Within Reason - #119 Jim Al-Khalili - The Strange World of Quantum Physics

Episode Date: September 1, 2025

Jim Al-Khalili is an Iraqi-British theoretical physicist and science populariser. He is professor of theoretical physics and chair in the public engagement in science at the University of Surrey. Learn more about your ad choices. Visit megaphone.fm/adchoices

Jim Alkalili, welcome to the show. Pleasure to be here. Once upon a time when doing a lecture at the Royal Institution, you spoke about the famous double-split experiment, this strange quantum phenomenon whereby something sort of appears to, light appears to act as a wave and a particle at the same time. And you issued a challenge. You said, now, if anybody thinks that they can offer

a commonsensical explanation for this kind of thing, then do send me an email. How did that work out for you? I wish I hadn't done that. I'd forgotten that I wasn't just talking to the three or four hundred people in the audience at the Royal Institution. But obviously it gets recorded and goes out on their YouTube channel. To this day, I mean, that's about 10 years ago now. To this day, I receive, on average, one or two emails a week from people saying,

I've solved the two slits mystery. You know, where's my Nobel Prize? No, we've tried that. Yeah. There's a lot that's strange about quantum mechanics. Maybe we'll get into a few more of those things later on. But the American, the great American physicist Richard Feynman said that the two-slit experiment sort of encompasses the central mystery of quantum mechanics.

We know that if when you send light through to slits and you get interference patterns, light and dark fringes, what's weird is that sending particles, indeed even entire atoms through, you get the same wave-like behavior when you send particles through. So I went through in this lecture, I went through how come, you know, quantum mechanics is this wonderful, powerful theory that has revolutionized the world, helped our understanding of the subatomic world, and yet at its heart there's this counterintuitive idea manifesting in this two-slith experiment that we still can't explain properly. Yeah.

Yeah. Quantum mechanics is stereotypically thought of. the sort of weird, spooky, kind of creepy, leaves room for new ages to get in and say there's all kinds of, you know, this could explain telepathy and this and that. Like, there was certainly a time when we start discovering these strange quantum phenomena and we think, now this is completely inexplicable. It still has that reputation today, but it's been, you know, a hundred years. Is it still legitimate to say, as Richard Feynman also famously said, you know, anyone who thinks they understand quantum mechanics, doesn't. doesn't understand quantum mechanics. Is it still true to say that it is just weird and creepy and spooky? Not all physicists and indeed chemists who also used quantum mechanics

would agree on this. I would say yes, it is as mysterious as it's ever been. The fact is it's also been such a powerful mathematical theory. It's helped us invent all sorts of things, you know, developing, understanding semiconductors, developing microchips and computers and small smartphones and lasers and so on. It works and it tells us how the atomic and subatomic world behaves. But at its heart, it is still mysterious. And we do not yet have an agreed upon explanation of how it's the only theory in all of science that seems to have got away with not requiring a narrative and interpretation to explain the mathematical formalism. We have half a dozen or more different ways of explaining it.

And you might say, well, so what? Because it hasn't stopped us doing the science, hasn't stopped us understanding the world. But at its heart, physicists and philosophers, I think, should still be worrying about this. Yeah. Well, people often talk about interpretations, the quantum mechanics, the Copenhagen interpretation, the many-world's interpretation, whatever. You don't hear people talk about different interpretations of gravitational theory in the same way. Why not? Like, what's going on with that?

Because we do have a single interpretation. I mean, when we think about Einstein developing relativity theory, his special theory in 1905, the mathematics for that was pretty much sorted out. People, you know, like Lorentz and Poincaray, had already developed the equations. What they didn't have was the correct interpretation. What is this telling us about reality? Einstein comes along. he doesn't come up with new mathematics, he comes up with the correct interpretation,

the narrative, the explanation of what this theory is telling us about the world, that nothing can go faster than light, that time and space must be unified into 4D space time. And Einstein is then credited with relativity theory. It's the interpretation, the explanation that's important. No other theory is like quantum mechanics where the maths works, describes the subatomic world, tells us about how atoms fit together and electrons fit orbit around the atomic nucleus and so on. But we have these half a dozen or more different ways of explaining what's going on and we can't agree on which one's right.

Yeah. Well, it's the difference between like a mathematical model that like predicts outcomes and I hesitate to use the word explains, but you know, let's say describes the various activities of subatomic particles. That is a separate question from the ontology. What is the stuff? Like, people will commonly think about, you know, particles being in two places at once and being here and there at the same time. And that's probably some kind of confusion between our mathematical models, which sort of act as though, you know, a particle is in two places at once. Whether it actually is or not is a totally different question. And I think you and I probably both agree that it's not actually in two places at once, but it's like our mathematics describes it as such because it's useful. So, does the quantum physicist of the modern era have much at all to say scientifically, beyond their, like, philosophical hunches and interpretation of the data, do they have anything like scientific to say about what is actually going on?

More so now than a generation or two or three ago. Sure. I mean, certainly when I started as a PhD student in the 80s, it was very much the case that, look, quantum. Quantum mechanics is a mathematical theory. It works. If you want to do philosophy, go and do philosophy. But a physicist uses it. So this is the Copenhagen view.

Sometimes it's simplified into the shut up and calculate interpretation. It works. Don't worry. You're pretty little head about how and why. We can't say anything. Copenhagen view says we can't say anything more about the quantum world beyond what we can measure and observe. What's going on behind the curtain before we look, we have nothing. to say about it. Not that it doesn't so much exist, but we have nothing to say about it.

