Where Does the Quantum World End and Ours Begin? | Quanta Magazine

Quantum mechanics may be one of the most precisely tested paradigms in science, but there’s still no universally accepted interpretation of what it tells us about reality. How do we get from the wave-like behavior of quantum systems to the solid, macroscopic world of objects and the universe at large?

Jonathan Halliwell, a professor of theoretical physics at Imperial College London, has spent his career probing the foundations of quantum theory. In this episode, he tells Steven Strogatz why decoherence — the process by which fragile quantum behavior becomes dispersed through interactions with the surrounding environment — is key to the transition from quantum to classical behavior, and he describes the “histories” approach that he uses to understand this. Along the way, Halliwell tackles some of the field’s deepest paradoxes. He explains why the quantum-to-classical transition doesn’t require a conscious observer, and he explores Einstein’s famous question of whether the moon is really there when no one looks. The conversation ends with Halliwell explaining how yoga and meditation help him sit with the mystery of competing perspectives in science.

Listen on Apple Podcasts, Spotify, TuneIn or your favorite podcasting app, or you can stream it from Quanta.

Transcript

[Music plays]

STEVE STROGATZ: Alright, here we go. I’m Steve Strogatz.

JANNA LEVIN: And I’m Janna Levin.

STROGATZ: And this is The Joy of Why.

LEVIN: A podcast from Quanta Magazine where we explore some of the biggest unanswered questions in math and science today.

STROGATZ: Yes, Janna, big questions. Today we got one of the biggest of all.

LEVIN: Okay, I’m excited. I’m ready. I’m braced.

STROGATZ: Yeah, brace yourself and buckle up. This is about foundations of quantum mechanics, one of the really paradoxical things that a lot of people worry about.

LEVIN: Yeah, it really is the great unsolved quandary. Quantum mechanics, the most precisely tested paradigm in all of science to the largest number of decimal points, and still there’s no coherent interpretation. I don’t know anyone who’s gonna claim to understand quantum mechanics fully.

STROGATZ: That’s what they say, right? That if you claim to understand it, it’s just a sign that you don’t understand it.

LEVIN: Yeah, exactly. It’s a dead giveaway.

STROGATZ:  But it’s such a surprising thing. As you say, the theory, especially in the more advanced flavor of quantum field theory or quantum electrodynamics, as you mention, it’s now been tested to something like, I don’t know, one part in 10 to the 12th or some astonishing precision.

LEVIN:  Yeah. Some astonishing number. Absolutely. And so we can send things to space. It works in your phones, and it works to that level of precision in an accelerator experiment, but we don’t actually have an interpretation that makes sense to us, and that might just be our big classical brains are just too lumbering to comprehend. But I never find that satisfying, and nobody’s ever made a great discovery by throwing their hands in the air and giving up, right?

STROGATZ: [Laughs] Yeah, well, we’re gonna be talking with a guy named Jonathan Halliwell, who is a professor of theoretical physics at Imperial College London, and he has given a lot of thought to these questions of foundations of quantum mechanics, but he came at it from quantum cosmology, so pretty close to your own area.

LEVIN:  Yeah, I knew Jonathan. Back in the day, we used to hang out in London. I was at Cambridge, and Jonathan, of course, had spent time at Cambridge and was at Imperial. It’s been a while, so I’m very interested to hear what his latest ideas are.

STROGATZ: Okay then, why don’t we just dive in. Take it away, Jonathan.

[Music plays]

STROGATZ: Welcome to The Joy of Why, Jonathan. It’s great to have you on the show.

JONATHAN HALLIWELL: It’s good to meet you and it’s good to be here.

STROGATZ:  I’d like to start by talking about the beginning of your career, which my understanding is that it was in an area we would call quantum cosmology, where quantum mechanics gets applied to the universe as a whole.

My question about that, first of all, what a mind-blowing idea. But also, like, when I took quantum mechanics in college or even when I used to read popular accounts of it, frequently we would hear about an observer and an experiment, and the observer is sort of outside the experiment, causing wave functions to collapse by making measurements and that sort of thing.

But when it’s the universe as a whole that’s the quantum object, the observer can’t be outside the system. And I just wonder if that’s the sort of thing that made thinking about foundations of quantum theory irresistible to you.

HALLIWELL:  That is essentially it. That’s why I started in quantum cosmology and ended up in quantum foundations, because of wondering about questions like that.

The original impetus for quantum cosmology is to do with the fact that the Big Bang model is based on quantum matter, but classical gravity. And quantum matter plus classical gravity, as you go backwards in time, as theorems due to Hawking, Penrose, and others showed, it must have a singularity initially, at least classically.

So a region of infinite gravity, infinite curvature. So that suggests that if it is still within the laws of physics, we need a quantum theory of gravity. And so perhaps the beginning of the universe is described by some quantum theory, which includes not just the matter modes, but also some modes of the gravitational field.

We don’t really know what a full quantum gravity theory is even yet, and this was 40 years ago when I was doing that work. But in quantum cosmology, what you can do is say, well, let’s just take a few elements of the gravitational fields, like the overall size of the universe, the scale factor, and maybe a few distortion parameters.

