved theory.
So then I worked out what the possible equations of motion for such a theory are. This was several years ago that I did this. I worked out, I think, that there are 13 possible second-order differential equations that are capable of being equations of motion for qubit field theory.
Then the question arose: what counts as a measurement? Because in ordinary quantum theory, if you measure something, you’re putting the value of it into another thing which then commutes with the original thing, so you can think of it as having a value, which if it’s a good measurement, it’ll be the same value as in the thing you were measuring. But in qubit field theory, that’s not true, because when you measure something, the result of the measurement will still not commute with the original thing. When you measure that, the non-commutativity will spread a bit like entanglement, but this is spreading a different thing. It’s spreading non-commutativity, and I couldn’t solve that problem.
So although the paper is on the arXiv, you can read it if you want to, it’s a nice little theory. I have no idea what it means physically, and I failed in finding a thing it could mean, so I never published it except on the arXiv.
Curt Jaimungal
Is it a non-local quantum field theory?
David Deutsch
No, no, it’s perfectly local. That is the right question, because normally when things don’t commute there’ll be problems with causality. But in qubit field theory, there are no problems with causality. It all works: no infinities, no non-localities. There is no Schrodinger picture for that theory. There’s only a Heisenberg picture. That seems to be an important thing, but I haven’t put my finger on exactly how.
Curt Jaimungal
Okay, so most of the time when people are thinking of combining general relativity with QFT, the mathematical problem is non-renormalizability. There are said to be three or so conceptual problems that are distinct from the non-renormalizability. One is just QFT requires a fixed background, like you mentioned: background independence is the issue here, conceptually speaking. Then there is, well, what does it mean for you to have a superposition of geometries operationally? And then there’s the problem of time that you mentioned.
Now the current leading theory of quantum gravity is string theory. At the time when you were a graduate student, it may not have been there. But either way, you have heard of it at some points during your career. What attracted you to it, or what did not attract you to it? What dissuaded you or persuaded you?
David Deutsch
I’ve never worked on it because I don’t think that progress in fundamental physics, one should never say never, but I don’t think progress in fundamental physics ever, or almost ever, comes by trying to find a better mathematical object and then wondering what kind of physics it means. For example, finding a different group for the fundamental particles to belong to, I don’t think you can find the answer to a sophisticated problem that way.
What you need to do is have an idea about what physical thing you want. For example, as I was saying with qubit field theory, you want the commutation relations of different field quantities not to be pathological. So you want everything to be smooth. Okay, now what kind of mathematics can give that to you? That’s the kind of thing that I think can make progress in physics. String theory, it seems to me, is entirely the other way around. It’s saying, suppose that the fundamental things in nature are not point particles but strings. Okay, now let’s find out what kind of a world that would look like. I can’t prove that that will never work, but I don’t think it can work.
Curt Jaimungal
So it was more their approach to landing on string theory that you disagreed with, rather than string theory itself?
David Deutsch
Yes. Well, string theory itself then just becomes trying to find some equations that will make it work. You should be trying to look for equations that do the physical thing that you think physics is going to be like.
Curt Jaimungal
Why should the approach matter? So let’s just analogize this to scaling a mountain. You think you should be hiking to find the mountain with your flashlight, and they think, no, you should be using a, you can tell I’m not a mountain climber, but what are those picks that they use? I did speak to Alex Honnold, who’s a rock climber, and I’ve already forgotten. Pitons, I believe their names are. But regardless, okay, sure, there are two approaches and you find something on the mountain. To me, it doesn’t matter how you got to what you found. You found it. So you just evaluate this. It’s not how you got there. It’s what the problem was.
Curt Jaimungal
You …
David Deutsch
As I said, as we were saying in the earlier part of the conversation, someone has to be passionate about it. Someone has to be obsessed with the problem and trying to solve it, not being expert at mathematics and making up a new mathematical thing and then throwing that over to the physicists and saying, is it this? And they say, no, it’s not that. Then you say, well, is it this? That’s the approach, though I shouldn’t really call it an approach. I mean, that’s not problem-based. That’s not somebody trying to solve a problem. Maybe you could say it’s somebody trying to solve someone else’s problem.
But from the physics point of view, the conceptual thing is fundamental. The conceptual thing is what motivates the whole procedure. You want to make the theories work, and you have an idea about how reality should be that would make it work, or what kind of reality would make that work, not what kind of equation of motion would make it work. I think you’ll never get there that way. If you tried to make general relativity by that method, you would absolutely never have got there, because you would never have had the theory of a dynamical spacetime. You’d just have been thinking of, what terms can we add to Newton’s laws to make it compatible with, let’s say, electromagnetism? Well, add a couple of terms, and you can do that. You might even get as far as the relativistic formulation of Maxwell’s equations, which might then get you to special relativity. I mean, this is already assuming a lot of luck, but you’d never get your general relativity, because the idea of a dynamical spacetime, a dynamical curved spacetime, was needed to make that progress. You’d never have found those equations without first having that idea.
