Why can't there be a time machine? Creating a time machine
There is probably no other topic in the world as exciting as time travel. For centuries, humanity has not only been interested in its meaning, etc., but also dreamed of a time machine. As a result, many famous science fiction writers have created incredibly interesting novels and time travel stories that have become real bestsellers.
But will we ever be able to create a time machine and travel to the future or the past? Is this possible in principle, or is all this a figment of our imagination and the dreams of scientists and science fiction writers? You won't believe it, but today we know how to build a time machine. So now it’s a matter of time - when we finally create a real time machine and go to the distant future.
In September 2015, cosmonaut Gennady Padalka returned to Earth from his last, sixth flight into space. On this day he broke the world record for the time a person spent outside earth's atmosphere. This astronaut was in space for a total of 879 days. That's 2.5 years in orbit! During this time, spent in Earth's orbit at enormous speed, cosmonaut Gennady Padalka became a real time traveler, once again testing Einstein's theory of general relativity in action.
When Padalka returned to Earth for the last time, he essentially found himself in the future. True, he ended up in the future for only 1/44 of a second. This is exactly how much faster time passed for him during all 879 days spent in Earth’s orbit, compared to time for all of us who were on Earth all this time. That is, literally, cosmonaut Gennady Padalka traveled through time during all his flights... into the future.
As a result, our Russian cosmonaut turned out to be a fraction of a second younger than all those who remained on Earth all this time. As you can see, such time travel turned out to be very simple and did not involve the use of charged plutonium on the DeLorean car, which became famous after the release of the Back to the Future film trilogy.
The secret of Gennady's time travel is high speed in Earth's orbit, where time flows faster. In fact, if our astronaut had the opportunity to move in space for all 879 days at the speed of light, when he landed on Earth, he would literally find himself in the future, since many years would have passed on Earth during this period.
That is, according to Einstein's theory of relativity, the higher your speed, the slower time flows for you. Accordingly, if you move at near-light speed, not only time will slow down for you, but also all physical processes in the body. And when you return to Earth, you will find that in your absence, time on Earth has moved forward much further, and your peers have noticeably aged.
As a result, since the discovery of Einstein, who determined that time in our Universe is relative (that is, time flows differently for each of us), humanity, in fact, has learned the main “ingredient” of traveling to the future. It's about about speed. So if you want to literally travel into the future today, all you have to do is figure out how to get to near-light speed.
How can you travel through time scientifically?
Until the 20th century, it was believed that time is unchangeable and that for each of us it flows the same way, that is, that it is absolutely throughout the entire Universe. Accordingly, it was generally accepted that time travel was impossible. In the 1680s, Isaac Newton began to think about the nature of time, establishing that time flows regardless of external forces or your location. As a result, for many years the scientific community took as a basis all of Newton’s teachings about the movement of bodies and the passage of time.
But two centuries later, the scientific world expected a revolution in knowledge.
In 1905, the young scientist Albert Einstein developed the special theory of relativity, using his theory of general relativity as a basis. Einstein defined many new concepts related to time.
He established that time in the Universe is elastic and depends on speed, deceleration or acceleration depending on how fast an object or person is moving.
In 1971, an experiment was conducted that confirmed that time flows slower for us on Earth than for those moving above it at a faster speed. Moreover, the higher above the Earth we move at higher speeds, the faster time flows for us.
During this experiment, scientists sent four atomic clock instruments (cesium atomic clocks) into flight. This watch flew around the Earth. Next, the clock readings were compared with the same clocks that were on Earth at that moment. The experiment confirmed Einstein's theory that time flows faster for objects or people flying at speed above the Earth. Thus, as a result of comparing the clock readings, it turned out that the clocks that flew around the Earth went nanoseconds ahead compared to the clocks on Earth during the experiment.
By the way, your smartphones have one interesting technology that also confirms Einstein’s theory.
"WITHOUT EINSTEIN'S GENERAL THEORY OF RELATIVITY
OUR GPS/GLONASS SYSTEM WILL NOT WORK" .
We are talking about a satellite navigator (GPS, or GLONASS system) built into our phones, which, with the help of satellites in Earth orbit, receives a signal about the location of our smartphone.
After all, due to the fact that satellites in orbit move at high speed and are far from the Earth, it turns out that time moves faster for them than for our smartphone located on Earth. As a result, it is periodically necessary to synchronize the time of navigation equipment on Earth and the electronics used on the satellites. Otherwise, the satellites would incorrectly determine our location.
By the way, in addition to the fact that time is relative for each of us, Einstein calculated the exact speed of light, which is 300,000,000 meters per second. Einstein also established that this is the speed limit in the Universe. That is, according to Einstein's theory, nothing in the world can move faster than the speed of light.
The last idea of the great scientific thinker was that gravity also slows down time. Einstein discovered that time runs faster where gravity is weaker. For example, time moves slower on Earth, the Sun, and Jupiter than in outer space because these planets have a greater force of gravity (gravity), which affects the passage of time. Accordingly, the passage of time, as you see, is influenced not only by the speed of an object in space, but also by the force of gravity.
For example, time at the top of Everest passes faster than time at its base. If you take an atomic clock, one of which you place at the top of a mountain, and the other one you leave lying at the foot, then exactly 24 hours later, the clock at the top will advance by nanoseconds. That is, in essence, the watch on Mount Everest will travel to the future. True, for a negligibly short time. This is possible due to the fact that the force of gravity at the top of the mountain will be weaker than at the foot.
Time machine of the subatomic world - Already a reality
But why did the Russian cosmonaut end up in the future just 1/44 of a second? The thing is that it moved in Earth orbit for 879 days at a speed of 27,000 km/h. As you can see, compared to the speed of light, at which time stops, the speed in low-Earth orbit is negligibly small to literally send an astronaut hundreds of years into the future. In fact, the astronaut made a leap into the future for an insignificantly short time.
Now let's see what happens if we create spacecraft, which will be able to fly faster than geostationary objects that currently orbit the Earth. No, as you can see, we are not talking about a commercial airliner capable of flying at 1,000 km/h, or a rocket flying to the ISS at 40,000 km/h. Let's think about an object that could accelerate to almost the speed of light, which is almost 300,000 km per second.
