# Q & A: equal numbers protons and electrons

Q:
I am not a physicist, just an interested engineer.Why are there equal numbers of electrons as protons in the universe? Just after the big bang energy created electron/positron pairs, proton/antiproton pairs as well as other matter/antimatter pairs. I understand that the antimatter disappeared and we don't know how nor why (many theories). But creating unequal numbers of electrons and protons (with their antiparticles) still satisfies the conservation of the charge. Is there another conservation law that caused the numbers of electrons and protons to be equal?I hope my question is clear.
- Bassam Baroudi (age 66)
San Jose, CA
A:

As you say the total charge is zero, and that doesn't change under creation/annihilation of particle/antiparticle pairs. After almost all the antimatter was gone, almost all the charge was in the form of positive protons and negative electrons. So their numbers have to be almost equal.  Before almost all the antimatter  positrons and anti-protons were annihilated, there was no requirement for the numbers of electrons and protons to be equal since the other charged particles were also present.

Mike W.

(published on 05/14/2018)

## Follow-Up #1: charge conservation

Q:
That does not answer the question. If the number of Proton/antiproton pairs was different than the number of the electron/positron pairs, then when the antiprotons and the positrons were gone (somehow) then the number of protons and electrons left over would not be equal.
- Bassam Baroudi (age 66)
San Jose, CA
A:

If the only charged particles are protons (+) and electrons (-) then the only neutral possibility is for their numbers to be equal. As you say, if the numbers of proton/antiproton pairs and electron/positron pairs started off different, then getting to this new neutral state requires processes other than simple electron/positron annihilation and proton/antiproton annihilation. There are also processes involving larger numbers of particles, not simple pairs in and pairs out. The real mystery is why the antimatter/matter processes were as much out of balance as they were. Even though that imbalance is very slight, if I understand correctly it is larger than expected from the known processes which violate matter/antimatter symmetry (the "C" symmetry  of CPT).

Mike W.

(published on 05/16/2018)