Can Humans Travel Through Wormholes? with Prof. Juan Maldacena

You have fallen into event horizon, with John Michael Gaedea.

In today's episode, John is joined by Juan Maldicina.

Juan Maldicina is the Carl P. Feinberg professor at the School of Natural Sciences at the Institute for Advanced Study.

Professor Maldicina's work focuses on theoretical physics and specifically on quantum gravity,

string theory and quantum field theory. He has proposed a relationship between quantum gravity and quantum field theories that elucidates various aspects of both theories.

He's studying this relationship further in order to understand the deep connection between black holes and quantum field theories.

He's also exploring the connection between string theory and cosmology.

Dr. Juan Maldicina, welcome to the program.

Hello, how are you?

Now, Dr. you recently wrote a paper about a very old subject that goes back to, well, Einstein and Rosen.

The idea of a traversable or at least the existence of a wormhole in the universe as we understand it.

Now, your paper seems to center around a different sort of model than the standard model of the universe. This is the Randall-Sunder model.

What exactly is that model and how is it different than the standard model?

Well, that's a model which originally postulated that you had an extra dimension.

So you have the four dimensions that we see and then there is an extra fifth dimension that is actually very large.

But the space in the extra dimension is highly current.

It turns out that this model has also can be viewed alternatively as having nowhere four dimensional space.

Somehow coupled or interacting with a theory that has light or massless particles.

A kind of dark sector that has massless or light particles.

Now, what creates the massively twisted nature of this fifth dimension?

Well, that's something that is postulated in this model.

And there could be some more microscopic models, like let's say models with extra dimensions such as in string theory and so on.

In those models, there is some energy or maybe some magnetic type fluxes in the internal dimensions that could generate this type of negative curvature.

Within the standard model, there's always been a problem with wormholes that they, if they could exist, they collapse very rapidly.

But in this other model, how are they held open?

Yeah, maybe I'll perhaps start by reviewing a little bit the ideas around traversable wormholes. If that's okay.

Pile myths.

Yeah.

So in general, relativity, the geometry of space time is curved and can have in principle different shapes.

However, the shapes are constrained by the equations.

The equations roughly say that the average curvature of the geometry should equal the matter density.

Now, in principle, one can imagine geometries that look quite weird, so quite strange.

So you could have a geometry where you take two far away regions in space time and you cut out these regions and you somehow connect them to each other.

And at the level of geometries, this is an allowed geometry. I mean, it's a possible geometry.

There are some geometries that look a bit like this that are solutions of Einstein's equations in the vacuum.

Those geometries were found by Einstein and Rosen in the 1930s.

And they look basically like two black holes from the outside. So from the outside, they look like two black holes.

But if you go inside, they are connected. So the inside of these two black holes is common. It's common for both of them.

So if someone jumps into one black hole and another person jumps into the other black hole, they can meet inside.

So that's sometimes called the Einstein-Rosen wormhole or Einstein-Rosen bridge.

And it's a connection in space, but the solution is time-dependent.

And as you evolve the solution in time, this interior region collapses.

And inside, it looks a bit like a big crunch. So a region where the space time is collapsing and shrinking to nothing.

So anyone, if these two people who go into the interior, they can meet, but they will eventually die in this collapse in the universe.

So it's not a very happy place to go.

And in fact, you can actually, so you can wonder whether there could be a solution where this doesn't happen, where somehow you have faraway regions.

And then you can send a signal through this wormhole to a faraway place faster than this piece of light.

This is a possibility that people have speculated a lot, especially science fiction writers and so on, because it will allow you to violate the principle of maximum speed.

It will allow you to travel faster than the speed of light.

But this geometry is in principle, you can think about that you can write them down.

You can, as geometries, they have nothing wrong, but they do not obey Einstein's equations with matter that always the standard principles that matter should obey.

So classical matter obeys a certain positive energy condition, which forbids any kind of wormhole, so any kind of connection between two separate points, any kind of topology of space time with little handle and so on.

Those things are forbidden by just the Einstein equations and classical general relativity.

And sorry, and classical matter.

Now, once you have one to matter, the energy conditions are a little laxer, so you can sometimes have a little bit of negative energy.

And people then return to this question of traversable wormholes and wondered whether in quantum mechanics, they would be allowed, whether once you consider quantum effects, maybe you could have those perhaps those science fiction wormholes that allow you to travel faster than the speed of light.

Now, people also did research on this question and the conclusion is that these are still not allowed. So it's still not possible, even with the quantum effects, if those quantum effects are constrained by the loss of let's say quantum field theory and special relativity quantum field theory is a kind of theory that results from the union of quantum mechanics and special relativity.

