Quantum Galileo Experiment Confirms Equivalence Principle for Superposed Atoms

Physicists have demonstrated that the principle of equivalence—a cornerstone of general relativity—holds true for quantum systems in a state of superposition. The experiment, led by Ron Folman of Ben-Gurion University, provides the first empirical evidence that quantum particles respond to gravity in a manner consistent with classical relativistic predictions.

Using a newly developed Quantum Galileo Interferometer (QGI), the research team utilized approximately 20,000 rubidium atoms maintained as a Bose-Einstein condensate. The team placed these atoms into a quantum superposition of two distinct trajectories: one in free fall and one held stationary against the pull of gravity. According to the study published in Science Advances (DOI: 10.1126/sciadv.aec8045), the trajectories reached a maximum separation of approximately 7.5 micrometers, with the longest flight duration lasting two-thousandths of a second. The resulting measured phase of the quantum wave aligned closely with theoretical expectations, falling within 2.5 percent of the prediction.

The Tension Between Two Worlds

For decades, a fundamental rift has existed between quantum mechanics, which governs the subatomic realm, and general relativity, which describes gravity and the curvature of spacetime. A primary point of contention is the principle of equivalence, which posits that the experience of free fall is indistinguishable from being at rest in deep space. While this concept is central to Einstein's relativity, testing it within a quantum framework has been historically difficult. It requires an interferometer capable of maintaining atoms in different gravitational states without destroying their delicate quantum coherence.

Implications for Quantum Gravity

This result establishes a critical empirical checkpoint in the quest for a unified theory of physics. By confirming that the equivalence principle applies to superposed systems, the QGI experiment validates that gravity interacts with quantum wave functions as predicted. As Ron Folman noted, the experiment showed for the first time that the equivalence principle can work within a quantum system in a superposition.

While the study does not fully resolve the incompatibility between relativity and quantum mechanics, it sets the stage for more ambitious tests. The ability to maintain coherence across different gravitational trajectories suggests that the laws of relativity are not immediately violated by quantum behavior.

The Path to Nanodiamonds

Looking forward, the research team intends to scale the experiment by replacing atoms with significantly heavier masses. The team plans to use nanodiamonds—which are 10 orders of magnitude heavier than rubidium atoms—in a similar superposition within the next four to five years. This next phase will test whether gravity eventually causes quantum collapse, a hypothesis proposed by physicists Roger Penrose and Lajos Diósi. Observing such a collapse would provide a direct test of whether gravity is quantized, potentially offering the first glimpse into the true nature of quantum gravity.