Voyage to Epsilon Eridani 3: Time

So how do you prove that a fast-moving clock runs more slowly than a stationary one?

You could send a very accurate clock on a journey.

In October 1971, two physicists flew four atomic clocks on commercial airliners twice around the world, once in an easterly direction, and once in a westerly direction. These clocks were accurate to within a few nanoseconds. (There are 1 000 000 000 nanoseconds in a second.) Using calculations from the special theory of relativity, the physicists predicted how time measured by the clocks on the planes would differ from time measured by clocks on the ground. The experimental results matched the predictions, confirming that time dilation actually does take place with real clocks.

In 1975, physicists tried another experiment. They sent an atomic clock up in a U.S. Navy plane that traveled for 15 hours at 270 knots, or 140 meters per second—about 0.47 millionths the speed of light. As the plane flew back and forth over Chesapeake Bay, the onboard clock was compared to a clock on the ground by laser signal. During the flight, the onboard clock lost 5.6 nanoseconds, exactly the amount predicted by special relativity.

You could measure the lifetime of a muon.



	This spiral marks the path of a muon in a streamer chamber. Photo: CERN Photo

Muons are subatomic particles that live for just 2.2 microseconds. (There are 1 000 000 microseconds in a second.)

Muons are created when cosmic rays traveling through space strike molecules in the atmosphere, some 10 kilometers above Earth’s surface. Even moving at nearly the speed of light, a muon should only be able to travel about 700 meters before it decays, so you might think no muons could ever reach Earth.

Not so! Many muons make the entire 10 kilometer trip. From the perspective of Earth, these high-speed particles live ten times longer than they would if they were stationary, and consequently, they can travel ten times farther.

In 1966, physicists at CERN—the world’s largest particle physics facility—repeated this natural occurrence in a laboratory.They created muons and sent them zooming around a ring at 0.997 times the speed of light. Those high-speed muons survived twelve times longer than did muons at rest.