Quarks and leptons, the building blocks of matter, are staggeringly small—less than an attometer (a billionth of a billionth of a meter) in diameter. But zoom in closer—a billion times more—past zeptometers and yoctometers, to where the units run out of names. Then keep going, a hundred million times smaller still, and you finally hit bottom: This is the Planck length, the smallest possible unit in the universe. Beyond this point, physicists say, the very notion of distance becomes meaningless.