00:00:00.000 Welcome to Topcast and for my continuing series.
00:00:29.960 On the fabric of reality, we're up to chapter two shutters.
00:00:34.320 For what I imagine will be a relatively short episode.
00:00:38.160 No reason for that, I flagged in a previous episode because I've already completed approximately
00:00:44.040 a five-part series on the multiverse, which is essentially what this is about.
00:00:52.040 As I've also said previously, and in various other forums, this was the chapter that
00:00:57.240 had the most profound impact on me when it came to reading the fabric of reality.
00:01:03.000 And the reason for that, I suppose, is twofold.
00:01:05.720 On the one hand, I was struggling when I first read this chapter to understand quantum theory.
00:01:11.800 I was studying it at university and the undergraduate level, and we were working through
00:01:16.680 problem sets and doing exercises as you do in university physics classes.
00:01:22.120 And although I could mechanically work through how to get the answer, I didn't understand
00:01:27.600 I didn't understand exactly what the Schrodinger wave equation, for example, was telling me
00:01:33.240 I didn't understand how to interpret the experiments, one of which David is going to
00:01:37.840 explain to us today, which I'm going to go through again.
00:01:40.920 So in this confusion, this haze of disillusionment, I suppose, with the way in which I was
00:01:50.440 By both my lecturers at university and by other popularizers of science, who'd written books
00:01:56.440 on the subject, like Paul Davies, for example, who writes some very exciting and interesting
00:02:00.360 books about the touch upon, at least, quantum theory.
00:02:03.880 I never felt like I was getting an explanation that made any sense.
00:02:07.960 And when I read the explanation here in chapter two shadows, it made sense.
00:02:13.640 And so that was why it was astonishing to me on the one hand of why it had such a profound
00:02:17.720 impact on me, because he finally, for the first time I got it, I felt as if now I understood
00:02:25.240 what quantum theory was an explanation of, what it was telling us about reality.
00:02:32.280 On the other hand, it was the fact that what it was telling us about reality was so astonishing.
00:02:37.120 I kind of knew that it had to be astonishing in some way, shape or form, because I'd read
00:02:41.720 various other so-called interpretations, and they were wacky to say the least.
00:02:47.120 Things like the human mind or consciousness was somehow involved in fundamental physics.
00:02:53.160 It didn't ring true to me, because I didn't think there could be a place, really, for
00:02:57.240 something as emergent and complex and large, as the human brain, giving right to the mind,
00:03:06.120 It evoked what Einstein called spooky action at a distance.
00:03:10.560 I, like Einstein, not to put myself in the same category, but I rejected this idea I regarded
00:03:16.240 it as spooky, this idea that if you're observing something, then the experiment goes
00:03:21.040 in one direction, and if you're not observing something, the experiment goes in a different
00:03:25.600 I felt either this was a prosaic claim about the way in which light interacted with
00:03:30.560 matter, who cares, or it was a weird claim about if you passively think of something, then
00:03:37.800 that can affect a physical system somewhere other.
00:03:40.720 Anyway, I wasn't buying it, I didn't understand it, but here we actually get the explanation.
00:03:47.160 So I'm going to do an abridged reading today of the chapter, and go through the double
00:03:55.040 slit experiment, the way that David puts it in his words, and I've done this before actually
00:03:59.680 in the beginning of infinity series, but we'll do it again here, because this is the
00:04:05.080 So here we are, chapter two, shadows, and David writes, there is no better, there is no
00:04:12.280 more open door by which you can enter into the study of natural philosophy than by considering
00:04:17.240 the physical phenomena of a candle, Michael Faraday, a course of six lectures on the chemical
00:04:24.800 In his popular Royal Institution Lectures on Science, Michael Faraday used to urge his
00:04:29.200 audiences to learn about the world by considering what happens when a candle burns.
00:04:34.240 I'm going to consider an electric torch, or flashlight instead.
00:04:38.080 This is quite fitting for much of the technology of an electric torch is based on Faraday's
00:04:43.680 I'm going to describe some experiments which demonstrate phenomena that are at the core
00:04:51.200 Experiments of this sort with many variations and refinements have been the bread and butter
00:04:57.520 There is no controversy about the results, yet even now some of them are hard to believe.
00:05:05.640 They require neither specialized scientific instruments nor any great knowledge of mathematics
00:05:10.760 Essentially they involve nothing but casting shadows, but the patterns of light and shadow
00:05:15.400 that an ordinary electric torch can cast are very strange.
00:05:19.480 When considered carefully they have extraordinary ramifications.
00:05:23.840 Explaining them requires not a just new physical laws but a new level of description
00:05:28.520 and explanation that goes beyond what was previously regarded as being the scope of science.
00:05:33.080 But first it reveals the existence of parallel universes.
00:05:37.560 What conceivable pattern of shadows could have implications like that?
00:05:42.360 Imagine an electric torch switched on in an otherwise dark room.
00:05:46.600 Light emanates from the filament of the torch's bulb and fills out part of the cone.
