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Puzzle #3
#3. What is the lifetime of the proton, and how do we understand it?
It used to considered gospel that protons, unlike say, neutrons, live forever, never decaying into smaller pieces. Then in the 1970's, theorists realized that their candidates for a Grand-Unified-Theory (GUT), merging all the forces except gravity, implied that protons must be unstable. Wait long enough, and, very occasionally, one should break down.
The trick is to catch it in the act. Sitting in underground laboratories, shielded from cosmic rays and other disturbances, experimenters have whiled away the years watching large tanks of water, waiting for a proton inside one of the atoms of water to give up the ghost.
So far, the fatality rate is zero, meaning that either protons are perfectly stable or their lifetime is enormous - an estimated billion, trillion, trillion years, or more....
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