And then you can quantize these models of the beginning of the universe using relatively standard techniques. It’s all kind of heuristic. I’m not sure how much faith I would put in it, but at least gives you a picture. And what comes out of that, which is very appealing, is there is a non-singular beginning of the universe which is quantum in nature.

That’s the beauty that comes out, at least in simple models. So in the sort of older classical model, thinking geometrically, the beginning of the universe has a singularity like a cone. A cone has a sharp point at the end. So that’s what a classical Big Bang model looks like. But in the quantum picture, as you go to very small scales, the very smallest bit becomes quantum, and then instead of the sharp point, you can actually close off with a kind of smooth hemisphere.

So the geometry we have in mind is sometimes called a shuttlecock geometry. This idea was due to Stephen Hawking and James Hartle. It’s called the no boundary proposal of the universe. This has caused a huge amount of interest then 40 years ago, and it’s still very much an explored idea. It’s one of the very few models of how the universe actually began.

So that’s why that’s interesting. Now, to your question, is this really quantum? What about observers, collapse of the wave function, and so on? There aren’t any observers there to do the measurement or to collapse the wave function. So what is actually happening is that there are observers now. What we make measurements on things we can look at now, like the microwave background or the expansion of the universe, or more precisely, the detectors and the photographic plates that actually measure those.

We look at those, we measure those, and in some sense, we collapse the wave function when we actually do those measurements. But the question then is: how do you relate those measurements made today, which we would call records, basically, how do we relate those to the Big Bang in the past? And the answer to that is there was a new reformulation of quantum mechanics developed by James Hartle, Murray Gell-Mann, and others, called the histories approach to quantum mechanics, or more precisely, decoherent histories approach.

It tries to get away from ideas of external observers and collapse of the wave function. We’re actually looking at present records and looking for the correlation between them. So decoherent histories approach is a way in quantum mechanics of actually linking records in the present in terms of what we think happened in the past. It’s a bit roundabout, but it avoids having to actually say that the past actually happened in some sense, or the wave function collapsed in the past.

STROGATZ:  Well, good. You’ve given us a lot to chew on already, and so let us start building up to some of these notions that you’ve touched on here. Decoherence will be part of our discussion. We are gonna be talking about foundations of quantum mechanics. There are a few old chestnuts that are sort of irresistible, sort of inevitable.

One of them being Schrödinger’s unfortunate cat. I wonder if you could start our discussion by reminding us about that feline. What was that thought experiment supposed to illustrate?

HALLIWELL: Well, it was supposed to illustrate a phenomenon in quantum physics, which is that a system, a particle, a cat, could be in two different states simultaneously.

I’m slightly averse to talking about cats, actually. It’s a sort of nomenclature that’s stuck, to the point that non-physics friends of mine go, “Ugh,” when this actually comes up, and I sort of understand that, actually.

STROGATZ: I’m assuming this is coming from the place of being an animal lover or just an ethical person. What was your objection to Schrödinger’s cat?

HALLIWELL: Both of those. It’s a phrase that’s kind of entered the discussion at a very early stage when people didn’t think about these, let’s say, more ethical things. I mean, you couldn’t introduce that sort of nomenclature these days, I don’t think. But it’s a small point. I don’t want to distract.

STROGATZ: OK [laughs]

HALLIWELL: Let’s take a simpler example, which is essentially the same, and that’s the famous double-slit experiment …

STROGATZ: All right.

HALLIWELL: where you prepare a particle in a sort of outgoing wave state. Like, you just have a source of, say, electrons, and you shoot them at a screen which has got two holes in, say, a millimeter apart, and the far side there’s a screen.

And what happens is that the wave goes through both of the holes, and then it sort of gives an interference pattern, a set of light and dark patches on the screen. And then you might say, well, actually it’s really an electron. It’s a particle. It’s a lump. It’s like a billiard ball. Surely it just went through one or the other. And if you actually block one of the slits or block the other slit, then the interference pattern goes away. So if you actually make the assumption that it’s a definite particle that goes through one or the other, you don’t get the interference pattern anymore.

So in a very real sense, you can have what you think is a solid particle that very genuinely actually goes through both sides and creates the interference on the other side. Let me give you a nice pictorial analogy. There’s a lovely book by, uh, Yakir Aharonov on quantum paradoxes, and on the cover it’s got a picture of a man skiing through a forest in the snow.

And he’s got parallel tracks, and at one stage he looks back to see where his tracks have gone, and he observes that his tracks have gone both sides of a tree. And, and that, that is obviously impossible for a skier, but that’s exactly what happens in quantum physics. The electron looks back and notices the tracks in the interferometer where it really has gone through both sides.

STROGATZ:  I have not heard that analogy before. It’s an interesting geometry you describe. So let me just make sure we’re getting it with this electron experiment that you described earlier with the double slit. When we hear about wave particle duality, like people are used to light as a wave, and if there are enough photons, we can make this kind of continuum approximation in our minds.

