00:00:00.000 Welcome to a topcast, episode 23, chapter 11 of the beginning of Infinity, the multiverse.
00:00:20.920 This is the chapter that personally I've been looking forward to the most and we're going
00:00:25.960 to approach it a little bit differently, background for a start is a bit different.
00:00:31.480 This episode is going to contain far less reading from the beginning of Infinity.
00:00:36.360 I don't think this is necessarily crucial for helping us to understand what's happening
00:00:40.000 in chapter 11, but it will just help things along a little bit perhaps.
00:00:44.400 I don't just want this chapter to blow right by you, as Sam Harris has said of David Deutsch's
00:00:50.400 way of explaining certain powerful but subtle concepts, because once you have understood
00:00:56.400 this chapter, it's one of the deepest explanations of quantum theory that exists out there
00:01:00.760 anywhere, whether in popular science form or even proper scientific text form.
00:01:07.080 So this chapter not only tries to explain the new way of understanding quantum theory,
00:01:11.760 which is basically via the multiverse, but also a new way of understanding the multiverse
00:01:19.800 So we're really looking at the very cutting edge way of trying to understand reality as explained
00:01:25.240 to us by quantum theory, and the very latest that David Deutsch has published on this.
00:01:31.760 David's given talks like the one title you can look this up online called Apart from
00:01:36.400 Universes, which attempts to convey some of the other perhaps more striking and surprising
00:01:41.600 things about the multiverse, apart from the multiple universes bit.
00:01:45.920 As we'll come to see, David's claim that this is actually among the more mundane parts
00:01:50.600 of quantum theory will make sense, when I say mundane parts, the fact that there are
00:01:54.360 multiple universes is really not the most interesting part of the multiverse theory.
00:02:00.600 So to my mind, this chapter really is a level up from the typical explanations of quantum
00:02:04.760 theory, and even a level up from the typical explanations that you hear from other science
00:02:09.880 communicators and physicists when they try and explain the multiverse.
00:02:13.840 And for this reason, it's why I think we need to take a little step back, and that's
00:02:16.960 what I'm going to do here and now in this episode.
00:02:19.760 Also, if you're listening on audio, this one contains a lot of images, videos, and
00:02:25.360 animations to help things along, in particular, I'm going to be trying to explain some experiments,
00:02:30.800 three experiments in particular, and without the visuals, one may really struggle in places
00:02:35.920 to understand what I'm describing in certain points.
00:02:38.920 Also, if you're interested, there's essentially a preamble to this episode, an episode
00:02:43.680 If you like, that was episode 22 of top-cast, it's an audio form only, and the purpose
00:02:47.800 of that is in part to provide some additional material to support these episodes, though
00:02:54.080 One charge leveled against the multiverse is that it cannot be tested, so it fails Popper's
00:02:58.480 criterion of falsifiability, but that's incorrect.
00:03:02.160 We'll be discussing that issue throughout these episodes, but if you can't wait, then
00:03:06.080 episode 22, the audio episode, that's available, and I give some details there about
00:03:11.160 the falsifiability of the multiverse, the testability of the multiverse, against rival interpretations
00:03:17.720 and, in fact, the testability of the multiverse, against classical physics.
00:03:21.600 Now, there's a question that David answers in the next chapter, chapter 12, the chapter
00:03:29.000 And that's about why the quantum multiverse, as first explained by Hueverit back in
00:03:34.160 1955, is still not taken seriously today by a majority of physicists, although that does
00:03:42.280 The answer to the riddle, why don't all physicists agree with the multiverse is basically
00:03:49.080 But most people watching or listening to this will not be physicists, and so we've got
00:03:55.480 On the one hand, the multiverse remains a minority opinion among physicists, and this might
00:04:04.640 If the supposed expert consensus seems to be against the multiverse among professionals
00:04:09.840 in the field, shouldn't I, as a non-physics, take that as a bit of a red flag?
00:04:14.480 The answer is no, and the reason why, as I alluded to, will be provided primarily in the
00:04:20.160 Okay, I'll have details on that in the next chapter.
00:04:22.720 So if I sit for now to say that the reason is bad philosophy, and I will be covering
00:04:28.000 some of that in these next few episodes about the multiverse, to give a taste of what
00:04:32.400 went wrong, physicists were quite understandably initially unable to comprehend what was going
00:04:37.800 on with a bunch of problematic observations from various experiments in the early 1900s that
00:04:45.920 What was happening in those experiments, the observations that they were making, were so
00:04:50.480 counterintuitive, and so many ideas were floated to try and explain it, that many working
00:04:55.720 quantum theorists simply gave up even trying to understand it at all, and became what
00:05:02.880 Now an instrumentalist is someone who regards a theory as useful, only insofar as it can
00:05:10.600 This has not really happened before ever in the history of science.
00:05:13.440 It will be rather like when Niels Bohr, who came up with some of the early foundational parts
00:05:19.440 of atomic theory, like for example, his model of the atom called the Bohr atom, helps
00:05:25.240 us to explain why the flames of certain elements when they burn or certain compounds at
00:05:30.400 all, when they burn, have a particular color, so if you're looking at fireworks and you
00:05:34.520 see a green firework, that's usually because of a copper atom there, somewhere other burning.
00:05:40.000 It could be a copper salt, for example, that's burning.
00:05:43.040 Copper salt tend to burn with a green flame, pure copper itself burns with a green flame.
00:05:47.400 If you were to powder up, copper metal and set it on fire, it catches fire very readily,
00:05:55.480 Well, it's similar to why yellow when your burning wood would contain carbon and it burns
00:06:02.760 Different materials burn with different colored flames, of course most of us are only
00:06:06.120 familiar with wood burning, and so we see that characteristic yellow flame or characteristic
00:06:11.200 red flame in some cases, but every other element has its own characteristic colored flame.
00:06:18.320 Well, on Bohr's model of the atom, the reason why is because the electrons orbit the nucleus
00:06:23.760 at different energy levels, and as the electrons move up and down those energy levels,
00:06:28.320 they can either absorb photons of light, or they can emit photons of light.
00:06:32.800 And so if you have a lot of heat, then what's going on is the electrons are being given
00:06:36.640 additional energy, okay, by the flame, by the fire, and when they are not up to a higher
00:06:42.240 energy level, because now they've got higher energy due to heat, then they're unstable
00:06:47.520 They want to be at the most stable, whole ground state, and so they tend to fall back down,
00:06:51.680 so they're originally at a higher atomic level, a higher atomic energy level, and they fall
00:06:56.160 back down to a lower atomic energy level, and in that process they emit a photon, that
00:07:00.320 photon is characteristic of the atom in which the electron finds itself, okay, that
00:07:08.720 Well, at the time when Bohr postulated all this, no one could see atoms at all.
00:07:14.400 There was no chance of seeing atoms at that time.
00:07:16.520 There was no such thing as a scanning tunneling electron microscope, which allows us to
00:07:22.880 If you're an instrumentalist, what you would have said at that time is that's an interesting
00:07:29.160 However, I don't believe in the existence of atoms, because I cannot see them, and no
00:07:34.840 So we can use your model in order to predict the colors of flames when elements burn, but
00:07:40.560 we are not forced to believe in the existence of atoms.
