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Relativity effects on photons

  1. Feb 20, 2008 #1
    question: do the effects of relativity that would manifest on objects approaching the speed of light (increasing mass, decreased time, etc) also affect photons? Photons are, after all perhaps the only things that routinely travel at that rate.
  2. jcsd
  3. Feb 27, 2008 #2
    photons are the main part of the electormagnetic spectrum which travels at c. everything else travels relative in order to keep the constant for all observers.
  4. Mar 2, 2008 #3
    What I am wondering about is are relativity effects present on photons. I.e., a space craft approaching the speed of light will experience increasing mass, decreased physical dimension, time dilation etc. The question is do theses effects occur with photons, in any way.
  5. Mar 2, 2008 #4
    Photons always travel at the speed of light.
  6. Mar 2, 2008 #5


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    ...and have zero mass.
  7. Mar 3, 2008 #6

    George Jones

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    The frequency, wavelength, colour, and energy of light depend on the frame of reference.
  8. Apr 23, 2008 #7
    So... by "frame of reference" do you mean perhaps relative velocity?
  9. Apr 23, 2008 #8


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    Relative velocity of the speed of light from any and all frames of reference will always be c. So, no.
  10. Apr 24, 2008 #9


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    However, two different inertial reference frames have a relative velocity with respect to each other, which is how we know they're different. And even though a pulse of light travels at the same speed in both reference frames, it has different energy, wavelength, etc. in the two frames. For frequency and wavelength this is known as the relativistic Doppler effect.
  11. Apr 25, 2008 #10
    This is surely a case where representing a pulse of light as a particle (or particles) is misleading. As when Isaac Newton described light as pulses or corpuscles from his prism experiment splitting light into colours. And got into problems over how many different coloured corpuscles exist.
  12. Apr 25, 2008 #11
    My understanding is yes. For example, the "rest mass" of a photon is zero, but its "relativistic mass" is not. The relativistic mass can be any finite value between zero and infinity.

    If you fiddle around with Einstein's equations, you'll see that if something travels at the speed of light, its relativistic mass will be its rest mass multiplied by infinity. So, for anything with non-zero rest mass, the relativistic mass would become infinity (which is why things with mass can never reach the speed of light). Conversely, if you start with zero rest mass (as with a photon) and multiply by infinity, you don't get infinity - instead you get an indeterminate finite number (i.e. anything between zero and infinity). This is why photons can travel at the speed of light, and this is why they can have different relativistic masses (e.g. photon of blue light has more relativistic mass than photon of red light).
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