Do you think this is impossible in our nature? It turns out not. Of course, it is still very, very early to talk about any large object that can be accelerated to near-light speed. But we have learned to accelerate subatomic particles to the speed of light, literally sending them into the distant future. We are talking about the most high-tech project of scientists from many countries of the world in the entire history of mankind - the Large Hadron Collider, which can accelerate subatomic particles to almost the speed of light.
Believe it or not, this particle accelerator is capable of accelerating protons to 99.999999% of the speed of light. At this speed, relative time moves approximately 6,900 times slower compared to their stationary observers.
“THE LARGE HADRON COLLIDER...REGULARLY SENDS
SUBATOMIC PARTICLES INTO THE FUTURE.”
So, yes, we have learned to send atoms into the future. Moreover, scientists have been doing this for already last decade quite successfully. But sending a person into the future is another matter.
But the most interesting thing is that given the fact that scientists have learned to regularly move particles at the speed of light, it is conceptually possible to send a person to travel into the future. The fact is that human travel into the future is really possible and is not prohibited by any law of physics.
In fact, in order, for example, to send a person to 3018, today it is enough to put him in a spacecraft and accelerate the shuttle to 99.995 percent of the speed of light.
Let's assume that such a ship has been created. So, imagine boarding a supership like this that is sent to a planet 500 light-years away (such as the recently discovered Earth-like planet Kepler 186f, which is 500 light-years away). For those who do not know or do not remember, let us remind you that 500 light years is the distance that light will travel in 500 years of its journey. Knowing the speed of light, you can calculate the incredible distance at which the Kepler space telescope managed to discover a planet with characteristics similar to Earth.
So now let's imagine that you board a spacecraft that is flying to the planet Kepler 186f. Next, your ship accelerates to the speed of light and flies for 500 years, moving almost at the speed of light. Having approached the planet, your ship turns around and flies back to Earth for another 500 years at the same near-light speed.
As a result, the entire journey will take you 1000 years. When the ship returns to Earth, it will be already 3018.
But wait, how can you survive in this spaceship for 1000 years? Surely people can't live that long?
This is where Einstein's theory of relativity comes to the rescue. The thing is that when you move 500 years (by earthly standards) towards the distant relative of the Earth at the speed of light, time will flow slower for you than for all the inhabitants of the planet.
So, when moving at near-light speed, your clock on the ship and all your processes in the body will slow down. For example, your clock on a spaceship will tick at 1/100th the speed of a clock on Earth. That is, having traveled a distance of 500 light years and the same amount back, you will age only 10 years, whereas on Earth 1000 years will pass during your journey.
But this is just a theory and our fantasies. Yes, as you can see, time travel is theoretically possible. It's real. Unfortunately, there is always a huge gap between theory and reality. After all, today we cannot build a spaceship that could accelerate almost to the speed of light. So how do we overcome the challenges of creating a time machine?
Will humanity soon be able to build a ship that can travel at the speed of light?
As you can see, in order to travel to the future, we need a spaceship that can accelerate to near-light speed. True, this is very difficult to implement. After all, there are huge engineering obstacles. Firstly, today humanity is still far from being able to build such a spaceship capable of traveling at the speed of light.
The fact is that today the fastest spacecraft ever created by mankind is solar probe "Parker", which will soon be launched into space. This space probe will be able to accelerate to a maximum speed of 450,000 miles per hour (724,204.8 km/h). Yes, it will be the fastest object created by man throughout its history. But compared to the speed of light, this speed is negligible. For example, at this speed you could get from Philadelphia to Washington in just 1 second. But during this time the light will cover the same distance 8 times.
Now imagine how much energy is needed to accelerate a spaceship to the speed of light. What fuel, then, is best used to generate incredible energy that could accelerate the ship to near-light speed?
Some scientists and astrophysicists propose using highly efficient anti-matter fuel (antimatter-based fuel) for such a spacecraft. By the way, many scientists around the world believe that such fuel could indeed be potentially invaluable in interstellar travel.
But beyond fuel, there is an even bigger problem for interstellar travel. We are talking about the safety of people who will travel at the speed of light. After all, such a spaceship will have to carry sufficient quantity supplies for crew members embarking on interstellar travel (food, water, medicine, etc.). But to ensure long-term travel in space, the ship must be large enough. As a result, the larger the ship, the more energy it will need to accelerate to the speed of light.
In particular, when accelerating to the speed of light, it must be taken into account that the acceleration must be smooth, since otherwise the people on the spacecraft will receive too much overload during acceleration, which is life-threatening.
But then it would take too much time to accelerate the ship to near-light speed. After all, in fact, the ship can be slowly accelerated, adding a little speed so that the overload experienced by the ship’s crew for a long time does not exceed 1g (usually, when we are on Earth, we experience this overload).
Thus, it may take too long to reach the speed of light, which will significantly increase travel time. And this ultimately minimizes the possible travel time to the future.
For example, using our example of traveling a distance of 500 light years with smooth acceleration, as a result of which the g-force will not exceed 1 g, our flight will take the clock on a spaceship not 10 years, but already 24 years. But nevertheless, if you move at near-light speed to a distance of 500 light years and back, you can still get to the year 3018.
Unfortunately, to create such an incredible space vehicle with such specifications, humanity will still need a lot of time, resources and, of course, a lot, a lot of money. But the same can be said about other large-scale, ambitious projects that seemed impossible just a few decades ago. We're talking about the gravitational wave detection project and the Hader Large Collider. Today these projects are already a reality and do not surprise anyone.
So who knows what awaits us in the coming decades. After all, it is quite possible that the next scientific megaproject will be the creation of a time machine (a spaceship capable of accelerating to the speed of light).
Is it possible to travel back in time?
But in the time machine we described, which may someday become a reality, travel to the future takes place in real time. That is, if you get into a spaceship today and accelerate to the speed of light, the time of your clock and the clocks of people on Earth will tick in reality. The only difference is that your clock will slow down while traveling.
As a result, the spaceship, which is a time machine, essentially throws you into the future in real time, but not back. That is, on such a spaceship you will not be able to go back in time. But is it even theoretically possible to time travel to the past?
