Wormholes
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Hello. In 1957, the American physicist John Wheeler coined the term wormhole, which we understand now as a potential shortcut between two points across the universe. It's an idea that sounds like science fiction alone, and it may well be, where wormholes offer a way to move between distant galaxies and even distant times.

Yet as scientists test whether wormholes could exist, they also test the laws of physics, in which those applying to the huge can differ so much from those of the small, in case, as Einstein hoped, one unifying law might apply across both.

With me to discuss the ideas around wormholes are Andrew Ponson, Professor of Cosmology at Durham University, Katie Clough, Senior Lecturer in Mathematics at Queen Mary University of London, and Toby Wiseman, Professor of Theoretical Physics at Imperial College London. Toby Wiseman, before we explore wormholes, can you give us some idea of how large the universe is and therefore why shortcuts might be desirable? Indeed I can. It is extremely large.

The starting point for thinking about how large it is is to think about how old it is. And as we understand it, it's about 14 billion years old.

So the universe started with a Big Bang, which is where we understand time and space originated as we see them today, 14 billion years ago. It's been expanding ever since. And the furthest out we can see in our universe is really the distance that light has traveled in that 14 billion years.

So when we look at the furthest objects away from us, we're looking back in time and the furthest we can see are the objects that were very early on after the Big Bang.

And roughly speaking, when you do the calculations, that's something like 14 or of order 14 billion light years away. And for some context, a light year is an extremely long distance. So it takes eight minutes for light to reach us from the sun. So that's eight light minutes to the nearest star other than the sun. It's about four years. So four light years.

So the universe is roughly tens of billions of light years across. So huge. So Toby, where's the wormhole supposed to be in the middle of all this? So it really depends on how big wormholes are. If they're very small, they might be around us. If they're microscopic, you know, subatomic in scale, there might be wormholes passing through this room now undetected. Or

On the other hand, if they were really large, large enough that they're in the realm of science fiction, they're probably very, very far distant from us, perhaps in our galaxy somewhere, but perhaps in distant galaxies, perhaps billions of light years away. If they were too close, certainly there are none in our solar system. Otherwise, I suspect we would have seen them and the dramatic effects that would be seen around them. Can you talk about what we hope to find outside? You've used the...

14 billion years, but outside the universe, how big is that? Well, that's an excellent question, and we don't honestly know the answer. So we can see approximately tens of billions of light years, and because the universe is only 14 billion years old, we can't see any further than that. Light simply hasn't had time to reach us.

Now, the universe, of course, we think goes on well beyond what we can see. And in the modern theories of cosmology, particularly what we call inflationary cosmology, we think it's massively bigger than what we're actually able to see.

But we just don't know. We can't see it. And actually, because in recent years we've understood that the universe is accelerating in its expansion, what people call dark energy is driving an acceleration, actually it looks like we're probably limited in how much we'll ever see.

So the sort of objects that we see most distantly today, we'll see a little bit more if we waited a few billion years, not us particularly, but maybe our future generations may see slightly more, but not that much more. It may be that we're limited and stuck in what we can see, unless, of course, there's some way to travel, say wormholes that might allow us to go even further.

The numbers here are huge, unimaginable really. They're huge anyway. The universe being a billion, billion, billion light years. I just think, what does that mean? What do you think it means? So yes, I mean, 50 billion light years, which is how far it is. If you put that in metres, it's something like 1 followed by 26 zeros.

and then meters, so many, many meters. It's just vast. It's difficult to imagine. I'm not very good at imagining things, to be completely honest, and fortunately, because of the mathematical description we have of the universe, we don't have to...

be able to sort of comprehend those numbers in order to understand the physics and understand our observations. But it is mind-boggling. I mean, it's impossible to comprehend. I defy anyone to. Andrew, Andrew Ponson. As we'll hear, the idea of wormholes was born from questions about the unimaginably large and small. What do we need to know about Newton here?

Well, Newton in many respects is a foundation stone for how we understand physics today. And of course, he was working at a very different time in the mid 17th century when the whole enterprise of science or natural philosophy was quite young.

but he set a sort of pattern in play, one that we're still working with today, which is the idea of unification. So instead of trying to explain all different things that we see with completely different explanations, Newton was one of the first great scientists to really try and simplify and explain very complicated big things in the simplest possible way.

And specifically what he looked at was what we now know as gravity. And the unification he was able to make is between the phenomena that we experience here on Earth, where if I drop something right here, it'll clang on the floor...

and the movement of planets in the heavens, which on the face of it are two completely different things. You know, Johannes Kepler had studied the movement of planets extremely accurately. It was known how they move around in the sky. But the idea that this could be explained by the same basic idea as gravity here on Earth was a really remarkable leap of the imagination, if you like.

But it turns out to be, to a large degree, correct. However, despite the fact that that is something that we still rely upon, this idea of unification, and we still strive for today, the details of what Newton proposed are not quite right.

And it's not his fault at all. He just didn't have access to the kind of knowledge and mathematics that we have access to today. But in particular, the way he conceived of gravity just doesn't really fit with developments that came later. Why not? Well, in particular, there were developments, if you fast forward to, say, the mid-19th century, seeing how electricity and magnetism, other forces, behave differently.

And the work of people like Faraday and Maxwell also unified a lot of these phenomena so that magnetism and electricity and even light came to be described in a unified way, in much the same way as Newton wanted to do.

