(Image: Matthew Richardson)
THE BIBLE of fundamental physics, The Review of Particle Physics, devotes several pages to the ways a proton might fall apart. Each “decay mode” comes with an estimate of how long you might expect to wait to see one of these particles, bedrocks of the atomic nucleus, disintegrate that way. The units are 1030 years – thousands of billions of billions of billions of years. Our stripling universe is a mere 13.8 billion years old, so this is a judicious, scientific way of saying that no one has ever seen a proton decay.
It’s not for want of looking. Super-Kamiokande, a 50,000-tonne tub of ultra-pure water under a mountain in central Japan, is one place where researchers are watching out for the pop and fizzle of a dying proton. Late last year, the team published its latest estimate for how often one particularly eagerly sought decay mode occurs: at most once every 5.9 × 1033 years.
You’d think people would be pleased that the particles we’re all made of are stable. Physicists too: their “standard model” of particle interactions firmly indicates that protons, as the lightest particles constructed of three quarks, should never decay.
So why is “never” not good enough? The answer is that few believe the functional but ramshackle standard model is up to snuff. So-called grand unified theories provide a more coherent account of three of nature’s forces – gravity remains aloof – at the price of the proton decaying. “It’s the signal of a grand unified theory,” says Benjamin Allanach, a theorist at the University of Cambridge.
Not only that, but these ambitious models ...