All photons have the same speed. In "space" (I mean when there is practically no matter) a photon will travel a distance faster than a photon in matter. But the speed is still the same.

Do 2 photons have the same proper time after the same chunk of time (in different environment)?

(because the speed is the same but the distance traveled isn't).

  • $\begingroup$ If we are talking about different mediums, doesn't it mean that at least one of the photons was "absorbed" and then reemitted repeatedly? So, does it even make sense to compare the frames of all these different photons? $\endgroup$
    – NiRVANA
    Oct 22, 2019 at 14:57
  • $\begingroup$ On further thought, photons are bosons, so basically they are indistinguishable. So, how do you even attach a reference frame(assuming one could) to a photon and differentiate it from others? It's a funny afterthought. $\endgroup$
    – NiRVANA
    Oct 22, 2019 at 14:59

1 Answer 1


As you can see from one of my previous answers, photons do not have a proper time, but you can use a λ affine parameter, that increases monotonically along the lightlike worldline.


Now you are saying that a photon will travel faster in space then in a medium. In reality, photons always travel at speed c in vacuum, when measured locally.

Now when a photon enters a medium, it still travels in vacuum between the atoms/molecules, at speed c.

It is the wavefront of the whole EM wave, that is built up by a herd of photons in a coherent way, that slows down in the medium, due to the interactions of the individual photons with the atoms/molecules in the media. That interaction takes time.

The simplest picture is that light always travels at the speed of light. But in a material it travels at the speed of light until it hits an atom. It is then absorbed and re-emitted in the same direction, which takes a small amount of time. The more this happens, the slower the effective average speed. The denser the material, the more atoms there are in the way.


As the medium gets denser, the number of interactions will be more, it will take more time from the individual photons and the wavefront will slow down more.


  • $\begingroup$ In my question I mean : if can be "in a photon" will I see "the end of the universe" out of the same time in diffent environnement ? $\endgroup$
    – Henry
    Oct 21, 2019 at 20:54
  • 1
    $\begingroup$ @Henry photons do not have a reference frame. You cannot say thing like when I travel with the photon what will I see. $\endgroup$ Oct 21, 2019 at 21:18
  • 1
    $\begingroup$ why the downvote? $\endgroup$ Oct 21, 2019 at 21:18

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