I was always uncomfortable about that. And I felt somehow, you know, not that I was out on a limb, but it wasn't the prevailing attitude to worry. I think something must be going on. You mentioned, you know, the ontology, you know, there is something, some objective reality out there. That wasn't even regarded as, you know, cut and dried. You know, quantum mechanics just tells us what we, you know, the results are making predictions about the results of measurements. Most physicists today, I would say, maybe not my generation, but younger than me, are a bit more concerned. There is something going on. I want to understand what it is. That should be the job of the physicist. As John Bell famously said, the job of the physicist is to understand

the world. Contra Niels Bohr, who in one of your books, you sort of blame for this functionalist view of science to some degree. Because I mean, Neil's Borr's a great hero of mine. Yeah, yeah. I mean, if Neil's Borne and John Bell, I'm siding with Bell. Yeah, because they have this conversation, which I've been talking about so much recently, about what science actually does. And in fairness, I am more in line with the view that science is functional, that it only describes

and doesn't really explain or tell us about the nature of what things are. But I'm definitely in your camp that, whether you call it science or not, we should be interested in what stuff actually is. actually is. I mean, when I spoke to Sabina Hosenfelder, I was asking her about if I blew a trumpet and I discovered that every time I blow this trumpet, a red light turns on over there. And I thought, that's interesting. And I figured out that if I blow it harder, if it's louder, the light is brighter, and if it's a different note, it's a different color. And I worked out exactly what was going on. And so I came up with this predictive machine, perfectly accurate. I can, if I play this node

at this volume, the light will do this. And I got it right every single time. And somebody asked me, like, why does the light turn on? I said, well, because I blew the trumpet. It's like, but why does it do that? And Sabina Hosenfeld has sort of said to me, that's it. That's enough. That's what science does. And I'm like, how could you not go further?

I couldn't sleep at night if that's what science was about. You know, if all we cared about was, I remember someone telling me that they wore, you can wear these copper bands bracelets that seem to have some medical curative power. And it's the same with a lot of alternative medicines. But it works. We don't care if it's, you know, what the mechanism is. The fact that it works is enough. For me, as a physicist, no, I want to understand how and why.

If all you care about is applying some mathematical equation, that for me is, I don't know, that's engineering. Yeah, yeah. A physicist wants to understand how. How did the trumpet get to the light? What is the interaction, the force between them that's causing that to happen? But to what degree do you think that science can answer that question fundamentally? Like what stuff actually is? Because the reason I ask is because, you know, if I were to ask what a table is,

you might tell me this table is made out of wood and the first view what words made out of and so on and so forth. We get down to an atom. An atom's made out of an electron. Well, what's an electron? And at this point, a lot of philosophers of science will say that the only answer is something like, well, an electron is a negatively charged particle. Well, what does that mean? Oh, it means it acts in this way.

It repels other negatives. Yes. But that's not what it is. That's what it does. And so at a fundamental level, especially in the quantum realm, where we're dealing with what are hopefully, or if they're not, hopefully we'll find at some point atoms in the true sense of being atomic, like fundamental particles. And we're asking what those things are, not what they do, not how they behave or how

they relate to each other, but what they are. Do you think that is a question for the physicist? I think it is, and I think they should try. It may be that we're peeling back layers of the onion. And it's also true that there are different scales at which certain explanations are sufficient. I don't need to understand the workings of the standard model of particle physics to work out how a washing machine works. At the level of table, it's made of wood and wood has this property. That may be enough for my explanatory satisfaction, but I can go deeper and I can talk about it being made of atoms and atoms being made of smaller particles.

And it may well be that there's more to come. But I don't, I think had we stopped, had physicists said, no, it's enough to know that a table is made of wood, we wouldn't understand then the nature of atoms. We have tried to go deeper and very often we have arrived at what objective reality. is at a deeper scale. That may not be the end point. Of course, there may not be an end point. We may never reach it. We may not be able to.

But there is a truth about the true nature of reality. There's another concern that people have about, well, all we are ever doing is perceiving the world and so building models in our heads. You know, that's epistemology rather monotology. But there is a real word out there. And I think the job of physicists is to get as close as possible to that objective. truth. Yeah, I find it interesting. I've heard such a variety of views, not just on whether it's accessible to science, but what that fundamental stuff might be. You know, I have Brian Green

telling me it might be vibrating strings. I have Philip Goff telling me it might be consciousness itself. I have theists telling me that it's being and that all participates in God. And the one thing that I've realized is that the people who seem least confident in like having a view on what the thing is, the ontology, I tend to be the scientists I speak to. Because either because they think that explanation consists in, like, descriptions and models and mathematical predictions and stuff, like Sabina Hossenfeld did, or because they recognize that it's a question that's very difficult not to be a bit agnostic on. And it's like the philosophers who sort of have that kind of confidence. But at the very least, I agree with you that it should be a project for whoever you are. I think I would side more with that latter view.

Yeah. that there's a certain built-in agnosticism in the scientific method that we should not be so confident and sure of our explanations because something may come along tomorrow and overturn those views. A good scientist is one who's prepared to change their minds. When it becomes an article of faith or belief, you know, you are so confident that string theory is the correct description of reality, then you're not doing science properly because you need evidence. physics is an empirical pursuit, and we need data and we need to observe, and we need to be prepared to change our minds. So physicists acknowledge that there have been revolutions in science all the time overturning previous ideas. Yeah. So what is it about quantum mechanics that makes it so subject to different interpretations and difficulty and weirdness? Is that

something intrinsic to quantum mechanics, or is that just because it's the newest thing that we're interacting with? Or is it actually about quantum mechanics itself? It is. I think we've, you know, after 100 years and it's not taking us that long, but I mean, very soon we realize that it's something intrinsic to the description of the quantum world. The way the quantum world behaves is very, and it's not just that it's far removed from our own everyday senses and experiences. It really is strange. And it's like the bump under a fitted carpet. You can move that bump around. You can shove it behind the sofa, behind the TV, so it doesn't show up. But that bump is somewhere. You can say that bump is many worlds branching out. You can say it's a non-local field a debroy bone mechanics. You can say the quantum wave function spontaneously collapses. You know, there are lots of ways of experience. But that weirdness is there. It's where you leave it, where you decide the weirdness originally. is what we can't agree on.