But it gradually became clear by the end of the 19th century and beginning of the 20th century that the unification of electricity and magnetism and light was not compatible with Newton's unification of gravity and the way that he was describing space and time within that unification. And the idea of wormholes emerged from Einstein's work trying to reconcile the large and the small, right?

What was that work? Yeah, so he was trying to reconcile electromagnetism. So that by the end of the 19th century was really very well understood. And it was understood that electricity and magnetism and light are all different parts of the same unifying idea. But he was trying to unify that with Newton's conception of space and time.

And Newton had the idea that space and time are a sort of arena in which interesting things happen. So they're a sort of fixed background, almost like a grid, if you imagine a sort of grid being laid out on a page in a maths lesson or something. That was Newton's conception of space and time. They're a grid-like structure.

And it began to be realised by Einstein, but by other people as well. There were other people working on this as well, that this didn't work. You couldn't simultaneously make sense of electromagnetism and retain this very fixed, rigid structure for what we understood by space and time. Does this bring in his theory of general relativity? Yes.

Yes. And how does that work? So in the end, the culmination of this work was Einstein's theory of general relativity, which really, in a nutshell, there are two big differences compared to Newton's vision. One is that space and time need to be brought together. So actually, this is another unification that what we think of in our everyday experience as space and what we think of as time, which seemingly two very different phenomena exist,

are actually part of the same thing, a unified idea called space-time. And if you try to understand one in the absence of the other, you're only led to contradictions. So that was one part of the idea. The other part of the idea is that once you can understand space and time together, you also need to understand that they can be distorted. So unlike Newton's idea of a fixed grid...

You now have something much more elastic, much more dynamic and alive and sort of taking part in the universe rather than just being an arena against which interesting things happen. Thank you very much. Katie, Katie Glover, one of the challenges is so much is unpredictable. Why is that?

So one of the main challenges that we still have is another unification, linking to what Andrew was saying, is to try and unify quantum physics and classical physics. In classical physics, this is the kind of physics that we're mostly familiar with. So if I throw a ball and I know everything about the ball and how I threw it, it will follow a particular path and it will end up in a particular place.

And if I throw another ball in the same way, the same ball in the same way, it will always take the same path and end up in the same place. And Einstein's theory of general relativity is like that. It's classical. So when we talk about planets moving, black holes moving, they follow particular paths through space. But the quantum picture is not like this. So in quantum physics, things are different. If I

fire an electron across the room, it can actually take all possible paths. So there's some very small probability of it flying to the moon and back again. It's just really tiny. And actually, we only know where it's ended up when we make a measurement of that particle. So when we measure the particle, we find out where it's gone. But if I repeat the experiment and I keep firing electrons across the room, then unlike with the ball, every time I take the measurement, I'll find it in a different place.

And so what the quantum theories predict is the probability of making a measurement of the position of that particle and not the particular path that it will take. And so the real difficulty is in how to unify these pictures. So we actually think that the quantum picture, although in some ways it sounds...

you know, it sounds more far-fetched. It's actually the more fundamental one. And the classical picture is something that's just an approximate description of nature for sort of larger objects. So the challenge for general relativity is really that that should be some kind of approximation, some classical approximation of some quantum theory. And we have no idea what that quantum theory is.

And Einstein linked up with Rosen, another physicist, and he set their hair running on the idea of wormholes.

That's right. So yeah, so actually Einstein really didn't like this idea of quantum physics. He didn't like this idea that we couldn't predict precisely where we were going to find the particle. And other ideas about entanglement of particles, that somehow particles can, a measurement of a particle in one place can affect the measurement of a particle somewhere a long way away. So he and his colleague Rosen came up with this idea that they called bridges between sheets.

and that we would now recognise that as being wormholes. But their idea wasn't really to sort of travel to distant parts of the galaxy. They had in mind that these would be particles. So this would somehow explain particles in a classical way. And...

these Einstein-Rosen bridges that they constructed. It was a really nice idea, but it actually didn't really work. So we now know that particles are not small wormholes or small black holes. But nevertheless, it was a kind of an idea that gave birth to a lot of this idea of wormhole physics. And the name Einstein-Rosen bridge is now given to the wormhole that you find in black hole space times. So there's a relationship between black holes and wormholes.

So it's actually, if you take the full mathematical solution for a black hole, you find that there's a wormhole in it. So it's not a wormhole that you can travel through. The full solution for a black hole actually is that the black hole connects two separate universes.

And so you can't go through the black hole to the other universe. But what you could potentially do is two people, one from each universe, could jump into the black hole and then they could exchange information. It would be a one-way trip for them. So they would end up at the singularity of the black hole and they would be torn apart by tidal forces. So it would be... You probably wouldn't get many volunteers for people to do it. But in principle, you know, you could...

have two people jump in and be able to communicate with each other. So in that sense, it is a bridge, you know, it is a wormhole between two separate parts of potentially the same universe or different universes.

Can you just graphically describe exactly what you imagine when you use the word wormhole? So I think we often have a picture in mind of a wormhole in two dimensions. So we usually think of it as being a sort of a sheet that's a funnel. And then the other end of the funnel connects to another sheet. And

And so when we think of going through the wormhole, we start on one of the flat sheets and then we move through this funnel and we come out on the other sheet. And that's a two-dimensional representation of the four-dimensional space-time. So I think in terms of how they would look, they wouldn't look like that, but that's the picture that I always have in my mind of wormholes. You've talked about billions here and there. Yes.

