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Light speed

  1. Nov 6, 2014 #1
    My apologies for the numerous times this has probably been posted, but I wasn't able to find a great answer through the search tool alone.

    Please correct me on anything I state incorrectly, and direct me to the answer..

    My question is, if the speed of light is 299,792,458 m/s, its speed is measured with respect to what frame?
    To say that it doesn't have a frame of reference makes no sense, because to have a speed, it has to have a distance traveled in a specified duration, right? With respect to what?

    Thanks for any help,
  2. jcsd
  3. Nov 6, 2014 #2
    And yes, I've found numerous posts and pages that answer this, but I have a few questions that I can't quite phrase, and will wait for a reply to formulate a response or a counter that will (hopefully) lead me to the answer.
  4. Nov 6, 2014 #3


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    The universe does not do a great job of adhering to what we humans think "makes sense", it just is what it is.

    Light travels at the same speed in all inertial frames of reference and does not have a frame of it's own. "Frame of reference" MEANS a frame in which an object is stationary, but light is never stationary.
  5. Nov 6, 2014 #4
    Couldn't a frame of reference be the surroundings of light, the surroundings being at rest?
  6. Nov 6, 2014 #5


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    Staff: Mentor

    Any and all (inertial) frames.

    If you and I are moving at .5c relative to one another, we can choose to think of me at rest while you're moving in one direction, or you at rest while I'm moving in the other direction. But I will measure the speed of any and all light signals to be c relative to me, and likewise you will measure the speed of any and all light signals to be c relative to you.
  7. Nov 6, 2014 #6
    So, if light moves at it's own speed, regardless of the observer, then how is it affected by gravity, if gravity is related to both the space and time of other, independent inertial frames?
  8. Nov 6, 2014 #7


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    If you want to actually measure the speed of light (without knowing ahead of time what it is), you must make a round trip measurement using a single clock or timer and a ruler. You put the light source at one end of the ruler, a mirror at the other end of the ruler and you measure how long it takes for the light to propagate to the mirror and back. Then you do a little arithmetic and calculate what the "average" speed was. You have to make sure the ruler, clock, mirror, and light source are all inertial, that is not accelerating or moving with respect to each other but you don't have to explicitly devise a frame or be cognizant of what a frame is. Everyone who performs that above measurement, no matter their state of motion, as long as it is not accelerating, will get the same answer. We are assuming that effects due to gravity are negligible.

    Can't you have a ruler to specify the distance traveled (times two) and the clock or timer measure the duration? That is, for a round trip.

    With respect to the ruler? Does that qualify?
  9. Nov 7, 2014 #8


    Staff: Mentor

    In GR "gravity" is the effect that mass (more correctly stress-energy) has on spacetime. Light still propagates through spacetime, so it is still affected by "gravity".
  10. Nov 7, 2014 #9


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    Staff: Mentor

    Note that gravity has no affect on light's local speed, so it's effect is different from what you probably think (direction and energy, but not speed).
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