Yes. And so people will have ideas. There'll be people listening who are super familiar with quantum mechanics and some who think to themselves, okay, I know about quantum mechanics. I've heard of the double split experiment. Something about particles and waves at the same time, two places at once, entanglement. But as we've already indicated, some people have misconceptions about what's like. So can you help us to dispel one or maybe a couple of important misconceptions about

why quantum mechanics is weird and then give us an indication of some of the things that actually do make it a bit weird. Well, I mean, very often one of the most often asked questions I get when I give public lectures is quantum mechanics and consciousness. And this goes back to work of Roger Penrose and Hammeroff talking about the nature of consciousness having some quantum origin. And most quantum physicists will say, look, quantum mechanics is mysterious

consciousness is mysterious that doesn't mean the two are connected it may be they're connected but we're a long way from being able to see that and people say oh you know entanglement quantum entanglement this weird property that Einstein

didn't like it called it spooky action at a distance two separated particles can nevertheless be part of the same quantum state and therefore instantly in connection communication with each other People say, oh, well, that will explain telepathy, or that'll explain why twins somehow can sense, you know, when the other one is something to happen, the other one, they can sense it, because their brains are quantum entangled. The fact is, these strange quantum phenomena and mechanisms are very much confined almost always to that subatomic realm.

They are ephemeral, delicate features that disappear very quickly. We now, in modern terms, we say the quantum effects decohere. The quantum weirdness, they're being in two places at once business, dissipates, like heat dissipating from a hot object when you put it in the freezer, very quickly once you scale up. So when you scale up to our everyday world, the idea that you can invoke quantum mechanics to describe telekinesis or telepathy or the nature of consciousness or alternative medicines, it doesn't

fit with the science. Just because quantum mechanics is weird, doesn't mean we're allowed to invoke it to explain any other mysteries we don't understand. Some people might say, for example, it's a little bit weird that some birds, for example, seem to be able to measure the, whatever it is, the magnetic pole of the earth and use it to navigate. And it's a little bit strange that they seem to have this ability to navigate just based on this magnetism of the earth that we can't perceive. And as far as I understand, especially with some of your work in quantum biology, which is really exciting. One great theory for the way that they do that involves quantum entanglement in the eye, right?

And so people might say, okay, let's not. jump the gun, let's not say anything without evidence, but doesn't this weirdness at the quantum level at least open the door to the possibility of a lot more weird kind of macro level stuff? It does, and I think this is where our earlier conversation about wanting to understand how something happens, with the ideas that these birds and certain other mammals have this magneto reception, first of all, that was discovered back in the 70s before anyone thought about a quantum origin for it, you know. And even then it was, it was pooh-pooh, despite the research being published in one of the top journals in science. In fact, the journal called science,

people were arguing, how can, you know, the Earth's magnetic field is so weak. It's one thing sticking you in an MRI scanner and mapping the body, because there's a huge magnetic fields. The Earth's magnetic field is so weak. How could it possibly have any effect? on chemistry inside living organisms. And yet they found evidence that these birds really could sense the orientation of the earth magnetic field, not like a normal compass, but simply how far away from the north or south pole they were. The lines of field, you know, the iron filings, if you were to scatter around the earth,

assuming the earth is a big magnet, whether they're parallel with the ground or vertical. birds could sense that and no one knew how that could happen where was this built-in compass inside these creatures we know it works but how quantum entanglement inside a protein called cryptochrome inside the bird's retina

is far-fetched but it's the only theory that we can actually explain step by step you know there's a whole chain of you know your your trumpet to the light. There's a whole sequence of mechanisms in the real physical world that start off with this strange quantum mechanism entanglement that will explain how these birds get directional information. It may not be true, but here is an example, but you're looking at, you know, two electrons spinning in different directions within a molecule inside the retina. That's down at

the quantum level, that doesn't mean you can scale up and say, well, quantum entanglement can explain telepathy, for example. It is strange to think that there are these, like, effects and truths and mechanisms that just disappear as you get bigger. And the question that raises for me is that, I mean, famously, the big trouble is that we've got this quantum mechanics over here, and we've got our macro classical physics over here, like Einstein's theory of relativity. don't want to go together.

Maybe we might want to talk about why they don't go together, but most people know that they just don't somehow. And the goal is this theory of everything, or something that will at least unify those two explanations. Yeah. Is it possible that for some reason that we don't yet understand, things do just work differently at the quantum level, such that you will never have a quantum theory of gravity or some classical understanding of quantum mechanics, because that sounds weird, but it also

sounds really weird to me to say that there are all of these things like entanglement and tunneling and stuff that if you increase the size enough, they just vanish into thin air. Yeah, I mean, there are two issues. One is, you know, how do you move smoothly from the quantum world to our everyday classical world? And that doesn't require going on to Einstein's general relativity. Right. It's just simply where is the boundary between what is quantum or is classical.

And I think we're starting to understand that. Okay. As you get more and more quantum systems accumulating, then they become more, more entangled and it's more and more difficult to keep hold of the delicate quantum effects. It gets washed out. You get what's called decoherence. The early pioneers of quantum mechanics, Mealsbaud, Heisenberg, people like that, talked about there being a sort of a cut, some hard boundary. between the quantum world and our classical Newtonian world. And they said, you know, how do you get something from the quantum realm, the entanglement,

the two places at once business, to our sensible results? We never see two people in the same, you know, one person in the same two places. And they've called it some irreversible act of amplification, which is just vague. Now we're starting to understand it's to do with increasing entanglement, increasing decoherence, where the quantum weirdness leaks away, and as a physical mechanism, as something we can understand. What we don't yet know is how to unify quantum mechanics with the more accurate picture of the large scale reality, which is Einstein's general theory of relativity, Einstein's theory of gravity. And there, it doesn't work simply because the

mathematics, the theories are very different. They just don't mesh together. It's an interesting points that you mentioned that maybe they don't, maybe each to their own domain and we have to live with that. But there are situations where you would want both to explain what's going on. For example, the singularity at the center of a black hole. Sure. Or the big bang. Or even if you think about it, imagine a single electron in a superposition, a quantum superposition or being in two places at once. Essentially, you're saying it's not the electron is doubled up. It's one electron, but its quantum state is spread out.

