Originally Posted by
wogoga
Sunlight should indeed be incoherent, because sunlight emerges from thermal radiation, and thermal radiation is considered spontaneous emission. So the phase of one photon should be independent from all other photons.
Not "should be".
Is.
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However
photons, as social particles, tend to emerge and travel in coherent groups. And the longer they travel next to one another, the more they become coherent, by exchanging momentum and energy.
First off, your source is basically religious, not scientific. Second, you seem to be confusing the coherence effects that lead to lasers with thermal emissions. This is wrong, very wrong. Stimulated emission does lead to coherence, but it can never dominate emissions unless you've got population inversion, and thermal emissions from a star are very much NOT inverted thermal populations. Third, photons interact with each other very little. As ben explains above, the photon-photon interaction is basically non-existent within the optical range.
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So even if sunlight (in a given direction) should not be coherent on Earth, it certainly will become coherent after having travelled some years, as starlight from our nearest stellar neighbors is highly coherent.
It is spatially coherent at long distances simply because the waves have traveled so far that they're basically all traveling parallel to each other. You can only get spatial decoherence when light arrives at your detector from different directions. This is a purely geometric effect, it involves zero interactions between photons. A distant star subtends so little solid angle from our view that it's about as good a point source as you could hope for.
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Huge areas of mutually coherent photons separated by "fissures", i.e. boundaries between photons of different phase shift (and maybe of slightly different frequencies).
This makes no sense whatsoever.