Curt Jaimungal
So what if the string theorist says, well, who cares about what motivated us to get to our answers? Firstly, we’re a diverse group of people. We all have different motivations. It’s unclear to speak of the motivation of the field of string theory itself. But regardless, look, David, we the string theorists have given you AdS/CFT correspondence. We’ve revolutionized our understanding of quantum information and black holes. We’ve developed holographic dualities that are now used in condensed matter physics. And sure, that latter case is not string-inspired, but it’s not string-contingent. Okay, still there are tools that we’ve developed that pure mathematicians use, and so on. So is this not evidence that we’re on the correct track? What is your response to that?
David Deutsch
Well, there’s never evidence that one is on the right track. If there was such a thing, then one could move further along the track. I’m not qualified to judge mathematics, so there might be very beautiful mathematics in string theory which sets up alternate realities that are like ours in some way and unlike ours in another way. I can’t prove to you that when they keep fiddling with it, it won’t eventually resemble our one or be our one.
But to get an answer without first having the problem to which that is the answer is, I think, very rare. Even when you cite examples like Alexander Fleming working on bacteria and he found penicillin, it really wasn’t like that. He had an idea which had finding a therapeutic chemical in the landscape of what he was looking for. He wasn’t specifically trying to find penicillin, and it was because he was in that landscape that he recognized the accidental discovery as being relevant.
If somebody had said to him, let’s say two or three years before, here’s a petri dish, what do you see? He might well have said, I don’t know, there are just some bacterial colonies on there. What am I supposed to look for? And then somebody might have said, well look, there’s a patch here where the bacteria aren’t going, and then he might have made further progress. But an idea of that kind, and a proposed solution to a problem, first a conception of a problem, and then a proposed solution to that problem, come before a viable theory that solves it, or that addresses it, partly solves it.
So in the case of string theory, I don’t see that it has solved any existing problem. What they’re hoping for is that some mathematics that resembles the existing mathematics will come out of it and will have desired properties. But I don’t know, maybe the right theory of quantum gravity has infinities. Maybe they’re a good thing. Maybe we ought to have more of them, or whatever.
Curt Jaimungal
Okay, so I imagine the rejoinder from the string theorist is: okay, you say that we haven’t solved any problems, but look, string theory is the only framework that’s been developed where quantum mechanics and gravity coexist without mathematical contradictions, and every other approach either breaks fundamental symmetries or has these contradictions. Is that not progress to you, David?
David Deutsch
Well, it is mathematical progress. But that it might solve the conceptual problems is a hope, and I keep saying I can’t prove that that’s not going to be fulfilled. Maybe it’ll be fulfilled tomorrow. Also, it’s not up to me to tell other people what to work on. They should work on it for whatever reason they like, and the funding entities should fund it for whatever reason they like.
Curt Jaimungal
So getting back to committees, I imagine that the Manhattan Project had a committee. I’m not a historian and I haven’t looked into that, so I’m nowhere near an expert in the Manhattan Project, other than watching Oppenheimer. What was different about their committee?
David Deutsch
About their committee? Yeah, I’m not well up on the history either, though I have seen Oppenheimer. I don’t know how accurate that was. We’re in the same boat, right? I think that was a very unusual organization, and it did not run on this kind of committee pattern.
For a start, nobody applied to be on the Manhattan Project. They were picked. Somebody would come and see you and say, do you want to work on work of national importance? It will involve a lot of sacrifice on your part, and we can’t tell you what it is, but it is of national importance. This was during the war, so a lot of people said yes. A lot of people then went there and never found out what it was about, because they were not employed at the center of the research. They were just supporting researchers. They were just told, get this machine to work, never mind why. And then there were the lab-assistant-level people who were just told, keep that dial between this number and this number, and turn this knob and press this button all day, every day, and don’t tell anyone what you’re doing. And they did, except a few who were Soviet spies.
Fortunately, Stalin didn’t have the wherewithal to make use of the knowledge himself before the end of the war. If there had been Nazi spies, it would have been a much bigger catastrophe. It was a catastrophe as it was, but the Nazis had an atom-bomb project underway with Heisenberg at the head, and if he’d been told a few of the secrets of the Manhattan Project, he could have done it. He’s later said he didn’t want to, but I don’t believe him.
Curt Jaimungal
You mentioned in your interview with Sam Altman that you keep a list on your computer of progress in fields where there’s been significant progress, but you thought you couldn’t have achieved that progress. I believe you said the World Wide Web was one, and, I’m sorry, not AGI, but being able to converse generally in natural language with something. So I want to know more about this list.
Curt Jaimungal
Tell me about this list. Well, should I bring it up on my computer screen and tell you a couple of the other things? Please.
David Deutsch
Well, obviously I was wrong, and being wrong could be a spur to inventing something. I’ve got the list up here in front of me. Another one was Mathematica, Stephen Wolfram’s program, because I thought there could never be a general-purpose application interface that would allow you to define your own mathematical notation. Most serious uses of mathematics depend on making your own notation as you go along, but Mathematica can. I didn’t think it was possible.
