An international team of researchers, including Nobel Prize-winning physicist Sir Roger Penrose, has directly observed the long-predicted gravitational effects on falling quantum objects for the first time. The experiment shows that Einstein’s equivalence principle remains consistent with quantum behavior under the conditions tested. Led by scientists from Ben-Gurion University of the Negev, Ulm University and the University of Oxford, the study was published Sept. 2 in Science Advances.
The breakthrough could help scientists better understand the relationship between quantum mechanics and Einstein’s theory of gravity. Quantum mechanics describes the behavior of atoms and other microscopic particles, while Einstein’s theory of gravity explains falling objects, planetary motion and the large-scale structure of the universe. Although both theories have been highly successful, physicists still lack a complete framework that unifies them.
The new experiment examined how gravity changes the quantum properties of atoms. The results matched predictions based on extending Einstein’s equivalence principle to quantum objects, providing a direct experimental connection between quantum physics and gravity.
Testing Einstein’s equivalence principle with quantum atoms
Einstein’s equivalence principle states that gravity should effectively disappear locally for an observer in free fall. A person falling inside an elevator, for example, would experience weightlessness. The principle has been tested with exceptional precision using ordinary matter, but testing it with quantum objects is significantly more challenging.
Quantum objects behave like waves and can exist in superpositions, meaning they can follow multiple paths simultaneously. To investigate how gravity affects these quantum waves, the researchers developed a device known as a quantum Galileo interferometer.
The instrument split the quantum waves associated with ultracold atoms into two paths. One part of the wave was held nearly stationary, while the other was allowed to fall freely under gravity. The researchers then recombined the two waves and measured the differences caused by their separate experiences.
The experiment took place at Ben-Gurion University using a cloud of rubidium atoms cooled to temperatures just above absolute zero. The atoms were controlled near the surface of a specially engineered atomic chip.
Splitting atoms into two quantum paths
The experimental team, including doctoral student Orr Dobkowski, used microwave pulses to place the ultracold rubidium atoms into quantum superpositions. This enabled each atom to behave as though it were traveling along two paths at the same time.
Tiny electrical wires built into the atomic chip produced a precisely controlled magnetic field. When one part of the atomic wave interacted with the field, the researchers generated an upward force that balanced the downward pull of gravity. That portion of the wave therefore remained stationary relative to the laboratory and the Earth.
The other part of the wave was pushed upward using carefully calibrated magnetic pulses. It was then transferred into a state that was largely insensitive to magnetic fields, allowing it to move freely under gravity along a path similar to that of a ball thrown into the air.
After the free-fall motion ended, another controlled magnetic pulse brought the two portions of the atomic wave back together. The waves interfered with one another, allowing the researchers to measure the tiny difference in quantum phase accumulated while one part fell and the other remained suspended.
Gravity creates a measurable quantum phase
The measured quantum phase matched the value predicted when Einstein’s equivalence principle is applied to a quantum wave. The result provides a direct laboratory demonstration of how gravity leaves a measurable imprint on a quantum system.
Quantum particles have previously been used to measure gravitational effects. However, the researchers say this is the first experiment to directly measure the predicted quantum phase generated by a freely falling quantum object.
Lead author Professor Ron Folman of Ben-Gurion University of the Negev said: “This is a difficult experiment and an extensive investigation into one of the most fundamental questions in physics: How can gravity, as explained by Einstein’s theory of relativity, and quantum theory be integrated into a single understanding of the universe?”
“These two pillars of modern physics have so far eluded all attempts to construct a unified theoretical framework, but this complex experiment provides further clues about how such a unification might eventually be achieved,” Folman added.
Study co-author Professor Vlatko Vedral of the University of Oxford’s Department of Physics said the experiment pushes quantum mechanics into one of its most important frontiers: gravity.
“There is no consistent theory explaining why quantum physics fails,” Vedral said. “This experiment shows once again that the predictions of quantum mechanics continue to hold under new and challenging conditions.”
What the quantum gravity experiment does—and does not—show
The findings do not establish a unified theory of quantum mechanics and gravity, nor do they prove that gravity itself is quantum. Instead, they show that Einstein’s equivalence principle remains compatible with quantum mechanics within the range tested by the experiment.
The results also do not rule out ideas proposed by study co-author Professor Sir Roger Penrose of the University of Oxford. Penrose has suggested that quantum mechanics could eventually break down if a sufficiently large object remained in a quantum superposition for a long enough period.
The latest experiment did not involve objects large enough or superpositions lasting long enough to test that possibility. Researchers hope, however, that the quantum interferometer technique can eventually be adapted for heavier objects, including nanodiamonds. Experiments exploring those possibilities are already underway at Ben-Gurion University of the Negev.
The international research team included scientists from Ben-Gurion University of the Negev, the University of Oxford, the University of Southampton, the German Aerospace Center’s Institute for Quantum Technology in Ulm, Ulm University and Texas A&M University.
Source: www.sciencedaily.com