2018-06-19 Markus Arndt Are There Many Worlds? David Deutsch in Conversation with Markus Arndt
Duration: 00:14:26
Transcript
UnConference narrator/interviewer
So, at the end of April this year, I was honoured to be invited to Wolfson College in Oxford to document a series of informal presentations and conversations concerning the role of probability in physics. This so-called UnConference was sponsored by the Utopia Foundation. Perhaps the most polarising issue to emerge over the course of the discussions was the many-worlds interpretation of quantum mechanics, which seemed to pitch some of the more practically minded experimental physicists like Markus Arndt from the University of Vienna against those big-picture theoretical physicists like David Deutsch from Oxford who were more willing to embrace this grand view of ever-branching parallel universes. So, for the sake of this video, I asked Markus and David if they wouldn’t mind hashing out their differences over their lunch break in one of the corridors at Wolfson, and I began by asking Markus how he conceived of the tensions here.
Markus Arndt
The fundamental problem that we all try to solve is why is there this unitary evolution of quantum mechanics which seems to explain everything very naturally, and out of a sudden during a measurement this evolution has to be reduced, collapsed in the Copenhagen interpretation, and that’s I think something that David doesn’t like. He wants to have everything in the same mathematical formalism, but if you follow it through, it leads to realities which seem to multiply. And then my question to you is, what is really the meaning of reality to you? Because I experience only my single reality here.
David Deutsch
Yes, you put it in terms of how do we make sense of the unitary evolution compared with what we see at a measurement and so on. I would want to start before that. I think we want to understand the world, we want to understand how the world is, and that is not necessarily what we perceive. Our perceptions are at the end of a long chain of physical processes of which themselves we only have scientific knowledge or indirect knowledge. So I would start with the question, how do we explain quantum phenomena like interference? Not how do we make sense of quantum theory, which gives the right prediction, but first before that, how do we explain quantum phenomena? So there’s an interference process and we have an interference pattern which we can see without any quantum mechanics that the result of the experiment cannot be explained by the events that we see. Now this is not very unusual, this happens a lot you know in physics and ultimately every observation is made very indirectly, so we have to infer things that are not there. Although infer is the wrong word, we have to conjecture explanations. So that’s where I would start.
Markus Arndt
But what makes you then certain that there must be a parallel world in that sense, or many, infinitely many parallel worlds instead of either some hidden variables, although of course as an experimentalist I know that some kinds of hidden variables have certainly been ruled out. And do we need the notion of reality more than the notion of information? Is so to say the many worlds thing an information world or a reality world in a thingy sense?
David Deutsch
Yes, so that’s a very interesting point about the relationship of information to reality because many people think that the concept of information is somehow prior to physics, that the laws of information are like mathematical theorems, they must be so. Whereas I tend to the view that what information does and what it can do in the world is determined by physics and therefore in regard to physics, physics is a theory of the world, of the reality of the world, not about the information. There is this view that quantum mechanics only tells us what we will see and it’s silent about everything else, everything else is just mathematical formalism. But I think that that ultimately leads to solipsism and it is no good philosophically, but it’s even more important maybe for a physicist, it’s no good for finding out what the next theory will be. If you just think a theory is predictions of experiments then we would never have got from Kepler’s theory to Newton’s theory and from Newton’s theory to Einstein’s because they differ from each other in what in predictive terms is a tiny amount, but in explanatory terms it’s an enormous amount, changes our whole view of the universe.
Markus Arndt
But in terms of predictions coming back to the many worlds and the predictive power of the many worlds, I’m sitting here, I can only probe my local world, even within my local world I seem to experience all these funny quantum superpositions where you would probably say another part of the multiverses branching into me again or into my world again, but this process is kind of experimentally, at least at the moment, very difficult to access and the question is do you have a good proposal how to do it?
David Deutsch
Yes, so I mentioned interference experiments, so interference, in an interference experiment we don’t, even with this notion of seeing things through the explanation, we only see that the, let’s say the photon exists in two instances rather than that the whole world does and …
Markus Arndt
When you talk about your experience at the moment as you say we can only probe whether an experimental outcome is caused by a single history or by multiple histories of an atom, a molecule, I mean you in your talks you gave a wonderful example of very large objects from an atomic point of view exhibiting interference, so existing in more than one instance.
David Deutsch
When we have quantum computers we will be able to have very large, very complex entities existing in superpositions, so in principle I suggested long ago before this was remotely on the cards experimentally that if we had a quantum computer on which an artificial intelligence program was running, say with human level artificial intelligence, then this entity would be able to experience interference in its own consciousness.
Markus Arndt
Well some people would say that your consciousness would collapse your reality.
