Light Scattering: Free Electron Explanation

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    Light Scattering
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Discussion Overview

The discussion centers on the scattering of light by free electrons, particularly in the context of the early universe and the formation of neutral hydrogen. Participants explore the mechanisms of scattering, the implications of "freezing out," and the relationship between scattering and energy loss.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • Some participants explain that light is scattered by free electrons until they "freeze out" and form neutral hydrogen, which significantly alters the interaction with visible photons.
  • It is noted that free electrons have a higher probability of scattering visible-light photons compared to electrons bound in hydrogen atoms.
  • One participant raises a question about whether the scattering process results in energy loss for the electrons with each scattering event.
  • Another participant clarifies that the cooling of the universe was primarily due to its expansion rather than the scattering process itself.

Areas of Agreement / Disagreement

Participants generally agree on the role of free electrons in scattering light and the concept of "freezing out." However, there is some contention regarding the causes of cooling and the implications of scattering on energy loss, indicating unresolved aspects of the discussion.

Contextual Notes

The discussion involves assumptions about the behavior of photons and electrons under varying conditions, and the dependence on the definitions of terms like "freezing out" and "scattering." There are also unresolved questions regarding the energy dynamics during scattering events.

GarryS
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Light is scattered many times by free electrons. The scattering goes on until the electrons freeze out, after 300,000 years. Can anybody explain this to me.
 
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The key is that free electrons have a much higher chance of scattering visible-light photons than do electrons bound in hydrogen atoms. The "freezing out" means that the temperature gets cold enough that the electrons in effect stick to protons and make neutral hydrogen. At that point, the "veil lifts", because the bound electrons do not interact strongly with the visible photons of the CMB at that time. This means the time it takes a photon to scatter off a bound electron in a hydrogen atom is longer than the age of the universe at that time (some 380,000 years), and if it doesn't scatter in that time, it probably won't later on either, because the density just keeps dropping with time.
 
Ken G said:
The key is that free electrons have a much higher chance of scattering visible-light photons than do electrons bound in hydrogen atoms. The "freezing out" means that the temperature gets cold enough that the electrons in effect stick to protons and make neutral hydrogen. At that point, the "veil lifts", because the bound electrons do not interact strongly with the visible photons of the CMB at that time. This means the time it takes a photon to scatter off a bound electron in a hydrogen atom is longer than the age of the universe at that time (some 380,000 years), and if it doesn't scatter in that time, it probably won't later on either, because the density just keeps dropping with time.

Does this mean that the scattering process takes away some energy of the electron each time?
 
The cooling wasn't due to the scattering process, it was due to the expansion of the universe. That cooled the radiation and the electrons, and allowed the atoms to form.
 

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