Is it possible to travel in time to the past? Time machine: myths and real facts about the possibility of time travel

It doesn't take much intelligence to travel through time. Each of us moves about 24 hours ahead every day. Another thing is that this movement remains as unintentional as it is inevitable. Unlike space, we cannot at will stand up and move so many “steps” into the past or future... or can we?

The idea of ​​the flow of time, as something unchanging, constant, eternal and uniform, sits somewhere very deep in our psyche. We measure it in seconds, hours, years, but the duration of these intervals can vary. Just as a river flow, which in fact is often compared to the flow of time, can either accelerate at sudden changes or slow down, spreading widely, time itself is subject to change. This discovery was perhaps the key in the scientific revolution that took place in 1905-1915. performed the work of Albert Einstein.

The impermanence of time originates in its complex relationship with space. Three spatial dimensions and one time form a single, inseparable continuum - the stage on which everything that happens in our world unfolds. The complex interweaving and interaction of these four dimensions with each other gives us hope that travel into the past and future is still possible. To gain power over time, you just need to tame space. How is this possible?

Just forward

For simplicity, let's imagine that the continuum of our Universe includes not four, but only two dimensions: one spatial and one temporal. Every object, from a photon to Donald Trump, moves along this continuum at a constant speed. No matter what he does, whether he is crossing the Galaxy or answering questions from journalists while sitting on a chair, the overall speed of his movement remains the same - to simplify, we can say that the sum of the speeds at which an object moves is always equal to the speed of light. If the president does not move in space, then all the energy of his movement goes into moving along the time axis. If a photon moves through space at the speed of light, then it has no energy left for a while, and for these particles time does not move at all.

We can say that movement in space “steals” movement from time. If Donald Trump accelerates - gets on a plane and crosses the Atlantic at a speed of about 900 km/h - he will slow down his movement in time and end up somewhere 10 nanoseconds in the “future”, in the time that for his “internal clock” It hasn't arrived yet. The current record holder for being in space, Gennady Padalka, during 820 days on the ISS, during which he moved at a speed of about 27.6 thousand km/h, moved into the future by several tens of milliseconds. By reaching 99.999% of the speed of light in a year, you can travel 223 “normal” Earth years into the future.

This flow of motion from space into time and back should be extended to gravity. In the description of the General Theory of Relativity, gravity is a deformation of the space-time continuum, and in the vicinity of a black hole (and any other gravitating object), all four dimensions are “bent”, and the stronger the attraction, the more strongly. Time moves slower near the Earth's surface than in orbit, and ultra-precise satellite clocks are advancing by about 1/3 of a billionth of a second per day. This movement into the future is much more noticeable for bodies located near more massive objects.

The supermassive black hole at the center of our Galaxy weighs about 4 million Suns, and if we start cutting circles in the vicinity of it, then after some time - when on our spaceship In just a few days, we may find ourselves in a Universe several years older than us. Again, in the future. As we understand, Einstein’s formula easily allows for such movements, although in practice they are as complex as it is difficult to reach a speed close to light, or to survive in the vicinity of a supermassive black hole. But what about the past?

Back and Up

By and large, traveling back in time is even easier to arrange than traveling forward: just look at the starry sky. The diameter of the Milky Way is about 100 thousand light years, and the light of more distant stars and galaxies can take millions or billions of years to reach us. Looking around the night sky, we see flashes of the past. The Moon as it was about a second ago, Mars - about 20 minutes ago, Alpha Centauri almost four years ago, the neighboring galaxy the Andromeda Nebula - 2.5 million years ago.

The farthest limit available for this kind of “movement” in time is more than 10 billion years: a picture of that incredibly distant era can be seen in the microwave range, like traces of the cosmic microwave background radiation of the Universe. But, of course, such travels will not satisfy us; there seems to be something “unreal” about them compared to how such movements look in science fiction. You select the desired era on the screen, press the button - and...

Interestingly, Einstein's equations do not impose restrictions on such targeted trips into the past. Therefore, some theorists, discussing this, assume that when moving at a speed greater than the speed of light, time in this reference frame will flow in the opposite direction relative to the rest of the Universe. On the other hand, Einstein’s theories still prohibit such movement: upon reaching the speed of light, the mass will become infinite, and in order to accelerate the infinite mass even a little faster, infinite energy will be needed. But, most importantly, the introduction of such time machines can violate an equally fundamental cause-and-effect principle.