We learn about interference. But what I remember reading about that seems so astonishing is that even if you turn down the intensity of light so much that there’s effectively only one photon in the chamber at a time, or in your case, single electron in some sense like this amazing skier can sort of interfere with itself.

HALLIWELL: That’s absolutely it. Initially with this double slit experiments they might have said, “Well, okay, there’s a whole swathe of particles going through.” But indeed, when you turn it right down so that there can only ever be one particle in the apparatus, it still has that property.

And in fact, they now can do interference experiments with pretty big things, like containing thousands of atoms. They’re borderline visible with the naked eye actually, some of the effects. I mean, this links to sort of macroscopic quantum experiments, and, it does lead to this question of how big a system can you interfere with itself?

The fundamental issue really beneath all of this, it is actually about the worldview at the quantum level. There are all these weird effects, but behind that, there is this worldview that the quantum world is actually about waves, and their intuitive properties are just not the same as those of particles. Just to give another simple example, if a police car comes past w- with its siren blaring and if you’ve got the doors and the window open, then the sound comes in both the door and the window. There’s nothing unusual about that. But if it’s a solid object that appears to come in through both the door and the window, that would be rather striking. But that is, in principle, true because wavelike is the fundamental nature of matter at the microscopic level.

STROGATZ: So in this wave-particle duality, you, it sounds like you’re arguing that for best intuition about quantum theory, we should be thinking waves?

HALLIWELL: Yes, people often say sometimes a particle, sometimes a wave. What actually quantum theory would say, always a wave. But sometimes the wave can be very compressed around a single point.

STROGATZ: Ah, I see.

HALLIWELL: So you can have a water wave, for instance, which is a single tightly peaked wave. If it comes in towards the beach, supposing there’s a low barrier, like a wall just beneath the surface, so one wave then becomes two: one being reflected, one transmitted. And that also literally happens in scattering experiments in quantum physics. You shoot in some particles uh, some bounce back, some keep going. So what you think was a single particle ends up in two separate bits.

STROGATZ: You touched earlier on what I hope will be one of our central questions in this discussion, which is how do we go in this transition from the quantum world with interference patterns and a wave-like nature of matter and energy to the macroscopic world that we’re used to? The one that we live in. I mean, why don’t we see interference in everyday scale objects like the moon or ourselves?

HALLIWELL:  Yeah, that is a, a big question. I mean, so the first feature is what’s called graining. So waves look smooth from a long way away. The ocean looks almost smooth from 30,000 feet, but when you get close, it’s not like that at all.

So that’s one of the things is from a coarse grain scale, the wave-like pattern goes away. But it’s more concrete than that. There are certain quantities which describe a physical system which are very slowly varying, and some which vary extremely rapidly. Suppose you have a big massive particle that’s being bombarded by molecules.

Like, there’s the famous Brownian motion example where you have, like, a pollen particle, which is kind of big compared to the atomic scale, being bombarded by molecules that makes it jiggle around a bit. The big particle will be effectively classical. What happens is the bombardment by other systems, which we o- often call an environment, it actually, it kills the interference, is the phrase we use. It suppresses the interferences that create the wave-like properties by kind of smoothing things out and destroying the wave-like nature.

STROGATZ: That’s a perfect segue. The fragility of quantum coherence, quantum interference patterns. I like your word bombardment because although it’s a very vigorous and almost violent word, it gives the right intuition that quantum systems are always at risk of being hit by stray photons, gas molecules, tiny electric and magnetic fields. There’s all kinds of ways that, as you say, the environment can jostle or mess up this delicate quantum state. So this is to do with this word you used earlier, decoherence. Is that right?

HALLIWELL: Yes. Destruction of interference or the reversal of coherence, yeah.

STROGATZ: So is it decoherence then that is the key to making a system stop acting in its pristine quantum character, look more classical?

HALLIWELL: That is exactly it. We’ve had 100 years of quantum theory, since modern quantum theory, which started with Schrödinger. And, so the first journey was from the classical world into the quantum, and that took us to Schrödinger’s famous equation and all these weird quantum effects.

But it was also important to do the return journey, which said, if everything’s quantum, my table isn’t, or what happened? And when you come back to the coarse-grain world, why is that not in fact quantum even though it’s made of very quantum stuff? That is the process of decoherence. That the quantum effects are actually very fragile, as you say. You have to work hard to maintain them. You need sort of isolated chambers and so on. And it is quite hard to find quantum effects which persist.

I mean, there are some like properties of electrically conducting materials and so on have relatively robust properties. But in terms of the really kind of, let’s say, out there freaky stuff like superpositions, those are quite easily destroyed. For all sorts of reasons, I spent a lot of time looking at different types of quantum effects and understanding which ones get killed quickly and which ones actually are quite robust.

STROGATZ: And so I guess I wanna ask something related to that, that when we speak about decoherence, is it that the system its quantum behavior becomes inaccessible to us? Through sort of the mixing or the entanglement with the environment causes us to lose our ability to know, or is it something deeper than that? It’s not just a matter of our inaccessibility of knowledge.

