Einstein's Theories of Relativity & Time Travel

Albert Einstein's theories of Relativity, shown below, laid the foundation of Time Travel. The following examples show how we can go to the future using these theories.

1) Theory of special relativity:
According to Einstein's special theory of relativity, the speed of light is the same for all observers, regardless of their relative motion or of the motion of the source of the light. As you travel faster, the time runs slower. If you travel closer to the speed of light, you will travel to the future compared to someone on earth.

Example 1: Time dilated by speed
Your friend is sitting on a chair on Earth looking at the clear sky and he is stationary (he is actually in motion around the center of the Earth because of the rotation of the Earth on its axis - the Earth is also revolving around the Sun - the Sun is in orbit around the center of our Galaxy and finally the Galaxy is moving towards other galaxies). But for the reference purpose, let us assume that he is stationary. You are driving around in a spaceship which is moving at a velocity "v" which is 99% of the speed of light. When you cross your friend, he turns his flashlight on. According to Einstein's theory of special relativity, the speed of light (c) will be same for both you and your friend. This happens because of time dilation that occurs when someone approaches the speed of light. In this case, time runs slowly for you.

Speed of light (c) = Distance travelled (d)/Time taken (t)

When your friend will look at distance travelled by light, he will divide it by the time elapsed in his watch. You will see less distance travelled by light and divide it by the time elapsed in your watch. Since your watch will be running slower than your friend's watch, less time will pass for you and that is why you will come up with the same speed of light. This all means that if you are travel at 99% the speed of light and come back to the Earth after one year, the time passed on the Earth will be seven years. So, when you come back to the Earth, you have travelled to the future by six years.

2) Theory of General Relativity:
General relativity adds gravity to special relativity (because Special Theory of Relativity is valid only for systems that are not accelerating). General Theory of Relativity describes gravity as a curvature of spacetime caused by the presence of matter. According to Theory of General Relativity, time gets dilated by gravity and the clocks in a gravitational fields run slow compared to clocks not in gravitational field. As the gravitational field becomes stronger, the clocks run even slower. A black hole is extremely massive and dense object in space that has strong gravitational force. It is a region of spacetime formed by a star collapsing under its own gravity. Black holes are black, because nothing, not even light, can escape from its extreme gravity. Around a black hole there is a boundary called “event horizon”. It is the point of no return and anything that goes inside will not come out. Due to gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it. So if you go close to black hole and come back, you have travelled to the future.

Example 2: Time dilated by gravity
Same scenario - your friend is sitting on a chair looking at the clear sky and he is stationary. You are in a spaceship moving towards a black hole. As you approach the black hole, time slows down for you. You put your brakes on and take a U-turn just before touching the event horizon and come back to the Earth. More time has passed on the Earth and less time passes for you. For example, 6 years have passed according to your clock & 10 years have passed on Earth's according to your friend's clock. So, when you come back to the Earth, you have travelled to the future by 4 years.