00:00:29.000 When I was a child, I remember being told that in the distant past, a very learned person
00:00:50.440 might hope to understand everything that was understood.
00:00:55.760 Whereas now, because of specialization, because so much is known, that's impossible.
00:01:00.000 That one person can only understand a very small fraction of what's known.
00:01:03.520 And I really didn't believe this, I didn't want to believe this.
00:01:06.200 And I envied the ancient scholars who might have aspired to knowing everything that was
00:01:13.800 And what I meant by knowing everything that was known, or understanding everything that was
00:01:18.000 understood, is not that they knew in detail everything that happened, that they had lists
00:01:23.600 of things which they remembered, that's very far from what I meant.
00:01:27.840 I meant that they understood all the explanations that were known.
00:01:33.000 And I believe that we are not heading away from an era in which one might understand all
00:01:41.680 the explanations that are known, but towards it, because we are continually unifying and broadening
00:01:59.440 The universe that we see around us is real, but it's only one small facet of reality.
00:02:06.280 The whole of reality consists of many such universes, and they're all equally real, and
00:02:26.120 I studied physics at Cambridge, and then, and after that I was always on the borderline between
00:02:34.600 I studied quantum field theory in an astrophysics department, and then I studied, I came
00:02:44.240 to the multiverse theory in the relativity center in the University of Texas at Austin,
00:02:51.920 and then I was in the mathematical institute in Oxford.
00:03:01.720 All my research has always been on the borderline between two subjects.
00:03:04.880 I suppose the reason for that is that I'm interested in unifications and in broadening
00:03:21.960 I was in my office, I've never actually been here before.
00:03:41.760 Because I work at home, so I've never used my office.
00:04:08.840 The multiverse theory comes about as an explanation of the predictions of our best theory
00:04:22.440 Quantum mechanics makes very accurate predictions, the most accurate predictions that any
00:04:29.440 But if you want to explain why these predictions are so, how these physical events come
00:04:34.360 about, there's no alternative but to postulate that what we see around us is not the
00:04:40.080 whole of reality, that reality is much more varied and has a great multiplicity.
00:04:49.600 But why would you know these multiple universes?
00:04:54.400 Suppose you take one of the classic experiments of quantum mechanics, for instance, the
00:05:05.000 Now I can show you interference on a simple laser which we've done.
00:05:14.680 What is it that forces us to believe in parallel universes?
00:05:21.880 This is an experiment, the very old one that's been first done hundreds of years ago.
00:05:30.280 Long before quantum theory was even thought of, it just involves shining light through
00:05:35.320 This is just a piece of cardboard with little pin prick holes in it.
00:05:47.880 This is a laser pointer that we use in giving lectures.
00:05:55.160 I shine the laser through the hole and there's, we can see a little spot on the screen
00:06:07.720 You'd expect to see just, if there's one hole, you'd expect to see one spot.
00:06:12.680 You see quite a complicated pattern, it's called an interference pattern.
00:06:19.560 Suppose we, what would happen if we made a second hole?
00:06:23.280 Well, here's another place on the screen where there are two holes.
00:06:28.360 We get a completely different pattern, and through three holes is a different pattern
00:06:51.080 This experiment has been known for many decades before quantum theory was even thought of.
00:06:57.640 But the key to quantum theory is what happens when we perform this experiment with very
00:07:05.040 Suppose we take the laser and we put in front of it a dark filter, very dark filter so that,
00:07:11.720 so that very few photons actually get through, maybe only one per second.
00:07:16.480 And then we can't see that anymore, but we have instruments that can see it.
00:07:23.640 So we put the instruments in where we know the light beam is.
00:07:27.240 And when the detector is in a place where photons are arriving, it will start clicking,
00:07:35.360 If we move it over slightly to a dark place in the pattern, it will stop clicking.
00:07:44.120 Now, the surprise is that even with this one photon at a time coming through the holes,
00:07:55.440 the pattern is exactly the same as it was when we were shining the bright laser light
00:07:59.920 Well, the question is, how do we explain the fact that we see the same pattern that
00:08:06.160 we detect the same pattern on the screen when there is only one photon at a time going
00:08:11.160 through here as we did when the bright laser light was shining on it?
00:08:16.480 You see, this one photon, the one we detect, must have gone through one of those two holes.
00:08:22.800 And the part, the place where it lands on the screen, depends on whether there is one
00:08:28.840 hole, two holes, or indeed three holes, however many holes there are, each number of holes
00:08:35.440 makes a completely different pattern on the screen.
00:08:38.200 So even when there is only one photon at a time coming through, the place where it lands
00:08:48.000 And what we must conclude from this is that something is coming through those other holes
00:08:52.840 and shoving our photon aside, and that process is interference.
00:08:59.280 Well, we can do further experiments to find out what that something is.
00:09:04.560 The conclusion that we draw is that this thing behaves like light in every way we can detect
00:09:10.160 experimentally except one, and that is we can't detect it.
