One of the greatest paradoxes of quantum physics may have lost its leading interpretation of science


Gravity is unlikely to cause quantum collapse, suggesting an underground experiment at Italy’s Gran Sasso National Laboratory.

Tommaso Guicciardini / Science Source

By George Muser

This is one of the strangest theories of quantum theory: a particle can be in two places at once — yet we can ever see it here or there. Textbooks state that the act of looking at a particle “collapses”, as if it appears only at random from its two locations. But physicists quarreled over why this would happen, if indeed it did. Now, one of the most admirable mechanisms for quantum collapse — gravity — has suffered a setback.

The hypothesis of gravity traces its origins to the Hungarian physicists Kérolihai frigues in the 1960s and the Lajos deoci in the 1980s. The basic idea is that the gravitational field of any object is outside quantum theory. It is placed in odd combinations of different states, or “superspecifications”. So if both a particle is made here And There, its gravitational field tries to do the same – but the field cannot withstand long-term stress; It collapses and carries the particle with it.

Noted Oxford University mathematician Roger Penrose endorsed the hypothesis in the late 1980s, as he says, it removes the anthropological assumption that measurement itself somehow causes collapse. “It happens in physics, and it’s not because someone comes and sees it.”

Nevertheless, the hypothesis seemed impossible to check with any realistic technique, notes Diosi, now at the Wigner Research Center, and a co-author on the new paper. “For 30 years, I was always criticized in my country for speculating on something that was completely inaccessible.”

New methods now make it doable. In the new study, Diósi and other scientists looked for one of several ways, whether by gravity or some other mechanism, that a quantum collapse would manifest itself: a particle that collapses, rotating randomly, of which it Heats up the system. . “It’s like you’ve given a kick to a particle,” says Sandro Donady, co-author of the Frankfurt Institute for Advanced Studies.

If the particle is charged, it will emit a photon of radiation as it floats. And many particles subject to the same gravitational gravity will emit uniformly. “You have an amplified effect,” says Cătălina Curceanu, co-author of the National Institute for Nuclear Physics in Rome.

To test this idea, the researchers built a coffee cup-shaped detector from a crystal of germanium. They looked for additional X-rays and gamma ray emission from protons in the germanium nucleus, which create electrical pulses in the material. Scientists chose this part of the spectrum to maximize amplification. They then wrapped the crystal in lead and placed it 1.4 kilometers underground at the Gran Sasso National Laboratory in central Italy to shield it from other radiation sources. Over 2 months in 2014 and 2015, he saw 576 photons, close to 506 from naturally occurring radioactivity, he reported here today Nature physics.

By comparison, Penrose’s model predicted 70,000 such photons. “You should see some collapse effects in the germanium experiment, but we don’t,” Curceanu says. This suggests that gravity is not, in fact, shaking particles from their quantum superposition. (There was also a bottleneck in use, although not ruled out, by demolishing mechanisms that do not involve gravity.

To confirm the result, physicists need to engineer those superpositions directly, as it does to rely on random natural phenomena, says Ivete Fuentes of the University of Southampton: “You should, in theory, have larger particles. Must be able to make a superposition. So let’s do it. ” She says her team is working to create clouds of 100 million sodium atoms at temperatures above absolute zero.

Although Penrose praises the new work, he thinks that it is not really possible to test his version of the model. He says he was never comfortable with particle-to-particle, as they can cause the universe to gain or lose energy, which violates the basic principle of physics. They have spent epidemic lockdown to build a new and improved model. He says, “It does not produce heating or radiation.” “” In this case, gravity may be the cause of the collapse, yet may hide its tracks.

Other factors, such as interactions between germanium protons and electrons, can also inhibit the signal, says theoretical physicist Manali Derakhansi from Rutgers University, New Brunswick. Overall, they say, if gravity causes collapse, the process needs to be more complex than the originally proposed Penrose. “An argument might say that … juice is not worth the squeeze.”