Two new experiments confirm Einstein’s gravity principle holds at quantum scale

Two independent experiments published this week in Science Advances have reinforced one of the central pillars of Einstein's general relativity, showing that the equivalence principle — the idea that gravity treats all objects the same — extends into the quantum realm.

A Quantum Object in Free Fall

In one experiment, researchers at Ben-Gurion University of the Negev, the University of Oxford, and Ulm University built a device they call the Quantum Galileo Interferometer to make the first direct measurement of the quantum phase accumulated by a freely falling atomic wave packet. The team cooled rubidium-87 atoms into a Bose-Einstein condensate and used microwave pulses to split each atom's quantum wave into two paths: one held stationary relative to Earth using magnetic fields, and another allowed to fall freely under gravity.thebrighterside+1

When the two wave packets were recombined, their interference pattern revealed roughly 80 radians of accumulated phase over more than five hours of measurements, matching the value predicted by applying the equivalence principle to quantum mechanics. "This experiment pushes quantum mechanics into one of its most intriguing frontiers, gravity, and shows that, once again, its predictions hold," said University of Oxford physicist Vlatko Vedral.interestingengineering+1

Atoms in Orbit

Separately, a team led by Ming-Sheng Zhan at the Wuhan Institute of Physics and Mathematics tested the weak equivalence principle aboard China's space station. The researchers cooled two rubidium isotopes to near absolute zero and released them inside an atom interferometer, where laser pulses split each cloud into two quantum paths before recombining them. After collecting data over 280 days in orbit, the isotopes were found to accelerate identically to within about five parts in 100 million — roughly three orders of magnitude more precise than any previous atom-based test in microgravity.phys+1

The orbital environment provided a key advantage: continuous free fall around Earth allowed the atoms to be observed far longer than any ground-based laboratory permits.phys

What Remains Unanswered

Neither experiment unifies quantum mechanics with general relativity. The Ben-Gurion team's result confirms a specific phase relationship consistent with the equivalence principle but does not prove gravity itself has quantum properties. The space station measurement narrows the space in which any violation of the principle could hide, but does not close it entirely.thebrighterside+1

Both teams point toward future work that could push into more demanding territory. The Quantum Galileo Interferometer group is working toward experiments with nanodiamonds — objects massive enough to test whether gravity-related collapse of quantum superpositions occurs, as proposed by Sir Roger Penrose. Zhan's team says longer free-fall times and quieter platforms could improve orbital precision further still. For now, Einstein's century-old principle has passed two more tests unscathed.phys+1