HALLIWELL: I think most would take the view it’s about inaccessibility. See, what would happen in the Schrödinger’s cat experiment is that you have a microscopic system which goes into a superposition of two states of something, like a couple of spins, and then those in turn are, en- bit tangled with a large system, and in this case, the unfortunate cat.

Now in practice, in a realistic experiment, any large system would be very quickly decohered by its, uh, immediate environment, by surrounding photons. But the entanglement, the quantum stuff, is actually still there. It’s just scattered far and wide. So if you have a small particle, like the Brownian motion particle, that is in some sense still actually quantum even though it’s scattered, but there’s so much information. You know, all the bits of information are just out there beyond the horizon that you can probably even never get them.

STROGATZ: Huh! It feels like you’re bringing the observer in a little bit, that it’s not that quantum nature is lost, it’s that it leaked into the room, or it leaked into the environment in general, and it’s hard for us to measure at that point.

HALLIWELL: True. There was a lot in the early days of quantum theory about does the observer in person play some role in this? And I think John Bell, who was a famous early writer on these things, he spoke about this actually with a certain level of rhetorical overkill, I would say. You know, what do we mean by the observer? And, you know, in practice it means that there is some environment somewhere, some surrounding system that actually stores the information.

But it does still leave the question, could you have, let’s say, some sort of super-observer with actually unlimited information processing capacity that could actually measure everything that’s going on? And in principle, that’s true. There is actually, let’s say, an information theoretic dimension to this.

But, part of it is that, as you probably know, quantum systems have vastly greater information storage capacities than classical ones. That’s one of the ways of characterizing quantum physics.

STROGATZ: You haven’t said the word “quantum computer” yet, but this is what you’re hinting at?

HALLIWELL: Essentially, yes. And it is to do with information storage capacity or processing capacity of quantum systems. It’s not an area in which I’m expert, but it’s an important reference point when someone discovers a new quantum effect. The first question is is it a resource? Can we do something that we can’t do with classical systems? And indeed, we can in many cases.

[Music plays]

LEVIN:  Wow. Okay, so you got into the observer in quantum mechanics. This is one of the terrible impacts of quantum mechanics on pop culture, this idea that an observer was required for quantum mechanics to make sense, that the observer collapsed the wave function the way that people talked about it.

Maybe the way to say it is things were in superposition until the observer rendered the quantum object in a definite state. And that’s just a total red herring in many ways, and I believe Jonathan probably went to great pains to explain how you don’t need a human consciousness. But it led to so much stuff out there, just ephemera out there about consciousness. And quantum mechanics that has been very hard to whack back actually.

So even now, people will still say, “Ah, but, you know, quantum and consciousness.” Those are not connected ideas. And I believe Jonathan’s done really important work to separate that out, to say the observer could be just the environment, a thoughtless environment, an unthinking collection of molecules.

STROGATZ: I certainly grew up in reading pop accounts of quantum physics, hearing a lot about the observer, certainly a lot about the collapse of the wave function. I’m not so sure it’s totally been put to bed, though. What you’re talking about is what used to be called the Copenhagen interpretation, right? From Niels Bohr and his school.

LEVIN: Right.

STROGATZ: The original attempt to not exactly make sense, but give some kind of philosophical account of what’s going on with this measurement problem. It’s out there still.

LEVIN: It’s still an issue. I think the Copenhagen interpretation and the many worlds interpretation are still out there. I think decoherent histories is maybe an alternative to both. So in the Copenhagen interpretation, something causes the superposition to collapse and a definite state to be assumed by the superposed particle, let’s say its location.

Now, in the many worlds, that’s not what happens. What happens is you, the observer, or the hot room or the cat, whichever it is, becomes superposed and branches off, and the two possibilities coexist in some meta world, some many worlds interpretation and, and all possibilities continue to be realized branching over and over again. That is surprisingly popular in quantum mechanics and completely confounded.

STROGATZ: Very surprising.

LEVIN: Very surprising! Now, to my mind, and I don’t know if I have this 100% correct, it’s been a long time since I’ve spoken to Jonathan about these ideas, but the decoherent histories is neither of those. It says, “Look, there are so many interactions, so many quantum probabilities that it just can’t keep its shape in that way.”

The quantum-ness doesn’t go away. It just becomes, in some sense, less distinct on these macroscopic scales where there’s so many histories, which is like saying so many particles macroscopic. And I think that’s where he’s going. I don’t know if you took a sampling of quantum physicists where they’d land on all of these.

STROGATZ: Well, it could be almost like one of these religious questions where, there’s, in addition to the believers, there’s the atheists, the agnostics, and what have been called the apatheists, right? Who just don’t care. So it could be, I mean, I think it’s fair to say that there are a lot of quantum physicists who feel, “I’m not interested in foundations. I wanna make measurements and do my experiments and get results and build new devices.” You, you can get surprisingly far without worrying about foundations.

LEVIN: Yeah, we can launch things into space.

STROGATZ: Yes, we can.