So the standard model of particle physics is a quantum field theory and extensions of the standard model that people normally consider are also quantum field theories in this sense.

And they, they are the energy can be negative but not too negative and in particular, not negative enough to allow traversable wormholes that would allow you to travel faster than the speed of light.

So let me call those science fiction wormholes.

So that's too bad, but in some sense it's too bad for science fiction writers, but in some sense it's good for the loss of physics because general relativity is based on the idea that there is a limit for the speed of propagation of signals.

And it would have been funny if this non trivial topologies, which would be allowed, but if those non trivial topologies were allowed, then general relativity itself would violate the principle that it was based on.

So it would it would be a funny situation, but interestingly enough, this funny situation does not happen and it doesn't happen thanks to an interesting interplay between properties of quantum matter and Einstein's equations.

Now what can happen is that you can have wormholes traversable wormholes, but the time it takes you to traverse the wormholes or to go through this non trivial topologies or through this you should imagine the wormhole as a little tunnel in space time where the tunnel has two entrances that are at some distance from each other.

The tunnel is some geometry that connects the two and so you enter in one mouth of the wormhole and through this tunnel you come out in the other mouth of the wormhole.

Now an important question is how long it takes you to travel through this wormhole.

So if the two mouths are at some distance the wormholes that are forbidden are those that where you the time it takes you to travel is shorter than the time it takes you to travel between the mouths in the outside space time.

So those are the science fiction wormholes. However, you can have wormholes where they have this geometry, but the time it takes you to travel from one mouth to the other through the wormhole is longer than the time it takes you to travel outside.

So those are those are fine and they are allowed and in fact you can find solutions that have this properties.

And this this this were found very recently they were based first on an idea by Gau Jeffress and Wald who noticed that and there are certain circumstances the quantum effects of quantum mechanics can lead to negative energy that can make the original Einstein Ross and wormhole they can change it a little bit and make it slightly traversable.

Then in subsequent papers with other people will include myself and others we propose the with me lacking and pop up we propose some solutions in four dimensions that use just the fields of the standard model such as the physics of the standard model which has that physics you can construct the wormhole solution is just a solution but has the task is funny geometric shape of the wormhole.

However, the in using purely the fields of the standard model we could only construct solutions which are very very small so smaller than the smallest distances we can see together we can see today with our best microscopes the best microscopy is the large pattern collider.

And they involve also things like magnetic monopoles and things like that but not magnetic monopoles itself but magnetically charge black holes which would be very difficult to produce in the standard model I mean they were very principle possible but very difficult to produce.

So those are some solutions and then we were wondering that the paper you mentioned came out of thinking about whether these solutions could be possible if we assume certain properties about the new physics so physics that we haven't yet seen.

So the question is could the loss of physics be such that we could have bigger traversable wormholes that are big enough in particular whether they could be big enough for a person to go through the answer was that it is possible if you assume a certain dark sector so by dark sector is set of particles or part of physics that does not interact directly with the known physics with the standard model through standard model interaction.

And so if the dark sector is of this particular random syndrome type that we mentioned before then it is possible to find traversable wormholes which are big enough for a person to go through so that's basically some of the background so I tried to summarize some of the background for this paper and what this paper was doing.

In the paper you mentioned that the wormhole under this specific condition under these assumptions the wormhole must still be very clean or it collapses so if you have particles entering it you're going to have to make sure that those aren't there right.

Yeah yeah yeah let me discuss a little bit more the geometry of this wormhole they have an interesting property.

So as I said before they are kind of like a tunnel and I said I said before that the time it takes you to travel through the wormhole is longer than the time it takes you outside but it is only longer for the person who stays outside.

For the person who travels through the wormhole this time is very short this is possible thanks to the fact that the is possible due to two effects.

First there is a large so called gravitational redshift so that means to say that the time inside the wormhole runs very slowly.

Second there is sort of like a large gravitational potential that pushes the traveler gives a very large speed to the traveler when it travels when this person travels along the wormhole.

So these two effects together they give rise to a kind of time dilation effect that implies that the time as felt by the traveler himself or herself is much shorter than the time that an outside person measures.

It's roughly similar it's very similar to the issue of the twins the twin paradox right so let me just remind you of the twin paradox so.

In the twin paradox you have two twins that are born at the same time and one remains at rest and the other travels with very high speed far away and then returns.

The twin that returns is much younger than the twin that stayed that stayed at rest and so the twin that travels is younger because it was traveling at high velocities.

Here we have a similar effect the person who travels through the wormhole if we had two twins and one remains travels outside the wormhole and the travels through the wormhole.

