00:00:00.000 So, at the end of April this year, I was honored to be invited to Wolfson College in Oxford
00:00:06.400 to document a series of informal presentations and conversations concerning the role of probability
00:00:11.920 in physics. This so-called unconference was sponsored by the Utopia Foundation.
00:00:17.840 Perhaps the most polarizing issue to emerge over the course of the discussions was the
00:00:22.800 many worlds interpretation of quantum mechanics, which seemed to pitch some of the more practically
00:00:27.520 minded experimental physicists like Marcus aren't from the University of Vienna.
00:00:32.480 Against those big picture theoretical physicists like David Deutsch from Oxford,
00:00:36.800 who were more willing to embrace this grand view of ever-branching parallel universes.
00:00:41.920 So, for the sake of this video, I asked Marcus and David if they wouldn't mind
00:00:45.600 hashing out their differences over their lunch break in one of the corridors at Wolfson.
00:00:49.600 And I began by asking Marcus how he conceived of the tensions here.
00:00:53.520 The fundamental problem that we all try to solve is why is there this unitary evolution
00:00:58.960 of quantum mechanics, which seems to explain everything very naturally.
00:01:02.240 And out of a sudden, during a measurement this evolution has to be reduced, collapsed in the
00:01:09.760 Copenhagen interpretation and that's I think something that David doesn't like.
00:01:12.640 He wants to have everything in the same mathematical formalism.
00:01:16.880 But if you follow it through, it leads to realities which seem to multiply.
00:01:21.920 And then my question to you is, what is really the meaning of reality to you?
00:01:27.600 Because I experience only my single reality here.
00:01:32.640 Yes, you put it in terms of how do we make sense of the unitary evolution
00:01:37.920 compared with what we see at a measurement and so on.
00:01:43.200 I think we want to understand the world, we want to understand how the world is.
00:01:48.000 And that is not necessarily what we perceive, our perceptions are at the end of a long
00:01:55.760 chain of physical processes of which themselves we only have scientific knowledge or indirect
00:02:06.240 So I would start with the question, how do we explain quantum phenomena like interference?
00:02:14.160 It's not how do we make sense of quantum theory which gives the right prediction.
00:02:19.760 But first, before that, how do we explain quantum phenomena?
00:02:23.040 So there's an interference process and we have an interference pattern which we can see without
00:02:31.520 any quantum mechanics that the result of the experiment cannot be explained by the events that we see.
00:02:41.360 Now this is not very unusual, this happens a lot, you know, in physics and ultimately
00:02:50.240 So we have to infer things that are not there, although it infers is the wrong word.
00:03:03.360 But what makes you then certain that there must be a parallel world in that sense,
00:03:08.720 or many, infinitely many parallel worlds, instead of either some hidden variables,
00:03:14.640 though of course there's an experimentalist, I know that some kinds of in variables
00:03:19.680 And do we need the notion of reality more than the notion of information?
00:03:23.920 It's such a say the many worlds thing, an information world or a reality world in a thingy sense.
00:03:30.240 Yes, so that's a very interesting point about the relationship of information to reality
00:03:36.720 because many people think that the concept of information is somehow prior to physics,
00:03:43.200 that the laws of information like mathematical theorems, they must be so.
00:03:49.280 Whereas I tend to the view that what information does and what it can do in the world is determined
00:03:55.680 by physics and therefore in regard to physics, physics is a theory of the world of the
00:04:03.040 reality of the world, not about the information. There is this view that quantum mechanics
00:04:10.240 only tells us what we will see. And it's silent about everything else.
00:04:15.360 Everything else is just mathematical formalism.
00:04:18.800 But I think that ultimately leads to solipsism and it is no good philosophically.
00:04:24.000 But even more important maybe for a physicist, it's no good for finding out what the next theory
00:04:29.440 will be. If you just think a theory is predictions of experiments,
00:04:36.160 then we would never have got from Kepler's theory to Newton's theory from Newton's theory to Einstein's
00:04:41.040 because they differ from each other in what in predictive terms is a tiny amount.
00:04:48.000 But in explanatory terms it's an enormous amount changes our whole view of the universe.
00:04:52.480 But in terms of predictions coming back to the many worlds and the predictive power of the many
00:04:57.840 worlds. I'm sitting here, I can only probe my local world. Even within my local world I seem
00:05:04.560 to experience all these funny quantum superpositions where you would probably say another
00:05:10.320 part of the multiverse is branching into me again or into my world again. But this process is
00:05:15.760 kind of experimentally at least at the moment very difficult to access. And the question is,
00:05:23.040 Yes, so I mentioned interference experiments. So interference in an interference experiment we don't
00:05:28.880 even with this notion of seeing things by through the explanation. We only see that the let's
00:05:37.360 say the photon exists in two instances rather than rather than that the whole world does.
00:05:44.400 And when you talk about your experience at the moment, as you say, we can only probe whether
00:05:51.760 an experimental outcome is caused by a single history or by multiple histories of an atom,
00:05:58.880 a molecule, I mean you in your talks you get wonderful example of very large objects
00:06:05.360 from an atomic point of view exhibiting interference or existing in more than one instance.
00:06:12.800 When we have quantum computers we will be able to have very large, very complex entities
00:06:20.880 existing in superpositions. So in principle I suggested a long ago before this was remotely
00:06:27.600 on the cards experimentally that if we had a quantum computer on which an artificial intelligence
00:06:35.200 program was running, say with a human level artificial intelligence, then this entity would be able
00:06:41.600 to experience interference in its own consciousness.
00:06:48.000 Well some people would say that your consciousness would collapse your reality in a certain point.
