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The galactic year

Our galaxy is a vast rotating system of stars and gas. This includes our star, the Sun. Like all stars, the Sun is orbiting the galactic centre with a period estimated between 220 and 250 million years. This period is convenient in a number of respects and serves as the definition of a galactic ‘year’, denoted GY in the following.

According to the most recent estimates, the universe (and hence time) started just over 60 GY ago. It is commonly believed that following a period of rapid expansion known as inflation, our galaxy formed just over 50 GY. Our star, the Sun, is a relatively latecomer in galactic chronology, appearing just over 18 GY ago. Therefore, the Sun has orbited the galactic centre close to 20 times in its lifetime so far.

The decay lifetime of the proton

The proton is one of the fundamental particles that define our observable world. Protons are, as far as is known, completely stable. However, various proposed ‘Grand Unified Theories’ (GUTs) predict that protons can decay into other particles. The essential fact that permits us to discuss such a concept is that protons are no longer believed to be ‘elementary’, but bound states of more elementary objects called quarks and gluons. GUTs describe the interactions of those more elementary objects and not protons per se.

Motivated by GUT predictions, physicists have attempted to observe proton decay. What helps is Avogadro’s number, about 6 x 1023, which is relatively colossal and a measure of how many atoms there are in a handful of matter. The sheer scale of Avogadro’s number, when factored in with the predicted enormously long lifetime of the proton, means that vast numbers of protons can be monitored over accessible timescales such as years. If just one of these protons does something unusual, such as decay during the course of being monitored, then hopefully that could be observed and allow meaningful limits to be placed on that lifetime. No such events have been observed, and so currently, the best estimate is that the proton half-life cannot be less than about 1034 years. This is about 1026GY or about 1024 times the current estimated age of the universe.

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