How to shake hands with yourself, who has flown in from the future? Why can a lion suddenly materialize in your room? How to get to another Universe, visit a time machine and what is a "white hole"? Sergey Krasnikov, a senior researcher at the Star Physics Laboratory of the Main (Pulkovo) Observatory, spoke about this.
Sergey Vladilenovich, what is a wormhole?
- There is no very strict definition. Such definitions are needed when you prove some theorems, but there are almost no rigorous theorems, therefore, they are mainly limited to figurative concepts, pictures. Imagine that we took out a ball from our three-dimensional space in one room and took out exactly the same ball in another room, and glued the resulting boundaries of these holes.
Thus, when we in one room step inside this former ball, which has become a hole, we will emerge in another room - from a hole that formed in place of another ball. If our space were not three-dimensional, but two-dimensional, it would look like a sheet of paper to which a pen is glued. The three-dimensional analogue and its development over time is called a wormhole.
How are wormholes studied?
- This is a purely theoretical activity. Nobody has ever seen wormholes, and, in general, there is no certainty that they exist at all. They began to study the wormholes, starting from the question: are there such mechanisms in nature that would guarantee us that such holes in nature cannot exist? These mechanisms were not found, so we can assume that wormholes are a real phenomenon.
Is it possible, in principle, to see a wormhole?
Promotional video:
- Of course. If in a locked room you suddenly crawl out of nowhere a person, then you observe a wormhole. Wormholes as an object of study were invented and promoted by the American theoretical physicist John Wheeler, who, with their help, wanted to explain, no more, no less - electric charges. Let us explain. Describing a free electric field from the point of view of theoretical physics is not a very difficult task.
But it is very difficult to describe an electric charge from the same point of view. An electric charge appears in this sense as a very mysterious thing: some substance, separate from the field, of unknown origin, and it is not clear how to handle it in classical physics. Wheeler's idea was as follows. Let's say we have a microscopic wormhole, which is riddled with lines of force - from one end these lines enter into it, and from the other they exit.
An outside observer who does not know that these two ends are connected by lines of force, such an object will perceive it as a simple sphere in space, will examine the field around it, and it will look like the field of a point charge. Only the observer will think that this is some kind of mysterious substance that has a charge, etc., and all because he does not know that in fact it is a wormhole.
Of course, this is a very elegant idea, and many have tried to develop it, but they have not made much progress, because electrons are, after all, quantum objects, and naturally, nobody knows how to describe wormholes at the quantum level. But if we assume that the hypothesis is correct, then wormholes are more than an everyday phenomenon, everything that is associated with electricity will ultimately be tied to them.
Exotic matter is a classical concept of physics that describes any (usually hypothetical) substance that violates one or more classical conditions, or does not consist of known baryons. Such substances can have such qualities as negative energy density or repel rather than attract due to gravity. Exotic matter is used in some theories, such as the theory of the structure of wormholes. The most famous representative of exotic matter is the vacuum in the region with negative pressure produced by the Casimir effect.
What kind of wormholes are there?
- From the point of view of theoretical travel, there are passable and impassable wormholes. Impassable ones are those through which the passage is destroyed, and this happens so quickly that no object simply has time to go from one end to the other. Of course, the most interesting for study is the second type of wormholes - passable. There is even a beautiful theory that says that what we used to think of as supermassive black holes in the centers of galaxies are actually the mouths of wormholes. This theory is almost undeveloped and, naturally, has not yet found any confirmation, it exists, rather, as a kind of idea. Its essence is that outside of the wormhole, you only see that in the center of the galaxy there is a certain spherically symmetrical object, but you cannot say what it is - a wormhole or a black hole.because you are outside this object.
In fact, they can be distinguished only by one parameter - mass. If the mass turns out to be negative, then this is probably a wormhole, but if the mass is positive, then additional information is needed here, because the black hole may turn out to be a wormhole. Negative mass in general is one of the central moments of the whole wormhole story. Because in order to be passable, a wormhole must be filled with what is called an exotic substance - a substance in which, at least in places, at some points, the energy density is negative.
At the classical level, no one has ever seen such a substance, but we know for sure that it can exist in principle. Quantum effects have been registered that lead to the appearance of such a substance. This is a fairly well-known phenomenon and it is called the Casimir effect. It was officially registered. And it is connected precisely with the existence of negative energy density, which is very inspiring.
The Casimir effect is an effect consisting in the mutual attraction of conducting uncharged bodies under the influence of quantum fluctuations in a vacuum. Most often, we are talking about two parallel uncharged mirror surfaces located at a close distance, but the Casimir effect also exists for more complex geometries. The reason for the effect is the energy fluctuations of the physical vacuum due to the constant creation and disappearance of virtual particles in it. The effect was predicted by the Dutch physicist Hendrik Casimir in 1948 and later confirmed experimentally.
