Keith Schwab builds bridges. By most people's standards, they are very small bridges indeed: around 8 thousandths of a millimetre long and 200 millionths of a millimetre wide, visible only under a microscope. But to Schwab's eye, these objects are huge. That's because he is hoping to see them behave according to the rules of quantum mechanics -- rules that allow for bizarre, counterintuitive behaviour such as being in two places at once. Quantum mechanics is generally thought to govern objects such as individual atoms, not lumps of matter like these bridges, which contain tens of billions of atoms.

It is an ambitious goal. But Schwab, based at Cornell University in Ithaca, New York, is one of several experimentalists seeking to probe one of the great conundrums of modern physics: the quantum--classical transition. Here, the fuzzy quantum world somehow gives way to the solid, definite certainties of the everyday 'classical' world as we go up the scale from atoms to apples. If these experiments manage to confirm current theories of this transition, they could turn long-standing preconceptions about quantum theory on their head.