Well, an electron has mass, and we know from general relativity, that mass causes space time to bend. Ever so slightly for the case of an electron, because it's a very tiny mass. Nevertheless, that would mean that space time is in a superposition of bending in two different places. Wow, yeah. Now, if we want to talk about space time, we need general relativity,

curvature, curvature, and yet the superposition thing is quantum mechanics. So there's an example, even though it's be far too tiny to actually check experimentally, in principle, you need both theories to explain it. That's interesting. And so you can't just say, well, we'll never have to worry about general relativity. Maybe in practice we will never will, but in principle, we should always need some combined theory. never considered the effect of the mass of quantum particles. I read it, I mean, I read it somewhere.

Someone else very clever, pointed this out. Cool to think about. Good example. I'll use that. But then, you know, some people might say, like, well, there are all kinds of paradoxes of space which seem to indicate that maybe there is just a smallest possible distance. And maybe that's somehow related to space time and that, like, if you get small enough or if you get to the smallest possible distance, it's also the smallest possible unit of space time.

Therefore, it can't be, you know, bent or shapen in any way because it's as small as it can get. At the very least, it's a bloody mystery, isn't it? I mean, it is. And, you know, and we have ideas and people can spend their whole careers pursuing a particular way of explaining things, whether it's something like string theory or, you know, what goes on down at this so-called plank scale. But the deeper you go, the harder it is to test your ideas. And ultimately, as I mentioned before, physics is an empirical science. We need data, observation, experimental evidence to tell us that our theory is on the right tracks. We don't know how to do that when we are probing reality at such a scale.

You've been doing a lot of work on time recently. And when we talk about this, one thing that comes to mind for me, as someone who's not a scientist, and so I don't know if this is relevant at all, but part of the problem with quantum mechanics is, for example, all these entangled particles seeming to be able to communicate instantly across space. The interesting thing about relativity for me is that it shows that time is not linear, time travels at different rates depending on like mass spending space time. But on the quantum level, to talk about this weirdness of like instantaneous information travel, does that kind of assume that there is a universal present?

In other words, is the problem of time, relevant to the inconsistency between relativity and quantum mechanics? I believe, yes. I think we're struggling to reconcile not just quantum mechanics with general relativity, but also with the other big idea in physics thermodynamics. Yeah. And it's strange that each of those three big pillars of physics describes time in a different way.

So quantum mechanics, like Newtonian mechanics, in fact, you know, classical mechanics that we learn at school regards time simply as a label, as a parameter, it's called coordinate time. So it's basically, here's an equation

that describes how something changes. I can work out what it states is at a particular moment. Then if I change that T in the equation, T for time, to some other value,

I can crank the handle and work out what that system's doing at that later time or at an earlier time. That's all time is. It's just a parameter that goes into an equation.

General relativity says, no, time isn't, just a number. Time is a dimension. It's part of the fabric of four-dimensional space-time. It's a real thing. In fact, it's all times coexist. And the thermodynamics says, no, time isn't a dimension. It's not a number. It's an arrow. It's a direction pointing from past to future, in the direction of increasing so-called entropy. So I think until we reconcile these very, very different pictures of physical time, we're not going to be able to reconcile those theories themselves together. I think time may not be the central thing that needs to be solved to get it, but I think

it's part of that question. Until we understand these disparate different ways of describing time, we're not going to come up with a theory of everything. Yeah, I mean, time sort of looms over everything. You can always forget that it's there for some time when you're talking about atoms and how they interact and stuff. And suddenly you realize that all of that is happening and the conversation that we're having is also happening through time. Aristotle famously said that when he's not asked to define it he knows what time is

but the moment someone asks him to like explain it or say it or define it he's like I haven't the foggiest I think it's quite boring when people bring physicists on podcasts and ask things like do aliens exist or like you know why is quantum mechanics so weird and one of the questions that's along that line

is like what is time man but I think unavoidably and I'm not necessarily asking for for an overview of the philosophy of time. But for you, with the work that you've been doing, what do you think time is? How can we start talking about it? Well, I start with, you know, what a number of philosophers do,

which is to divide it into two categories. There's physical time, which is the time that appears in quantum mechanics, and relativity, thermodynamics. It's the objective, the time that's out there that we try to understand. And then there's what we call manifest time, our psychological time. and it's one of those subjects we're embedded in time. It's so difficult to extract yourself from it

and look at it and study it objectively. We can't help but being embedded in time. And it's the manifest time, psychological time, that gives us this notion that time flows, that time, that there's a now that's real and that the past has gone and the future has yet to exist.

And it's, I think, one of the main problems is how do you reconcile our psychological perception of time, this manifest time, with physical time. Physical time says there's no such thing as flow. That's just an illusion. But then some physicists and philosophers will go even further and say, look, the whole time itself is just an illusion. Some will even go as far as saying time is just something we invented to order events or to measure intervals between one and event and another. I think that's, I don't agree with that. I think time is real. I think it exists in

the same way that space exists, but not in the way that Newton believed, which said that time was absolute, there's some cosmic clock ticking by the seconds, minutes, and years, independently of us. Certainly with a relativity theory, which probably is our best theory of time, space time is real. It's a fabric of reality. Reconciling that with the idea that the equations of physics are time symmetric and that the time comes in just as a parameter, or that time has a direction from past the future, that is still a problem in studying the nature of time and it

has been since Aristotle and before. Yeah, and it's funny because, you know, you can say all of that and I still feel like I'm left with this question, but like, what is it? Like, what is the thing that we're talking about? You know, like you say that

there's this idea that we move through time. Okay, well, if time is a dimension like spaces, well, I can kind of feel like I can move through space. I can move through space. I can see what that's like. I'm moving my hand.

But I feel like I'm moving through time. But if I try to pay attention to what's it like? What's it like to wave my hand left and right? Well, I can feel it. I can move it. What's it like to move through time? Maybe because we can't turn it off, it's kind of impossible to know. So I'm not sure what like even mental image to have.

