# 2013-05-08 - Closer To Truth - What Does Quantum Theory Mean [YouTube](https://www.youtube.com/watch?v=0CwPa0tScf8) Duration: 00:08:12 ## Transcript ### Robert Lawrence Kuhn 00:00:00 - 00:00:22 David, it was difficult enough for me to hear about the many-worlds interpretation of quantum theory with its continuous branching of innumerable different universes. Now I'm told that all these different universes have always existed and always will exist, and it's very difficult to understand. ### David Deutsch 00:00:22 - 00:00:57 What's happening to them, and the reason that they used to be thought of as a branching or splitting, is that they become differentiated from each other. At the beginning of time, perhaps the Big Bang or whatever was the beginning of time, the universes may all have been identical, and physical processes happening in them caused them to differentiate according to the laws of motion of quantum mechanics. So the total number, as it were, remains constant, but the degree of differentiation between them increases very rapidly, and that's what we call the arrow of time. ### Robert Lawrence Kuhn 00:00:57 - 00:01:06 Well, that is unbelievable. Now what kind of number can express the total number of them? Is it a finite number? ### David Deutsch 00:01:06 - 00:01:42 It's, yes, we have, we can't answer that question definitively because it depends on quantum gravity, which is a theory that we don't have yet. But there is very good reason to believe that the total number of different universes is finite, while the actual total number of universes is infinite. So they keep dividing up among themselves, and there's no limit to how finely the multiverse can subdivide itself. But the total number of distinct different universes with different contents is a finite, though enormous, number. ### Robert Lawrence Kuhn 00:01:42 - 00:01:51 And as they differentiate from each other, what happens to all of them? In other words, I think I'm living in one. ### David Deutsch 00:01:51 - 00:02:38 Yes. On the gross scale of human experience, most of the elements of human experience, once they have separated in what we usually perceive as a random event, like a coin toss or whatever, or winning the lottery, once they've separated, they no longer interact, or rather their effect on each other becomes exponentially small, so that it's immeasurable. The smaller the scale you look at, but it never goes away, the interaction. It is, in fact, always there to some very, very tiny degree. But the smaller the scale you look on, the more important these interactions between different universes become, that they are what is called in quantum theory interference processes, because the universes interfere with each other. ### Robert Lawrence Kuhn 00:02:38 - 00:02:51 Help me to understand the difference between the universe and the multiverse, in that the multiverse can have many universes in it, but has more than what's in the universe. ### David Deutsch 00:02:51 - 00:03:23 Yes. So the multiverse is the name that this theory gives to the whole of physical reality. And it is a very complex object that is not described by the normal kind of numbers that we are familiar with, like three cats, three dogs, that kind of thing. But instead, it's described by mathematical entities that describe vast numbers of these objects at the same time, in the same formalism, and their interactions with each other. ### Robert Lawrence Kuhn 00:03:23 - 00:03:33 And when we select among them as they differentiate, is that a retroactive process, because they were all there to begin with? ### David Deutsch 00:03:33 - 00:03:40 No. The tendency is that universes that have been identical become different. ### Robert Lawrence Kuhn 00:03:40 - 00:03:42 Because of the processes in each one? ### David Deutsch 00:03:42 - 00:04:35 Because of the processes, yes, in each one, although really, when they're identical, we must regard that as a process across all of them. What's happening all the time as well, though, is that they rejoin. But they rejoin on a microscopic scale, which adds up to, in our experience, to a different physics that we call classical physics, and which can be approximated by these numbers that take only one value at a time. So on a microscopic scale, there are these quantum mechanical type numbers that take multiple values simultaneously. And then they are approximated on a large scale by an ensemble, a lot of universes that look like classical physics and barely interact with each other. ### Robert Lawrence Kuhn 00:04:35 - 00:04:51 OK. Let's assume this is reality. What are some of the implications of it for the things that we think are very fundamental? For example, time. If there is a multiverse, what is the implication for the fundamental nature of time? ### David Deutsch 00:04:51 - 00:06:31 One of the exciting prospects about multiverse theory is that it sheds light on some rather notoriously difficult problems at the foundations of physics, one of which is time, as you've just mentioned. So people often ask about the multiverse, OK, so there are lots of copies of me. Which one is the real me? Which one am I? And how can there be more than one I, since I have a unitary consciousness, don't I? What people often don't notice is that this same question has been asked since time immemorial by philosophers wondering about time. Is the I of 10 years ago, who perhaps did something very embarrassing that I wouldn't do today, is that really me or is it not me? Or if a criminal repents, is it the same person? And should that person be punished and so on? These philosophical issues are all about whether the different times, whether the entities at different times are the same entity or not. Well, it turns out, amazingly enough, that in quantum gravity, we don't have a theory of quantum gravity yet, but in the approximate theories that we think might be like quantum gravity, the different times, the snapshots of the entire universe all at different times, appear in the theory in exactly the same way that the different universes at any one time do. In fact, there's no fundamental difference between them. If we use relativistic transformations, we can transform the one into the other. And so there's no fundamental difference between different times of the same universe and different universes at the same time. ### Robert Lawrence Kuhn 00:06:31 - 00:06:43 That sounds remarkable. That sounds like a radical transformation of the classical Einsteinian four-dimensional block universe, which sort of sits all in the same four-dimensional space. ### David Deutsch 00:06:43 - 00:06:59 It is radically different, although it shares, of course, some things in common. But we don't know how to integrate them yet. But it seems to me that this is obviously a clue to how to integrate Einstein's relativity with quantum theory, which is an unsolved problem. ### Robert Lawrence Kuhn 00:06:59 - 00:07:13 So to state the remarkable thing again, which you say in normal language, but it's so startling, that different universes in the multiverse will deal with different times of the same sorts of events? ### David Deutsch 00:07:13 - 00:07:44 It's not that they deal with them. It's that the different times, that is, the universe yesterday, is the same kind of object as an alternative universe, as we might call it today. And in the way that this is described in these putative theories of quantum gravity, there is no fundamental difference between those. It's like one of them is different universes in the east-west direction, and the other is different universes in the north-south direction. ### Robert Lawrence Kuhn 00:07:44 - 00:07:54 And these are existing with some sort of, I don't want to say contemporarily, because that may confuse everything, but these are all existing, let me just say. ### David Deutsch 00:07:54 - 00:08:11 They're all existing on the same basis as each other. None of them is privileged relative to the other. As you rightly say, the flow of time is an emergent property of this thing, and we can't think of the multiverse as being in time. It's more that time is a feature of the multiverse.