00:07:44.160 The best explanation is not that atoms exist, we don't need a best explanation, that's
00:07:50.720 Science is merely about predicting the outcome of experiments, it's merely about allowing
00:07:55.000 us to say what color the flame will be when you burn a particular atom, not that atoms
00:08:00.240 It's about telling us that when you burn a particular substance like copper, you get a
00:08:03.760 green flame, and the reason why according to atomic theory is because the electrons are
00:08:08.320 moving up and down, but you're not forced to actually believe in the existence of atoms.
00:08:14.880 Indeed, at the time, there was some physicist, the chief among them was Ernst Mark, who
00:08:19.760 said at the time that he refused to believe in the existence of entities which could not
00:08:23.800 in principle be seen, and that was the status of atoms at that time.
00:08:28.080 But as David Deutsch has explained over and again, science is really about explaining
00:08:32.320 the scene, the color of certain flames, in terms of the unseen, the movement of electrons
00:08:39.280 So we explain the green flame burning in copper that we do see in terms of electrons moving
00:08:44.360 between orbitals around an atomic nucleus, which is a protest that we don't see.
00:08:48.840 This is absolutely key to appreciate in science as a general principle.
00:08:52.600 We explain this shining sun that we do see in terms of the smashing together of protons,
00:08:57.880 hydrogen nuclei, in the core of the sun to form helium nuclei, that we don't see.
00:09:02.720 We've got no hope of seeing that it's in the core of the sun at 15 million Kelvin, and
00:09:07.760 nothing can survive 15 million Kelvin if it was attempting to observe what was going on
00:09:15.000 We explain observations that we have of the cosmos, for example, that there is heat left
00:09:20.040 over at a temperature of 2.7 Kelvin above absolute zero, that permeates all of space.
00:09:25.440 We see that, we observe that, and we observe the so-called Hubble shift of galaxies, all
00:09:33.200 Many of the galaxies out there are red-shifted away from us and appear to be moving away
00:09:38.000 And finally, we see that around about 75% of all the atoms in the universe are hydrogen.
00:09:43.720 And about 24% or something like that are helium.
00:09:46.960 And this ratio is only explained by the big bang.
00:09:50.840 So we see these things, the ratio of the elements, we see the movement of galaxies, we
00:09:55.840 see heat left over in the universe, but we do not see the big bang.
00:10:02.000 We can't travel back in time to 13.7 billion years ago and see the big bang.
00:10:07.120 So we're explaining what we do see, those three things I mentioned, the three pieces of
00:10:10.920 evidence in terms of the unseen, the actual occurrence of the big bang.
00:10:15.480 And of course, David's favorite example here is dinosaurs.
00:10:19.560 And he's talked about this in many different places.
00:10:22.320 What we see are fossils, rocks, certain patterns in rocks.
00:10:27.320 Yeah, they look like dinosaurs, but only in retrospect, only once you know that dinosaurs
00:10:36.600 Likely, no one ever will see a dinosaur, what we see are rocks.
00:10:42.160 This is a series of rocks that people have dug out of the ground and assembled together
00:10:45.840 and my gosh, it looks like a T-Rex, but it's not a T-Rex.
00:11:07.320 They aren't even dinosaur bones, they're rocks, because over time, the dinosaur bone material
00:11:12.520 itself has been replaced by rocks, it's a form of metamorphic rock.
00:11:16.600 We don't see actual dinosaurs, we explain the scene, what we can see, the fossils, in
00:11:24.800 So one, the unseen stuff, is an explanation of the other, the scene stuff.
00:11:32.240 It's an interpretation, and the denial of the reality of dinosaurs, by say, some hardcore
00:11:38.080 creationists, they might say, it's a test of our faith, some creationists have asserted
00:11:45.600 It's irrational because it's far too easy to vary.
00:11:49.360 You can go all the way back to episode one of this talk cast series, all the way back
00:11:52.920 to chapter one of the beginning of an affinity for more on that.
00:11:57.040 So that's the philosophical context within which we need to catch this whole discussion
00:12:01.240 of the multiverse, the explanation of our observations of certain experiments in the field
00:12:07.600 I'm going to discuss three experiments before getting to the beginning of the infinity
00:12:14.200 The first one is known as Millican's experiment, and it demonstrates the photoelectric effect,
00:12:19.720 and the photoelectric effect is about how light can actually move particles around.
00:12:25.840 And this is evidence that light is not a wave, it decides between two models of light.
00:12:32.280 On the one hand, the idea that light is a wave, and it cannot possibly knock particles
00:12:37.280 out of the way, it can only vibrate particles up and down, it can't knock them away.
00:12:42.760 And the particle theory of light, on the other hand, where the particles of light are
00:12:47.440 kind of like little billiard balls, and they can knock things out of the way.
00:12:50.920 They can physically collide, and that's a good way of thinking about things that particles
00:12:55.520 can actually physically collide and bounce off one another, whereas waves can go through
00:13:01.440 And if you've been to the beach, you've actually seen water waves do that.
00:13:04.440 Now, the second experiment is known as the Mark Zender Interferometer.
00:13:09.000 And in this experiment, we can't possibly explain the outcome, if you set up the experiment
00:13:13.600 just right, and I'm going to spend some time explaining that.
00:13:16.600 You can't explain the outcome without understanding that there must be two photons in
00:13:24.240 the apparatus, even though you only ever fire one, which is astonishing.
00:13:30.200 You fire one photon at the apparatus, and the only way to understand the result of the experiment
00:13:36.440 is by recourse to postulating the existence of a second equally real photon, and in fact
00:13:43.040 what's going on is a little bit more complicated than that.
00:13:45.440 The one photon differentiates itself into two groups of fungible photons.
00:13:53.160 We're going to have to unpack a lot of quantum theory in order to understand what I just
00:13:59.920 The third experiment is Young's Twin-Slit experiment, and this is the experiment that was
00:14:04.200 discussed in David's first book, Chapter Two, Shadows, and so I'm going to be doing a
00:14:08.800 little bit of reading from Chapter Two of the fabric of reality.
00:14:12.440 Okay, before I get to those three experiments, I'll just have a little personal reflection
00:14:17.280 here and told me for a moment, because there are many reasons today that people are becoming
00:14:21.800 more and more attracted to the work of David Deutsch.
00:14:24.800 Back in 1997, my own reason, and I think I speak for many others, was that I was struggling
00:14:30.280 at university to understand the basics of quantum mechanics.
00:14:34.400 What I was presented with in lectures as a physics undergraduate was the mathematical formalism.
00:14:39.640 We solved kind of puzzles or did exercise in quantum mechanics, rather like doing, you know,
00:14:45.280 But unlike school maths, which was just about abstract numbers lots of the time, in quantum
00:14:49.920 mechanics, at university, the numbers are supposed to represent parts of the physical world,
00:14:56.280 the symbols are supposed to represent parts of the physical world, how particles and other
00:14:59.760 things actually behaved, but it was near impossible to understand as it was presented
00:15:05.440 What I was offered was instrumentalism of the sort that I just mentioned before.
00:15:09.240 You have these formulae, and you have a process whereby you predict the outcome of the experiment,
00:15:15.920 but you're not really supposed to ask what's going on during the experiment.