Curt Jaimungal
Just a moment to linger on this notation aspect. What are you referring to? Do you mean, say, like Leibniz invented the little S that’s squished together for an integral, and that would be, I imagine, trivial to…
Curt Jaimungal
Program anything, even back when early computers came out, to display whatever notation you like.
David Deutsch
No, I mean things like, when working on quantum computers, I wanted to go over to the Heisenberg picture, which was an unusual thing to do, and I wanted a notation that was very suitable for the Heisenberg picture. So instead of using sigma matrices, I wanted to say a Q matrix, where Q was a function of T, a function of time, and then I wanted to have an automated thing to say: take the commutator of two Qs. The commutator of two Qs at the same time would be zero unless they were the same Q, in which case they’d be the Pauli algebra. And then at different times, the commutator would depend on how much the Hamiltonian had evolved one of them compared with the other one at an earlier time.
I did have a computer at the time. It was a home computer, and I had to write my own software for doing that for me. So I wrote a little program to manipulate these Q quantities. With Mathematica, I could just define the Q quantities and Mathematica could do it. I didn’t see how a general-purpose thing of that kind could exist, but Mathematica did. After getting Mathematica, I didn’t have to write my own program to manipulate things anymore.
Curt Jaimungal
Okay, tell me more about what’s on this list.
David Deutsch
Okay, so another important one was when I was first told about the laser guide star technique, for allowing telescopes to see through shifting atmosphere, I didn’t think that that could make much difference. I thought the difference that could make was very marginal, because the atmosphere affects the laser beam going up as well as coming down, so you don’t know what to correct for. I was in Dennis Sciama’s department as well in the early days of people doing this, and they were making the hardware and they explained to me how it worked. I was saying, I don’t see how that can be. They were trying to explain it to me. Perhaps they weren’t trying to explain it very well, but I came away with the idea that this wasn’t going to make much difference. It makes a lot of difference. So that was a piece of hardware, or experimental physics, where I underestimated the power of an idea.
Curt Jaimungal
What’s the latest on this list? X’s Community Notes? Okay. All right. Tell me about that.
David Deutsch
Well…
Curt Jaimungal
So…
David Deutsch
So a previous one was Wikipedia, which I didn’t think could work because it would get filled up with spam edits. I don’t know who thought that would work. It’s remarkable. Well, it worked for several years, and that’s why it’s on the list, but it’s now on the list crossed out, because now it no longer works. This failure mode has actually happened, but several years later. So I don’t know why it worked. I still don’t know why it worked when it did work, but I now know why it isn’t working anymore, and it was my original objection.
I thought that the Community Notes thing would suffer from the same problem, that the error-correction mechanism would itself get taken over, a bit like AIDS infecting the immune system, so that the immune system was not capable of combating AIDS. I thought that the trolls and the bad actors on X would find ways of gaming it. Okay, maybe they still will. But again, I thought it wouldn’t work at all. I thought it would make matters worse. But it didn’t. It has made matters better.
Curt Jaimungal
The issues you’re referring to regarding Wikipedia, are they of spam or of bias?
David Deutsch
Bias, except I don’t think bias exists. It’s error, either intentional or unintentional.
Curt Jaimungal
Interesting. Okay, let’s talk about Everett. I heard that you had a restaurant conversation with Everett. Is that story true, and do you mind retelling it if it is?
David Deutsch
Yes. Bryce DeWitt contrived to let me sit next to Everett when he visited Austin. I forget when it was, sometime in the 70s. The group of us, postdocs, graduate students, and professors, often did go out to one of the restaurants in Austin and have lunch together. On that occasion Everett joined us, and I had a long conversation with him over lunch.
Curt Jaimungal
What happened during that conversation? Did Everett say something that convinced you of many worlds?
David Deutsch
No, no. I was already, this is why Bryce DeWitt sat me there, convinced long before. But I was curious about what Everett thought about various of the issues that came up. The most important thing perhaps, for any historians watching this, there’s a sort of myth growing up about Everett, or two myths. One was that he didn’t think in terms of parallel universes, that he thought in terms of relative states. But he was the most enthusiastic person about parallel universes that I had met up to that point. He was very enthusiastic. The relative states were a thing that was imposed on him by his supervisor Wheeler. So that was one thing.
Another thing is that there was sort of folklore in physics at the time, I think people have realized that this is false, that he left physics because of the lack of reception of his ideas.
David Deutsch
But that’s not the case. He left physics because he wanted to make a fortune. And he did make one. I mean, he didn’t say this, but presumably he didn’t have any problems that he thought he could solve by remaining in academia, so he went into the consultancy business and worked for the Pentagon.
Curt Jaimungal
Wait, was it so that he could be rich or because he didn’t have problems to be solved?