00:09:17.120 It is there because it pushes aside the light that we can see, and it behaves like light,
00:09:23.040 and the only reasonable conclusion is that it is light of an invisible kind.
00:09:29.000 Now that doesn't take you all the way to concluding that there are many universes, but
00:09:33.800 the thing is quantum theory predicts all this, and in the equations of quantum theory,
00:09:40.640 these invisible bits of light appear in the equations, and they are treated on it in exactly
00:09:51.600 What's more, all particles behave like this, not just light, but even the particles in
00:09:55.960 the screen, the particles that you and I are made of, they all are affected, shoved aside
00:10:02.240 by counterparts of themselves that behave exactly like those particles, but cannot be seen.
00:10:10.000 They are real matter, real energy, real light that we cannot see, and so they are an entire
00:10:22.400 That's why we call it a parallel universe, and there are many of them.
00:10:26.600 In this experiment, we can see how many holes we make, there is a different pattern.
00:10:33.560 Even when one photon is going through, there is a different pattern for six holes, for
00:10:37.600 seven holes, for a hundred holes, for a thousand holes, and that means that there must
00:10:42.840 be something travelling through every one of those thousand holes, and making a difference
00:10:51.640 And that means there must be at least thousands of parallel universes.
00:10:55.240 In fact, the theory tells us there are a lot more than that.
00:11:20.520 At the time of the Big Bang, according to quantum mechanics, all the universes came
00:11:29.640 And then with the interactions between them, interference, and other interactions caused
00:11:40.200 All the other universes are as real as the one you and I are in now.
00:11:46.320 Suppose we have a lot of different and a lot of the same universes all existing together.
00:12:01.520 You have to start thinking of yourself as not as an entity existing in one universe, but
00:12:07.800 an entity existing in the multiverse as a whole.
00:12:11.640 And so there are other copies of you and of me in other universes.
00:12:16.800 Some of them, some of these copies, are completely identical to us.
00:12:20.640 Now in those cases, it's really just a matter of words to say whether they are other
00:12:24.680 universes or whether they're the same universe if they're completely identical.
00:12:28.640 There are other universes which are very like this one, but differ only in the position
00:12:35.840 Now those universes are interfering with ours and they are producing interference effects,
00:12:41.120 which we could detect in the laboratory if we wanted to.
00:12:44.000 And there are others which are very different where the interference is so small that we
00:12:47.360 will never see them, but they form part of the explanation of the things that we do see.
00:12:58.520 And there are some universes in which I'm sitting here talking to you about something
00:13:05.040 And there are other universes where I've just got up and gone to have a cup of tea.
00:13:18.080 What makes me go into multiple copies in multiple universes?
00:13:22.800 You already exist in multiple copies, initially they are all identical.
00:13:27.480 And then when a moment of choice happens, both the microscopic one and one made by you
00:13:31.800 consciously, these identical copies become different from one another.
00:13:36.840 So suppose you're walking in Oxford and you have to choose to go left or right?
00:13:42.960 In that case, for instance, it could be that in half the universes I go left and in
00:13:50.840 And could you say that the universe splits up as soon as you go left or right?
00:13:55.760 It's not a matter of splitting, this terminology of splitting was the way that the many
00:14:01.120 universes theory was originally introduced when they thought of there being a universe
00:14:09.040 But nowadays it's better, we find it better to think of there being just a certain number
00:14:13.360 of universes, perhaps an infinite number, already there.
00:14:17.000 And then half of them do one thing and half do another.
00:14:27.960 How do I know in which universe I am or how do I even know who I am, what I am if there
00:14:38.240 That is the same question that one of a pair of identical twins might say if there are
00:14:44.960 two identical twins called Joe and Jack and Joe might ask, well why am I Joe and him
00:14:54.120 Well, the answer is if you were Jack and he was Joe, you'd still be asking this question.
00:15:00.040 The thing is the fact of the matter is there are two of you and they are identical or
00:15:07.120 they were identical until they started to become different by having different things
00:15:12.080 And it's the same with you and your counterparts in other universes.
00:15:16.320 They start off identical, when they're identical it doesn't make any difference whether
00:15:22.360 But then when they start becoming different it is worth calling them different versions
00:15:29.600 Some of them in more distant parts of the multiverse will be so different from you that
00:15:44.400 We have to get used to the fact that new advances in science get further and further away
00:15:52.200 If you want science merely to predict the outcomes of experiments without telling you why,
00:15:57.000 then you don't need to believe in multiple universes.
00:15:59.080 In fact, you don't believe need to believe in multiple planets.
00:16:02.840 You don't need to believe that anything outside this room exists if you only want to predict.
00:16:07.840 And if you want to explain, then you must adopt the explanation which meets the facts and
00:16:13.800 in quantum mechanics there is only one explanation that meets the facts and that is the
00:16:19.120 We can see the behavior of things like photons being affected by things we can't see.
00:16:25.400 Our only choice is to say it behaves as if it were affected by those or to say it really
00:16:33.600 And it only makes sense to say it's really affected because something that doesn't exist
00:16:48.800 Somebody who is busy with logic and mathematics and quantum theory, it looks rather messy
00:16:56.400 Yes, well you have to remember this is my place of work.