Imagine that you are a rabid Hillary Clinton supporter and decide to go back in time to beat up petty Donald Trump and drive him away from politics forever. If it worked, and Donald, after such a “teaching” back in the 1950s, decided to completely focus on business or playing chess, then how would you even know about his existence, let alone become disliked by this politician? .. These paradoxes are well revealed by the cult film series “Back to the Future,” and many scientists believe that they make travel into the past fundamentally impossible. On the other hand, we can always reason and fantasize. Let's try?

Through the ring

Approaching a sufficiently large black hole causes time to slow down. Falling inside is hardly an option: this activity is too dangerous and will not keep both you and your time travel machine safe. However, there is an option in which a black hole may turn out to be a completely suitable “portal” to the past. It was pointed out by calculations carried out back in the 1960s by the famous (and then very young) New Zealand physicist Roy Kerr, who studied the gravitational field of rotating black holes.

In fact, if an ordinary spherical body is compressed to a critical radius and forms a black hole singularity, then the mass of the rotating body is influenced by centrifugal forces. This angular momentum does not allow the formation of an ordinary “point” singularity, and instead a very unusual singularity appears - in the form of a ring of zero thickness, but non-zero diameter. And if the singularity of an ordinary black hole is not avoided by anyone who dares to get too close to it, then an observer approaching a ring-shaped singularity may well “overshoot” it - and end up on the other side.

Some scientists suggest that these properties can make “Kerr” black holes a kind of antipodes to ordinary ones - somewhere, in another space-time, they do not absorb, but, on the contrary, throw out everything that got into them in ours. The lucky one who avoids complete disintegration in the ring-shaped singularity will end up somewhere completely different in place and time. Where? Alas, no control is provided for here yet: it depends. So far, we are not even sure of the existence of a singularity of such a suitable form, not to mention the ability to control their occurrence and exactly which parts of the space-time continuum they connect. Does this remind you of anything?

Burrows and strings

If we remember our simplified two-dimensional continuum, which contains only one time dimension and one space dimension, then it will be easy for us to imagine how its fabric not only deforms and bends, but also breaks - as in the vicinity of massive bodies and in the singularity of a black hole . But where do such gaps lead? Apparently, again, to a different part of the continuum, as if we took a flat two-dimensional sheet and folded it in half, punching “holes” from one surface to the other. No theory prohibits the existence of such holes in our four-dimensional space-time - objects commonly known as wormholes.

Practically, physicists have never observed them anywhere, but there are a number of models that describe such wormholes, and their authors include very authoritative figures, including the American Kip Thorne and the British Stephen Hawking. The latter believes that wormholes exist only on Planck scales, in the “quantum foam” of virtual particles that are continuously born and annihilated in the vacuum of space-time. Together with them, countless wormhole tunnels are born and crumble, which for a tiny fraction of a second - randomly - connect completely different areas of space-time, and disappear again.

In order to use such burrows for any benefit, they will have to be learned to stabilize and increase in size. Alas, calculations show that this will require colossal amounts of energy, unimaginable even for American President, nor for all of humanity in any more or less foreseeable future. Therefore, another semi-fantastic concept, developed in the second half of the 20th century, gives somewhat greater hope for free movement in time. Thomas Kibble, Yakov Zeldovich and Richard Gott - we are talking about cosmic strings.

They should not be confused with superstrings from another well-known theory: cosmic strings in Gott’s view are very dense one-dimensional folds of space-time that arose at the dawn of the existence of the Universe. To put it simply, the “fabric” of space-time in that era had not yet been “smoothed out,” and some of the folds that were then preserved to this day. They stretched to tens of parsecs, but are still unusually thin (about 10-∧31 m) and carry enormous energy (density about 10∧22 g per cm of length).

Thinner than an atom, cosmic strings penetrate the space-time continuum, exhibiting powerful, albeit locally limited, gravity. But if we learn to manipulate them, bring them together, twist and weave them, we can “tune” the space-time around us in any way we want. Such superpowers promise full-fledged movements into the past and future at will, need or mood. Unless there are fundamental prohibitions on this. Remember "Back to the Future"?

