Measurements of the Velocity of Light

Editor’s Note

Commenting on the earlier proposal of M. E. J. Gheury de Bray, Lord Kitchener asserted that observations suggesting a very slight slowing of the speed of light over time were probably due to error. Here Gheury de Bray responds. Although some techniques for measuring the speed of light could be criticized, he says, this cannot be said of his toothed-wheel method. He also notes that Simon Newcomb and Albert Michelson had independently reported similar results, which differed by just the amount expected if the speed of light were slowly changing. All the same, there seems now no evidence of such an effect, although the possibility of changes in physical constants over time is still debated.ft　　中文

M. E. J. Gheury de Bray suggests1 that measurements of the velocity of light show that it is changing according to a formula:

c = 299,900 – 3.855 T

(1)

T being measured in years from 1900.ft　　中文

Assuming that Planck’s constant and the energy E given out by an excited atom remain unchanged, the relation

Eλ = ch

(2)

shows that a change in c must be accompanied by a proportional change in λ. But the wave-length of the red calcium line has remained constant to within one part in five million for thirty years, only 0.5 percent of the change required by (1).ft　　中文

It is probable that there are unsuspected systematic errors in the determinations of c.

Kitchenerft　　中文

　　*

The decrease of velocity of light deduced from the red-shifts is of the order of 1 km./sec. in 6,000 years, or 1 in 600,000,000 for thirty years, which is quite consistent with the apparent constancy of the wave-length mentioned by Lord Kitchener. Our observations, being affected by unsuspected systematic errors, and covering only a third of a century, give what is probably a greatly exaggerated rate of slowing down. The remarkable fact is that all the determinations are unanimous in indicating the existence of such a variation, and the red-shifts, if interpreted so as to escape from fantastic results, confirm it.ft　　中文

The Table 1 of the communication referred to cannot be dismissed on the ground of “unsuspected systematic errors”. It is admitted that the method of the revolving mirror may suffer from physical bias, but no such reproach can be levelled against the toothed wheel method, which is only open to objections of a physiological nature. These can be readily overcome by substituting for the observer’s eye a photo-electric sensitive device. While France and the United States share between them practically the whole initiative in the measurement of c, Great Britain has only to her credit a conspicuous failure. Is there in this country no one who can redeem it from this position and settle this question, which lies at the basis of physical science, considered in its broadest aspect?ft　　中文

Two observations, of Newcomb (1882.7∶299,860) and of Michelson (1882.8∶299,853) agree so closely that, if we consider that they were made by different observers, working independently with different instruments and different techniques, in different places, they must be extremely accurate, despite their large probable errors. It is significant that the second in date gives a lesser value of c.ft　　中文

M. E. J. Gheury de Bray

(144, 945; 1939)

Kitchener: Trinity College, Cambridge.

M. E. J. Gheury de Bray: 49, Great Thrift, Petts Wood, Nov. 2.

Reference:

  1. Nature, 144, 285 (1939).