View Single Post
Old 27th August 2014, 02:14 PM   #10
Ziggurat
Penultimate Amazing
 
Ziggurat's Avatar
 
Join Date: Jun 2003
Posts: 43,071
Originally Posted by wogoga View Post
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.

Quote:
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.

Quote:
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.

Quote:
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.
__________________
"As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law
Ziggurat is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top