David Deutsch
Yes, so if that happened that would refute the Everett interpretation or as I would say would refute quantum theory and that would be a very interesting problem and that’s one of the reasons why scaling up both the size and the complexity and the mass of phenomena that are experimentally observed but can only be explained by quantum theory is very important. I fully agree because we need to do that. And we just need to close the gap between that and the AI because the AI would not be having this conversation or at least the AI would not be able to make the argument that you just made. It would have to say I’ve only got evidence of many worlds on the scale of my mind but not bigger. So and I guess that will always be true.
Markus Arndt
Yeah but there’s something in the formulation or phrasing of our sentences where I’m getting also doubtful at least when I talk about these things about superpositions I always make these quote unquote when I say a particle is at the same time here and there because there are two words that I don’t understand, three words that I don’t understand. First the word is reality, the second what does time really mean and the third one what does space really mean. And we don’t have any experimental evidence that the particle is at the same time here and there, we just have the physical description, the quantum mechanical description, that the wave function behaves as if, and how can we make the step to the many worlds then?
David Deutsch
I think we have something slightly more than that. Again you come from the theory, but I think prior to the theory we have the experience that this thing cannot be explained by single trajectories. Somebody, you know, we don’t have to believe quantum mechanics to see that, so we rule out single trajectory explanations and that we have before we have quantum mechanics. If we didn’t have quantum mechanics it would be a mystery. We would say there simply is no explanation.
Markus Arndt
Well hypothetically could this maybe be explained by some very weird folding of space time in this case so not really making another multiverse or another branch of the multiverse but really a new 3D or 4D space time or another 11D? Could that jump through a shortcut through another higher dimension so to say from one end to the other?
David Deutsch
Yes is the answer. Calling the multiverse many universes is a bit of a misnomer because the whole point of it is interference and many parallel universes would indicate separate universes and that these universes that our universe splits into two every time a quantum event happens whereas actually it’s only the electron or the photon or something that’s splitting. Now if this can be explained in another way than by quantum theory then another thing we know from just you know without theory with just elementary reasoning about the experiments is that this other thing has to be immensely complicated. It’s sort of two to the power of the complexity of what we see and again when we have fully fledged quantum computers we will have computations going on whose results cannot be explained by any history of the computer that has it single-valued, so when that number is greater than the number of atoms in the universe, which will easily be attained as soon as we have quantum computers, so yes there could be another explanation in terms of folded universes or other dimensions or what but those things would have to be as complicated as the many universes and as some people were saying at the other Everett conference these things would contain other instances of people or they would contain things whose shapes were the other instances of people and so on and other instances of the quantum computer which would be interacting with our instance and so on so it’s really a matter of terminology then whether you call that a multiverse or parallel universes or a much higher dimensional reality than in classical concepts.
Markus Arndt
And in your multiverse concept um well there are again zillions of branches and hypothetically you could also be here and in this other part of the branch of the multiverse you could be in a different place, different time, different internal state, yeah. What does that tell us about your identity? Does that affect anything about how you feel as a human so to say um
David Deutsch
It certainly has to be taken into account but logically it’s the same issue as am I the same person that I was 10 years ago or that I will be in 10 years time. If we go back to when I was a baby, I certainly was not the same person as I am today. There is a continuity between uh me then and me now but that doesn’t mean I’m the same person because I am quite a different person, I have changed drastically. But the relationship between the past and the present is one kind of thing, the relationship between the present and future is a different kind of thing, the relationship between uh one Everett branch and another is a different kind of thing, but in all cases what makes us say that those things are real is the explanations that we have for here and now although other scientists would say uh shut up and …
Markus Arndt
Measure, yes, calculate actually um just don’t talk about things that you cannot see, and the other part of the other branches you don’t see, so why do you dare not to shut up?
David Deutsch
Yes, well first of all I think that that attitude involves saying that there are certain questions about reality that you’re not allowed to ask. You’re allowed to ask how the experiment was prepared, you’re allowed to ask what will the results be, you’re not allowed to ask how were the results brought about by the preparation. So therefore it’s not an explanation in my terms. And as for shut up, that’s really another way of trying to evade the consequences in terms of reality, like my favorite example is of the dinosaurs in the past. So there are people who say nobody ever saw a dinosaur, nobody ever will, and therefore it’s just a frivolity to say that they really exist. At most we can say fossils behave as though dinosaurs existed but no paleontologist would accept talking that way even though there is no experimental way of disproving that manner of speaking. And that’s because paleontologists are only interested in paleontology because they want to know what really happened, they’re not, you know, if they were interested in fossils they would be geologists.
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