1 00:00:00,180 --> 00:00:04,830 Hello and welcome to this series on physics and philosophy from the University of Oxford. 2 00:00:05,430 --> 00:00:11,940 Imagine your favourite football team playing a Premier League final on the same night that your favourite band are headlining at a festival, 3 00:00:12,750 --> 00:00:17,640 and you're lucky enough to be offered tickets for both. The decision is tough, so you decide to flip a coin. 4 00:00:18,210 --> 00:00:24,900 It lands on heads and you go to the match. However, lately you're left wondering What if you chose the concert instead? 5 00:00:25,590 --> 00:00:28,980 What if the coin had landed on tells and perhaps it did? 6 00:00:29,430 --> 00:00:34,290 Then maybe a parallel world in which the coin did land on tells you did go to the constant stead. 7 00:00:35,260 --> 00:00:39,100 The idea of parallel worlds has often been explored in the realm of science fiction. 8 00:00:39,610 --> 00:00:43,090 Today, it seems that the concept may be emerging out of modern physics, too. 9 00:00:43,960 --> 00:00:50,470 I'm anchored to an open and I'm speaking to Dr. David Wallace, tutorial fellow in Philosophy at Balliol College, Oxford. 10 00:00:51,360 --> 00:00:57,330 Dr. Wallace started off as a theoretical physicist with his first dphil thesis on relativistic quantum theory. 11 00:00:58,020 --> 00:01:04,020 However, his interests in the conceptual and foundational aspects of physics soon led him towards a philosophy of physics. 12 00:01:04,350 --> 00:01:08,400 And a second thesis is on quantum theory, according to the average interpretation. 13 00:01:09,180 --> 00:01:12,420 His book, The Emergent Multiverse, was published earlier this year. 14 00:01:12,960 --> 00:01:19,500 In this, he discusses the Everitt interpretation of quantum mechanics, more commonly known as the many worlds theory. 15 00:01:20,250 --> 00:01:20,850 In your book, 16 00:01:20,850 --> 00:01:29,730 you write that the idea of many worlds and parallel universes is an emergent property of the Everett's interpretation of quantum mechanics. 17 00:01:30,120 --> 00:01:36,179 Could you please explain exactly what this interpretation is? Show in quantum mechanics? 18 00:01:36,180 --> 00:01:40,259 What's going on? Essentially, this. This is our best theory of the very small. 19 00:01:40,260 --> 00:01:47,100 It's remarkably effective as a theory, and it seems to say some very strange things about the microscopic world. 20 00:01:47,100 --> 00:01:52,799 So it seems to say that atoms aren't just here or here, but they're somehow here in here at the same time. 21 00:01:52,800 --> 00:01:56,910 Or maybe they're going this way and that way at the same time. And that's weird. 22 00:01:57,180 --> 00:02:01,139 But okay, we can perhaps live with the idea that the microscopic world is weird, 23 00:02:01,140 --> 00:02:05,310 where because of large, slow moving systems, we're not supposed to find the macroscopic intuitive. 24 00:02:05,790 --> 00:02:11,429 But the problem is, what the theory also says is that when you look at the microscopic world, 25 00:02:11,430 --> 00:02:20,280 that kind of neither this nor that, but both at once magnifies up to the level of the device doing the measuring. 26 00:02:20,280 --> 00:02:22,499 So we don't just have atom here and here. 27 00:02:22,500 --> 00:02:27,170 At the same time, we have needed a measurement device pointing this way and pointing that way at the same time. 28 00:02:27,510 --> 00:02:29,940 And if I look at the measurement device and the theory says, 29 00:02:30,180 --> 00:02:35,240 I see this and that at the same time, and that doesn't just seem weird, that seems pathological. 30 00:02:35,250 --> 00:02:39,900 It's not clear we can even make sense of the idea of a needle pointing in two directions at the same time. 31 00:02:40,140 --> 00:02:46,560 Even if we can make sense of it, then it just seems obvious that we don't in fact see needles pointing in two directions at the same time. 32 00:02:46,950 --> 00:02:49,590 So it looks as if something's profoundly wrong with the theory. 33 00:02:49,800 --> 00:02:55,350 And the way this is normally fixed in textbook quantum mechanics is just to introduce a new kind of ad hoc rule that says, 34 00:02:55,350 --> 00:02:59,970 suddenly, when you make a measurement, the rules of quantum mechanics are suspended and we do something different. 