Special Relativity Time Dilation phenomenon experiment

The experiment with m-Mesons, is a classic experiment on the time dilation phenomenon, performed by B.Rossi and D. Hall in 1941 .

In the experiment, cosmic rays entering the earth's atmosphere from space were monitored - in particular the production of particles termed 'm-Mesons'.

Here are the essential details of this experiment:

1) A m meson is a charged particle that decays into an electron or positron , a neutrino and an anti neutrino.

2) As produced by cosmic rays the mesons travel through the atmosphere at speeds very close to that of light.

3) With the help of a scintillation counter, the arrival of the mesons may be detected and at a measured time later, their decay into an energetic electron. Observation of the second stage means that the meson has stopped in the detector so the decay of mesons AT REST is being recorded. Detector showed 568 counts that were obtained in one hour at the top of Mt Washington (6300ft above sea level.)

4) Since the mesons travel at nearly the speed of light, the time axis can be relabelled in thousands of feet and then we can see how many mesons should reach sea level if they decay in the same way in flight as they do at rest: On this basis if the detecting equipment is taken to sea level, 27 counts should be recorded in one hour. Accepting the result of this stage means we can go on to predict what fraction of a group of mesons should be lost through decay in a trip of a given distance (d) and duration~ (d /c) .

5) The scintillation counter is now taken down the mountain to sea level. At sea level a full hour's count is taken : Instead of 27 we have 412 mesons left at sea level. 412 counts corresponds to about 0.7msec on the decay clock. 0.7 divided by 6.3 equals 1/9. These mesons moving at near light speed keep time at 1/9 the rate they do when they are at rest with respect to us. To the observer on the ground, the mesons survive the journey in far greater numbers than one would predict from studying their decay at rest. The time-dilation factor of 9 corresponds obviously to a particular value of meson velocity v.