General relativity says that time exists throughout the universe, and the speed of its flow can change depending on the place and circumstances. According to the theory of relativity, there are 2 factors that can affect the speed of the passage of time, these are gravity and the speed of movement in space.

### Let's start by slowing down time while moving:

An astronaut named Scott Kelly spent a year aboard the ISS. After returning to Earth, he turned out to be 2 minutes younger than his brother Mark, that is, he actually made a trip to the future.

Why did this happen? According to the special theory of relativity , all moving objects experience time dilation. The speed of an airplane, or a high-speed train, of course? not enough to feel a significant slowdown in time. But the higher the speed, the more noticeable the difference.

But at cosmic speeds, time dilation becomes significant. Each cosmonaut on board the ISS, which has a speed of 7.5 km / s, travels in time to the future. And the deceleration effect accumulates, respectively, the longer a person is in orbit, the farther he will go into the future.

In theory, if we can learn how to create ships that move at tremendous speeds, we can also learn to travel in time. If the speed of the ship is 0.995 the speed of light, which does not contradict the laws of physics, then 1 year on the ship will be equal to 10 years on Earth. And this is the "time machine".

### Now let's move on to gravitational time dilation .

The general theory of relativity says : the closer an object is to a massive body, and the more massive this body is, the slower time flows for a given object. If we take a couple of synchronized clocks, some are placed on Earth at sea level, and others on board the ISS, then they will desynchronize quite quickly (the clocks on Earth will lag behind) and the longer we observe, the greater the difference between the clock readings. This effect has to be taken into account when creating all space technology.

Based on the foregoing, we can conclude that if instead of the Earth there is a much more massive object, like a black hole, then the difference will be more significant.

### How to use this to create a time machine?

To begin with, let us give a more precise definition of a time machine: it is a machine into which a person can enter at time t₀ , spend some nonzero time t in it , and when he leaves this machine, he will find himself at time t₁ that is not equal to t₀ + t.

Let's simulate the situation that our planet is at a distance of 10 astronomical from the black hole. We go to the black hole, stop reaching a distance of 0.1 astronomical units from it and return. The closer we would fly to the black hole, the slower time would flow for us, and the culmination occurred at the moment of the reversal.

It turns out that if 1 year has passed for the crew (for example), then 2 years have passed for people on Earth. Thus, it turns out that the ship traveled in time to the future for 1 year. This ship absolutely satisfies the definition of a time machine that we gave above.

### Is it possible, on the contrary, to go to the past?

Traveling to the past is much more complicated than traveling to the future. In theory, closed time-like curves, which indicate the possibility of travel to the past, appear as solutions to the Einstein equation in four-dimensional space-time.

But many scientists, including the famous Stephen Hawking, believe that if quantum effects are taken into account in general relativity, loopholes that allow the existence of world lines crossing themselves will disappear.

Hawking believed that the laws of nature would prevent travel back in time. This is the basis of his hypothesis about the Security of chronology. However, until the moment when the theory of quantum gravity is developed, which describes all the observed phenomena, it is impossible to judge the possibility of travel to the past with absolute certainty.

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