Einstein took literally the idea of the older German physicist Max Planck that light could be treated as if it came in little packets.

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.

When the speed of light was fixed, measurements that had previously been constant—an object’s mass, its size, and even the passage of time it experienced—became variable.

It all comes down to the relativity of simultaneity.

On the Electrodynamics of Moving Bodies

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

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

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.

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