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Speed of Light relation with time dilation

  1. Apr 12, 2013 #1

    Want to know if constant speed of light is a result cause by time dilation? When your time slow down as you approach speed of light you take a longer time to measure light speed in your spaceship, there for you think speed of light relative to your current speed is still c?

    When one measure speed of light do they consider their own current speed and time dilation, compare to maybe CMB?

    If one is near speed of light can one see his own image on a mirror put at left/right side of the direction he is going? If he can doesn't this mean speed of light become square root of his current speed^2+C^2?

    Sorry for bad english
  2. jcsd
  3. Apr 13, 2013 #2


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    Hello, and welcome to the forums.
    No, it is not.
    No matter how fast your spaceship is going, assuming your thrusters are turned off so that you are just coasting, you cannot tell that anything different is happening to you or your clocks. And any measurements you make of the speed of light will yield the same result as any other condition of coasting at any other speed. This is an example of the Principle of Relativity (PoR).
    No, no consideration is necessary. Simply another example of PoR.
    Looking in a mirror that is traveling with you will be the same under all states of coasting. You will have no way of knowing that you are traveling by looking in a mirror. Otherwise, the PoR would be violated and none of us would be in Physics Forum for Relativity.
    No, it just means you can't tell the difference between all states of coasting at any speed in any direction.
    Last edited: Apr 13, 2013
  4. Apr 13, 2013 #3
    That is assuming you do not know there is time dilation if you make adjustment to your clock similar to what they do with the GPS satellites wouldn't you be able to tell light rate of displacement is the same because you are measuring it at a slower time?
  5. Apr 13, 2013 #4


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    You are misunderstanding how time dilation works. It's something that happens to people moving relative to you, not to you. Two things to consider:
    1) You right now are moving at .99c; that's relative to some observer in some far distant galaxy. You are also moving at .44c relative to some other observer in some other galaxy. You are moving at a mere few kilometers/second relative to someone on Mars. And (assuming that you're sitting still now, and ignoring the rotation of the earth) you are not moving relative to me. Does any of this matter to you at all? Of course not - for you time is passing normally.
    2) If you are at rest (Relative to what? Think about it for a moment) and somebody zooms past you in a spaceship moving at .8c you will observe that his clock is running slow relative to your clock. Your clock is of course running normally - see #1 above. However from the point of view of the guy in the spaceship, he is at rest, you're moving backwards from him at .8c, his clock is running normally, and it's your clock that is running slow.
  6. Apr 13, 2013 #5


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    An orbiting spaceship is just another example of a spaceship coasting where you would still measure the speed of light to be the same as in any other situation where your spaceship was coasting. It doesn't matter if you know anything at all about Time Dilation or any other aspect of any theory about light or relativity or any of their assumptions, postulates or principles, you will still make the same measurement of the speed of light.

    Now if for some reason you don't believe your clock is working correctly and you make an adjustment to it, then of course you will get a different value for the speed of light when you make a measurement. But why would you do that?
  7. Apr 13, 2013 #6
    Cause i want to know the relation of time dilation with C? So with time dilation in mind I sync my time with a reference point time to make sure time dilation does not affect my reading, then i take a measurement with a different velocity compared to the reference point(synced time) to show that if there is no time dilation C will not be constant thus show constant C relation with time dilation.
  8. Apr 13, 2013 #7


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    If you want to measure the speed of light, you have to carry your own clock and use it without regard to any other clock. You cannot use some other "reference point time" such as is provided by GPS or internet time or the time delivered to your cell phone and there is no point in syncing your clock to any other clock because when you measure the speed of light, you're going to take the difference between two times on your clock (one at the start of the measurement and one at the end).

    So it won't matter whether or not you sync your clock prior to you changing your speed (with your thrusters on) as long as you turn them off during the time you make your measurement. You could make one measurement while you are at rest with what you are considering your "reference point time", then take off to a high speed and then coast (thrusters off) and make another measurement with the same apparatus you used the first time and you will get the same answer for the speed of light. Meanwhile, someone else could also make another measurement of the speed of light using the "reference point time" clock and they will get the same answer you got.

    Do you have any doubt about this?
  9. Apr 13, 2013 #8
    No doubt about the result you get will be the same this way but you did not consider time dilation effect under different velocity.

    I make a measurement on earth lab I got C, I make one on ISS lab with a clock I bring up from earth(affected by time dilation) I got C, but i know that ISS time is slower than earth so I make a measurement with a clock that is use to communicate with earth that's currently sync to earth time(unaffected by time dilation) I get a different result. I want to use earth time for a measurement to show that without time dilation C will not be constant.
  10. Apr 13, 2013 #9


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    I don't have to consider time dilation effect under different velocity when measuring the speed of light. If you have no doubt about that, then I don't know why you keep asking about it.
    I didn't know the ISS time is slower than earth time. Can you provide a reference for that?

    I already said that if you use a different clock you will get a different measurement for the speed of light.

    Maybe I should ask you what you understand Time Dilation to be. What does that mean to you?
  11. Apr 13, 2013 #10


    "After our 6 months in space, we will have actually aged slightly less than everyone else on the ground because of an effect called time dilation. It isn't by much (about 0.007 seconds), but it is one side benefit of flying in space!"
  12. Apr 14, 2013 #11


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    I think the problem is much more complex than your link would indicate. See this link. This website points out that we on earth are also subject to gravitational time dilation so why would you want to use one of our clocks as the "reference point time"? I keep saying that each measurement of the speed of light requires its own clock for this very reason.

    I get the impression that you believe that clocks in one situation, for example, on earth, are better or more accurate or more real than clocks in other situations for example traveling at a high speed or in orbit around the earth and that you want to conclude that the measurement of the speed of light by some clocks is legitimate while others need to be explained. Is that where you are going with this?
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