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Coherency And Other.

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Posted by Alexander on January 25, 2001 17:04:59 UTC

Stars radiate light because of spontaneous emission of Hydrogen atoms in the plasma in their photosphere. This plasma is hot, and atoms/ions of hydrogen frequently collide with each other and electrons around. Due to this lifetime of energy levels is short, and radiated photons are small (about 0.1-1 mm in length, or what they call coherence length is in the range 0.1-1 mm).
In so-so He-Ne lases a photon bounces between mirrors separated by 30-50 cm a few times before it gets out, so the length of such photon is several meters, so coherence length is sousand times more than of stars' photons.
Sphericity of the wavefront of a photon has nothing to do with its coherence length. A light photon can be billion miles across but less than millimiter in length.

About absorption of light by matter. You know that early Universe was dense, hence opague for radiation. Which means, that e/m radiation in it was constantly absorbed by matter (electrons, quarks, neutrinos, etc), reradiated, reabsorbed,... many times over. That is how you get it thermalised with matter - both had abou the same temperature (average energy).And that is the reason why we do not see relic radiation from early history of Universe - it all long gone (absorbed by matter). But during expansion density drops, and transparency of matter increases. After about 2x10^13 sec (0.6 million years) since the expansion began the temperature of Universe dropped below 3000 K and electrons bonded to protons can no longer absorb radiation. Since then, radiation (with about 3000 K temperature) travels the Universe with very little absorbtion, and we see it today (not as 3000 K light, but due to red shift as microwave). So during entire inflation and big bang radiation was absorbed by matter, and only 0.6 million years afer radiation "decoupled" from matter.

About omega. Omega is the ratio of actual mass (or density) of the Universe to the critical one. And in the mass (or density) we include mass (density) of all kind of stuff: physical matter, radiation, gravitational fild, magnetic field, neutrinos, etc. - anythig having energy (thus mass m=E/c^2, thus gravity). Thus transformation of matter into light (annihilation) and back (matter production) which take place should not bother you - total energy of Universe (thus the amount of bending gravity in it) does not depend on the content - wheather the Universe is made of 1 kg of light or 1 kg of protons, or 1 kg of neutrinos, or 1 kg of mixture.

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