The ghosts and the machine

DEEP beneath the plains of Illinois, in a man-made cavern filled with racks of scientific equipment, someone has spray-painted a white circle onto the bare rock wall. Stand in front of it and you are standing in the path of the most powerful beam of neutrinos in the world, which is emerging from a nearby particle accelerator at Fermilab, America’s main particle-physics laboratory. With any other kind of accelerator, standing in the beam would have spectacular and fatal consequences. But your correspondent was not vapourised—nor, several weeks later, has he developed either cancer or superpowers.

And that is the point: neutrinos are ghostly things. Billions a second stream through every cubic centimetre of space. But because they feel only the two weakest of the four fundamental physical forces—gravity and the aptly named weak nuclear force, rather than electromagnetism and the strong nuclear force—they hardly interact with the rest of creation.