Avengers endgame, back to the future, and so many amazing sci-fi movies are based on time travel. Apparently, everyone loves them, but have you ever wondered if it could happen in real life? Most people would say no and answer that it is just some fantasy. However, physicists would say differently. 

There is something called ‘time dilation’ in relativity according to which time can slow down or move faster depending upon velocity(special relativistic time dilation) or gravity(gravitational time dilation). 

Special relativistic time dilation

First, let’s talk about special relativistic time dilation. According to Einstein’s special theory of relativity, time in a moving frame appears to run slower. You may have heard this famous spaceship example. If you travel in a spaceship with speed closer to that of light, and return after 10 years of travel, on earth about 30 years would have passed! Talking in scientific terms, in a particular frame of reference, suppose that two events occur at the same point in space. The time interval between these events, as measured by an observer at rest in this same frame (which we call the rest frame of this observer), is  Δt₀ . Then an observer in a second frame moving with constant speed relative to the rest frame will measure the time interval to be Δt where- 

The denominator is always smaller than 1( u<c), so  Δt   is always larger than  Δt₀    .Thus we call this effect time dilation(time getting elongated).

Now, let’s come to the other type of time dilation- gravitational. Gravitational time dilation is a form of time dilation, where the difference in elapsed time is due to the gravitational potential. The lower the gravitational potential (the closer the clock is to the source of gravitation), the slower time passes, speeding up as the gravitational potential increases (the clock getting away from the source of gravitation). Albert Einstein originally predicted this effect in his general theory of relativity and it has since been confirmed by tests of general relativity. For example, imagine that you are approaching a black hole which has gravity due to its mass(the gravitational potential is decreasing as you are moving towards the source of gravity). If your friends observe your clock, it would be seen as running slower and slower as you move ahead. Since the gravity is so strong, for them it would appear that you are moving towards the event horizon forever!  In simple terms, the closer you are to some strong source of gravity, the slower time passes for you. 

Earlier, in our spaceship example, the spaceship is moving at very high velocity, and when it returns back, there is a 20 year gap. We can say that the spaceship traveled 20 years back in time. I get it, not quite fulfilling your expectations.

You see, time travelling has consequences. To further limit the possibility of time travel, we have Einstein’s equation- E=mc² . To travel at high speeds we need a lot of energy to be synthesized within a fraction of a second, which is not possible currently. 

Conclusion

So, time travel is ‘possible’, we just haven’t figured it out yet. Don’t let down your childhood fantasies . Remember that once mobile phones were a fantasy but now they are a reality. I am sure that the future won’t let us down. 

https://en.wikipedia.org/wiki/Time_dilation
http://www.exactlywhatistime.com/physics-of-time/relativistic-time/#:~:text=In%20relativity%2C%20time%20is%20certainly%20an%20integral%20part,must%20be%20flexible%20and%20relative%20to%20accommodate%20this.