00:15:21.240 The maths allows you to simply predict the outcome of the experiment.
00:15:27.160 It's an instrument for predicting what happens in the experiment, but you don't ask what
00:15:33.840 Now if we didn't get instrumentalism, maybe 5% of the other time, the lecturer or the
00:15:37.160 tutor would mention something like wave particle duality.
00:15:40.680 And in fact, this is even what our high school teachers today will do.
00:15:43.520 I'll talk about wave particle duality that I photon or an electron is sometimes a particle
00:15:50.280 and sometimes a wave or simultaneously both a particle and a wave at the same time.
00:15:55.200 Now given that a particle is something that's isolated at one point in space and a wave
00:16:02.680 You can't simultaneously be isolated at a point and spread out at the same time.
00:16:06.640 This violates the law of the excluded middle, it violates logic, but in fact, some people
00:16:11.040 who teach quantum mechanics will say, well, classical logic doesn't apply.
00:16:15.920 And of course, if you're going to give up on classical logic, you're really giving up
00:16:18.560 on reason altogether, okay, you're entering into woo woo land.
00:16:22.320 We must obey the laws of logic, otherwise we're speaking nonsense, but whatever the
00:16:28.560 case, we were reassured that if we were completely confused that it was no cause for concern
00:16:33.720 really, because very famous physicists would say things like, if you think you understand
00:16:38.520 quantum mechanics, you don't understand quantum mechanics, and this was supposed to be reassuring.
00:16:43.800 But I was doing a physics degree back then, and I wanted to understand this stuff, irrationality
00:16:49.960 didn't sit well with me, it was one of the reasons I sort of migrated into philosophy
00:16:54.800 I was thinking, how can something be isolated at a point, not spread out in space and
00:17:04.240 Now I read some of Paul Davies books on quantum mechanics, he wrote the mind of God, he wrote
00:17:08.800 various other books that had a strong science component to them in particular, quantum theory.
00:17:14.400 He was often a bit of a mysterious, he really reveled in the mystery of quantum mechanics,
00:17:19.560 which was interesting, but I wanted to solve the mystery.
00:17:26.720 But to Paul Davies credit, in many of his books, he actually relayed theories of many
00:17:34.000 This is a fantastic one, the ghost in the atom, and it's got interviews with a lot of
00:17:40.480 And one of the physicists that is interviewed in the book is David Deutsch.
00:17:45.960 And in the interview, he explains the multiverse, and he explains how it's testable.
00:17:52.960 David was the first one to come up with an experimental test for the multiverse, against
00:18:03.560 I remember at the time when I did read that book with the interviewer, David, I was confused
00:18:09.800 But I'm going to explore what he does say as we move through this series, in particular
00:18:14.720 Now, it was sometime in around 1997, when I picked up this book, although not exactly
00:18:20.920 this one, my original fabric of reality, a little paperback, I lent to a friend, and it never
00:18:28.520 So I've got many other copies now, that's the nice hardcover book version.
00:18:33.080 In this book I've mentioned before, it has an excellent recommendation from Paul Davies
00:18:39.160 And so seeing Paul Davies recommend this book, of course, I picked it up straight away.
00:18:45.240 And in that book, in chapter two, Shadows, David explains the multiverse perfectly clearly.
00:18:53.600 And it was then that I was completely convinced.
00:18:55.960 I finally felt that I understood who was right about how to understand quantum theory.
00:19:03.000 There was the people on the side of the multiverse, it was the only one that made sense.
00:19:06.840 We didn't have to give up logic, we didn't have to give up common sense, we didn't
00:19:10.760 have to give up any other part of science or rationality.
00:19:14.440 We just had to accept the fact that the universe, that reality rather, was much, much
00:19:21.800 And this wasn't particularly, I mean, it was cool, it was kind of really interesting,
00:19:27.360 but it wasn't so shocking that I was going to say, no, that's ridiculous, I can't possibly
00:19:34.160 It was no more shocking to me than I guess as a child, learning, there are other planets
00:19:39.680 All later on, learning that there are other planets going around other stars, or that there
00:19:46.680 Well, now we know there are other universes, this was no, this was cool, but it wasn't
00:19:53.720 a great shock to me, but some people are still shocked.
00:19:57.240 It's certainly amazing, but lots of stuff is amazing, it's amazing that we're made of
00:20:02.320 It's amazing that galaxies exist beyond our own, that the universe is as big as it is,
00:20:08.080 that cells, tiny as they are, are so remarkably complicated, that all of life on Earth, apparently,
00:20:13.600 has been extinguished almost completely, around about five times in the history of the planet.
00:20:22.560 It's amazing that the center of the Earth is solid iron, but the outer core of the Earth
00:20:27.160 is liquid iron, and that generates a magnetic field that protects us from the sun.
00:20:32.200 So I guess, you know, amazement is not a reason to reject something, it's a reason to
00:20:38.800 And I don't find it any more amazing that there are these parallel or almost parallel
00:20:44.000 universes out there than any of those other things that I've just mentioned.
00:20:46.920 They're just amazing, the amazing parts of science.
00:20:49.320 But one reason to reject a scientific theory is when it violates logic, and the other
00:20:56.240 interpretations of quantum theory do exactly that, they violate logic or common sense,
00:21:03.840 So we don't reject things because they're amazing, not in science or anywhere else, but
00:21:08.560 we can reject things if they violate common sense, and especially if they violate logic,
00:21:14.640 and a lot of these things do violate, a lot of these alternatives do violate logic.
00:21:39.320 If you're trying very hard to understand a thing, and the explanation you're getting isn't
00:21:43.360 working, it's not necessarily all your fault, or at least not exclusively your fault, it
00:21:48.400 can be very much the fault of the explainer, and that might be quite right.
00:21:52.960 You could be proficient at one part of a particular subject, and completely ignorant
00:21:58.000 I think it is a bit of a travesty that quantum theory is professional physicists don't
00:22:05.360 all endorse the multiverse, and again we'll come to that next chapter.
00:22:11.520 I did make Paul Davies once, back in about the year 2003 in Sydney, we were coincidentally
00:22:16.800 at a pub together for an event called Science in the Pub, and the debate raging on stage
00:22:21.080 was between a group of astronomers about the definition of a planet, a matter that would
00:22:28.800 But I wasn't really interested in that matter, as soon as I saw Paul Davies.
00:22:32.920 I immediately asked about, as soon as I went up to him and introduced to myself, I asked
00:22:40.120 And sadly, plays under this conversation was, and interesting as it was, and excited as
00:22:43.760 I was to speak with him, and I got his autograph, and we've discussed lots of other things.
00:22:49.720 The response about the multiverse was essentially, I don't have time to explain why it's
00:22:53.080 not correct, but later I was able to read some more of Paul's thinking and watch some
00:22:58.960 more interviews with him, and he thought, and this is probably one of the most common
00:23:04.400 criticisms of the multiverse, that it violates Occam's razor.
00:23:08.920 So there's too many things to explain the one thing that we do observe.
00:23:14.160 But this, understanding of Occam's razor is, in my opinion, completely fallacious.