35 00:03:00,570 --> 00:03:04,110 But no one's ever thought that's a terribly satisfactory way of setting up a scientific theory. 36 00:03:04,440 --> 00:03:11,429 And an interpretation of quantum mechanics is a slightly awkward term for a way of sorting out this 37 00:03:11,430 --> 00:03:16,620 kind of problem in quantum mechanics and making quantum mechanics a slightly better behaved theory. 38 00:03:17,130 --> 00:03:24,150 So perhaps to give some perspective on how the many worlds theory is playing, there is more or less need to see it in contrast with alternatives. 39 00:03:24,540 --> 00:03:33,120 So the the normal moves other than the many well theory you might take you might call a change the physics move or change the philosophy move. 40 00:03:33,300 --> 00:03:36,990 So change the physics move says look, something's clearly broken. 41 00:03:36,990 --> 00:03:43,170 We have a theory, any theory that predicts that needles are in place at once or people see two things and ones must be wrong. 42 00:03:43,590 --> 00:03:49,860 And so we better we better come up with a new theory that work better than quantum mechanics and doesn't have this problem. 43 00:03:50,220 --> 00:03:56,280 Now, philosophers like to change the physics strategy, but physicists don't like it much because quantum mechanics is a very successful theory and 44 00:03:56,280 --> 00:04:00,150 people are really reluctant to change really successful theories unless there's some concrete, 45 00:04:00,480 --> 00:04:03,480 experimental reason to design the change. 46 00:04:03,480 --> 00:04:04,980 The philosophy theory says, Look, 47 00:04:05,190 --> 00:04:11,519 all of this confusion is coming because you want to think about your scientific theory as a way of understanding the world. 48 00:04:11,520 --> 00:04:16,800 But that's the wrong way of thinking about science and scientific theory is just gadgets to predict what experiments do. 49 00:04:18,490 --> 00:04:20,620 That kind of strategy, if you like, counts, 50 00:04:20,620 --> 00:04:25,330 has changed the philosophy because it's changing the philosophical sense we normally make of scientific theories. 51 00:04:25,690 --> 00:04:30,340 Physicists like to change the philosophy theory because it gets them out of awkward philosophical and conceptual puzzles. 52 00:04:30,760 --> 00:04:34,089 But philosophers don't like it much because it seems to get in the way of what 53 00:04:34,090 --> 00:04:37,149 seems to be a very clear fact about science across the whole of science, 54 00:04:37,150 --> 00:04:43,480 which is that the point of science is to learn how things work. It's not the way we do science so as to understand what experiments say. 55 00:04:43,510 --> 00:04:50,799 We do experiments to understand science. So the many worlds theory tries to kind of avoid that kind of dichotomy in the way it does. 56 00:04:50,800 --> 00:04:56,710 It is basically saying, look, what would it really be like if a needle was here, in here at the same time? 57 00:04:56,830 --> 00:05:01,660 What would it really be like if I saw this result and that was out at the same time? 58 00:05:01,810 --> 00:05:05,350 And the claim is that what it would be like for me to see two results at the same 59 00:05:05,350 --> 00:05:09,130 time is for there to be two mes one seeing one without one seeing the result. 60 00:05:09,400 --> 00:05:16,030 And what it would be for you to hear me telling you one result and hear me telling you the other result at the same time would be for there to be two. 61 00:05:16,030 --> 00:05:19,690 Use one. Hear me tell you one thing and one is we tell you the other thing. 62 00:05:20,110 --> 00:05:24,219 And so the idea is that when you take seriously the idea that somehow we can be 63 00:05:24,220 --> 00:05:27,850 doing two things at the same time and then just dismiss it as obviously silly. 