In general, in quantum science, negative energy density is a fairly common thing, with which, for example, Hawking evaporation is associated. If such a density exists, we can ask the following question: how large is the mass of the black hole (the parameter of the gravitational field created by it)? There is a solution to this problem that is applicable to black holes - that is, objects with a positive mass, and there is a solution that is applicable to a negative mass.
If there is enough exotic matter in the wormhole, then outside the mass of this object will be negative. Therefore, one of the main types of "observation" of wormholes is the tracking of objects in relation to which it can be assumed that they have a negative mass. And if we find such an object, then with a fairly high degree of probability we can say that this is a wormhole.
Wormholes are also divided into intra-world and inter-world. If we destroy the tunnel between the two mouths of the second type of holes, we will be able to see two completely unrelated universes. Such a wormhole is called interworld. But if we do the same and see that everything is fine - we stayed in the same Universe - then we have an intra-world wormhole. These two types of wormholes have a lot in common, but there is also an important difference. The fact is that the intra-world wormhole, if it exists, tends to turn into a time machine. Actually, it was against the background of this assumption that the last surge of interest in wormholes arose.
In the case of an intra-world wormhole, there are two different ways to look at a neighbor: directly through the tunnel or in a roundabout way. If you begin to move one mouth of a wormhole relative to the other, then, in accordance with the well-known paradox of the twins, the second person, returning from the trip, will be younger than the remaining one. On the other hand, when you look through the tunnel, you are both sitting in laboratories that are motionless, from your point of view, nothing happens, your clocks are synchronized. Thus, you have the theoretical opportunity to dive into this tunnel and get out at a moment that, from the point of view of an external observer, precedes the moment when you dived. The delay brought to an appropriate degree will give rise to the possibility of such a circular travel in space-time,when you return to your original departure point and shake your previous incarnation's hand.
The twins paradox is a thought experiment with which they try to "prove" the inconsistency of the special theory of relativity. According to SRT, from the point of view of "stationary" observers, all processes of moving objects slow down. On the other hand, the principle of relativity declares the equality of inertial frames of reference. On the basis of this, reasoning is built, leading to an apparent contradiction. For clarity, the story of two twin brothers is considered. One of them (the traveler) goes on a space flight, and the second (stay-at-home) remains on Earth. Most often, the "paradox" is formulated as follows:
From the point of view of a couch potato, the watch of a moving traveler has a slow passage of time, so when returning, it should lag behind the clock of a couch potato. On the other hand, the Earth was moving relative to the traveler, so the homebody's clock should lag behind. In fact, the brothers are equal, therefore, after returning, their watches should show the same time. However, according to the SRT, the traveler's watch will be lagging behind. This violation of the apparent symmetry of the brothers is seen as a contradiction.
What is the fundamental difference between a wormhole and a black hole?
- First of all, I must say that there are two types of black holes - those that were formed as a result of the collapse of stars, and those that existed initially, arose with the emergence of the Universe itself. These are two fundamentally different types of black holes. At one time there was such a concept as "white hole", now it is rarely used. A white hole is the same black one, but evolving backward in time. Matter just flies into a black hole, but it can never escape from there. From a white hole, on the contrary, matter only flies out, but you cannot get into it in any way. In fact, this is a very natural thing if we remember that the General Theory of Relativity is symmetric in time, which means that if there are black holes, white holes must also exist. Their totality is a wormhole.
What is known about the internal structure of wormholes?
- So far, in this sense, only models are being built. On the one hand, we know that the appearance of this exotic matter may have been discovered even experimentally, and still there are a lot of questions. The only model of a wormhole known to me that is more or less consistent with reality is the model of an initially evaporating (since the beginning of the Universe) wormhole. Due to this evaporation, such a hole remains passable for a long time.
What exactly are you working on?
- I am engaged in purely theoretical activity, what can generally be called the causal structure of space-time is the classical Theory of Relativity, sometimes semiclassical (as you know, quantum does not exist yet).
In the classical nonrelativistic theory, one can come up with sufficiently convincing evidence that time travel cannot exist, but in general relativity there is no such evidence. And Einstein, when he was just developing his theory, was aware of this. He wondered if there was some way to exclude this possibility. Then he did not cope with this task, as he himself later said. Although Einstein created a language to study this issue, the task remained academic. Interest in it exploded in the late 1940s when Gödel proposed a cosmological model containing such closed curves.
But since Gödel always offered something exotic, they reacted to this with interest, but without serious scientific consequences. And then, around the end of the last century, thanks mainly to science fiction - for example, the film "Contact" with Jodie Foster - interest in the topic of time travel with the help of wormholes was revived again. The author of the novel, according to which the script of the film was written, is a very famous astronomer, popularizer of science Carl Sagan.
He approached the matter very seriously and asked his friend, also a very famous relativist, Kip Thorne, to see if everything described in the film was possible from the point of view of science. And he published a semi-popular article in the magazine for American physics teachers "Wormholes as a tool for studying General Theory of Relativity", where he considered the possibility of time travel through wormholes.
