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Roger Penrose wants to know if the Copenhagen interpretation of quantum mechanics - which says that a system can exist simultaneously in two states until observed, when it must become one or the other - is correct.

Penrose proposes to put a tiny crystal into a superposition of two locations and see if the superposition persists for more than a tenth of a second. According to his theory, the crystal will have to decide between the two possibilities.

As the experiment involves large distances, it will take place in space, using an X-ray observatory called Constellation X. A single X-ray particle, a photon, will be sent to a semi-silvered mirror that will either reflect it or let it straight through.

If the photon goes straight through, it will hit the crystal, which is about the size of a speck of dust, and knock it a distance about the size of an atomic nucleus. The crystal will reflect the photon towards a second distant mirror, which will reflect the photon back to the semi-silvered mirror.

But if the photon is immediately reflected by the semi-silvered mirror, it will head into space towards the distant mirror, which will reflect the photon back to the crystal, which will bounce it back to the semi-silvered mirror. Both possibilities co-exist in superposition under quantum mechanics' rules.

But Penrose believes that the system will have to decide which way the photon goes within a tenth of a second. "Quantum mechanics in its present-day form says that superposition lasts forever until some disturbing influence comes along," Penrose says. "In my opinion, superposition does not last forever. It's common sense."