How long are these wormholes thought to be? Well, in principle, they could be as long as you like. There's no sort of specific length for it. For it to be useful as a way to get from one part of the universe to the other, you would like it to be short. So the kind of distances that Toby was talking about, you know, we don't want to travel those distances. We want to be able to take a shortcut.

And I think that's the interesting thing with wormholes is there's no reason that the tube has to be as long as the long way round. You know, the two ends can be separated by billions of light years, but the tube itself can be even just a light year long, say, or even shorter. You see these tubes like arteries joining bits of the universe that we can't see and can't even ascertain are truly there. Yeah.

And they're finding comparisons galactically separate from each other. Yeah, that's it. That sounds like a project, doesn't it? Yeah. Toby, can I come back to you? Maybe one thing that's of relevance here is, and I don't want to be a party pooper, but in Einstein's equations of general relativity, which are, as Andrew was saying, the modern way we understand gravity...

So Einstein says matter bends space and time around it. And it's the bending of that space-time that we then perceive to give a force of gravity as we move through that bent space-time. That replaces Newton's idea of there being a force. But one of the important things about Einstein's understanding of space-time is he wrote down this amazing equation that links how space and time is bent...

to the matter within it. What does that mean? What do you mean to the matter within it? So when there's matter, his equation tells you that space and time bends, and that's what gives rise to gravity. So for example, the Earth, if we put that into Einstein's equations, will bend space and time around it, and that's what gives us the perception of a gravitational field due to the Earth. That's why we're standing on the surface rather than floating.

However, Einstein's equations are really tricky to solve. So one of the... You can put any spacetime you like into his equations and his equations will tell you there's some matter that will support that bending of spacetime. So you can put your craziest spacetime in that you like.

But then we have to face the fact that matter has to obey its own laws, in particular quantum mechanics, as Katie was saying. And quantum mechanics doesn't allow, you know, it's pretty exotic, but it also has some

some reasonableness to it. You can't do anything. So the real challenge with Einstein's understanding of space-time, particularly when we start thinking about wormholes, is that you can write down these terribly exotic-sounding space-times that connect one very distant region to another through a peculiar space-time tunnel, as Katie was saying. But there has to exist some matter in our universe that will support that solution, that will drive that curvature.

And the sort of matter that you need for wormholes is, as you might have guessed, extremely exotic. And so the real conundrum in this area is, does there exist matter that could support these sort of wormholes of science fiction or not? Thank you. Can we take a step back, Andrew, to John Wheeler, who coined the term wormholes? How did he arrive at coining it?

Well, John Wheeler is a fascinating character and had an enormous impact on theoretical physics in the mid-20th century. And one of his many talents was to come up with great names for things. And so, I mean, he's credited with

popularising the term black holes, for example. I don't think he actually came up with that one. But once you're sort of into the idea of a black hole, which is the idea that there can be a bit of space that is sort of sucking in things in such an extreme way. Yes, a collapsed star. And it sucks things in in such an extreme way that nothing can get out, not even light.

And so the word black hole sounds pretty good for that. It's probably a fairly small jump to go from there and say, well, if it can go in and then come out somewhere else, well, it's a bit like a wormhole. I mean, John Wheeler was just full of sort of enthusiasm for these very exotic ideas. It had actually come from a deeply practical interest in nuclear fusion. Like many American theoretical physicists of the time,

He was deeply involved with the project to build nuclear weapons. But when he sort of emerged from that, many of those people started turning their attention to astrophysical questions, which is a more natural step than it might at first seem, because stars...

are nature's fusion reactors. They are giant nuclear reactors out there in space. And so it was quite natural for people emerging from this programme to start thinking about how fusion, how nuclear reactions proceed in space and what happens when you run out of fuel. So this is how he got interested in all of these topics. But he actually ended up picking up the baton from Einstein and Rosen, the kind of stuff that Katie was describing. You know,

Einstein and Rosen were trying to explain reality, essentially, to try and unify our ideas of what matter is with Einstein's older ideas about what space-time is and how gravity works. And in effect, John Wheeler ended up picking up that programme and attempting to do something a little bit similar in the wake of Einstein's death.

It ultimately turned out to be an unsuccessful programme amongst many very successful things that Wheeler did. But he did leave a lot of very inspired people. And so although the programme he was pursuing was perhaps not quite on the right lines, its inspiration lives on. And one of the ideas that came out of it that really flourished was this idea of wormholes and what they might teach us about what do these equations really mean?

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Well, I'm going to turn to you, Katie, for this. What evidence is there that these wormholes do exist? Well, this could be a very short answer because, in fact, there's no evidence that they exist.

However, there is some serious work, depending on how you look at it, serious or not serious work, to try and understand whether we could observe wormholes. So the real test for proving something exists is that we observe it. And

Actually, wormholes, it depends a little bit on what wormhole you're talking about. So there are different models depending on if they're made of different things and how they've been constructed. But wormholes can look a lot like a black hole from the outside. So if they look a bit like a black hole from the outside, how would they appear in our observations?

And actually, we're in a kind of golden age of gravitational observation. We've been taking pictures of the supermassive black hole at the centre of our galaxy, and we've been receiving gravitational wave signals that we're now able to detect in

from black holes that are in spiraling with each other and merging in sort of big collisions of these black holes. And so some people have been asking the question, using these observations, can we tell if these are really black holes or maybe they're actually, you know, one end of a wormhole that's colliding with another wormhole? And these would actually give potentially differences in the signal that we would see, the signal that we detect.

