An international team of scientists has demonstrated for the first time how gravity influences a fundamental property of individual atoms, providing experimental support for one of Albert Einstein’s key principles at the quantum level. The research, involving researchers from Ben-Gurion University in Israel as well as institutions in the United Kingdom, Germany, and the United States, represents a significant step toward reconciling the theory of general relativity with quantum mechanics.
The study, published in Science Advances, focused on the equivalence principle—a cornerstone of Einstein’s theory of gravity that states there is no distinguishable difference between the effects of gravity and acceleration. Traditionally, this principle is illustrated by the observation that objects in free fall accelerate toward Earth at 9.8 meters per second squared, or equivalently, that objects inside an accelerating elevator experience a similar force when gravity is absent. While this concept has long been tested on macroscopic scales, applying it to quantum particles has proven elusive.
The researchers conducted their experiment using approximately 20,000 rubidium atoms cooled to near absolute zero temperatures, just above minus 273.15 degrees Celsius, to minimise thermal motion. Each atom was prepared in a state in which it traveled simultaneously along two separate paths—a quantum property known as superposition. One path was held stationary through magnetic manipulation, effectively counteracting gravity, while the other was allowed to fall freely, akin to a ball tossed upward and then dropping back down under gravity’s influence.
By recombining the two paths, the team observed shifts in the "quantum phase"—the wave-like oscillations that describe the atom’s state—which resulted in interference patterns detectable by sensitive imaging equipment. These patterns yielded direct evidence that the atom’s wave components were affected differently by gravitational free fall, aligning with the predictions derived from Einstein’s equivalence principle.
Professor Ron Folman of Ben-Gurion University, the lead author, highlighted the significance of the findings, stating that while general relativity and quantum mechanics have been the twin pillars of modern physics, efforts to unify them into a single framework have remained challenging. He suggested that the experiment offers new insights into how such a unification might be approached.
The study also involved Nobel laureate Sir Roger Penrose and others, underscoring its prominence within the physics community. By validating the influence of gravity on quantum phases, the research provides a clearer picture of how gravitational effects operate at atomic scales. This advance could open pathways for future exploration into the fundamental laws governing the universe and the intersection between quantum mechanics and gravitational theory.