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Is the speed of light constant in a vacuum?

  1. May 5, 2013 #1
    Hello everyone!

    Just something I was thinking about today that I haven't been able to shake. I recently read that space is not actually a perfect vacuum, but that it contains low density particles, plasma, electromagnetic fields and so on. Is 'hard vacuum' the correct definition, or is it okay just to use vacuum?

    I'd like to know why these low density particles, fields, and plasma don't interact with the light or slow it down in any way (or do they?) While I'm on the subject of particles, do virtual particles exist in space, and do they interact with light?

  2. jcsd
  3. May 5, 2013 #2


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    Science Advisor

    The vacuum, as defined, contains no particles or plasma. Electromagnetic fields are classically unimportant, since Maxwell's theory is linear and one electromagnetic field does not affect the propagation of another.

    It's true that even the space between galaxies/clusters/etc. is filled with an extremely tenuous gas. However, the density in these regions is so low that the effect on the propagation of light is (mostly) negligible.
    Virtual particles do interact with light. In particular, they destroy the linearity of magnetic fields and mean that light does interact with other light, albeit very weekly. This effect, however, is much too small to be seen in astrophysical experiments and can only be seen in very precise particle colliders.
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