Some scientists believe (not all, for example, Hawking proved that traveling into the past is impossible) that traveling into the past is also possible. But to do this, you need to find a place where you can bypass the laws of physics.
The most interesting thing is that there can be such places in the Universe.
For example, purely theoretically, traveling into the past is possible through a wormhole (wormhole in space-time), through which one can get into the past.
The problem is different - to find a similar place in space where there is a wormhole connecting a rift in space-time. Unfortunately, in most cases, such burrows disappear within nanoseconds after their appearance.
Meanwhile, according to Einstein's theory of relativity, such wormholes are real. The fact is that such wormholes can form as tunnels crossing through curved space-time. Theoretically, through such holes it is possible to send a beam of light to a certain point in space. Accordingly, theoretically, a beam of light can be sent into the past.
Fiction? Not at all. Look at the sky at night and you will see the light of thousands of stars that reached your eyes only today, despite the fact that many stars ceased to exist billions of years ago. The thing is that these stars are located at a great distance from us, and also, given that our Universe is constantly expanding, it turns out that the light of many stars came to us from the past.
Thus, as you can see, theoretically sending someone into the future is much more realistic than sending someone into the past. Therefore, in the future, most likely, scientists will be willing to send someone into the future first, rather than into the past. Unfortunately, this will not happen in the near future. After all, for this, humanity will still need to come up with a superfuel capable of accelerating the ship to near-light speed.
However, as you can see, traveling to the future is real and possible. But this requires huge funding. According to many scientists, if today many states united and financed a project to create a spaceship capable of moving at the speed of light, then within 20 years such a ship would become a reality.
Well, for now, to enjoy the effect of a time machine, we can only review famous films about time travel, as well as re-read various popular science fiction books.
Moreover, many films actually show what space travel through time might look like. For example, watch the old original Planet of the Apes movie, where the astronauts thought they were on another planet like Earth, run by apes instead of humans.
But in fact, the astronauts arrived on the same planet Earth in the future, where for some reason monkeys seized power on the planet. Essentially, in this film, the astronauts arrived in the future of planet Earth as their journey through space was accomplished at the speed of light. This movie accurately depicts Einstein's theory of special relativity and shows how man can travel into the future.
Who didn’t read H.G. Wells’s novel “The Time Machine” as a child? Who didn't feel excited when watching the film of the same name? The possibilities of a person controlling a time machine are limitless, and for the most part people consider such a project feasible in the distant future, when scientists discover something there. But how realistic are these expectations? Are there laws preventing such travel? Are there at least suggestions about the possible mechanism of such a machine?
In order to determine the possibility of time travel, it is necessary, first of all, to decide what time is. But there is a difficulty with this - there is no consistent definition of time. I thought about time back in 400 AD. Blessed Augustine.
"Time is one of the greatest mysteries of the Universe. The river of time carries us all away, without exception, regardless of our desire and even against our will."
“How can there be these two times, past and future, when the past no longer exists and the future does not yet exist? And if the present always remained the present and did not go into the past, then it would no longer be time, but eternity.”
“If the Lord is omniscient and omnipotent, then is He bound by the passage of time?” “The Lord is omnipotent and therefore cannot be limited by anything, including the passage of time; therefore, he must exist outside of time.”
Much of Augustine's reasoning has not lost its relevance today. There are few theories explaining the phenomenon of time. Newton believed that time is independent and unchangeable, flowing at times at the moment infinitely in one direction. After the tremendous success of special and general relativity, it became clear that time and space are inextricably linked together. But why time is destined to play a special dimension, what the physical meaning of this is, is still not clear. There were different assumptions. N.A. Kozyrev believed that time is a physical process and time can have a mechanical effect on devices. A.I. Veinik believed that time is a special chronal field that affects objects. Oros di Bartini considered time to be three-dimensional. The successes of physicists in explaining the special status of time are so rare that a breakthrough in this direction cannot be expected in the near future. But, despite this, discussions about the possibility of building a time machine are gradually migrating from the novels of science fiction writers to serious articles by theoretical physicists.
Van Stockum's time machine.
In 1937, W. J. Van Stockum found a solution to Einstein's equations that made time travel possible. He calculated that if he took an infinitely long cylinder and spun it to a speed close to the speed of light, it would drag the matter of space-time with it. (This “frame-dragging” effect is also known as “frame-dragging” and is designed for rotating black holes.)
Any brave soul who dared to walk past the cylinder would be sucked inside at fantastic speed. At the same time, it would seem to an outside observer that that person exceeded the speed of light. Although physicists themselves noticed this danger in 1937, when W. J. Van Stockum found a solution to Einstein's equations, which made time travel possible. He calculated the effects of an infinitely long rotating cylinder. Although it is physically impossible to build an object with infinite dimensions. Van Stockum himself did not understand then that by flying around the cylinder, in fact, you can go back in time, to the moment preceding the moment of departure. The faster the cylinder spins, the further you can travel back in time (with the only limitation being that you could not travel back to a point in time before the creation of the cylinder itself). Unfortunately, Van Stockum did not fully develop his theory; during World War II, he died fighting in the Royal Netherlands Air Force against Germany.
Kurt Gödel's time machine. 
In 1949, the talented mathematician Kurt Gödel proposed a mathematical model of a time machine. He found one of the solutions to Einstein's gravity equation, more complex with some parameters of the gravitational field or space-time curvature, which humanity has never encountered. Kurt Gödel, an Austrian mathematician, worked with Albert Einstein at the Princeton Institute for Advanced Study.
He suggested that the entire Universe rotates. Like the case of Van Stockum's rotating cylinder, everything is carried along by spacetime. Such a natural time machine. For example, a Universe of our size, according to Gödel, would have to complete one revolution every 70 billion years, and the minimum radius for time travel would be 16 billion light years. However, when time traveling to the past, you must travel at just below the speed of light.
Einstein did not welcome the possibility of time travel in theory, which led to logical paradoxes, similar to the grandfather paradox. He wrote: "Kurt Gödel's work, in my opinion, represents an important contribution to the general theory of relativity, especially to the analysis of the concept of time. The problem addressed in the work bothered me even during the creation of the general theory of relativity, and I never achieved success in its resolution... The “earlier-later” distinction is erased when considering points of the Universe that are far apart from each other in the cosmological sense, and when taking into account the direction causal connections the paradoxes that Mr. Gödel speaks about arise... It will be interesting to figure out whether they can be discarded due to insufficient physical justification.”
