Proton decay

I’ve heard that it’s very important to know whether protons decay in ten to the thirtith years. Why

The philosophers have only interpreted the world in various ways; the point, however, is to change it. (Karl Marx, 1845)

We need to know so we can change their brand of toothpaste if necessary.

Physicists have discovered four fundamental forces in nature: gravity, electromagnetism, the weak nuclear force, and the strong nuclear force. They would like to reduce this list to just one by demonstrating that they are all manifestations of a single force. Steven Weinberg and someone else (whose name I can’t remember but it was Das Salaam or something from the University of Trinidad) won the Nobel Prize for combining the weak force and electromagnetism, usually referred to as the electroweak force.

Proton decay demonstrates a sought-for connection between the strong nuclear force and the electroweak force. If protons decay very slowly they do so because of the existence of massive, undiscovered particles which are derived from the combined strong and electroweak forces. The particles are too massive to be created in an accelerator but proof of their existence may be demonstrated by proton decay.

When you try to add gravity you get other complications and then you come up with real crazy ideas like string theory.

BTW, you don’t watch a proton for 10^30 years and wait for it to decay. You gather 10^30 protons and watch them for a year. One should decay. You can see this is a non-trivial experiment. It involves large quantities of protons (usually water or carbon tetrachloride) and sensitive detectors. You have to shield them from outside influences like cosmic rays to ensure you’re seeing actual proton decays. And you do have to wait a long time for results.

IIRC, the experiments done so far have demonstrated 10^30 years as a lower limit to the proton half-life. No answers yet on what the true value is, but the theorists are getting nervous that the value is higher than expected.

“If ignorance were corn flakes, you’d be General Mills.”
Cecil Adams
The Straight Dope

There ain’t so many protons in (pure) carbon tet are there?

Sure there are. For the purposes of this experiment it doesn’t matter that they’re bound up in the carbon and chlorine nuclei. The big factors are transparency (the proton decays create minute flashes of light – that’s how they’re detected) and availability in large quantities.

Sorry, bad chemists joke.

There are 74 protons per carbon tetrachloride molecule. That means you need a little under 22500 moles of CCl[sub]4[/sub] to get 10[sup]30[/sup] protons, which is about 2.2 cubic meters.

Ideally you’d use a swimming pool full of the stuff so that you’d get several decays per year instead of just one.

Since neutrons don’t decay inside stable nuclei, but only in their free state, I’d have thought that you did need free protons (hydrogen) for the experiment.