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A proton is about eighteen hundred times heavier than an electron, though their charge matches.

Physics Facts, sourced to NIST.

What it means

The two particles balance each other exactly in charge, which is why atoms are electrically neutral, and yet one of them is nearly two thousand times the mass of the other. Nothing in the standard model explains why the ratio takes the value it does. It is measured to extraordinary precision and simply put into the theory by hand, which physicists find unsatisfying and have not yet fixed.

The claim comes from NIST, and the page it comes from was read on 2026-08-27 to confirm it still says this.

This is not the kind of claim that goes out of date, which is why it was set as a puzzle rather than a fact with a shelf life.

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