How can a particle be its own antiparticle?

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In summary: It's just a term we use to describe a certain type of reaction. In summary, the process of particles meeting their antiparticles and producing other particles is known as particle annihilation, but it can also be referred to as reaction or decay depending on the specific circumstances. It is ultimately just a label and does not have any physical significance.
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TL;DR Summary
matter, anti-matter pair
I seek an explanation as to how a particle can be its own anti-particle. I would think the instant such a particle comes into existence, it would self-annihilate.
 
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  • #2
RJ Emery said:
Summary:: matter, anti-matter pair

I would think the instant such a particle comes into existence, it would self-annihilate.
Why? There is absolutely no rationale for such an argument.
 
  • #3
If a particle meets its antiparticle it is possible that the two react. If the reaction products are massless or much lighter (which is again possible but not guaranteed) we typically call this reaction "annihilation". The classical example is electron+positron -> 2 photons.
Such a reaction always needs two particles. If you have a single particle then it might be able to decay. As an example, a Z boson (which is its own antiparticle) will decay quite quickly, typically to a particle+antiparticle pair. A W boson will decay very fast as well, but it cannot be a particle+antiparticle pair as the W boson has an electric charge. A photon (which is its own antiparticle as well) can't decay, it is stable.
 
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  • #4
mfb said:
A photon (which is its own antiparticle as well) can't decay, it is stable.
Can two photons annihilate? I would think not because they don't interact very strongly. But they can interact. Delbruck scattering is an example. I'm confused.
 
  • #5
mfb said:
A photon (which is its own antiparticle as well) can't decay, it is stable.
There is the maxim that "if a process does not violate a conservation law, it should happen".
How to pinpoint the conservation law violated by decay of a photon into several?
You can take care of energy conservation (resulting photons combined have the same energy as the initial), momentum conservation (products combined have the same momentum, meaning they travel in the same direction), angular momentum conservation (suitable state of polarization).
Classically it´ s obvious - a plane wave of a given frequency cannot spontaneously change its frequency. But viewing it as a photon subject to conservation laws only, which one specifically forbids it to do such an absurd thing?
 
  • #6
Paul Colby said:
Can two photons annihilate?
Yes. Since an electron-positron pair can annihilate to two photons, two photons (given sufficient invariant mass for the pair) can reverse this process. What the cross section is is another matter.
 
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  • #7
Paul Colby said:
Can two photons annihilate? I would think not because they don't interact very strongly. But they can interact. Delbruck scattering is an example. I'm confused.
They can react (and we have found reactions), but I don't think "annihilation" is a good name for that.
snorkack said:
Classically it´ s obvious - a plane wave of a given frequency cannot spontaneously change its frequency. But viewing it as a photon subject to conservation laws only, which one specifically forbids it to do such an absurd thing?
There is some symmetry that tells you the cross section is exactly zero, I forgot the name.

There is also an interesting consistency argument that you can find in more detail with the search function: If a photon can decay, how does its lifetime depend on the energy? It turns out it's impossible to make that decay Lorentz invariant without absurd consequences.
 
  • #8
mfb said:
They can react (and we have found reactions), but I don't think "annihilation" is a good name for that.
In QED a positron and electron can annihilate to form 2 photons. Just run the reaction diagrams backward in time and I'm good to go. So yeah, looks like annihilation of photons to me.
 
  • #9
##H\to\gamma\gamma## is a decay but ##\gamma\gamma\to H## is not. ##e+p \to n+\nu## is electron capture but ##n+\nu \to e+p## is neutrino capture. There is no rule that would give time-reversed processes the same name.
 
  • #10
mfb said:
There is no rule that would give time-reversed processes the same name.
Then what are the rules? Particle meets antiparticle to produce other particles is the definition of particle annihilation hocked up by Google when queried. Is there a more technically correct one?
 
  • #11
What I wrote earlier:
mfb said:
If the reaction products are massless or much lighter (which is again possible but not guaranteed) we typically call this reaction "annihilation".
But ultimately it doesn't matter. You can call annihilation what you want as long as it doesn't lead to confusion.
 

1. How can a particle be its own antiparticle?

This phenomenon is known as particle-antiparticle symmetry, where a particle and its antiparticle have the same mass, spin, and other properties, but opposite electric charge. This is possible because particles and antiparticles are actually different manifestations of the same underlying concept, known as quantum fields.

2. How do we know that particles have antiparticles?

The existence of antiparticles was first predicted by theoretical physicist Paul Dirac in the 1920s, and later confirmed experimentally in the 1930s with the discovery of the positron, the antiparticle of the electron. Since then, numerous other antiparticles have been discovered and studied, providing strong evidence for their existence.

3. Can a particle and its antiparticle exist at the same time?

Yes, in certain circumstances, a particle and its antiparticle can exist simultaneously. This is known as pair production, where a high-energy photon can spontaneously create an electron-positron pair. However, this is a temporary state, as the particles will eventually annihilate each other and release energy in the form of photons.

4. Why is it important to study particles and antiparticles?

The study of particles and antiparticles is crucial for understanding the fundamental building blocks of our universe and the laws that govern them. It also has practical applications in fields such as particle physics, nuclear medicine, and quantum computing.

5. Is there an equal number of particles and antiparticles in the universe?

This is still an open question in physics. According to the Big Bang theory, equal amounts of particles and antiparticles should have been created in the early stages of the universe. However, this is not the case, as there is a significant excess of matter over antimatter in the observable universe. The reason for this imbalance is still a mystery and an active area of research in particle physics.

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