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Is this the one way speed of light?

  1. Aug 13, 2015 #1


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    Hello PF.
    A stationary observer generates two pulses of light. The pulses are 2 seconds apart according to his watch.

    What is the distance between the two pulses? Assume he marks out 10 metres and places himself half way at the 5 metre mark. If the light pulses go past and align with the 10 metre marks, the distance between the light pulses is 10 metres. Being equidistant from the 10 metre marks he records simultaneous measurements. The odds are the light and the 10 metre marks won't al;gn. But the 10 metres can be altered. ( 9,8,11,12 etc. ) until they align with the light pulses. thereby giving the distance between the pulses.
    With distance and time we have speed. One way speed?
  2. jcsd
  3. Aug 13, 2015 #2


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    Odds ? There is no probability involved !
    You just before this wrote he records simultaneous measurements ?
  4. Aug 13, 2015 #3


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    I guess I'm not following what you are saying. You say "the distance between the light pulses is 10 meters", but you have stated that the distance between the light pulses is 2 light seconds. Compared to 2 light seconds, 10 meters is approximately zero.
  5. Aug 13, 2015 #4


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    Staff: Mentor

    How do you establish that the measurements are in fact simultaneous? All techniques for determining simultaneity are directly or indirectly based on the assumption that the one-way speed of light is equal to the two-way speed.

    The "one-way speed of light problem" isn't that we can't "measure" the one-way speed of light. We can, just as you did: choose a definition of "simultaneous" so that we can measure the time between emission and reception and then speed=distance/time will see us home. The problem is that when our measured one-way speed comes out the same as the measured two-way speed that doesn't tell us that the speed of light is the same in both directions. It tells us that we chose a definition of "simultaneous' that assumed that the two speeds are equal.
  6. Aug 13, 2015 #5


    Staff: Mentor

    What does this mean?
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