proton-electron collisions

[DrPizza]

Fine tuning my understanding of this has been annoying me lately; I can find few references. Sometime around 1930, Dirac suggested there could be proton-electron annihilations; it was part of his hole theory, if I recall correctly. Oppenheimer and others showed him that it doesn't happen; if it did, there'd be no matter left after a fraction of a second. That ultimately led to the hypothesis, later verified, of a positively charged particle called the proton.

So, we know it doesn't happen. If it did happen, we could calculate the rate at which it happened; but we see that matter is stable, so we know it doesn't happen. But, WHY doesn't it happen? I keep overlooking something, and every source I've read more or less says the same thing - we know it doesn't happen, because we're here.

I'm hoping one of you points out something that I'm just not thinking of at the moment, and I realize I'm just having one of those "oh, duhhh" moments.

 

[Farmer]

I know nothing of proton-electron annihilation, but I think the proton was discovered prior to Dirac's QFT work, experimentally, by Rutherford and company.

I will summarize what I know of the problem you pose, which is based on undergrad-level understanding of physics and reading a few chapters of Zee's nutshell book:

Unlike Schrodinger equation, the relativistically compatible Dirac equation (it describes the relativistic quantum dynamics of electrons) allows for negative energy eigenvalues. This was profoundly troublesome for Dirac because that would indicate essentially the possibility of infinite energy (if you could keep decreasing the energy state via "annihilation" of photons). Inspired by Pauli exclusion, Dirac postulated that all the vacuum state (the "normal" state) is one where negative energy states were already filled with electrons (or some fermion), hence removing the difficulty of occupying negative energy states (this is called the "Dirac sea," see Wikipedia). This also resulted in him declaring the existence of antiparticles, i.e., an electron missing from the negative energy states (a "hole") would be perceived as an extra positive charge.

After googling this : http://books.google.com/books?id=KC...7;20electron annihilation oppenheimer&f=false

Apparently, Dirac believed this "hole" to be the proton, since it was the opposite charge to the electron. Of course, I don't know why/how Oppenheimer believed it to be the positron, or why symmetry in mass is important. I'll look into it more. The google text above indicates Oppenheimer wrote a paper on this.

Maybe you should ask your local physics professor?

 

[Mark R]

Conservation of lepton number?

 

[Abwx]

Farmer gave some good clues.
A proton "colliding" an electron will not produce any anihilation,
since these particles are opposed in electric charge only,
each one having the same charge but opposed in sign.

But if the collision involve an electron and its symetrical ,
i.e , a positron (an electron with positive electric charge) ,
then the masses are completely converted (not anihilated)
in masseless particles that will travel at light speed, litterally,
since theses resultant particles are photons..

The general field theory stipulate that particles can emerge
from the vaccum , as it doesn t violate the conservation principle, since what is gained at a end is lost as the other one .

 

[DrPizza]

Duhhh, that'll do it Mark R, thanks.