One of the tenets of quantum physics is the concept of superposition, which, like in the oft-lectured case of Schrödinger’s cat, occurs when a particle exists in multiple states and locations at once. A common misinterpretation of the phenomenon remains that particles seemingly maintain this dual existence until observed. In fact, the particle’s collapse of superposition into a single observable state is not due to human observation. The exact cause has been under scrutiny for decades, and even the widely favored gravity hypothesis has always faced skepticism. This skepticism reached a new high in a September 2020 study involving some of the very proponents of the gravitational explanation, throwing the entire concept of superposition into contention.
This skepticism reached a new high in a September 2020 study involving some of the very proponents of the gravitational explanation, throwing the entire concept of superposition into contention.
The idea that gravity limits superposition first rose to prominence in the mid-20th century through the work of physicists Károlyházy Frigyes and Lajos Diósi. Roger Penrose, a 2020 Nobel Laureate in Physics, greatly contributed to this idea as well and continues to investigate it in light of the new discoveries. The gravity hypothesis posits that, when a particle is in superposition, its gravitational field undergoes significant stress, as the field also must be in two places at once. Unable to withstand this physical state, the gravitational field collapses, eliminating the second particle state.
Unfortunately, the technological ability to detect and confirm this gravitational collapse remained nonexistent until recently; this issue has historically been a source of skepticism. A group of scientists, including Diósi, devised an experiment to measure the behavior of the particle and the gravitational field.
Spanning between 2014 and 2015, the experiment took place in the Gran Sasso National Laboratory in Italy, which lies 1.4 kilometers beneath the Earth’s surface, far from large amounts of natural radiation. Avoidance of such external radiation was crucial in order to detect radiation emitted from particles in collapsing superpositions. Current understanding of physics requires that, upon gravitational collapse, particles move erratically, releasing energy in the process. When charged and moving, the particle emits radiation in the form of a photon. By observing a collection of particles, experts sought to amplify the radiative signal, finally creating a potential way to sense this gravitational behavior.
It seemed simple, there was no significant increase in energy; therefore gravity, which required such energetic behavior, was not the answer to the collapse of superposition
This collection of particles — in the form of a germanium crystal — emitted electrical pulses as a result of instances of elevated radiation from the germanium protons experiencing superpositional collapse. An outer layer of lead and an inner layer of electrolytic copper enclosed the crystal to further decrease exposure to unrelated, natural radiation. In particular, the scientists looked for the more amplified gamma rays and X-rays. They already knew that the rate at which the germanium atoms emitted these rays in all directions related to detectable energies released. After two months of experimentation, results were orders of magnitude smaller than expected, and the team only detected 576 photons released, which was very close to the natural radiative state of 506 photons. In other words, the team did not detect the sudden increases in energy expected with gravitational collapse; there was no evident link between the gravitational field and superposition.
It seemed simple, there was no significant increase in energy; therefore gravity, which required such energetic behavior, was not the answer to the collapse of superposition. This hypothesis had lasted decades and was seemingly gone in an instant, yet the results are not quite that simple. While the results of this experiment may not entirely eliminate the gravity hypothesis, they acknowledge a multitude of shortcomings: a continued lack of effective gravity-sensing techniques, the idea that gravity could conceal its behavior, and simply a lack of understanding of superposition. Scientists still do not completely disregard gravity, as hope remains that improved technology and unimagined methods can reinstate the hypothesis. Currently, experts are attempting to manufacture superpositions of large groups of particles so that they no longer have to depend on unobvious and natural radiative tendencies. The question of whether gravity, or another unnoticed interaction, conceals the energetic effects also remains relevant. Furthermore, the implications of energy loss through gravitational collapse might challenge energy conservation.
Although the gravity hypothesis faces many challenges, scientists are only more motivated to rework the supporting models and science. Questions and previously unnoticed behaviors brought forth by this experiment, contribute to another tenet of science — the simple persistence toward greater knowledge. Whether or not gravity is the answer, its consideration has enhanced the investigations of quantum physics and likely will continue to do so in the future.
Nature Physics (2020). DOI: 10.1038/s41567-020-1008-4
