Deep in a disused zinc mine in Japan, 50,000 tonnes of purified water held in a vast cylindrical stainless-steel tank are quietly killing theories long cherished by physicists. Since 1996, the photomultiplier-tube detectors (pictured above) at Super-Kamiokande, an experiment under way a kilometre beneath Mount Ikeno, near Hida, have been looking for signs that one of the decillion (1033) or so protons and neutrons within it (of which a water molecule contains ten and eight respectively) has decayed into lighter subatomic particles.
That those tubes have, in the more than 20 years the experiment has been running, failed to do so is a conundrum for physics, and one that is becoming more urgent with every passing month. Grand unified theories (GUTs), thought since their genesis in the 1970s to be the most promising route to understanding the fundamental forces that bind matter together, predict that protons and neutrons should occasionally disintegrate in a way that breaks what was previously regarded as an iron law of physics—namely that the number of baryons (a class of particle that includes both protons and neutrons) in the universe is constant.
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