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Wave propagation and space.

  1. Jun 11, 2013 #1
    Whats the effect of shrinking and stretching of space on the propagation speed of electromagnetic waves. This is in reference to the direct correlation of the propagation speed to the fundamental properties of empty space viz. permittivity and permeability of free space. Do the permittivity and permeability of empty space change when it is shrunk or stretched significantly ?
    Ofcourse the effect can't be tested in a lab so far or can it ?
    c = 1/√ε0*μ0
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  3. Jun 11, 2013 #2

    Simon Bridge

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    Welcome to PF;

    No, it isn't.

    IIRC the evidence is indirect from observation of distance galaxies.

    It's been a while since I've visited this subject though and the state of the art may have moved on since then. My previous thinking (in a nutshell) was: since the Hubble expansion is in the metric, and E-Mag is Lorentz invariant, then I would not expect E-Mag constants to change with the metric.

    I have seen the occasional paper suggesting an expansion model that assumes some alteration to c, but none I've seen appear to add anything to existing theory.
  4. Jun 13, 2013 #3
    There are many possible answers to your question as I read it....since you posted this in cosmology, I'm wondering what you had in mind via the term 'propagation speed'. 'Speed' in curved spacetime is not well defined because neither time nor distance are well defined.

    Some possible answers:

    The speed of light is always 'c' locally. Locally, c = 1/√ε0*μ0, as you posted.

    Light at the Hubble radius, where recession velocity is c, is not closing any distance towards us right now, but will be in the future.

    Light just inside the particle horizon is receding from us right now, but can reach us in the future.
  5. Jun 13, 2013 #4


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    Bear in mind, in an expanding universe, we have a cosmic event horizon; which is not necessarily the same as the particle horizon.
  6. Jun 13, 2013 #5


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    Light just inside the Cosmic Event Horizon is receding from us right now but can reach us in the future. Assuming it's aimed at us. The CEH is currently 16.4 Gly

    Light just inside the particle horizon is receding from us right now, and always will recede, and can never reach us. The particle horizon is currently around 46 Gly.
  7. Jun 14, 2013 #6


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    The confusing part about the CEH is light from the particle horizon can reach us in the past, but, light from the
    CEH cannot reach us in the future.
  8. Jun 14, 2013 #7
    yes...thanks...{edit: Glad someone is checking on me!!}

    I saw this rather nice explanation somewhere and saved it.....too bad I didn't utilize it!!

    So this 'particle horizon' is the surface of last scattering from where CMBR originates....

    I think we can say for our universe, with our expansion 'an observer WILL find'...

    [and I'll be lucky if I remember to hit 'submit reply' key' !!!!]
    Last edited: Jun 14, 2013
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