Invariance of Light

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>[LB]: If you want to measure the time it takes a light beam to travel 0.299 meters why is it not possible to use *one* clock and two photo detectors - one to start the clock and one to stop it?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

Think about it. How can the photo-detector 0.299 meters away start a local clock except by sending a signal to the local clock at light-speed or slower? How could the clock get started before the photon to be detected arrived?

The only way to avoid another delay in the system is to have a local clock at each event so there is no propagation delay. The experiment with length 0.299 meters is no different in principle than the same experiment with a length of a lightyear. The measurement will depend on how the two clocks are synchronized.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>I believe this means (using LR instead of SR to interpret your measurement) that two clocks *can* be synchronized and then separated<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

Moving clocks changes them in SR and in LR. If the distance traveled is fixed, the net change in time is the same whether the clock is moved rapidly for a short time or very slowly for a very long time. -|Tom|-

Jan, The momentum of a photon causes chemical reactions, heating, lighting, and a lot of other effects. How is the speed of light to be measured in that pile of effects?

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>
[tvf]
How can the photo-detector 0.299 meters away start a local clock except by sending a signal to the local clock at light-speed or slower? How could the clock get started before the photon to be detected arrived?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>
I envisioned both detectors connected to the same clock with identical wires that were each, for example, 1 meter long. The signal delay between the detector and the clock would then be the same for both detectors and would cancel out.

No?

Regards,
LB

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>[LB]: I envisioned both detectors connected to the same clock with identical wires that were each, for example, 1 meter long. The signal delay between the detector and the clock would then be the same for both detectors and would cancel out. No?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

No. <img src=icon_smile.gif border=0 align=middle> One signal would be traveling upstream against an aether, and the other would be traveling downstream. Defining the delays as equal is equivalent to assuming that the speed of light is the same in both directions. -|Tom|-

The invariance of the speed of light is POSTULATED in SR. Therefore any attempt to prove or disprove it within SR will fail. It is like building a kind of mathemathic around a set of axioms, then trying to prove the axioms using the tools developed.

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>
The invariance of the speed of light is POSTULATED in SR. Therefore any attempt to prove or disprove it within SR will fail. It is like building a kind of mathemathic around a set of axioms, then trying to prove the axioms using the tools developed.
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How frustrating this is. We have a postulate that cannot be verified in any way, no matter how hard we try. Like hal said, it is simply an axiom and cannot be proven. But isn't physics a science where mathematical theory should be tested until there is no doubt in our minds that the theory adequately approximates the experiments and phenomena we study? Why did we accept the second postulate in the first place?