EVIDENCE FOR TIME DILATION & LENGTH CONTRACTION

Experiments Involving Muons

Muons are cosmic ray particles formed in Earth’s atmosphere. When formed, they travel at velocities near the speed of light towards Earth's surface. However, due to their extremely short half life, majority of muons would decay before reaching the surface. 

 

In an experiment conducted at Mount Washington, quantities of muons were measured at the summit and bottom of the mountain. These numbers were then compared.

A greater proportion of muons was detected at sea level compared to what was originally predicted by considering the muons' average velocity and half-life at rest (1.5 µs) 

Evidence for Time Dilation

This observation can be accounted for using time dilation. When muons are moving near the speed of light (0.98c), their half-life is increased when measured by a stationary observer on Earth. This dilated half-life as observed by an Earth-bound observer allows muons to reach the surface of Earth before decaying.

Evidence for Length Contraction

This observation can also be accounted for using length contraction. In the  muons' frame of reference, the distance between the top of Mount Washington and sea level becomes shorter due to length contraction.

Therefore, in the same amount of lifetime before decaying, more muons can reach Earth's surface.

Hafele-Keating Experiment

In 1971, Joseph Hafele and Richard Keating deonstrated time dilation using  caesium-beam atomic clocks.

 

Twelve clocks were used in total. Four clocks were flown on a plane in an eastward direction, four were flown in a westward direction, and the last four remained on Earth. After being flown twice around Earth, the times on the three groups of clocks were compared.

The atomic clocks, flown eastward, moved slower (as observed from an Earth-bound observer) and consequently 'lost time'. In other words, a shorter time elapsed on these clocks compared to those on Earth.

In contrast, the atomic clocks flown westward, moved faster and gained time. In other words, a longer time elapsed on these clocks compared to those on Earth.

This experiment is not the best to discuss as evidence for special relativity as the effects of general relativity also affects the experimental data. However, after the effect of general relativity are accounted for, the observed differences in time were consistent with predictions made using time dilation equations. 

 

Evidence of Special Relativity from Particle Accelerators 

In 2014, an experiment was conducted to demonstrate time dilation using lithium ions travelling at 0.338c in a particle accelerator. 

The time interval between excitation of electrons in lithium ions and their return to ground state was measured when lithium ions are travelling at 0.338c and at rest. 

Physicists found that the interval was longer for moving lithium ions compared to those at rest, as measured by a stationary observer in the laboratory. This difference in time was consistent with time dilation. 

 

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