PURPOSE AND PLAN 21transmission of light waves and which was abolished by Einstein. The question
has often been asked whether or not Einstein disposed of the aether because he
was familiar with the Michelson-Morley experiment, which, with great accuracy,
had demonstrated the absence of an anticipated drift of the aether as the earth
moved through it without obstruction (6a). The answer is that Einstein undoubt-
edly knew of the Michelson-Morley result (6d) but that probably it played only
an indirect role in the evolution of his thinking (7a). From 1907 on, Einstein often
emphasized the fundamental importance of the work by Michelson and Morley,
but continued to be remarkably reticent about any direct influence of that exper-
iment on his own development. An understanding of that attitude lies beyond the
edge of history. In (8) I shall dare to speculate on this subject.
Two major figures, Lorentz and Poincare, take their place next to Einstein in
the history of special relativity. Lorentz, founder of the theory of electrons,
codiscoverer of the Lorentz contraction (as Poincare named it), interpreter of the
Zeeman effect, acknowledged by Einstein as his precursor, wrote down the Lor-
entz transformations (so named by Poincare) in 1904. In 1905, Einstein, at that
time aware only of Lorentz's writings up to 1895, rediscovered these transfor-
mations. In 1898, Poincare, one of the greatest mathematicians of his day and a
consummate mathematical physicist, had written that we have no direct intuition
of the simultaneity of events occurring in two different places, a remark almost
certainly known to Einstein before 1905 (6b). In 1905 Einstein and Poincare
stated independently and almost simultaneously (within a matter of weeks) the
group properties of the Lorentz transformations and the addition theorem of veloc-
ities. Yet, both Lorentz and Poincare missed discovering special relativity; they
were too deeply steeped in considerations of dynamics. Only Einstein saw the cru-
cial new point: the dynamic aether must be abandoned in favor of a new kine-
matics based on two new postulates (7). Only he saw that the Lorentz transfor-
mations, and hence the Lorentz-Fitzgerald contraction, can be derived from
kinematic arguments. Lorentz acknowledged this and developed a firm grasp of
special relativity, but even after 1905 never quite gave up either the aether or his
reservations concerning the velocity of light as an ultimate velocity (8). In all his
life (he died in 1912), Poincare never understood the basis of special relativity (8).
Special relativity brought clarity to old physics and created new physics, in par-
ticular Einstein's derivation (also in 1905) of the relation E = me^2 (7b). It was
some years before the first main experimental confirmation of the new theory, the
energy-mass-velocity relation for fast electrons, was achieved (7e). After 1905 Ein-
stein paid only occasional attention to other implications (7d), mainly because
from 1907 he was after bigger game: general relativity.
The history of the discovery of general relativity is more complicated. It is a
tale of a tortuous path. No amount of simplification will enable me to match the
minihistory of special relativity given earlier. In the quantum theory, Planck
started before Einstein. In special relativity, Lorentz inspired him. In general rel-
ativity, he starts the long road alone. His progress is no longer marked by that