Abstract
I CANNOT agree with Mr. Newman that an instrument which measures time in terms of the mass of fallen sand is not a clock. If such an instrument is passed by a recognized observatory, I should be glad to know on what authority Mr. Newman would reject it. If it is accepted, then the mass must be measured, directly or indirectly, whenever it is used, whether moving or at rest. A consideration of what an observer would see on a dial is not sufficient to determine such a measurement, as perhaps the following example will show most convincingly. Suppose we wish to find how mass varies with velocity. Insert a body in an apparatus which indicates its mass by a pointer reading on a dial. Now recede from the apparatus with velocity, v. So long as the dial can be seen its reading will, clearly, appear the same. Hence if Mr. Newman is right, mass is invariant. It is not “equally legitimate to read a measuring rod by counting its divisions or weighing a piece of it”, because the standard of space measurement in physics is defined as the length, and not the weight, between two fixed marks.
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DINGLE, H. "The Relativity of Time". Nature 144, 1047 (1939). https://doi.org/10.1038/1441047a0
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DOI: https://doi.org/10.1038/1441047a0
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