LEVIN: I mean, we can get really far.

STROGATZ: So we have a lot of ground to cover, and we will get into everything from macroscopic realism to possibly the mythology of the Big Bang after the break.

[Music plays]

STROGATZ: Welcome back to The Joy of Why. We’re joined by theoretical physicist Jonathan Halliwell.

So, at this point, I’d like to get into this notion of macroscopic realism that I know has been of interest to you, you’ve worked on extensively. Let me start by bringing up a quote of Einstein that is close to what I think we wanna talk about.

But one of Einstein’s famous quotes in thinking about paradoxes of quantum theory in the old days was, “I like to think that the moon is there even if I’m not looking at it.” Back in the days when Niels Bohr was drilling in the idea that you had to think about the observer, and does it make sense to say that an object has properties like a certain velocity or a position, or its spin state even before you measure it.

And Einstein thought, “Of course, I wanna believe that the moon is there even when I’m not looking at it.” Tell us what we mean by macroscopic realism, and what’s missing in this little analogy of Einstein and the moon?

HALLIWELL:  Is the moon really there when no one looks? That is indeed exactly what’s going on. In fact, the first experimental paper to test macrorealism, I think it was called No Moon There or something like that.

So, there’s a whole set of things here which I can try and sketch. So though on the one hand, these tests of macroscopic realism, it’s a version of what is classicality, and it’s designed for single systems that you measure sequentially in time. So when you measure something and the outcome you get is not the real outcome of what it actually is, but it might depend on what’s going on somewhere else.

This is what is called contextuality. Now, this is what happens in Leggett-Garg tests of macroscopic realism. What Leggett-Garg tests are doing is trying to eliminate classical explanations of quantum results, when you do a two-time measurement. When you’ve got a very quantum situation, you get something that would classically be quite paradoxical.

To go on to what macrorealism is, what we do is have a simple system with, say, it could be a spin, which just has two values, or it could be something more complicated by, you know, approximate position measurements of a particle like the moon, and then you just measure it a bunch of times and you compute average positions or average spins.

The important thing is you also have to compute correlations between different spins, like are two spins the same? And then from those you can actually try and assess are these correlations, are these measurements consistent with a classical model? And by a classical model, what you really mean is an underlying system in which the thing you measure has a definite value, so the spin is either plus or minus, or the position is definitely this or definitely not that.

And it seems like such a simple thing, but this is the idea of definiteness of value that actually is probably not true in quantum theory, and you prove that it’s not true by assuming that it is, and then deriving some contradictory consequences.

STROGATZ: So, when you speak about definite values, a particle or a quantum system having definite values, as opposed to what?

HALLIWELL: That’s an excellent question. In quantum physics, you have the possibility of a so-called superposition state, where a particle can, for instance, be in two places at the same time. So position, we would say, is not definite. But it’s a little bit more nuanced than that. I mean, it could be a probabilistic distribution of being here or here.

But quantum physics, you can do a measurement that shows it definitely is in two places at the same time, and then you can measure it in each place and find it’s definitely there and definitely there. So sometimes people say like misty states or indefinite states.

The only thing you can say is that if you assume it’s indefinite states and you assume also various measurements can’t influence each other, then you find a contradiction. Therefore, the assumption that it was in a definite state is false. So it’s indirect actually. You can only refute the worldview of definite states. It’s very elusive concept. And it’s the most important one in quantum physics, actually, I would say, conceptually.

STROGATZ: The idea of definite states versus superposition or…?

HALLIWELL: Versus superposition, but it goes even beyond that.  Sometimes with a superposition, you can still find an underlying model which has definite states. It does come down to this notion of contextuality. Another way you can explain away indefinite states by saying when you make a bunch of measurements, if they depend on each other in ways that you hadn’t noticed, that’s what an indefinite state might look like.

Except that in quantum physics, we do these experiments in such a way that there isn’t a dependent on context. So two distant particles in an entangled state, you arrange it so they can’t possibly signal. It still looks like they’re talking to each other. I mean, that’s really the true mystery.

There is a quantum mechanical explanation. The quantum mechanical explanation is it’s described by a wave, and the wave is a non-local thing that spreads out over space. And it’s only when you try to think of it as two separate particles that it looks like two particles with different states. The wave is like a hologram, and every little bit of the hologram contains a whole. That’s the best analogy, actually.

STROGATZ: I’m just wondering when you earlier mentioned the double-slit experiment, if we were to say that the particle definitely did go either through the left slit or the right slit, if that were true, it would leave a two-humped probability distribution on the recording screen, not a set of interference fringes?

HALLIWELL: It would change the interference pattern. In fact, you can take a double slit experiment and turn it into one of these Leggett-Garg tests for macrorealism.

STROGATZ: Is that right?

HALLIWELL: So you make a measurement of the first time. Did it go through the left slit or the right? And then you look at the screen at some small localized area and say, “Did it arrive here or not?”

So, you’ve got simple histories at two times, and then you do a bunch of measurements of correlations and averages, and well, what you find is that if you assign definite values to these, then you end up with a contradiction, or more specifically, you end up with a probability which is negative.

