[pdonis]

Note that this article is assuming, for purposes of argument, that photons actually have mass, equal to the current upper limit for possible photon mass based on experiments. But the experiments are all consistent with the current theoretical belief that photons have zero mass; and if they have zero mass, the concept of "lifetime" for a photon (and indeed the concept of "photons frame of reference") is not even well-defined.

[tinus_hn]

If they are moving with the speed of light no time passes for photons so ‘lifetime’ has no real meaning either.

[pdonis]

> If they are moving with the speed of light no time passes for photons

No, the concept of "time passing" for an object moving at the speed of light has no meaning. That's why "lifetime" has no meaning for a photon if it has zero mass.

[omarchowdhury]

Wouldn't c have to be infinite for "no time to pass", since light is still traversing space at a finite speed?

[ben_w]

No, because the rules of Lorenz invariant field theories mean that the infinity you’re (presumably) expecting is mapped, via the joy of hyperbolic geometry, onto the propagation speed of that field.

Or, to put it another way, the speed of light is where time dilation and length contraction run into each other and it all goes zero-divided-by-zero.

[pdonis]

> Wouldn't c have to be infinite for "no time to pass"

You're using a different notion of "time passing". Yes, we, observing photons, certainly observe them to take a finite time to cover a finite distance. But that is not the same as the concept of "time passing" for the photons themselves, for example according to a clock that the photons carried along with them; that is the concept that has no meaning.

[omarchowdhury]

If the photons were a conscious subject, would that mean, from their point of view, they are everywhere (everywhere being defined as the entire path the photon will take from its genesis to its final destruction or absorption), all at once? Because if no time is passing for them, doesn't that mean no space is being traversed either?

[etaoins]

The flip side of time dilation is Lorentz contraction. As you approach the speed of light objects in your direction of motion will become shortened from your frame of reference.

For example, there are particles from cosmic rays that should not be able to make it to the surface without decaying. However, they're detected all the time. Two valid ways to think of this are:

1. From the Earth's frame of reference time moves more slowly for the particle. This slows down the process of decaying.

2. From the particle's frame of reference the Earth's atmosphere is considerably shorter so it doesn't need to travel as far.

Things get a bit hairy to talk about once you actually reach the speed of light. One way to think of it might be from the photon's frame of reference its entire path has become infinitely short so it had no distance to travel at all.

[pdonis]

> If the photons were a conscious subject

They can't be. You can't make a conscious subject out of something that has zero mass and moves at the speed of light, and therefore does not have a well-defined concept of "time passing".

> if no time is passing for them, doesn't that mean no space is being traversed either?

It is not the case that no time is passing for a photon. What is the case is that the concept of "time passing for a photon" is not well-defined. That concept not being well-defined means that "space being traversed for a photon" is not well-defined either. That does not mean "no space is being traversed". It means the concept of "space being traversed for a photon" is not well-defined.

[Yajirobe]

No, outside observer will measure clocks of photon run slower by a factor of γ (Lorentz factor), which (in scalar form) is equal to 1/sqrt(1 - v^2 / c^2 ). It diverges at v = c.

[pdonis]

> outside observer will measure clocks of photon run slower by a factor of γ

This is not correct since "clocks of photon" is a meaningless concept (at least if photons have zero mass, which they do according to our best current models).

[Yajirobe]

Obviously, I did not mean to imply that you can attach a clock on a photon. I thought this goes without saying.

[Florin_Andrei]

Speed of light is for-all-practical-purposes infinite in the reference frame of the moving object (i.e. photon), thanks to special relativity.

[pdonis]

> Speed of light is for-all-practical-purposes infinite in the reference frame of the moving object (i.e. photon), thanks to special relativity.

This is not correct, because there is no "reference frame" for a photon moving at the speed of light. The concept of "reference frame" is not well-defined for such objects.

[misnome]

This conversation sounds like it's running into the linguistic barrier that we tend to think of "speed of light" in terms of, well, light, so becomes recursive when talking about photons with mass, which naturally travel slower than what should be thought as "the speed of massless particles".