It had been expected that a proton's half-life was 1029 years. Unfortunately, no proton decay has been found yet (spring 1999 CE), which is a bit of a problem. This rules out the theoretical model that was advocated at first. The current favorite prediction for the half-life of a proton is 1032 years or so. Now, it is true that the longer the proton lifetime is found to be, the more likely that the standard model is incomplete. However, this does not mean that we know the proton lifetime. At present (spring 1999 CE) the Kamiokande instrument is the most sensitive on Earth. It should be able to detect proton decay for a half-life as long as 1034 years. A proton should decay into a positron and a pi-zero meson. A side effect of proton decay should be a distinctive light emission caused by neutrinos traveling through a very large tank of very pure water.
Current experiments are based on detecting these faint light emissions. The water itself provides
a large population of protons. A mere 18 grams of water would have about
6.02*1023 (a mole) of water molecules and each molecule would have 10
protons. Since proton decay is almost certainly well modeled using a uniform
probability distribution, so examining 2*1032 protons and observing a mean rate of decay of
one proton per year should be comparable to watching 20 protons for 1032 years and observing about
10 protons decay. (See
references.)
The theory of proton decay predicts that after many half-lives, all protons in the universe will cease to
exist. Current estimates of the age of the universe are on the order of 15*109. The
half life of protons is in excess of 2.5*1032 so the 15 billion year age of
the universe is a rather irrelevant 1.5*10-23rd of the total time from the origin of all protons until half the original number of protons decay. After
the decay of all protons, some cosmologists have described the resulting state of everything as a very cold,
very sparse, very dull soup of elementary particles.
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