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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?

Lou
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2 Answers2

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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.

Mikael Fremling
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In the interior of the star the electrons are stripped from the hydrogen atoms, so absorption shouldn't be occurring, but scattering where the photons change direction would still happen. The photon would do a random walk scattering many times before reaching the cooler outer layer of the star where absorption and remission processes would more likely. The millions of years estimate is likely too long, but with some assumptions hundreds to thousands of years would be more likely. This is discussed some on a NASA web page.

UVphoton
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