Can photon-photon interactions alone produce black body radiation?

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Discussion Overview

The discussion centers around whether photon-photon interactions can lead to the production of a Planckian distribution of energies in a system of electromagnetic radiation that is initially out of equilibrium. The scope includes theoretical considerations of photon interactions, thermal equilibrium, and the implications of boundary conditions in a closed system.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that bouncing off the walls of a container would quickly lead to thermal equilibrium, while others argue that photon-photon interactions are extremely rare and may not significantly affect the energy distribution.
  • One participant suggests that photon-photon interactions can change the number of photons, while another counters that energy conservation would prevent this from happening in a perfectly reflecting system.
  • There is a discussion about the nature of interactions with the walls, with some asserting that these interactions would lead to temperature effects on the photons, while others maintain that the assumption of perfectly reflecting walls negates this consideration.
  • A later reply emphasizes the importance of considering the question abstractly, suggesting that in a closed universe composed only of photons, photon-photon interactions could potentially thermalize the photon gas.
  • One participant references an abstract that suggests photon-photon interactions may lead to thermalization, indicating that this perspective aligns with their inquiry.

Areas of Agreement / Disagreement

Participants express differing views on the role of photon-photon interactions and the effects of boundary conditions, indicating that multiple competing perspectives remain without a consensus on the issue.

Contextual Notes

Limitations include assumptions about the nature of the walls and their temperature, the rarity of photon-photon interactions, and the implications of boundary conditions in theoretical scenarios.

Rap
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Suppose you have a container with perfectly reflecting walls, containing electromagnetic radiation that is not in equilibrium (i.e. does not have a Planck distribution of energies.) Will photon-photon interactions (QED and/or gravitational) produce a Planckian distribution after a sufficiently long time?

I don't care how long - fifty gazillion times the age of the universe, I don't care.
 
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I suspect that bouncing off the walls would lead to thermal equilibrium very quickly. Photon-photon interactions (except right after the big bang) are extremely rare.
 
mathman said:
I suspect that bouncing off the walls would lead to thermal equilibrium very quickly. Photon-photon interactions (except right after the big bang) are extremely rare.

No, bouncing off the walls does not change the energy of a photon. If the energy distribution were out of equilibrium, it would remain out.
 
Sure it will. It can even change the number of photons.
 
Vanadium 50 said:
Sure it will. It can even change the number of photons.

I don't see how - the energy has to be conserved, so if one photon hits, two photons reflect, then they are both of lower energy. That's not perfectly reflecting.
 
Rap said:
No, bouncing off the walls does not change the energy of a photon. If the energy distribution were out of equilibrium, it would remain out.
Bouncing off the wall means an interaction takes place with the material in the wall. The wall is at a temperature - hot photons hitting the wall will tend to cool off, while cold photons tend to heat up.
 
mathman said:
Bouncing off the wall means an interaction takes place with the material in the wall. The wall is at a temperature - hot photons hitting the wall will tend to cool off, while cold photons tend to heat up.

But this is counter to the assumption of perfectly reflecting walls. You are changing the subject.
 
Photon-photon interactions (except right after the big bang) are extremely rare.
But they do take place. And he gave us fifty gazillion times the age of the universe, so one scatter every 13.7 billion years is still fifty gazillion collisions.
 
Rap said:
But this is counter to the assumption of perfectly reflecting walls. You are changing the subject.

There is no such thing. The walls have to be made of something. That something will have a temperature.
 
  • #10
mathman said:
Bouncing off the wall means an interaction takes place with the material in the wall. The wall is at a temperature - hot photons hitting the wall will tend to cool off, while cold photons tend to heat up.

What is a "hot photon" and a "cold photon"?
 
  • #11
mathman said:
There is no such thing. The walls have to be made of something. That something will have a temperature.

You are missing the point of the question. The reflecting walls are a way of bounding the system in order to ask a question. They are not the point of the question. The question is whether photon-photon interactions will thermalize a photon gas that is out of equilibrium.

Assume a photon gas of practically infinite extent so that boundary conditions are practically negligible - will photon-photon interactions thermalize that photon gas?

Assume a closed universe composed only of photons, so that there are no boundary conditions - will photon-photon interactions thermalize that photon gas?

Do you see what I am asking?
 
  • #12
Rap said:
You are missing the point of the question. The reflecting walls are a way of bounding the system in order to ask a question. They are not the point of the question. The question is whether photon-photon interactions will thermalize a photon gas that is out of equilibrium.

Assume a photon gas of practically infinite extent so that boundary conditions are practically negligible - will photon-photon interactions thermalize that photon gas?

Assume a closed universe composed only of photons, so that there are no boundary conditions - will photon-photon interactions thermalize that photon gas?

Do you see what I am asking?

This abstract http://meetings.aps.org/Meeting/DPP11/Event/152079 would suggest that the answer is yes, which I don't think is unexpected.
 
  • #13

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