If the wormhole exists as a hole that goes into something at one end and comes out of the other,

What would you suggest it does look like, the universe looks like when it comes out? Yeah, so what you want is you want the wormhole to be not a black hole. So ideally, the throat of the wormhole should be outside any kind of event horizon, because otherwise you're going to get trapped in the event horizon area. So you want it to be somehow bigger than a black hole would be. And then, yeah, then what would happen is you would fly...

fly into the wormhole and you would really come out in another place. So wormholes from the outside would look something like a big soap bubble. So they wouldn't be a tunnel. So as I was saying earlier, in our mind, we sometimes have this image of a sort of funnel, a two-dimensional surface that's a funnel. But in real space, they would actually be a kind of a bubble that you would fly into that bubble

and you would come out of another bubble somewhere else in the universe. But what is the speculation about there's something else you'd come out to?

But that's exactly it. So that's the danger in a way of flying into a wormhole is you don't know where you're going to end up. And you might be very disappointed. So, I mean, you could hope that you would end up in another part of our own galaxy and that would enable you to explore somewhere else. But there's no reason why you might not come out in a completely separate universe. So it's obviously very speculative, but you...

as I say, as you fly in, you kind of don't know where you're going. Although in principle, you could actually see through the wormhole. So light can also travel through the wormhole if you can travel through the wormhole. So you could imagine that you could kind of look through with a telescope and try and see if you recognize the stars there, and maybe you can match them to another part of the galaxy. And then that might give you enough confidence to fly through it.

So the universe you're talking about is just one of many universes. Yeah, so it could be. Yeah, so it could be you go through and you end up in our universe, but it could also be you go through and you end up in a different universe. Can you take that on, Toby? LAUGHTER

I'm a very conservative sort of person. And yeah, I mean, some of the things we've learned in the last sort of 15 years or so, maybe even longer, is that in reality, the sort of matter that you would need for the sort of wormholes that Kate is describing in science fiction that we could actually travel through is

According to what we understand about matter at its sort of most fundamental quantum levels, we don't believe that that sort of matter exists or we have no indication that that sort of matter exists in our universe that could create the sort of tunnels that humans could pass through.

Interestingly, it is the case, and there was serious work a few years ago now, it is the case, rather surprisingly, that theoretically a description of a wormhole was given that was pretty mathematically consistent, actually using the sort of matter that we know does exist in our universe in a very quantum state, so the sort of matter that we know exists by particle physics experiments, for example, and

And it was very surprising. But the big caveat is these wormholes would be absolutely tiny. So subatomic. So there is no sense in which humans could pass through them.

Of course, we don't really know what's out there. We're actually quite limited in our understanding of the wider, you know, the larger scales in our universe. We can't visit them. We can only look at them, peer at them with our various telescopes. And so we don't really understand what's out there. It's likely that there aren't wormholes that we could pass through out there, but I suppose you never know. And according to, if we just take Einstein's theory, Einstein doesn't rule them out.

But no one has seriously tried to create a wormhole in the laboratory. I think, too, one of the challenges would be, you know, if we think about a black hole, for example, if we think about a black hole that would form if the sun collapsed, all of the matter in the sun is required to to create a black hole that

that's just a few kilometres across. Now, if we wanted a wormhole that we could safely travel through, it would have to be bigger than us, because otherwise we wouldn't fit. So it would probably need to be something like kilometres across if we were to have any chance to go through it.

And even if the sort of exotic matter existed that could create it, you would need an awful lot. You would need, you know, the sort of amounts of mass that the sun has, I would imagine, to support something like that. So Einstein's theory is really a theory of the macroscopic. To really bend space and time, you need a lot of matter. So, Andrew, what if there is this wormhole?

What do you think it would tell us about the shape of the universe or the universes? Well, you would want to pass through it and find out. I mean, I think the stunning thing about general relativity as a theory is that it allows space and more specifically space and time taken together, space-time together.

to take on such a wide variety of shapes. And so we've been talking in analogies. It's very hard not to talk in analogies because it's so hard to visualize this in any concrete way. But, you know, the way to think about this would be if you imagine our universe as being somehow a sheet of paper...

And in fact, when we take measurements of our universe out on very large scales, it does appear to be flat in that sense. So we don't have any current sense that the universe as a whole has a large scale curvature to it. So on large scales, the universe does appear to be flat. However, what the presence of a wormhole would indicate is

is that it is nonetheless possible to imagine this as a flat sheet of paper which you then imagine putting a fold into. So in some sense space itself is still flat because the structure of the flat paper still exists. But seen from another perspective, it's kind of folded back on itself again.

And then, I think Katie was talking about this earlier on, you can then imagine creating a funnel that goes from one side of the sheet to the other. And now you can kind of visualise that while the overall structure of space hasn't really been altered by this, that it still looks like a sheet of paper. If you were an ant crawling around on this sheet of paper, you wouldn't particularly notice that it had been folded. But

But just right around the wormhole, suddenly something... You can really tell something is very strange about the structure of the space in that very particular area.

So traversing a wormhole, it would bring you out somewhere else. And then you would know that there is this complexity in what we sometimes call the topology of the space. So there's a sort of added complexity to our universe that at the moment we don't have any evidence is actually there. What revelations are going to come from this if something happens in the way that you're indicating had to happen?

