Time travel–the concept of going back in time to the past or forward to the future– has been an intriguing topic since ages. In Hindu mythology, the story of King Raivatakakudmi (Mahabharata), who travelled to heaven and was astounded to find that many decades had passed on earth, sounds the picture of time travel.
What is time? Most of us think of time as an absolute, which passes at a constant rate for all objects in the universe.
But physicist Albert Einstein described time as an illusion, which is relative and varies according to observers, depending on the speed of travel and gravity.
Approaching the speed of light, a person on a spaceship would be much younger than his twin at home. This dilemma can be explained by Einstein’s postulates of special theory of relativity, that is, uniformity in the laws of physics for everyone and uniformity in the speed of light for everyone.
To understand the twin paradox, let’s imagine a space-time dimension, where a beam of light is allowed to reflect and go forth perpendicularly between two sets of stationary mirrors by an observer, A. Here observer A is inside the set-up and sees the beam of light as the letter I.
Let’s replicate the same setup for another observer B, but this time, along with observer B, the sets of mirrors are travelling at a constant speed. Here observer B also sees the beam of light as the letter I, but observer A sees the beam of light as the letter M in moving frame.
The beam of light had travelled a larger distance in the travelling mirror from reference frame A. This is possible only if the time for observer B slows down to allow the light beam to travel a greater distance.
In other words, speed is the distance per unit time, and speed of light is constant for all observers (Einstein’s postulate).
At a glance from reference frame A, the distance travelled by B is more, so the value of time must be more to keep the speed of light constant. In other words, observer A experienced more time than the time experienced by observer B.
Einstein’s general theory of relativity explained that gravity can bend light. Anything that has a mass on four dimensional space time causes a dimple or curvature on space time, which allows the object to accelerate when it falls from outer surface.
As we say, time slows down at greater speed, the clock on Jupiter runs more slowly than on earth because of its strong gravitational field.
It is hard to say time travel to the past is physically possible.
But time travel to the future is an extensively observed phenomenon and well understood in the framework of the special and general theory of relativity.
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