64 00:05:27,850 --> 00:05:32,409 That is, if you think hard about it, rather than just appealing to our intuitions, 65 00:05:32,410 --> 00:05:38,200 which are a really unreliable way of doing science, then you realise that the quantum theory isn't contradictory at all. 66 00:05:38,290 --> 00:05:41,499 It's just telling us something very, very strange about the universe, 67 00:05:41,500 --> 00:05:45,399 which is that quantum mechanical processes cause the universe to branch into many, 68 00:05:45,400 --> 00:05:51,300 many copies so that all these copies of a universe start together at the same time, 69 00:05:51,310 --> 00:05:57,070 for example, with the Big Bang or the new universe has been continuously made all the time. 70 00:05:57,370 --> 00:06:04,449 It's more like the second than the first, but this is kind of the place where my kind of jargon title, 71 00:06:04,450 --> 00:06:08,079 Emerging Multiverse, comes into play at the level of the fundamental physics. 72 00:06:08,080 --> 00:06:11,520 There's just quantum mechanics. At the level of the fundamental physics. 73 00:06:11,530 --> 00:06:17,680 There's no sense of the universe is a totally separate classical, as in definite things happening universes at all. 74 00:06:18,190 --> 00:06:24,459 The description of the world as being made up of classical universes is something 75 00:06:24,460 --> 00:06:28,570 that happens when we look at the theory at a certain level of approximation, 76 00:06:28,900 --> 00:06:32,049 just as our fundamental physics doesn't contain any chairs or tables, 77 00:06:32,050 --> 00:06:38,830 chairs and tables of a kind of structure inside physics that we observe in certain circumstances and certain regimes say, 78 00:06:38,830 --> 00:06:43,930 likewise, the classical universes have that character. But that all being said, 79 00:06:44,500 --> 00:06:48,549 the kind of events that cause the world have the structure of multiple universes 80 00:06:48,550 --> 00:06:51,910 rather than one universe didn't happen all at once at the beginning of time. 81 00:06:51,910 --> 00:06:55,210 They're happening all the time as quantum mechanical. 82 00:06:56,560 --> 00:06:59,640 Multiplicity get magnified up to classical multiplicity. 83 00:06:59,950 --> 00:07:05,169 So every time some process causes a quantum situation that's here and here and here and there, 84 00:07:05,170 --> 00:07:08,750 at the same time, the turn into a macroscopic situation that's here now. 85 00:07:08,770 --> 00:07:11,950 At the same time, that's when a new universe branching happens. 86 00:07:12,400 --> 00:07:16,600 There's not a new law of physics. It's an understanding of what's going on in the existing laws of physics. 87 00:07:17,320 --> 00:07:21,790 So what exactly does it mean for there to be another universe where the alternative happened? 88 00:07:22,150 --> 00:07:26,770 Do these universes ever interact with each other? Could I ever find myself in a parallel universe? 89 00:07:27,900 --> 00:07:31,410 They almost by definition don't interact with each other. 90 00:07:31,510 --> 00:07:34,680 What I mean by that is that the reason in quantum mechanics, 91 00:07:34,680 --> 00:07:39,509 we have took seriously the idea that somehow the atom could be here and there at the same time was because 92 00:07:39,510 --> 00:07:45,390 the the here bit of quantum state of the atom interacts with that bit of the quantum state of the atom. 93 00:07:45,750 --> 00:07:48,930 And that interaction is what physicists call interference. 94 00:07:49,230 --> 00:07:54,420 And it's the phenomenon of interference that fundamentally gives the the experimental results 95 00:07:54,420 --> 00:07:57,810 that tell us that quantum mechanics is just fundamentally different from classical mechanics. 96 00:07:58,290 --> 00:08:04,980 But it turns out to get this kind of interference between the two places that the atoms at the same time is quite a delicate matter. 97 00:08:05,280 --> 00:08:09,120 Dealing with atoms is quite difficult. Doing it with big molecules is very, very difficult. 98 00:08:09,510 --> 00:08:17,550 Doing it with viruses is almost impossible. Not trying doing it with entire humans is going to be completely impossible. 99 00:08:17,880 --> 00:08:21,930 And what normally happens is, unless when you make a measurement of the microscopic world, 100 00:08:21,930 --> 00:08:28,830 unless you control that measurement incredibly carefully, more and more and more and more systems start ending up into place at the same time. 101 00:08:29,580 --> 00:08:32,940 Again, the jargon term for this is entanglement. More and more entanglement happens. 102 00:08:33,390 --> 00:08:41,160 And very, very quickly you get to the stage where there's no prospect whatsoever of interference between the two branches of reality, if you like. 103 00:08:41,340 --> 00:08:44,040 That's the point to which we start using the many world language. 