Does Quantum Mechanics Break Einstein's Rules? – Quantum Reality (3/3) with Jim Al-Khalili

[00:00:00]Many of my colleagues don't like the term quantum weirdness, but tough luck.

[00:00:07]Quantum mechanics is weird.

[00:00:11]Welcome to part three of our deep dive into what quantum mechanics tells us about the nature of reality.

[00:00:21]In this episode, we ask whether there's experimental evidence for quantum mechanics strangest implications and what it would mean if one particular interpretation of quantum mechanics turned out to be true.

[00:00:37]What would it mean for future quantum technologies, for the search for alien life, and for the various versions of ourselves who might exist right now in some parallel universe?

[00:00:54]But first, a quick recap.

[00:00:58]We saw how quantum mechanics seems to shake the principles of classical physics [music] to their core.

[00:01:06]It appears to break determinism. [music] Take identical particles, subject them to identical conditions and identical [music] forces, and they still won't behave the same way as each other.

[00:01:20]And quantum [music] mechanics seems to undermine objective reality, making the act of [music] observation central to the story.

[00:01:30]A subatomic particle is governed by a wave function which [music] describes it as being in an uncertain blur of many possible states [music] all at once known as superp position. But the moment we observe it, the wave [music] function collapses to one definite state as if reality only sharpens into focus when we look.

[00:01:56]We saw how three interpretations part of a movement known as quantum [music] realism try to resolve these problems.

[00:02:05]First, there's the many worlds interpretation.

[00:02:08]This says the [music] wave function never collapses. It remains in the superposition of all states.

[00:02:17]This implies a plethora of parallel realities. But this multiverse [music] is deterministic and real.

[00:02:28]Then there's pilot wave theory. This says subatomic particles are guided by real but invisible waves. Everything [music] unfolds deterministically.

[00:02:40]The reason particles appear to be in a superp position of many states is because we currently don't know precisely where on the wave they start.

[00:02:50]So we can't be sure where they'll end up.

[00:02:56]And [music] thirdly, there's objective collapse. This says there's a subtle faint [music] vibration permeating all of space called a continuous localization field. This nudges [music] the wave function of a quantum system at random moments causing it to collapse.

[00:03:18]Thus, quantum systems are still indeterministic, but they do retain objective [music] reality.

[00:03:27]Nature at the quantum level does its thing even if no one's observing [music] it.

[00:03:36]So, these are three [music] different ways of dealing with indeterminism and subjectivity.

[00:03:43]But there's a further huge conundrum [music] to come.

[00:03:47]A third and devastating blow to classical physics.

[00:03:52]The third profound idea that quantum mechanics has something to say about is called locality.

[00:04:00]Now locality is pretty easy to explain.

[00:04:02]If something happens over here, then it can't instantly affect something far away. Its influence has to spread out gradually through space. And the maximum speed this influence can spread out is the speed of light. That's the maximum speed in our universe. So there can be no signaling or sending of messages faster than light.

[00:04:24]What if our universe were non-local?

[00:04:27]What if things could travel faster than light? Well, this leads to some rather strange situations. Think of the following example. If I were to fire a gun that shoots a bullet that could travel much faster than light and I shoot my producer Paul over there, then if my cameraman Andy is flying past us at some high speed, below the speed of light, but close to the speed of light, he will see the events happening very differently. He could see Paul falling to the ground before I fired the gun.

[00:04:59]Basically, he would see the effect coming before its cause. In principle, he could come and stop me from firing the gun in the first place, but it's too late by then. It doesn't make sense. So, non-locality violates causality. It breaks this connection between cause and effect, and no one wants that.

[00:05:23]Einstein specially hated non-locality and one aspect of quantum [music] mechanics seemed to imply it was worryingly real entanglement.

[00:05:35]For example, two electrons can share a single quantum state, say describing the direction of their spins.

[00:05:44]Until measured, that spin is undefined.

[00:05:48]But measure one and the other snaps into a definite [music] spin direction as well.

[00:05:56]It looks as if the first electron is instantly telling the second what to do, however far apart they are, communicating faster than the speed of light in defiance of locality. [music] Einstein famously dubbed this spooky action at a distance.

[00:06:16]But is it real?

[00:06:19][music] The road to answering this question of whether entanglement implies non-locality starts with a big hero of mine, the Northern Irish physicist John Stewart Bell. He explained the puzzle with a playful analogy.

