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One way speed of light

  1. Apr 22, 2012 #1
    Has the one way speed of light been measured?

    All the measurements of the speed of light that I am aware of are two way or round trip measurements.
     
  2. jcsd
  3. Apr 22, 2012 #2

    DrGreg

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    The one way speed of light can be any value you like, depending how you synchronise the two clocks required. If you have already synchronised them using the standard Einstein Synchronisation Convention, then you already know the one-way speed equals the two-way speed by definition, so there's no need to do the experiment.

    If, instead, you synchronised the clocks using "ultra slow clock transport", then performing the experiment would be a way of testing whether the theory of relativity is correct or not. I don't know if anyone has actually done this.

    A number of people have claimed to measure the one-way speed without using two clocks, but when you analyse their method, it turns out they have really measured the two-way speed without realising it.
     
  4. Apr 22, 2012 #3

    phinds

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    there are a couple of LONG threads on this topic, if you want to do a forum search but the bottom line is exactly what DrGreg said.
     
  5. Apr 22, 2012 #4
    If it is impossible to measure the one way speed of light how did the group in Italy measure the one way speed of neutrinos? And what one way speed of light did they compare it to?
     
  6. Apr 22, 2012 #5

    phinds

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    I'm sure they compared it to the two-way speed of light divided by two. How they measured it is the subject of a LONG thread on this forum. Do a forum search.
     
  7. Apr 22, 2012 #6
    Does the Einstein synchronisation method use the one way speed of light to synchronise clocks?
     
  8. Apr 22, 2012 #7

    jtbell

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  9. Apr 22, 2012 #8

    phinds

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    Google is your friend
     
  10. Apr 22, 2012 #9

    DrGreg

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    They synchronised the two clocks at either end using GPS, which ought to have been equivalent to Einstein synchronisation, but it looks like there was an error in their equipment, so the synchronisation was incorrect (as far as we can tell at this stage).
     
  11. Apr 22, 2012 #10
    Thanks jt, I'm not sure if I've seen that specific link before, but I have googled it and seen other articles on it. That particular link doesn't seem to be opening for me at the moment, though.

    I was just looking to confirm if my understanding was correct; my understanding is that it does rely on the one way speed of light. If the answer is no, then I need to search a little further, if the answer is yes, then I don't need to, for that particular question.

    Based on my understanding that the synchronisation does rely on the one way speed, I just thought that the formulation "If you have already synchronised them using the standard Einstein Synchronisation Convention, then you already know the one-way speed equals the two-way speed by definition, so there's no need to do the experiment" was a bit strange, as the question was has it ever been tested.

    Little things like that cause a bit of confusion.
     
  12. Apr 23, 2012 #11

    jda

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    The old experiments using two cogwheels mesured, in laboratory, the one-way speed of ligh without clocs.
     
  13. Apr 23, 2012 #12
    Indirectly. If you use the two way measurement to determine the one way speed by dividing it in half, you still have to transmit both that information and a synch signal to the distant clock. On receiving that signal the clock is then set appropriately for that one way speed.
    In principle, if the exact distance is known, a simple one way synchronization based on the calculated time from distance/c should exactly corrospond with clocks synchd by the standard convention.
    Of course the two way method has the added advantage of automatically providing its own exact distance measurement.
     
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