00:23:19.440 Occam's razor is not about increasing the number of entities, it's about increasing the
00:23:27.520 It would be like saying, we shouldn't postulate the existence of other planets out there
00:23:33.760 that we cannot see, just because we can see some here in our solar system, and in out to
00:23:43.000 No reason to think that the entire galaxy is filled with planets, let alone galaxies in the
00:23:51.440 We're proliferating the number of planets beyond all reason.
00:23:56.000 It's quite reasonable to presume there are other planets out there in the universe.
00:24:00.560 Even what we've already observed here in our solar system, and indeed in our local
00:24:07.040 So I think it's completely wrong when people talk about Occam's razor in this way.
00:24:11.400 Now, even when some of physicists write about the multiverse, they can still kind of
00:24:17.480 I don't generally mention names, but I've been, I've been called out by some people
00:24:23.400 watching these videos that I don't mention names enough, so I'll mention one more name.
00:24:27.680 Another person I respect greatly, Sean Carroll, Sean Carroll, Quantum Physicist.
00:24:32.840 He's a supporter of the many worlds interpretation as well, he supports the multiverse.
00:24:36.960 He gave a quite good defence of it recently on the infinite monkey cage.
00:24:41.440 They had an episode if you haven't listened to that podcast, it's quite entertaining.
00:24:45.920 Robin Ince and Professor Brian Cox have that, and they did one on coincidentally the
00:24:51.320 most recent one, so I'm recording this now in February of 2020, so if you're looking
00:24:58.400 for it some years hence, the infinite monkey cage in about February 2020 was all about quantum
00:25:06.040 Both Brian Cox and Sean Carroll support the many worlds interpretation.
00:25:11.640 But I've heard Sean in other situations, give interviews about the multiverse.
00:25:17.360 I think he was on both Joe Rogan and Sam Harris, and during those interviews he insinuated
00:25:24.320 that the only problem with the multiverse is it's not testable, so this is kind of disappointing.
00:25:32.560 He doesn't understand, there's two tests at least that I'm aware of of the multiverse
00:25:39.520 theory, and we're going to get to those, I regard as test of the multiverse theory.
00:25:43.640 One that was discussed in my previous episode about the fact that if you run an interference
00:25:51.520 experiment, multiple times, certain kinds of interference experiment, in particular the
00:25:55.480 double slit interference experiment for single particles, you don't get the same outcome
00:26:00.160 each time, you don't get the same outcome each time because you're finding yourself
00:26:06.000 Science should produce the same results every single time you run the experiment, but
00:26:09.400 this is a case where you don't get the same result every single time, and that would indicate
00:26:13.360 that you're approximating all the different possibilities, all the different universes
00:26:18.680 This is a test, okay, and I'll get to that again later, and so the problem with Sean's
00:26:26.760 claim that the multiverse is untestable is he then thinks simultaneously that because it's
00:26:33.760 not testable, he thinks that this is a reason that falsification is overrated, or a reason
00:26:40.040 even that physics has moved beyond falsification, and I heard Sabine Hoffenstutter say the
00:26:48.840 A lot of physicists have said this, I should qualify that, a lot of theoretical physicists
00:26:53.200 have been saying this, that the the Paparian criterion of demarcation of falsification
00:26:59.760 is old hat, we don't need it anymore, we've moved beyond it now.
00:27:05.200 But in the case of Sean, he endorses the multiverse, but for the wrong reasons in part,
00:27:11.560 and Brian Cox he does endorse the multiverse in some moods, but with hesitation it seems
00:27:17.320 to me and hedges and with sort of dodges in one of his recent books, I don't know that
00:27:25.320 even uses the term multiverse, just claims that about simultaneous realities occurring
00:27:31.160 It's the same thing, it's the same concept, it's like multiple histories as well, it's
00:27:34.200 basically multiple universes just with different language being used.
00:27:39.200 Okay, so that's enough here, I'm just I guess emphasizing how difficult it is to find good
00:27:47.160 My own sources here moving forward right now come down to four of them, okay, there's four
00:27:50.720 sources I'm going to be using for these, firstly my own recollections and recent survey
00:27:55.400 the fundamentals in textbooks and stuff, secondly Paul Davies book, okay, so I'm actually
00:28:01.360 going to be going to this and what David says in this book here in his interview with
00:28:09.080 David Wallace is my third source, okay, excellent professor David Wallace, professor of
00:28:15.200 philosopher of physics, and of course David Deutsch in the beginning of infinity, but not
00:28:20.400 only beginning of infinity this time around, but the fabric of reality, and substantially
00:28:24.960 his paper, the structure of the multiverse as well.
00:28:28.400 Okay, so let's move into the first of our experiments, and the first experiment is the experiment
00:28:35.000 that demonstrates that light is in fact a particle.
00:28:38.560 Okay, so this is a cartoon of the photoelectric effect, and we can see here where, and
00:28:46.480 it's from physics education technology at the University of Colorado, so you can type
00:28:51.120 in PHET and you can find this online and play with it, and you can see here we can play
00:28:56.400 with the intensity of the light, putting out some red light there, and over here what
00:29:02.000 we've got is some metal, okay on the left hand side there, and the metal is having red
00:29:06.440 lights on it, nothing's happening, nothing's happening, red light has a very long wavelength,
00:29:11.760 it's got low energy, the photons have low energy, and so even if you turn up the intensity
00:29:15.760 which means you're shining more and more photons of light, you're throwing more and
00:29:20.360 more photons there, nothing's happening, nothing's happening because none of the photons
00:29:23.840 have enough energy to knock the electrons out of the way, now if I reduce the wavelength
00:29:28.320 I increase the frequency, increase the energy, there's a point at which, look electrons
00:29:33.600 come out of that surface, this is called the photoelectric effect, of course you can do
00:29:37.640 this in real life, and actually I've done this in real life, there's a video in mind
00:29:42.280 of me doing this in real life, it shows how there's this thing called the threshold frequency,
00:29:49.000 so below the threshold frequency no light gets emitted, okay, around about there, but
00:29:56.000 if you increase the frequency then you do get emission of electrons, the intensity never
00:30:02.440 makes any difference if you're below that threshold frequency, okay, so that's the photoelectric
00:30:09.040 effect, and so what's going on there is if light was a wave, if it was truly a wave,
00:30:16.880 now I guess a way to think about this is if you're a, you're a swimmer, you're a surfboard
00:30:21.240 rider, and you're out beyond the breakers on the ocean, and then you're bobbing up and
00:30:25.440 down as the waves pass, you're not being, you don't tend to be carried one way or the
00:30:31.000 other, by the wave, you just move up and down, you vibrate in place, you don't get carried
00:30:36.560 horizontally, by the wave, you can't be knocked out of position, by the wave unless it's
00:30:41.040 breaking and then in that case it ceases to be a wave and the normal way we think about
00:30:45.200 them, waves also tend to pass through one another as well, they can pass through matter
00:30:49.520 just a sound can vibrate through a window, it doesn't necessarily, it doesn't cause the
00:30:54.800 window to move, now on the other hand particles can collide one with another, and they
00:31:00.120 can knock each other out of the way, so if light is a wave then as it strikes the surface
00:31:07.440 of the metal here and it could be cesium, it could be sodium, we use active metals because