[00:06:38]Bell published a famous paper entitled Berleman Socks and the Nature of Reality. He was referring to a colleague of his at CERN called Reinhold Bertleman who had this idiosyncratic habit of always wearing oddcoled socks.

[00:06:54]So if you saw one of his socks and what color it was, then you'd immediately know that the other sock will be a different color.

[00:07:03]So imagine I'm Myrtleman.

[00:07:07]Let's say you knew that I was wearing different color socks and I show you my left sock and it's a glorious orange.

[00:07:14]Then you would know immediately that my right sock will be a different color and it's blue. Right? There's nothing magical about this since I put a left sock an orange sock on my left foot and a blue sock on my right foot this morning and there's no connection between them. But what if I were wearing quantum socks? Now each sock wouldn't be orange or blue, but would be both orange and blue at the same time. They'd be in what we call a quantum superp position.

[00:07:46]And it's only when we look do we force the sock to make up its mind which one it's going to be. You force it to make a choice. [music] Now let's say my socks are entangled as well.

[00:08:00]The electrons I mentioned earlier were entangled to have the same spin. My socks are entangled. So they must always be different colors. If I observe one to be orange, the other must be blue and [music] vice versa.

[00:08:17]So my left sock is both orange and blue.

[00:08:21]But when I look, I see that it's orange and that forces my right sock to be blue. Before that, the right sock was both orange and blue. Only by observing the left sock have I changed the nature of the right sock.

[00:08:39]Here's the puzzle that Belle identified.

[00:08:41]Either option one, everything happens locally, which could mean that Bertman has a rule to always wear [music] mismatched socks, so the colors are mixed from the start.

[00:08:54]Or maybe that one sock communicates to the other locally without breaking the speed of light.

[00:09:02]Or option two, [music] they are quantum socks that are instantaneously non-locally connected.

[00:09:13]Belle's genius was to figure out how to check which [music] option was going on with quantum particles. He came up with a statistical [music] test now called the Bell inequality which you can run on real entangled particles like electrons or photons.

[00:09:31]I'll get into the details another time, but the punchline is this. The Bell inequality can tell [music] which explanation is true. It can tell if there's a local rule connecting particles or if there's spooky action linking them instantaneously when far apart.

[00:09:52]It took decades to run that test in the lab. John Clauser in the 1970s, Alan Aspect in the 80s and [music] Anton Xylinger in the '90s did it with ever more sophisticated experiments. [music] Eventually, in 2022, they all won the Nobel Prize for confirming Bell's inequality is indeed violated. Quantum mechanics is undeniably non-local.

[00:10:20]The electrons are somehow communicating instantly.

[00:10:26]Einstein's nightmare had come true.

[00:10:29]Spooky action at a distance is real, whether or not there are hidden rules.

[00:10:35]Which leaves a huge problem for our quantum realists.

[00:10:40]How do they make peace with Einstein [music] and relativity which forbids faster than light [music] or super lumininal signaling?

[00:10:50]Objective collapse [music] answers this by saying, "Yes, entangled particles do communicate instantaneously, but there's no possible way to exploit that [music] to send a useful faster than light signal. So for all intents and purposes, locality [music] is preserved."

[00:11:11]And the reason is that you cannot control the collapse. So and and what what does that mean? It means that if you have an entangle system, I can decide how when to collapse it because on my side of course because I can I can I create an interaction between this part of the entangle pair and a device.

[00:11:29]So I can decide when to change the wave function but I cannot decide how to change the wave function and that this a bit of a mathematical technicality to to explain and that prevents signaling. But I want to make a comment. So I feel confident in saying that there is a tension there is a problem between quantum mechanics and relativity. Abd Abdar Shimoni a great philosopher of science talked about peaceful coexistence. So I agree on the co coexistent part. I don't agree on the peaceful part. There is so it's not peaceful it's there is a problem. There is a problem between quantum mechanics and relativity because of non-locality.

[00:12:05]We are not understanding the fully the problem and even less the possible solution.

[00:12:10]>> [music] >> Things take a much stranger turn with pilot wave theory. Yes, if the particles [music] start out near where the guiding wave peaks, then as with objective collapse, faster than light signaling is not possible.

[00:12:29]But pilot wave theory allows particles [music] to be distributed anywhere on the wave.

[00:12:36]and such distributions could in principle violate locality.

