An experiment has been performed to demonstrate the relativistic time dilation as a large effect, using only comparatively simple equipment. μ-mesons incident on top of Mt. Washington, New Hampshire, were selected to have speeds in the range between 0.9950 c and 0.9954 c. The number of these which survived to reach sea level was measured in Cambridge, Massachusetts. The number expected without time dilation was calculated from the distribution of decay times of these μ-mesons (i.e., the mean life as measured in both this experiment and others) and from the known distance of descent. The observed time dilation factor is 8.8±0.8 to be compared with the effective time dilation factor calculated for mesons of these speeds in our detection geometry 1/(1 − v2/c2)12 = 8.4 ± 2.

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