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.

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

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

The light speed barrier is a kind of discontinuity in reality—we can’t assume that things will continue in a steady fashion after passing through it (if that were possible).

From this simple idea would come Einstein’s masterpiece, general relativity. This builds on the concept of special relativity to take in the real world where we aren’t restricted to steady motion, and acceleration and gravity play their part. But general relativity is much more than an enhanced version of the laws of motion. It describes the behavi

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.

Since 1895, when Wells published his book, science has moved on with frightening speed. And that progress has included the theories that makes time travel possible in principle.

But Wells transformed it into science fiction (even though that term was yet to be invented), a fictional concept of practical, solid achievement, opening up speculation about how time travel might be achieved and what the implications of traveling back to the Crucifixion, or visiting the far distant future of humanity, might be.

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.