David Deutsch
Yeah, sorry, I was speaking too glibly. It’s because he wanted interesting problems, and he found interesting problems in a different way, in optimization. I don’t know what it was exactly.
Curt Jaimungal
What was he like personally?
David Deutsch
Very intense. Very, very smart. I mean, very quick on the uptake, and quick in jumping from stepping stone to stepping stone. Chain smoker. That upset everybody even then, even in the 70s. People smoked, but not the whole time like that, and he was a chain smoker.
Curt Jaimungal
Do you think he understood his many-worlds theory beyond just unitary evolution?
David Deutsch
Yes, yes. For a start, he’d thought deeply about the problem of probability. He got that wrong, and Bryce DeWitt had a better theory, which was also wrong, and my theory, which is right. I developed that because Bryce DeWitt told me that his version was wrong, and he explained it to me. I have told this story many times. I used to go and see him in his office in Texas, and whatever we were talking about, at some point he would say, well, there’s this problem of probability. He would write on the board what the problem was, and then I would say, okay, I see the problem, I’ll think about it. By the time I got home, I’d forgotten what the problem was, and this happened several times, until finally, I have it in mind as about the fifth time, but maybe it was only the second time or something. Anyway, finally I went home and I still remembered what the problem was when I was at home. Then I worked on it, and then I solved it. This is now called the decision-theory approach to probability in quantum theory.
Curt Jaimungal
I see. Okay. What were Wheeler’s and DeWitt’s and Graham’s roles when developing or promoting Everettianism?
David Deutsch
Well, this is a very complicated story and you need to ask a historian. But as far as I know, in short, Wheeler hated the many-worlds interpretation, as it was called then. But Wheeler was a good supervisor and wanted to give his student every possible opportunity to get his work seen as far as possible. It was Wheeler who sent Everett’s paper to DeWitt, and DeWitt wrote a scathing response saying, there’s a problem with this, there’s a problem with that, there’s a problem with that, and he ended up saying, and finally, I don’t feel myself split. Everett wrote back his famous reply saying, Galileo didn’t feel the Earth move, but it does.
That persuaded DeWitt, and he then became for several years the major backer of Everettian quantum mechanics. He got together this book, The Many-Worlds Interpretation of Quantum Mechanics, Princeton University Press, which contains every paper even remotely relevant to Everett, in not a very thick book at the time. He is responsible for me and many other people getting interested and taking the theory forward. So DeWitt, as I said, had a theory of Everettian probability which didn’t quite work, and he knew it didn’t quite work and he wanted to fix it.
David Deutsch
I don’t know why he didn’t fix it himself. He was doing some highly mathematical things above my head at the time. DeWitt had a kind of attitude that it was kind of obvious that this was the right interpretation, and he didn’t have any interest in working on it. It’s backwards-looking.
That was true of Everettian quantum theory for many years, that everything was focused on trying to explain again and explain again and again to reluctant people why the prevailing view is untenable, why the Everettian view is illuminating and it’s the only possible one that will work. The trouble is that that attitude, as I have often said, is a bit like if biologists had spent all their time proving and reproving that creationism doesn’t work and that you should have Darwinism. Yes, you can keep doing that, but what was needed was to improve Darwinism and make further progress. They wouldn’t have made further progress if they’d kept on just engaging with this problem of why the creationists are wrong.
So Everettians tended to, by the way, I think no one was a full-time Everettian. They were all working on other things as well. They did spend a lot of intellectual effort on killing and re-killing these zombie theories that were already dead. Until relatively recently, a lot of progress has been made on Everett.
After the probability thing, there’s also the question of the structure of the multiverse, which I think is still an unsolved problem. We don’t have an equivalent geometry of the multiverse, the same in the sense that we do for spacetime. We can say spacetime is a four-dimensional pseudo-Riemannian manifold with metric, so we can say what it is mathematically, which is different from saying it obeys Einstein’s equations. Here are the equations it obeys. That’s a different thing. With Everett, we do not have that statement. The multiverse is, we don’t have that yet. We only know it in special cases like measurement and quantum teleportation and so on, but we don’t have a general theory of what the multiverse is in general.
Curt Jaimungal
Speaking of people who are skeptical, and then you constantly have to disprove or correct their misunderstanding of many worlds, I was speaking to Leonard Susskind on this podcast. I asked him about many worlds, and he said there are several technical questions that people who are believers in many worlds have, sorry, that I have toward them, that they aren’t able to answer. He listed two that I recall. One was that branches in quantum theory can recombine. He said this whole notion of completely separate branches is, well, he questions that. We’ll get to that. And then number two, he said, well, what about if you have, sure, you can have half a branch here, half a branch there, and the Born rule says 50-50, but what if it’s two branches and then one is one-third and the other is two-thirds? Does the universe split into three? And then irrational numbers, and so on.