00:06:52.640 So if that happened that would refute the average interpretation or as I would say would refute
00:06:59.040 quantum theory. And that would be a very interesting problem and that's one of the reasons why
00:07:08.560 scaling up both the size and the complexity and the mass of phenomena that are experimentally
00:07:15.040 observed but can only be explained by quantum theory is very important.
00:07:22.400 And we just need to close the gap between that and the AI because the AI would not be having
00:07:29.600 this conversation or at least the AI would not be able to make the argument that you just made.
00:07:35.040 It would have to say I've only got evidence of many worlds on the scale of my mind but not bigger.
00:07:45.600 Yeah but there's something in the formulation of phrasing of our sentences where I'm getting
00:07:52.080 also dealt with it is when I talk about these things about superpositions I always make these
00:07:56.160 quote unquote when I say a particle is at the same time here and there because there are two
00:08:02.000 words that I don't understand three words that I don't understand. First the word is so reality.
00:08:07.280 The second what does time really mean and the third one what does space really mean and we don't
00:08:11.600 have any experimental evidence that the particle is at the same time here and there we just have
00:08:16.640 the physical description the quantum mechanical description that the way function behaves as if
00:08:22.640 and how can we make the step to the many worlds then?
00:08:24.800 I think we have something slightly more than that. Again you come from the theory but I think
00:08:31.680 prior to the theory we have the experience that this thing cannot be explained by single
00:08:38.800 trajectories. We don't have to believe quantum mechanics to see that so we rule out single
00:08:46.640 trajectory explanations and that we have before we have quantum mechanics. If we didn't have
00:08:53.200 quantum mechanics it would be a mystery. We would say there simply is no explanation.
00:08:57.760 Well hypothetically could this maybe be explained by some very weird folding of space time in
00:09:05.600 this case so not really making another multi-verse or another branch of the multi-verse but really a
00:09:10.560 new 3d or 40s space time or another 11 general space time. Could that jump through a shortcut
00:09:18.640 through another higher dimension so to say from there to the other? Yes it's the answer. Calling the
00:09:26.240 multi-verse many universes is a bit of a misnomer because the whole point of it is interference
00:09:34.400 many universe parallel universes would indicate separate universes or these universes that our
00:09:41.520 universe splits into two every time a quantum event happens whereas actually it's only the
00:09:47.040 electron or the photon or something that's splitting. Now if this can be explained in another way
00:09:53.440 then by quantum theory then another thing we know from just you know without theory with just
00:10:02.000 elementary reasoning about the experiments is that this other thing has to be immensely complicated.
00:10:08.480 It's sort of two to the power of the complexity of what we see and again when we have fully
00:10:15.040 fledged quantum computers we will have computations going on whose results cannot be explained by
00:10:22.480 any history of the computer that has it single valued so when that number is greater than the
00:10:32.160 number of atoms in the universe which will easily be attained as soon as we have quantum computers.
00:10:37.200 So yes there could be another explanation in terms of folded universes or other dimensions
00:10:41.440 or what but those things would have to be as complicated as the many universes and as some
00:10:49.840 people were saying that the other ever a conference these things would contain
00:10:55.680 other they would contain other instances of people or they would they would contain things
00:11:00.720 whose shapes or other instances of people and so on and other instances of the quantum computer
00:11:06.880 which would be interacting with our instance and so on. So it's really a matter of
00:11:12.000 terminology then whether you call that a multiverse or parallel universes or a much higher
00:11:18.960 dimensional reality than in classical concept. And in your multiverse concept
00:11:27.360 well there again zillions of branches and hypothetically you could also be here and in this other
00:11:34.320 part of the multiverse you could be in a different place, different time, different internal
00:11:40.400 state. What does that tell us about your identity? Does that affect anything about how you feel
00:11:46.160 as a human sort to say? It certainly has to be taken into account but it's logically
00:11:53.360 it's the same issue as am I the same person that I was 10 years ago that I will be in 10 years
00:12:00.560 time. If we go back to when I was a baby I certainly was not the same person as I am today
00:12:07.680 there is a continuity between me then and me now but that doesn't mean I'm the same person
00:12:14.560 because a car person I have changed drastically but the relationship between the past and the
00:12:21.280 present is one kind of thing the relationship between the present and future is a different kind
00:12:26.560 of thing the relationship between one branch ever at branch and another is a different kind
00:12:33.760 of thing but in all cases what makes us say that those things are real is the explanations that
00:12:40.720 we have for here and now. Although other scientists would say shut up and measure for the
00:12:47.120 calculator actually don't talk about things that you cannot see and the other part of the
00:12:53.840 other branches you don't see so by to you they are not too shut up. Yes well first of all I think
00:13:00.000 that that attitude involves saying that there are certain questions about reality that you're not
00:13:07.200 allowed to ask. You're allowed to ask how the experiment was prepared, you're allowed to ask what
00:13:13.520 will the results be. You're not allowed to ask how were the results brought to bounds by the
00:13:19.360 preparation so that's what's the therefore it's not an explanation in my terms but and as for
00:13:26.080 shut up that's really another way of trying to evade the consequences in terms of reality like
00:13:36.960 my favorite example is of dinosaurs in the past so there are people who say nobody ever saw
00:13:45.200 dinosaur nobody ever will and therefore it's just a frivolity to say that they really exist
00:13:52.000 that most we can say fossils behave as though dinosaurs existed but no paleontologist would
00:13:59.680 accept talking that way even though there is no experimental way of disproving that that
00:14:06.000 manual speaking so and that's because paleontologists are only interested in paleontology
00:14:12.800 because they want to know what really happened not they're not you know if they were
00:14:17.040 so interested in fossils they would be geologists.
