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Study of 1919 eclipse sparks talk of terms and terminology

In “Testing Relativity from the 1919 Eclipse—A Question of Bias” (PHYSICS TODAY, March 2009, page 37), Daniel Kennefick makes synonyms of two words—observation and experiment—that traditionally have described different ways to gain knowledge of the physical world.

In his first paragraph, Kennefick describes Arthur Eddington’s work on the 1919 eclipse as an observation, but elsewhere he usually calls it an experiment. He mentions “observation” or some form of it 5 times in that first paragraph and does not mention “experiment.” Yet “experiment” or forms of it appear 19 times elsewhere in the text of the article where “observation” or forms of it appear 3 times.

In December 1919 Eddington wrote in the preface to the second edition of his Report on the Relativity Theory of Gravitation (Fleetway Press, 1920), “I think it may now be stated that Einstein’s law of gravitation is definitely established by observation.” Eddington appears never to have used the word “experiment” to describe results of the 1919 eclipse expedition, but he does use it to describe anticipated work on Fraunhofer lines, which agrees with traditional understanding of the word. And Albert Einstein himself, in early October 1919, reported that he had received provisional eclipse results of the “Beobachtung” (observation). ¹

Having spent 18 years doing experiments on water waves, I am aware, along with Kennefick, that measurement and insight—seeing the link between existing knowledge and the observed phenomenon—are forms of observation accompanying experiment. However, experiment differs essentially from observation by prescribing the values of variables believed to be relevant.

Although ongoing changes in dictionary definitions tend slightly to agree with Kennefick’s usages, I think the traditional distinction between observation and experiment is logically necessary for the results of the 1919 eclipse expedition. The data obtained then are observations, as Eddington and his contemporaries called them.

Reference

1. 1. A. Einstein, The Collected Papers of Albert Einstein, vol. 7, M. Janssen et al., eds., Princeton U. Press, Princeton, NJ (2002), p. 200.

 

Two items interestingly coincide in the March 2009 issue of PHYSICS TODAY. Daniel Kennefick’s article (page 37) on the 1919 eclipse observations to test predictions of gravitational light deflection vindicated the traditional conclusion in favor of general relativity and Arthur Eddington’s role in the matter. Charles Day’s Search and Discovery story (page 14) concluded that a cosmological term in Albert Einstein’s equations can account for present data on the acceleration of cosmic expansion.

Eddington was a strong proponent of the cosmological term even after the discovery of the expansion of the universe removed Einstein’s original rationale for it and many physicists had rejected it dogmatically. He argued that the term had a fundamental character because its “cosmical constant” Λ provided a universal standard of length, and he asserted in his picturesque way that “to drop the cosmical constant would knock the bottom out of space.” ¹  (Italics in the original.)

The speculative theories Eddington developed in later life have tended to prejudice physicists against his views, but he was surely right that the cosmological term should not be regarded as a mere fudge factor. If Einstein had not introduced it to make a static universe possible, someone eventually would have realized that it was a legitimate addition to the original field equations. There are even purely affine generalizations of Einstein’s Riemannian theory, such as Schrödinger’s, that not only allow but demand a cosmological term. ² Wolfgang Pauli’s rejection of Schrödinger’s version precisely because it required a cosmological term is an example of the dogmatism I mentioned above. ³

I hope present observations of the acceleration of cosmic expansion will convince physicists to be more open-minded. Eddington may once again be vindicated.

References

1. 1. A. Eddington, The Expanding Universe, Macmillan, New York (1933), p. 148.
2. 2. E. Schrödinger, Space-Time Structure, Cambridge U. Press, New York (1960), chap. 12.
3. 3. W. Pauli, Theory of Relativity, G. Field, trans., Pergamon Press, New York (1958), p. 225.