So I know that these issues have been solved, or at least have answers, and have had answers for decades. My question is two parts. One, I would like you to actually answer those questions for people who are in the audience who are like, yeah, those sound like reasonable objections. But then number two, someone like Susskind, who’s in the field of fundamental physics…
Curt Jaimungal
He said they’ve never been able to answer this. So it’s either that he’s asking people who are rudimentary in many-worlds theories, or he’s not asking them, or he’s not listening to them, or something like that. I don’t know what’s going on there, and I don’t know if you see that as well with many of these people who are skeptics saying, look, they’re never able to answer this, and you’re like, I’ve had answers to these, you’re just not listening. I guess that’s a psychological question which you perhaps don’t want to go down. But either way, what is the answer to those two critical questions?
David Deutsch
We’ll get to the psychoanalyzing question later. So that was the one about recombination of branches. Well, this is going right back to Everett. One of the things that was kind of mistaken in Everett’s view is that although he made the enormous bit of progress of regarding a measurement as a quantum process instead of regarding it as a classical process that gets an answer, so the measurement process is a process like any other, and then you see that there are superpositions of the observer as well as the system, and then you can see how the correlations happen and so on. However, although he analyzed the measurements that way, he analyzed the measurements in terms of what happened at the beginning and what happened at the end. He didn’t actually ask what happens during the measurement, at the time when the branches are forming. People later did that in the 1980s. I had a go at doing it in the late 70s and early 80s, and my proposal was rubbish, but at least it kept me interested in the subject. Then some philosophers actually were the people that persuaded me what the right answer is, which is that the branches are emergent properties.
Branches don’t appear in the fundamental theory. Instead of, like in Everett’s way of doing it, you had one world and then three worlds, let’s say one world and a million worlds, what really happens is that there isn’t really one world. Even when you have a single pure state of a system, the state of a particle being at a particular place also includes, within that unity, a diversity. The more the particle is in one place, the more its momentum is different. So there is no such thing as there being one world at the beginning. There is always a continuum of universes or worlds, but they’re only worth calling universes when they subsequently evolve independently of each other. Typically that happens when there’s been a measurement. Before there’s a measurement, when there’s just a particle, a wave packet sitting there…
David Deutsch
Yes, there’s lots of momenta. There’s lots of positions all happening at once. Nothing is ever sharp. But you can’t say that there are different momenta in different universes, because all those universes are interacting with each other. You should only call something a universe when it is causally autonomous. In other words, it’s behaving exactly as it would if the others were not there. That’s what happens after a measurement.
It’s been a bit long-winded, but the answer is that the rejoining of universes is what happens in an interference experiment. During the interference experiment, you can’t speak of universes, because the different branches are affecting each other. Precisely, the interference is precisely the fact that the different paths of a single photon around an interferometer are not behaving as if the other were not there. When they come together, they do something completely different if the other one is there from what they would do if the other one weren’t there. That’s the whole interference phenomenon.
The picture you can have in mind is that there’s a continuum in some kind of entity that’s a bit like spacetime, but in Hilbert space or something, in the multiverse, which we don’t know how to classify mathematically yet. Then that continuum just differentiates itself into two, and as it’s differentiating there is no moment of split. What happens is that branch A is affecting branch B less and less. When they have separated enough, like when you have made the measurement and you’ve copied it or something, then they’re hardly affecting each other at all. They’re affecting each other only to the level of or something. So then you can speak of those things as different universes. That happens after a measurement. During interference, and also in the general case, you can’t speak of universes. You can only speak of the multiverse and the multiplicity of values of things within the multiverse.
Now briefly speaking: probabilities. Basically, why do we need probabilities at all? The answer within physics is basically because we need to know when we have refuted a theory. If the theory says that there’s a probability of of X happening, and the rest of the probability is all about Y happening, why can we be confident that Y will happen and that we will never see X, even though we know that in the multiverse some of us will see X? Why should we expect Y to happen and not X?
You only have to give an account that synthesizes probability in certain special cases, like when there’s a thing to expect. That only happens after a measurement, at the time when the universes have decohered. In fact, this should have been obvious: we know that when the universes have not decohered, they don’t even obey the probability calculus. Rather, the physical world does not obey the probability calculus when you’re in the middle of an interference phenomenon. There’s a…
Curt Jaimungal
Probability of a half that this will happen, and probability of a half that that will happen, and at the end there’s a probability of one quarter, one quarter, one quarter, one quarter, when they…
David Deutsch
Pass through another beam splitter, and that’s simply not true. The probabilities do not add up in the way that the probability calculus says relative probabilities ought to behave. But you want to have relative probabilities behaving properly when there has been a measurement and you’re actually looking at what happens, rather than thinking about it theoretically: what is the particle doing?
If you look at it that way, then it turns out that quantum theory with the Born rule removed, having no reference to probability, just stripped-down quantum theory without probability, and then you take classical decision theory, which is about things like if you prefer A to B and you prefer B to C, then you prefer A to C. If you take classical decision theory and take the probability rule out of that, the rule being that you should prefer the thing that has the highest expectation value of your utility, you take that out because you’ve taken probability out and therefore there’s no such thing as an expectation value.
