In one second, light travels 300 000 km, most of the way from Earth to the Moon. In one nanosecond, a billionth of a second, it travels 30 cm, the length of a sheet of paper. In one attosecond, a billionth of a nanosecond, it travels 3 angstroms, about the length of a water molecule.

Attoseconds—or more precisely, tens to hundreds of attoseconds—are the natural time scale of the motion of electrons. Whereas atoms and molecules typically move on the scale of tens to hundreds of femtoseconds, electrons are about three orders of magnitude lighter and faster. Their motions include processes, such as tunneling and photoionization, that were once thought to be instantaneous, or at least too fast ever to be measured.

But the attosecond regime is now measurable, thanks to decades of theoretical and experimental work by dozens of scientists. For this year’s Nobel Prize in Physics, the Royal...

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