00:05:51.880 In order not to complicate the experiment with reflected light, the walls of the room should
00:05:58.600 Alternatively, since we are only imagining these experiments, we could imagine a room of
00:06:02.760 astronomical size, so there is no time for any light to reach the walls and return before
00:06:09.520 Figure 2.1 illustrates the situation, but it is somewhat misleading.
00:06:14.080 If we were observing the torch from the side we should be able to see neither it nor,
00:06:20.440 Invisibility is one of the more straightforward properties of light.
00:06:23.800 We see light only if it enters our eyes, though we usually speak of seeing the object in
00:06:28.760 our line of sight that last affected that light.
00:06:33.840 Yes, this can be sometimes surprising to people who hear it for the first time that light
00:06:42.840 What that means is that if light is passing in that direction, let's say, and not entering
00:06:47.600 your eye, then there's no way that you can possibly detect it.
00:06:50.680 You can't see it, which is why the laser bolts in Star Wars should be invisible.
00:06:57.800 Now, we don't know what these laser bolts technically speaking consist of, and Star Wars
00:07:02.840 might very well be a magical universe, separator from our own operating via different physical
00:07:09.520 Indeed, the existence of the force probably suggests that that is the case, but presuming
00:07:13.520 that you could indeed have bullets made of light coming from storm trip of guns, then you
00:07:19.640 wouldn't be able to see them unless, of course, they're not actually made of light, which
00:07:23.080 is quite possible after all, they're moving much, much slower than light, indeed, they're
00:07:33.160 You can only see stuff that light reflects off.
00:07:37.720 And when you see stuff, what you're seeing are the photons entering your eye.
00:07:41.520 If a laser is shone from here to there, you don't see the laser beam.
00:07:53.320 I can see the little dot over there, and if I put the laser behind me, I'm not sure if
00:07:57.080 that'll sharpen my screen or whatever, but here you can probably see it on my forehead
00:08:02.320 Now, if I pointed out you, then you can see it, right.
00:08:05.600 So lasers, as they go from one place to another, you can't actually see the photons.
00:08:11.520 So explain those pictures where you can see the laser light.
00:08:13.800 That's because you've got some sort of smoke or mist or something in the room, and it's
00:08:18.240 reflecting off the laser ray, beam, whatever you want to call it.
00:08:23.120 The photons of light are crashing into matter particles, particles of dust, usually of some
00:08:29.160 So the other thing to say about light, especially light, such as comes out of a normal
00:08:33.520 And this is something we all have experience with, is that if you are close to the source
00:08:38.440 of light, if you're close to where the torch is, then the torch is bright, and the further
00:08:43.040 you move away from the torch, the more dim it becomes.
00:08:47.920 And the reason is that the light is spreading out over an ever-greater area.
00:08:53.440 And so it's getting spread more and more thinly, like butter being spread over more
00:08:58.560 Let me go back to the book at this point, and David Ryan's.
00:09:01.960 Can light really be spread more and more thinly without limit?
00:09:07.080 At a distance of approximately 10,000 kilometers from the torch, its light would be too
00:09:11.400 faint for the human eye to detect, and the observer would see nothing.
00:09:17.880 But what about an animal with more sensitive vision?
00:09:21.960 Frogs eyes are several times more sensitive than human eyes, just enough to make a significant
00:09:28.960 If the observer were a frog, and it kept moving further away from the torch, the moment
00:09:33.160 at which an entirely lost sight of the torch would never come.
00:09:37.640 Instead, the frog would see the torch light begin to flicker.
00:09:41.240 The flickers would come at irregular intervals that would become longer as the frog moved
00:09:47.360 But the brightness of the individual flickers would not diminish.
00:09:50.800 At a distance of 100 million kilometers from the torch, the frog would see on average
00:09:55.320 only one flicker of light per day, but that flicker would be as bright as any that it
00:10:04.240 Frogs cannot tell us what they see, so in real experiments, we use photomultiplires,
00:10:08.520 light detectors, which are even more sensitive than frog's eyes, and we thin out the light
00:10:12.480 by passing it through dark filters, rather than by observing it from 100 million kilometers
00:10:27.920 There comes a point at which the light begins to flicker.
00:10:31.480 In a similar way, gold is an infinitely malleable.
00:10:34.760 Now, I've used this example myself, and it's an interesting psychological trick that
00:10:40.960 I actually thought this analogy was something that I sort of came up with, but no, here
00:10:46.120 it is gold is mentioned here in the fabric of reality.
00:10:49.000 So I have absorbed it then, only just reading it here again now, do I realize that,
00:10:53.320 oh, the reason why I keep using gold is my go-to example of what stuff is quantized
00:11:01.120 So if you hammer out a metal, the degree to which you can hammer it out of the ease with
00:11:06.000 which you can hammer it out is known as the malleability of that material.
00:11:13.800 You can spread it so thin, of course, that you can get that gold leaf stuff that is extremely
00:11:21.840 But you can't make gold infinitely thin, because you eventually get down to the gold
00:11:27.880 And once you get down to the gold atom, if you were to try and break that down still
00:11:33.680 You'd have half of a gold atom, which is no longer gold.
