Einstein, perhaps the greatest physicist in history, formulated the Theory of Relativity in 1905. Though complex, one of its practical aspects can be discussed rather simply. This is the principle at issue: Clocks in a moving frame of reference run slower than clocks in a stationary frame of reference. For example, if one observer stays on Earth with a first clock, and a second observer enters a spaceship with a second clock, when the spaceship is accelerated to near the speed of light (186,000 miles per second), the second clock will run at a slower rate than the first clock that is stationary on Earth.

This phenomenon is called the Twin Paradox: If the two observers are twins, the second twin in the spaceship will age at a slower rate than the twin who remains on the Earth. The practical implications are enormous: When the second twin returns home to Earth, he/she will be younger than the other twin!

The Twin Paradox, which is one of the implications of the Theory of Relativity, has been confirmed. Scientists confirmed it by accelerating a muon, which is an extremely small particle-like entity that is smaller than an atom, up to near-light speed. They used a particle accelerator for this task. A muon has a remarkably short life span, something less than a second, so ordinary humans cannot observe them in everyday life. With the proper instruments, physicists can observe them.

The Twin Paradox would be confirmed, they postulated, if the lifetime of the muon were extended. As the Theory of Relativity predicts, the muon had a longer lifetime when it was moving at near the speed of light. Physicists had made a tremendous confirmation of Einstein’s theory!