Time dilation, in the form of the so-called Twin Paradox, is a popular and long-standing plot element in science fiction, dating back at least to Robert Heinlein’s classic YA novel “Time for the Stars,” first published in 1956.

In the novel, one of two twin brothers goes on a voyage of interstellar exploration aboard a ship capable of traveling at nearly the speed of light. The other remains on Earth. When the starship returns and they meet again, the star-faring protagonist is still young, while his twin brother has grown old.

The underlying phenomenon, relativistic time dilation, is not only a basic consequence of Einstein’s theory of relativity. It has also been demonstrated experimentally many times, as the National Institute of Standards and Technology points out.

## Getting Relative

The discovery of time dilation and of the theory of relativity itself has roots going back generations beyond Heinlein’s novel, to the late 19th century. According to Space.com, in 1887, a physicist and chemist — Albert Michelson and Edward Morely, respectively — attempted to determine the Earth’s precise movement through the universe by measuring the speed of light as observed in various directions.

At the time, physicists assumed that space had a fixed geometric framework, called the “ether,” that light flowed through and which Earth was also moving through, orbiting the sun and also being carried along by the sun’s own motion through interstellar space.

But to Michelson and Morely’s surprise, their instruments measured the speed of light as the same in all directions. They checked their instruments; these worked fine but still gave the same results. Physicists were left puzzled for nearly 20 years, until a young Albert Einstein came up with an explanation that seemed completely bizarre but that also worked.

Einstein showed that the speed of light really IS the same for all observers, no matter where they are or how fast they are going. The rigid cosmic “ether” that Michelson and Morely had taken for granted did not exist. Instead, our measuring instruments — both yardsticks and clocks, along with everything else in the universe — turned out to be “rubbery” in a sense, seeming to stretch or shrink so that the measured speed of light always came out the same.

And in 1905, Einstein published his results, calling it the theory of special relativity.

## What’s so Special About Special Relativity?

Einstein’s original theory was called “special” relativity because it did not attempt to cover everything. It dealt only with the effects of high speeds on matter, setting aside the implications of acceleration or gravity and, thus, was formally applicable only to special cases.

Nevertheless, much of what popular culture associates with Einstein and relativity — including the speed of light as the cosmic speed limit and the famous equation E=mc^{2} — falls within the scope of special relativity. This also includes the time dilation effects of very high (relative!) speeds, including the “Twin Paradox” of interstellar travel at relativistic speeds.

In 1915, Einstein expanded his work to incorporate gravity and acceleration; this enlarged and “generalized” version of the theory was aptly called general relativity. However, for decades, general relativity was regarded as a bit like the Andromeda Galaxy — awesome but remote, far from any practical applications. As Universe Today notes, general relativity’s greatest claim to fame in popular culture was that only a handful of people in the world could understand it.

Over time, though, general relativity has come to seem a bit less abstruse. The discovery of black holes — objects so dense and massive that not even light can escape from them — was a big win: One of the strangest theoretical predictions of general relativity turned out to be for real.

## Black Holes and Time Dilation

As it turns out, general relativity also has its own implications for time dilation. In general relativity, the force of gravity is indistinguishable from acceleration, and both can cause clocks to run slower (as measured by an outside observer). If one of two twins orbited close to the event horizon of a black hole, that person would age more slowly than their sibling who remained on earth.

We do not have the technology to achieve this (yet!) any more than we can achieve the “old-fashioned” time dilation of a traveler going nearly the speed of light. But per Science News, atomic clocks have reached such a high level of precision that we can measure the time-dilation slippage between clocks that are one millimeter apart (about 1/25 of an inch). The clock that is one millimeter lower (closer to Earth’s gravitation) runs a tiny bit slower, just as general relativity predicts it should.

Score one for Albert Einstein!

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