Paradoxes and their resolution

Violation of cause-and-effect relationships when traveling into the past can confuse not only philosophers, but also any reasonable physical and mathematical calculations. Most famous example This is the “murdered grandfather paradox,” first described in science fiction back in the 1940s. The book by the French writer Rene Barjavel tells how a careless time traveler killed his own grandfather, so that subsequently he could not be born, fly into the past and kill his grandfather... Here any logic begins to fail: a broken chain of causes and effects arises, which Neither science nor our everyday experience accepts this.

One solution to this paradox may be “post-selection” of events in the Universe itself. In other words, once in the past, the traveler will not be able to do anything that would disrupt the correct course of causes and effects. The gun won’t work, or he won’t find his grandfather, or a thousand other accidents, oddities, embarrassments will happen, but the flow of things will not allow the Universe to be thrown off its measured course. But in general it is difficult to imagine any action in the past that would not have far-reaching consequences. Let us recall another term that comes from science fiction - the “butterfly effect”, which indicates the property of some systems to amplify a minor influence to large and unpredictable consequences. Perhaps the post-selective solution to the paradoxes of time will not allow us to travel through it anyway.

However, there is another approach, much more promising. According to the popular Multiverse hypothesis today, any possible (and impossible) option can be realized in the universe, they just all “diverge” into different parallel universes. You can go back in time and shoot your grandfather, and he really won’t give birth to your father, and he won’t give birth to you, but in a different way, parallel world. Just like somewhere out there Donald Trump may lose the election, or not be born at all, or turn out to be a famous cyclist. Just as somewhere there are worlds inhabited by green thinking jellyfish or generally subject to other laws of physics.

Thus, time travel paradoxically leads us to the problems of the fundamental structure of the space-time continuum. For problems that can only be finally resolved by the first experience of truly moving into the past, it is a pity that in our world this incredible event will have to wait for an indefinite period of time.

From the era of Queen Victoria to the present day, the concept of time travel has captivated the minds of science fiction lovers. What is it like to travel through the fourth dimension? The most interesting thing is that time travel does not require a time machine or something like a wormhole.

You've probably noticed that we are constantly moving through time. We move through it. On basic level concept time is the rate of change of the Universe, and regardless of whether we like it or not, we are subject to constant change. We get old, planets move around the sun, things break down.

We measure the passage of time in seconds, minutes, hours and years, but this does not mean that time flows at a constant speed. Like water in a river, time passes differently in different places. In short, time is relative.

But what causes temporary fluctuations on the path from cradle to grave? It all comes down to the relationship between time and space. A person is able to perceive in three dimensions - length, width and depth. Time also complements this party as the most important fourth dimension. Time does not exist without space, space does not exist without time. And this couple connects into the space-time continuum. Any event that occurs in the Universe must involve space and time.

In this article we will look at the most real and everyday possibilities travel through time in our universe, and also less accessible, but no less possible ways through the fourth dimension.

Train - real car time.

If you want to live a couple of years a little faster than someone else, you need to manage space-time. Global positioning satellites do this every day, beating the natural course of time by three billionths of a second. Time passes faster in orbit because the satellites are far away from the Earth's mass. And on the surface, the mass of the planet carries time along with it and slows it down on a relatively small scale.

This effect is called gravitational time dilation. According to general theory Einstein's relativity, gravity bends space-time, and astronomers use this consequence when studying light passing near massive objects (we wrote about gravitational lensing here and here).

But what does this have to do with time? Remember - any event that occurs in the universe involves both space and time. Gravity not only tightens space, but also time.

Being in the flow of time, you will hardly notice a change in its course. But quite massive objects - like supermassive black hole Alpha Sagittarius, located in the center of our galaxy, will seriously bend the fabric of time. The mass of its singularity point is 4 million suns. Such a mass slows down time by half. Five years in orbit of a black hole (without falling into it) is ten years on Earth.

Movement speed also plays a role important role in the speed of our time. The closer you get to the maximum speed of movement - the speed of light - the slower time passes. The clock on a fast-moving train will begin to be “late” by one billionth of a second towards the end of the journey. If the train reaches the speed of 99.999% of light, one year in a train car can transport you two hundred and twenty-three years into the future.