00:11:37.520 So as David writes, and I'll pick up from where he talks about this, you're right.
00:11:42.400 So the only way in which one can make a one atom thick gold sheet even thinner is to space
00:11:47.640 the atoms further apart, with empty space between them.
00:11:51.680 When they are sufficiently far apart, it becomes misleading to think of them as forming
00:11:57.440 For example, if each gold atom were on average several centimeters from its nearest neighbor,
00:12:01.760 one might pass one's hand through the sheet without touching any gold at all.
00:12:06.480 Similarly, there is an ultimate lump or atom of light, a photon.
00:12:11.200 Each flicker scene by the frog is caused by a photon striking the retina of its eye,
00:12:17.320 So what David's talking about here is, of course, quantization.
00:12:22.040 This concept that there is the smallest possible unit of stuff.
00:12:26.520 In the case of gold, the smallest possible unit of stuff is the gold atom.
00:12:31.000 In the case of light, it is the photon of light, smallest particle of light.
00:12:35.200 Case of electricity, the smallest possible particle is the electron.
00:12:39.200 In the case of water, it's the water molecule and repeat for all stuff that's made out
00:12:43.560 of matter, anything that appears in the so-called standard model of particle physics.
00:12:48.680 All the stuff that you can see around you is going to have a smallest possible unit
00:12:55.720 And I say everything, of course, I mean the everything that's made out of pure substances
00:13:01.840 So if you have in the simplest case, salt water, then the smallest possible unit of salt
00:13:09.520 This smallest possible unit are two discrete things, one of which is called the salt lattice.
00:13:14.960 Well, really, it's made of two ions, the sodium and the chloride, which are bonded together
00:13:19.800 via electrostatic forces, and the water molecule.
00:13:23.200 And so these two things constitute the units out of which the mixture, salt water is
00:13:28.840 So a lot of the things you're seeing in your environment, of course, are mixtures, including
00:13:31.480 human beings, for example, which are made up of lots of pure substances, each of which
00:13:40.480 What doesn't appear to be quantized or what we don't know about the quantization of?
00:13:45.440 Things like gravity, it has been suggested that the standard model should contain a graviton.
00:13:51.120 I don't know why, but this is just what people who want to quantize spacetime or quantize
00:13:58.560 gravity talk about, they talk about a particle gravity, never been observed and doesn't appear
00:14:03.600 to explain anything because we have an explanation of gravity, which is in terms of spacetime,
00:14:11.400 Under that theory, under general relativity, we don't have a smallest unit of time or a
00:14:14.800 smallest unit of space or a smallest unit of spacetime, et cetera.
00:14:18.320 But this idea of quantization in physics within the realm of quantum physics is the thing
00:14:24.760 There is a quanta, a smallest particle of something.
00:14:29.840 Okay, so I'm skipping, as I say, a number of pages there and I'm picking up where David
00:14:34.280 says, I have made an assumption about light, namely that it travels only in straight
00:14:40.280 From everyday experience, we know that it does, for we cannot see around corners, but careful
00:14:45.000 experiments show that light does not always travel in straight lines.
00:14:52.880 This is hard to demonstrate with a torch alone just because it is difficult to make very
00:14:59.120 These practical difficulties mask the limits that fundamental physics imposes on the sharpness
00:15:04.680 Fortunately, the bending of light can also be demonstrated in a different way.
00:15:09.040 Suppose that the light of a torch passes through two successive small holes in otherwise
00:15:14.240 opaque screens, as shown in figure 2.4, and that the emerging light falls on the third
00:15:21.520 If the experiment is repeated with ever smaller holes and with ever greater separation
00:15:25.440 between the first and second screens, can one bring theumbra, the region of total darkness,
00:15:31.360 ever closer without limit to the straight lines through the centers of the two holes.
00:15:36.200 Can the illuminated region between the second and third screens be confined to an arbitrarily
00:15:41.720 narrow cone, in Goldsmith's terminology, we are now asking something like how ductile
00:15:57.360 Long before the holes get as small as a 10,000th of a millimeter, in fact, even with holes
00:16:02.880 as large as a millimeter or so in diameter, the light begins noticeably to rebel.
00:16:08.280 Instead of passing through the holes in straight lines, it refuses to be confined and spreads
00:16:12.840 out after each hole, and as it spreads, it frays.
00:16:16.800 The smaller the hole, the more the light spreads out from its straight line path, intricate
00:16:23.480 We no longer see simply a bright region and a dark region on the third screen with a
00:16:27.400 penumbra in between, but instead concentric rings of varying thickness and brightness.
00:16:33.600 There is also color, because white light consists of a mixture of photons of various colors,
00:16:39.000 and each color spreads and frays in a slightly different pattern.
00:16:43.120 Figure 2.5 shows a typical pattern that might be formed on the third screen by white light
00:16:48.480 that has passed through holes in the first two screens.
00:16:52.000 Remember, there is nothing happening here but the casting of a shadow.
00:16:55.920 Figure 2.5 is just a shadow that would be cast by the second screen in figure 2.4.
