[Letters to Editor]

Abstract

I THINK Dr. Powell's letter is based on a misunderstanding. I did not say that it was impossible to detect experimentally the slowing-down of a moving clock. My point was that whatever slowing-down was detected would not necessarily be in the Fitzgerald ratio unless the clock was an ‘ideal’ one, and that we could not check the relativity requirement with regard to time measurement by experimenting on actual clocks, because if they did not confirm it, the proper deduction would be simply that they were not ideal. The “lack of symmetry between time and space” thus implied is not at all “surprising”. It exists because we can make an ‘ideal’ measuring rod but not an ‘ideal’ clock. You cannot, in fact, define the standard unit of length without implying the existence of the measuring instrument—the material bar. The unit of time, on the other hand, cannot be embodied in a material instrument. The best we have, the rotating earth, itself needs slight correction, and experiments on relatively moving earths are, to say the least, difficult. (Incidentally, I wonder if Dr. Powell and Prof. Campbell have considered the case of relatively moving sundials?)