In 1968, academician A.D. Aleksandrov proposed to evaluate the physical conditions under which Kurt Gödel’s time machine is realized. It turned out that to implement this mechanism, a speed slightly less than that of light was required, or a density of matter surrounding the time machine of the order of 10^28 g/cm^3, which is consistent with Gödel’s own conviction that it is impossible to build a time machine not because of logical contradictions, but only for technical reasons.
Kip Thorne's Time Machine 
In 1988, Kip Thorne proposed new model time machines. Kip Thorne, along with his colleagues Michael Morris and Ulvi Yurtsiever, Thorne announced that a time machine could be constructed if strange forms of matter and energy, such as “exotic negative matter” and “negative energy,” were somehow obtained. Why were such exotic forms of matter necessary? In order to cut out a wormhole from space that connects two sections of space “A” and “B” in the shortest possible way. Section B of the wormhole must be launched at near-light speed, then slowed down, and returned back at the same speed. In portal A, time will move at a normal pace; in portal B, time will slow down. It is proposed to dive into portal B, almost instantly reach A, and then, as quickly as possible, rush to portal B through outer space. This will result in a time loop, which is equivalent to a time machine. 
However, placing portal B in a strong gravitational field will lead, in accordance with the equivalence principle, to a similar result. However, there is some difficulty here that may be decisive. The direction of the arrow of time in the wormhole not only does not coincide, but has almost the opposite direction in relation to the arrow of time in the rest of space. Maybe this is the ban that will nullify attempts to build a time machine.
The fact is that Stephen Hawking, an English astrophysicist who studies the problems of the beginning and end of the Universe and black holes, once set a task for his colleagues: to find a law that prevents time travel. None of my colleagues could find a justification for such a ban, called “protection of history.” 
There are other fundamental difficulties in creating Kip Thorne's time machine. This is the creation of a large amount of negative energy. Small quantity so-called negative energy can be observed in experiment. The theoretical possibility of such an experiment was proved by the Dutch scientist Henrik Casimir in 1933, showing that two uncharged parallel metal plates can create negative energy. In 1948, this minuscule force was actually measured, which proved the real possibility of negative energy. The Casimir effect uses quite unusual property vacuum. According to quantum theory, empty space is filled with “virtual particles”, and this is possible thanks to the Heisenberg uncertainty principle, which allows that the primordial classical laws can be violated if these violations are short-lived. For example, thanks to the uncertainty principle, there is some possibility that an electron and a positron could come into existence out of nothing and then annihilate each other. Since the parallel plates are very close to each other, these virtual particles cannot freely enter the space between the plates. Thus, since there are many more particles around the plates than between them, this creates an outward force that pushes the plates slightly towards each other. This effect was precisely measured in 1996 by Stephen Lamoreaux of Los Alamos National Laboratory.
Negative energy is also contained in a black hole - at its “event horizon”. As Jacob Bekenstein and Stephen Hawking proved, a black hole is not perfectly black because it emits energy, albeit slowly. This is because the uncertainty principle makes it possible for radiation to tunnel through the incredible gravity of a black hole. But since such a black hole loses energy, over time the “event horizon” narrows. Usually, if positive matter (for example, a star) is thrown into a black hole, the “event horizon” expands. But if we throw negative matter into a black hole, the “event horizon” will narrow. Thus, the emission of energy from a black hole creates negative energy near the "event horizon".
The Universe and Mizner's time machine. 
Misner proposed a simplified model of the Universe, with which it would be easier to do mathematical calculations. Let's imagine a certain room. The opposite walls of this room have mirror image properties and are identical to every point of the opposite wall. You can pass through walls, but in this case, having disappeared, say in the left wall, we immediately appear on the right. Also, the points on the front wall of the house are identical to the points on the back wall, and the points on the ceiling are identical to the floor points. This way, walking in any direction will cause you to pass through one of the walls and emerge on the opposite side. Now let's imagine that the walls are slowly converging at speed X1. Now, having passed the wall with our own speed X0, we will exit the opposite one with speed X0 + X1. Repeating this again, the speed will increase to X0 + X1 + X1. And so on each time until your speed reaches the speed of light. Stephen Hawking has carefully studied Misner space. He discovered that, from a mathematical point of view, the right and left walls were almost identical to the two mouths of the wormhole portal. In this way, a wormhole is formed, identical to the one necessary for the functioning of the time machine. However, he noticed a contradiction. If you use a flashlight, the light beam will become more blue-shifted each time it passes through the wall. Then the shift of radiation will occur in the ultraviolet part of the spectrum, then in the X-ray, and until the moment when the radiation of the flashlight becomes so energetically expressed that its gravitational influence itself begins to shift the walls of the room. This will lead to the collapse of the room - the Universe. Thus, Stephen Hawking found the main obstacle to creating a time machine - the radiation at the input of the time machine will be amplified so many times (re entry) that it will quickly reach an energy level comparable to the beginning of the Universe, the Big Bang. This radiation will simply burn any creature trying to pass the threshold of the time machine.
Gott's time machine. 
In 1991, J. Richard Gott of the Princeton Institute proposed another solution to Einstein's equations that allowed time travel. This new solution, without having to look for negative energy, is next to black hole or accelerate to the speed of light. Here we must say about cosmic strings - exotic formations that could remain after the Big Bang in the Universe. The diameter of these formations is less than the diameter of the atomic nucleus, but the length can reach millions of light years. Moreover, their mass is enormous, given their monstrous density. Due to the super-strong gravitational field, these formations “cut out” a “cone” in space. While studying Einstein's equations, he noticed that the space around cosmic strings has the topology of a cone. This means that having described a circle around the cone, we will note that the circumference is less than if the cone were straightened out on a plane - there is no cut out sector. Having described a circle around the cosmic string, travelers will notice that their path has become shorter due to the “cut out section” of space-time. But this does not make such a journey time travel. It's another matter if these cosmic strings move in relation to each other. The direction of time of the second string will be a combination of time and spatial changes of the first. Now, if the traveler moves with the first string, then the interacting second string will shorten both space and time. And if the speed of convergence of cosmic strings is comparable to the speed of light, then the effects of “contraction” of space and time will create conditions for the emergence of time loops, that is, a time machine. Gott recalls: “When I discovered this solution, I was extremely excited. The solution used only positive matter, which moved at no faster than the speed of light. By comparison, solutions that attract portals require the presence of a more exotic negative energy-dense matter ( that is, something that weighs less than nothing)."