00:17:03.200 If light travelled only in straight lines, there would be only a tiny white dot, much smaller
00:17:08.000 than the central bright spotting figure 2.5, surrounded by a very narrow penumbra.
00:17:14.400 Outside that, there would be pure umbra, total darkness, pausing their my reflection
00:17:23.800 At this point, nothing to a science student, in particular a physics student, as I was
00:17:30.320 20-plus years ago, was particularly surprising in what David had said.
00:17:35.120 It was a little bit idiosyncratic, I thought perhaps, that he was invoking frogs and
00:17:38.960 torches, and I thought it was a nice harking back to the fact that, yes, Michael Faraday
00:17:44.480 would explain stuff through contemplation of the operation of how a candle produced light,
00:17:50.880 and so that was lovely, but so far nothing particularly surprising.
00:17:56.960 Pretty much run-of-the-mill science and the same kind of way in which things would be explained
00:18:03.120 in a clear way with analogies, such as the analogy of light being quantized, to gold
00:18:09.800 being quantized, but now we're about to get into the part where, as I've said before,
00:18:15.360 the sense of vertigo comes in, for anyone who hasn't heard this description of the experiment
00:18:21.280 before, much less the explanation which is to come.
00:18:25.360 The description of the experiment, including the result, is rather strange.
00:18:32.880 The first part, if you've even done high school physics, won't seem to be that mysterious,
00:18:39.920 But how it works with single photons, well that is the challenging part for us.
00:18:49.200 Figure 2.6 shows that, roughly its actual size, a part of the pattern of shadows cast three
00:18:55.400 metres from a pair of straight parallel slits in an otherwise opaque barrier.
00:19:00.640 The slits are one-fifth of a millimetre apart and illuminated by a parallel-sided beam
00:19:05.920 of pure red light, from a laser on the other side of the barrier.
00:19:13.080 Only because the precise shape of a shadow also depends on the colour of the light in which
00:19:19.440 White light, as produced by a torch, contains a mixture of all visible colours, so it can cast
00:19:27.400 Therefore, in experiments about the precise shapes of shadows, we are better off using
00:19:33.920 We could put a coloured filter, such as a pane of coloured glass, over the front of the
00:19:37.760 torch, so that only line of that colour would get through.
00:19:41.000 That would help, but filters are not all that discriminating.
00:19:44.200 A better method is to use laser light, for lasers can be tuned very accurately to emit
00:19:49.240 light of whatever colour we choose with almost no other colour present.
00:19:54.000 If light travelled in straight lines, the pattern in figure 2.6 would consist simply
00:19:59.160 of a pair of bright bands, one-fifth of a millimetre apart, too close to distinguish on
00:20:04.160 the scale, with sharp edges and with the rest of the screen in shadow.
00:20:08.800 But in reality, the light bends in such a way as to make many bright bands and dark bands
00:20:15.520 If the slits are moved sideways, so long as they remain within the laser beam, the pattern
00:20:22.120 In this respect, it behaves exactly like an ordinary, large-scale shadow.
00:20:29.560 If we cut a second identical pair of slits in the barrier, interleaved with the existing
00:20:34.720 pair, so that we have four slits at intervals of one-tenth of a millimetre.
00:20:40.000 You might expect the pattern to look almost exactly like figure 2.6, after all, the first
00:20:44.880 pair of slits by itself cast the shadow in figure 2.6, and as I have just said, the second
00:20:51.240 pair by itself would cast the same pattern, shifted about a tenth of a millimetre to the
00:20:58.080 We even know that light beams normally passed through each other, unaffected, so the two
00:21:02.400 pairs of slits together should give essentially the same pattern again, though twice as
00:21:09.400 In reality, though, what happens is nothing like that.
00:21:13.400 The real shadow of a barrier, with four straight parallel slits, is shown in figure 2.7a.
00:21:20.240 For comparison, I have repeated below it the illustration of the two-slit pattern, figure
00:21:26.320 Clearly, the four-slit shadow is not a combination of two slightly displaced two-slit
00:21:32.640 shadows, but has a new and more complicated pattern.
00:21:35.960 In this pattern, there are places, such as the point marked x, which are dark on the
00:21:40.480 four-slit pattern, but bright on the two-slit pattern.
00:21:43.680 These places were bright when there were two slits in the barrier, but when dark, when
00:21:47.680 we cut a second pair of slits for the light to pass through, opening those slits has
00:21:52.520 interfered with the light that was previously arriving at x.
00:21:56.760 So adding two more light sources darkens the pointed x, removing them, illuminates it again,
00:22:04.360 Now, at this point, I still recall, to some extent, the excitement of reading that.
00:22:10.920 Even though I didn't know the explanation yet, I'd read that sort of thing before, but
00:22:14.200 probably not quite as well explained exactly what the problem was.
00:22:18.760 But this idea that if you open up more places that are light to come through, you actually
00:22:26.200 There's more shadows, even though you've got more light sources.
00:22:29.040 You had two previously, two places that are light to come through, and you had a certain
00:22:36.000 And then you open up more places that are light to go through, more gaps, more slits,
00:22:45.640 It's like opening up more places that are light to get through, darkens the screen.