In fact, hypothetical travel to the future in the future is built on this idea, forgive the tautology. But what about the past? Is it possible to turn back time?

Time travel to the past

Stars are relics of the past.

We found out that traveling to the future happens all the time. Scientists have proven this experimentally, and this idea forms the basis of Einstein's theory of relativity. It is quite possible to move into the future, the only question remains “how fast”? When it comes to traveling back in time, the answer to this question is to look at the night sky.

Galaxy Milky Way about 100,000 years wide, which means light from distant stars needs to travel thousands and thousands of years before reaching Earth. Catch this light, and in essence, you are simply looking into the past. When astronomers measure cosmic microwave radiation, they peer into space as it was 10 billion years ago. But is that all?

There is nothing in Einstein's theory of relativity that rules out the possibility of traveling back in time, but the very possibility of a button that could take you back to yesterday violates the law of causality or cause and effect. When something happens in the universe, the event gives rise to a new endless chain of events. The cause always comes before the effect. Just imagine a world where the victim died before the bullet hit his head. This is a violation of reality, but despite this, many scientists do not exclude the possibility of traveling to the past.

For example, it is believed that moving faster than the speed of light can send people back to the past. If time slows down as an object approaches the speed of light, could breaking this barrier turn back time? Of course, as we approach the speed of light, the relativistic mass of the object also increases, that is, it approaches infinity. It seems impossible to accelerate an infinite mass. Theoretically, warp speed, that is, the deformation of speed as such, can deceive the universal law, but even this will require a colossal expenditure of energy.

What if time travel to the future and past depends less on our basic knowledge of space, and more on existing cosmic phenomena? Let's take a look at a black hole.

Black holes and Kerr rings

What's on the other side of a black hole?

Spin around a black hole long enough and gravitational time dilation will throw you into the future. But what if you fall right into the mouth of this space monster? We have already discussed what will happen when diving into a black hole. wrote, but did not mention such an exotic variety of black holes as Kerr ring. Or the Kerr black hole.

In 1963, New Zealand mathematician Roy Kerr proposed the first realistic theory of a spinning black hole. The concept involves neutron stars - massive collapsing stars the size of St. Petersburg, for example, but with the mass of Earth's sun. We included neutron holes in the list of the most mysterious objects in the Universe, calling them magnetars. Kerr theorized that if a dying star collapsed into a rotating ring of neutron stars, they would centrifugal force will prevent them from turning into a singularity. And since the black hole will not have a singularity point, Kerr believed that it would be quite possible to get inside without the fear of being torn apart by gravity at the center.

If Kerr black holes exist, we could pass through them and exit into a white hole. It's like the exhaust pipe of a black hole. Instead of sucking in everything it can, the white hole will, on the contrary, throw out everything it can. Perhaps even in another time or another Universe.

Kerr black holes remain a theory, but if they do exist, they are portals of sorts, offering one-way travel to the future or past. And although an extremely advanced civilization could evolve in this way and move through time, no one knows when the “wild” Kerr black hole will disappear.

Wormholes (wormholes)

Curvature of space-time.

Theoretical Kerr rings are not the only possible shortcuts to the past or future. Science fiction films - from Star Trek to Donnie Darko - often deal with theoretical Einstein-Rosen bridge. These bridges are better known to you as wormholes.

Einstein's general theory of relativity allows for the existence of wormholes, since the great physicist's theory is based on the curvature of space-time under the influence of mass. To understand this curvature, imagine the fabric of space-time as a white sheet and fold it in half. The area of ​​the sheet will remain the same, it itself will not deform, but the distance between the two points of contact will clearly be less than when the sheet was lying on a flat surface.

In this simplified example, space is depicted as a two-dimensional plane, and not the four-dimensional one that it actually is (remember the fourth dimension - time). Hypothetical wormholes work similarly.

Let's move into space. Mass concentration in two different parts The universe could create a kind of tunnel in space-time. In theory, this tunnel would connect two different segments of the space-time continuum with each other. Of course, it is quite possible that some physical or quantum properties do not allow such wormholes to arise on their own. Well, or they are born and immediately die, being unstable.

