An international team that includes Nobel Prize-winning physicist Professor Sir Roger Penrose has directly observed a long-predicted gravitational effect in a falling quantum object for the first time. The finding shows that a central principle of Einstein’s theory of gravity continues to agree with quantum behavior under the conditions tested. The research, led by Ben-Gurion University of the Negev, the University of Ulm and the University of Oxford, was published September 2 in Science Advances.
Modern physics rests on two remarkably successful frameworks. Quantum mechanics describes the unusual behavior of atoms and other extremely small objects, while Einstein’s theory of gravity explains falling bodies and the large-scale structure of the Universe. Despite their individual success, physicists still do not have a complete theory that brings the two together.
The new experiment explores a region where these two descriptions overlap. Researchers measured a specific change in the quantum properties of atoms as they moved under the influence of gravity. The effect matched the prediction that follows when Einstein’s equivalence principle, one of the foundations of his theory of gravity, is extended to a quantum object.
Testing Einstein’s Equivalence Principle
The equivalence principle says that gravity should effectively vanish locally for an observer in free fall. A person falling freely in a lift, for example, would experience weightlessness. The principle has been confirmed with extraordinary precision using ordinary matter, but testing it directly with quantum objects has been much more difficult because quantum objects can behave like waves and can effectively follow more than one path at the same time.
To make such a test possible, the team built an instrument called the Quantum Galileo Interferometer. The device allowed the researchers to split the quantum wave associated with an atom into two separate paths. One part could be kept in place while the other was allowed to fall freely. The two were then brought back together so the researchers could determine how gravity had affected the falling wave.
The experiment was carried out at Ben-Gurion University using clouds of rubidium atoms cooled to temperatures just above absolute zero. The atoms were manipulated near the surface of a specially designed atom chip.
Splitting an Atom Into Two Quantum Paths
The experimental team, including PhD student Or Dobkowski, began by using microwave pulses to place the ultracold atoms into a quantum superposition. This effectively allowed each atom to follow two paths at once.
Tiny electrical wires built into the chip then generated carefully controlled magnetic fields. One part of the atomic wave interacted with the magnetic field, allowing the researchers to create an upward force that precisely balanced the downward pull of gravity. As a result, that portion of the wave remained stationary relative to the laboratory and the Earth.
The other portion was pushed upward using a precisely controlled magnetic pulse. It was then switched into a state that was almost unaffected by the magnetic field, allowing it to move freely under gravity, following a ballistic trajectory similar to a ball tossed into the air.
Once the falling motion was complete, another carefully controlled magnetic pulse brought the two parts of the atomic wave back together. The reunited waves interfered with one another, giving the researchers a way to measure the extremely small difference in quantum phase that had accumulated while one part was falling and the other remained fixed.
Gravity Leaves a Measurable Quantum Signature
The quantum phase measured by the researchers matched the phase predicted when Einstein’s principle is applied to this type of quantum wave. The experiment therefore provides a direct laboratory connection between quantum physics and Einstein’s description of gravity.
Quantum particles have been used in earlier experiments to measure gravity, but the researchers say this is the first direct measurement of the predicted quantum phase produced by a freely falling object.
Lead author Professor Ron Folman (Ben-Gurion University of the Negev) said: “This is a unique paper, in the sense that it combines a hard experiment with a far-reaching theoretical interpretation, about one of the most fundamental questions in physics: How can gravity (described by Einstein’s theory of relativity) and quantum theory, be unified into one understanding of the universe? These two pillars of modern physics have so far eluded all attempts at a unified theoretical framework, but this complex experiment gives more hints as to how such a unification may be achieved.”
Study co-author Professor Vlatko Vedral (Department of Physics, University of Oxford) added: “We have no consistent theory telling us why quantum physics should fail. This experiment pushes quantum mechanics into one of its most intriguing frontiers, gravity, and shows that, once again, its predictions hold.”
What the Experiment Does and Does Not Show
The finding does not provide a unified theory of quantum mechanics and gravity, and it does not demonstrate that gravity itself is quantum. Instead, it shows that Einstein’s equivalence principle remains compatible with quantum mechanics within the range explored by the experiment.
The results also do not disprove an idea proposed by study co-author Professor Sir Roger Penrose (University of Oxford). Penrose has argued that quantum mechanics could eventually break down when sufficiently massive objects remain in quantum superpositions for long enough periods.
The current experiment did not involve objects massive enough, or superpositions lasting long enough, to test that possibility. However, the researchers hope the new technique can eventually be extended to much heavier objects, including nanodiamonds. Experiments designed to investigate that possibility are already underway in the same group 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; German Aerospace Center, the Institute of Quantum Technologies, Ulm; Universität Ulm; and Texas A&M University.
