A team of researchers from Germany, Italy and Hungary has tested a theory that suggests gravity is the force behind quantum collapse and has found no evidence to support it. In their paper published in the journal Nature Physics, the researchers describe underground experiments they conducted to test the impact of gravity on wave functions and what their work showed them. Myungshik Kim, with Imperial College London has published a News & Views piece in the same issue, outlining the work by the team and the implications of their results.
Quantum physics suggests that the state of an object depends on its properties and the way it is measured by an observer; the thought experiment involving Schrödinger’s cat is perhaps the most famous example. But the theory is not universally accepted—physicists have wrangled for many years over the notion, with some arguing that it seems a bit too anthropocentric to be real. Behind the theory is the concept of waveform collapse, by which the observation of a particle, as an example, makes it collapse. To help make sense of the idea, some physicists have suggested that the force behind waveform collapse is not a person taking a look at a particle, but gravity. They suggest that gravitational fields exist outside of quantum theory and resist being forced into awkward combinations such as superpositions. A gravitational fieldforced to do so soon collapses, taking the particle with it. In this new effort, the researchers devised an experiment to test this theory in a physical sense.
The experiment consisted of building a small crystal detector made from germanium and using it to detect gamma and X-ray emissions from protons in the nuclei of the germanium. But before running the experiment, they wrapped the detector in lead and dropped it into a facility 1.4 kilometers below ground level at the Gran Sasso National Laboratory in Italy to prevent as much extraneous radiation from reaching the sensor as possible. After two months of testing, the team recorded far fewer photon hits than theory would suggest—indicating that the particles were not collapsing due to gravity, as theory had suggested.