According to Stephen Hawking, the ten most interesting facts from the life of which we recently presented to you, wormholes can exist in quantum foam - the shallowest medium in the Universe. Tiny tunnels are constantly being born and torn, connecting separate places and times for short moments.

Wormholes may be too small and short-lived for human travel, but what if one day we can find them, hold them, stabilize them, and enlarge them? Provided, as Hawking notes, that you are prepared for feedback. If we want to artificially stabilize a space-time tunnel, the radiation from our actions can destroy it, just as the backflow of sound can damage a speaker.

We're trying to squeeze through black holes and wormholes, but maybe there's another way to travel through time using a theoretical cosmic phenomenon? With these thoughts, we turn to physicist J. Richard Gott, who outlined the idea of ​​the cosmic string in 1991. As the name suggests, these are hypothetical objects that could have formed in the early stages of the universe.

These strings permeate the entire Universe, being thinner than an atom and under strong pressure. Naturally, it follows that they provide gravitational pull to everything that passes near them, which means objects attached to the cosmic string can travel through time at incredible speeds. If you pull two cosmic strings closer together, or place one of them next to a black hole, you can create what is called a closed timelike curve.

Using the gravity produced by two cosmic strings (or a string and a black hole), a spacecraft could theoretically send itself back in time. To do this, one would have to make a loop around the cosmic strings.

By the way, quantum strings are currently a very hot topic. Gott stated that to travel back in time, you need to make a loop around a string containing half the mass-energy of an entire galaxy. In other words, half the atoms in the galaxy would have to be used as fuel for your time machine. Well, as everyone well knows, you cannot go back in time before the machine itself was created.

In addition, there are time paradoxes.

Time Travel Paradoxes

If you killed your grandfather, you killed yourself.

As we have already said, the idea of ​​traveling into the past is slightly clouded by the second part of the law of causation. Cause comes before effect, at least in our universe, which means it can ruin even the best-laid time travel plans.

First, imagine: if you go back in time 200 years, you will appear long before you were born. Think about it for a second. For some time, the effect (you) will exist before the cause (your birth).

To better understand what we are dealing with, consider the famous grandfather paradox. You are an assassin who travels through time, and your target is your own grandfather. You sneak through a nearby wormhole and approach the living 18-year-old version of your father's father. You raise the gun, but what happens when you pull the trigger?

Think about it. You haven't been born yet. Even your father hasn't been born yet. If you kill your grandfather, he will not have a son. This son will never give birth to you, and you will not be able to travel back in time to complete the bloody task. And your absence will not pull the trigger, thereby negating the entire chain of events. We call this the loop of incompatible causes.

On the other hand, one can consider the idea of ​​a sequential causal loop. Although it makes you think, it theoretically eliminates time paradoxes. According to physicist Paul Davis, such a loop looks like this: a mathematics professor goes into the future and steals a complex mathematical theorem. After that, he gives it to the most brilliant student. After this, the promising student grows and learns in order to one day become the person whose professor once stole a theorem.

Additionally, there is another model of time travel that involves the distortion of probability when approaching the possibility of a paradoxical event. What does this mean? Let's get back into the shoes of your girlfriend's killer. This time travel model could kill your grandfather virtually. You can pull the trigger, but the gun won't fire. The bird will chirp at the right moment or something else will happen: quantum fluctuation will prevent the paradoxical situation from taking place.

And finally, the most interesting thing. The future or past that you go to could simply exist in a parallel Universe. Let's think of this as the paradox of separation. You can destroy anything, but this will not affect your home world in any way. You will kill your grandfather, but you will not disappear - perhaps another “you” will disappear in a parallel world, or the scenario will follow the paradox patterns we have already discussed. However, it is quite possible that this time travel will be disposable and you will never be able to return home.

Completely confused? Welcome to the world of time travel.

Just imagine how many useful things could be done if we could travel in time! Kill Hitler, change dollars, convince yourself not to drink last night, convince Hitler to drink last night! But our heroes were busy with completely different things.

Father Pelligrino Ernetti

Father Pellegrino Ernetti, a Benedictine monk, lived almost his entire life in a monastery on the island of San Giorgio. He was a practicing exorcist and chairman of the prepolyphony department at the local conservatory. But, apparently, when he was not busy exorcizing demons and ancient music, he still had free time, because Father Ernetti invented the chronovisor - a device that allows you to travel back in time to see historical events with my own eyes.

