Time travel is always a major climax of many science fiction movies. Actually, time-travel is a theoretical application first derived from Einstein’s Special Theory of Relativity. The concepts of twin paradox in Special theory and concept of gravitational time dilation in General theory are two checkpoints in the time-travel.
What amuses me most is the concept of ‘back-time travel’ or traveling to the past. Generally imagining, people see the interesting ideas of raveling in past or in future as the same thing. They usually imagine a machine like a regular car with both reverse and forward gears. However, to some scientists like Stephen Hawking (and me also), travelling back into the past is not possible.
To understand why, I would like to take you in a science fiction. Suppose a very big cosmic body (probably another planet or comet) strikes our Earth and our Earth vanishes including us. The heavenly cosmic body, thereafter the collision, somehow travels to the past (probably by passing through a wormhole).
Let’s recall what had happened till now in our story: Our planet was somehow attacked a very large body and then after vanishing us it travels to the past by a wormhole.
Now, the climax starts. Suppose that heavenly body, which is in the past now, collided with a giant star and vanishes. This directly implies that the body was destroyed before it will collide with our planet at present! In other words, we were destroyed by a heavenly body which does not exist!
What will happen now?
Only two possibilities are there. Firstly, our Earth dramatically ‘appears’ again after destruction or secondly, this theory of travelling back in time is not possible.
I rely on second. You?