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A relativity paradox

  1. Feb 24, 2013 #1
    Well its a paradox to me haha. If light has a maximum speed but yet every one object in the universe is moving relative to another... 1. Wouldnt I technically be moving at the speed of light since the light emitted from my iphone are zooming past me at 300,000,000 m/s?

    2. If no, then there has to be a way to have absolutely no directional motion in space, right? (Not on earth of course)
  2. jcsd
  3. Feb 24, 2013 #2
    you cant assume light ray to be stable...
    since it is the basis of relativity that a photon moves wid the velocity which is actually 299792458 m/s irrespective of the speed of the observer...

    reply ur comments
  4. Feb 24, 2013 #3


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    No, because you cannot use light as a reference frame for measuring other velocities. When we say something like "A's reference frame" or "relative to A" we mean the reference frame in which A is stationary. Light travels at speed c in all inertial reference frames, so there is no frame in which light is stationary. Therefore we cannot speak of velocities "relative to light."
  5. Feb 24, 2013 #4
    Wow, so if you were moving at say, 200,000,000 m/s, then a photon would appear to be moving 300,000,000 FASTER than you are?
  6. Feb 24, 2013 #5


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    1) When we say that speed is always relative to something, that's a short way of saying that we can pick a frame of reference in which the speed of an object is zero, or any other speed we please. Use the ground as the frame of reference, and the car is moving past at 60 mph; choose the car and we'd say it was at rest while the road and the scenery were moving backwards at 60mph; meantime the guy watching from Mars and considering himself at rest says that the earth, the road, the car are all moving at a few miles a second relative to him.

    But (many threads over in the relativity sub forum - search for "rest frame of a photon") light moves at speed c in all frames; there is no frame in which light is at rest or even moving at any other speed than c.

    Therefore, you can say that the light from your iPhone is moving at c relative to you, but you cannot say that from the point of the light, it is at rest and you're moving relative to it. Light doesn't have a rest frame or "point of view".

    2) No, that is not right. It would be right if we could find a frame in which the light was at rest... But we can't.

    [Edit: Looks like I don't type fast enough, or I type too much :smile: Jtbell said the same thing while I was still keying in my response. Silly iPad :smile;]
  7. Feb 24, 2013 #6

    Doc Al

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    Staff: Mentor

    Not really.

    If you were traveling at 200,000,000 m/s with respect to some reference frame, a photon would still travel at 300,000,000 m/s with respect to that frame. (And it will also travel at that speed with respect to your frame, in which you are at rest.)
  8. Feb 24, 2013 #7


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    200,000,000 m/s relative to WHAT?

    But the answer to your question, after we make it more precise, is "yes".
    Try this: two spaceships are flying in the same direction through empty space. They are moving at 200,000 m/s relative to each other. Both will measure the speed of a passing flash of light to be c. Yes, that merits a "wow", and you will also want to google for "relativistic velocity addition" or look for the many threads on this topic in the relativity forum.
  9. Feb 24, 2013 #8


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    What do you mean by "appear"? How do you plan to watch a photon?
  10. Feb 25, 2013 #9
    As funny as that is, it's a great point / question...at any speed.
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