Time Dilation: An Experiment With Mu Mesons 1962 PSSC; David Frisch, James Smith, MIT Physics

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“Using the radioactive decay of cosmic ray mu-mesons shows the dilation of time in a filmed experiment which takes place on top of Mt. Washington, N.H. and at M.I.T.in Cambridge, Mass. Explains how data are taken to determine the time distribution of the decays of mu-mesons at rest. Determines the counting rate for mu-mesons with speeds of about .99 the speed of light which arrive on top of Mt. Washington and measures the number that survive to reach sea level. Deduces from the experimental results that the mesons, moving at .99c, keep time at about 1/9th the rest rate. Features David H. Frisch and James H. Smith.”

Originally a public domain film, slightly cropped to remove uneven edges, with the aspect ratio corrected, and one-pass brightness-contrast-color correction & mild video noise reduction applied.
The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and/or equalization (the resulting sound, though not perfect, is far less noisy than the original).

Wikipedia license:

In the theory of relativity, time dilation is a difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from a gravitational mass or masses.

A clock at rest with respect to one observer may be measured to tick at a different rate when compared to a second observer’s clock. This effect arises neither from technical aspects of the clocks nor from the propagation time of signals, but from the nature of spacetime…

Time dilation of moving particles as predicted by special relativity can be measured in particle lifetime experiments. According to special relativity, the rate of clock C traveling between two synchronized laboratory clocks A and B is slowed with respect to the laboratory clock rates. This effect is called time dilation. Since any periodic process can be considered a clock, also the lifetimes of unstable particles such as muons must be affected, so that moving muons should have a longer lifetime than resting ones. Variations of experiments that actually confirmed this effect took place in the atmosphere or in particle accelerators. Other time dilation experiments belong to the group of Ives–Stilwell experiments measuring the relativistic Doppler effect. See also Tests of special relativity…

Theory

The emergence of the muons is caused by the collision of cosmic rays with the upper atmosphere, after which the muons reach Earth. The probability that muons can reach the Earth depends on their half-life, which itself is modified by the relativistic corrections of two quantities: a) the mean lifetime of muons and b) the length between the upper and lower atmosphere (at Earth’s surface). This allows for a direct application of length contraction upon the atmosphere resting in inertial frame S, and time dilation upon the muons resting in S′…

Frisch-Smith experiment

A much more precise experiment of this kind was conducted by David H. Frisch and [James] Smith (1963), who measured approximately 563 muons per hour in six runs on Mount Washington. By measuring their kinetic energy, mean muon velocities between 0.995 c and 0.9954 c were determined. The target was located in Cambridge, Massachusetts with a difference in height of 1907 m, which should be traversed by the muons in about 6.4 µs. Assuming a mean lifetime of 2.2 µs, only 27 muons would reach this location if there were no time dilation. However, approximately 412 muons per hour arrived in Cambridge, resulting in a time dilation factor of 8.8±0.8.

Frisch and Smith showed that this is in agreement with the predictions of special relativity: The time dilation factor for muons on Mount Washington traveling at 0.995 c to 0.9954 c is approximately 10.2. Their kinetic energy and thus their velocity was diminished until they reached Cambridge to 0.9881 c and 0.9897 c due to the interaction with the atmosphere, reducing the dilation factor to 6.8. So between the start (≈ 10.2) and the target (≈ 6.8) an average time dilation factor of 8.4±2 was determined by them, in agreement with the measured result within the margin of errors (see the above formulas and the image for computing the decay curves).

Other experiments

Since then, many measurements of the mean lifetime of muons in the atmosphere and time dilation have been conducted in undergraduate experiments…