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Objective state reduction is a mechanism proposed by Roger Penrose to explain why macroscopic objects do not remain in a superposition. It goes a little something like this:

Because a particle or group of particles distorts spacetime, if it is in a superposition of states then there are two separate distortions of spacetime around it. This means that the object has slightly different amounts of gravitational energy in both of its two states. This means that the superposition causes an uncertainty in the amount of energy in the system containing the superpositioned object. Since one of the consequences of Heisenberg's Uncertainty Principle is that a large uncertainty in a system's energy can only exist for a short amount of time, and that that amount of time is inversely proportional to the system's energy, the energy uncertainty must cease to exist, and the only way in which that can happen is if the object ceases to be in a superposition. Pow! The object is forced to choose one of the two positions, and decoheres.

Superpositions have been observed on fairly large scales, right up to the size of individual buckminsterfullerene molecules, so some quantum physicists doubt that Penrose is correct.

However, Roger Penrose conjectures that as the objects get bigger, the amount of time in which they can stay in a superposition gets smaller. He reckons that an item slightly smaller than a micrometre would decohere in less than a tenth of a second, and that as the object gets bigger that amount of time is cut even smaller and smaller. Penrose suggests an experiment in space using X-ray mirrors and a crystal on a carbon filament to see whether or not he is correct. He has dubbed the experiment FELIX, short for Free-orbit Experiment with Laser Interferometry X-rays.

Reference: NewScientist, March 9, 2002, issue 2333.

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