That essentially, a Leggett-Garg inequality or a Bell inequality, they are essentially probabilities for something. But they actually turn out to be negative, which means that actually there isn’t an underlying probability, which means you were wrong to assume that they have definite values.

In Leggett-Garg, we’re trying to actually explain that sequential measurements obey very non-classical ideas as long as we can justify this idea about invasiveness. I’ve spent quite a lot of time in my book on this to try understand it, but it’s very, very hard. The whole area is just riddled with loopholes.

STROGATZ: It’s interesting this word invasiveness that you just used. What it calls to mind for me is like people used to explain things like the uncertainty principle by giving a picture of a measurement disturbing a system, like an electron or a photon would knock the system off course. The act of measurement could disturb the system, right?

It became a kind of credo of old-fashioned Copenhagen interpretation of quantum theory. And it seems like the Leggett-Garg tests, if I’m understanding you, are maybe saying something deeper, that even if we take a lot of care to measure a system without disturbing it, that we do a non-invasive measurement, the classical picture could still fail. Am, am I getting that right?

HALLIWELL: It is something like that. So, in the Bell case, if the particles are quite close together, you can always say, “Well, you got this quantum result, but actually a classical system that just signals really quickly would give the same.”

Now, in Leggett-Garg tests, you could do two sequential measurements, and if you get the expected quantum result, you could always say, “Well, the first measurement you actually, you were clumsy.” That’s the word we use.

STROGATZ: It’s a useful one.

HALLIWELL: In Bell tests, it’s called the signaling loophole. In Leggett-Garg we call the clumsiness loophole, where if you do a classical clumsy measurement it will actually explain the quantum results. So that is true. You can cook up models that actually do that.

So it’s not particularly a quantum issue here. It’s really, can you eliminate a classical model which has some invasive measurement?

But there is a quantum measurement aspect also to do with the wave function that’s affected, but you can actually factor that out. So I mean classically supposing, uh, you’re speeding down the highway and a policeman has a radar gun, and they bounce the radar off your car and that’s how they work out the speed.

The bounce actually slows the car down a tiny, tiny little bit. There’s a change of momentum but it’s absolutely minuscule. But when you do that for a quantum particle, then the bouncing off of a photon can actually be quite significant.

There is a way that Leggett and Garg first suggested for creating this non-invasiveness, and they called it an ideal negative measurement, where supposing this particle has two options at the first time, then you attach a detector on option A but not to option B.

And if the detector doesn’t click, you say, “Well, option B happened,” but the detector wasn’t attached to B, so we didn’t actually disturb anything. Classically, that would actually be non-invasive. But there’s a tricky debate there saying, “Well, it’s really quantum, and it’s actually a wave function, and the wave function goes through both slits at the same time,” so it was still invasive at a quantum level.

So a certain number of pirouettes have to be performed in order to actually get past that.

STROGATZ: But so how far have experiments pushed these kinds of ideas? Like, are we still in the microscopic world of photons and atoms and tiny crystals?

HALLIWELL: Sure. I mean, what has been done is interferometry experiments for pretty big systems. I think Vlatko Vedral from Oxford, who’s an expert on this thing, he talked about Schrödinger’s virus as the sort of scale we might be getting up to.

Those experiments simply confirm there’s an interference pattern, when you do a double slit experiment. Leggett-Garg tests they do more than that. They not only confirm quantum, but they rule out alternative classical explanations, which is a stronger thing to do.

This is sort of interesting. You can still get an interference pattern, but there’s a classical model for it actually, if the interference isn’t too big. The thing is, interference is created by the fact that the wave goes through two holes, and what come out is two waves. They’re not probabilities, they’re waves that can be all positive and negative, and they can constructively interfere, so they get bigger. And that’s like a classical effect actually, ’cause if you add two probabilities, they get bigger.

But they can destructively interfere, so they cancel each other out. So you can get two waves coming in and you get nothing, which can’t happen with probabilities. Now, if the destructive interference isn’t too big, you can still model them as like classical probabilistic flows, and that would satisfy the Leggett-Garg inequalities, which actually means that there is a classical model of interference patterns in a certain limited regime.

So for one of these big interferometers experiments with big particles, to really say this is definitely quantum and not pseudo-classical, you’d need to violate a Leggett-Garg inequality, basically. That can all be done in principle. I mean, even just with the data from existing experiments, basically.

STROGATZ: Let me start backing away from some of the more theoretical points and get into some more personal things. One of them is just a matter of opinion, really, but I bet you have a strong opinion, which is so far no experiment has contradicted quantum mechanics or quantum field theory. I’m not sure how far we should push this, so why do we keep testing it? What are we hoping to learn by pushing these quantum tests to larger and larger systems?

HALLIWELL: Yes, that is a very good point. It is very, very striking indeed that there isn’t, in terms of experimental predictions that have been measured to, you know, many, many, many decimal places, there are no contradictions in the formalism. To the point actually that I know experts in quantum foundations who, who’s even said to me, “Why do you work on Leggett-Garg inequalities which confirm quantum behavior when,” in their words, “we know quantum physics is true anyway?”