Like when I think of space time, for example, in the physical sense of like the three dimensions of space and being warped by. objects of mass. I imagine, as I imagine, most people do, like, a bunch of lines running through space, like making all these cubes that grow and expand. That's probably not accurate, right? But it's close enough that I can kind of think about things in that term. But when someone says time, and you're picturing in your head, like moving through time, literally like, what imagery are you thinking of? What kind of thing are we getting at here? Yeah. I mean, typically in physics, we talk about 4D space time, or what we do is throw away one other dimensions of space.

Yeah. So we only have two dimensions of space for one of time. So then we have a 3D block. It's called the Block Universe. Yeah. That allows us then to maybe get a sense of what space time, 4D space time means. That's before you even start bending it, warping it due to gravity. But in that picture, you're right.

You know, you can look at space and say, well, I can get from one place to another. I can be over here. I can be over there. I can get over there. I can go back there again at a different time. but you can't visit different times and yet the block universe would say all times coexist

just as all points in space coexist. So what is it about what we call now? The present moment that seems to us according to Manifest Time to be drifting along, flowing along the time axis.

That is all we can do. And then even we think about in cosmology, we talk about the universe and time. Finally is described by general relativity, 4D space time. So, yeah, but the universe is expanding. Well, it's space that expanding, not time. And the only way to get that picture is you start off with Einstein's equations,

and you manipulate them to get a new equation, called the Friedman equations, which has space stretching over time. And that's an equation like Newton's equations or quantum mechanics. Their time appears like a coordinate that shows it. So we have to revert to things changing over time in order to get any sense of them. What general relativity tells us about time is not something we can visualize, other than mathematically. And so much about how we understand these things, I think it's down to how we visualize it. I mean, at one point in somewhere you, I think, I'm sure it was you who wrote about this.

I thought it was quite funny the way he said, well, one thing we know about time is that, you know, time seems to move forward. And then you were like, or does it? because you could say that if you imagine yourself is still, and time is like moving past you, then time actually moves backwards, and so you were sort of moving through it as it goes. And I thought, oh, God, yeah, okay, fair enough. Like, both of those seem like completely equally legitimate ways of thinking about it. Time moves forward, time moves backwards. And I began to realize how much of what I understand about time is just based on the image I have.

Yeah, we think about the river of time flowing from the past to the future. But actually, it's not. Explowing from the futures of the past. If you're on a boat moving along, drifting along the river of time, you're looking backwards. You're looking at the past receding away from you into the distance. The future coming at you, you can't see it's behind your back. And that makes a lot more sense in terms of the fact that we can remember the past. But we can't remember the future, right?

It makes more sense to look at it that way. You've talked about this block universe, which you can sort of imagine being God, like zooming out, seeing the universe. first, including the beginning of time and the end of time, all at once right in front of you. Philosophers have, for a long time, talked into, I think it's McTaggart, who popularized these different ways of thinking about time, A, and B and C series, and most people are familiar with the first two, the A and B theories of time, so the A theory of time broadly suggesting that the present is what exists, the past does not exist, the future does not exist, and we are actually moving through time, and that's like real thing.

sensical view of time. This B theory of time suggests that the past does exist and the future does exist and you get this image of the block and for some reason we are like conscious of just a part of it at one point, something like that's going on. But you have these sort of two views of physics. It seems to me that general relativity, which seems to get rid of the ever present, the ever-present present, the common present between everybody, your time moves differently to my time and whatnot, that seems to me to point towards this block universe, because time is this thing which we can warp and move into and move out of in various ways. But having not looked into it much further than that thought, do you think that Einstein's views about time and

interpretations about space time necessarily lead to this B-theory, block universe of time? Or can we still salvage the A theory? I think we can still salvage the A theory. I think the block universe idea in which you're right that, you know, there is no universal present moment. You know, what is now for me is not a now for someone. And we know in relativity theory, in fact, I've taught this for years, something called the relativity of simultaneity. And the example is always two people in spaceships passing each other close to the speed of light. And they see two flashes of light. And one sees the two flashes, you know, A before B. And one sees the two flashes, you know, A before B and the other sees the flash of B before A.

So which one happened first, you know, cause and effect and so on. You can't violate causality. If something is causal, if A is the cause of B happening, then A must always happen before B for all observers. So I think this fuzziness in the now and the order of events around the now is very limited. I think the block universe can be a helpful tool, but to say that that fuzziness about no universal now means all times coexist, I think it takes it too far. For me, Einstein's idea of a block universe and all times existing is a tool that is useful for us, but it doesn't

reflect reality itself. I'm far more in favour of trying to find a way that physical time can map onto our manifest time, which is the one about only the present moment being real. Yeah. I mean, it's interesting. You know, you say, the past has happened. It's gone. It only ever exists in records that we access in the present moment.

The actual past, it doesn't exist anymore. The future hasn't happened yet. It doesn't exist. The present moment is simply the edge of the shadow between the past and the future. So in itself, it doesn't have any duration. So it also doesn't exist. Well, I've just done away with the whole of time then. Yeah, that's right.

And it also seems sort of paradoxical, because if you just say, it's easy to say, well, the past doesn't exist. But if the past doesn't exist, then, okay, like, you know, Shakespeare doesn't exist. And neither does, I don't know, neither does Mac Miller, the rapper who died a few years ago. Two of excellent examples. Yeah. Yeah, but there are still things that we can say about them. We can say that one was after the other, and that still remains true. So it's not like it doesn't exist at all or in any sense that we can't talk about.

But it exists in the presence, in the sense that it exists as records in our memories, in books, in photographs, in films. Everything that has happened continues to have an existence in every subsequent present moment. stored in records but in reality it doesn't any longer have a real existence. But it's weird isn't it that like most people will be aware of the fact that it takes

a very small but measurable amount of time for any kind of sensory input. You know the words you're speaking, the light that's bouncing off the objects in this room to travel to my eyes or my ears and go into my brain and do their funny little calculations and stuff

to the extent that like I don't even know what it means to say that the present exists. Because if you were to sort of pause time, we're in a time slice, and you ask me, what's the present? I would say whatever visuals I experienced in that time slice and whatever noise is in my ear in that time slice. But those are actually products of the past. The actual things happen before you're conscious.