Will a highly developed space civilization discover suitable cosmic strings in space moving towards each other at a speed of 99.999999996% of the speed of light? Some theorists deny the very possibility of the existence of such exotic matter, which no one has yet observed. However, even if they exist, their collisions are an even rarer event. Gott proposes to influence the cosmic strings in such a way as to create a loop that self-disintegrates due to its gravity. Then, by flying around the disintegrating loop, you can go back in time. However, he himself admits the difficulties along this path: “A collapsing loop of cosmic string, large enough for you to fly around it and go back at least a year into the past, would have to have the mass-energy of more than half the entire galaxy.” .
Ronald Malet's time machine. 
“In my work, I found another way. It turns out that according to Einstein’s theory, gravity can be created not only by matter, but also by light. If gravity can influence time, and light can create gravity, then it is natural that light can also influence time So my idea is that light should be used to manipulate time. My time machine would look like light, a cylinder of constantly circulating light.
Let's imagine that the coffee in this cup is a certain amount of space, and the spoon is a circulating sheaf of light. Now, when I start stirring the coffee with a spoon, I can see what is happening to the coffee. A ray of light moving in a circle does the same thing to space: space begins to wrap around it and creates a funnel. But if you rotate fast enough, then not only space, but also time will behave like this. According to Einstein's theory, time and space are inextricably linked, and what you do with space will invariably affect time. So the funnel will be not only spatial, but also temporal. This will allow you to travel through time."
"What I didn't know when I started working on the project is that there are certain limitations. For example, if I turned on the device today, a time loop would begin to form and I would leave it in this state for 10 years, then who -I could travel 10, 7, 5 years ago, to the time in which the machine was turned on. However, it would be impossible to travel to earlier times, because a time machine did not exist then. Thus, time travel is possible. , but only from the future, when the device is turned on, during that period and not earlier.
This explains why we have never seen time travelers from the future - because a man-made time machine in our time has not yet been built and turned on. In other words, I will never, for example, be able to visit my father, which I would really like. However, I must say that since I have achieved something in the theory of the issue, my father would be proud of me. My passion allowed me to succeed, but it was not all-consuming because I now know that in addition to wanting to control time, it is also important to live in time. And even if time travel is possible in practice and we are all masters of our fate, we all have only the present, and it is important to live this present to the fullest. This is what I learned while working on my project."
Amos Ori's time machine. 
According to Amos Ori of the Israel Institute of Technology in Haifa, space can be twisted enough to create a local gravitational field that resembles a donut of certain dimensions. 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 what will it look like in real world quite difficult to describe. 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.
Back in 1895, H.G. Wells' science fiction novel "The Time Machine" was published. The idea appealed to everyone, regardless of their level of education and social status. Dreamers happily traveled into the future and became observers of the latest technological achievements or returned to the past, trying to influence the course of events. Some not only dreamed, but also made attempts to recreate such a machine. What is there! Rumor has it that Einstein's Theory of Relativity arose in the wake of general hysteria around the time machine.
How long until time travel?
A century has passed since then, and interest in intertemporal movement has not waned. The concept of a time machine has entered the lexicon of science fiction writers as an undeniably existing device of the future. No, no, but attractive headlines appear in the newspapers: time? Exclusive photo!" And this means that people (some in the garage, some at the table in a scientific laboratory) are still struggling to create a real prototype. Their zeal is also fueled by the fact that almost all the equipment of the future, imagined by H.G. Wells, already exists : space rockets, stealth scouts, laser and others. So maybe it’s very close?
Time machine physics
The very idea of such a travel device can hardly be called fantastic. Any object follows a temporary path - it appears, ages, wears out, disappears. On the other hand, no one fully controls this process. Yes, technological progress has reached the point that a person can extend its service life. But turning back time is a question of another level. Therefore, a time machine (how to make one) is not just a task of moving forward in the flow of oncoming traffic of minutes and hours, but of reversible piloted movement.
Next question: moving where?
The Earth moves around the Sun. Its coordinates in space are constantly changing. In addition, the Earth rotates around its own axis, which means that any object on its surface, rushing into the future or past, may end up at that time, but in the wrong place. In outer space, for example. It turns out that it is necessary not only to figure out how to make a time machine, the instructions for which are unknown, to create some kind of device that will count down minutes in reverse order or in accelerated mode, but also to take into account spatial coordinates, which may be unpredictable. This means that the definition of a time machine can be confidently supplemented: it is a device for reversible manned movement in a time-dependent spatial coordinate system.
Einstein's experiment
It turns out that every modern person can easily derive the exact definition of a time machine. With creation, things are not so rosy. Although it cannot be denied that humanity has achieved some successes along this path. For example, the same Einstein was able to construct a system that could hide an entire aircraft carrier not only from the screens of tracking equipment, but also make it disappear from the field of view of observers and spectators for as much as 5 minutes. However, the crew members found it difficult to say where they had been all this time, or whether they had been anywhere at all. The experiment can hardly be called successful. The entire team soon died from an unknown disease. On the other hand, Einstein proved that man can control time. All that remains is to understand: is a real time machine possible, how to make it?
A modern view of time
Einstein's theory of relativity shows that time and space have a certain surface that is invisible to the human eye. This surface is extremely heterogeneous in composition and shape, has depressions and convex formations, wrinkles and cracks. A well-known example of a time cavity is a black hole.
Scientists have been working on their study for more than half a century. And they came to the conclusion that they are present not only in outer space, but also in the smallest particles of everything that surrounds us - atoms. These wormholes appear to be intertemporal tunnels that connect two points, shortening not only the distance between them by millions of times, but also slowing down time in proportion to the distance. They are not stable. They disappear and appear. And what causes these disappearances remains a mystery.
