Simple Nature - Light and Matter

p / Apparatus used for the testof relativistic time dilation describedin example 2. The prominentblack and white blocks arelarge magnets surrounding a circularpipe with a vacuum inside.(c) 1974 by CERN.is essentially a heavier version of the electron. Muons undergoradioactive decay, lasting an average of only 2.197 µs before theyevaporate into an electron and two neutrinos. The 1974 experimentwas actually built in order to measure the magnetic propertiesof muons, but it produced a high-precision test of time dilationas a byproduct. Because muons have the same electric chargeas electrons, they can be trapped using magnetic fields. Muonswere injected into the ring shown in figure p, circling around it untilthey underwent radioactive decay. At the speed at which thesemuons were traveling, they had γ = 29.33, so on the average theylasted 29.33 times longer than the normal lifetime. In other words,they were like tiny alarm clocks that self-destructed at a randomlyselected time. Figure o shows the number of radioactive decayscounted, as a function of the time elapsed after a given stream ofmuons was injected into the storage ring. The two dashed linesshow the rates of decay predicted with and without relativity. Therelativistic line is the one that agrees with experiment.q / Colliding nuclei show relativisticlength contraction.392 Chapter 7 Relativity