And even if you say, okay, but it's the vibration caused by the thing you're looking at. Yeah, but even that has to be processed through the brain. And then I can't even imagine what it would be like in a time slice because my brain wouldn't function if there were no time. And so I just don't know what it means to say the present. And I don't know what the difference is between now and now, especially the difference between those two things now. I don't know what the difference between those two things is.

Like, I have no idea what this thing is that we're going through. And so I suppose what I'm interested in is, as a physicist, is this kind of weird, almost philosophical problem of just being able to figure out what it is that we're dealing with, a prerequisite of doing a physics of time, or like with quantum mechanics, can we kind of figure out ways to describe time and use it in our mathematical calculations whilst shelving the weirdness of not really knowing what the present is and whether the past exists? I think there are similarities. I think, you know, in the same way that, as you say, we can use quantum mechanics and go a very long way in describing a lot of physics and chemistry

without worrying about whether there are parallel realities or not. Yes, we can do a lot in physics involving time and the properties of time without being able to answer questions about what is now. We can certainly talk about events taking place and we can talk about intervals between events and how long that interval is, maybe something that we don't have different observers who are, for example, sensing different gravitational fields or moving very fast relatives of each other, won't agree on. We can order events.

We can talk about light cones and events having a future light cone, all events that it could possibly have caused or influenced and past light cone, all events that could have possibly influenced or caused it. We can go a long way in physics in using time to understand reality. I'm not as frustrated about not getting to the essence of what time is as I am about not getting to the essence of what the origin of quantum weirdness is. I want to be able to know for someone to discover the correct interpretation of quantum mechanics, which I believe it should be. Because I'm a quantum realist, I believe there's an objective reality out there, and I therefore believe, There's a correct way of describing how nature does things, regardless of our plethora of different interpretations.

So I'd like to be able, before I die, to know what is actually going on in the quantum world. We may never get an answer, but I'd like to. In the same way, I'm not so worried. It doesn't keep me up at night thinking, what is time, what is now, and so on. Maybe because I'm sort of reconciling myself to the idea that we are so embedded within time that we're never going to be able to. understand it, although it's, it's a, it's a challenge that we should take on and continue to work on. Yeah, yeah. How much time do we have for time? That's the question. You used a phrase earlier, which is used all the time, there it is again, the so-called arrow. Yes. Of time, which as far as

I'm aware, refers to the fact that time seems to have a direction. But beyond that, like, what is this arrow of time? Why is it mysterious? Where does it crop up as a concept? It crops up in thermodynamics that developed and statistical mechanics developed by people like Maxwell and Boltzmann in the 19th century and others, where they show that things happen a certain direction in time in a way that they don't happen in the opposite direction. And it's normally associated with the increase in disorder, what we call entropy. You take a pack of cars that's unshuffled and you shuffle it and it will become more mixed up. won't unshuffle itself over time.

So it's statistical inevitability that things move in a certain direction. Boltzmann talks about having molecules of gas in a box, all sort of congregating in one corner of the box, over time they will spread out. If you spray an aerosol can, you have molecules just by the aerosol can, and if you give it some time, they'll spread across the room. But what you don't see is these molecules spread across the room coming back and then finding their way into the nozzle of the aerosolcan. randomly coalescing under that lamp over there. And the basic idea there is that because it's highly improbable. It's not that's impossible. Yeah.

It's just highly, highly improbable that will happen. So there's a directionality. We can also think about it in terms of not just moving from order to disorder, but from moving from a system being away from equilibrium, moving towards equilibrium, towards thermodynamic equilibrium, you know, when everything is, you know, batteries run out, we get older. balls roll down hills and so on and so on. That all has a directionality. The problem is that all our fundamental laws of physics, our equations of physics that describe how things change,

are all symmetric in time. You can crank the handle one way and starting from a particular moment, you can say, what happens if I evolve this system according to this equation into the future? It'll arrive at some other. The system will be in a different state. What if I crank the handle backwards and run it an equal length of time into the past, it will also evolve to the identical state that it would evolve to into the future. So time is symmetric in both directions. So the big mystery is where does this irreversibility of time come about that we see all around us? The way we get around it is to say that, well, you have to start with the system in a special state

that's off equilibrium, an unshuffled pack of cards, molecules in the corner of a room, and if you move forward in time, entropy increases. Yeah, but what about when you move backwards in time also increases? Well, what if that special moment

we shove it all the way back and stick it at the Big Bang? That's the first moment in time. Now, all we have is forward motions. We only ever see entropy increases. There was no time before the Big Bang, according to standard theories of cosmology, and therefore you don't have to worry about the time symmetry. So this is what's called the past hypothesis,

and a lot of cosmologists are perfectly happy with that. For them, that has solved the problem of getting irreversibility from time symmetric laws of physics. A few questions come to mind for me, right? And it's all got to do with the fact that the laws of thermodynamics in this degree are, as you said, probabilistic. That is, I mean, you can imagine if I spray this aerosol can, you can imagine, say there were just a hundred

molecules that come out of the aerosol can, and they are all randomly vibrating in different directions. They bump into each other sometimes that randomly vibrating. Just because there are a hundred of them, if you give it enough time, even though it's all random,

they will, on average, just spread out and go off in their different directions. In principle, they could all, very low chance, they could all happen to vibrate in the same direction, and all of them just go one way as a group. It's incredibly unlikely. Could happen in principle.

