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Could it be possible to measure the speed of light using the Doppler effect of light? I would propose an experiment where a light source is moving towards a photo detector at a constant speed, due to the Doppler effect the light detected would be blue shifted. Then the light source stops moving instantaneously at a fixed point away from the detector, causing the light to shift towards the red end of the spectrum. If you where to measure the time where the light observed is not shifting towards "blue" or stable "red" but is actually shifting from "blue" to "red" could you calculate the speed of light?

There are plenty of hurdles I can imagine with this setup. I also know that our units by convention are pegged to the speed of light. I just want to know if in theory this setup could work, and if not why not?

Matthew
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  • Is it possible to stop instantaneously? – JMLCarter Dec 26 '21 at 12:20
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    Does this answer your question? Can One-Way Speed of Light be Instantaneous? – Dale Dec 26 '21 at 12:36
  • The setup you describe is interesting. There are parallels with the Pound-Rebka experiment. Now: the concept of stackexchange is that duplicate questions are redirected. So: in order to submit this setup for examination you need to avoid the concept of one-way-speed-of-light. Let's say that you make the source oscillate by introducing a mechanical oscillation, resulting in an oscillation of the amount of doppler shift. Can a source like that be used to find the distance to some detector? (At the detector end you can tell from the oscillation of the doppler shift when the source reversed.) – Cleonis Dec 26 '21 at 12:58
  • @Dale sorry but this does not answer my question. Its unfortunate that my question is marked as duplicate, i do not see how link is an answer to the question i am asking. My proposed setup is not a topic in the "answer". – Matthew Dec 26 '21 at 13:37
  • @cleonis thank you for the reference to Pound-Rebka experiment i did not know that, very interesting. – Matthew Dec 26 '21 at 13:37
  • @JMLCarter I know there are practical problems. I stated clearly in my question if this setup could work from a theoretically point of view #thought-experiment. – Matthew Dec 26 '21 at 13:41
  • @Matthew While there are parallels with the Pound-Rebka experiment, there is also a fundamental difference. Shared: source and detector are separate (hence the EM-radiation travels one way), and the measurement involves doppler shift. Different: Pound-Rebka does not involve time resolution. I just wanted to suggest ideas for presenting the setup while avoiding explicit mention of one-way-speed-of-light. – Cleonis Dec 26 '21 at 13:44
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    @Matthew the setup is irrelevant. There is no possible way to measure the one way speed of light. It is not a matter of clever setup and the answer explains why. Every person who asks this question thinks the setup matters, the answer is that it doesn’t. There is no point in addressing the individual setup when it is irrelevant to measuring the one way speed of light – Dale Dec 26 '21 at 13:57
  • @Dale I am sure you are right, but the link you provide does not properly explain this to me. I do not have a physics degree, i am just curious and trying to learn but you are robing me of that. I understand that you are not willing to answer my question but do not block me from learning or force me to go back to uni every time i have a question you are not willing to explain. If you want physics.stackexchange.com to be a boys club where only physicists are allowed to post questions and get answers thats fine, but no one asked for my credentials when i signed up. – Matthew Dec 26 '21 at 15:07
  • @Matthew I do sympathize with your feeling here. Sometimes when learning, the most difficult part is asking a good question. Any question of the form “can I measure the one way speed of light by setup X” is a bad question. The X is irrelevant, the one way speed of light cannot be measured by any setup as explained. A good question would be to go through several of the “one way” questions that are answered with “the X is irrelevant” and explain what leaves you confused with those answers. This should be done without proposing any new X because that just leads back to “X is irrelevant” – Dale Dec 26 '21 at 15:46
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    @Matthew In any setup for measuring a speed of propagation/motion a part of the procedure is to disseminate time, so that the clocks at the start point and at the end point are synchronized. Time can be disseminated in multiple ways, examples are by way of transport of clocks, or by exchanging timing signals. In the setup that you suggest time is disseminated by way of modulation of the light itself. At the source the amount of doppler shift is changed in a known way. At the detector: by matching the pattern of doppler shift the time of emission can be reconstructed. Continuing... – Cleonis Dec 26 '21 at 17:54
  • @Matthew ...continued The point about special relavity is that while time can be disseminated in multiple ways (and each of those ways has its own pitfalls), in the end all forms of time dissemination end up with the same result. The general procedure to analyse a proposed setup is to address the question: 'How is time disseminated in this particular setup?', and then you work out how that particular form of time dissemination ends up with the same result as Einstein synchronization procedure. – Cleonis Dec 26 '21 at 17:54

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