THE NEW KINEMATICS 14!- The laws of physics take the same form in all inertial frames.
- In any given inertial frame, the velocity of light c is the same whether the light
be emitted by a body at rest or by a body in uniform motion.
FitzGerald and Lorentz had already seen that the explanation of the Michel-
son-Morley experiment demanded the introduction of a new postulate, the con-
traction hypothesis. Their belief that this contraction is a dynamic effect (molec-
ular forces in a rod in uniform motion differ from the forces in a rod at rest) was
corrected by Einstein: the contraction of rods is a necessary consequence of his two
postulates and is for the very first time given its proper observational meaning in
the June paper.
What is so captivating about the Einstein of 1905 is the apparent ease and the
fraicheur with which he introduces new ideas. If free radiation consists of light-
quanta, then the emission and absorption of light should also go by discrete steps;
if van't Hoff's law holds for solutions, then it should also hold for suspensions; if
the velocity of light does not seem to depend on the velocity of the emitter, then
why not make that into a postulate? Steps like these were the result of very hard
thinking, yet the final product has that quality of greatness of looking easy if not
obvious.
The big question was, of course, the compatibility of the two postulates, about
which Einstein had the following to say in his review article of 1907 [E3]: 'Sur-
prisingly, it turned out that it was only necessary to formulate the concept of time
sufficiently precisely to overcome the first-mentioned difficulty [i.e., the Michel-
son-Morley result, which Einstein did mention for the first time in this 1907
paper]. All that was needed was the insight that an auxiliary quantity introduced
by H. A. Lorentz and denoted by him as "local time" can be defined as "time",
pure and simple.'
There are as many times as there are inertial frames. That is the gist of the
June paper's kinematic sections, which rank among the highest achievements of
science, in content as well as in style. If only for enjoyment, these sections ought
to be read by all scientists, whether or not they are familiar with relativity. It also
seems to me that this kinematics, including the addition of velocity theorem, could
and should be taught in high schools as the simplest example of the ways in which
modern physics goes beyond everyday intuition.* (If only I could make a similar
recommendation for the case of quantum theory. .. .)
I briefly recapitulate the content of the new kinematics.* In a given inertial
frame an observer A measures his position xk relative to the origin by means of
rigid rods, using (as Einstein states explicitly) 'the methods of Euclidean geome-
try.' A second observer B does likewise for B. Then A's clock at JA is synchronized
with B's clock at JB by means of light signals. If A's clock is synchronous with
*See, for example, the excellent popular yet rigorous account by Born [Bl].
"More details are found in standard texts, e.g., [Ml] and [P3].