Same thing in the real world, with the actual number of molecules there are. Like they spread across the room because they're all vibrating and fluttering around, but in principle it would be possible for them all to go one way rather than the other. So this law, quote unquote, that entropy always increases, is actually just a really, really strong probabilistic prediction, right? Yeah. So, I suppose one question that comes to mind is, if this is what we're using to get rid of the problem that our physical laws work one way and work the other, if I were to actually, like, understand the position of all of those molecules and the way in which they were vibrating in principle, and I described, you know, their trajectory coming out of the aerosolcan into the room, can I not also just reverse that? once I'm actually certain of exactly how everything's vibrating and I get rid of the random element,

if I reverse that equation, wouldn't that also work in reverse and therefore, like, you know, the molecules would all shoot back up into the, into the can, if you know what I'm getting at? Yeah, I mean, so this is, some people who, so the physicist Carlo Rovelli talks about this. He refers to it as, you know, when you're not, when you don't see all the details of the motions of the individual molecules, you have this blurred vision, this myopic view of this system. And so you can't tell apart all the different arrangements of the molecules of gas in a box when it's in thermal equilibrium. You know, it all looks pretty much the same.

But you can tell if all the molecules are up in one corner. So that's a special state that you can distinguish from thermal equilibrium. zooming in and knowing the direction of all the molecules, yes, any arrangement is as likely as any other. It's just that there are a lot more ways of arranging the molecules of the aerosol spread out as a high entropy. There's more ways of arranging it and you can work them all out and one can move to it and you can calculate. it's just as likely to go to this as to that. It's just as likely to go from a particular arrangement spread out to all of them congregating at the nozzle of the aerosol can

as it is for them to all spread out in a different way. Okay. But all the different ways of spreading out, there are many, many, many of them. And we tend to congregate them all together and say those are all the indistinguishable macrosestates within that macroscate that we call a state we call thermal equilibrium. So it's still, you don't get away from this idea that congregating at the nozzle is an unlikely special type of state.

Yeah, see, because it's satisfying to me that we have this one thing at least that works one way in time versus the other, but it's the one so-called law that is like probabilistic, that isn't like, because I guess what I'm asking is if all physical laws are reversible and you say well that's a problem and the way we solve that is by referring to thermodynamics which isn't actually a law it's not actually if p then q it's like this is probably going to happen it seems to me like if i like reversed all of the in fact laws that were governing you know this entropic increase i could in principle just reverse that too well i would argue that yes it's probabilistic in the sense that entropy increasing in this probabilistic view you

view, Boltzman entropy, is what we'd call statistical inevitability. It's much more likely to it. For me, it's a law that says it's much more likely to go in one direction than another. There is an arrow of time. The fact that there is a small possibility that points in the other direction, there's an imbalance. And for me, that imbalance is a directionality. It's not an absolute. It can never go back. Much more likely to go in this direction than that direction. that's where I'm pointing. I'm pointing in the direction of more likely. Gotcha.

And that's the arrow. Now, I have something I talk about in my book. It doesn't come out until next year, but I spent ages trying to get my head around this. I would wonder whether this fundamental arrow of time is somehow baked into the universe. There is a direction at his time. And then you might ask,

yeah, but how about all these, fundamental underlying time symmetric equations of physics and laws of physics, well, the thing is they only ever apply to isolated systems, to systems that are not interacting with their surroundings. So for an isolated system, yes, everything is time symmetric. And sure, if it's away from equilibrium, it moves towards equilibrium, but equally you could run it time backwards and it'll move towards equilibrium as well. Right. But, All these, the time symmetry only applies to isolated systems. Our universe, everything in our universe, apart from the universe itself, is not an isolated system. It's an open system where entropy will always increase because systems are interacting with their surroundings. So it may be that directionality is built in at a fundamental level, which means that time itself must be built in at a fundamental level to reality. And it's time, time, so it's not how do you get from time symmetric equations to irreversible arrows of time, but how do you get from an irreversible arrow of time to time symmetric equations?

Well, you're isolated. Isolation. Stick a pendulum in a box in a vacuum. It'll carry on swinging forever in principle. And if you ran that video backwards, you wouldn't tell whether, you wouldn't know that it was running backwards because it's time symmetric. Open it up to the air and friction will cause it to slow down and dampen down. Suddenly it's an open system, suddenly there's directionality. Yeah.

So it's like a problem of restricted scope, I suppose. Yeah. Okay, that's interesting. And I think I can make some sense of that, and I am trying my best here. One other question that came to mind was that we've sort of spoken about two areas in which probability

rears its ugly head. One is time. We talk about time, and we say defining quality of time for a physicist, thermodynamics and thermodynamics is actually probabilistic. Another area where probability famously comes up all the time and perhaps unavoidably is in quantum mechanics. And both of those are dealing with like the activities of very small little things.

Is there some connection to be drawn from the fact that quantum mechanics seems to have an inbuilt probability, at least in certain interpretations like the Copenhagen interpretation, seems to have this probabilistic element, but that we can use to make sort of calculation, but is ultimately probabilistic, and that probability is baked into the only thing we can use to measure time is moving in one direction versus the other, even though, like, on a macroscopic level, we can treat it like a law.

Yeah, the entropy always increases. Even though we can always treat quantum mechanics on the macro level like a law, you know, atoms act in this way, is there some connection between the probabilistic nature of quantum mechanics and the probabilistic nature of entropy and dime? Or is that just like two completely different isolations? related, you know, croppings up of probability? I'm not sure.

I mean, one of the things that I'm interested in looking at is the increase in entropy down at the quantum level. So when a quantum system interacts with its surrounding environment, it becomes increasingly entangled with its environment, and decoherence can set in if, you know, if the environment has macroscopically distinguished. we say, state, so you can tell them apart. So there is a directionality that just comes from quantum mechanics. But I think the probabilistic nature of quantum mechanics itself, of the quantum wave function, is deeper.

The Copenhagen view, by the way, doesn't, interpretation, doesn't have this built-in understanding of what probability is. That's simply what's called the Bourne postulate is, you know, here's the Schroding equation. and here's how we explain what the wave function in quantum mechanics is and this is how probabilities come about so it's like a recipe that was put on top

of the mathematics of quantum mechanics the realist interpretations try very hard to understand where this probabilistic nature of quantum mechanics comes from the one that many people like is the many worlds Everettian interpretation

but that many physicists argue is still struggling to understand where probabilities come from. If there's two events, sorry, if a quantum particle can do two things, one with a likelihood of 99% and one with 1%. It can either go left with 99% probability and right with 1% probability.

