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When reviewing a refractive index chart there is a relationship to the density of the medium and the velocity of electromagnetic radiation through the medium. The lower the density the greater the velocity. The less stuff in light's path the faster it goes. The vacuum medium, the QED electromagnetic field sometimes called the photon field, propagates light at 300 million m/s.

My question in very layman's terms is this: Stuff in light's path slows it down, but if there is no stuff then there is no field and propagation stops. How does modern physics look at this transition zone from a very low density field to zero density. Would a less dense photon field increase the speed of propagation? Does the photon field have a refractive index?

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  • $\begingroup$ As it stands, it's quite unclear what you're asking. I think the answer to your question might be this: light does not interact with itself, so having a "bath" of photons would not impede the propagation of any one of them (if this bath is in a vacuum -- there are situations where photons can effectively interact, and such is the realm of nonlinear optics). $\endgroup$
    – Bob Knighton
    Commented Feb 6, 2019 at 19:58

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if there is no stuff then there is no field and propagation stops

This is not correct. Light propagates in vacuum, there is no need for stuff in order for light to propagate. So if there is no stuff then light proceeds at its maximum speed: c.

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  • $\begingroup$ Doesn’t light need the QED photon field? $\endgroup$
    – Lambda
    Commented Feb 7, 2019 at 0:40
  • $\begingroup$ Light IS the QED photon field. $\endgroup$
    – Dale
    Commented Feb 7, 2019 at 1:03
  • $\begingroup$ I know light, photons, are excitations within the field. Doesn’t the field exist in the dark. Isn’t the field present throughout the universe. It must be expanding rapidly with the expansion of the universe. The field is something. $\endgroup$
    – Lambda
    Commented Feb 7, 2019 at 18:45

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