The two would meet the one that travels outside will reach the other mouth of the wormhole earlier but once the finally the twin that goes through the wormhole emerges from the wormhole that twin would be much younger than the previous one.

So in other words the twin going through the wormhole passes through the wormhole momentarily but if you're going from in the normal universe from one end of the wormhole to the other it takes thousands and thousands of years.

So it's all once again relative perspectives.

Yeah that's right so with particular parameters we gave an example in the paper where you could have wormhole where the distance between the two months of the wormhole would be roughly like 10,000 years.

So the time to go through the wormhole as seen from the outside would be longer than that but the time going through the wormhole would be like one second so it's a very very drastic example.

And it is something that in theory you would be able to survive traveling through this wormhole so you are accelerated to very high speeds and if the wormhole is very clean as you were saying before then you can you can do that.

But if there is a little speck of dust that enters through the other mouth of the wormhole and hits you then then you're then you're dead so it's dangerous.

So always keep everything clean inside this now are there limits so say you can create somehow can create a wormhole like this.

Is there any distance limit in the universe to where it's just simply too far or is or could you just literally go anywhere in the observable or an observable universe.

Is there some sort of limit as to you know how much can you twist this geometry right yeah if you if you were yeah in principle there are limits I guess you.

If if we were in flat space in empty flat space and you take it really very very far then the effect of this effect of the negative energy is not strong enough to keep it open.

So that's one limit now there are more practical limits in our universe which is that even if you somehow could make this geometry just the fact that there is a cosmic microwave radiation that could enter this wormhole that radiation would accumulate inside and eventually also make it close or prevent the traveler traveler from traveling through without dying.

There is also the fact that this extra dimension we're talking about would get heated up by some of the heat in our universe so these are all issues that make it impractical today so today by today I mean this time the evolution of the universe maybe even in the very far future might be possible.

But even all of that is assuming that there is a dark sector and there are some lots of physics which which which which allow them which allow these wormholes.

Something I should emphasize is that this type of dark sector and so on was considered before and it's it's consistent with all the principles of special and general relative and quantum mechanics special relativity and quantum mechanics so it's a you could build as a type of quantum field theory.

So it's consistent with the loss of physics with the very general principles of physics it's not somehow consistent with the standard model and gravity but it is consistent with general principles of physics so that we had to invoke some extra unseen matter to find the solutions.

So I just think I think we would be with just a security of what is vaguely possible but you should not expect that you will be able to make them or find them in our own universe anytime soon.

Now that leads to a question there's always been floating around the discussion regarding wormholes there's always been this concept that you would need some kind of negative mass or exotic matter something like that to begin to think about creating one of these things.

That could be traversable. Is that also applicable to the wormhole in this Randall syndrome model?

No, no, so when people talk about exotic matter in the context of wormholes what they mean is matter that has a very large relatively large negative energy and it's a kind of negative energy which we think it's not allowed by the very general principles of special relativity and quantum mechanics.

So it's not that it's not allowed by the matter we find experimentally in our universe by the matter of the standard model but it's also not allowed by the matter of most extensions of the standard model that people normally think about.

So it's you know in the history of physics in the 20th century there were some principles that were laid out like quantum mechanics special relativity and general relativity that have held on till today.

The particular content matter content has not I mean the beginning of the 20th century well they were on electrons and protons neutrons and many new particles were discovered all these new particles of the world discovered they all obey this general principles of quantum field theory which is just the word we used to describe theories that are joining together special relativity and quantum mechanics.

Now we early in the discussion we covered that these would look like black holes right even in this sort of.

The Einstein-Rosen ones they would look like black holes what would these these and yes so this ones would look somewhat.

Yeah for to discuss what this look like I will need to discuss type of black hole first which are the so called charge black holes.

These are black holes that carry some charge so it could be some magnetic charge for example and then such black holes have to have a mass bigger than a certain threshold that depends on the mass on the charge story.

And when they have the minimum possible mass they develop a very long throughout a very long neck let's say and then the horizon is at the bottom of this neck you should imagine them as a kind of big gravitational well like a very deep well and at the bottom of the world you have the horizon so that's how these black holes look like.

Now the one holes we were talking about they look they start let's say looking similar to two black holes like this but if you go in instead instead of finding the horizon and the interior geometry and so on what happens is that this this long this long potential wells are just simply connected to each other before they encounter the horizon so the final geometry has no horizon.

And that's what allows you with allows a person who's traveling to go in and then come out again on the other side so that's roughly how they look like.