Curt Jaimungal
You …
David Deutsch
Then you put the two together: quantum theory without probability and decision theory without probability. Put them together, and they tell you what a rational person would decide in the case where there is some substantial amplitude for two or more things to happen. In those cases it gives the Born rule, but you don’t have to postulate it.
Curt Jaimungal
So when you were developing early quantum computing, was Everett’s interpretation important to you, or was it a driving force behind the idea of quantum advantage?
David Deutsch
Completely crucial. I couldn’t have done it if I’d been thinking in the old way. I think it’s a very interesting theory.
Curt Jaimungal
Does that prove to you in your mind that research in quantum foundations or philosophy of quantum mechanics actually drives scientific breakthroughs?
David Deutsch
Well, it did in this case. But I don’t think that’s a proof of anything. As I said some time ago, it very often happens that fundamental things cause practical improvements as well, eventually. But I don’t think it’s tenable to say that that’s why one should think about the fundamental things. It’s a thing that can happen. Sometimes I think that often happens, perhaps even very often. I don’t know how to characterize it, but it certainly happened to me in this case. If I had thought of quantum theory in the wave-function-collapse way, I wouldn’t have thought of quantum computation.
In fact, at the time when I was persuading people of quantum computation, that it’s a thing and they ought to think that way, a lot of them didn’t want to make this change in their conceptualization. I remember talking to Landauer in his office when I think I was either just about to publish my first paper on that or just after, and I gave a talk. He very kindly invited me to give a talk, even though he very much disapproved of the theory at the time. He was saying to me, no, this is just something you write down on paper. This can never work, because when the wave function evolves in this way and then there are two systems…
The door was partly open in his office. He had quite a small office for such an eminent man. He grabbed the door and said, so when you shut the door, slam, and he slammed the door, he said, you see, it’s not coming back. If this was quantum, it would bounce back. So basically in modern terms he was explaining why quantum error correction is impossible. But it isn’t impossible. Then I infuriated him by saying, that’s a problem, that’s a problem that will be solved.
Curt Jaimungal
And you said this behind the closed door?
David Deutsch
No, no, I didn’t. That would have been a good joke. I said that’s going to be solved. In his mind it was a fundamental problem that will never work. Asher Peres made the same objection to me at the Broadway conference. He said, I was writing stuff up on the blackboard, and he said, yeah, but that won’t work in real life because there will be more effort to correct those errors than what you’re correcting. Again I said, technical problem. That is going to be solved, and indeed it was solved by Peter Shor very soon afterwards.
Curt Jaimungal
Yes.
David Deutsch
Now the reason I thought it was a technical problem and they didn’t is because I was thinking Everett and they were thinking collapse.
Curt Jaimungal
Is the universal wave function as physically real to you as the camera, or your laptop, that’s in front of you, and the microphone? Is it more real? What are David Deutsch’s ontological commitments?
David Deutsch
So I try not to have commitments. Like W. W. Bartley said, we should retreat from commitment. I also try not to have beliefs either. Let me put it this way around: my theory that this computer that I’m talking into now exists has the same status in my mind as the theory that many copies of it exist in other universes. In both cases, that status is that there are no rivals to that theory that I know of that aren’t nonsense. I mean, there are only nonsense rivals.
So you could call that belief, but that suggests that I want it to be true, or that I would resist it not being true if an argument were presented. I hope and expect that that is not the case. Whereas the opponents of Everett do have beliefs, and that, I think, is what is… Well, again, I said I wouldn’t be psychic, so I’ll try not to be psychic.
Curt Jaimungal
Let me be psychic. It seems like beliefs to you is synonymous with dogmatic beliefs.
David Deutsch
Yes, although dogmatic belief is a belief that nothing could persuade you otherwise. That’s absolute dogmatic belief. But I’m against having a ten percent dogmatic belief, saying, I’m pretty sure that it would take a lot to persuade me that that’s true. I don’t have anything on that scale, and I don’t think that stuff on that scale really exists. It’s a misconception about how thinking works.
The way thinking works is to have a problem, then attempt to solve it, then criticize the attempted solutions. If you’re lucky enough to come to a place where there are no criticisms left that you can think of, then you don’t accept the theory. You just are in the position of not being able to think of another criticism, so you go and work on something else.
Curt Jaimungal
So when David Deutsch has an interpretation of Everettian quantum mechanics, and Sean Carroll has an Everettian quantum mechanics, and David Wallace has an Everettian quantum mechanics, are these different theories? Are there disagreements between you and Sean Carroll and David Wallace?
David Deutsch
There certainly are disagreements, but the things we’re disagreeing on are very, very minor compared with the difference between Everettian theory and all the other theories. For example, David Wallace thinks that there are substantive assumptions behind my proof of the equivalent of the Born rule in the decision-theoretic approach to quantum probability, and I don’t think there are substantive assumptions. So he naturally works on trying to make clear what those assumptions are, to analyze them, to see what the arguments for and against those assumptions are. I don’t think such assumptions are needed. So that’s the difference between me and David Wallace.
