Thursday 21 June 2018

general relativity - How to identify a "measuring rod", and how to compare separated "measuring rods" with each other?


The notion of "measuring rod" has appeared in PSE here and there, and outside PSE as well.


As far as I understand (and as perhaps all who refer to this notion do agree on), important constituents of any one "rod" are "its two ends", being two distinct and (in general) separate material points (or principal identifiable points, or for short: participants).


Therefore the following questions remain to identify measuring rods among all rods, and to explain their use:




  1. Do all rods, i.e. all pairs of distinct participants, in all trials, make up a measuring rod, constituting "its ends"?,


Or else:



  1. How should be determined for a given rod, i.e. for a given particular pair of distinct participants, in a particular trial under consideration, whether these two had made up a measuring rod, constituting "its ends", or not?


And:



  1. How should be determined whether two distinct measuring rods, in one particular trial, had been equal to each other, or not; i.e. especially if they had only at most one end in common, or if only at most one end (of one of the two measuring rods being considered) had been coincident with one end of the other?





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