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I have an idea for checking the isotropic speed of light in all directions that, as far as I can tell, does not involve clock synchronisation or mirrors or so called mechanical rigid bodies or conveyor belts, which I know don't really exist. The idea seems somewhat simple to me which leads me to believe that I must be overlooking something, so perhaps a clever mind here can enlighten me. Or failing that, present me with a Nobel prize... lol.

Suppose we take a strobe light that emits a flash at a constant rate of say 10 flashes per second, and place it at some good distance from an observer. Initially the observer is stationary relative to the flashing strobe light. The stationary observer at a distance records the flashes of light from the strobe at the expected calibrated rate of 10 flashes per second. This is as expected, regardless of the speed of light.

The observer then sets out at a constant velocity towards the strobe light. (Yes there will be some initial acceleration to get to a constant velocity from a stationary beginning and hence there will be some time dilation. Don't think it matters in this case, as we are not relying on synchronising anything.)

Assuming the speed of light from the strobe to the observer is C, the observer should now be able to detect a change in the rate of observed strobe flashes. The observed time between strobe flashes should decrease, because the observer's distance from the strobe is decreasing ever so slightly between each strobe flash.

Alternatively, taking the edge case, if the speed of light was infinite in the direction of the strobe light to the observer, then the observer would not observe any change in the rate of strobe flashes when the observer moves towards the strobe at constant velocity, because if the speed of light was infinite, any change in distance to the strobe becomes irrelevant in terms of time delay between the two. One would observe the flashes at the same moment they occur at the strobe, irrespective of the distance from the strobe.

The experiment could then be reversed with the observer moving away from the strobe light to check the results.

In this way, we should be able to verify the isotropic speed of light, or alternatively, discover that it is in fact not isotropic.

Thanks. Tristan.

Tristan
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  • To interpret what you see, you have to know how fast your clock is moving relative to the clock that controls the strobe. Good luck determining that. – WillO Sep 01 '22 at 11:02

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Note first that no physicists claim that the speed of light is anisotropic or should change value depending on its direction. The speed of light has the same value in every direction. It is isotropic.

But in your experiment, moving toward the light, the rate of flashes would increase and vice versa moving away. The only difference would be that you'd notice blue shift in each pulse toward the light, and then red shift while moving away.

The isotropy of the speed of light has been verified. For example, in the Michelson-Morley experiment.

You may have confused isotropy with the one-way measurement of the speed of light versus the "round trip" method, which has been the subject of debate.

joseph h
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  • Michelson-Morley can't verify isotropy because isotropy is a convention that is not subject to verification. What it does verify is a certain algebraic relation between speed in one direction and speed in the diametrically opposite direction (at least if one assumes that speed in any given direction is independent of location). – WillO Sep 02 '22 at 02:05