7
The New Kinematics
7a. June 1905: Special Relativity Defined,
Lorentz Transformations Derived
- Relativity's Aesthetic Origins. Without a carrying medium, light can as little
be seen as sound can be heard. Such was the sensible prejudice of nineteenth cen-
tury physics. The better light was understood, the more circumscribed became the
properties of its medium, the aether. The best of all possible aethers, it appeared,
was one which blows through man and his planet as they speed through this
absolutely immobile medium. When light turned out to be a transverse wave phe-
nomenon, the aether had to be declared quasi-rigid.
The special theory of relativity divested the aether of its principal mechanical
property, absolute rest, and thereby made the aether redundant. As Einstein put
it in the introduction to his June 1905 paper (referred to in this chapter as the
June paper), 'the introduction of a "light-aether" will prove to be superfluous
since, according to the view to be developed [here], neither will a "space in abso-
lute rest" endowed with special properties be introduced nor will a velocity vector
be associated with a point of empty space in which electromagnetic processes take
place' [El].* Special relativity represents the abandonment of mechanical pictures
as an aid to the interpretation of electromagnetism. The one preferred coordinate
system in absolute rest is forsaken. Its place is taken by an infinite set of preferred
coordinate systems, the inertial frames. By definition, any two of these are in uni-
form motion with respect to each other. The preference for uniformity of relative
motion makes this version of relativity a special one.
In the spring of 1905, even before the completion of the relativity paper, Ein-
stein had written to his friend Conrad Habicht, 'The fourth work [i.e., El, the
fourth paper Einstein published in 1905] is available only in draft form and is an
electrodynamics of moving bodies in which use is made of a modification of the
tenets about space and time; the purely kinematic part of this work will surely
interest you' [E2]. Small wonder that Einstein would draw his friend's attention
to the kinematic part. In its entirety, the June paper consists of an introduction,
five sections on kinematics followed by five sections on electrodynamics, no refer-
ences, and one acknowledgment. The kinematic part contains the complete first
principles of the special relativity theory.
*For an English translation of this paper, see [SI].138