Now traveling through the wormhole say we're in a spacecraft going through it this stable traversable wormhole yeah it would we would just let there would just be instantaneous right you would enter and then come out the other side again relative to your position and inside the wormhole.

Regardless of how much time has passed in the universe or the normal universe anyway now this makes this like any other sort of a sort of conceptual idea of wormholes it makes it a time machine you're traveling into the future by going through the wormhole correct.

Yeah you're traveling to the future in the same sense that the in the twin paradox one of the twins is traveling to the future so he's younger than the others when he gets to the end point.

So that in itself sort of resembles a black hole interior right as you pass the event horizon the black hole you are looking down at the future as you approach the singularity and therefore you must go towards the singularity because it's in your future is that the same with the wormhole is there a point of no return where you can't actually stop yourself from going to the wormhole.

No no there is no such thing for a wormhole so this collapsing universe that is inside the black hole interior is only a property of the black hole so when there is a horizon and so on in the case of of the wormhole you cannot any point decide to do something different and you will travel through the wormhole but the main difference between the two is that in the black hole if you go in you die and the wormhole you go in and you come out the other side and you die.

But you are in the future so you can't ever go back to the past you're still.

No you cannot go back yeah yeah yeah yeah so general relativity and so on it allows you to travel to the future so you can travel to the future faster.

I mean we are constantly traveling to the future right but there's certainly let's say speed at the speed that we're clock ticks and it is possible to go faster by moving very rapidly.

Like in the example of the twins we're going through those wormholes and so on being in the presence of a high gravitational field you also travel to the future faster.

But and that's allowed what's not allowed is to travel to the past or to travel at speeds larger than the speed of light so that those are the things that are not allowed.

But this doesn't violate that you're not going faster than speed of light.

No this doesn't violate that no.

You are still it is consistent with everything else in the universe in that you cannot go faster than light.

You cannot go backwards in time but you go forwards and by going forwards you can end up at a certain point.

Now with this wormhole say someone is actually traversing it and you are an observer on the outside.

Could you actually see the person traveling across space or would they be invisible in some of their higher dimension or something like that.

I mean could you actually watch their progress across the universe.

No basically you cannot watch them. It's like someone mashing two big wells and you see someone going into one well and comes out of the other well.

But you don't see how the wells are connected.

In a certain sense it's somewhat similar to a wormhole as the holes that worms make.

Imagine an actual wormhole so we have the surface of the ground and there is one entrance of the wormhole and another.

And it's somewhat similar to that.

Now with this sort of model of multi-dimensional space which is something that keeps coming up whether it's string theory or whatever model you want.

You are essentially traveling through another dimension more or less.

You are sort of taking a shortcut through a very convoluted warped area of space time that we can't see.

Is that the right way to visualize it?

Yeah, the particular solutions that we discussed you can think about them that way.

Yes, it looks you could imagine that there is a fifth dimension with let's say something like a constant gravitational field.

And then you have this little tunnel similar to an actual wormhole that goes through the extra fifth dimension.

So that's one point of view and it's perfectly valid point of view.

So the most interesting aspect of this is perhaps not this but something that might be a little hard to appreciate.

But the interesting point is that there is another point of view where you can replace this extra dimension by some matter fields, some matter that lives in the original four dimensional space.

In this point of view what happens as you go through this wormhole is very interesting.

But perhaps before we discuss it in more detail, I'd like to explain a little bit more about black holes because it's important to have this picture of black holes.

So far we've discussed the black hole as geometry as a space time geometry and that's the description we have in general relativity.

However, once you start thinking about general relativity and quantum mechanics, you are led to think about thinking about black holes in a slightly different way as thinking of the black hole as being some object that obeys the loss of quantum mechanics with some finite number of degrees of freedom.

So you can build us a very complicated atom with huge, huge number of very complicated material, let's say.

And from that point of view, when a person falls into a black hole, what happens is that it goes and interacts with this material and it gets destroyed by the dynamics with this materials like jumping in a pool of acid where you get completely dissolved.

And so that's the picture of the quantum mechanical picture of black holes.

So in this picture, the traversable wormhole would look like the two black holes would be entangled with each other.

So you have some fluid, let's say this acid we were talking about where and the particular quantum states of this acid is completely entangled between the two pools.

Let's say the two pools of acid, two droplets are like the two black holes.

And then when a person, when the traveler goes into one of these pools of acid gets dissolved and then there is a kind of quantum fluctuations or radiation that travels through particles in the embedded space time, goes into the other pool of acid and causes the person to reassemble in the in the other pool of acid and emerge from the pool of acid.

Something that looks very strange, but that's the picture you get and you can understand this picture in quantum mechanical terms as a version of quantum teleportation.

