MARGINALIA

# Lost in Einstein's Shadow

Einstein gets the glory, but others were paving the way

Principle of Relativity

Einstein didn't call his creation "the theory of relativity," but it was indeed based on two postulates, the first being the "principle of relativity," the supposition that any experiment done on a train moving with constant velocity should give the same result as an identical experiment done on the ground.

It wasn't Einstein's idea. The great French mathematician Henri
Poincaré enunciated the principle of relativity at least as
early as 1902 in his popular book *Science and Hypothesis.*
We know from Einstein's friend Maurice Solovine that the two pounced
on Poincaré's book, indeed that it kept them "breathless
for weeks on end." It should have. In *Science and
Hypothesis*, Poincaré declares: "1) There is no
absolute space, and we can only conceive of relative motion; 2)
There is no absolute time. When we say that two periods are equal,
the statement has no meaning; 3) Not only have we no direct
intuition of the equality of two periods, but we have not even
direct intuition of the simultaneity of two events occurring in two
different places."

These ideas lie at the heart of relativity, and it is hard to
imagine that they did not have a profound effect on Einstein's
thinking. But Poincaré not only speculated—he
calculated, and in the same weeks that Einstein was writing his
paper on relativity, Poincaré completed a pair of his own.
The major one is quite remarkable. Mathematically, he has more than
Einstein does. Among other things, he notes that time can be viewed
as a fourth dimension (something Einstein doesn't do, by the way),
he predicts the existence of gravitational waves 10 years before
Einstein does and, perhaps most remarkable of all, he writes down an
expression exactly equivalent to *E* = *mc*
^{2} several months before his rival. But he fails to
interpret it.

Poincaré's paper, alas, is that of a mathematician. Right at the start he sets the speed of light equal to a constant, "for convenience." The second, and revolutionary, postulate at the basis of Einstein's relativity is in fact that the speed of light is always observed to be the same constant, regardless of the speed of the observer. Perhaps if Poincaré had been less a brilliant mathematician and more a dumb physicist he would have seen that the whole edifice stands or falls on this "convenience." He didn't.

Not long ago I had the opportunity to give a colloquium on these and related matters at a major university. Among the 50 or so physicists in the audience, not one had read Einstein's original papers, yet alone Poincaré's. As I said, physicists are notorious for taking history on faith. Such insouciance, though, has not stopped physicists from repeating for several generations the usual platitudes about the history of their field. One might make a case that science is inherently anhistorical—certainly recent editions of undergraduate physics texts are entirely bereft of meaningful history. But if the history of science has any relevance to the doing of it, surely it is to remind us that science is a collective enterprise and to engender in us a humble awareness that the landscape of science would appear very different had the vast unrecognized majority never existed.

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