The famous thinker is confident that someday our technological potential will reach such a level that we will not only be able to take advantage of these intertemporal tunnels, expanding them to human size, but also synthesize our own tunnels.
Hawking claims that the next goal of scientists who have conquered distances will be to step into the past in the truest sense of the word. Humanity will learn to use wormholes for time travel.
Games with time
Time is the master of all things. And now the object of his desires is a time machine. How to create it? A person’s task is to move the cup of power in his direction. And if in reality this issue is still unsolvable, then in the virtual world everyone can manage time. For example, the real-time simulator of building your own universe Minecraft (this simulator involves the player moving around the universe in real time, which becomes a problem during the game and the gradual expansion of the world) allows each player to get their own time machine. What is missing in our world that is in the Minecraft project? How to make a time machine? Use mods that are widely available. In general, if our scientists had some secret channels to access data about space, we would also have a time machine.
Metaphysical time travel
While scientists and gamers are inventing tools for their own meditation master, they are taking a different path: traveling metaphysically.
Each body located in our world leaves behind a certain trace, woven from the energy of its life and vibration - the reaction of space to its movement. Experienced yogis know how to tune into this trace, claiming that the path they have chosen makes it possible to travel to the past and back. By the power of thought. Are they telling the truth? The answer is ambiguous. Shamans, clairvoyants, and people interested in magic talk about metaphysical time travel. They claim that reality is a kind of time machine. How to make it work? It is enough to achieve harmony with yourself, learn to renounce outside world, meditate and practice daily - and everything will work out: any person will be able to follow the path of time back and forth.
What about the future? Is it possible to get the golden key to the treasured door with the help of meditation? Real, but what kind of future will the observer find himself in? After all, this is an extremely changeable space, which is influenced by many events. Every minute, every second functionally depends on thousands of variables. If you predict the conditions and make a certain decision, you can learn about its further development. This method is used by clairvoyants and shamans. It’s as if they choose the right corridor out of thousands and walk along it, predicting the future.
Time for an experiment
Every person has at least once thought about how to make a real time machine in order to meet himself from the past. Look at yourself and maybe give some advice. In this regard, so-called “time capsules” have gained wide popularity, in which a group of people or someone alone composes a message for themselves in the future. The message may contain a video recording or things that evoke some emotions in the sender - in general, some information from the past that will be interesting to receive.
After a while they open up and, as it were, establish contact with themselves from the past. People on both sides of time are united by anticipation and thoughts. A local time continuum is created - a real time machine in action. How to do it is no longer a question.
« Each of us has a time machine: what takes us into the past are memories; what carries into the future - dreams»
Herbert Wells. "Time Machine"
What does a person dream about if his head is not occupied with war and mercantile ambitions? He dreams about his future, about the stars, about the well-being of those around him. This fact was reflected most colorfully in our area during the existence of Soviet Union, when state propaganda within the framework of the Cold War and the space race convinced people that science was the engine of progress. And there was nothing wrong with that.
Having seen the successes of mankind in the exploration of outer space, as well as achievements in other fields of science, people began to dream of what previously seemed only fantasy. For example, about eternal life and youth, perpetual motion, traveling to the stars and other galaxies, understanding the language of animals, levitation and even a time machine. However, science has again intervened in the matter, which time after time clips the wings of dreamers with its formulas, which prove that some dreams are unrealistic:
Creation perpetual motion machine of the first kind is impossible within the framework of the law of conservation of energy. The first law of thermodynamics prohibits us from doing this, so we can only wait for the next breakthrough theory in the field of physics and mathematics.
Understanding the language of birds and animals is, for obvious reasons, still a fantasy. Scientists are only in the early stages of deciphering the sounds animals make. The greatest success has been achieved in deciphering the language of dolphins, but this is still more like a ghostly future.
We will not be able to live forever, because our cells are programmed to die. There are no adequate theories about reprogramming yet and are not expected, so human life is only possible.
You can endlessly smash the dreams of humanity against the rocks of science, but there are things that are not prohibited by science. For example, time travel. One of the craziest, at first glance, ideas turns out to be real, because it does not contradict modern laws physics.
Humanity's first thoughts on time travel
It is impossible to establish when a person first thought about returning to the past or going to the future. Most likely, this thought has visited many throughout the entire existence of our species. Another thing is the rejection of ordinary dreams and an attempt to describe the idea of time travel within the framework of the relativity of time periods. And it was not scientists who were the first to notice this, but science fiction writers. Creative people are not constrained by scientific frameworks, so they can give free rein to their imagination. In addition, it turned out that most of the writers’ prophecies regarding our future came true.
In literature, time travel was described depending on the era in which its creators lived. For example, in novels of the 18th century, when religion still retained its weight in society and prevailed over other facts, writers associated everything unusual with divine intervention.
The first science fiction book about time travel is considered to be Samuel Madden's novel “Memoirs of the 20th Century. Letters concerning the State governed by George VI... Received by revelation in 1728. In six volumes.” In the book, which was written in 1733, the main character received letters describing events from the end of the 20th century, which were brought to him by a real angel.
The appearance of the "Time Machine"
The first mention of a certain man-made mechanism that allowed time travel appeared only at the end of the 19th century. In 1881, a story by American journalist Edward Mitchell, “The Clock That Went Backward,” appeared in one of the New York scientific journals. It talks about young man, who was able to travel back in time using an ordinary room clock.
Edward Mitchell is considered one of the founders of modern science fiction. He described in his books many inventions and ideas long before they appeared on the pages of other science fiction writers. He talked about faster-than-light travel, the invisible man, and more before anyone else.
In 1895, an event occurred that turned the world of fantastic prose upside down. In the English magazine The New Review, the editor decides to publish the story “The Story of the Time Traveler,” the first major work of fiction by H.G. Wells. The name “Time Machine” did not appear immediately, and was adopted only a year later. The writer developed the idea of the story “The Argonauts of Time,” written in 1888.
“The idea of the possibility of time travel arose in 1887 after a certain student named Hamilton-Gordon, in the basement of the School of Mines in South Kensington, where the meetings of the Debating Society were held, gave a report on the possibilities of non-Euclidean geometry based on the book Hinton "What is the fourth dimension"
A distinctive feature of the novel is that some moments of the protagonist's time travel were described using assumptions that later appeared in Albert Einstein's general theory of relativity. At the time of writing the story, it did not even exist.