If we actually found evidence for a wormhole really existing in our universe, it would be totally transformative in many different ways. But one of the ways is that it would teach us something about the nature of matter. As I think we've already alluded to, the very existence of a wormhole, especially on large scales, directly tells you about what kind of matter, what kind of materials exist.

can exist in nature. And that link, by the way, was brought out by a really fascinating Indian physicist called Ray Chowdhury. He approached relativity in a very different way to how people had before. He was working in the 1950s and he very directly showed that certain behaviours, like, for instance, being able to go into something like a black hole and then turn around and come out through another hole,

very directly imply the existence of very exotic materials. It was kind of startling at the time because it so directly links the behaviour of things. He was actually thinking about the universe as a whole, but it equally applies to wormholes. Very directly links that behaviour to the kinds of materials that

that have to be there supporting that structure. So it would very directly tell us that these very exotic materials exist, but I think it would also be a revelation in terms of just, you know, our ability to think about future space projects and so on. But, I mean, I have to say I'm also very sceptical that we would ever actually discover one of these things on large scales in the real universe.

Well over to you, Katie. What do you learn from computer simulations? I think Toby mentioned, and people always say this, that the equations of general relativity are really complicated to solve. So actually to solve them, we often need computers. And this is something that's taken a big role recently with these observations of gravitational waves from black holes merging. Because to calculate the type of signals that we would see, we need to do these computer simulations.

So in the context of wormholes, when I was saying about, you know, we want to look for evidence of wormholes, we can do the same thing where we can look at what signals we would get if these wormholes were merging, and then potentially we could go and look in the data and see if they're there.

But we can also use simulations as kind of an extension to our thought experiments about gravity. So gravity and general relativity are full of these kind of thought experiments where we think of things that maybe we physically don't think that they're possible, but we explore things

the idea of them happening and we think about where that would take us. And wormholes is a nice example where we can think about doing simulations and exploring the consequences of different models of exotic matter, for example. So one nice thing about computer simulations is that they don't care about

matter being exotic or not, you just tell it how the matter behaves and it will go and simulate it. So we can imagine that, okay, even if we can't create this in a laboratory, if we can create a simulation of it, how will it behave? And what kind of features will it have? Will it be stable? Will it collapse? And I think this is quite an exciting way to extend what we know.

Well, just to take one step back for a second, Toby... Well, assuming that we discover some exotic form of matter that supports these things... Can you explain what you mean by exotic? Exotic, well, when you start writing down these space times that would have wormholes in them, and you put that into Einstein's equations...

Einstein's equations tell you that the matter has to have negative energy. What does that mean? So, well, energy in some sense is the potential to do stuff to other things. And in classical physics, matter always has positive energy. Negative energy is a very strange concept. It's sort of the ability for things to do stuff to that object. And in classical physics, there isn't matter with negative energies. Energy is always positive, but...

In quantum mechanics, and quantum mechanics, as we've already said, as Katie said, it's a pretty strange theory, and it does allow strange things to happen.

within limits. And one of the things it does allow is negative energies to exist. So there's something very famously discovered in the late 40s called the Casimir effect, which is actually precisely where matter under certain circumstances, where it becomes quite quantum, allows negative energies. And it's precisely this sort of negative energy, which is very exotic from a

has to be a very quantum type of matter, but it's exactly that that was, for example, used in these recent theoretical explorations of wormholes, albeit the microscopic variety, but to allow them to solve the equations. So very exotic matter, but still could exist.

If we allow ourselves sufficiently exotic matter to have big wormholes that actually people could pass through, as Katie was saying, they would look sort of presumably like a portal, a big sphere in space. A little like a black hole would look roughly like a sphere. It would be a round region of space where if you look close to it, everything around it would look extremely distorted.

you would see an extreme distortion of everything behind it, as if you were looking through a very powerful lens. You might even see yourself. So a black hole would suck you in, but assuming wormholes that were useful could exist, they would not necessarily suck you in. I mean, it's very difficult to understand how such a thing could exist without...

destroying you with forces but if some future civilization could could solve that problem with this exotic matter it would then look like a portal you would you would go near it it would look like a sphere which would look extremely distorted and as you were sort of looking into the sphere you would see images coming from the far side of the wormhole but in an extremely distorted form

And provided it was big enough that you could pass through, so probably I'm guessing would have to be kilometers big, you would then pass through what would look like a pretty amazing light display, I think, out, as Katie said, to the other side where you would turn around and you would see behind you this sphere, again, very distorted. And you would then pass into presumably some perhaps distant part of the universe.

Finally, then, what do we learn from possibilities of wormholes existing, Katie? I hope that from this discussion that people can take away from it that we learn a lot, right? I mean, just everything we've discussed, such a lot of rich physics, quantum physics, classical physics, general relativity. We really explore a lot of territory just with this topic of wormholes. So I think we do learn a lot.

And I think it's useful to think about them in this way, sort of challenging our ideas and challenging what we can do. So we don't know the answers in research. You know, we have to make some wrong guesses before we get to the right answer. We can't go straight to the answer. Maybe we have to travel through a few wormholes to get to the right solution. So I think it's OK to work on wormholes.

And it's interesting and it can bring us things as long as we kind of keep our feet a bit on the ground and we're clear about what we're doing and why and about the limitations of what we're doing. What position do you take on this, Andrew?