[00:12:45]For a sense of why, imagine the following. There's a box of coins at the edge of the galaxy. Half are heads, half are tails. This is the standard quantum arrangement, equivalent to particles being near the guiding waves peaks.

[00:13:02]Next to the box is a detector that measures the proportion of heads. It currently reads a half. Now, a super luminal signal arrives from entangled coins on Earth, which flips these coins.

[00:13:18]Our detector, of course, is unaware of this change. No useful signal has been sent. Locality survives.

[00:13:29]Now imagine a non-standard arrangement say around 16th our heads and the rest are tails. This is equivalent to particles being far from the guiding waves peaks.

[00:13:42]Now the same super luminal signal arrives instantly and flips the coins.

[00:13:50]This time the detector immediately registers the change. We've sent a useful message. Locality fails.

[00:13:59]>> The statistics of what you see here would respond instantaneously to what's done far away. And super luminal signaling would be possible. And of course, when you have super luminal signaling, then relativity is also going to fail. You would I mean, I should say physicists often um are horrified by this. They say, "Well, if you have super luminal signaling, then you're going to violate causality because according to relativity, if I have a super luminal signal going on in this frame of reference for a moving observer, the signal could be going back in time." And you can then get into various paradoxes.

[00:14:43]That's very simple to resolve that objection because in relativity, um, people synchronize clocks using light signals.

[00:14:54]And it's a bit of a long story, but long story short, um, if we were able to control these superal signals, we would use those signals to synchronize clocks. And there wouldn't be any paradox. Even for a moving observer, the clocks would be synchronized in a way that the signal is super luminal and instantaneous in all for all observers.

[00:15:22][music] Many worlds is in some ways the best at retaining and dealing with locality.

[00:15:29][music] But the way it does it is well, it needs you to stretch your imagination.

[00:15:35]Imagine there's a device that can produce a pair of entangled electrons. One traveling towards me and the other traveling towards my colleague Tim who's in a lab on the other side of the galaxy. Now, uh the electrons are going to be represented by coins. So the idea is that if my electron is spin up, so will Tims. If my electron is spin down, Tims will be spin down. So if I if my coin is is is measured with my measuring device to be heads, Tim will also have heads.

[00:16:10]If mine is tails, his will be tails. The the thing about entanglement is that each electron or in this case each coin is both heads and tails at the same time.

[00:16:20]The problem about non-locality, this is what uh upset Einstein so much is that when I measure my coin and see that it's heads up, then instantly tins will also be heads up. Somehow the influence has broken the speed of light barrier and affected tins coin even though it's on the other side of the Milky Way galaxy.

[00:16:43]So how does the many worlds interpretation deal with this? Well, it says that when I make a measurement, my superp position of coin being up and down at the same time is destroyed.

[00:16:54]But there are now two universes. By measuring, I split reality in two. In one universe, I see uh uh heads. In another universe, I see tails.

[00:17:06]The two Tims in each universe remain identical until he looks.

[00:17:12]But he's not going to affect anything when he does look at his his coin. When I see mine is heads in that universe, when Tim looks at his coin, he will also see heads. In the other universe, if I see tails, Tim will also see tails.

[00:17:28]There's never any influence traveling faster than the speed of light, breaking the rules of relativity, which Einstein was so worried about. In this sense, many worlds advocates will say it's perfectly local, provided, of course, you're happy with there being two realities, two universes.

[00:17:49]So, many worlds says there's no non-locality.

[00:17:53]Objective Collapse says non-locality is real but inaccessible.

[00:17:59]Pilot wave theory embraces it.

[00:18:03]But now, let's move on to the $64 million question.

[00:18:07]What's the evidence?

[00:18:10]We've heard theoretical arguments for three different interpretations of what quantum mechanics is telling us about reality.

[00:18:17]But theory isn't enough. We also need experimental or observational evidence.

[00:18:24]So, is there any evidence that one of these theories is correct? or is there even any promising avenues of research that might lead to evidence? Well, let's find out. Let's start with pilot wave theory.

[00:18:38]Anthony Valentini believes there could be evidence hidden in these images of the so-called cosmic microwave background radiation. This was created shortly after the Big Bang and fills all space.

[00:18:54]Current theories say the temperature of this radiation should be just under 3° Kelvin with slight variations which are the same at all [music] scales and directions.

[00:19:07]In fact, in very wide angle views, the temperature variation [music] appears to be lower than expected.

[00:19:15]And also large scale temperature variations should look the same [music] in all directions but don't seem to even taking into account the fact that the earth is moving through this microwave background.

