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Can antiparticles only be created in pair production? How/which laws of physics prohibit direct conversion of say an electron to a positron? A neutron to an antineutron?

I have seen a comment that it is thermodynamically impossible. True? How exactly? Any other ways it is theoretically impossible?

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    $\begingroup$ Changing an electron to a positron would contradict conservation of charge. $\endgroup$
    – Peter
    Jan 22 at 1:31
  • $\begingroup$ @Peter Yeah just figured that, so added about neutrons too. $\endgroup$
    – Greendrake
    Jan 22 at 1:31
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    $\begingroup$ The neutral $K$ meson oscillates between matter and antimatter, so what you are saying is not true in general. $\endgroup$
    – FrodCube
    Jan 22 at 1:37

2 Answers 2

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It depends on the particular kind of particle. Assuming the Standard Model holds, then:

  • Electrons can't convert to positrons because that would violate conservation of charge.
  • Neutrons can't convert to antineutrons because that would violate conservation of baryon number.
  • When permitted by conservation laws, any particle can and generally does convert to its antiparticle. Examples include neutral kaons, $D^0$ mesons, and $B^0$ mesons. Kaon oscillations in particular have been measured with exquisite sensitivity, and provide some of the strongest known constraints on physics beyond the Standard Model.
  • The story for neutrinos is more complicated, but to oversimplify it a bit, we still don't know whether they can or can't convert to their antineutrinos.
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    $\begingroup$ It's worth noting that electrons converting to positrons would also violate conservation of lepton number. $\endgroup$
    – Sandejo
    Jan 22 at 1:54
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    $\begingroup$ One should add that conservation laws have come from experimental observations and are as extra axioms to pick up from the general mathematical solutions of the model those solutions that fit data and observations. $\endgroup$
    – anna v
    Jan 22 at 5:06
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    $\begingroup$ @annav conservation laws are as experimental as theoretical: Noether's theorem is the basis for the latter. Also remember the drive to avoid Bohr's proposal about non-conservation of energy in beta decay that led Fermi to postulate existence of the neutrino. $\endgroup$
    – Ruslan
    Jan 22 at 11:14
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    $\begingroup$ @Ruslan if it were not for the experiments, there would not be any of the known conservation laws , in my opinion. I know that the beauty of mathematics convinces many physicists that mathematics creates reality instead of mathematics models reality. $\endgroup$
    – anna v
    Jan 22 at 13:40
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    $\begingroup$ @annav experiments are just a bunch of numbers and techniques. All the logic that generalizes the patterns in these numbers and lets us declare "laws" is contained in the theory. So unless you have both experiments and theory, you won't have any laws, including those of conservation of anything. $\endgroup$
    – Ruslan
    Jan 22 at 13:49
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What prohibits a particular interaction from occurring are the applicable conservation laws which must be obeyed. In your two examples:

  1. The conversion of an electron to a positron would violate the law of conservation of charge, and would also violate conservation of lepton number, meaning the reaction $e^+\rightarrow e^-$ is forbidden.

  2. The conversion of a neutron to an antineutron, since baryon number is not conserved, $n\rightarrow \bar n$ is also forbidden.

You'll find that other interactions are allowed only if the relevant conservation laws are obeyed.

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