I mean, I violently agree with what Katie said. But I think what's so interesting about it is that if you look at the history of physics, it is a history of being very bold and not being afraid to tackle these seemingly crazy ideas. And, you know, relativity, even the bits of relativity that are very well verified, are

has very strange consequences, things that to us seem very counterintuitive and yet experiments show that they are true. For example, the time travel into the future, you can actually show experimentally that that is going on within experiments. You can construct and test. Quantum physics is another example. It's a very strange world and yet experiments show it's correct. And you get there by a combination of experiment but also bold supposition.

and wormholes, well they certainly are bold supposition and probably they're leading us to an understanding of why they're not possible more than anything else but I think it's good that we're doing it Toby, finally

So I also very much agree. I think one of the most interesting questions is really, as we've discussed, how exotic can matter be? How strange and bizarre is quantum mechanics allowed to let matter and energy be? And we've learned a lot. So whilst...

One might, in the back of one's mind, be thinking about wormholes. The actual many of the questions come down, the sort of hard maths questions come down to what does quantum mechanics restrict matter or how does it restrict matter? And actually a lot of serious progress has been made on that.

And also, you know, what is allowed? How strange can it be? And we've learned really quite a lot about things like the Casimir energy, the strange things that matter can do. And matter, of course, particularly very quantum matter, that could be part of technology in the future. So whilst it may be the sort of almost science fiction, rather vague questions that drive that precise question,

study of matter, that study of matter could lead to all sorts of technology, engineering, you know, many years down the line. Well, thank you very much. Thanks to Katie Clough, Toby Wiseman and Andrew Ponton.

Next week, it's anarchism in the USA, how a bombing in Chicago in 1886 reverberated around the world. That's the Haymarket Affair. Thank you for listening. And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin and his guests. What do you regret not having time to say, Toby? Well, I think...

One of the things that perhaps it's nice to say is just how remarkable our understanding of the universe is, given Einstein's theory of general relativity. I mean, we've talked about how wormholes work.

probably don't exist. But at the same time, maybe it's worth emphasising that perhaps the most exotic phenomenon that we could imagine, which is the Big Bang, where the whole universe was scrunched into sort of infinitesimal size in the past, that really did happen as far as we understand.

And I mean, that's more exotic than anything. That's so unimaginably exotic that I think we don't even bother to try and imagine it. It's difficult. Also, black holes out there, as Katie was saying, you know, we're pretty sure they exist now. We've got amazing observations of them. Mathematically, they've been proven to exist. And these are incredible structures, you know, incredibly complex.

bizarre regions of space-time where they're incredibly curved and if you fall inside you encounter what Katie mentioned before, the singularity of the black hole where space and time are literally sort of torn and you would be crushed to infinitesimal size and all of this

we really do believe is out there. So it's an absolutely remarkable universe, incomprehensible universe really. So the fact that perhaps wormholes don't exist shouldn't dull our wonder. And yet you make calculations about it, you talk about it as if you're talking about going down the street to a shop. What gives you the confidence to do that?

Well, so years and years of experiment versus theoretical understanding of the laws of nature. So...

The history of physics is really one where new theories are proposed, they're tested by experiment. Experiment maybe discovers some defect in the theory, the theory is modified. But where we've landed up is a very interesting place where we have mathematically a description of our universe, which is incredibly elegant and beautiful. And to those who perhaps are not so keen on maths, they may disagree with this, but to those who enjoy mathematics...

These are theories that describe pretty much everything we've ever observed. And you could write them on a T-shirt. In fact, famously, the particle lab called CERN in Geneva has a wonderful T-shirt that has the laws of physics written on the T-shirt.

So they're incredibly simple mathematical structures, which we, as I think Katie and Andrew would agree, we as theoretical physicists agree,

feel have been discovered rather than constructed by us. They've been revealed and uncovered over years. And it's that simplicity and the depth to which they enable us to understand everything we've really observed from the very large to the very small that is what gives us some confidence in these theories. I think the thing that occurs to me when sort of reflecting on this as a topic as a whole is just how rich

physics theories can be. And I mean, of course, we're specifically talking about general relativity here, but I think you can apply it to other theories as well, in the sense that you might imagine, you know, Einstein writes down some equations, what we now call the Einstein field equations, and they're supposed to describe general relativity. And that's it, right? That's the theory. And yet, here we are more than 100 years later,

still trying to understand what the implications of that equation are. And it's very compact. It looks very simple on the page, but

actually tracing through what are the consequences of that and how does it change our understanding of the world then plays out over so far 100 years. And, you know, it's not just Einstein's theory that's like that. Certainly understanding what Newton's account of gravity and more generally the universe meant took hundreds of years. There's still people really, you know, there are still aspects of even Newtonian physics that

that we don't yet fully understand. And certainly there's aspects of electromagnetism that we've discussed. So I think wormholes really bring into sharp focus the way that you can have a theory, and in some sense it's specified, it's written down on the page, and yet there is still so much to understand. Personally, that's why I find physics exciting, so that's why I like this discussion. What about you, Katie?

So I wanted to give a sense of excitement about what's coming in terms of new data for gravitational physics.

So I mentioned that we were living in a kind of golden age of gravitational data, but we're really only at the beginning. So we've been able to observe these gravitational waves from events happening in our universe, but we're really only looking at a very narrow frequency range. So we're sort of in the days compared to electromagnetic observations where we were just looking in visible light through a telescope at one frequency.

And with electromagnetic observations, as we opened new parts of the spectrum, so as we had radio telescopes, X-ray telescopes, we always discover something new about the universe that often we didn't expect.