So quantum teleportation is some phenomenon that can exist when you have entanglement. And in this case, you have the entanglement between these two black holes and so you can view this as an example of quantum teleportation.

And it was really by thinking about these examples of quantum teleportation and so on, that all these recent, recent examples of tremors that were wrong, so wormholes came about.

But there's a difference between this and traveling through a wormhole, because quantum teleportation you're essentially being deconstructed and then reconstructed, whereas through the wormhole you're not, right?

That's right. So the difference is in how you feel, but what we claim is that it is the same. So we claim these two pictures are the same, but certainly you feel, certainly with this kind of teleportation you never feel bad, you never feel that you died or or anything.

So in some other kinds of teleportation, the person would feel that he or she dies, but then he or she is reconstructed on their side with all the memories intact and so on and having forgotten that he or she felt that.

They died, so that's the difference between the two. I see. Yeah, but I think the, so the interesting point is that in this case is the idea is that you could have this very mild type of teleportation where you don't feel anything special when you are being teleported and that's a surprise.

That could have implications on things like communications and non-animat matter so you could send supplies to someone or you could communicate to someone, but you're still beholden to that.

The speed of light limit. Yeah, the speed of light limit. Yeah, yeah, yeah, you could use it for that.

You could use that. You could use it for that, but I have to say that if you want some practical use that there are more practical ways of communicating.

But one feature that this type of communication has is that it's very secure. So people had discussed quantum teleportation in the context of secure communication.

And so there are lots of quantum mechanics somehow for be anyone from if dropping on the communication.

Here, this security is due to the fact that the message is somehow passing through this through this wormhole that is not accessible to the observer that stays outside, or at least not very easily accessible.

It's related to the question you asked me before of what the person outside would see. So they will not really see you traveling through the wormhole race.

To first approximation, let's say they will not see you travel through the wormhole and for that reason the communication is secure.

This is related to the security of the communication. Yes, and I can't imagine that anyone could ever come up with a way to break that sort of level of security that where your communication is inside of a wormhole travels through other dimensions.

It would be very difficult to break, but it's also very difficult to produce this kind of entanglement. So it's not that we're going to see any answer.

Communicating in that way would probably even given the security not be worth it.

Probably not, yeah. The reason we are thinking about this is more to learn about the features of space time.

So to learn should by knowing that this is possible in space time, we are kind of seeing the kind of tricks that space time can do.

Quantum mechanicals, the interrelation between quantum mechanics and space time can do all these various tricks.

And I think this will teach us something about space time.

Now my last question for you is in regards to a quantum mechanical version of this very, very tiny one.

Yeah, of this type of a black hole.

Is there any way we could study that and see if these sorts of things are present in nature?

I mean, can we get down to some sort of a scale since this can be bigger, you know, more than anything that's usually thought of as a as a wormhole in quantum mechanics?

Is this would this be easier to study and look for if these happen in nature than say a standard model wormhole?

These objects are difficult to produce, I think, and we haven't come up with a simple way of producing them, but by simple, I mean, even possible within the lifetime of the universe or something like this.

So we found them as solutions, but we don't know whether they are simple to produce.

So I mean, even some of the ingredients are a bit exotic. So even for the standard multiple black holes that the wormholes that are ice in the standard mold, they involve this magnetically charged black holes, which themselves are not terribly easy to produce, but there are at least.

There are some scenarios where maybe they could exist. So they haven't been detected yet, but it is suddenly something that could in principle exist.

But then you would have to bring two such magnetically charged black holes and bring them relatively close to each other and wait for a long time for the wormhole to form.

And we don't know exactly how long it should wait for the wormhole to form. I don't know if these answers your question.

Fascinating stuff. Well, thank you for joining us today, Doctor. I hope you'll come back sometime when you release another paper and maybe it will become easier with time figuring out whether if we could ever create a wormhole or not.

But it definitely is interesting, especially in regards to understanding space time.

Sure, and it's a pleasure talking to you.

John, are you aware that the opossum has a name for you?

No, I don't speak possum. I can't understand a word he says. What is this name?

He calls you, troding as human.

What? He's the one that came out of a box, not me.

From his perspective, you are the one that came out of a box, and knowing you is probably a chief box.

I remember that differently. Quantum mechanics is confusing.

Well, I suppose there has to be at least some differences between the two of you.

Yeah, well, one is that I'm not a quantum mechanical possum, Anna.

And on that note, next week we'll be joined by two greats of speculative science fiction.

A word's galore here, folks. It is Gregory Benford and Larry Niven. See you then.