00:31:12.560 they tend to release electrons more easily, then if you've got low energy light, something
00:31:18.000 like red light, then what should happen is if you wait long enough then electron should
00:31:23.600 come out because they will slowly absorb the wave energy and then electrons will come
00:31:29.440 off the surface, but that's not what happens, what you need to do is to exceed a certain
00:31:34.520 frequency, exceed a certain energy the photons, because on the particle theory of light,
00:31:41.120 what's going on is a physical collision and once you have enough energy, enough kinetic energy,
00:31:47.080 you can physically knock the electron out of the atom, and that's really what's going
00:31:51.600 on here, a physical collision between a particle of light and a particle of electricity
00:31:56.880 namely the electron, and you can detect these electrons, now you can go to my YouTube
00:32:03.800 channel, I guess this is one of the first videos I ever made, and this is an hour
00:32:09.680 long video, not really recommending anyone watch it unless you're really keen, all about
00:32:16.280 the photoelectric effect experiment, and so I take a bit of kit, the actual apparatus
00:32:20.880 and go through it, go through shining light onto a cesia bit of cesium metal that's
00:32:26.040 hidden inside of a black box, so it's difficult to see the full details, but I do go through
00:32:32.640 it in excruciating detail, all the way down to calculating what Planck's constant is by plotting
00:32:42.400 a whole bunch of data here, so it's interesting if you're a high school physics student
00:32:49.280 or physics undergraduate perhaps, but otherwise, just know that it's there, if you're really
00:32:54.920 curious about the photoelectric effect, it's there as an hour's worth of me talking
00:32:59.680 about the photoelectric effect, okay, so that's the photoelectric effect, that shows that
00:33:05.880 light genuinely is a particle, now this is the Mark Zender interferometer, and it's
00:33:16.240 available at this website, you can play with it yourself, so we're going to try and understand
00:33:23.440 this and take a few minutes to understand this, what's going on, over here I can fire photons
00:33:32.160 from a laser or something, one at a time, and they're going to hit something right here
00:33:37.040 that I'm indicating called a beam splitter, in other words for a beam splitter is a half
00:33:41.680 silver mirror, and half silver mirror is like, you know, one of the, when you watch TV shows
00:33:48.880 and the detectives are interviewing the criminal, and the criminals in the room locked
00:33:54.720 up with an interrogator, a single interrogator, and then behind the glass, behind one
00:34:01.240 way or two way glass rather, all the other detectives watching what's being said, well
00:34:07.520 that mirror, that's a half silver mirror, you can see through in one direction, but not
00:34:13.120 in the other direction, so long as the room where the interrogations take place is lit
00:34:17.480 very brightly, and the people in that room can't see the others that are on the other
00:34:21.880 side of the mirror, okay, so that's what, that's what this thing here is, the beam splitter,
00:34:29.000 the beam splitter is a half silver mirror, and you can make a beam splitter to send through
00:34:35.360 10% of the light, or 20% of the light, or 50% of the light, and this is what's used
00:34:40.640 in this experiment, is a beam splitter where for every photon that you send through, it's
00:34:45.560 got a 50-50 chance of either going through or being reflected, okay, on the front of the
00:34:51.520 beam splitter is silver, okay, it's got metal there, highly shiny, reflecting material,
00:34:57.800 and so photons can bounce off it, but also on the back there's glass, okay, some photons
00:35:04.000 have enough energy to go through, some by chance, and some will bounce off, okay, alright,
00:35:09.880 so firing a photon will result in 50% going through the half silver mirror, and 50% bouncing
00:35:17.160 off the half silver mirror, and then from there we've got two other mirrors, we've got
00:35:21.560 mirror one and mirror two, and these are normal mirrors, okay, these aren't half silver
00:35:24.360 or anything like this, so 100% of photons that hit here will go this way to detect a
00:35:29.640 two, and the other half will go to some mirror one and through to detect a one, okay, so
00:35:35.520 if we fire a photon, well we don't know which way it's going to go, it's going to go
00:35:39.160 through here, so let me just fire one and see what happens, fire, don't worry about the fact
00:35:44.760 that it's appearing to split in two, just notice that it's set off to detect a two, and so in
00:35:50.200 that case it appears the common sensing would be it's gone bang bang and over here, the photon
00:35:56.840 apparently hasn't gone that way apparently, okay, let's do it again and see what happens,
00:36:03.720 and it's being detected at to detect a one this time, and we've got one one, so we've got a 50
00:36:08.440 50, okay, well let's just continuously fire and see if we maintain that 50, because 50 50 would make
00:36:14.520 sense, right, if there's 50% going through the beam splitter and 50% bouncing off the beam splitter,
00:36:21.000 then we should expect these numbers to roughly approximate 50 50, or a bit out of 50 50 at the moment,
00:36:27.640 but if we continue to let it run or we fast forward, let me fast forward, let's stop the continuous,
00:36:33.320 and let's do 100 all in one go and see what happens, okay, we've got five and four out of nine,
00:36:37.480 so that's close to 50 50, if we got exactly 50 50, that would be a bit strange, 55, 54 out of 109,
00:36:46.280 so that's a pretty good 50% rate in detector one and 50% in detector two, so if our makes perfect
00:36:52.520 sense, all right, clear, now I'm going to take a second half silver mirror, okay, a second beam splitter
00:37:01.320 and put it there, now what do we expect happens, well you might expect okay, well 50% go this way,
00:37:09.800 bounce, bounce, bounce, bounce, okay, or through, and 50% go this way, bounce, bounce, and could
00:37:17.560 either go that way or that way, so a quarter of the photons coming this way will end up in detector one,
00:37:25.800 a quarter will end up in detector two, and if the photons being transmitted through this first
00:37:29.720 beam splitter, a quarter will end up in detector one, so we should still end up with 50 50,
00:37:34.120 it still looks symmetrical, doesn't it, let's see what happens, let's fire one and see what happens,
00:37:40.040 okay, detector two, fire another one, detector two again, continuous fire,
00:37:52.680 well would you look at that, it seems like detector one's not being set off ever,
00:37:57.000 what the heck is going on, and if I stop the continuous and we just do a hundred all in one go,
00:38:06.920 yes 100% go to detector two, they're always and only ever going to detector two,
00:38:14.680 why, this animation gives some clue, it's showing that the mirror is causing the photon that is
00:38:23.480 coming out of the laser or wherever to literally split into two photons, now technically speaking
00:38:31.560 we will build the cat right now, what's going on is that the photon is not a single photon
00:38:39.560 in a single universe, it's a multiverse object, it's a photon that exists in many uncountably
00:38:46.520 infinite probably, universes, and when an encounter is the half silver mirror, it splits into two
00:38:52.600 fungible groups, two groups, that are differentiated only by the fact that one of them heads
00:38:57.080 towards mirror one, and the other group heads towards mirror two, they both combine them at
00:39:02.920 beam splitter number two, and that results in them all going to detector two, now but why,
00:39:08.280 why should it be detector two and not detector one, couldn't it just be a simply be detector one,
00:39:13.400 why isn't it 50% so just a point of clarification or emphasis, the photoelectric effect tells us
00:39:21.480 that there's no such thing as half a photon, you don't get half photons, so if you were to place
