Einstein 1920: Experience and the Special Theory of Relativity

Reference: Einstein’s 1920 Book

This paper presents Part 1, Chapter 16 from the book RELATIVITY: THE SPECIAL AND GENERAL THEORY by A. EINSTEIN. The contents are from the original publication of this book by Henry Holt and Company, New York (1920).


The heading below is linked to the original materials.


Experience and the Special Theory of Relativity


Maxwell-Lorentz theory holds very well for optical phenomena, and so does the theory of relativity derived from it.



The law of motion as it applies to electrons is experimentally known with accuracy.



The nature of electrons is not known. It doesn’t seem to have a structure in the classical sense, but it seems to contract in the direction of motion.

The theory of relativity leads to the same law of motion, without requiring any special hypothesis whatsoever as to the structure and the behaviour of the electron. We arrived at a similar conclusion in Section XIII in connection with the experiment of Fizeau, the result of which is foretold by the theory of relativity without the necessity of drawing on hypotheses as to the physical nature of the liquid.

The theory of relativity brings absolute motion into picture by using the datum of c.

The second class of facts to which we have alluded has reference to the question whether or not the motion of the earth in space can be made perceptible in terrestrial experiments. We have already remarked in Section V that all attempts of this nature led to a negative result. Before the theory of relativity was put forward, it was difficult to become reconciled to this negative result, for reasons now to be discussed. The inherited prejudices about time and space did not allow any doubt to arise as to the prime importance of the Galilei transformation for changing over from one body of reference to another. Now assuming that the Maxwell-Lorentz equations hold for a reference-body K, we then find that they do not hold for a reference-body K’ moving uniformly with respect to K, if we assume that the relations of the Galileian transformation exist between the co-ordinates of K and K’. It thus appears that of all Galileian co-ordinate systems one (K) corresponding to a particular state of motion is physically unique. This result was interpreted physically by regarding K as at rest with respect to a hypothetical æther of space. On the other hand, all co-ordinate systems K’ moving relatively to K were to be regarded as in motion with respect to the æther. To this motion of K’ against the æther (“æther-drift” relative to K’) were assigned the more complicated laws which were supposed to hold relative to K’. Strictly speaking, such an æther-drift ought also to be assumed relative to the earth, and for a long time the efforts of physicists were devoted to attempts to detect the existence of an æther-drift at the earth’s surface.

With zero inertia, the velocity of aether is infinite and not zero. It can act as a reference point though, which it does in the theory of relativity.


The unique reference-body K is light per the theory of relativity. In Michelson-Morley’s experiment, we are actually comparing the speed of light to itself in the two perpendicular directions. Any change in time would naturally cancel out because speed of light shall adjust itself accordingly. So, naturally, no “aether-drift” shall be visible.



We do not know if the contraction in length occurs due to speed, but we do know that inertia must reduce.


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