Re: Einstein, Hawking, and a myth about relativity

[Stephen Speicher <sjs@COMPBIO.CALTECH.EDU>]

On Tue, 10 Jun 2003, Hugh Haskell wrote:

> One of the little projects I got assigned in grad school was to
> analyze the data that Eddington got in 1919 that he was so sure
> verified Einstein's prediction of gravitational bending of
> light.  ... I would have to say that either Eddington was
> deliberately fudging his result, or, more likely, he wanted the
> result to be in accordance with Einstein's theory so badly that
> he saw what really wasn't there.

I must take exception to this conclusion, for several reasons.
First, the actual measurements were not solely the responsibility
of Eddington, and several of the contibutors to the experiment
and the published paper [1] also performed the measurement tasks.

The Astronomer Royal, Sir Frank Dyson, himself studied the plates
in detail ("After a careful study of the plates I am prepared to
say there can be no doubt that they confirm Einstein's
prediction." [1]) I hardly think that that such disparate people
as Eddington, Dyson, Sir Joseph Thompson, and A.C. Crommelin
conspired to see "what really wasn't there," much less being
joined by some supposed mutual motivation of "want[ing] the
result to be in accordance with Einstein's theory so badly."

(Also, please note that in the referenced paper [1] Eddington
goes out of his way to make clear that, in his own view, it is
not even Einstein's theory which was validated, but the
mathematical law: "This effect may be taken as proving Einstein's
_law_ rather than his _theory_.")

Second, I do not know what particular data Hugh used in grad
school, but the results from the two different locations varied
widely.  The observers in the Principe expedition used a
telescope such that on the photographs taken 1 sec. of arc was
equal to about 1/1500 of an inch.  Their observations were
hampered by clouds in the skies, and of the 16 photographs taken
and processed one was deemed suitable for measurement.  The
measurement was done with a micrometric measuring-machine, and
after all appropriate allowances were made, the result clearly
ruled out Newtonian predictions, and closely matched the
prediction of general relativity. There were four unexposed
plates which were brought back to England from the Principe
expedition, and when these were later developed one plate was
deemed clear enough to be measured, and again Einstein's
predictions were met.

There were two telescopes used at Sobral; the first one was quite
similar to the telescope used at Principe, but the second was a
longer telescope of 4" aperture and 19 feet focal length. The
results from the former telescope were very poor, but the results
from the latter one were excellent, and the seven plates from
that scope at Sobral are the ones which Eddington refers to in
the following quote, part of the drama surrounding the
experiment.

    "There remained a set of seven plates taken at Sobral
     with the 4-inch lens; their measurement had been
     delayed by the necessity of modifying a micrometer to
     hold them, since they were of unusal size. From the
     first no one entertained any doubt that the final
     decision must rest with them, since the images were
     almost ideal, and they were on a larger scale than the
     other photographs. ... but the observers achieved
     success, and the perfection of the negatives surpassed
     anything that could have been hoped for.

    "These plates were now measured and they gave a final
     verdict definitely confirming Einstein's value of the
     deflection, in agreement with the results obtained at
     Principe." [2]

The point being that, not only was the measurment process an art,
but one must discriminate carefully between those plates which
provide the clearest and most precise information, and those
which do not. With all that said, I would agree that the
definiteness of the results were most likely somewhat overstated,
given the error bounds of the plates and the observations. Of
course, later experiments confirmed the effect to a degree not
even imagined by this adventurous group in 1919.

[1] "The Reflection of Light by Gravitation and the Einstein
Theory of Relativity," Joseph Thompson, Frank Dyson, A.C.
Crommelin, A.S. Eddington, _The Scientific Monthly_, Volume 10,
Issue 1, pp. 79-85, Jan. 1920.

[2] Sir Arthur Eddington, "Space, Time & Gravitation: An Outline
of the General Relativity Theory," p. 118, _Cambridge University
Press_, 1920/1987."

--
Stephen
sjs@compbio.caltech.edu

Ignorance is just a placeholder for knowledge.

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