128 RELATIVITY, THE SPECIAL THEORY
the absolute velocity, and if it is not the velocity relative to the aether, it can always
be the velocity relative to a new unknown fluid with which we would fill space.'
He gently chides Lorentz for his accumulation of hypotheses, and then he goes
beyond Lorentz in treating local time as a physical concept. He considers two
observers in uniform relative motion who wish to synchronize their clocks by
means of light signals. 'Clocks regulated in this way will not mark the true time,
rather they mark what one may call the local time.' All phenomena seen by one
observer are retarded relative to the other, but they all are retarded equally (Poin-
care points out) and 'as demanded by the relativity principle [the observer] cannot
know whether he is at rest or in absolute motion.' Poincare is getting close. But
then he falters: 'Unfortunately [this reasoning] is not sufficient and complemen-
tary hypotheses are necessary [my italics]; one must assume that bodies in motion
suffer a uniform contraction in their direction of motion.' The reference to com-
plementary hypotheses makes clear that relativity theory had not yet been
discovered.
Poincare concluded this lecture with another of his marvelous visions: 'Perhaps
we must construct a new mechanics, of which we can only catch a glimpse,... in
which the velocity of light would become an unpassable limit.' But, he added, 'I
hasten to say that we are not yet there and that nothing yet proves that [the old
principles] will not emerge victoriously and intact from this struggle.'
The account of Einstein's precursors ends here, on a note of indecision. Lorentz
transformations had been written down. Simultaneity had been questioned. The
velocity of light as a limiting velocity had been conjectured. But prior to 1905 there
was no relativity theory. Let us now turn to what Poincare did next, not as a
precursor to Einstein but essentially simultaneously with him.
6c. Poincare in 1905
All three papers just mentioned are qualitative in character. Poincare, one of the
very few true leaders in mathematics and mathematical physics of his day, knew,
of course, the electromagnetic theory in all its finesses. He had published a book
on optics in 1889 [P10] and one on electromagnetic theory in 1901 [Pll]. In 1895
he had written a series of papers on Maxwellian theories [P12]. From 1897 to
1900 he wrote several articles on the theory of Lorentz [PI3]. All this work cul-
minated in his two papers completed in 1905. Both bear the same title: 'Sur la
Dynamique de 1'Electron.' The occurrence of the term dynamics is most signifi-
cant. So is the following sequence of dates:
June 5, 1905. Poincare communicates the first of these two papers to the Aca-
demic des Sciences in Paris [PI4].
June 30, 1905. Einstein's first paper on relativity is received by the Annalen
der Physik.
July, 1905. Poincare completes his second paper, which appears in 1906 [PI5].
The first of the Poincare papers is in essence a summary of the second, much