The proton of protium has never decayed in an observation, so scientists believe that protium is a stable isotope. New theories of particle physics predict that a proton can decay, but this decay is very slow. The proton is said to have a half-life of 10 years. If proton decay is true, then all other nuclei that are said to be stable are actually only "observationally stable", i.e. they look like they are stable. Recent experiments have shown that if proton decay does occur, it would have a half-life of 6.6 × 10 years.