[00:19:29]In pilot wave theory you would expect to see both kinds of anomalies. It it's tantalizing in that the kinds of anomalies that are being reported are the same kinds of anomalies we expect to see. Do they actually match the details of the predictions where we can't tell?

[00:19:48]>> Anthony hopes new data from satellites will reveal more.

[00:19:53]He's also looking at the possibility that X-ray photons emerging from the accretion discs of [music] black holes might not follow the current quantum rules as might particles created in high energy colliders. So let's see. [music] Meanwhile, Simon Saunders is confident that we already have [music] evidence for many worlds. For him, locality ought to be preserved because there's so much evidence for it. In fact, all the evidence in support of relativity points to [music] it. And the only way locality can be preserved in the face of quantum entanglement [music] is for many worlds to be true.

[00:20:39]There is overwhelming evidence [clears throat] if you've got locality.

[00:20:43]If further we can't change the past and when we think we act freely, we do act freely. It's not that the past is controlling us in what we do somehow.

[00:20:54]Then given Bell inequalities violated, it follows that remote experiments do not have unique outcomes, which isn't quite saying it follows that the Everett interpretation is [laughter] true, but it does seem to suggest that something very part of the core of Everett and the many worlds approach must be correct given Bell inequalities are violated. I'll bet my house on that. I really will. So actually I was going to ask is one of the things I wanted to get to just [clears throat] towards the end is is is where can one see a signal for this interpretation. Yes. And what you're saying is we've seen it.

[00:21:34]>> Yes indeed. Is this violation of balon equalities >> is the best evidence for many worlds.

[00:21:41]>> It's the almost direct evidence for many worlds. set up. So, we have to just >> And as for objective collapse, all they need is evidence for their continuous localization field, which they claim is causing wave functions to collapse.

[00:21:57]>> A team in Southampton are hunting, but it's not easy.

[00:22:00]>> That looks like on the inside >> cuz temperature down here is 4 Kelvin.

[00:22:04]Temperature up here is 300 Kelvin. So, we need a lot of thermal isolation between them, and that's what these are for. Then we have our vacuum chamber.

[00:22:12]>> The experimental challenge, much as in gravity wave detectors, is to remove all known disturbances.

[00:22:21]Everything from collisions with air molecules to trucks driving by. And so the purpose of all this machinery is to isolate a tiny chamber from the rest of the universe.

[00:22:35]We use these just basic lead pots. So this is normal normal lead that you probably have heard about but isn't in our pipes anymore in a nice like cup.

[00:22:45]Inside that we put a magnet ranging from either a millimeter wide down to 100 microns wide. And when it is cooled to 4 Kelvin that lead becomes superconducting. And that means that has a very interesting property for us which is that it repels all magnets. So the magnet is levitated. So it's not touching anything, no matter it's in a vacuum. So there's no gas molecules around it. And it is down to not absolute zero but very close with this such that there is very little in the outside world affecting it. And then we are test we are trying to remove as much motion from that as possible and then observe its basic dynamics. And if we can do that and account for all the noise sources that we see in it, we can check off um many like seismic noise that gets through and everything. And if that leaves us with any noise, it's a very good indication that there is a background noise affecting our experiment that we cannot shield, which is this continuous spontaneous localization field.

[00:23:45]>> But how far close do you think you are?

[00:23:48]>> If it's just at the upper bound of the measurements, it could be within a year.

[00:23:52]But if it's very far into the bound, it could be many years of iterations getting a slightly larger magnet, more sensitive squid, even colder temperatures, and we just have to hope the universe has chosen a high bound for CSL.

[00:24:10]Well, the jury is clearly still out, [snorts] but let's assume that one of these theories is the correct one, that it does describe the true nature of reality at a fundamental level. What would this mean and what would the consequences be?

[00:24:28][music] There will of course be huge technological implications which we could probably only dimly foresee. The most obvious would be super luminal signaling across large distances in space, our solar system, the wider universe. You would be able to signal instantaneously. You would no longer be bounded by the speed of light.

[00:24:54]>> Anthony believes this might help us answer that most beguiling of questions.

[00:24:59]Are we or aren't we alone in the universe?

[00:25:04]The search for extraterrestrial intelligence as we have it today assumes that advanced aliens that may be technologically millions or even billions of years ahead of us are still signaling to each other using radio waves.

