electric-charge conservation

Rub a balloon on your hair and it picks up charge — but the charge was not made from nothing; it was scraped off the hair, which is left with the opposite charge. The total amount of electric charge in the room did not change. This is charge conservation, one of the most ironclad rules in all of physics: in any process whatsoever, the total electric charge you start with equals the total you end with.

Electric charge comes in positive and negative, and you add it up like a balance sheet, with the positives and negatives partly cancelling. In a particle reaction you tally the charges of everything going in and everything coming out, and the two tallies must match exactly. Crucially, charge can still appear and disappear in particle form — a high-energy photon (charge zero) can turn into an electron (charge -1) and a positron (charge +1), seemingly creating charge out of light. But the new charges are equal and opposite, so the total is still zero, just as it was before. You can never create a lone charge with no partner.

Charge conservation is exact as far as anyone has ever measured, and it is woven directly into the theory of electromagnetism: it follows, by Noether's theorem, from a deep symmetry of the electromagnetic field. There is also a beautiful consequence — because charge can never disappear, the lightest charged particle, the electron, can never decay. There is simply nothing lighter for its charge to go to, so it must live forever. Experiments confirm the electron is stable to lifetimes vastly longer than the age of the universe.