Curt Jaimungal
Assumptions in this case, is that just axioms or something else?
Curt Jaimungal
Okay, so is there a rigorous list of axioms of your specific Everettian approach somewhere?
David Deutsch
So I don’t believe in axioms, but David Wallace has written down how he characterizes my approach. In his book, he has what he thinks are the axioms behind my approach. I don’t think physics should work that way. Axioms are a bit like definitions. They’re something you can come back to after you’ve got a theory, but working forward from them, the axiom never completely captures the theory anyway. We know that from Gödel and so on. We know that Peano’s axioms don’t tell you everything that’s possible to know about the integers. But you can have a conception of the integers and you can say, oh, this new axiom someone’s proposed brings the theory closer to my conception of the integers.
Curt Jaimungal
Well, I was going to ask you what the axioms of constructor theory are …
David Deutsch
Oh, well, the basic axiom of constructor theory, if you can call it an axiom, is that the laws of physics can be characterized by specifying a dichotomy between physical processes that can be brought about by something else, which is a constructor, and those that cannot be brought about. Once you’ve stated that dichotomy with all conceivable physical transformations, you’ve stated the laws of physics.
So then the constructor-theory research program is, on the one hand, to reformulate all existing laws of physics in those terms, saying what is possible to bring about and what is impossible to bring about, and then to formulate purely constructor-theoretic laws that are over and above that, which are a bit like the laws of quantum theory are at a level above the dynamics of particular systems. Quantum theory doesn’t refer to any particular system. It just has a theory of what laws of physics can say. They have to have a space of states, and they have to have a Hamiltonian, or you would have a Lagrangian, however you phrase it.
Constructor theory is a level above that. It’s a law about laws, but also a law about laws about laws. That’s actually how I first thought of it, because I first thought of it as an extension of the theory of quantum computation. In a way, the theory of quantum computers contains the whole of the rest of physics, since the universal quantum computer can simulate any other physical system. The set of all motions of the universal quantum computer is in one-to-one correspondence with the set of all possible motions of anything. So in a way, the study of physics is the study of the possible motions of the universal quantum computer.
Well, then I realized that that wasn’t right, because you still have to have a theory of which programs of the universal quantum computer correspond to which physical systems, and that is not contained in the abstract theory of the universal quantum computer. So I wanted to have an extended theory of the universal quantum computer that included saying which program corresponds to which physical system. I then built on that and so on, and eventually got down to the core of the issue, which was the dichotomy between things that can be brought about and those that can’t.
Curt Jaimungal
Speaking about interpretational splits as a framework or theory develops, for constructor theory, since I think almost more than a decade now, has it developed in a manner that you’re largely happy with, you agree with, or are there…
David Deutsch
No, other people who are, you’re on the wrong constructor branch. Yeah, so you’re right to say that the development of constructor theory since I first thought of the idea has been mainly the correction of errors in it, mainly the correction of ways that I thought were viable which turned out not to be viable.
In fact, Chiara Marletto first came to me after I’d given a talk at the Clarendon Laboratory about my ideas of constructor theory. She came up to me at the end of the lecture and said, that thing you said can’t be true because so and so. I said, oh, yeah, right. Okay, thanks. Then I invited her, and eventually we ended up working together on the theory. We ended up solving that and many other things. The first thing that resulted in was Constructor Theory of Information, which is the only constructor theory of something that we’ve completed so far. You could say that we’ve also got a constructor-theory version of quantum theory, but you may or may not think that. For information, we made real progress, and we unified classical and quantum information via constructor theory. It couldn’t have been done otherwise.
Curt Jaimungal
As for how it’s developed with other people involved in constructor theory, there’s now a “program of constructor theory,” largely speaking. Do you look at that field with pride and happiness, or are there some children that you’d kick out of the house?
David Deutsch
I like being baffled. I haven’t really worked on quantum computers ever since I stopped being baffled by the field. There’s still plenty of things to be baffled with on the experimental side, but I’m useless at experimental physics. There’s nothing I can really contribute to on the theoretical side nowadays. In constructor theory, no, it’s not satisfactory yet, but I don’t think that there are crass errors in it anymore.
Curt Jaimungal
Okay, now before my last question of what advice do you have for people who are watching, and as I mentioned they comprise professors, researchers, graduate students, but also laypeople, I have a question from Scott Aaronson here about free will. Scott said to me, David once remarked that he’s certain that free will exists and equally certain that it has nothing to do with quantum mechanics. He wants you to expound on that. What could possibly be the source of such certainty on either account?
