Length contraction and time dilation in spacetime

  • Thread starter Quinto
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I would normally take that term as more general than just referring to one-way light speed. I understand the context better from your latest post. (I do wonder what the original word Einstein used in German was that got translated as "propagation".)
1907: "Fortpflantzungsgeschwindigkeit" ;)

For me that discussion is pointless (or at least, exhausted). As we know, Einstein assumed in 1905 that - once more: "from experience"! - the two-way speed of light is a universal constant (the speed of light in vacuum), or in German: "die Lichtgeschwindigkeit im leeren Raume".
- http://fourmilab.ch/etexts/einstein/specrel/www/

[edit:] I'll do one last attempt to explain this, as this "bug" keeps on returning...

The 1-way speed of light rays in all directions can only be made a constant by means of appropriate clock adjustments if the 2-way speed of light rays in all directions is a constant (not by definition, but by measurement!).

If that does not help, I give up. Perhaps Peter can explain it clearer! o0)

A nice example of the constancy of the speed of light (Einstein could not cite it in 1905/1907 as it had not yet been performed) is the Sagnac experiment. No matter how fast the light source and mirrors move, the propagation speed is unaffected ( in vacuum, and practically in air; it's slightly more complex with a glass fibre). Consequently light can be used to measure the speed of rotation.
- for elaboration see https://en.wikipedia.org/wiki/Sagnac_effect ; regretfully the current version has "invariance" which could be confusing as only a single inertial frame is concerned ("constancy" would be clearer in this context).
 
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PeterDonis
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I don't see how there can be any doubt that he is talking about the one-way speed of light, can you?
No. I wasn't questioning what Einstein was talking about in the paper; I was just failing (until harrylin reminded me) to interpret the phrase "propagation of light" in that context.

The 1-way speed of light rays in all directions can only be made a constant by means of appropriate clock adjustments if the 2-way speed of light rays in all directions is a constant (not by definition, but by measurement!).
Agreed.
 

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