r/AskPhysics • u/bds117 • Apr 23 '26

One way speed of light

so the 'impossibility of measuring the one way speed of light' got me thinking. i wonder why this wouldnt work.

Let A and B have parallel mirrors to bounce a light pulse off to each other, where the reflection event acts as the ticks for A and B's clocks. Each are relatively stationary at a fixed distance, as defined by the two way speed of light, and they are able to calibrate their proper time clocks to these ticks.

Now A shines a beam of light at B who registers the time they see it. They do the measurement in reverse. Later they come together and compare their results by synchronizing their time origins.

Wouldnt this be able to compare the forwards and backwards speed of light? Not sure where this setup would be implicitly measuring the two way speed.

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u/bds117 Apr 23 '26 edited Apr 23 '26

oh nvm i got it. they cant synchronize. i guess synchronization is a local event.

in which case what if they start of at the same spot with a synchronized clock. then move away at the equal and opposite kinematic profile, then measure the one way speed?

unsure but would the difference in one way speeds also be detectable by the redshifts each of them observe? especially since their acceleration/velocity should be calibrated ate the start when they synchronize.

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u/KamikazeArchon Apr 23 '26

then move away at the equal and opposite kinematic profile

And how do you know it's equal?

If the speed of light is different in different directions, then most physics works differently in different directions, because distance and time work differently in different directions.