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How can one know what guarantees the PoR without first deriving it? Before 1904 Lorentz kept the unknown factor l in his discussions, leaving the question open if what we now call Lorentz contraction is indeed the right solution.Thanx Harrylin. By force-fit, I meant "not by logical deduction, but rather by insertion". IOWs, if you assume you know the required final form of the equations to guarantee the PoR, you "make it happen" during derivation. From what you say, Lorentz obtained his linear coefficients by deduction, as Einstein did. I'll have to read thru his paper more closely.

Probably you mean with "LET" Lorentz's 1904 paper, which Einstein summarized in 1907 together with his 1905 paper as the new theory that is based on the PoR (and which he later renamed "SR").Wrt the PoR ... Consider a dual pan balance whereby the line joining the pan midpoints are aligned with the balance's propagational path. Assume the pans are separated by a very long beam, and the wonder beam cannot bend. Further assume that "someone who believes himself at rest in Lorentz's master aether frame" records the balance's motion at luminal v thru the aether. Now, LET and SR produce the very same solns, so in either case, observers at rest with the balance should always witness the same result(s) ... ie, it's presumedly not possible to distinguish between SR & LET by experiement. Now, let's say 2 weights of identical mass drop from the sky, always of identical velocity andstrike the balance pads AT ONCE"per an observer at rest with the balance". What would each theory predict wrt the balance beam tilting upon impact?

SR says ... the balance beam would not tilt.

LET (I think) would say ... the balance beam tilts, because the 2 weights do NOT strike the pads AT ONCE per an aether frame observer "if they strike the pads AT ONCE per the observer moving with the balance thru the aether". The aether POV is always right.

Wrt LET theory ... How is it that the PoR is upheld by LET theory? It seems to me that ... even though the LT results are the same in either case, they do not mean the same thing. The PoR "only appears to be upheld" (per LET) from a kinematic standpoint, but not with regards to force, and thus not with regards to energy.What is wrong with my reasoning here?

If all the coefficients were logically obtained by deduction, then I would agree. However, it just seems to me that the PoR is upheld kinematically, but not beyond that. How do you explain the scenario I pose above?

GrayGhost

To be frank, I did not carefully read your example as any such discussion or paradox that I know of can be rephrased by replacing "aether" or "aether frame" by "rest frame". For a correct understanding of SR it is essential to realise that according to SR you may assume any inertial frame to be "truly in rest" so that the laws of nature should be valid wrt it, without any frame jumping.

Thus rephrased in interpretation-free SR:

"the 2 weights do NOT strike the pads AT ONCE per a rest frame observer if they strike the pads AT ONCE per the observer who is moving with the balance".

Note:a wonder beam that cannot bend cannot exist in SR

Harald