The novel "The Time Machine" by H.G. Wells popularizes the idea of time as the fourth dimension, making it possible to travel by analogy with the three spatial dimensions.

Einstein's groundbreaking paper on the theory of relativity introduces the idea of time dilation—the first hint that, in both real physics and science fiction, time is interchangeable with space.

The physicist Arthur Eddington first introduces the concept of the "arrow of time," and its relationship to entropy, in his book entitled "The Nature Of The Physical World".

Together with Nathan Rosen, Einstein shows that under certain circumstances it is possible to have a shortcut between two different points in spacetime (a wormhole) even between the past and the future.

Two American experimenters, Herbert Ives and G.R. Stilwell, confirm the reality of time dilation by observing fast-moving particles inside a TV-liker cathode ray tube.

Physicist Frank Tipler designs the first real-time machine (at least on paper). According to the design, a Tipler cylinder would use a series of rotating neutron stars to produce a closed time-like curve.

Stephen Hawking suggests there may be an undiscovered law of nature that prevents closed time-like curves, preventing time travel into the past.

Stephen Hawking throws a party for time travelers that gets a lot of publicity, but only after it takes place. Unfortunately, no time travelers turn up to the party.