00:22:52.120 And I don't know exactly what my psychology was back at the time of reading this, but
00:22:56.840 I guess it might have been something like, I'm about to be given another confusing account
00:23:06.080 I've even conducted this experiment myself before, and I've had lecturers tell me about
00:23:10.680 the results of the experiment and what it could mean, and I haven't understood anything
00:23:16.640 So I guess I had my hopes were pretty low for being given a realistic account, a clear
00:23:24.000 account, a logical account that I would be able to actually explain to other people.
00:23:30.160 After all, that's a measure of whether or not you understand something.
00:23:32.960 If you can actually explain to someone else that such that they then walk away, nodding
00:23:36.120 their head and going, oh, now I also get it, then that means to some extent that you
00:23:40.800 At the very least, you've convinced yourself you understand it, even if you might have some
00:23:45.320 Okay, so backtracking a little bit and reading on David wrote.
00:23:51.400 These places were bright when there were two slits in the barrier, but when dark, when
00:23:55.560 we cut a second pair of slits of the light to pass through, opening those slits has interfered
00:24:00.800 with the light that was previously arriving at X.
00:24:04.240 So adding two more light sources, darkens the pointed X, removing them, illuminate it again.
00:24:11.720 One might imagine two photons heading towards X and bouncing off each other, like billiard
00:24:17.360 Either photon alone would have hit X, but the two together interfere with each other so that
00:24:25.520 I shall show in a moment that this explanation cannot be true, nevertheless.
00:24:34.240 Something must be coming through that second pair of slits to prevent the light from the
00:24:42.880 We can find out with the help of some further experiments.
00:24:46.680 First, the four-slit pattern of figure 2.7A appears only if all four slits are illuminated
00:24:56.040 If only two of them are illuminated, a two-slit pattern appears.
00:24:59.760 If three are illuminated, a three-slit pattern appears, which looks different again.
00:25:04.440 So whatever causes the interference is in the light beam.
00:25:07.920 The two-slit pattern also reappears if two of the slits are filled by anything opaque,
00:25:13.800 but not if they are filled by anything transparent.
00:25:16.160 In other words, the interfering entity is obstructed by anything that obstructs lights.
00:25:26.200 But it can penetrate anything that allows light to pass, even something as impenetrable
00:25:34.240 If complicated systems of mirrors and lenses are placed anywhere in the apparatus, so
00:25:37.280 long as light can travel from each slit to a particular point on the screen, what will
00:25:40.400 be observed at that point will be part of a four-slit pattern.
00:25:43.560 If light from only two slits can reach a particular point, part of a two-slit pattern
00:25:50.560 So whatever causes interference behaves like light, or just pausing there, what we mean
00:25:57.480 by interference is something affecting that light that would have struck X, but which
00:26:05.440 So remember, in figure B, we've got the pattern that happens that occurs with two slits.
00:26:12.560 And that bit that's light then goes dark when you add two more slits, additional places
00:26:19.800 So something's interfering with light that otherwise would have hit X in picture B.
00:26:26.360 Now those photons that we're heading towards point X, but which don't make it to point
00:26:30.680 X in figure A, when you've got those two extra slits, has been interfered with something's
00:26:38.120 And so there's some reason why it hasn't gotten there when it would have got there.
00:26:45.200 Whatever causes interference behaves like light.
00:26:47.560 It is found everywhere in the light beam and nowhere outside it.
00:26:50.320 It is reflected transmitted to a block by whatever reflects, transmits or blocks light.
00:26:54.600 You may be wondering why I'm laboring this point.
00:26:59.520 That is, what interferes with photons from each slit is photons from the other slits.
00:27:04.440 That you may be inclined to doubt the obvious after the next experiment.
00:27:11.640 What should we expect to happen when these experiments are performed with only one photon
00:27:18.840 For instance, suppose that our torches moved so far away that only one photon per day is
00:27:25.920 What will our frog observing from the screen see if it is true that one had a few years
00:27:30.280 with each photon is other photons, then shouldn't the interference be lessened when the photons
00:27:36.320 Should it not cease altogether when there is only one photon passing through the apparatus
00:27:40.920 We might still expect penumbras since a photon might be capable of changing course when
00:27:45.040 passing through a slit, perhaps by striking a glancing blow at the edge.
00:27:48.760 But what we surely could not observe is any place on the screen, such as X, that receives
00:27:53.800 photons when two slits are open, but which goes dark when two more are opened.
00:27:59.520 Yet that is exactly what we do observe, however sparse the photons are, the shadow pattern
00:28:06.120 Even when the experiment has done one photon at a time, none of them is ever observed
00:28:10.240 to arrive at X when all four slits are open, yet we need only close two slits for the flickering
00:28:18.160 Could it be that the photon splits into fragments, which after passing through the slits
00:28:24.880 We can rule that possibility out too if again we fire one photon through the apparatus,
00:28:29.440 but use four detectors, one at each slit, then at most one of them, ever registers anything.
