B4    Muons 1: An Experimental Confirmation

For a long time there existed hardly any experimental confirmation to Einstein's relativity theories. This changed dramatically with the development of atomic clocks and modern electronics starting around 1960, providing a boost to relativity theory as an area of research.

A first confirmation of relativistic time dilation was found in the extended half-life of fast-moving muons (B.Rossi and D.B.Hall 1941). These come into being at a height of 10 to 20 km over the earth's surface, when high-energy particles of cosmic radiation strike atoms in the terrestrial atmosphere. Muons differ from electrons in that they have a much larger mass and are unstable. Slow muons have an average life span or 'half-life' of 1.52 µs. The extr
emely fast muons, which are produced by the cosmic radiation, move nearly at the speed of light and should therefore according to Newton travel about 1.52·10-6 · 3·108 m, approximately 456 m, during their life time. The flow of muons should therefore be halved, if one decreases the altitude by 456 m. However it actually decreases much more slowly. Since the muons are created at an altitude of about 15 km, they must travel 33 times the distance 456 m to reach sea level. Out of 233 muons only one should reach our detector. However this does not fit the observed density of the muon stream: in Germany approximately 200 muons per square meter per second are counted at sea level.

CERN scientists tested time dilation of muons quantitatively much more exactly in 1975. Large quantities of muons were produced and captured with 99.942% the speed of light in a special storage ring. It showed that their half-life at this speed amounts to 44,6 µs, in complete agreement with our formula of B2 (do the math!). More details are provided in [11-13f].

Muon storage ring at the Brookhaven National Laboratory, USA