According to the priest, he wanted to attend the opera “Trieste,” which created a sensation in Rome in 169 BC. There are testimonies from people who were present at the launch of the chronovisor. Pellegrino Ernetti's friend, the priest Francois Brun, even wrote the book “The Chronoprojection Machine - the New Secret of the Vatican,” which tells how he listened to Napoleon’s speeches and saw the crucifixion of Christ. To date, there is no information about the chronovisor, and everything that is offered on Avito is a pathetic fake.

Billy Meyer

Swiss Billy Meier's first contact with aliens, according to his testimony, occurred when he was five years old. An alien from the Pleiades constellation named Sfat contacted the boy and replaced Billy’s father. (And who among us has not suspected from time to time that his dad is also from another Galaxy!)

Then, when Sfat died, Billy was contacted by the Pleiadian Ascetic, with whom he kept in touch for 11 years. In 1975, when Billy reached puberty, Sfat’s granddaughter, Semjase, came to him. Don’t ask what she taught Billy, we don’t know ourselves; he didn’t talk about it, but he presented the public with many photographs of his alien friends and their spaceships.

Aliens came to him both from the past and from the future, as well as from parallel measurements, warning of future world cataclysms. Concerned about the well-being of earthlings, Billy reported on the upcoming Third World War, which was supposed to begin in November 2006, then in 2008 and finally in 2010. But, apparently, Billy’s friends are not very skilled in earthly history, because we are writing these lines to you from 2016.

Charlotte Ann Moberly and Eleanor Jourdain

Two schoolteachers of exemplary reputation went on an excursion to Versailles in 1901 and, getting lost in the gardens, ended up in 1792, right in the middle of French Revolution. They claimed to have seen Marie Antoinette in her last days before arrest. The Queen was sitting in front of the Petit Trianon and drawing when an armed mob of mobs marched on Paris.

Returning to the present, Charlotte and Eleanor wrote a book about what happened to them and called it "The Adventure". The book, of course, was immediately criticized, and the teachers were accused of fraud. Critics used a modern map of Versailles as evidence of deception. Charlotte and Eleanor described how, before going back in time, they crossed a bridge that did not exist in their time.

However, a map of Versailles from the 18th century was later discovered, on which the bridge described in the Adventure actually existed. However, as it turned out later, neither Charlotte nor Eleanor were specialists in the history of France and, before writing the book, they studied the issue with teacher meticulousness.

John Titor

A hero of internet forums in the early 2000s who claimed to be from the year 2036. John's final destination was 1975, and the goal was an IBM 5100 computer, needed in the future to destroy a computer virus aimed at destroying the world. It’s even strange that the name John Connor was never mentioned.

When asked how he ended up in 2000, John replied that he dropped in to see his family, since the Third world war, as a result of which America will be subjected to nuclear bombing by Russia. And the bombing was probably just what was needed, since the Americans in 2036 needed computers from the 70s.

Bob White

In 2003, a lot of people received an email asking for help in creating a large strain module with a generator and an induction motor (or something like that). To those who responded to the letter, the author willingly and in detail told his theory about time travel and methods for creating a device for their implementation. The author of the letter scheduled a meeting for his followers in a small town in Massachusetts on July 9, 2003, to which he conveniently failed to show up. We hope he was able to return to his planet. Or to a psychiatric hospital.

Victor Goddard

RAF Marshal Sir Victor Goddard was simply a magnet for the paranormal. In 1935, while flying in his biplane with an open cockpit, he encountered turbulence, during which he observed a strange picture as he flew over an abandoned airfield: the landscape below him seemed to have changed, there were planes standing on the airfield where no one should have been , and mechanics in blue clothes scurried between them. This surprised Goddard, since at that time all mechanics wore brown uniforms. Of course, none of his colleagues believed him, and the story was forgotten until four years later, when the Air Force actually changed the color of the uniform from brown to the same shade of blue that Goddard had seen.

The concept of a time machine conjures up images of an implausible device that is too often used in science fiction plots. However, according to Albert Einstein's general theory of relativity, which explains how gravity works in the universe, time travel is not just a figment of the imagination. And if time travel is a plot twist in movies, what about in reality?

