Diamonds May Not Be Forever

Experiments could prove that all matter eventually decays

If a physicist says he is being sent off to a salt mine these days, he may not be joking. He could be heading 40 km (25 miles) east of Cleveland, where an 81-ton digging machine is carving a huge cavity in a salt mine 600 meters (2,000 ft.) below the ground. When excavation is completed, the cavern will be lined with synthetic rubber and filled with 10,000 tons of exceptionally pure, filtered water. Then, about two years from now, physicists will begin looking in the pool for flashes of light that could signal the decay of protons, confirm a unifying theory of nature, and end the cherished notion that matter is permanent.

Protons, which along with neutrons form the nuclei of atoms--and hence the bulk of the matter in the universe --have long been regarded as permanent fixtures on the subatomic scene, members of a family of heavy particles known as baryons. If they happened to collide with still other subatomic particles, one thing was certain: the number of baryons coming out of such interactions was always the same as the number going in. To put it in the language of physics, there was conservation of baryon number.

Now that idea is being challenged by, among others, Physicists Steven Weinberg and Sheldon Glashow of Harvard and Pakistani Abdus Salam, winners of this year's Nobel Prize in physics for showing an underlying unity of two of nature's four basic forces: electromagnetism and the so-called weak force, which governs some forms of radioactive decay within the atomic nucleus. In carrying their work further to relate these two forces to a third --the strong force (which binds the atomic nucleus together)--they and other researchers determined that such unity requires a net loss of baryons when certain particles collide. In other words, the proton must decay into lighter subatomic fragments. By most physicists' reckonings, protons have a mean life of around 10,000 billion billion billion (10^3-^2) years (more than half of them will disintegrate in that time). Thus out of 10^3-^2 protons, only one is likely to decay each year. The problem: how to detect that rare disintegration.

Enter the subterranean reservoir, as well as similar experiments at a South Dakota gold mine, a Utah silver mine and a Minnesota iron mine. Based on the number of protons in the cavity's water (more than 10^3^3), Physicists John Vander Velde of the University of Michigan, Frederick Reines of the University of California at Irvine, and their colleagues figure that there should be about 200 decay "events" per year.

Each dying proton would shoot off two decay products: most likely a positron (or positively charged antielectron) in one direction, a neutral pion in the opposite. Hurtling through the clear water faster than light travels through it, the fleeting particles will leave distinctive cone-shaped wakes of light, which should be detected by one or more of the 2,000 photomultiplier tubes lining the reservoir walls. Cosmic rays can produce similar flashes, but most of them are blocked by the thick layer of earth above the chamber. An occasional will-o'-the-wisp particle called a neutrino also may cause flashes. But its light pattern is different, and the detecting system should be able to distinguish it from those produced by disintegrating protons.

If proton decay is indeed detected, it will help establish a unity among the strong force, electromagnetism and the weak force. That would leave only nature's fourth force, gravity, outside the unified field theory sought in vain by Einstein in his later years. Perhaps most startling of all, it will set an absolute limit on the life of all matter. Says Physicist Larry Sulak: "If proton decay is true, then dust doesn't go to dust and diamonds are indeed not forever." qed

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