Nature constants: How variable are they really?
Over a period of seven years, PTB scientists compared the time display of two extremely accurate time bases – caesium and ytterbium atomic clocks. Their results are in line with a similar experiment at the British metrology institute NPL (National Physical Laboratory), as published in the latest issue of the Physical Review Letters.
Just having recognised that the universe is expanding, physicists in the years about 1930 started to speculate if in such a dynamic universe constant values are possible after all. Possibly even the so-called nature constants would be subject to variations over time? If so, would it be possible to use such variations to draw conclusions on the structure and the development on the universe? Since this time, scientists across the world were searching for related hints within their observations in astro- and geophysics. These resulted in conclusions which however were inconsistent and in part even contradictory.
Over the past years, atomic clocks have reached an accuracy that enables them to provide a contribution to the solution of this question even over a relatively short period: If the conclusions were correct, two atomic clocks based on different elements would expose a slight but predictable deviation – if the nature constants indeed would be moving targets. With this method, PTB scientists verified the constancy of an important natural constant over time – the ratio of the masses of a proton and an electron. A proton has 1836-times the mass of an electron; in addition to electromagnetic forces it also is subjected to the so-called strong interaction, which is responsible for keeping the atom nuclei. Case the natural constant would actually be not so constant, one could register how the relative strengths of these forces would change which in turn would affect the masses of the particles involved.
The mass of the electron determines the frequency of optical atomic clocks while the mass of a proton is reflected in the frequency of a caesium clock. During the development of atomic clocks over the past years, the caesium clock and the ytterbium clock have been compared over and over again, and with increasing accuracy. Currently these clocks are among the most exact atomic clocks in the microwave and the optical frequency range. The data gathered from these comparisons allowed the conclusion that the electron / proton mass ratio does not exhibit any verifiable change up to a relative uncertainty in the order of 10 by the power of -16 per year. Extrapolated over the age of solar system this translates into a potential change of about a ppm – for practical uses it thus can be regarded as a constant.
See also https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.210802
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