Is the number of photons in the universe fixed

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Discussion Overview

The discussion revolves around the question of whether the number of photons in the universe is fixed, particularly in the context of the Cosmic Microwave Background (CMB) and photon interactions with electrons. Participants explore the implications of photon creation and absorption in atomic processes and the effects of interactions in different environments.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant cites a Wikipedia article stating that CMB photons have been propagating since photon decoupling, suggesting a fixed number of photons in that context.
  • Another participant challenges the idea that the behavior of CMB photons can be generalized to all photons, arguing that different sets of photons can behave differently in various environments.
  • A participant clarifies that the CMB refers specifically to photons that have not interacted with anything since their creation, implying a specific context for the discussion of photon numbers.
  • It is proposed that photons can be created or transformed through various processes, such as electron transitions or interactions that can lead to the creation of lower energy photons, indicating that the number of photons is not fixed.
  • One participant provides an example of how holding a black cardboard to the sun can alter the photon population by converting higher energy photons into lower energy ones, further supporting the idea that photon numbers can change.

Areas of Agreement / Disagreement

Participants express disagreement regarding the notion of a fixed number of photons in the universe. There are competing views on the implications of photon interactions and the context of the CMB, leading to an unresolved discussion.

Contextual Notes

Participants highlight the complexity of photon interactions, including the potential for photons to split or be transformed into other forms of energy, which complicates the question of whether the number of photons is fixed.

bobsmith76
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It says clearly at the wiki article regarding the CMB that: "The photons that existed at the time of photon decoupling have been propagating ever since, though growing fainter and less energetic, since exactly the same photons fill a larger and larger universe."

I don't see how that's true. As I understand things when an electron descends from a higher orbit it releases a photon. When electrons are in a lower orbit they have less energy and that loss of energy is transferred to the photons so that the amount of energy in the universe is conserved. It appears to me then that photons are created from electrons and later they are absorbed by electrons when they ascend to a higher orbit. It seems like I've unwittingly answered my question. Still, I want it to be sure. Is it accurate to say that

any time an electron releases a photon by descending an orbit that same photon must later be absorbed by a different electron ascending to a higher orbit?
 
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I don't see how this possibly follows.

The statement about one group of photons doing one thing - in this case, traveling in empty space in an expanding universe - says absolutely nothing about what an entirely different set of photons in an entirely different environment are doing.
 
bobsmith76, I believe the Wikipedia statement is referring specifically to the CMB photons, and simply means that what we detect as the CMB comes from original photons that have never interacted with anything over the 13.75 Gy since their creation.
 
bobsmith76 said:
any time an electron releases a photon by descending an orbit that same photon must later be absorbed by a different electron ascending to a higher orbit?

Not at all. It could split into an electron/positron pair or some such as it travels along, and there are many interactions with materials that could occur to convert it into a whole bunch of lower energy photons, and so on. So the number of photons in the universe is nothing like fixed. If you just hold up a big bit of black cardboard to the sun you will be helping to increase the photon population, since you will be converting higher energy visible photons into a larger number of lower energy infrared photons. Or perhaps you will decrease the total population since you store a bunch of those photons as heat before they get re-emitted. Anyway you get the idea.
 
Kurros,

thanks for the reply. i appreciate your help.
 

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