# Constancy of light speed implied in time and clocks !

1. Oct 25, 2009

### myspacetime

From "The conceptualization of time and the constancy of the speed of light", Vasco Guerra and and Rodrigo de Abreu.

I just came across this article which proves that the speed of light had to be constant by just assuming "good" clocks record time correctly irrespective of orientations.

Consider 2 similar (Feyman) clocks that record a "tic-tac" round trip period with a light photon reflected perpendicularly by 2 parallel mirors. Rotating one 90 degree gives the equivalent of the Michelson Morley experiment ! Thus the constancy of the speed of light.

Seems brilliant, but I am no physicist. Can someone comment.

Are these authors first with this idea.

2. Oct 25, 2009

### George Jones

Staff Emeritus
3. Oct 25, 2009

### Al68

Sure it would follow that the speed of light is constant if we assume those "good" clocks keep proper time. Just like it follows that those clocks will keep proper time if we assume the speed of light is constant.

But, a priori, without an assumption that the speed of light is constant, what basis is there to assume those clocks will keep proper time? If we don't assume the speed of light is constant, we have no reason to believe they will, and no explanation as to why they would even if we had experimental data.

Logically, the clocks would keep good time because the speed of light is constant, not the other way around.

4. Oct 26, 2009

### myspacetime

Nothing is assumed about proper time or about any frames comparisons. Only assumption is all type of clocks (and orientation) give the same reading. Each of the two Feynman light clocks could have been accompanied by a any similar precision mechanical clocks to collaborate their readings which should confirmed those two light clock must give equal readings irrespective of motion, allowing even under acceleration !

The two light clocks in the OP giving equal readings would imply that the 2-way average light speed would be the same in the two different directions. The experimenter of the Michelson Morley experiment expected the light speed in the perpendicular beams to travel with different speed due to the ether speed of the light source.

I only vaguely understand the original articles. What I think it stressed seems to be "time" is only as meaningful as the "best" clocks we agree on and time is nothing but readings of such clocks. Without clocks, there is no notion of time. The ancient "clock" is the recurrence of sunrise and sunset.

So the original article's premise is the notion of time and its inalienable dependence on clock readings would have logical contradictions if the speed of light is not constant. Thus a proof of constancy through refutation.

5. Oct 26, 2009

### myspacetime

We could argue about the "goodness" of the two Feynman clocks, that such light clocks are not "good" if we don't know if the speed of light in both directions are the same.But in this manner we could always use the "badness" of clocks to reject any findings. In the case of the experiment which involved taking one clock around the world and comparing it with a clock at home, we could just dismiss the clocks as unreliable. They are good when calibrating each other and "bad" when one goes round the world. So any such experiment has no meaning !

Finally, then, what is time and clocks ?