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Which direction did they measure a meter?

  1. Apr 30, 2007 #1
    Okay, a meter is the distance traveled by light in 1/299792458th of a second. It's relative, so the actual measure of a meter changes depending on your velocity (I won't even go into "Relative to what?", though I do sort of wonder if one could find a 0 point of velocity by measuring light going at equal speeds in all dimensions...).

    My point is there has to have been some point where we've used light to get an extremely accurate measure of a meter for whatever purpose. Now, where did they measure it, when, and in what direction were they facing the light? These things have a negligible effect on the matter, but it's still there...were they even taken into consideration, or is it akin to neglecting wind resistence in elementary physics problems? How big of a factor would the earth's angle and motion be?
  2. jcsd
  3. Apr 30, 2007 #2


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    Are you worried about how the Earth's gravity curves space-time, or do you think that you will measure a different speed of light when you are moving at a different velocity?
  4. Apr 30, 2007 #3
    I'm saying that if c is what we base a meter off of, and c is always perceived as c to you, no matter how fast you're going, does that not mean that the closer your speed is to the speed of light, the smaller your meter would become? And, if that's true, would things such as the Earth's rotational and revolutional speeds have an effect on our physical measurement of a meter? If they do, about how much would this effect be (comparing extreme cases)?

    Is that a little more clear?
    Last edited: Apr 30, 2007
  5. Apr 30, 2007 #4


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    No. Your speed is always zero with respect to yourself and C is always C, so the difference is always C. So no, the meter's length doesn't depend on your speed.

    That's kinda the whole point of the 1st postulate that all the laws of the universe are the same in all inertial reference frames. It means that as long as your frame is inertial, you can't tell one from another with any experiment in a closed box.
  6. Apr 30, 2007 #5
    An one meter meter-stick is always one meter when you and the meter-stick are in same inertial frame. No matter how close you are to the velocity of light, you will measure the same length as both of you have no relative velocity.

    But when you and the meter-stick are in different inertial frame; think someone is running away from you with the meter-stick in his grip then you and that someone will not agree with the length of meter-stick. The length will apear to be shorter to you than the person in flight(who is stationary relative to the stick).
    You know that length contraction equation is; L=L0(1-v^2/c^2)

    L= the length of meter-stick by your measurement
    L0= the length of meter-stick in its own inertial reference frame
    v= relative velocity between two frame
    Last edited: May 1, 2007
  7. May 1, 2007 #6
    Relativity hadn't been developed at the time that the metre was first defined though, surely? So did they see this as a problem - or at least a curiosity - when they did do the definition?

    It's 'metre', by the way.
  8. May 1, 2007 #7


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    Not always. It is a "meter" here in the US.

    Note also that a "second" was also "invented" before we defined it with the Cs transition.

    I think the OP has some misconception about the basic postulates of Special Relativity.

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