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I have 2 questions about relativity

  1. Feb 17, 2008 #1
    First of all, velocity is a relative phenomenon. When we say velocity of an object is s m/s, it is relative to some other object or a frame of reference (fixed to an object).

    1)According to the Theory of relativity, the speed of light is always equal to 3*10^8 m/s. But this velocity is considered relative to what?

    2) When two photons are moving in opposite directions, what is the velocity of one photon with respect to the other?
  2. jcsd
  3. Feb 17, 2008 #2
    1) c is an universal constant. That's the basic ingredient of special relativity ! So, c is constant with respect to "EVERYTHING" !

    2) The photon speed is always c. One cannot exceed this speed, which is another consequence of the basic theorem of special relativity. Just try this for instance : Use the Lorentz transformations to calculate how fast one observer is moving with respect to 2 others. Of these 2 other observers, one is not moving and the other has some speed v. So the third one is moving with some speed v' with respect to the second one, which is moving with speed v compared to the third one.

    In classical physics, you will find out that one can just add up the velocities linearly. In special relativity this will not work : so speed does not add up linearly ! This is one of the biggest differences between Newton and special relativity.


  4. Feb 17, 2008 #3


    Staff: Mentor

    Photons don't have their own rest frame, so all we can say is that in any inertial reference frame the speed of each photon will be c. Since their direction is opposite then the distance between the two photons will be increasing at 2c. Note that this is not the same as saying what is their velocity wrt each other.
  5. Feb 17, 2008 #4
    I'm going to jump in here with a question too.

    Is only light at the constant c or the whole electromagnetic spectrum?
    and if space is warped, is it warped in the frame too or does it remain the same?
  6. Feb 17, 2008 #5
    1) Relative to any inertial observer.

    2) The relatavistic velocity addition formula give the the answer as 0/0. In other words indeterminate. This is not a contradiction to (1) because no physical observer can move at c relative to any other observer.
  7. Feb 17, 2008 #6
    Yes, the constant speed c applies to the whole electromagnetic spectrum.

    As far as space warping is concerned the term is vague and often used in hand waving arguments. It is better to be more specific by what you mean by warping. If you mean the curvature of space due to gravity then we are talking about General Relativity and not Special Relativity. An observer that is stationary in a gravitational field would notice the curvature unless they are restricted to a infinitessimally small volume. Technically the frame of that observer is not an inertail frame as they are experiencing accleration. You would have to clear if you are talking about an observer that is free falling or stationary in the gravitational field.
  8. Feb 17, 2008 #7
    With the exception of an uniform gravitational field, wouldn't tidal forces still be detected?
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