Einstein phenomenon
Since ancient times, man has perceived the space around him as the value of three dimensions: length, width and height. Talking about time was the lot of philosophers; only in the 17th century the concept of time was introduced into science as a physical quantity, but scientists, including Newton, perceived time as something unchangeable and linear.
Newtonian physics assumed that clocks placed in any part of the Universe would always show the same time. Scientists were satisfied with the current state of affairs, because it is much easier to carry out calculations using such data.
Everything changed in 1915, when Albert Einstein stood up at the podium. The report on the Special Theory of Relativity (SRT) and the General Theory of Relativity (GRT) brought Newton's perception of time to its knees. In his scientific works time existed inextricably with matter and space and was not linear. It could change its course, speed up or slow down, depending on conditions.
The supporters of the Newtonian universe gave up. Einstein's theory was extremely logical, all the basic laws of physics continued to work flawlessly in it, so the scientific community could only accept it as a given.
« Imagination is more important than knowledge. Knowledge is limited, while imagination embraces the whole world, stimulating progress, giving rise to evolution».
Albert Einstein
In his equations, the scientist presented the curvature of space-time caused by the gravitational component of matter. They took into account not only geometric features objects, but also the density, pressure and other factors that they possess. The peculiarity of Einstein's equations is that they can be read both from right to left and from left to right. Depending on this, the perception of the world around us and the interaction of space-time will change.
First representations of time travel
After the scientific community recovered from the shock, it began to actively use Einstein's work in its research. Astronomers and astrophysicists were the first to become interested, because the theory of relativity worked for the Universe around us, which will undoubtedly help answer a number of questions that were previously considered rhetorical. At the same time, it turned out that the scientific works of the German physicist allow for the possibility of the existence of a time machine, even several of its types.
Already in 1916, the first scientific works on time travel with theoretical justification appeared. The first to announce this was a physicist from Austria, whose name was Ludwig Flamm, who at that time was only 30 years old. He was inspired by Einstein's ideas and tried to solve his equations. Suddenly it dawned on Flamm that with the curvature of space and matter in the Universe around us, peculiar tunnels could arise through which we could pass not only within space, but also within time.
Einstein warmly accepted the young scientist's theory and agreed that it met all the conditions of the theory of relativity. Almost 15 years later, he was able to develop Flamm's reasoning, and he, together with his colleague Nathan Rosen, were able to connect two Schwarzschild black holes with the help of a space-time tunnel that widened at the entrance, gradually narrowing towards its middle. In theory, one can travel through such a tunnel in the space-time continuum. Physicists called such a tunnel the Einstein-Rosen bridge.
To people outside the scientific world, Einstein-Rosen bridges are known by the simpler name “wormholes,” which was coined by Princeton scientist John Wheeler in the mid-20th century. The name “wormholes” is also common. This expression quickly spread among supporters of modern theoretical physics and very accurately reflected holes in space. Traveling through a wormhole would allow a person to cover vast distances in much shorter periods of time than traveling in a straight line. With their help, one could even go to the edge of the Universe.
The idea of "wormholes" has inspired science fiction writers so much that most science fiction since the mid-20th century tells us about the distant future of humanity, where people have mastered the entire space and easily travel from star to star, meeting new alien races and interacting with some of them. them into bloody wars.
However, physicists do not share the writers’ optimism. According to them, traveling through a wormhole may be the last thing a person sees. As soon as he falls beyond the event horizon, his life will stop forever.
In his book The Physics of the Impossible, the famous scientist and popularizer of science Michio Kaku quotes his colleague Richard Gott:
« I don't think the question is whether a person in a black hole can go back in time, the question is whether he can get out of there to show off».
But don't despair. In fact, physicists still left a loophole for romantics who dream of traveling through space and time. To survive in a wormhole, you just need to fly faster than the speed of light. The fact is that according to the laws of modern physics this is simply impossible. Thus, the Einstein-Rosen bridge is impassable within the framework of today's science.
Development of the theory of time travel
If traveling through a “wormhole” allows us, in theory, to get into the future, then with our past in this regard everything is much more complicated. In the mid-20th century, Austrian mathematician Kurt Gödel once again tried to solve the equations created by Einstein. As a result of his calculations, a rotating universe emerged on paper, which looked like a cylinder, in which time ran along its edges and was looped. Such a complex model is difficult for an unprepared person to even imagine, however, within the framework of this theory, it was possible to get into the past if you circled the universe along the outer contour at the speed of light or higher. According to Gödel's calculations, in this case you will arrive at the starting point long before the start itself.
Unfortunately, Kurt Gödel's model also does not fit into the framework of modern physics due to the impossibility of traveling faster than the speed of light.
Kip Thorne's Reversible Wormhole
The scientific community did not stop trying to solve the equations of the theory of relativity, and in 1988 a scandal occurred that put the whole world on its ears. One of the American scientific magazines published an article from the famous physicist and expert in the field of gravity theory, Kip Thorne. In his article, the scientist stated that he and his colleagues were able to calculate the so-called “reversible wormhole”, which will not collapse behind the spacecraft as soon as it enters it. For comparison, the scientist gave an example that such a wormhole will allow you to walk along it in any direction.
Kip Thorne's statement was very reliable and supported by mathematical calculations. The only problem was that it went against the axiom that lies at the foundation of modern physics - events of the past cannot be changed.
The so-called time paradox of physics was jokingly called “the murder of grandfather.” This bloodthirsty name quite accurately describes the scheme: you go back in time, accidentally kill little boy(because he pisses you off). The boy turns out to be your grandfather. Accordingly, your father and you are not born, which means you will not go through the wormhole and kill your grandfather. The circle is closed.
This paradox is also called the “Butterfly Effect,” which appeared in Ray Bradbury’s book “A Sound of Thunder” long before scientists developed the theory, in 1952. The plot described the story of a hero who went on a journey into the past, into the prehistoric period, when giant lizards reigned on the earth. One of the conditions of the journey was that the heroes had no right to leave the special path, so as not to cause a time paradox. However, the main character violates this condition and leaves the path, where he steps on a butterfly. When he returns to his time, a terrifying picture appears before his eyes, where the world he knew before no longer exists.