And that's what we're going to be doing over the next sort of 10, 20 years. We have new detectors being built that will be in space, new detectors, other ground-based detectors that will detect gravitational waves in different frequency ranges. So we're really opening this window on the universe in this messenger that is gravitational waves. And that gives us an opportunity to learn about gravitational physics,

And it's my great hope that when we do this, we will discover something unexpected. You know, maybe a wormhole, maybe not a wormhole. But I really hope that we see something unexpected in these future observations. Going back to the start, you all say that in the first second, in the first minute or so,

more happened than at any other time, more change, more... Did that big bang, as it's been called, did that apply to all the universes, all the universes that you can conceive, or just the universe simply? The one that's nearest us. The one.

I mean, there are different theories. I mean, when it comes on to the multiverse, which is what you're talking about, where there are multiple universes, there are different theories about multiverses. So it really depends on exactly which theory you then home in on. But it's entirely possible that this is a sort of ongoing process. So although we see our universe as being 14 billion years old, give or take,

And, you know, so it had a very definite start at a particular moment. It's entirely possible that there are more universes being born right now. We don't have, as it stands, any evidence that that's the case, but we also don't have any evidence that it's not the case. So it could be an ongoing process on a sort of much larger, even more unimaginable scale. Toby, are you going to join in this? LAUGHTER

Are you sitting back being wise and silent? Well, yes, there's a lot we don't know. I think is probably what's worth emphasising, which exactly as Andrew said, which is why it's so exciting. So we have these gifts that we've been given of understanding of Einstein's beautiful theory, of our understanding of particle physics. And so now our generation of scientists have these amazing things that

but there are still mysteries out there. We don't even understand, for example, the acceleration in our universe, even in our universe. So just, just our universe. Um, and you know, that's for various reasons, particularly, uh,

involving quantum mechanics, that's a very mysterious thing that we don't understand. Some people believe that Einstein's equations are actually wrong on the largest scales in our universe. They do a very good job, for example, for our solar system and they're well tested and they are correct in detail on the sort of solar system scales, but some people believe perhaps dark matter and dark energy suggest even they're wrong.

So there's an awful lot we don't know about our universe. You know, how it, the very, very earliest stages of what happened, we don't really understand. The very largest scales, I think I would argue we don't fully understand. So there's a lot we don't understand. But on the other hand, there's a tremendous amount we do understand. And perhaps the most surprising thing is that it all comes down to some mathematical equations that you can write on a T-shirt.

Do you want to come in there, Katie? Yeah, I would just echo that there's a lot that we don't know. So one sort of speculative idea that's quite fun to think about is the idea that universes are born in black holes. So when you have the singularity of one black hole, so a star collapses, that somehow instead of having a singularity, you actually balloon out into a new universe.

So maybe the sort of the collapse of a star can be the birth of a new universe. And I've always kind of liked that. But I think I like it for sort of purely aesthetic reasons and not for any good scientific reason. But I think, you know, all of these ideas are such fun to think of. And I think these kind of ideas are what has brought us into physics in the first place, often from watching sort of science fiction programs and things like that. So I think we should keep our sense of

joy and wonder in the research that we do because it's very powerful. There's been this talk that physics in one sense is as imaginative as science fiction. What do you make of that?

I mean, I agree. I think, you know, physics is a fundamentally very imaginative endeavour and it has been right from the start. So we were talking about Newton and his unification of the heavens with earthly phenomena. I mean, if that's not a leap of the imagination, I don't know what is. It's an incredible leap of the imagination. Or if you go on to, I think, Wiemann's

we mentioned Faraday and Maxwell and their ideas where you take electric phenomena, you know, which were only very rudimentarily understood at the time and magnetic phenomena, you know, the thing that makes your compass needle swing around and light and

three seemingly totally different things and find a way to describe them all at once. There's another leap of the imagination. And Einstein, you know, with bending space times, another leap of the imagination. So I think this is sometimes under-emphasized that physics, and I think science more broadly, requires a huge amount of imagination as well, of course, as experimental rigor and finding the right balance. Because we've been

right on the edge of that, you know, when you talk about wormholes because you're so far away from experiment. But there has to be a balance and it's good when we let our imaginations fly as long as we remember that there's a real universe there as well and we have to go back and reference that from time to time. Well, I might say...

Actually, I think it's creative rather than necessarily imaginative. So certainly to, you know, to understand these theories and to create new ones is very much a creative process. But actually, particularly when we talk about these fundamental science, our understanding of the universe really is a mathematical one.

And so it's a sort of creative... That was Copernicus, wasn't it? Indeed. And of course, Newton. We've mentioned Newton a lot. Newton didn't just give us gravity. He gave us essentially the notion that you could really analytically control physical phenomena through mathematics. And he developed calculus in order to do that. So he gave us everything in one go. It was remarkable, really.

And that's essentially the theme that then for hundreds of years is dominated, that, of course, you know, you can imagine wonderful things and you have to be creative in working with the mathematics. But at the end of the day, the mathematics sort of keeps you honest. You know, you can only do things that you can mathematically show or try and understand. And so it's, you know, it actually doesn't require you to be a wildly imaginative person, right?

It requires you to be a creative person, but I'm not sure... You know, it's wonderful to sit back and imagine these phenomena, but I think we would never understand the universe in the way it is today if it wasn't for the fact that it didn't require imagination. You know, actually, it's all there in the mathematics.