00:28:35.800 Since in such an experiment we never observe two of the detectors going off at once, we
00:28:39.760 can tell that the entities that they detect are not splitting up, so if the photons do
00:28:44.600 not split into fragments and are not being deflected by other photons, what does deflect
00:28:50.920 When a single photon at a time is passing through the apparatus, what can be coming through
00:29:00.320 We have found that when one photon passes through the apparatus, it passes through one of
00:29:04.520 the slits, and then something interferes with it, deflect it in such a way that depends
00:29:11.200 The interfering entities have passed through some of the other slits.
00:29:14.720 The interfering entities behave exactly like photons, except that they cannot be seen
00:29:21.800 pausing there, just my reflection, just a psychological reflection.
00:29:25.080 So I think at this point, this is where the sense of vertigo really begins to happen.
00:29:30.120 It's, you're recognizing that he is David is offering for you, served up on a platter,
00:29:41.000 At this point, you don't understand the full explanation, but you're getting a hint of
00:29:45.760 So everything will be understood at this point.
00:29:48.560 Everything is logical, realistic, it makes sense.
00:29:52.280 We're not having the wall pulled over your eyes, at no point, at least to me, do I have
00:29:57.400 But wait, what about, what about, what about, because he's answering any of the objections
00:30:04.080 If you have any objections throughout this, please write a question in the comments
00:30:10.480 It's very interesting to try and clarify if you're not too sure let's keep going.
00:30:16.240 David writes, I shall now start calling the interfering entities photons.
00:30:20.880 That is what they are, but for the moment it does appear that photons come in two sorts,
00:30:26.000 which I shall temporarily call tangible photons and shadow photons.
00:30:31.280 Tangible photons are the ones we can see or detect with instruments, whereas the shadow
00:30:35.320 photons are intangible, invisible, detectable only indirectly through the interference
00:30:42.440 Later we shall see that there is no intrinsic difference between the tangible and shadow
00:30:47.160 Which photon is tangible in one universe and intangible in all other parallel universes,
00:30:51.800 but I anticipate what we have inferred so far, is only that each tangible photon has an
00:30:56.920 accompanying retinue of shadow photons and that when I photon passes through one of
00:31:01.640 our four slits, some shadow photons pass through the other three slits.
00:31:06.480 Since different interference patterns appear, when we cut slits of the other places
00:31:10.040 in the screen, provided that they are within the beam, shadow photons must be arriving
00:31:15.680 all over the illuminated part of the screen, whenever a tangible photon arrives.
00:31:20.160 Therefore, there are many more shadow photons and tangible ones.
00:31:25.240 Experiments cannot put an upper bound on the number, but they do set a rough lower bound
00:31:28.640 in a laboratory, the largest area that we could conveniently illuminate with a laser
00:31:32.480 might be about a square meter, and the smallest manageable size for the holes might be
00:31:36.600 about a thousandth of a millimeter, so there are about one trillion possible hole locations
00:31:42.400 Therefore, there must be at least a trillion shadow photons accompanying each tangible one.
00:31:48.560 Thus we have inferred the existence of a seething, prodigiously complicated hidden world
00:31:54.960 They travel at the speed of light, bounce off mirrors, are refracted by lenses and are
00:31:58.640 stopped by opaque barriers or filters of the wrong colour.
00:32:01.800 Yet, they do not trigger even the most sensitive detectors, the only thing in the universe
00:32:06.120 that a shadow photon can be observed to affect is the tangible photon that it accompanies.
00:32:14.920 So at this point, I think I understood interference, and you should understand interference
00:32:21.360 Interference is these photons that you see, colliding, physically colliding with these photons
00:32:28.680 that cannot be seen, these photons that cannot be seen, push aside the photons that
00:32:34.040 you can see, because if they weren't there, then the photon would have just continued
00:32:38.680 off through that double slit and landed in one of two places behind those two slits.
00:32:44.360 That's the single photon version of reality, the classical universe, version of reality.
00:32:51.280 We're in a multiverse, back to the book, we're almost there at where we have to accept
00:32:59.520 David Wright's shadow photons would go entirely unnoticed if it were not for this phenomena
00:33:04.200 and the strange patterns of shadows by which we observe it.
00:33:08.400 Sometimes it's not a special property of photons alone.
00:33:11.240 Quantum theory predicts and experiment confirms that it occurs for every sort of particle.
00:33:16.440 So there must be hosts of shadow neutrons accompanying every tangible neutron, hosts of
00:33:20.960 shadow electrons accompanying every electron, and so on.
00:33:24.360 Each of these shadow particles is detectable, only indirectly, through its interference
00:33:29.480 with the motion of its tangible counterparts, pausing there.
00:33:33.440 Okay, so at this point I think I accepted the multiverse account because I knew about doing
00:33:45.280 In fact, I'd read about it being done with oxygen atoms.
00:33:49.720 So it's clear to me at this time that, well, not only do you have this retinue of photons
00:33:56.960 following around every photon you can see, you have a retinue of electrons following
00:34:00.600 every electron that you can detect and oxygen atoms, and therefore you just conclude that,
00:34:05.920 well, every single particle of matter has a retinue of unseen counterparts.
