Ripples in space divide classical and quantum worlds

WHY can't we be in two places at the same time? The simple answer is that it's because large objects appear not to be subject to the same wacky laws of quantum mechanics that rule subatomic particles. But why not - and how big does something have to be for quantum physics no longer to apply? Ripples in space-time could hold the answer.

The location of the boundary between the classical and quantum worlds is a long-standing mystery. One idea is that everything starts off as a quantum system, existing in a superposition of states. This would make an object capable of being, for example, in many places at once. But when this system interacts with its environment, it collapses into a single classical state - a phenomenon called quantum decoherence.

Brahim Lamine of Pierre and Marie Curie University in Paris, France, and colleagues say that gravitational waves may be responsible for this. These waves in the very fabric of the universe were generated by its rapid expansion soon after the big bang, as well as by violent astrophysical events such as colliding black holes. As a consequence, a background of ripples at very low amplitudes pervades space-time.
Lamine and colleagues calculated how this fluctuating space-time might contribute to quantum decoherence. They found that for systems with very large mass, such as the moon, decoherence induced by the gravitational waves would have caused any quantum superposition to dissipate immediately. At the other end of the scale, such waves would have a negligible effect on massless photons.

To test whether gravitational waves do in fact cause the decoherence seen in large objects, the researchers suggest using a set-up called a matter-wave interferometer in which molecules are made to pass through multiple gratings. The wave-like nature of the molecules causes them to diffract, and the diffracted waves interact to give rise to an interference pattern. Quantum decoherence destroys this pattern, so in principle this could provide a test for whether the decohering effect of background space-time fluctuations matches predictions. Such a system would have to be completely isolated to rule out other effects.

This is, however, impossible in practice - with today's interferometers, at least. Experiments pioneered by Anton Zeilinger, Markus Arndt and colleagues at the University of Vienna, Austria, have been able to generate interference with beams of 60-atom carbon buckyballs, but even with molecules of this size the effect of gravitational waves would be too small to be observed.