See Inside A Matter of Time

How to Build a Time Machine [Preview]

It wouldn't be easy, but it might be possible

In Brief


  • Traveling forward in time is easy enough. If you move close to the speed of light or sit in a strong gravitational field, you experience time more slowly than other people do--another way of saying that you travel into their future.
  • Traveling into the past is rather trickier. Relativity theory allows it in certain space time configurations: a rotating universe, a rotating cylinder and, most famously, a wormhole--a tunnel through space and time.


TIME TRAVEL HAS BEEN A POPULAR SCIENCE-FICTION THEME SINCE H. G. WELLS WROTE his celebrated novel The Time Machine in 1895. But can it really be done? Is it possible to build a machine that would transport a human being into the past or future?

For decades time travel lay beyond the fringe of respectable science. In recent years, however, the topic has become something of a cottage industry among theoretical physicists. The motivation has been partly recreational—time travel is fun to think about. But this research has a serious side, too. Understanding the relation between cause and effect is a key part of attempts to construct a unified theory of physics. If unrestricted time travel were possible, even in principle, the nature of such a unified theory could be drastically affected.

Our best understanding of time comes from Albert Einstein's theories of relativity. Prior to these theories, time was widely regarded as absolute and universal, the same for everyone no matter what their physical circumstances were. In his special theory of relativity, Einstein proposed that the measured interval between two events depends on how the observer is moving. Crucially, two observers who move differently will experience different durations between the same two events.

The effect is often described using the “twin paradox.” Suppose that Sally and Sam are twins. Sally boards a rocket ship and travels at high speed to a nearby star, turns around and flies back to Earth, while Sam stays at home. For Sally the duration of the journey might be, say, one year, but when she returns and steps out of the spaceship, she finds that 10 years have elapsed on Earth. Her brother is now nine years older than she is. Sally and Sam are no longer the same age, despite the fact that they were born on the same day. This example illustrates a limited type of time travel. In effect, Sally has leaped nine years into Earth's future.

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