00:34:13.240 We live in a universe, a reality, where much of it is unseen, which again, it's an astonishing
00:34:20.640 fact, but it's also consistent with the history of ideas of their universe just getting
00:34:30.440 The scene, in terms of the unseen, indeed, as David says, going back to the book, it follows
00:34:36.400 that reality is a much bigger thing that it seems, and most of it is invisible.
00:34:41.120 The objects and events that we and our instruments can directly observe are the mere
00:34:47.160 Now tangible particles have a property that entitles us to call them collectively a universe.
00:34:52.960 This is simply the defining property of being tangible.
00:34:55.480 That is, of interacting with each other and hence of being directly detectable by instruments
00:34:59.520 and sense organs made of other tangible particles.
00:35:02.720 Because of the phenomena of interference, they are not wholly partitioned off from the
00:35:06.000 rest of reality, that is, from the shadow particles.
00:35:09.000 If they were, we should never have discovered that there is more to reality than tangible
00:35:13.320 But, to a good approximation, they do resemble the universe that we see around us in
00:35:17.360 everyday life, and the universe referred to in classical pre-quantum physics.
00:35:21.600 For similar reasons, we might think of calling the shadow particles collectively a parallel
00:35:26.800 For they, too, are affected by tangible particles, only through interference phenomena.
00:35:33.200 For it turns out, that shadow particles are partitioned among themselves in exactly the same
00:35:36.640 way, as the universe of tangible particles is partitioned from them.
00:35:40.040 In other words, they do not form a single homogenous parallel universe, vastly larger than
00:35:45.080 a tangible one, but rather a huge number of parallel universes, each similar in composition
00:35:49.640 to the tangible one, and each obeying the same laws of physics, but differing in that
00:35:53.800 the particles are in different positions in each universe.
00:35:57.280 It's pausing there, and this is where it helped me understand the Schrödinger wave
00:36:03.880 Because the Schrödinger wave equation gives you the distribution of all the positions
00:36:13.800 And previously, I'd of course been taught that well, when you observe, this of course
00:36:17.720 is the collapse of the wave function, and the wave function is the thing given by the
00:36:23.240 And you make an observation, and then you find that all of the different possibilities
00:36:28.520 They all the others disappear, which makes you think, why does the equation describe all
00:36:33.080 of these other possible positions if only one of them really exists?
00:36:38.040 And usually, you're given the non-explanation that, well, this is just the formalism that
00:36:42.840 just helps you to predict the outcome of experiments, don't worry.
00:36:46.160 It doesn't really mean anything in actual reality, there's no point asking about actual
00:36:50.840 What this says here is that, in fact, the Schrödinger wave equation, the wave function,
00:36:55.400 is telling you that all these particles really do exist.
00:36:58.560 Before and after, before, during and after, an experiment is conducted.
00:37:02.640 They really do exist when you make an observation, you find that in which universe you are,
00:37:11.480 But all the others absolutely exist, and they continue to exist.
00:37:16.880 The distribution of particles changes over time, but they all exist.
00:37:20.880 I'm skimming apart there where David talks about terminology, this idea of the universe
00:37:25.240 versus multiverse, and he writes, so I'll just go to the bit where he explains the distinction.
00:37:32.440 A new word multiverse has been coined to denote physical reality as a whole.
00:37:37.560 Single particle interference experiments, such as I have been describing, show us that
00:37:40.760 multiverse exists, and it contains many counterparts of each particle in the tangible universe.
00:37:46.360 To reach the further conclusion that the multiverse is roughly partitioned into parallel
00:37:50.160 universes, we must consider interference phenomena involving more than one tangible
00:37:54.760 The simplest way of doing this is to ask, by the way, of a thought experiment, what must
00:37:57.920 be happening at the microscopic level when shadow photons strike an opaque object.
00:38:03.360 We know that because the interference ceases when an opaque barrier is placed in the
00:38:06.400 parts of the shadow photons, but why what stops them, we can rule out the straightforward answer
00:38:11.120 that they are absorbed like tangible photons would be by the tangible atoms in the barrier.
00:38:15.400 For one thing, we know that shadow photons do not interact with tangible atoms.
00:38:19.800 For another, we can verify by measuring the atoms in the barrier or more precisely by
00:38:23.680 replacing the barrier with the defector that they neither absorb energy nor change the
00:38:27.200 state in any way unless they are struck by tangible photons.
00:38:32.600 To put that another way, shadow photons and tangible photons are affected in identical ways
00:38:36.600 when they reach a given barrier, but the barrier itself is not directly, is not identically
00:38:42.600 Pause in other words, or in my words, if you are shining a torch at the wall, we know
00:38:48.280 already given David's explanation that coming out of that torch are the photons you can see,
00:38:53.320 and a lot to not, I would say, the more modern understanding of conduct areas uncountably
00:38:58.400 infinite fungible instances of these other photons are also coming out of the torch.
00:39:03.560 But when it hits a wall and opaque barrier, the opaque barrier that you can see is only
00:39:08.560 absorbing the photons that you can see, so how are all the other shadow photons being
00:39:14.880 Well, only because there must be shadow barriers there as well.
