A watched pot, for atoms.

A watched pot never boils, the saying goes. It isn’t true of pots. For atoms, something like it is.

In 1990, Wayne Itano, David Wineland and their colleagues at the US National Institute of Standards and Technology in Boulder, Colorado, reported an experiment with about 5,000 beryllium atoms. Each atom was missing one electron, which let a trap of electric and magnetic fields hold it. They cooled the atoms with lasers to within a quarter of a degree of absolute zero, and then they set them a task.

A pulse of radio waves, a quarter of a second long, nudges each atom from one inner state to another, as a switch goes from one setting to the other. Checked only once, at the end, nearly all of them, about 99 in every 100, had made the jump.

Then they checked on the atoms while the radio waves were still working. Each check was a flash of ultraviolet light a few thousandths of a second long. An atom still in its first state glows in that light; an atom that has jumped stays dark. So every flash asks every atom the same question: have you jumped yet?

Checked twice, half the atoms made the jump. Checked 16 times, about one in ten. Checked 64 times, too few to measure. The more often they looked, the more the atoms stayed as they were.

Physicists call it the quantum Zeno effect, after the Greek thinker Zeno, whose arrow, seen at any single instant, is not moving. They have long disagreed about how best to define the effect, and so about whether Boulder showed it. What the atoms did is not in doubt. In 2012 Wineland shared the Nobel Prize in Physics for ways of measuring and steering single quantum systems, trapped atoms like these among them.

In the very small, a look is never nothing.