David Deutsch
Rather like with belief, I certainly don’t want to be certain of anything. On introspection, I think if I was presented with a good argument on either of those points, I would be open to it. But in both cases, I think the idea that free will doesn’t exist, or that free will requires quantum theory, are both in the status that there is no such position. You can say maybe it has something to do with quantum theory, but there is no actual theory, or even in principle, apart from Penrose’s, which I think is wrong for other reasons. There is no actual theory of how quantum theory could produce free will, or philosophical theory about how free will could not exist. And I think that the philosophical…
Curt Jaimungal
Problem.
David Deutsch
Problem that people have with free will is that they have a conception of free will which, by definition, violates the laws of physics. Although they don’t often say it like this, it’s basically: free will is the human capacity to override the laws of physics. I don’t think anything overrides the laws of physics. We might be wrong about what the laws of physics are, again, like Penrose thinks, but I don’t see in that case a proposal for different laws that would help with the problem of free will.
Now I think that free will has to do with knowledge. The problem that does make sense about free will is that we have an intuition, and it’s in all our explanations of human behavior and so on, that when we make a choice, not a random choice, but a choice that we have thought about, we have brought something new into the world.
For example, when Einstein was inventing general relativity, he was bringing that into the world. It didn’t exist before. The theory of general relativity did not exist before Einstein thought of it, and ultimately if you think it did, then you’ve got to say it was in the big bang. I don’t think it’s a tenable view, philosophically or physically, that all the knowledge that’s ever going to be created in the world was in the big bang, and that all that happens is that it’s being made real in some sense. Although why one time should be more real than another time, I don’t know either, because that seems to violate relativity as well.
So I think there is such a thing as bringing something new into the world, and the thing that you’re bringing into the world is knowledge, or explanatory knowledge. The thing that didn’t exist before, even if the equations of general relativity existed, like apparently Hilbert had the equations before Einstein, but he didn’t know what they were about, he didn’t understand the physics problem, which again touches on the thing we were talking about earlier. Einstein was seized of the physical problem, and he eventually came up with the equations. I think that the discovery of general relativity was the discovery of the explanations of what the equations mean, which actually came before the equations.
It’s the same with everything. When you decide that you want to have a curry for dinner tonight, and it was possible that you would have chosen something else, that decision is something new you have brought into the world. When children learn their native language, the language which they learn is different from everyone else’s, and it has been an act of creativity to bring that language, which is unique to them, into the world. You seem to be puzzled, but if I make a list of 20 words and ask you to define them, you will not produce the same list of 20 definitions as anyone else on the planet. So everyone has a different language in mind, and it’s a bit of a miracle that we can communicate with each other.
The reason we can is basically error correction, and again creativity, because we need not mechanical error correction, but creative error correction. So creativity brings something new into the world, and then there is no problem with how come you’re violating the laws of physics, because it’s not new trajectories of the electrons that you’ll bring into the world. It’s new knowledge, which can only be understood at an emergent level. So that’s my answer, I think, to both questions. I’ve forgotten what they were now.
Curt Jaimungal
Well, one was about advice to prospective students and researchers and so on.
David Deutsch
Right. Again, I don’t know, because I’d have to be psychic to be able to second-guess someone else’s decisions about their own life. But at the most general scale, I would say: go for the thing that is fun rather than the thing that you think will lead to fun, or lead to some other benefit, heaven forbid some other benefit that isn’t fun. That is extremely dangerous. The more prophecy you have to make to justify your present choice, the more error-prone it’s going to be and the more different from the reality that is going to happen.
Curt Jaimungal
What do you mean the more prophecy you have to make? What do you mean?
David Deutsch
Well, if I decide to work on LLMs because I think that LLMs are going to give rise to AGI, and I do that because I think that AGI is going to be terribly dangerous and that we have to understand it well, then I’m prophesying various things in the future according to some theory that I have now. But that theory is going to change. If that theory doesn’t change over the period we’re talking about, 10, 20 years, then I won’t have discovered anything, or nobody will have discovered anything if that landscape of ideas doesn’t change. So it’s better to make decisions according to the shortest possible timescale of prophecy.
Curt Jaimungal
I see. So in the case of someone predicting about AGI, thinking it’s an important issue, let me attempt to solve that now, that would be different if they were passionate about trying to solve the issue of AGI.
David Deutsch
Exactly. I imagine you would say, yeah, that’s your curiosity. That’s the fun you refer to. Go after that. Don’t try to leapfrog ahead one decade, let alone two decades, and then think backward from there, because you could be incorrect, most likely. If you have an idea for AGI, you can work on it today. You can drop what you’re doing and work on that instead. That’s the sort of thing you should be doing.
Curt Jaimungal
Professor, it’s an honor to speak with you. It’s something I’ve wanted to do for years, and I’m more than honored to have spent a couple hours with you. I appreciate that you took some time out of your day to spend with me.
David Deutsch
Well, it’s been fun.
Curt Jaimungal
To say it oppositely, hopefully next time we speak I would like to talk about consciousness, as that got brought up toward the end, and also the philosophy of science. Thank you.
David Deutsch
You’re welcome. Fun chatting.
Curt Jaimungal
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