With the lower limit for the lifetime of a proton is described to be 100 billion trillion times longer than the age of the universe, do protons really last forever?
By: Ringo Bones
Those Madison Avenue “Mad Men” hired by DeBeers may have been a little way off the mark when they made a bold advertising claim that “A diamond is forever” – well, at least on a human timescale. But in the world of theoretical physicists – which we are also a part of – there is something that may indeed really last forever and could potentially even outlast our own universe.
Given the current experimental evidence obtained so far, theoretical physicists has reached a current consensus that the lower limit for the lifetime of a proton – which forms part of the atomic nucleus of ordinary baryonic matter – is described to be at least 100 billion trillion times longer than the age of our universe – which current experimental observations pegged it to be about 13.8 billion years old. For almost 40 years, Scientific American magazine has published several articles on various experiments – some of them are even elaborately grandiose in scale – to determine the absolute lifetime of a proton.
In particle physics, proton decay is a hypothetical form of radioactive decay in which a proton decays into lighter subatomic particles, such as a neutral pion and a positron. As far as particle physics knows, proton decay has yet to be observed and there is currently no experimental evidence that proton decay even occurs.
In the Standard Model, protons – a type of baryon – are theoretically stable because their baryon number is conserved, that is under normal circumstances; however there is that “chiral anomaly”. Therefore protons will not decay into other particles on their own because they are the lightest – and therefore the least energetic – baryon.
Some theoretical studies beyond the Standard Model, grand unified theories (GUTs) explicitly break the baryon number symmetry, allowing protons to decay via the Higgs Particle, magnetic monopoles or new X-bosons. Proton decay is one of the few observable effects of the various proposed grand unified theories. To date, all attempts to observe a proton’s decay so far have failed, but some theoretical physicists have proposed that the continuously accelerating expansion of our own universe since the Big Bang might affect the apparent stability of the proton – maybe perhaps 100 billion years from now.