00:39:26.360 a detector before mirror one or before mirror two and perform the experiment you would find
00:39:33.160 that only one of the detectors ever went off, only one of the detectors would ever go off,
00:39:38.360 you would only ever detect one photon, this demonstrates that the photon does not split in half in
00:39:44.680 our universe, so when I move forward in this explanation and talk about the splitting of the photon,
00:39:50.760 what I mean is the splitting of the multiverse object which is called the photon, we exist in a
00:39:56.040 multiverse, and so therefore when the photon encounters the half-silvered mirror, in half of the
00:40:03.720 universes it heads towards mirror one and half of the universes it heads towards mirror two,
00:40:08.840 now you and your detector are only in one universe, so you cannot possibly detect both
00:40:15.080 simultaneously both the parts simultaneously, but the way that we reason towards the existence
00:40:20.840 of both parts is the fact that the photon only ever ends up in detector two, so the multiverse
00:40:27.240 object that is the photon does in fact take both parts, you can only ever detect one path,
00:40:34.760 but it is the existence of both parts that explains why the interference happens such that
00:40:41.480 the photon only ever goes and lights up detect two, okay, so this is caused by something
00:40:49.880 known as the phase of the electrons, so phase has something to do with wavelength, but when we
00:40:55.240 thinking about photons we really just need to think of phases of quality of the photon,
00:41:00.120 there's such a thing as superposition, so here's a little animation here, I just googled,
00:41:07.400 and you can see you can imagine these are two kind of water waves that encounter one another,
00:41:13.720 when the crest here coincides with the crest there, the addition of the two causes a big crest there,
00:41:20.680 when you've got a crest and a trough, then they cancel each other out, this thick blue line
00:41:25.560 is showing what's called the resultant, the net effect of the two waves above adding together,
00:41:30.760 so there's a point where you get zero, the two waves can add together to give zero,
00:41:37.880 and so that's precisely what's happening over here, the two photons that could travel towards
00:41:47.880 detect two, sorry, detect a one, cancel each other out, they destructively interfere,
00:41:54.520 but the photons that are heading towards detect two constructively interfere,
00:41:59.800 why should that be? Well, if you know a little bit of physics,
00:42:04.120 reflections from mirrors cause a phase change, so if the photon that bounces off here,
00:42:11.480 or it's originally kind of an up, we think of it as an up bit, rather than as a down bit,
00:42:16.840 okay, so we've got troughs and crests, if the phase of that photon is up as it comes out,
00:42:25.960 then it will be down as it sort of flicked it off, it changes from being up to being down,
00:42:31.880 and then as it reflects off this mirror again, reflection causes a phase change, it goes back to being
00:42:36.440 up, and so it's up as it goes through here. Now, the one that's transmitted here in the other
00:42:44.680 universe, as it goes through the beams that are here, there's no phase change because of
00:42:51.720 no reflection, there is a phase change over here, so if it was up coming out here, then it's down
00:42:58.040 here. The key to understanding this, and this is a very subtle point to get into the technical
00:43:05.400 details, is that the second beam splitter here, the reflection that can happen there is different
00:43:13.640 to the reflection that can happen here. Here, the metallic side that reflects is encountered
00:43:23.800 before the glass, so it's on the outside of the glass, so the reflection happens and you get
00:43:28.680 this phase shift at this point. However, when a photon passes through glass first, and then hits the
00:43:39.000 rear of the beam splitter causing reflection up to detect the one, the phase change is different,
00:43:46.760 in fact doesn't happen, the phase change doesn't happen there. Now, there are other complications
00:43:52.760 with this, namely that as light in the form of photons passes through glass, it is refracted,
00:44:01.640 refracted means bent, and when it's bent, the optical path length, the distance that it travels,
00:44:08.600 is technically further than what it is through air. The net consequence of all these
00:44:14.440 factors, the refraction through the glass, the fact that one of the photons encounters the silver
00:44:21.560 side first, and the other photon encounters the, encounters the glass first, has the overall effect
00:44:33.880 of causing destructive interference to detect the one, and constructive interference to detect
00:44:39.000 the two. And so we end up with all of the photons kind of detected two. What on earth has it
00:44:44.680 got to do with the multiverse? Remember, we only fired one photon here. In order to explain why
00:44:52.440 that one photon, always ends up with detected two, we have to invoke the existence of a second
00:44:59.640 photon, traveling up to mirror one, that combines with the photon in such a way that it always ends
00:45:08.840 up detected at detected two, and never it detected one. We have to postulate the existence of
00:45:14.600 two photons to explain what one photon is doing. The one photon splits into two groups,
00:45:20.440 and then recombines into one. It's more profound than that. The one photon splits the universe
00:45:27.640 into two, and then recombines the universes into one. That's what interference is.
00:45:34.440 Interference is the rare example of where differentiation between universes in the multiverse
00:45:40.040 has happened, and then they're recombined together again. Now, if you're not convinced by that,
00:45:45.880 don't worry. We've got a third experiment to talk about, and I think this really is the
00:45:51.080 clincher. It was certainly the clincher for me. Before I leave here, let's just talk about this
00:45:58.440 excellent paper here. Hopefully, you can just see that on the screen. How does a Mark Zenda
00:46:04.120 interferometer work by Zethi Adams and Tocknell in teaching physics? From the physics department
00:46:13.320 at Westminster School, it's excellent. It goes through a complete explanation, not in terms of
00:46:19.160 the multiverse, but if you're interested in the algebra here and the mathematics is what it's
00:46:22.680 going on, it's a very simple algebraic argument about the physical makeup of those beam splitters,
00:46:30.200 and how the phase of the light changes, the phase of the photon changes. It's very short. It's
00:46:36.360 only three pages with some diagrams there, but if you're interested in the details of this, if you're
00:46:41.320 an undergrad or if you're at school trying to understand this or you want to understand this,
00:46:45.320 it's a great little introduction there, but not in terms of the multiverse. No one ever uses the
00:46:50.200 multiverse. Okay, so now I'm going to do some reading from the fabric of reality. To try and really
00:46:56.360 push this home, this idea that the universe is larger than we think. Okay, so this from the fabric
00:47:04.200 of reality, chapter two, page 39, and David writes, figure 2.6. It should be up on the screen now.
00:47:11.720 It shows that roughly it's actual size, a part of the pattern of shadows cast three
00:47:17.960 meters from a pair of straight parallel slits in an otherwise opaque barrier. Now what might that
00:47:23.960 look like? So that's what this looks like. So the experimental setup is like this. We have a
00:47:30.120 source of photons. We can find them one at a time, and there's a barrier, and the barrier's just got
00:47:35.720 to very narrow slits there. And behind on the screen, we end up if we fire enough of them over time,
00:47:43.640 we end up with this thing called an interference pattern. Okay, and David's picture is a lot clearer
00:47:50.120 about what's going on there. The slits are one fifth of the millimeter apart and illuminated by a
00:47:56.040 parallel-sided beam of pure red light from a laser on the other side of the barrier.
