Actually, it is useful to divide this question into two: Is it possible to travel to the future? Is it possible to travel to the past? We know the answer to the first question. Yes, it is possible to travel to the future! This is a result of Einstein’s famous theory of relativity. Moreover, this is not a theoretical claim, and this is a part of our lives…
For example, according to GPS satellites that help us find our way around, we travel into the future for 38 microseconds (1 microsecond, one millionth of a second) every day. This may be a very small number, but if we do not take this into account, we will not be able to accurately determine our position.
According to the theory of relativity, time can flow at different rates for observers in different conditions. For example, time passes more slowly in an object moving relative to you while you are stationary. This is such a small difference that it is not noticeable at low speeds. However, as the speed difference approaches the speed of light, this effect becomes visible. This phenomenon is known as the shortening of time and has been demonstrated experimentally. One can easily travel to the future using the shortening of time.
For example, suppose that in 2018, a 20-year-old astronaut embarks on a five-year journey in a spaceship that moves at 99.5 percent of light. The astronaut will return to Earth at the age of 25. But the Earth will be in 2068, not 2023. Thus, the astronaut will have traveled 50 years into the future. Their peers with whom they went to primary school will be 70 years old. It is also possible for the astronaut to travel further into the future by accelerating to higher speeds. The famous 1968 film Planet of the Apes is based on the idea of travel in time just like this.
Speed is not the only factor affecting the flow of time. Gravity also bends space-time, changing the flow of time. Time slows down as gravity increases. This is exactly why we travel to the future according to satellites. Gravity on Earth’s surface is much higher than in space. Therefore, time passes more slowly on Earth’s surface than in empty space. Since the mass of the Earth is not very large, this slowdown can only be detected by very sensitive clocks such as atomic clocks, and time travel cannot be made that makes a practical difference.
However, time can be slowed noticeably by traveling to a place with high gravity, such as neutron stars or black holes. A spaceship that enters and exits such an environment can travel centuries or even millions of years into the future. The time travel that took place in the movie Interstellar was exactly that kind of time travel.
So, is it possible to travel to the past? The answer to this question is not as clear as the first question. There are solutions to the equations of relativity that allow travel to the past. However, it is a matter of debate whether the conditions that allow these solutions can be physically realized. If one can travel faster than the speed of light, time begins to flow into the past according to the special relativity equations and the person travels to the past.
However, according to the theory of relativity, no mass object can reach the speed of light, so it does not seem possible to exceed the speed of light. Another physical system that could allow travel to the past is space-time bridges known as wormholes.
These can be thought of as tunnels allowed by Einstein’s equations, with entrance at one point in space-time and exit at another point in space-time. If we can create wormholes, it is possible for us to travel to any time and place in the past. But to create such time corridors, we must have matter with negative energy known as exotic matter. For now, we do not know of such an item.
Another structure that can make a journey into the past possible is the Tipler cylinder, put forward by Frank Tipler. According to Tipler’s calculations, a time machine can be created if an elongated cylinder 10 times the mass of the Sun is made and this cylinder can be accelerated to rotate several billion times per minute.
A spaceship following a spiral path around this disk can travel to the past or the future, depending on its direction. The cylinder Tipler uses in the calculations is infinitely long. However, according to him, a very long and slightly faster rotating cylinder can also enable time travel.
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