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Hughes–Drever experiment
Hughes–Drever experiments (also clock comparison-, clock anisotropy-, mass isotropy-, or energy isotropy experiments) are spectroscopic tests of the isotropy of mass and space. As in Michelson–Morley experiments, the existence of a preferred frame of reference or other deviations from Lorentz invariance can be tested, which also affects the validity of the equivalence principle. Thus these experiments concern fundamental aspects of both special and general relativity. Unlike Michelson–Morley type experiments, Hughes–Drever experiments test the isotropy of the interactions of matter itself, that is, of protons, neutrons, and electrons. The accuracy achieved makes this kind of experiment one of the most accurate confirmations of relativity (see also Tests of special relativity).
==Experiments by Hughes and Drever==
Giuseppe Cocconi and Edwin Ernest Salpeter (1958) theorized that inertia depends on the surrounding masses according to Mach's principle. Nonuniform distribution of matter thus would lead to anisotropy of inertia in different directions. Heuristic arguments led them to believe that any inertial anisotropy, if one existed, would be dominated by mass contributions from the center of our Milky Way galaxy. They argued that this anisotropy might be observed in two ways: Measuring the Zeeman splitting in an atom, or measuring the Zeeman splitting in the excited nuclear state of using the Mössbauer effect.
Vernon W. Hughes ''et al.'' (1960) and Ronald Drever (1961) independently conducted similar spectroscopic experiments to test Mach's principle. However, they didn't use the Mössbauer effect but made magnetic resonance measurements of the nucleus of lithium-7, whose ground state possesses a spin of . The ground state is split into four equally spaced magnetic energy levels when measured in a magnetic field in accordance with its allowed magnetic quantum number. The nuclear wave functions for the different energy levels have different spatial distributions relative to the magnetic field, and thus have different directional properties. If mass isotropy is satisfied, each transition between a pair of adjacent levels should emit a photon of equal frequency, resulting in a single, sharp spectral line. On the other hand, if inertia has a directional dependence, a triplet or broadened resonance line should be observed. During the 24-hour course of Drever's version of the experiment, the Earth turned, and the magnetic field axis swept different sections of the sky. Drever paid particular attention to the behavior of the spectral line as the magnetic field crossed the center of the galaxy. Neither Hughes nor Drever observed any frequency shift of the energy levels, and due to their experiments' high precision, the maximum anisotropy could be limited to 0.04 Hz = 10−25 GeV.
Regarding the consequences of the null result for Mach's principle, it was shown by Robert H. Dicke (1961) that it is in agreement with this principle, as long as the spatial anisotropy is the same for all particles. Thus the null result is rather showing that inertial anisotropy effects are, if they exist, universal for all particles and locally unobservable.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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