One of quantum physics' greatest paradoxes may have lost its leading explanation

It's one of the oddest tenets of quantum theory: a particle can be in two places at once -- yet we only ever see it here or there. Textbooks state that the act of observing the particle "collapses" it, such that it appears at random in only one of its two locations. But physicists quarrel over why that would happen, if indeed it does. Now, one of the most plausible mechanisms for quantum collapse -- gravity -- has suffered a setback

The gravity hypothesis traces its origins to Hungarian physicists Karolyhazy Frigyes in the 1960s and Lajos Diosi in the 1980s. The basic idea is that the gravitational field of any object stands outside quantum theory. It resists being placed into awkward combinations, or "superpositions," of different states. So if a particle is made to be both here and there, its gravitational field tries to do the same -- but the field cannot endure the tension for long; it collapses and takes the particle with it.

Still, the hypothesis seemed impossible to probe with any realistic technology, notes Diosi, now at the Wigner Research Center, and a co-author on the new paper. "For 30 years, I had been always criticized in my country that I speculated on something which was totally untestable." New methods now make this doable. In the new study, Diosi and other scientists looked for one of the many ways, whether by gravity or some other mechanism, that a quantum collapse would reveal itself: A particle that collapses would swerve randomly, heating up the system of which it is part. "It is as if you gave a kick to a particle," says co-author Sandro Donadi of the Frankfurt Institute for Advanced Studies. If the particle is charged, it will emit a photon of radiation as it swerves. And multiple particles subject to the same gravitational lurch will emit in unison. "You have an amplified effect," says co-author Catalina Curceanu of National Institute for Nuclear Physics in Rome.

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