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It's pretty simple so there must be something obviously wrong with it but what is it? Starter is an electric signal emitter that sends a pulse to the laser source and the detector, starting a counter in both, it is located at the exact halfway point between them.

So basically you send out the starter signal that starts a counter clock in both ends at the same time (or does it?) then you fire the laser. When you fire the laser the counter at the source stops. When the detector detects the laser the counter there stops too. Now you have the time it took for light to get from source to detector. What is the problem?

LaserSource-----------------------------Detector

            <----starter---->
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  • $\begingroup$ "...starts a counter clock in both ends at the same time then you fire the laser. When you fire the laser the counter at the source stops." - Why bother having a counter at the laser source if it stops immediately? $\endgroup$
    – Poo2uhaha
    Jan 14, 2021 at 11:13
  • $\begingroup$ I guess I wasn't clear enough, I shoot the laser after the counters are started at both ends.I need both counters to tick at least once to get a measurement. $\endgroup$
    – Saffer
    Jan 14, 2021 at 11:16
  • $\begingroup$ How does the clock at the detector know when the laser has been fired? $\endgroup$
    – Dan Pollard
    Jan 14, 2021 at 11:24
  • $\begingroup$ It only knows to stop counting when the detector detects the light. $\endgroup$
    – Saffer
    Jan 14, 2021 at 11:31
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    $\begingroup$ The electric signal sends a pulse, which is EM radiation, which propagates at the speed of light. The main problem with all these setups is clock synchronisation: moving the clock causes it to desynchronise, whilst sending any type of signal to sync the clocks ends up relying on the speed of light itself. $\endgroup$
    – Eletie
    Jan 14, 2021 at 12:21

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If I've understood yours setup correctly: the electric signal sends a pulse, which is EM radiation, which propagates at the speed of light. So you've already assumed the speed of light is isotropic in the setup.

The main problem with most of these proposed experiments is clock synchronisation: moving the clock causes it to desynchronise, whilst sending any type of signal (regardless whether it's EM or not) ends up relying on the the speed of light or some sort of clock synchronisation convention. Think about Lorentz transformations & velocity when considering your other suggestion about using rods: remember that all speeds are affected, not just light.

This is covered in great detail in Conventionality of synchronisation, gauge dependence and test theories of relativity, Anderson et. al. https://www.sciencedirect.com/science/article/pii/S0370157397000513.

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