00:17:03.160 I sit at my computer and write the books that I've been writing and then when I'm working
00:17:07.800 on other kinds of research, if I receive some papers, then my filing system is my floor.
00:17:19.280 Like immortality or perpetual motion, time travel is one of the enduring fantasies of the
00:17:41.360 But until recently, the view held by scientists has been that time travel is impossible.
00:17:47.760 Not because there is any law of physics for bidding it, but because if it were possible,
00:18:05.440 Ever since childhood, I wanted to travel in Doctor Who's Time Machine.
00:18:13.440 More recently, I made a filming which I had the privilege of travelling in the actual
00:18:17.880 TARDIS of Doctor Who to explain how the many universes interpretation of quantum mechanics
00:18:25.480 solves the problems which in science fiction are usually thought to exist with time travel.
00:18:33.320 The obstacles, the paradoxes, which are usually thought to prevent time travel, actually
00:18:38.000 do not prevent it if we perform a proper quantum mechanical analysis.
00:18:49.840 Does physics give the possibility of time travel?
00:18:57.080 The first is, is it physically possible to build a time machine that is a path from the
00:19:06.640 And the answer there is it's still an open question, but as far as we know, there are
00:19:11.840 physical processes which would allow the tearing of the fabric of space time in such
00:19:19.840 a way that there would be a pathway into the past.
00:19:22.760 This would involve something like a rotating black hole, some very violent event in space
00:19:30.200 The other half of the question is, if we had such a path, would it be possible actually
00:19:36.320 to travel on it and what would then happen in regard to the paradoxes?
00:19:41.160 For instance, what if we try to change the past?
00:19:43.720 Would the past be as it is recorded or would it get changed?
00:19:47.400 If, for instance, I go into the past and prevent myself from ever entering the time machine
00:19:54.080 or from even building the time machine, then what would happen?
00:20:01.760 If so, who was it that went back into the past to stop me?
00:20:32.320 There's apparently a paradox and the resolution of the paradox is that when one does
00:20:36.840 things like that, when one goes into the past and changes things, the result of that is
00:20:42.360 that one has actually gone into the past of a different universe.
00:20:52.120 I don't think I would because when you travel in time, one thing we do know is that you
00:21:03.360 So at the very least what would happen is that I would end up in a universe where there
00:21:08.760 was another copy of me, the copy who hadn't set out in a time machine, and in that
00:21:14.880 universe there'd be two copies of me, and in my original universe there'd be no copies
00:21:19.720 That would mean that all my friends and the people I know in this universe, they would
00:21:26.800 see me entering the time machine and not come out.
00:21:30.800 And from their point of view, I would be lost forever.
00:21:34.240 There would be another group of the same people in another universe.
00:21:37.920 They would see one copy of me, and they would see a second copy arising coming out of
00:21:46.240 And from then on, they would be in the company of two copies of me.
00:21:50.120 And they might not consider that to be an improvement on one.
00:22:03.920 The fact that we can't directly detect parallel universes is to some degree an accident
00:22:15.800 If you wanted to detect the motion of the Earth with your human senses, you wouldn't
00:22:25.640 The way we detect the motion of the Earth is, for instance, by taking a long pendulum,
00:22:30.600 Foucault's pendulum, and letting it hang for a while.
00:22:35.960 And in the cause of a few hours, we see the plane of its rotation changing.
00:22:41.400 And this is because the plane actually stays the same, and the Earth is rotating underneath
00:22:46.760 So this is an instrument which detects the rotation of the Earth.
00:22:50.920 If, by chance, our senses had contained an instrument equivalent to this pendulum, we would
00:23:00.640 In a similar way, if our senses happen to work through quantum interference, then we would
00:23:06.240 be able to feel directly the influence of parallel universes.
00:23:11.200 So saying that you don't feel other universes is like the inquisition saying to Galileo
00:23:16.640 that they don't feel the Earth moving beneath their feet.
00:23:19.640 Well, the reason why they don't feel the Earth moving beneath their feet is because Galileo's
00:23:27.040 But if they do a different experiment with the Foucault's pendulum or with telescopes,
00:23:31.280 they will indirectly be able to tell that the Earth is moving.
00:23:37.160 The core of every scientific advance is a better explanation.
00:23:41.800 Now quantum mechanics and the many universe interpretation are our best explanation of the
00:23:49.920 In fact, I would say they are the only current explanation of the physical world at the
00:24:00.000 We don't understand it fully yet, and I don't think one ever understands a theory fully
00:24:07.120 One day there will be a successor to quantum theory, just as quantum theory itself is
00:24:11.600 the successor to classical theories of physics.
00:24:15.000 And then we will understand the full implications of quantum theory and where it went wrong.
00:24:21.160 And I would expect, again, on the basis of what usually happens in science, we would expect
00:24:27.520 that future theory to be even strangers in quantum theory.
