quantumaniac:  Unsolved Physics Questions : The Lifetime of a Proton Until only a few decades ago, the lifetime of protons was considered infinite. Unlike the unstable neutron, protons were assumed to never breakdown into smaller particles. During the 1970s this view radically changed. The 1970s was an exciting decade for Physicists, with new breakthroughs like the Standard Model circulating the Physics world. Physicists soon realized that in order for this new unified theory (the Standard Model) to work mathematically, protons would have to be unstable. Although it may seem counterintuitive, mathematically speaking if you wait long enough a proton should breakdown into lighter subatomic particles - such as a positron and a neutral pion.   Since then, many Physicists have been studying proton decay - but with no results so far. A proton has never actually been seen breaking down, which proves that either protons never actually decay or their lifetimes are extremely long, perhaps billions or even trillions of years! Sitting in underground laboratories such as the Super-Kamiokande Experiment pictured above, these Physicists have been staring at these large tanks of water waiting for a proton to decay. Although they cannot be sure of the incredibly long lifetime quite yet, the current estimate of the half-life of a proton is about 1.1 * 1034 !
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