Quarks and leptons, the building blocks of matter, are staggeringly small. Even the largest quarks are only about 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, approximately 1.6 x 10-35 meters, believed by physicists to be the shortest possible length in the universe. Beyond this point, they say, the very notion of distance becomes meaningless.
How small are we talking? It would take more Planck lengths to span a grain of sand than it would take grains of sand to span the observable universe.