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Coherece Length Is Just A Photon Length

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Posted by Alexander on November 5, 2001 00:13:33 UTC

when it (photon) is propagating in free space. Let me explain this in more simple terms.

Suppose, you have an atom radiating a photon during 1 nanosecond, so electron spirals from one level into another during 1 ns and creating oscillating electric field around atom. Moving with c during this time interval, electric field covers the distance about one foot, right? After the electron settled on a new state, no more variable electric field is created. Thus, spherical shell of e/m field with the thickness about one foot is propagating from atom in all directions. That is what we call a photon with "the coherence length equal 1 ft". You can modify this field by cutting shorter chunks out of it (=making the same photon shorter or longer).

Relationship between a charge and created by it electric field written using Maxwell equation is incomplete (although is very usefull where quantum effects can be ignored). It does not take into account uncertainty principle, contribution of which is small for large objects but can't be ignored for small objects (= small energies of field). Adding UP to Maxwell equations results in QED (quantum electrodynamics versus classical one when you ignore UP). Quantum electrodynamics derives thus the wave function of variable electric field (=photon) from equation of static field of electric charge plus Lorents transformation into non-accelerating observer system of reference plus UP.

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