SUBTLE IS THE LORD 115theory of relativity 'must be accorded a generous acceptance' and gave a clear
expose of Lorentz transformations and their consequences for the Michelson -
Morley experiment and for the experiment of Armand Hippolyte Louis Fizeau
on the velocity of light in streaming water. Then follows his summation: 'The
existence of an ether appears to be inconsistent with the theory.. .. But without
a medium how can the propagation of light waves be explained?... How explain
the constancy of propagation, the fundamental assumption (at least of the
restricted theory) if there be no medium?'
This is the lament not of a single individual but of an era, though it was an era
largely gone when Michelson's book came out. Michelson's writings are the per-
fect illustration of the two main themes to be developed in this and the next two
chapters. The first one is that in the early days it was easier to understand the
mathematics of special relativity than the physics. The second one is that it was
not a simple matter to assimilate a new kinematics as a lasting substitute for the
old aether dynamics.
Let us turn to the influence of the Michelson-Morley experiment on Einstein's
initial relativity paper [E5]. The importance of this question goes far beyond the
minor issue of whether Einstein should have added a footnote at some place or
other. Rather, its answer will help us to gain essential insights into Einstein's
thinking and will prepare us for a subsequent discussion of the basic differences
in the approaches of Einstein, Lorentz, and Poincare.
Michelson is mentioned neither in the first nor in any of Einstein's later
research papers on special relativity. One also looks in vain for his name in Ein-
stein's autobiographical sketch of 1949 [E6], in which the author describes his
scientific evolution and mentions a number of scientists who did influence him.
None of this should be construed to mean that Einstein at any time underrated
the importance of the experiment. In 1907 Einstein was the first to write a review
article on relativity [E7], the first paper in which he went to the trouble of giving
a number of detailed references. Michelson and Morley are mentioned in that
review, in a semipopular article Einstein wrote in 1915 [E8], again in the Prince-
ton lectures of 1921 [El], and in the book The Meaning of Relativity [E9] (which
grew out of the Princeton lectures), where Einstein called the Michelson-Morley
experiment the most important one of all the null experiments on the aether drift.
However, neither in the research papers nor in these four reviews does Einstein
ever make clear whether before 1905 he knew of the Michelson-Morley experi-
ment. Correspondence is of no help either. I have come across only one letter,
written in 1923, by Michelson to Einstein [M13] and none by Einstein to
Michelson. In that letter, Michelson, then head of the physics department at the
University of Chicago, offers Einstein a professorship at Chicago. No scientific
matters are mentioned. The two men finally met in Pasadena. There was great
warmth and respect between them, as Helen Dukas (who was with the Einsteins
in California) told me. On January 15, 1931, at a dinner given in Einstein's honor
at the Atheneum of Cal Tech, Einstein publicly addressed Michelson in person