Traveling forward in time, according to Einstein's theory, is absolutely possible. Essentially, physicists have managed to send tiny particles called muons, much like electrons, forward in time by manipulating the gravity around them. This doesn't mean that the technology to send people forward into the future will be possible in the next 100 years, but still.

1. Wormholes

Astrophysicist Eric Davis from International Institute Advanced Research EarthTech in Austin thinks it's possible. All you need is a wormhole, a theoretical passage through the fabric of space-time predicted by the theory of relativity.

Wormholes have not yet been proven, and if they are ever found, they will be so small that even a person, let alone a spaceship, cannot fit into them. With all this, Davis believes that wormholes can be used to travel back in time.

Both general relativity and quantum theory offer several possibilities for travel - for example, a "closed timelike curve" or a path that shortens space-time, that is, a time machine.

Davis argues that modern scientific understanding of the laws of physics "is teeming with time machines, that is, numerous solutions to the geometry of space-time that allow time travel or have the properties of a time machine."

As you can imagine, a wormhole would allow a ship, for example, to travel from one point to another faster than the speed of light - almost like in a warp bubble. This is because the ship will arrive at its destination before the light beam, taking a short path through space-time. The vehicle will thus not violate the universal speed limit rule imposed by light, since the ship itself does not travel at that speed.

Such a wormhole could theoretically lead not through space, but also through time.

“Time machines are inevitable in our physical space-time,” Davis writes in the paper. - "Passable wormholes turn on time machines."

However, Davis adds, turning a wormhole into a time machine won't be easy. It will take titanic efforts. This is because once a wormhole is created, one or both of its ends will need to be accelerated in time to its destination, which follows from general relativity.

2. Time machine: Tipler's cylinder

To use the Tipler Cylinder time machine, you need to leave Earth in a spaceship and travel into space to the cylinder that is spinning there. When you get close enough to the surface of the cylinder (the space around it will be mostly warped), you will need to circle it several times and return to Earth. You will arrive in the past.

How far into the past depends on how many times you orbit the cylinder. Even though your own time may seem to be moving forward as usual while you are going around the cylinder, outside the distorted space you will inevitably move into the past. It's like walking up a spiral staircase and finding yourself one step lower with each complete circle.

3. Donut vacuum

According to Amos Ori of the Israel Institute of Technology in Haifa, space can be twisted enough to create a local gravitational field, which resembles a donut of certain sizes. The gravitational field forms circles around this donut, so space and time are tightly twisted.

It is important to note that this state of affairs negates the need for any hypothetical exotic matter. Although it is quite difficult to describe how it will look in the real world. Ory says that mathematics has shown that at regular intervals a time machine will form inside the donut in a vacuum.

All you need is to get there. In theory, it would be possible to travel to any point in time since the time machine was built.

4. Exotic matter

In physics, exotic matter is matter that is somehow different from normal matter and has some “exotic” properties. Because time travel is considered non-physical, physicists believe that so-called tachyons (hypothetical particles for which the speed of light is a state of rest) either do not exist or are incapable of interacting with normal matter.

But when negative energy or mass—that exotic matter or matter—twists spacetime, all sorts of incredible phenomena become possible: wormholes, which can act as tunnels connecting distant parts of the universe; warp drive, which will allow faster-than-light travel; time machines that will allow you to travel back in time.

5. Cosmic strings

Cosmic strings are hypothetical 1-dimensional (spatially) topological defects in the fabric of space-time, left over from the formation of the universe. With their help, in theory, fields of closed time-like curves can be formed, allowing one to travel into the past. Some scientists propose using “cosmic strings” to build a time machine.

If you bring two cosmic strings close enough to each other, or one string close enough to a black hole, in theory it can create an entire array of “closed timelike curves.” If you make a carefully calculated "figure eight" on a spacecraft around two infinitely long cosmic strings, in theory you can end up anywhere, anytime.

6. Through the Black Hole

A black hole has an incredible effect on time, slowing it down like nothing else in the galaxy. Essentially, it is nature's time machine. If the flyby mission around the black hole were managed by a ground-based agency, the orbit would take them 16 minutes. But for brave people on board a ship that is close to a massive object, time would pass very slowly. Much slower than on Earth. Time would slow down by half for the team. For every 16 minutes they would only experience 8.

