When a journalist asked the British astronomer Sir Arthur Eddington if it was true that he was one of only three people in the world who could understand Einstein’s relativity theories, Eddington considered deeply for a moment and replied: “I am trying to think who the third person is.” In fact, the problem with relativity wasn’t that it involved a lot of differential equations, Lorentz transformations, and other complicated mathematics (though it did—even Einstein needed help with some of it), but that it was just so thoroughly nonintuitive.

In essence what relativity says is that space and time are not absolute, but relative to both the observer and to the thing being observed, and the faster one moves the more pronounced these effects become. We can never accelerate ourselves to the speed of light, and the harder we try (and faster we go) the more distorted we will become, relative to an outside observer.

Almost at once popularizers of science tried to come up with ways to make these concepts accessible to a general audience. One of the more successful attempts—commercially at least—was The ABC of Relativity by the mathematician and philosopher Bertrand Russell. In it, Russell employed an image that has been used many times since. He asked the reader to envision a train one hundred yards long moving at 60 percent of the speed of light. To someone standing on a platform watching it pass, the train would appear to be only eighty yards long and everything on it would be similarly compressed. If we could hear the passengers on the train speak, their voices would sound slurred and sluggish, like a record played at too slow a speed, and their movements would appear similarly ponderous. Even the clocks on the train would seem to be running at only four-fifths of their normal speed.