Confirming Einstein's Twin Paradox

The twin paradox is one manifestation of time dilation, a phenomenon predicted by Einstein's Theory of Relativity. According to this physical law, moving clocks essentially run at a slower rate than stationary clocks. If two twins each have clocks and one enters a spaceship that is accelerated almost to light speed, the clock in the spaceship runs slower than the stationary clock, the twin in the spaceship ages at a slower rate, and upon return to Earth, the traveling twin would be younger than the other.

Simple but indirect confirmation of Einstein's twin paradox occurs in nature. Muons are subatomic particles that have a short life span - about 2.19 microseconds. Physicists posed the question: How is it that a muon, which is created by a cosmic ray when it enters the Earth's atmosphere about 25 kilometers above the Earth's surface, can reach the surface of the Earth before decaying? It travels at just under the speed of light. Moving at light speed, a muon would decay in 660 meters, but the Earth-bound muon, which is moving at just under light speed, travels 25 kilometers without decay.

The explanation is simple: Time dilation is at work. The muon is moving at near light speed, so it is in a moving frame of reference, and its clock runs at a slower rate than a stationary clock on the Earth. According to the muon's clock - the clock of its moving frame of reference - the lifetime of 2.19 microseconds does not elapse on its 25-kilometer journey to the Earth.

Muons have also been accelerated artificially in a particle accelerator in order to confirm the twin paradox. Among others, Bailey and his team at CERN performed the muon acceleration experiment in 1979. As predicted, the accelerated muon had a longer lifetime than a muon at rest. In fact, its lifetime was 29.3 times longer than a muon at rest, direct evidence of the twin paradox. The accelerated muon experiment has also been conducted in the United States.

Other experiments have confirmed time dilation in a simpler way. Atomic clocks in airplanes and satellites have been compared to stationary clocks on the Earth to confirm time dilation. Exactly as the Theory of Relativity predicts, the clocks in the airplanes or satellites run slightly slower than the clock on Earth. The planes and satellites are moving at classical speeds rather than near the speed of light so there is no significant change in the aging of the object, but a slight difference in time is registered.