Ever dreamed of traveling somewhere in a different time? No, not at the usual speed with which we “boringly” move forward - second by second. Or:

• faster, so that you can climb far into the future, remaining at the same age;
• slower so that you can do much more than others in the same amount of time;
• in the opposite direction, so that you can go back to a past era and change it, perhaps changing the future or even the present?

This may sound completely sci-fi, but not everything on this list will be purely "fantastic": traveling through time is a scientifically possible process that is always with you. The only question is how you can manipulate it for your own purposes and control movement in time.

When Einstein proposed his theory of special relativity in 1905, the realization that every massive object in the universe must travel through time was just one of its astonishing consequences. We also learned that photons - or other massless particles - cannot experience time in their frame of reference at all: from the moment one is emitted to the moment it is absorbed, only massive observers (like us) can see the passage of time. From the position of the photon, everything is compressed into one point, and absorption and emission occur simultaneously in time, instantly.

But we have plenty. And anything that has mass is limited to always traveling less than the speed of light in a vacuum. Not only that, but no matter how fast you are moving relative to anything - whether you are accelerating or not, it doesn't matter - to you, light will always move at one constant speed: c, the speed of light in a vacuum. This powerful observation and awareness comes with a surprising consequence: if you watch a person moving relative to you, their clock will run slower for you.

Imagine a “light clock,” or a clock that works by reflecting light back and forth in an up and down direction between two mirrors. The faster a person moves relative to you, the greater the speed of light will be in the transverse (along) direction, and not in the up and down direction, which means the slower the clock will go.

Likewise, your watch will move slower relative to it; they will see time passing more slowly for you. When you get back together, one of you will be older and the other younger.

This is the nature of Einstein's "twin paradox". Short answer: Assuming you started out in one frame of reference (for example, at rest on Earth), and end up in the same frame of reference later, the traveler will age less because time will pass "slower" for him, and the one who stayed at home, will face the “normal” passage of time.

Therefore, if you want to accelerate through time, you will have to accelerate to near the speed of light, move at this pace for some time, and then return to your original position. We'll have to turn around a little. Do this and you can travel days, months, decades, epochs or billions of years into the future (depending on your equipment, of course).

You could witness the evolution and destruction of humanity; the end of the Earth and the Sun; dissociation of our galaxy; the heat death of the Universe itself. As long as you have enough power on your spaceship, you can see as far into the future as you want.

But getting back is a different story. Simple special relativity, or the relationship between space and time at a basic level, was enough to take us into the future. But if we want to go back in time, back in time, we need general relativity, or the relationship between spacetime and matter and energy. In this case, we regard space and time as an inseparable fabric, and matter and energy as something that distorts this fabric, causing changes in the fabric itself.

For our Universe as we know it, spacetime is pretty boring: it's almost perfectly flat, practically uncurved, and doesn't loop back on itself in any way.

But in some simulated universes - in some solutions to Einstein's general theory of relativity - it is possible to create a closed loop. If space loops back on itself, you can move in one direction for a long time, for a long time to get back to where you started.

Well, there are solutions not only with closed spacelike curves, but also with closed timelike curves. A closed timelike curve implies that you can literally travel through time, live in certain conditions and return to the same point from which you left.

But this is a mathematical solution. Does this mathematics describe our physical Universe? It seems not quite. The curvatures and/or discontinuities we need for such a Universe are wildly inconsistent with what we observe even near neutron stars and black holes: the most extreme examples of curvature in our Universe.

Our Universe may be rotating on a global scale, but the observed rotation limits are 100,000,000 times tighter than those allowed by the closed timelike curves we need. If you want to travel forward in time, you'll need a relativistic DeLorean.

But back? It might be better if you couldn't travel back in time to prevent your father from marrying your mother.

In general, to summarize, we can conclude that traveling back in time will always fascinate people at the level of idea, but, most likely, it will remain in an unattainable future (paradoxically). It's not mathematically impossible, but the universe is built on physics, which is a special subset mathematical solutions. Based on what we have observed, our dreams of correcting our mistakes by going back in time will likely remain only in our fantasies.