00:48:00.600 Why laser light? Not torchlight. Only because the precise shape of a shadow also depends on the color
00:48:05.560 of the light in which it is cast. White light, as produced by a torch, contains a mixture of all
00:48:10.200 visible colors, so it can cast shadows with multi-colored fringes. Therefore, in experiments about
00:48:15.000 the precise shapes of shadows, we are better off using light of a single color. We could put a
00:48:19.640 colored filter such as a pane of color glass in the front of the torch, so that only light
00:48:23.400 if that color would get through, that would help, but filters are not all that discriminating,
00:48:26.840 a better method is to use laser light. For lasers can be tuned very accurately to emit light
00:48:31.560 of whatever color we choose with almost no other color present. So there we can see the shadow
00:48:37.720 cast by a barrier containing two straight parallel slits, so this is what you get. If light
00:48:45.160 traveled in perfectly straight lines, the pattern in figure 2.6 will consist simply of a pair
00:48:50.520 of bright bands, so you'd end up with just two of them. One fifth of a millimeter apart, two
00:48:56.600 close to distinguish on this scale, with sharp edges and with the rest of the screen in shadow.
00:49:00.840 But in reality, the light bends in such a way as to make many bright bands and dark bands,
00:49:06.200 and no sharp edges at all. If the slits are moved sideways, so long as they remain within the
00:49:10.600 laser beam, the pattern also moves by the same amount. In this respect, it behaves exactly like
00:49:15.160 an ordinary large-scale shadow. Now what sort of shadow is cast if we cut a second identical
00:49:21.320 pair of slits in the barrier into leave with the existing pair, so we have four slits,
00:49:25.160 one tenth of a millimeter apart. You might expect the patterns would look almost exactly like
00:49:29.800 2.6. After all, the first pair of slits by itself cast a shadow in figure 2.6.
00:49:36.200 And as I have just said, the second pair by itself would cast the same pattern,
00:49:39.720 shifted about a tenth of a millimeter to the side, in almost the same place. We even know that light
00:49:44.600 beams normally pass through each other out affected. So the two pairs of slits together should
00:49:48.440 give us essentially the same pattern, though twice as bright and slightly more blurred.
00:49:53.080 Now a four slit apparatus, looking if it would be something like that. So here we've got four slits.
00:49:59.400 And then we say bright bits and dark bits. There are the shadows, and these are the bright bits.
00:50:04.040 If you actually do anything about this, what you see? So you increase the number of slits,
00:50:07.080 and you see this sort of... So just to hammer this point home, if you've got one slit,
00:50:12.520 you get this big long diffraction pattern. Lots and lots of light with one slit.
00:50:18.680 If you increase the number of slits, in other words, increase the number of places through which
00:50:22.200 light can come, you seem to increase the number of shadows. That's rather weird and more shadows
00:50:29.400 here. Less light. More slits, less light compared to one slit. That's phenomenal, isn't it?
00:50:36.040 Okay, well David's going to explain that. So let's keep going. Back to fabric of reality.
00:50:46.600 The real shadow of a barrier with four straight parallel slits is shown in figure 2.7i.
00:50:54.040 Okay, so over here in 2.7i, we've got a picture of these two things.
00:50:58.440 Picture A, that's the four slit one, and B is two slits. So David writes in fabric of reality,
00:51:08.120 the four slit shadow is not a combination of two slight displaced two slit shadows,
00:51:12.360 but has a new and more complicated pattern. And this pattern, there are places such as the point
00:51:16.920 marked x, which are dark on the four slit pattern, but bright on the two slit pattern.
00:51:22.040 These places were bright where there were two slits in a barrier, but went dark when we cut a
00:51:26.040 second pair of slits that were light to pass through, opening those slits is interfered with the
00:51:29.720 light that was previously arriving at x. So adding two more light sources, darkens the pointed x,
00:51:34.760 removing them, illuminates it again. How? One might imagine two photons heading towards x,
00:51:39.960 and bouncing off each other like billiard balls. Either photon alone would have hit x,
00:51:44.760 but the two together interfere with each other so that they both end up elsewhere.
00:51:48.760 I shall show in a moment that this explanation cannot be true. Nevertheless, the basic idea of it
00:51:53.800 is in a skyhole, something must be coming through that second pair of slits to prevent the light
00:51:57.640 from the first pair from reaching x. But what? We can find out with the help of some further
00:52:02.600 experiments. First, the four slit pattern of figure 2.78 appears only if all four slits are
00:52:07.880 illuminated by the laser beam. If only two of them are illuminated, a two-slip pattern appears.
00:52:12.600 If three are illuminated, a three-slip pattern appears, which looks different again.
00:52:15.880 So whatever causes the interference is in the beam. The two-slip pattern also reappears if the two
00:52:21.560 if two of the slits are filled by anything opaque, but not if they are filled by anything transparent.
00:52:26.040 In other words, the interfering entity is obstructed by anything that obstructs light,
00:52:29.400 even something as insubstantial as fog, but it can penetrate anything that allows light to pass,
00:52:34.600 even something as impenetrable to matter as diamond. If complicated systems of mirrors and lenses
00:52:39.480 are placed anywhere in the apparatus, so long as light can travel from each slit to a particular
00:52:42.920 point on the screen, what will be observed at that point will be part of the four-slip pattern.
00:52:47.320 If light from only two slits can reach a particular point, part of a two-slip pattern will be
00:52:51.480 observed there and so on. So whatever causes interference behaves like light. It is found everywhere
00:52:56.440 in the light beam and nowhere outside it. It is reflected, transmitted or blocked by whatever
00:53:01.080 reflects, transmits or blocks light. You may be wondering why I'm laboring this point, surely
00:53:05.080 it is obvious that it is light, but what interferes with photons from each slit is
00:53:09.000 photons from the other slits. But you may be inclined to doubt the obvious after the next experiment.
00:53:14.120 The denoument of the series. What should we expect to happen when these experiments are performed
00:53:19.800 with only one photon at a time? For instance, suppose that our torches move so far away
00:53:24.760 that only one photon per day is falling on the screen? What would be seen? If it is true that
00:53:29.960 what interferes with each photon is other photons, then shouldn't the interference be
00:53:34.040 lessened when the photons are very sparse? Should it not cease altogether when there is only
00:53:38.920 one photon passing through the apparatus at any one time? We might still expect
00:53:43.080 per numbers, since a photon might be capable of changing course when passing through a slip,
00:53:48.040 perhaps by striking a glancing blow at the edge. But what we surely could not observe
00:53:52.440 is any place on the screen, such as X, that receives photons when two slits are open,
00:53:57.320 but which goes dark when two more are opened? Yet that's exactly what we do observe.
00:54:02.280 However sparse the photons are, the shadow pattern remains the same. Even when the
00:54:05.960 experiment is done with one photon at a time, none of them is ever observed to arrive at X,
00:54:10.200 when all four slits are open. Yet we need only close two slits for the flickering
00:54:14.760 at X to resume. Could it be that the photon splits into fragments,
00:54:20.040 which after passing through the slits, change course and recombine? We can rule that
00:54:24.680 possibility out too. If again we fire one photon through the apparatus but use four detectors,
00:54:30.200 one at each slit, then at most one of them ever registers anything.
00:54:35.080 Since in such an experiment we never observe two of the detectors ever going off at once,
00:54:39.800 we can tell that the entities that they detect are not splitting up.
00:54:43.720 So if the photons do not split into fragments and are not being deflected by other photons,
00:54:49.080 what does deflect them? When a single photon at a time is passing through the apparatus,
00:54:54.680 what can be coming through the other slits to interfere with it? Let us take stock.
