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.

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

It all comes down to the relativity of simultaneity.

On the Electrodynamics of Moving Bodies

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.

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.

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

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

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.