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Does light experience relativistic effects?

  1. Oct 7, 2013 #1
    I have heard people argue that light doesn't experience time or space. That it is everywhere at once. I suppose that they base this on the relativistic idea that time and space are warped by speed, and that the closer an object gets to C, the more time slows down and space contracts relative to an observer in an inertial frame. Thus because light travels at C it would experience no time, and no space relative to all observers.

    But it seems to me that this would only hold true for objects with mass, but not for light. Since light always moves at C relative to everything else does it experience the relativistic effects of time dilation and length contraction? Thus it is indeed everywhere at once? Or is light immune to relativistic effects because it has no mass?

  2. jcsd
  3. Oct 7, 2013 #2
    Light is subject to relativistic effects; for example light is redshifted or blueshifted depending on the reference frame in which it is measured, light can bend around gravitational sources (e.g. gravitational lensing), and there are issues of simultaneity characteristic of relativity involving light, e.g. did two parallel but separated light beams pass through the ecliptic plane at the same time, or did one precede the other? So light obviously is not "immune" to relativistic effects.

    However, you seem to be asking about whether a light beam itself "experiences" time dilation or length contraction in the same way a moving observer would experience those phenomena. The answer to this question is that measuring time and space requires massive particles, and a system of only massless particles cannot have any real "experience" of time or space. For example, we can build a clock and accelerate it to any speed less than c, and observe the effect of time dilation on how it ticks--but we cannot accelerate it to c. Conversely, we cannot build a clock out of massless particles alone. So asking what a system of massless particles experiences is a contradiction in terms--all measurements must be made by massive observers with massive instruments.
  4. Oct 7, 2013 #3
  5. Oct 8, 2013 #4


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    That is the best answer I have ever seen to the question of why light does not experience time or space, thanks.
  6. Oct 8, 2013 #5
    Excuse me for continuing this thread, but is the redshifting and blueshifting of light really a relativistic effect? If I have two inertial observers in motion relative to each other, such that they pass each other at near the speed of light. Won't each see the other as being length contracted both before they pass, and after they pass? They won't see each other as being length contracted as they approach, and then length expanded as they recede will they? But isn't that what we see in the case of light? The light from each of our observers will be blueshifted in one direction and then redshifted in the other.

    For this reason I'm not sure that the redshifting and blueshifting of light can be said to be a relativistic effect. It seems to simply be the product of the motion of an observer relative to a source of light. Merely the doppler effect, not a relativistic effect. Or perhaps I'm missing something. Can you in some way clarify how redshifting and blueshifting is a relativistic effect?

  7. Oct 8, 2013 #6


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    Google for "relativistic doppler effect".
  8. Oct 8, 2013 #7


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    The relativistic Doppler effect is quantitatively different from the non-relativistic Doppler effect. Experiments confirm the relativistic Doppler.
  9. Oct 8, 2013 #8


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    In addition to the above (which deals with the kinematical redshift), electromagnetic radiation doesn't interact with the gravitational field in Newtonian gravitation whereas it does in GR i.e. the gravitational redshift of electromagnetic radiation comes from the coupling of the radiation to the gravitational field in GR, whereas such a coupling is nonexistent in Newtonian gravitation; a simple conservation of energy argument can derive the result.
  10. Oct 8, 2013 #9
    They will both measure each other to be length contracted by the same factor and this will be the same before and after they pass if the relative velocity is constant.
    Visually, they will see the the other as apparently longer when approaching and apparently shorter when receding. When an object is approaching, the light they see from the trailing edge of the object left at an earlier time than the light from the leading edge and vice versa for the receding object.
    The relativistic Doppler effect is basically the classical Doppler shift multiplied by 1/gamma where gamma is the relativistic time dilation factor. There is a caveat to this statement. The classical Doppler effect uses the velocities of the receiver and source relative to the medium. In relativity there is no medium for light so we have to use only the velocity of the source relative to the receiver. This can be done algebraically by setting the velocity of the receiver in the classical Doppler formula to zero, before multiplying by the time dilation factor.
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