The paper analyzes a form of the familiar "clock paradox" in which two observers separate and reunite, but in which the accelerations normally necessary at starting, reversing and stopping are eliminated, so that the problem lies wholly within the scope of the special theory of relativity. The importance of the problem lies in the fact that it embodies all the essential principles of that theory, about which recent correspondence in 'Nature' has shown much disagreement to exist. The points specially brought out are the following: (1) The theory is not basically concerned with a comparison between different observers but between different coordinate systems which are available to a single observer. (2) It is not basically concerned with a comparison between stationary and moving systems but between descriptions of a single system with respect to different arbitrarily chosen standards of rest. (3) The familiar expression, "time retardation," does not relate to a physical change experienced by a clock but to a comparison of times of an event by two clocks, at least one of which is not present at the event. When both are present the discrepancy vanishes. A similar remark applied to the "Lorenz contraction" of moving rods.
The author replies to correspondence connected with a previous article in which it was maintained that there is no foundation for the statement that the rate of a moving clock is reduced by a certain factor compared with that of a similar stationary clock. It is stated that the result of comparison between the stationary and moving clocks is quite unpredictable without an exact specification of the principle of operation or detailed mechanism of the clock. The experimental discovery of the dependence of length on velocity affects the whole of physics, and its effect is to show that certain conceptions thought to be independent and necessary are actually different forms of others already adopted. These results in no way remove the dependence of laws of physics on experience.
Calls for a re-examination of assumptions. Refers to ideas expressed by Sir George Thomson in his book "The Foreseeable Future;" and by W. H. McCrea, P. S. Epstein, and Einstein.
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