00:39:19.840 However, many shadow photons are, there are, that's how many shadow barriers they are,
00:39:27.080 Which leads you inexorably to the idea that everywhere there's matter, there are uncountably
00:39:33.760 infinite copies of that matter, existing, in a sense, in the same place at the same time.
00:39:40.440 But I'd refer you to the book if you need more about this chapter 2 is certainly worth
00:39:46.120 And I'll just read the final part of the chapter, I've skipped a fair bit, ignoring
00:39:54.000 stuff that David says there about instrumentalism, because I've concentrated on that before
00:39:58.920 in various episodes, so just in the last part of this chapter and David writes, so far
00:40:04.960 I have been using temporary terminology which suggests that one of the many parallel
00:40:09.640 universes differs from the others by being tangible, it is time to sever that last
00:40:14.960 link with the classical single universe conception of reality.
00:40:18.360 Let us go back to our frog, we have seen that the story of the frog that stares at
00:40:23.000 the distant torch for days at a time, waiting for the flicker that comes on average once
00:40:26.320 a day, is not the whole story, because there must also be shadow frogs in shadow universes
00:40:32.200 that coexist with the tangible one, also waiting for photons.
00:40:36.400 Suppose that our frog is trained to jump when it sees a flicker.
00:40:40.240 At the beginning of the experiment the tangible frog will have a large set of shadow counterparts,
00:40:45.400 all initially alike, but shortly afterwards they will no longer all be alike.
00:40:49.920 Many particular one of them is unlikely to see a photon immediately, pausing there, so
00:40:55.560 all of these frogs that are alike beforehand, fungible instances, indeed, so the beginning
00:41:03.920 of infinity sharpens this notion up with new terminology, but it's the same idea back
00:41:09.040 to the book, David writes, any particular one of them, the frogs, is unlikely to see a
00:41:13.560 throat photon immediately, but what is a rare event in any one universe is a common event
00:41:18.080 in a multiverse as a whole, at any instance somewhere in the multiverse, there are a few instances
00:41:23.840 in which one of the photons is currently striking the retina of the frog in that universe,
00:41:32.040 Because within its universe, it obeys the same laws of physics as tangible frogs
00:41:35.240 do, and its shadow retina has been struck by a shadow photon belonging to that universe.
00:41:40.040 One of the light-sensitive shadow molecules in the shadow retina has responded by undergoing
00:41:43.840 complex chemical changes to which the shadow frog's optic nerve has in turn responded,
00:41:51.120 It has transmitted a message to the frog's shadow brain, and the frog has consequently
00:41:59.080 Or should I say the shadow sensation of seeing a flicker?
00:42:03.120 If shadow observers, be they frogs or people are real, then the sensations must be
00:42:08.320 When they observe what we might call a shadow object, they observe that it is tangible.
00:42:13.160 They observe this by the same means, and according to the same definition, as we apply
00:42:18.280 when we say that the universe we observe is tangible.
00:42:24.600 So objectively, there are not two kinds of photons, tangible and shadow, nor two kinds
00:42:29.080 of frog, nor two kinds of universe, one tangible in the rest shadow.
00:42:33.040 There is nothing in the description I have given of the formation of shadows, or any
00:42:37.080 of the related phenomena that distinguishes between tangible and shadow objects, apart
00:42:41.920 from the mere assertion, that one of the copies is tangible.
00:42:45.960 When I introduce tangible and shadow photons, I apparently distinguish them by saying
00:42:50.160 that we can see the former, but not the latter.
00:42:55.200 While I was writing that, hosts of shadow davids were writing it too.
00:42:58.400 They drew a distinction between tangible and shadow photons, but the photons they called
00:43:04.880 And the photons they called tangible are among those, I called shadow.
00:43:09.360 Not only did none of the copies of an object have any privileged position in the explanation
00:43:13.440 of shadows that I had just outlined, neither did they have a privileged position in the
00:43:17.320 full mathematical explanation provided by quantum theory.
00:43:20.920 I may feel subjectively that I am distinguished among the copies as the tangible one,
00:43:26.200 because I can directly perceive myself and not the others, but I must come to terms with
00:43:30.600 the fact that all the others feel the same about themselves.
00:43:35.480 Many of those davids are at this moment, writing these very words, some are putting
00:43:46.440 And that, as I say, is the most affecting, for me, personally, of the chapters in the
00:43:52.760 fabric of reality, because it explains what's going on in this double-strict experiment.
00:43:59.880 And once you understand that, coupled with this concept of quantization in quantum theory,
00:44:05.400 you've gone a long way to understand in quantum theory.
00:44:07.440 Maybe not the mathematical formalism, but who cares about that unless you want to be
00:44:12.080 If you want to understand reality, the best that everyone who works in physics understand
00:44:18.400 reality, then after reading shadows, you will have a very good understanding.
00:44:24.360 In fact, you might understand a better than some physicist, in fact, a physicist who
00:44:27.400 deny this way of explaining what we see in terms of the unseen.
00:44:36.480 For more on this, find my multiverse series on YouTube, and until next, bye-bye.
