Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 24/11/2014 18:30:02
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David Feury asked the Naked Scientists:
I am trying to understand the "age" of a photon coming from Sol (our sun) to the Earth. I know that the travel time is about 8 min., but is the photon always "born" at the surface? Is there any gap between the time a photon is created and it bursts free of a stars surface?
Most curious, David F.
What do you think?
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Photons are also produced inside the sun. It can often take much, much longer than 8 minutes for a photon to get out of a star (it can be thousands of years!) because the star is so dense. There is a short article about it here: http://www.abc.net.au/science/articles/2012/04/24/3483573.htm
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Can one consider an absorption/emission chain reaction as being the same photon traveling through matter?
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Can one consider an absorption/emission chain reaction as being the same photon traveling through matter?
I guess probably not, but I'm not exactly sure how a model in which it is the same would differ from what could be observed.
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The spectrum of radiation that has been absorbed and re-emitted several times will be quite different from that of "virgin" photons that have been delayed by gravitation. The primary emission from nuclear reactions will have sharp spikes characteristic of those reactions, though presumably redshifted.
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What frame of reference are we using here :)
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You can't give a 'photon' a own 'frame of reference', so it is a trick question in one way. What we can do though, is to observe that 'light' seems ageless from our own frame of reference (as from Earth). We can see all the way back to the Big Bang, if that now means that some locally 'photonic intrinsic clock' doesn't 'tick, or 'ticks'? I would say light is eternally here :), well, time wise, or not? It seems to be able to define itself from the first light we can observe.
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Can one consider an absorption/emission chain reaction as being the same photon traveling through matter?
Agree. That is a "metropolitan legend"😊
I believe it's not physically meaningful to talk of the same photon.
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lightarrow
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If one shut down the fusion reactions in the sun, then it would remain hot and bright for a very long time.
That is essentially what a white dwarf is.
It is not that it would take millions or billions of years for a photon to reach the surface, but rather a slow radiation of heat over time as the core slowly cools, and to a large extent, the outer surface insulates the core.
The radiation of heat and light from fusion isn't much different.
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Can one consider an absorption/emission chain reaction as being the same photon traveling through matter?
Not at all. You're 100% right on this part.
However it can be said that light does take a certain time to move from the center of the sun to the surface and that amount of time is along time. Some estimates give about 4,000 years. The actual length of time depends on various assumptions.
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What frame of reference are we using here :)
Simple. It's implicit in the statement of the problem that the we're talking about the frame of reference in which the sun is at rest.