(Image: Matthew Richardson)
THE BIBLE of fundamental 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.
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It’s not for want of looking. , 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. : 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 , a theorist at the University of Cambridge.
Not only that, but these ambitious models predict the proton should live for somewhere between 1030 and 1035 years – just the range experiments are now probing. “We’re entering the really interesting area where you can start to rule these theories out,” says Allanach. “Or if they are correct, you should start seeing protons decay.” It could be a long wait: pushing the minimum lifetime up by a factor of 10 means watching for 10 times as long.
So should we be worried if one of the fundamental components of matter, the stuff that makes up you and me, proves to be a little wobbly? Not really: our bodies do not make good observatories for proton decay. “From my back-of-the-envelope calculation, it would be a rare person that would experience a proton decay in their lifetime,” says Allanach. And the universe has so many protons that we’ll be long gone before they run out.
“Should we be worried if one of the fundamental components of you and me is a little wobbly?”
In the meantime, here’s the best answer we have for the lifetime of a proton: a very, very long time.
Read more: “10 mysteries that physics can’t answer… yet”
This article appeared in print under the headline “How long does a proton live?”
