Build Your Own Time Machine

And because there is no special frame of reference, the view of the observer on the moving train is as valid as the view of the observer who isn’t moving. Either two events can be simultaneous, or one can come after the other, depending on how you are moving.

On the Electrodynamics of Moving Bodies

Anything moving at near the speed of light, Einstein realized, would experience phenomena totally different from the everyday.

Special relativity provides a double opportunity for time travel. If you travel at a sizable fraction of the speed of light, time for you slows with respect to the outside world. In effect, you move forward in time. And if you can travel faster than light, you will have a means to move backward in time.

Two absolute essentials of real time travel are the linkage of space and time, and the influence of gravity on the space-time continuum.

It all comes down to the relativity of simultaneity.

All movement has to be relative to something, and when Einstein was floating along with the sunbeam, it was not moving with respect to him.

However, most of the real mechanisms for time travel will involve moving spatially as well, reflecting the way that time and space are inextricably linked in the four-dimensional matrix of space-time.

This different way of looking at light not only explained the photoelectric effect, but also provided the foundations of quantum theory.