A beginner’s guide to time travel

Everyone can travel in time. You do it whether you want to or not, at a steady rate of one second per second. You may think there’s no similarity to traveling in one of the three spatial dimensions at, say, one foot per second. But according to Einstein‘s theory of relativity, we live in a four-dimensional continuum — space-time — in which space and time are interchangeable.

Einstein found that the faster you move through space, the slower you move through time — you age more slowly, in other words. One of the key ideas in relativity is that nothing can travel faster than the speed of light — about 186,000 miles per second (300,000 kilometers per second), or one light-year per year). But you can get very close to it. If a spaceship were to fly at 99% of the speed of light, you’d see it travel a light-year of distance in just over a year of time.