00:55:00.040 We have found that when one photon passes through this apparatus,
00:55:04.200 it passes through one of the slits, and then something interferes with it,
00:55:10.440 deflecting it in a way that depends on what other slits are open.
00:55:14.440 The interfering entities have passed through some of the other slits.
00:55:18.840 The interfering entities behave exactly like photons, except they cannot be seen.
00:55:26.440 I shall now start calling the interfering entities photons. That is what they are though
00:55:30.200 for the moment, it does appear that photons come in two sorts, which I shall temporarily call
00:55:34.760 tangible photons and shadow photons. Tangible photons are the ones we can see or detect with
00:55:39.640 instruments, whereas the shadow photons are intangible, they are invisible. Detectible only through
00:55:44.600 their interference effects on the tangible photons, later we shall see that there is no intrinsic
00:55:48.440 difference between a tangible and shadow photons. Each photon is tangible in one universe
00:55:53.080 in intangible and all the other parallel universes, but I anticipate. What we have inferred so far
00:55:59.240 is only that each tangible photon has an accompanying retinue of shadow photons,
00:56:03.400 and that when a photon passes through one of our four slits, some shadow photons pass through
00:56:07.480 the other three slits, since different interference patterns appear when we cut slits at other
00:56:11.320 places in the screen, provided that they are within the beam, shadow photons must be arriving
00:56:16.040 all over the illuminated part of the screen whenever a tangible photon arrives. Therefore,
00:56:21.560 there are many more shadow photons than tangible ones. How many? Experiments cannot put
00:56:26.040 an upper bound on the number, but they do set a rough lower bound. In a laboratory, the largest
00:56:31.240 area that we could conveniently illuminate with a laser light might be about a square meter,
00:56:34.680 and the smallest manageable size of the holes might be about a thousandth of a millimeter.
00:56:39.320 So there are about 10 to the power of 12, a trillion possible whole locations on the screen.
00:56:44.840 Therefore, there must be at least a trillion shadow photons accompanying each tangible one.
00:56:50.760 Thus we have inferred the existence of a seething,
00:56:53.240 religiously complicated hidden world shadow photons. They travel at the speed of light,
00:56:57.800 bounce off mirrors are refractable lenses and are stopped by opaque barriers or filters of the wrong
00:57:02.840 colour. Yet they do not trigger even the most sensitive detectors. The only thing in the universe
00:57:08.840 that a shadow photon can be observed to affect is the tangible photon that it accompanies.
00:57:14.200 This is the phenomenon of interference. Shadow photons would go entirely unnoticed
00:57:19.480 where it not for this phenomenon and the strange patterns of shadows by which we observe it.
00:57:26.920 Interference is not a special property of photons alone. Quantum theory predicts and experiment
00:57:31.320 confirms that it occurs for every sort of particle, so there must be hosts of shadow neutrons
00:57:36.840 accompanying every tangible neutron, hosts of shadow electrons accompanying every tangible
00:57:41.880 electron and so on. Each of these shadow particles is detectable only indirectly.
00:57:47.960 Through its interference with the motion of its tangible counterpart, it follows
00:57:52.680 that reality is much bigger than it seems and most of it is invisible. The objects and events
00:57:58.200 that we and our instruments can directly observe are the mirrors tip of the iceberg.
00:58:02.040 Now tangible particles have a property that enables us to call them collectively a universe.
00:58:07.160 This is simply their defining property of being tangible,
00:58:09.560 that is of interacting with each other and hence of being directly detectable by instruments
00:58:13.640 and sense organs made by other tangible particles. Because of the phenomenon of interference,
00:58:18.280 they are not wholly partitioned off from the rest of reality. That is, from the shadow particles.
00:58:23.000 If they were, we should never have discovered there is more to reality than tangible particles.
00:58:27.240 But to a good approximation, they do resemble the universe that we see around us in every day
00:58:30.760 life and the universe referred to in classical or pre quantum physics.
00:58:36.920 For similar reasons, we might think of calling the shadow particles collectively a parallel universe.
00:58:41.320 For they too are affected by tangible particles only through interference phenomena,
00:58:44.680 but we can do better than that for it turns out that shadow particles are
00:58:47.480 partitioned among themselves and exactly the same way as the universe of tangible particles
00:58:51.320 is partitioned from them. In other words, they do not form a single homogenous parallel
00:58:55.320 universe vastly larger than the tangible one, but rather a huge number of parallel universes,
00:58:59.720 each similar in composition to the tangible one and each obeying the same laws of physics,
00:59:04.120 but differing in that the particles are in different positions in each universe.
00:59:07.640 Or a mark about terminology. The word universe has traditionally been used to mean the whole
00:59:13.160 of physical reality, in that sense there can be at most one universe. We could stick to that
00:59:17.560 definition and say that the entity we have been accustomed to calling the universe,
00:59:21.080 namely all the directly perceptible matter and energy around us and the surrounding space is
00:59:25.480 not the whole universe after all, but only a small portion of it. Then we should have to
00:59:29.560 invent a new name for that small tangible portion, but most physicists prefer to carry on
00:59:33.640 using the word universe to denote the same entity that has always denoted, even though that
00:59:39.080 entity now turns out to be only a small part of physical reality. A new word, multiverse,
00:59:44.520 has been coined to denote physical reality as a whole. Single particle interference experiments,
00:59:49.240 such as I have been describing, show us that the multiverse exists and it contains many counterparts
00:59:55.240 of each particle in the tangible universe. To reach the further conclusion that the multiverse
1:00:00.280 is roughly partitioned in the parallel universe, we must consider interference phenomena involving
1:00:04.760 more than one tangible particle. The simplest way of doing this is to ask by way of thought
1:00:09.400 experiment what must be happening at the microscopic level when shadow photons strike an opaque
1:00:13.720 object. They are stopped of course. We know that because interference ceases when an opaque
1:00:18.040 barrier is placed in the paths of shadow photons. But why? What stops them? We can rule out
1:00:22.200 the straightforward answer that they are absorbed like tangible photons would be.
1:00:25.000 Okay, and then David goes on to say, well they are absorbed. The tangible photons are absorbed
1:00:31.240 by tangible barriers but it means that also the shadow photons are absorbed by shadow barriers.
1:00:38.760 And the shadow barriers exist inside shadow laboratories and the shadow laboratories are in a
1:00:43.640 shadow world and so on. And so this is the emergent multiverse idea. Okay, so there are three
1:00:52.600 experiments. There's some personal motivation and a hint about the multiverse. We still haven't
1:01:00.520 got to reading in chapter 11 of the multiverse but that will happen next time and we'll look in
1:01:08.760 some detail at the further experiment. Okay, it's more of a thought experiment but potentially
1:01:17.240 could be really done. An experiment to refute other ways of understanding quantum theory or
1:01:24.440 trying to understand quantum theory. Namely so-called collapse interpretations or the Copenhagen
1:01:29.800 interpretation. To refute those without refuting the multiverse theory, there is a test. There is a
1:01:35.880 test that David came up with and so we'll talk about that next time. But for now this has been
1:01:41.000 nearly an hour's worth of quantum theory. Until next time, bye. If you're finding my videos,
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