In the many world interpretation, universe branches out. And in one universe, we see it going along the path that had 99% probability. In the other universe, we see it going along the 1% probability branch. Yet both universes exist, one might argue, with equal likelihood. They're both definitely there. So where do the probabilities come in?

And a number of physicists are working on this, and there are ways of choice. But it's still a struggle. In other views like Birmingham mechanics, probability comes in simply because our inability to accurately enough measure the quantum properties of a system. Right. It's like a practical. Yes, it's actually a practical problem of, of, you know, like in, the butterfly effect. Yeah.

In chaos theory that, you know, you can't know the details, you need to know the details with infinite accuracy. Yeah. And so there's a built-in uncertainty that we can never get rid of. And that's how these probabilities come. But it's in, it is in fact a practical thing in the same way that like, if I were to throw a tennis ball, practically, if I was trying to work out where it landed, I'd have to sort of give a rough estimate.

Yes. Yeah, you could in principle work out exactly where it went, but practically you're not going to be able to do it. But if you want to know what it's going to do next into the future, the further in the future you want to predict how the system will evolve, the more accurately you need to know its present states and more decimal places. Right. At some point, at some point it becomes impossible to predict.

Yeah. And when you talked about the 99% going left and 1% going right, I instantly thought, okay, many worlds, there are 99 worlds in which it goes left and one which it goes right. But that's not, that's not correct, is it? No. Because I just wanted to say that in case anybody else thought that. It's not that there are 99 realities in which it goes left and one in which it goes right. It either goes left or right.

Or you might as well say, if that's the case, they might as well say 10 goes right and 9. Yeah, yeah, yeah. It's like there are two options right or left. And yet if there's a differential probability, the many worlds interpretation might be correct that there are two branching realities, but it hasn't accounted for why one is more. likely than the other, or what that even means if both exist. If both exist with equal certainty, yeah, where do the probabilities go? Man. Okay, one more question for you, which is this. Could time be an emergent property of the universe? And if so, what does that mean? What is an emergent

property? It's possible, yes, and I think a number of people working in the foundations of physics are starting to talk in that language. Emergence is a, is a quite a comprehensive. complicated thing to describe. The simplest way of describing emergence is, well, for consciousness is an emergent property. You know, it emerges once the neuronal connections in the brain become complex enough and the system become complex enough. A much simpler temperature be another example. Temperatures are a very good example. Temperature itself doesn't exist when you get down to the molecular level. It's just vibrations of atoms and molecules. Zoom out and you've got something called temperature.

The wetness of water is another one. Sure. You'd never appreciate wetness of water, however much you study a single molecule of H2O. You need trillions of them together for that property to emerge. The sponginess of a cake, perhaps, you know? Yeah. None of the ingredients or the atoms themselves are spongy.

Exactly. Put them together and zoom out and you get sponginess. So the idea is that time itself doesn't exist at a fundamental level. It's only when you zoom out that gradually it emerges. that it emerges from something more fundamental. What that something more fundamental is, I think, is still something that we haven't understood yet. I mean, a lot of physicists have tried. There's a famous equation called the Wheeler-Dewitt equation, which starts from more fundamental equations and relativity,

and it's an equation that doesn't have time at all in it. So it's a fundamental feature of reality. Time doesn't exist. So the fact that we perceive things changing in time must be an emergent property of the universe if the wheeler-do-wit equation is more fundamental. Interesting. I'm not sure. I'm not saying that I disagree with that. I'm yet to be convinced that time is emergent rather than fundamental.

I asked earlier if the relevance of time is important to the incompatibility between relativity and quantum mechanics because I've heard it discuss. in some contexts that, and like I said, I'm not a physicist, I might be wrong about this, but one of the problems might be the way that they both sort of deal with time. Quantum mechanics might require instantaneous, like, you know, the present is real and it's the only thing that exists, whereas relativity has this, this time block, this time traveling. If time is immersion, maybe it's just that like the quantum world acts at a small enough level that time hasn't emerged yet. And so it makes sense to say that, well, time functions this way on the quantum level, but once you're talking about general relativity, you're big enough that you have

this emergent quality of time, which is a property of space time. And I sort of wondered if having time as an emergent property at some higher level of complexity of atoms might help us to solve part of the problem as to why the big doesn't mesh with the small. Yeah, maybe. I mean, that would suggest that quantum mechanics is a deeper level of reality, right, where you know, where time has a more fundamental or there's some fundamental feature of the quantum world that isn't the time that we perceive, and that general relativity and how it sees time as part of 4D space time is emergent from a deeper quantum level. It may be. I'm not sure. I mean, all the research into finding a theory of quantum gravity, a theory of everything, there's still the debate. Do we start from quantum mechanics or more correctly quantum field theory? and do we move towards general relativity or do we, you know, so that's what, you know, string theorists would say, or do we start from general relativity, the space time is fundamental and we quantize it. That's what people are working in loop quantum gravity.

Yeah. So do you start from this end and move towards that end, all that, or do you start from both the meat in the middle, or do you scrap them all together and come up with something new? I mean, that's where we really are at with finding a theory of everything, that we don't know what our starting point is that we can say, well, let's start from here. We know this is a correct description of reality. Let's figure out how to get to the other end. Yeah. Oh, man, that's amazing. It's like, I feel like I feel like I'm sort of auditing physics because everybody keeps hearing about this theory of everything. It's like, you know, someone's told you they're going to build a grand palace or a new hotel or something and you come in and you speak to the building

and it's like, okay, how's it going? How are we getting on? And they're like, I hate to tell you this. We haven't even agreed on how to start. I haven't done the foundations. We'll get there eventually, I'm sure. And in part, that will be thanks to people like you and works like yours. Fourthcoming, it will all be in the description, all the information about the upcoming book about time and also some of the other works that we've already mentioned. Jim Alkalili, thanks so much for taking the time. It's been fun. It has been fun. Thank you.