Um, I have a problem with that. I mean, it’s this bordering on a kind of fundamentalism about accepting something as truth, that could be a wider discussion. But whatever theories are, they must be exposed to experimental tests and confirmation. I mean, really, absolutely.

I mean, it would be a wonderful thing if we started seeing differences. But it is still a really striking thing that there have been no differences with theory ever observed. I think behind this it is about what is the true interpretation of quantum physics, behind all these investigations, Bell inequalities, Leggett-Garg inequalities. What these things do, they eliminate certain very natural worldviews.

So Bell inequalities shows that quantum physics and experiment are consistent with the worldview of local realism. So the idea is that a particle has definite properties independent of what distant particles are doing. Similarly, Leggett-Garg is testing macroscopic realism. A particle has definite properties irrespective of measurements in the past or measurements in the future, for that matter. So we know what reality is not.

But there are other worldviews, if you like, other interpretations. One of the most famous, complements to these ideas is Bohmian mechanics, if you’re familiar with that one, or de Broglie–Bohm theory, which basically does take standard quantum mechanics, which is based on a wave function, but says the particle has a wave function, and it also has a position trajectory. Then you don’t need to worry about is the particle here or there? The particle definitely has a trajectory. It’s definitely somewhere. The wave function has a back seat as a kind of, they call it a guidance field, like a fluid that sort of flows along and tells the particle what to do, but the particle always has a definite position. And that’s very appealing, and it was invented actually shortly after quantum physics was invented.

And a lot of people like it because you know where you are, basically. You’ve got a definite path through. But it has a feature, which is that it’s non-local. Because there’s a wave function and a particle, that where the particle goes, the wave function can be spread out all over the place. So the guidance field for the particle can depend on very distant observers. It basically says that in an entanglement situation with distant particles, that one particle really can depend on what the other one is doing or what you measure on the other particle. But that might be it. I mean, you can have a world in which everything is definite, but it’s non-local. And that might also be true, actually. So that’s why I don’t believe in truth. I believe in multiple perspectives.

STROGATZ: Do you wanna expand on that idea? I, in some of my preparation for our conversation, I was told you have an interest and have practiced things like yoga, meditation. Is that related to the sorts of perspectives you’re describing here?

HALLIWELL: It’s not directly related, but in terms of the sort of attitude and perspective I take towards physics and scientific ideas, it sort of helps me in that sense. So I think in the early days when I wanted to figure out, you know, how does the universe actually start? We really want to know. Now I’m a lot happier to sit with the mystery.

When I’ve given popular talks, I have the sense that the general public interested in physics, feel discomfort at the lack of understanding. That they want the physicist to deliver something mythologically meaningful to them in terms of the Big Bang, and they’re dissatisfied. For me, I found a sort of other perspective in a more, let’s say, spiritual side, where I’m just happier to sit with the mystery of multiple perspectives.

There was an American cosmologist who’s now more of a sort of mythologist and writer called Brian Swimme, S-W-I-M-M-E, who you may have come across. And he’s basically… starts with the idea that all societies have creation myths of one form or another, and modern society doesn’t necessarily have that.

But he said, “Well, look, let’s take the Big Bang cosmology as a modern myth for our time. We don’t have to take it as true, but we can take it as a meaningful mythological tale from which we can draw a sense of guidance or purpose or perspective and so on.”

And I felt in popular talks I’ve given, I felt that actually filled a gap in things where people have the Big Bang, but it’s presented to them as, almost as truth, but it doesn’t really fulfill people to a degree that they actually want.

STROGATZ: It’s interesting to me the analogy between the perspective you’ve described with holding different possibilities in mind at the same time and being comfortable I wanna say sort of as a superposition. That you seem to resist collapse in your own thinking.

HALLIWELL: Yes. I mean, some of my non-physics friends, or even physics friends, they say, “Well, how is you as a sort of, you know, hard-nosed theoretical physicist, how can you entertain all of these more elusive things that are spiritual or mythological or whatever?”

Well, here’s the thing. You see in physics, of course, we had two major revolutions at the beginning of the 20th century, one in relativity, one in quantum theory. In relativity, suddenly space and time are not even anything like what you think it is. In quantum theory, matter is not what you think it is. So physicists had to get used to adjusting enormously their worldviews.

There’s always new perspectives coming along. For sure the next generation of scientists will have new ways of thinking about these things.

Quantum information is a very current vogue, and a lot of people turn the question of interpretation into quantum informational notions. So I, I think what we will see is kind of multiple ways of actually saying the same thing about the sort of bizarreness and mystery of quantum mechanics.

So I think at the end of the day, I believe in mystery. Sitting with the mystery and the uncertainty, and that, that is the most comfortable position. In fact, one of my meditation teachers said, I think, quoting from thousands of years, that a lot of these things are about the infinite one way or another, the beginning of the universe.