And I must say that then in science fiction, the idea of traveling through black holes was popular. But he understood that a black hole is an absolutely impenetrable object - travel through them is impossible, so he considered wormholes as an opportunity for time travel. Although this was known before, but for some reason people took his conclusions as a completely fresh idea, and rushed to investigate it. Moreover, the emphasis was on the presumption that a time machine cannot exist, but we decided to find out why. And quite quickly it became clear that there were no obvious objections to the existence of such a machine. Since then, larger-scale studies have begun, and theories have begun to appear. In general, since then I have been doing this too.
"Contact" is a 1997 sci-fi film. Directed by Robert Zemeckis. Main plot: Ellie Arroway (Judy Foster) devoted her whole life to science, she becomes a participant in a project to search for extraterrestrial intelligence. All attempts to search for extraterrestrial signals are fruitless, and the future of her project is at stake. Ellie is desperate to find support, but unexpectedly gets help from the eccentric billionaire Hadden. And here's the result - Ellie picks up the signal. Signal decoding shows that it contains a description of the technical device. Its purpose is not clear, but a place for one person is planned inside.
After the creation and launch of the device, Ellie embarks on a journey through the wormhole system and is transported, probably to a planet in another star system. Waking up there, on the seashore, she meets a representative of another civilization, who chose the image of her late father. Looking around, the heroine realizes that this area has been recreated by an alien mind in her mind in the image of a drawing she drew in childhood. The alien tells her that the device allows you to organize a system of interstellar communication routes, and the Earth from now on becomes a member of the community of civilizations of the Universe.
Ellie returns to Earth. From the point of view of outside observers, after the launch of the installation, nothing happened to her, and her body did not leave our planet. Ellie finds herself in a paradoxical situation. As a scientist, from the point of view of rigorous science, she cannot in any way confirm her words. One more circumstance is also revealed: the video camera attached to Ellie during the trip did not record anything, but the duration of the blank recording was not a few seconds, but 18 hours …
Is it possible to "make" a wormhole?
- Just about this there is a strict scientific result. This is due to the fact that there are no exact results on the study of wormholes. There is a theorem proved a long time ago, and it says the following. There is such a thing as global hyperbolicity. In this case, it does not matter at all what it means, but the point is that while and since space is globally hyperbolic, it is impossible to create a wormhole - it can exist in nature, but it will not work to make it ourselves.
If you manage to break the global hyperbolicity, then maybe you can create a wormhole. But the fact is that this violation in itself is such an exotic thing, so poorly studied and poorly understood that the side effect of the birth of a wormhole is already a relatively minor thing compared to the very fact that you managed to violate global hyperbolicity.
A very famous thing is taking place here, called the “principle of strict cosmic censorship,” which says that space is always globally hyperbolic. But this, in principle, is nothing more than a wish. There is no evidence of the correctness of this principle, there is just a certain inner confidence inherent in many people that space-time should be globally hyperbolic. If so, it is impossible to create a wormhole - you have to look for an existing one. Meanwhile, severe doubts about the fidelity of the principle of cosmic censorship were expressed by the author himself - Roger Penrose, but that's another story.
That is, to create a wormhole requires some serious energy costs?
- It's very difficult to say something here. The trouble is that when your global hyperbolicity is violated, then at the same time predictability is violated - this is practically the same thing. You can somehow geometrically change the space around you, for example, take a bag and put it in another place. But there are certain limits within which you can do this, in particular, the limit imposed by predictability. For example, sometimes you can say what will happen in 2 seconds, and sometimes not. The verge of what you can or cannot predict lies precisely in global hyperbolicity. If your space-time is globally hyperbolic, you can predict its evolution.
If we assume that at some point it violates global hyperbolicity, everything becomes very bad with predictability. Therefore, an amazing thing arises, for example, such that right here and now a wormhole can materialize through which a lion will jump out. It will be an exotic phenomenon, but it will not violate any laws of physics. On the other hand, you can spend a lot of effort, money and resources to somehow facilitate this process. But the result will still be the same - in both cases, you don't know if a wormhole will appear or not. In classical physics, we can’t do anything about it - if it wants, it will, it doesn’t want it, it won’t arise - quantum science doesn’t give us any clues in this matter.
The principle of "cosmic censorship" was formulated in 1969 by Roger Penrose in the following figurative form: "Nature abhors a naked singularity." It says that spacetime singularities appear in places that, like the inner regions of black holes, are hidden from observers. This principle has not yet been proven, and there are reasons to doubt its absolute correctness (for example, the collapse of a dust cloud with a large angular momentum leads to a “naked singularity”, but it is not known whether this solution of Einstein's equations is stable with respect to small perturbations of the initial data).
Penrose's formulation (a strong form of cosmic censorship) suggests that spacetime as a whole is globally hyperbolic.
Later Stephen Hawking proposed another formulation (a weak form of cosmic censorship), where only the global hyperbolicity of the “future” component of space-time is assumed.
Olga Fadeeva