Does light have an infinite lifetime?

Cosmic Future and Photon Dominance

• Several comments discuss a far‑future universe dominated by photons (and neutrinos), contingent on whether protons decay and electrons ultimately annihilate.
• Some note that this scenario is model‑dependent and untestable in practice, though it underpins ideas like conformal cyclic cosmology (CCC), where a photon‑only universe enables a new “cycle.”
• Evidence for CCC from CMB data is described as disputed due to methodological issues.

Proton and Electron Stability

• One side: in the Standard Model, the proton is exactly stable; decay is hypothetical. Experimental lower bounds on its lifetime have already ruled out many beyond‑Standard‑Model theories that predicted decay.
• Other side: many extensions of the Standard Model still expect proton decay at very long timescales, and some commenters think most high‑energy physicists still “expect” decay, though this is characterized as uncertain.

Photon Lifetime and Reference Frames

• Repeated clarification that special relativity does not allow a rest frame for photons; their “proper time” is undefined, not “zero.”
• Thus, statements like “from the photon’s perspective it is emitted and absorbed instantly” are called useful intuition at best, but technically ill‑posed.
• Theoretical work suggests known massless particles (photons, gluons) are stable; decay channels would violate conservation laws under current theories.

Light Speed, Media, and Refraction

• Light in media is explained either as:
  • A wave interacting with atoms, creating delayed re‑emission and an effective slower group velocity, or
  • Photons being continually absorbed and re‑emitted (or forming polaritons) that propagate slower than c.
• Photons themselves always move at c in vacuum; “slow light” is an emergent effect.

Are Photons “Things”? Identity and Memory

• Some argue the question “infinite lifetime” is almost meaningless: photons are excitations or “motion,” not enduring objects.
• Others highlight that individual photons are created and destroyed at emission/absorption; in mirrors and materials, incident photons are annihilated and new ones emitted, so mirrors do not “store” scenes.
• Identity of photons or electrons is described as fundamentally fungible; tracking a specific one is not meaningful in quantum theory.

Philosophical, Religious, and Sci‑Fi Angles

• Discussions reference science fiction futures (e.g., “photino birds,” “slow glass”), philosophical ideas about time as an emergent slice through a static spacetime, and religious metaphors equating light with divinity.
• Some participants enjoy these resonances; others object when they drift too far from the physics.

Meta: Style of Physics Communication

• One commenter criticizes pop‑physics articles for being long, qualitative, and nontechnical, wishing for more direct, paper‑level explanations and fewer paywalled sources.