And instead of worrying about “What it is”, you can ask, “Well, who am I in relation to that?” That’s the simple question, I think, which is there in a lot of spiritual traditions. So that’s where I am with physics. You know, how do I feel in relation to all this uncertainty and immensity?

STROGATZ: This has been a real pleasure. Thank you for spending time with us here on The Joy of Why. This has really been fascinating.

HALLIWELL: It has for me too. Well, thank you very much indeed.

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LEVIN: I’m glad Jonathan mentioned meditation. I wasn’t sure if I could mention it if he didn’t mention it. Jonathan used to do a lot of yoga. I remember this. This was an era when I was also doing a lot of yoga, and we would talk about that connection too. This kind of attempt to observe without tackling with hypotheses and conjectures and theories, right?

This dual modes that we were living. One is theorizing about the world precisely in an attempt to understand it. And one is this other mode of meditating on thoughts and not trying to understand it. And it seems to me that that’s kind of what he’s describing, this duality that he’s comfortable entertaining or or, you know, just experiencing.

STROGATZ: Mmm-hmm. I have to say, as someone who has not practiced meditation, I found myself wrestling with the word myth when he discusses creation myths. So we hear the word myth used a few different ways. Sometimes it’s used like, “Oh, that’s just a myth,” as a dismissive term. He’s clearly not using it that way. He says a creation myth gives people meaning, gives them purpose, helps them orient themself in the vast infinite mystery of the universe that we find ourselves in. So am I hearing him right, do you think? I mean, what do you hear when you hear creation myth?

LEVIN: Yeah, I was very surprised by that, too, even with what I just said, the idea of observing and tolerating mystery and a lack of understanding. But that is not the same as myth. Myth seems to me to be doing something very strong, which is making an unprovable declaration about something. Anyway, I can see why it’s a little uncomfortable to hear the word.

I mean, it was interesting to hear him talk about that. I’m uncertain, even in that conversation, if he’s suggesting that this is part of what people are looking for when they’re looking to understand the Big Bang, is they’re looking for a replacement or the myth itself. And whether or not people are searching for it, that’s certainly not what we’re doing when we’re practicing scientists trying to understand the origin of the universe.

You know, if you destroy all myths today and all memory of them and rebuild civilization, there’s gonna be different myths. But quantum mechanics is still gonna be there you know? Whether it’s described by matrices or anything else, quantum mechanics is still discoverable in a way that myth is not. I don’t know that… what do you think, Steve?

STROGATZ: Hmm. Well, okay, here was my question, when Jonathan speaks about multiple perspectives, it’s something that comes up a lot in quantum theory itself. So like for instance, going back to the very beginning, Schrödinger’s approach with wave mechanics. Before that, there was Heisenberg and Born and Jordan had this approach based on matrix mechanics. Dirac later had his way of doing it. Feynman eventually has his sum-over-histories way of doing it. I mean, in the case of those first two, matrix mechanics and wave mechanics, they’re two perspectives, but they turn out to be mathematically equivalent. There’s one quantum mechanics, right?

And so when Jonathan talks about the multiple perspectives, there’s really the one universe, okay, leaving aside the multiverse, the universe we seem to be in, our observable part of the whatever multiverse, there’s one thing. And if we have multiple perspectives, don’t they have to agree?

LEVIN: Yeah. Well, it’s really interesting because his early work on decoherence in quantum cosmology is precisely to formalize how a closed quantum system, there’s nothing external to the universe. It’s all within the closed system, and his careful and thoughtful work on that is exactly what made his early reputation, that you could do all of that within the universe itself. And, I thought that was quite profound, actually.

STROGATZ: Thank you. That’s a beautiful summary. I think we should leave it there.

LEVIN: There we go. Off into the wild blue yonder. Okay. Till next time.

STROGATZ: See you next time.

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LEVIN: If you’re enjoying The Joy of Why and you’re not already subscribed, hit the subscribe or follow button wherever you’re listening. You can also leave a review for the show. It helps people find this podcast. Find articles, newsletters, videos and more at quantamagazine.org.

STROGATZ: The Joy of Why is a podcast from Quanta Magazine, an editorially independent publication supported by the Simons Foundation. Funding decisions by the Simons Foundation have no influence on the selection of topics, guests, or other editorial decisions in this podcast or in Quanta Magazine.

The Joy of Why is produced by PRX Productions. The production team is Caitlin Faulds, Jade Abdul-Malik, Genevieve Sponsler, and Merritt Jacob. The executive producer of PRX Productions is Jocelyn Gonzales. Edwin Ochoa is our project manager.

From Quanta Magazine, Simon Frantz and Samir Patel provided editorial guidance with support from Samuel Velasco, Kit Sudol, Simone Barr, and Michael Kanyongolo. Samir Patel is Quanta’s editor-in-chief.  The episode art is by Chanelle Nibbelink, and our logo is by Jaki King and Kristina Armitage. Special thanks to Garth Avery at the Cornell Broadcast Studio.

I’m your host, Steve Strogatz. If you have any questions or comments, please email us at [email protected]. Thanks for listening.

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