[amluto]

I think this is sort of besides the point. If you build a box, paint the walls black, and put a flashlight in the box, then the photons coming from the flashlight are shorter lived than if you shine the flashlight into the sky on a cloudless day or night. Not shorter lived from their own perspective — shorter lived from an outside observer’s perspective. Sure, one could quibble about the choice of observer, but you would he hard-pressed to put an observer in the box who thinks the photons last very long.

[pixl97]

Why is the speed of causality beside the point?

lets take two magical particles that have clocks on it. One is a photon and the other is a neutrino. I send these off towards you in a perfect vacuum. When you receive these particles the clock on the photon will be 0. It will be be the exact same photon that left my emitter, it will not have changed in any way as it did not interact with anything along the way. And as long as you are not moving relative to me, you'll perceive the photon as the same color/wavelength I emitted it at.

Meanwhile that neutrino will arrive billions of a second later (well depending on our distance) and will have oscallated at least trillions of times if not far more. The clock on the neutrino will have ticked the difference between the photon arrival to the neutrino arrival.

Don't apply classical behavior to light-like objects. They play be different sets of rules.

[amluto]

This is all true, but the article isn’t about how long a photon thinks it lives or how much it experiences the passage of tone. It’s about whether the photon keeps going forever from the perspective of someone approximately at rest [0] in the universe (like astronomers on Earth!).

[0] General relativity has no preferred “at rest” frame, but the generally accepted FLRW model of the universe does. You can be at rest with respect to the universe, or you can be moving. If you are moving, distant objects in front of you will appear blue-shifted on average as compared to distant objects behind you.