104 00:08:44,610 --> 00:08:48,120 So it's not, if you like, that different bits of reality can't interact with each other. 105 00:08:48,450 --> 00:08:52,620 It's that the the fact that in practice you get to the stage where they're not 106 00:08:52,620 --> 00:08:56,630 going to do that is what makes it sensible to start using the language of words. 107 00:08:57,330 --> 00:09:04,230 So you couldn't ever experimentally verify this theory in the way that you could experimentally verify all the scientific theories. 108 00:09:04,680 --> 00:09:08,940 It kind of depends what you mean by experimentally verified. Get back to my change. 109 00:09:08,940 --> 00:09:11,540 The physics. Does it change the philosophy ideas? 110 00:09:11,550 --> 00:09:15,420 If you think that we have three three ways of doing quantum mechanics, one is just take quantum mechanics. 111 00:09:15,420 --> 00:09:19,710 Literally. That's the many worlds theory. One is change quantum mechanics for a new theory. 112 00:09:20,100 --> 00:09:23,730 And one is rethink the way we think of scientific theories. 113 00:09:23,940 --> 00:09:28,470 Now, you can definitely test the many world theory against and change the physics way of doing quantum mechanics. 114 00:09:28,860 --> 00:09:34,590 The change of physics, ways of doing quantum mechanics normally predict that this kind of two things at the same time goes away at some point, 115 00:09:35,220 --> 00:09:41,430 and that you can in principle look for if you were to ever fail to find these interference phenomena 116 00:09:41,430 --> 00:09:46,130 where two bits of reality do interact with each other that quantum mechanics predicts are there. 117 00:09:47,040 --> 00:09:51,690 If you if you were to ever fail to find those in a situation where the theory predicts that you ought to be able to see them, 118 00:09:51,930 --> 00:09:57,120 that would be a falsification of the of quantum mechanics taken literally. 119 00:09:57,120 --> 00:10:01,530 So if you like, it would be a falsification of the many wells theory. We have to get rid of them anyway else they that happened. 120 00:10:02,400 --> 00:10:08,520 And conversely, every time we make one of those experiments and the result is exactly what quantum mechanics says it should be, 121 00:10:08,760 --> 00:10:13,530 that's the situation where we say, okay, quantum mechanics taken literally many wells theory has passed that test. 122 00:10:14,160 --> 00:10:17,940 Could you ever test quantum mechanics against the change the philosophy idea? 123 00:10:18,210 --> 00:10:20,850 Now you couldn't that that's a very general fact in science. 124 00:10:20,850 --> 00:10:26,460 You can never test any theory against the idea that the theory is wrong, but it still gets the predictions right. 125 00:10:26,670 --> 00:10:28,200 So to take a silly example, 126 00:10:28,650 --> 00:10:37,740 we can test the theory that fossils of dead dinosaurs against the theory that fossils are created by some kind of particular random process. 127 00:10:37,740 --> 00:10:42,810 Because the theory, the fossils of the dinosaurs make lots of predictions about where we'd find fossils and how fossils are structured. 128 00:10:43,140 --> 00:10:47,040 You can't test the theory the fossils are dead dinosaurs against the theory. 129 00:10:47,040 --> 00:10:50,970 The fossils aren't dead dinosaurs, but behave exactly the same way as dead dinosaurs behave. 130 00:10:51,270 --> 00:10:56,340 That's basically why philosophers don't take these take philosophy strategies very seriously. 131 00:10:56,490 --> 00:10:57,690 They seem to, in some sense, cheat. 132 00:10:58,860 --> 00:11:07,980 So it seems that the Money Worlds theory is the only seemingly valid way of looking at the results of quantum mechanics providing us with. 133 00:11:09,350 --> 00:11:11,810 I'm always nervous about saying something quite that strong. 134 00:11:12,110 --> 00:11:19,160 I think the many worlds theory is the only way to make sense of quantum theory as a theory of the world as a whole. 135 00:11:20,090 --> 00:11:23,670 It's always sensible to think, well, maybe quantum mechanics is wrong. 