Development of Thorne's theory
Because of time paradoxes, it would be stupid to abandon the idea of Kip Thorne and his colleagues; it would be easier to solve the problem with the paradoxes themselves. Therefore, the American scientist received support from where he least expected it: from the Russian astrophysicist Igor Novikov, who figured out how to get around the problem with the “grandfather.”
According to his theory, which was called the “principle of self-consistency,” if a person finds himself in the past, then his ability to influence events that have already happened to him tends to zero. Those. The very physics of time and space will not allow you to kill your grandfather or cause the “butterfly effect.”
At the moment, the global scientific community is divided into two camps. One of them supports the opinion of Kip Thorne and Igor Novikov regarding travel through wormholes and their safety, others stubbornly deny it. Unfortunately, modern science does not allow either to prove or disprove these statements. We are also not yet able to detect wormholes in space due to the primitiveness of our instruments and mechanisms.
Kip Thorne became the chief scientific consultant on the famous science fiction film Interstellar, which tells the story of a man's journey through a wormhole..
Creating your own space-time tunnel
The wider the imagination of a modern scientist, the high altitudes he can achieve in his work. While skeptics deny any possibility of the existence of the Einstein-Rosen bridge, supporters of this theory offer a way out of the situation. If we are not able to detect a wormhole in our immediate vicinity, then we can create it ourselves! Moreover, there are already developments for this. For now, this theory is in the realm of science fiction, however, as we have already seen, most of the predictions of science fiction writers have come true.
Kip Thorne, along with his supporters, continues to work on the theory of wormholes. The scientist was able to calculate that it is possible to provoke the birth of a wormhole using the so-called “dark matter” - a mysterious building material in the Universe, which cannot be detected directly, but according to physicists, 27% of our universe consists of it. By the way, baryonic matter (the one that we are made of and can see) accounts for only 4.9% of the total mass of the universe. Dark matter has amazing properties. She doesn't emit electromagnetic radiation, does not interact with other forms of matter except at the gravitational level, but its potential is truly enormous.
According to Thorne, dark matter could be used to create a reversible wormhole large enough for a spacecraft to pass through. The only problem is that for this you need to accumulate so much dark matter that its mass will be commensurate with the mass of Jupiter. Humanity is not yet able to obtain even a gram of this substance, if the concept of “gram” is applicable to it at all. In addition, no one has canceled the need to travel at the speed of light, which means that despite all the achievements of mankind in the field of science, we are still at a cave level of development, and we are very far from real breakthrough discoveries.
Afterword
Ideas for inventing a real time machine, which would allow us to discover the mysteries of the past and see our future, are still unrealistic. However, this does not change the fact that the theory of relativity developed by Einstein continues to work for each of us. For example, finding a real time traveler will not be difficult even now. The faster a person moves, the slower time goes for him, which means that he is slowly but surely moving into the future. Airline pilots, fighter pilots, and especially astronauts working in orbit are real time travelers. Even if only by hundredths of a second, they were ahead of us, people living on Earth.
He launched the “Question to a Scientist” project, within the framework of which experts will answer interesting, naive or practical questions. In the new issue, Doctor of Physics and Mathematics Alexey Rubtsov talks about whether we can build a time machine.
Is it possible to create
time machine?
Alexey Rubtsov
Physicist, Doctor of Physical and Mathematical Sciences, Professor of the Department of Quantum Electronics, Faculty of Physics, Moscow State University, external researcher at the RCC
The question of the possibility of creating a time machine is a question of the universal applicability of the principle of causality and the closely related second law of thermodynamics. To put it simply in simple language, the principle of causality tells us that always and everywhere, in any frame of reference and for all phenomena, the effect cannot precede the cause. First thunder roars, and then a man crosses himself. The second law of thermodynamics, again deliberately simplifying, states that closed systems always change towards increasing disorder (entropy). For example, sugar dissolves in water over time because syrup has greater entropy than its constituent sugar and water separately. It takes energy to separate sugar and water again. (for example, heat the solution).
It is clear that the possibility of time travel would violate both of these laws: a man jumping a few seconds into the past could cross himself before a flash of lightning, and sending him back into the past sugar syrup, we would see how unmixed water and sugar arise from it by themselves.
Interestingly, no other physical laws establish the difference between the past and the future. Most of the equations do not change their form at all when changing the direction of the flow of time, the rest remain unchanged while simultaneously changing the direction of the time axis and the signs of several more physical quantities (simplest example This kind of systems are systems with magnetism, in which it is necessary to simultaneously change the sign of the time axis and the direction of the magnetic field).
It may well turn out that in the future we will hear about some kind of “quantum time machine.” But, unfortunately, this will not make time travel possible.
Thus, the principle of causality and the second law of thermodynamics in the modern picture of knowledge represent isolated statements - if suddenly it turns out that they do not hold, the rest of scientific knowledge will remain unchanged. An analogy can be drawn with Euclid’s fifth axiom: a theory based on the postulate of non-intersection of parallel lines correctly describes geometry on the plane, but the abolition of this axiom does not lead to disaster - the result is non-Euclidean geometry, which describes, for example, the properties of figures on the surface of a sphere.
The difference between physics and mathematics, however, is that mathematics is interested in any theories, and physics is only interested in those that describe our real world, which exists in a single copy. And in this real world, the principle of causality, apparently, is not violated. Of course, you can always think that we don’t notice these violations, but the likelihood of such a state of affairs is extremely low - like all fundamental laws, the principle of causality manifests itself in the most different aspects observable reality, and it would be difficult to ignore its violation.
One more thing needs to be said. Scientists love catchy names just as much as newspaper writers, and lately it has become fashionable to borrow terms from science fiction for new discoveries in order to attract the attention of the community to them. One of bright examples- the term “quantum teleportation”, corresponding to an absolutely real and very beautiful quantum information technology, which, however, has nothing in common with teleports from books and computer games. It may well turn out that in the future we will hear about some kind of “quantum time machine.” But, unfortunately, this will not make time travel possible.