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Problem caused by length contraction and relativity and personal expla

  1. Nov 22, 2013 #1
    The problem has been shown in the picture. Because of relativity, both the guys on the ground and on the vehicle would observe length contraction. So the destination of the guy on the vehicle---the galaxy, would be more closed to him. If the vehicle is replaced with the light, which means the distance between the photons and the galaxy would be 0 based on the formula of Lorentz's Transformation. But if this was possible, we can't measure the c=3000000000m/s, since the light can reach where we are immediately without time passes.

    Personal Explanation1:
    To solve such problem, I think the key is that photons do not have mass. So when the they speed up to c, their mass is still 0. Based on General Theorem Of Relativity, the space will not warp and no time dilation will be observed. ImageUploadedByPhysics Forums1385180746.725027.jpg
  2. jcsd
  3. Nov 23, 2013 #2


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  4. Nov 23, 2013 #3
    Agreeing with the above poster, but to clear up another misconception, time dilation isn't due to the warping of spacetime but rather to the Lorentzian character of flat spacetime.
  5. Nov 28, 2013 #4
    Ok, thanks
  6. Nov 28, 2013 #5


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    In GR, spacetime curvature is "caused" by energy and momentum, via the stress-energy tensor. Mass is only one form of energy. Photons do not have mass, but they do have energy and momentum, therefore they can create spacetime curvature.
  7. Nov 28, 2013 #6
    Hi Bruce:

    Any photons must travel at 'c' in the local frame. Also, the Lorentz transform only applies where relative speed between frames [observers] is less than 'c'. edit: in the absence of gravity...no curvature of spacetime....

    the speed of light is always 'c'..never 'instantaneous'.....

    as already posted, that is correct.

    photons always move at 'c' in free vacuum space.....So, for example, a photon emitted from an electron orbital begins at speed 'c'....and stays that way.
    Last edited: Nov 28, 2013
  8. Nov 28, 2013 #7
    But how to explain that phenomenon
  9. Nov 28, 2013 #8
    John Wheeler explains it this way:

    + ......or energy or momentum
  10. Nov 28, 2013 #9
    careful here .....gravitational time dilation exists:

    Two things affect the relative passage of time for different observers: relative speed as in special relativity [and associated length contraction] and in GR, relative gravitational potential.
    So in GR, for example, a clock in the bottom a building ticks more slowly relative to a local clock in the top of the building.

  11. Nov 28, 2013 #10


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    No scientific theory explains anything. GR is like a good map, it helps us move about in a terrain and predicts what we will see at different places. It is not trying to explain anything.

    I could give you a good explanation of why water boils in terms of atoms, heat, and so on. But then you could ask me to explain 'atoms' and we're back to square one.
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