Build Your Own Time Machine

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.

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

However, we can’t dismiss that “glittering metal framework,” as the time machine is first described in the 1895 novel—it is hugely significant. Although traveling in time was not a new idea even then, fictional time travel before that book had relied on dreams or magic

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.

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.

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.

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

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.