136 00:11:23,690 --> 00:11:25,970 Maybe there's some better theory that we should look for. 137 00:11:26,510 --> 00:11:33,740 And I wouldn't in any way want to denigrate the value of various lots and lots of different projects to try to do that. 138 00:11:34,070 --> 00:11:36,060 But in terms of the quantum mechanics we have. 139 00:11:36,080 --> 00:11:44,059 If you want to understand that theory as a theory of the world, not just a convenient black box that tells us what experimental devices will do then. 140 00:11:44,060 --> 00:11:47,030 Yes, I think the many wells theories, the only way to make sense of it in that sense. 141 00:11:47,900 --> 00:11:53,840 So is it the case that every decision I could possibly make has been made by me in a different universe? 142 00:11:54,470 --> 00:11:58,820 Perhaps it's worth saying a little bit about. What these these words are. 143 00:11:58,820 --> 00:12:06,440 From our perspective, it's not literally true that the thing that makes these wells is something like although in everyday decisions, 144 00:12:07,220 --> 00:12:11,690 it's they're made they're made by a particular physical process. 145 00:12:11,690 --> 00:12:18,440 They're made by this magnifying up to the classical macroscopic level of quantum uncertainties. 146 00:12:19,040 --> 00:12:28,010 However, a great many of the kind of things that determine at level decisions are things that ultimately can be grounded quantum mechanically. 147 00:12:28,220 --> 00:12:32,480 So in particular, something like the weather, whether it's sunny today or raining? 148 00:12:33,050 --> 00:12:36,170 Well, actually, at the moment in the United Kingdom, that seems to be deterministic. 149 00:12:36,170 --> 00:12:42,379 But ideally, whether it's only raining does seem to be determined ultimately by very chaotic phenomena in the atmosphere, 150 00:12:42,380 --> 00:12:45,720 which is to say very great sensitivity of the atmosphere to what? 151 00:12:45,740 --> 00:12:52,940 To earlier conditions. And then ultimately, that sensitivity is powerfully enough to magnify out quantum mechanical variations to this level. 152 00:12:52,950 --> 00:12:56,390 So, yes, there is a branch like this one, but where it's sunny rather have the raining. 153 00:12:57,170 --> 00:13:06,860 And as in your example, you opened the fall of a coin is determined by various issues about how the coin how exactly one 154 00:13:06,860 --> 00:13:11,209 position the coin to start with which in terms you can probably track to quantum mechanical 155 00:13:11,210 --> 00:13:17,030 uncertainties magnify that but there hopefully isn't a branch just like this one in which I 156 00:13:17,030 --> 00:13:22,430 spontaneously decide to do something stupid because hopefully my decision making is determined by. 157 00:13:23,430 --> 00:13:26,550 Some process that isn't just total quantum randomness. 158 00:13:27,000 --> 00:13:34,770 So they philosophers and writers of science fiction have an idea of possible worlds, which include every possibility, as we see it as. 159 00:13:35,760 --> 00:13:39,350 If you like, actors, agents, the many worlds framework isn't quite like that. 160 00:13:39,360 --> 00:13:44,310 It's a bit more tame, a bit more controlled, but it still in practice has a lot in common with it. 161 00:13:45,030 --> 00:13:52,530 I should also say that the ideal of many worlds, although it's been contentious in physics for a long time now, seems to be an idea that. 162 00:13:53,920 --> 00:13:59,980 Seems almost Hayden compared to some of the other aspects of many world stories that have come out of modern physics. 163 00:14:00,250 --> 00:14:04,000 So modern cosmology is filled with the idea that we live in a multiverse, 164 00:14:04,000 --> 00:14:09,340 a set of worlds which are not just emergent in my sense, but are fundamentally present in the physics. 165 00:14:09,340 --> 00:14:12,280 Some of them are in a world in which the laws of physics are supposed to be different. 166 00:14:12,820 --> 00:14:18,880 In many ways, the many worlds in quantum mechanics are quite tame, well controlled, well understood, said Wells. 167 00:14:19,090 --> 00:14:24,909 So perhaps it's not surprising that these days, while I wouldn't say the many worlds theory is a majority view in physics, 168 00:14:24,910 --> 00:14:29,559 it's probably tied for first place among the actual interpretations of quantum mechanics. 169 00:14:29,560 --> 00:14:34,150 The physicists who are interested in this question take seriously. Thank you very much, Dr. Wallace.