I was reading this article discussing how experiments have been able to observe proton decay: https://www.economist.com/news/scie...ry-fundamental-physics-frustrating-physicists It states that after concluding that there has been any evidence of proton decay in certain experiment, the lower bound of the average lifetime of a proton must be increased. Is this simply a calculation that given a certain number of protons being observed for a certain amount of time, the statistical spread of lifetimes must be such that it would be *probable* (perhaps 50%?) that a proton decay would have been observed if the average lifetime were a certain value (i.e., that becomes the minimum lifetime)? It would seem to me that the answer all along is that protons don't decay, but I suppose that it is impossible to prove a negative. What gives physicists the notion that protons are supposed to decay in the first place?