The Relativity of Time

The relativity of time, as set forth in Einstein's theory, has been affirmed once again, with new higher precision. Time dilation is the name for the notion that elapsed time as recorded by two observers with identical clocks will differ if one of the observers is traveling at a velocity v with respect to the other. The amount of dilation will become more noticeable as v becomes a larger fraction of the speed of light. In an experiment performed by Gerald Gwinner, Dirk Schwalm and their colleagues at the Max Planck Institute for Nuclear Physics in Heidelberg the clocks are lithium ions. The ions are struck by laser light from in front and from the back, putting them temporarily into an excited state and inducing fluorescence. By comparing the resonant laser wavelengths with the transition wavelength of the stationary ion, and by taking into account the Doppler effect (the apparent wavelength of a wave emitted from a traveling source will always be different from a stationary source owing to bunching or thinning of the wave crests - but this has nothing to do with relativity) the researchers can arrive at a value for time dilation. In the Heidelberg experiment, the lithium ions moved with a speed of 19,000 km/sec, or about 6.4 % of the speed of light (and corresponding to an energy of 13.3 MeV, the largest energy obtainable at the local heavy-ion storage ring). The precision of the new time dilation measurement, an uncertainty of 2.2 x 10^-7, is about a factor of four better than the best previous value. (Saathoff et al., Physical Review Letters, upcoming article; contact Guido Saathoff, 49-6221-516-547; website)