According to Einstein’s theory of relativity, the speed of light in a vacuum is an absolute constant, and modern physics is based on this assumption.
Not surprisingly, early measurements of the speed of light varied considerably, but by 1927, the measured values had converged to 299,796 kilometers per second. At the time, the leading authority on the subject concluded, “The present value of c is entirely satisfactory and can be considered more or less permanently established.”20 However, all around the world from about 1928 to 1945, the speed of light dropped by about 20 kilometers per second.21 The “best” values found by leading investigators were in impressively close agreement with each other. Some scientists suggested that the data pointed to cyclic variations in the velocity of light.22
In the late 1940s the speed of light went up again by about 20 kilometers per second and a new consensus developed around the higher value. In 1972, the embarrassing possibility of variations in c was eliminated when the speed of light was fixed by definition. In addition, in 1983 the unit of distance, the meter, was redefined in terms of light. Therefore if any further changes in the speed of light happen, we will be blind to them because the length of the meter will change with the speed of light. (The meter is now defined as the length of the path traveled by light in a vacuum in 1/299,792,458 of a second.) The second is also defined in terms of light: it is the duration of 9,192,631,770 periods of vibration of the light given off by cesium 133 atoms in a particular state of excitation (technically defined as the transition between the two hyperfine levels of the ground state).
How can the drop in c between 1928 and 1945 be explained? This remarkable episode in the history of physics is now generally attributed to the psychology of metrologists. Brian Petley, a leading British metrologist, explained it thus:
The tendency for experiments in a given epoch to agree with one another has been described by the delicate phrase “intellectual phase locking.” Most metrologists are very conscious of the possible existence of such effects; indeed ever-helpful colleagues delight in pointing them out! Aside from the discovery of mistakes, the near completion of the experiment brings more frequent and stimulating discussion with interested colleagues and the preliminaries to writing up the work add a fresh perspective. All of these circumstances combine to prevent what was intended to be “the final result” from being so in practice, and consequently the accusation that one is most likely to stop worrying about correction when the value is closest to other results is easy to make and difficult to refute.
Existing theories of varying constants, like Paul Dirac’s, assume that the changes are small, slow and systematic. Another possibility is that the constants oscillate within fairly narrow limits, or even vary chaotically. We are used to fluctuations in the weather and in human activities: newspapers and websites routinely report changes in the weather, stock-market indices, currency exchange rates and the price of gold. Maybe the constants fluctuate too, and perhaps one day scientific periodicals will carry regular news reports on their latest values.
The implications of varying constants would be enormous. The course of nature would no longer seem blandly uniform; there would be fluctuations at the heart of physical reality. If different constants varied at different rates, these changes would create differing qualities of time.
from "Science Set Free" by Rupert Sheldrake.