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While watching the rebooted Cosmos series, I heard Tyson say that a photon arriving to the Earth from the Sun might be millions of years old.

If I understood correctly, once it's emitted inside the Sun's core, the chance of it reaching the surface of the Sun is rather small because it's constantly being absorbed and emitted inside the Sun.

But isn't the correct way to say that the lifetime of a photon lasts from its emission to its absorption by the encountered atom? If this atom now emits a new photon, shouldn't we regard this simply as a new photon?

Or is that single exact photon really "wandering" around and being unable to get to the surface, without being absorbed at all?

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    $\begingroup$ You can't distinguish photons from one another, they are identical. Thus it doesn't make sense to ask whether a photon is "the same" as another photon. $\endgroup$ – Ruslan Aug 21 '17 at 17:39
  • $\begingroup$ @Ruslan In what sense is a gamma ray generated at the solar core indistinguishable from a visible photon received by us from the solar photosphere. $\endgroup$ – Rob Jeffries Aug 22 '17 at 0:09
  • $\begingroup$ @RobJeffries in the same sense as the gamma ray generated at the Big Bang is indistinguishable from a microwave photon coming with the CMB. The photon may have lost its energy in whatever way, but it still remains a quantum of the electromagnetic field, thus indistinguishable from the other quanta of the same field. $\endgroup$ – Ruslan Aug 22 '17 at 4:19
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    $\begingroup$ @Ruslan Agreed. There is no possibility that a CMB photon was emitted in the first second of the big bang. Neither is there any possibility that a photon from the Sun has come from the core. $\endgroup$ – Rob Jeffries Aug 22 '17 at 6:35
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Without having seen the series, I think what is envisioned is that a photon created in the innermost parts of the Sun (where the fusion happens) could take millions of years to reach the surface due to (really heavy) scattering in warm plasma that is the suns interior.

It's porbaly true that the lifetime of any individual photon is probably quite short, but if one can consider multiple absorption and emission processes as still referring to the same photon, then the whole ting makes sort of sense.

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  • $\begingroup$ Ok, right, I guess it makes sense that this packet of energy originated from the interior of the sun, even it's not the "same photon". $\endgroup$ – Lou Aug 21 '17 at 15:14
  • $\begingroup$ @Lou if instead of an active star you imagine a high-energy photon trying to escape a similar but cold environment, perhaps that is easier. The high energy photon(s) would be distinct due to its(their) energy, but would still be caught within the body due to constant absorption and emission. Of course realistically in that case the solid lattice of the cold body absorbs the entire photon as lattice vibrations rather than re-emitting... but you can get the picture by ignoring that minor inconvenience. $\endgroup$ – Asher Aug 21 '17 at 16:00

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