What is time travel?

Traveling forward in time is possible. But traveling back in time is impossible.

Traveling to the Future

Imagine a rest observer and a person who travels at a high speed. According to the theory of relativity, the traveler’s clock stalls more (relative to the rest observer) when the velocity of the traveler increases or when that person moves closer to a region with large gravitational force. The traveler’s clock freezes (according to the rest observer) when he/she attains the speed of light. But the traveler doesn’t notice any change in his clock as he moves along with it. It effectively means that time runs faster for the rest observer. If you watch the movie Interstellar, the protagonist’s daughter becomes older than himself after his voyage in the vicinity of a black hole (Gravitational Time Dilation). We can essentially consider that the protagonist “travels forward in time”. (Look at Twin Paradox)

Traveling to the Past

But traveling backwards in time is practically impossible. One cannot move faster than light. But you can “see” the past. If you observe the light coming from a place 13.7 billion light years away in a telescope, you can actually witness the beginning of the Universe. For those who don’t know, Scientists have estimated the age of the Universe to be around 13.7 billion years.