Photon and anti-photon annihilation?

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Discussion Overview

The discussion centers around the concept of photon and "anti-photon" annihilation within the framework of quantum field theory. Participants explore the nature of photons, their interactions, and the implications of energy conservation in processes involving real and virtual particles.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions how photons, being their own antiparticles, can annihilate each other, suggesting that if real photons could annihilate, they would do so frequently.
  • Another participant notes that the Number Operator for photons is not conserved, emphasizing that the sum of their energy is conserved instead.
  • A participant raises a concern about the energy of two photons appearing from the vacuum, questioning whether this implies one must have negative energy.
  • It is mentioned that photons can annihilate through processes such as \(\gamma+\gamma \rightarrow e^+ + e^-\), although the observation of this process has not been widely reported.
  • One participant claims that the annihilation process has been observed in experiments involving multiphoton light-by-light scattering, providing a reference to positron production from real photons.
  • Another participant reiterates the concern about the energy of photons appearing from the vacuum, while arguing that the creation of electron-positron pairs does not violate conservation rules due to their transient existence.
  • There is a discussion about the concept of "total mass-energy," with a participant asserting that energy-momentum conservation applies to real photons in scattering processes.
  • One participant clarifies that the experiment discussed involves real photons from laser sources, which possess positive energies.

Areas of Agreement / Disagreement

Participants express differing views on the nature of photon annihilation, the implications of energy conservation, and the validity of experimental observations. No consensus is reached regarding the specifics of photon interactions or the interpretation of energy in these processes.

Contextual Notes

Participants reference various assumptions about virtual versus real photons, energy conservation, and the nature of particle interactions, which may not be fully resolved within the discussion.

Goliatbagge
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Photon and "anti-photon" annihilation?

In quantum field theory pairs of virtual particles can appear from vacuum and quickly annihilate each other, for example an electron/positron or a proton/antiproton. But how does this work with photons?From what I know, the photon is its own anti-particle. So the "anti-photon" is just another photon. But how can a real photon annihilate an other "real" photon? Real photons are everywhere, so in that case photons would annihilate each other all the time?
 
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The difference between photon and other massive particles is that the Number Operator of photons is not conserved. What is conserved is the sum of energy of the photons not their number.
 
So if two photons appear from vacuum their sum of energy should be zero? Wouldn't that mean that one of them must have negative energy?
 
In principle, photons can annihilate, e.g., through the QED process \gamma+\gamma \rightarrow e^+ + e^-. I've never heard that anyone has been able to observe this process, however.
 
I think that this process has in fact been observed - even for real photons:

Positron production in multiphoton light-by-light scattering

... followed by
a collision between the high-energy photon and several laser photons to produce
an electron-positron pair. These results are the rst laboratory evidence for
inelastic light-by-light scattering involving only real photons.
 
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Goliatbagge said:
So if two photons appear from vacuum their sum of energy should be zero? Wouldn't that mean that one of them must have negative energy?

I don't believe so. Consider the creation of an electron and a positron. The total mass-energy would be positive, but because they only exist for an instant before annihilation, no conservation rules are violated.

Also, note that "real" photons are not created directly from the vacuum, but from accelerated charges.
 
Drakkith said:
Consider the creation of an electron and a positron. The total mass-energy would be positive, but because they only exist for an instant before annihilation, no conservation rules are violated.
Since energy is conserved, the total mass-energy will be zero, just like the vacuum they emerged from. Virtual particles don't have to obey the usual relationship between energy and momentum, and it's perfectly OK for one particle or the other to have negative energy.
 
What do you mean by "total mass-energy"? Of course the two real photons are onshell and the scattering process \gamma + \gamma \rightarrow e^+ + e^- obeys energy-momentum conservation. You can analyze the kinematics as usual with help of the Mandelstam variables.
 
The experiment is about a scattering process with REAL photons (originating from laser sources). Of course, these photons have positive energies (frequencies).
 

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