[00:25:21]Now from the point of view of pilot wave theory, signaling using radio waves is is as primitive as it would be to signal using water waves.

[00:25:35]If we found particles coming from a very distant planetary system, photons coming incoming from from a million light years away. and we find that these photons break the Bourne rule in for example a two slit experiment then I think we would those particles would be worth studying for signs of a possible message from a remote civilization for Simon the existence of parallel worlds represents a paradigm shift in our understanding of reality though perhaps not as we might imagine it. So most versions of you will be very similar to you [laughter] in in in choices in attitudes in responses and so forth. I mean the differences will be precisely those differences that you think might have arisen in your life by virtue of some quantum chance event. And some of them could be profoundly damaging like you know you get cancer or something you know that could be a chance quantum event that leads to you getting cancer and not getting cancer but but none of these things really except on maybe you go back to early childhood are really dictating your moral character for example you know or I mean so it's not quite the way it's often presented I think >> but Simon does believe there is one very dramatic consequence of the many worlds interpretation.

[00:27:15]It could change our view of mortality itself, [music] >> especially say someone who's died from cancer. Cancer is the sort of thing that could start from a quantum event. So, it's perhaps entirely plausible that there are versions of that person who did not die from cancer.

[00:27:34]And I think that might be really rather comforting. I would be very hesitate ever to say this to someone who was bererieved that I'd almost feel it would be appropriate for them to say don't come over with this nonsense about many worlds. This isn't going to make me feel better, you know. But perhaps because I do maybe take many seriously. I think I might find it comforting. This might be something.

[00:28:00]>> Yeah.

[00:28:02]Well, if I'm prepared to bet the house in the damn thing. [laughter] [music] >> And the consequence of objective collapse being true? Well, the [music] existence of an allervading noise might in fact limit some of the more ambitious quantum technologies we're currently working on. But we have to fight against a noise that ultimately will not allow you to make a computer as big as you wish. And the same with sensors. So quantum sensors work because they are sensitive to what you are looking for.

[00:28:33]But if there is a noise which is stronger than what you're looking for, the sensor doesn't work.

[00:28:41]So quite a journey from photons hitting [music] a screen to super luminal signaling all pervading cosmic noise and multiple versions of ourselves.

[00:28:55]Okay, to recap, where do we stand?

[00:28:58]Let's have a quick look at pros and cons for for each of them. Pilot wave theory.

[00:29:04]See, I like it. It says there's just one universe. Uh everything seems to be happening deterministically.

[00:29:12]However, it is very non-local. It says non-locality is real. The whole universe is interconnected instantaneously.

[00:29:20]And if pilot wave theory is the correct interpretation of quantum mechanics, then if we push it to its obvious conclusions, that tells us that Einstein's relativity is wrong. That the speed of light is not the upper limit in our universe. And many physicists, probably most physicists would argue that this is too much to swallow. Next, you have objective collapse. Again, sensible. It's uh it's non-deterministic, but at least it says there's just one universe. However, it requires modifying Schroinger's equation, adding what's called a nonlinear term to it, and we see no evidence for this yet. Schroinger's equation works perfectly well and has done so for a hundred years. Uh and it's predictions fit all observations and experiments perfectly. So why should we mess with it and add something extra?

[00:30:13]Again, many physicists would argue this is too much uh too heavy a price to pay.

[00:30:18]Then finally, there's Everett's many worlds interpretation.

[00:30:23]In a sense, this is the cleanest, the simplest of these quantum realist interpretations because it doesn't require any extras, any baggage. It says all you need is the Schroing equation.

[00:30:35]However, of course, it comes with this huge metaphysical baggage that says there are potentially an infinite number of parallel universes. Every time a quantum particle anywhere in the universe is faced with a choice, the whole of reality splits in two or more.

[00:30:54]And again, that seems to be a heavy price to pay. And wherever you look, there are problems and reasons for not liking a particular approach cuz nature doesn't care that there's lots of different ways of explaining the weirdness of quantum mechanics. That's our problem. Nature does things in a certain way. I've always said either there are parallel universes or there aren't. Either there's non-locality or there isn't and so on. Either the wave function spontaneously collapses or it doesn't. So there is an interpretation.

[00:31:29]We don't know which one it is. Normally it's physicists who are asked the question, what is your favorite interpretation? Well, here's your opportunity. What do you think? Add your comments and and suggestions below. Let us know what you think. Of course, at the same time, like and subscribe to this channel.