And so when, for example, in 1916, the first black hole solution was written down very shortly after Einstein wrote down his theory, it wasn't understood to be a black hole solution for 40 years later. But at the same time, there it was. It's an equation. You can write it down in a line. It describes bending of space time.

No one could have imagined that that object actually exists out there many times over with remarkable properties, the mass of the sun and so on. But yeah, yeah.

I think we finally found a point of disagreement, actually, which is an achievement because we've been agreeing so much on everything. But I mean, I think, OK, of course, you're right in some sense. But I mean, if you look at somebody like Faraday, for instance, Faraday did remarkable experiments on electricity and magnetism in the mid 19th century and had very little mathematical ability. And yet he was able to extrapolate. You know, he wrote about things like ray vibrations is what he called them.

And it was the idea that what he had been talking about, magnetic fields, which were these things that came out of his mind, they were not mathematically elucidated. And the fact that they could vibrate, and then this is what became our understanding of light. So I think people do make these leaps. Or black holes. You know, Mitchell was writing about black holes in the time of Newton, long before somebody came along and got the maths correct. So I don't know. I don't think it's as clear-cut as you say. No, maybe that's fair. Yes.

Is this where we bring in the idea of time travel or the possibility of time travel? We certainly can. So again, time travel. Well, so time travel is an amazing thing because if we go back to Einstein's special relativity, time travel is part and parcel of that theory. So whilst I've been quite...

I feel I've been a bit negative about the existence of wormholes as a way to travel across to some distant galaxy. What Einstein does say, and this is real physics that we know is true, is that if you travel extremely close to the speed of light, your perception of distance gets distorted. So while to us, say, the nearest star other than the sun is four light years away, it would take light four years to travel there,

If you're travelling very close to the speed of light, to you, it could take 10 minutes, according to Einstein. So actually Einstein does allow us to travel extremely long distances in a short time, and then if you turn around and come back, you have actually time travelled into the future.

So say someone goes off to Alpha Centauri, four light years away, very fast. To them, it might be 10 minutes there, 10 minutes back. But to us on Earth, we would have seen it take four years for them to travel there and four years back. So eight years. So 20 minutes versus eight years. So we would have got eight years older. They would have just experienced 20 minutes and they've effectively time travelled into our future.

I mean, I think most physicists are totally comfortable with this idea, right? Even though it sounds crazy when you say it out loud like that. But on the other hand, this is just time travel into the future. What's really striking about wormholes is that, at least in principle, they could allow time travel into the past. And if you now start thinking about all the fiction...

about, you know, what might happen if you could time travel into the past. Well, to make it dramatic, you could go back and you could kill your grandfather or something like that. And of course, if you went back in time and killed your grandfather, then how were you ever born? And suddenly you have a paradox on your hands. So if...

If it's possible to travel back in time, then physics is in real trouble. And so for that reason, a lot of people, a lot of physicists believe it shouldn't be possible to travel back in time. And yet wormholes are very, very closely connected to time machines. A lot of the same physics that enables you to understand how a wormhole might work

also allows you to understand that if you can create the right sort of matter, you can also create a time machine.

And in fact, as far back as the 1930s, the mathematician Kurt Gödel pointed out to Einstein, he said, you know, your theory allows somebody to create a time machine and go back in time. And I don't think Einstein took it terribly seriously, but it shows the kind of absurd territory we're in. And I think it comes back to the point Katie was making. A lot of the interest in these ideas is in trying to work out

What is it in physics that stops this from happening? Because we don't think it's reasonable, but it's also quite hard to pinpoint exactly why is it going to be prohibited. The producer Simon is about to make his entrance. Does anybody want tea or coffee? A tea would be lovely. A tea would be lovely, thank you. In Our Time with Melvin Bragg is produced by Simon Tillotson and it's a BBC Studios audio production.

I'm Gabriel Gatehouse, and from BBC Radio 4, this is Series 2 of The Coming Storm. There's a divide in American politics between those who think democracy is in peril and those who think it's already been subverted, hollowed out from the inside.

In order to understand the deep state, you must understand the organisations within the deep state. As America prepares to elect its next president, we go through the looking glass into a world where nothing is as it seems, where the storming of the capital was a set-up and the institutions of the state are a facade. It's all an illusion. Listen on BBC Sounds. Yoga is more than just exercise. It's the spiritual practice that millions swear by.

And in 2017, Miranda, a university tutor from London, joins a yoga school that promises profound transformation. It felt a really safe and welcoming space. After the yoga classes, I felt amazing. But soon, that calm, welcoming atmosphere leads to something far darker, a journey that leads to allegations of grooming, trafficking and exploitation across international borders. ♪

I don't have my passport, I don't have my phone, I don't have my bank cards, I have nothing. The passport being taken, the being in a house and not feeling like they can leave.

You just get sucked in so gradually.

And it's done so skillfully that you don't realize. And it's like this, the secret that's there. I wanted to believe that, you know, that whatever they were doing, even if it seemed gross to me,

was for some spiritual reason that I couldn't yet understand. Revealing the hidden secrets of a global yoga network. I feel that I have no other choice. The only thing I can do is to speak about this and to put my reputation and everything else on the line. I want truth and justice.

And for other people to not be hurt, for things to be different in the future. To bring it into the light and almost alchemise some of that evil stuff that went on and take back the power. World of Secrets, Season 6, The Bad Guru. Listen wherever you get your podcasts.