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Q & A: Electron/Proton Annihilation?

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Most recent answer: 11/28/2011
Q:
Electron/Proton Annihilation: If an electron and proton are placed in proximity to each other at rest and allowed to move together by Coulomb's law, why do the charges not annihilate (or cancel) each other in a similar fashion to electron/positron annihilation? I realize this does not happen but I would like to understand why. Thanks.
- Greg (age 24)
Jackson, TN, USA
A:
Nope, no can do.  It's because electrons and protons are different animals.  An electron can annihilate with a positron (anti-electron) and a proton can annihilate with an anti-proton, but not with each other.      Very serious experiments have searched in vain for the disintegration of a proton into a positron plus pions or gamma rays, which would be equivalent to the inverse of proton-electron annihilation.  
There is no fundamental reason for this, it's just a fact of life.  In quantum field theory electrons interact with other charged particle via the virtual exchange of  photons.  They can also interact with neutrinos and other leptons such as muons via the exchange of W bosons.   Protons, being charged, also interact with other charged particles as well as participate in strong interactions via exchange of so-called gluons. 
Now life gets complicated here. Since we know that the neutron does decay into a proton plus an electron plus an anti-neutrino, you should ask why doesn't the reverse happen.  It could, but the energetics are unfavorable.   The possibility of such a reaction is due to the slight mixing of the electromagnetic and weak interactions.
Sigh... it's tough.  There are no easy answers to your question. 

LeeH


(published on 11/23/2011)

Follow-Up #1: electron-proton collision

Q:
So....as the electron and proton move together what happens? Does the electron pass through the proton or go around? Also, does the electron enter the lowest energy shell and form Hydrogen? Thanks!
- Greg (age 24)
Jackson, TN, US
A:
The electron wave function (cloud) and the proton wave function overlap. That is, they both become fairly intense in the same spatial regions.

If the electron started out far from the proton, its wave function consists almost entirely of unbound components with more energy than any bound state. Therefore it cannot stick to the proton (i.e. form a hydrogen atom) unless it dumps some energy. The way that can happen is via emission of a photon. So sometimes that happens and an atom forms and sometimes it doesn't. When it doesn't, you could say that the particles pass through each other. The weird thing about quantum mechanics is that there's no detail of the prior state that determines which of those outcomes is seen. It's purely random.

Mike W.

unchecked answer- Lee's in Paris.

(published on 11/28/2011)

Follow-up on this answer.