Penrose's Model For Gravitational Collapse Of Quantum Superpositions Doesn't Work

The way that an observer making an observation triggers a collapse of a quantum physical wave function is a longstanding unsolved problem in physics. 

A recent experimental effort to see if quantum gravity effects triggered this in a theory promoted by Roger Penrose, but first proposed by Lajos Diósi, turns out not to be the answer to this question, which remains unsolved.

Roger Penrose proposed that a spatial quantum superposition collapses as a back-reaction from spacetime, which is curved in different ways by each branch of the superposition. In this sense, one speaks of gravity-related wave function collapse. He also provided a heuristic formula to compute the decay time of the superposition—similar to that suggested earlier by Lajos Diósi, hence the name Diósi–Penrose model. The collapse depends on the effective size of the mass density of particles in the superposition, and is random: this randomness shows up as a diffusion of the particles’ motion, resulting, if charged, in the emission of radiation. Here, we compute the radiation emission rate, which is faint but detectable. We then report the results of a dedicated experiment at the Gran Sasso underground laboratory to measure this radiation emission rate. Our result sets a lower bound on the effective size of the mass density of nuclei, which is about three orders of magnitude larger than previous bounds. This rules out the natural parameter-free version of the Diósi–Penrose model.

From Nature Physics via Science.org which explains the results as follows:

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 Károlyházy Frigyes in the 1960s and Lajos Diósi 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.

Renowned University of Oxford mathematician Roger Penrose championed the hypothesis in the late 1980s because, he says, it removes the anthropocentric notion that the measurement itself somehow causes the collapse. "It takes place in the physics, and it's not because somebody comes and looks at it." . . . 

In the new study, Diósi 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 Cătălina Curceanu of National Institute for Nuclear Physics in Rome.

To test this idea, the researchers built a detector out of a crystal of germanium the size of a coffee cup. They looked for excess x-ray and gamma ray emissions from protons in the germanium nuclei, which create electrical pulses in the material. The scientists chose this portion of the spectrum to maximize the amplification. They then wrapped the crystal in lead and placed it 1.4 kilometers underground in the Gran Sasso National Laboratory in central Italy to shield it from other radiation sources. Over 2 months in 2014 and 2015, they saw 576 photons, close to the 506 expected from naturally occurring radioactivity, they report today in Nature Physics.

By comparison, Penrose's model predicted 70,000 such photons. "You should see some collapse effect in the germanium experiment, but we don't," Curceanu says. That suggests gravity is not, in fact, shaking particles out of their quantum superpositions. (The experiment also constrained, though did not rule out, collapse mechanisms that do not involve gravity.)