[cygx]

Tomayto, tomahto. You can't parametrize a null geodesic by proper time, sure, but I don't see anything particularly wrong with calling the arc 'length'/spacetime interval between events along any particle trajectory a proper time, even when it's 0.

[platz]

https://www.reddit.com/r/askscience/wiki/physics/light_frame...

    (Δs)2 = 0

        It is wrong to then equate this to some sort of proper time Δτ, find that

    Δτ = 0

    for a photon, and thus conclude that photons always "experience" zero proper time. No. Remember that Δτ is defined as the 
    time difference between two events as measured by an observer (i.e., an inertial frame) that actually travels between the events. Photons 
    have no reference frames! So the definition of Δτ doesn't apply to null paths.

[cygx]

I disagree: While you cannot have a clock travelling alongside the photon, you can have a family of clocks travelling along light-like trajectories that have the photon trajectory as its limit.

But as I was alluding to with my 'tomayto, tomahto' remark, this is a question of semantics, and we're not really arguing about physics, but labels.

[platz]

fair enough