Quantum Galileo Experiment Tests Einstein’s Gravity With One Atom

What if there was a single rubidium atom, cooled to just above absolute zero, splitting into two versions of itself? One version remains perfectly still. The other falls, like a ball thrown skyward that then descends. When the two halves reunite, the subtle discrepancy between them reveals a previously unmeasured aspect of gravity. This is the entire experiment, and it has just been successfully conducted.

A team, spearheaded by Ben-Gurion University of the Negev in collaboration with Ulm and Oxford, published their findings in Science Advances on September 2. This peer-reviewed publication in a reputable journal presents a significant claim: they measured the quantum phase predicted by Einstein’s equivalence principle for a freely falling object, and the result matched. Notably, Nobel laureate Roger Penrose is among the authors, a detail that will prove relevant later.

The Atom That Fell Two Ways At Once

Quantum objects can exhibit wave-like behavior, and waves possess a phase, or essentially, a position within their up-and-down cycle. When an atom is split into a superposition, two wave-paths are created, each accumulating its own phase over time. Upon reunification, they interfere, much like ripples overlapping in a pond. The resulting interference pattern indicates the extent to which the two paths diverged.

To achieve this, the team constructed what they call the Quantum Galileo Interferometer. Microwave pulses placed each rubidium atom into two states simultaneously. Tiny wires on an atom chip generated magnetic fields, and here lies the ingenious part: one portion of the atomic wave experienced the magnetic field and was pushed upward with precisely enough force to counteract gravity, thus remaining stationary relative to the Earth. The other portion was shifted into a state that effectively ignored the magnetic field, allowing it to fall freely.

One half was held in place; the other half was in free fall. All from the same atom.

At the experiment’s conclusion, another pulse brought them back together. The phase difference between the stationary half and the falling half aligned with theoretical predictions when Einstein’s equivalence principle is applied to a quantum wave.

Why the Equivalence Principle is the Star Here

The equivalence principle posits that within a freely falling frame of reference, gravity locally vanishes. If you drop with an elevator, you experience weightlessness. Einstein based his entire theory of gravity on this concept, and it has been verified with extraordinary precision using everyday matter, dropped weights, orbiting satellites, and similar methods.

However, quantum objects presented a challenge. They don’t follow a single path; instead, they spread across multiple possibilities. So, what does “free fall” even signify for an entity existing in two places at once? No one had directly measured the phase a freely falling quantum object acquires under gravity until now. Previous experiments utilized quantum particles as sensitive gravity meters, which differs from observing the specific phase accumulated by a truly free-falling superposition.

Lead author Professor Ron Folman articulated the significance of this:

“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.”

What This Doesn’t Do

Here’s where the press cycle often blurs the lines, so let me be direct. This experiment doesn’t unify gravity and quantum mechanics. It doesn’t prove that gravity is quantum. It demonstrates that Einstein’s equivalence principle remains consistent with quantum mechanics within the specific tested regime, which means small masses, short timescales, and an object on the scale of a single atom.

Penrose has a long-standing argument that quantum mechanics might break down for sufficiently massive objects held in superposition for extended periods. In his view, gravity itself could cause the superposition to collapse. This experiment doesn’t contradict him. It didn’t approach the masses or durations his hypothesis requires. Therefore, both statements are simultaneously true: Einstein’s principle held, and Penrose’s conjecture remains untouched.

This isn’t a contradiction; it’s a mapping of the measurement’s scope, which doesn’t yet delve deeply into the profound questions.

The Heavier Objects Coming Next

The technique itself is more significant than this single result. The same Ben-Gurion group is conducting an experiment aimed at much heavier objects, including nanodiamonds. Placing an object with substantial mass into superposition and maintaining it long enough begins to probe the regime where Penrose believes the theory might falter. That is the door this apparatus opens. It’s a first step.

So, what truly happened? A team split an atom, allowed half of it to fall, and confirmed that a century-old principle survives interaction with quantum mechanics under gentle conditions. It’s a genuine measurement with a true scope, but no unification. The truly interesting challenge lies in the next experiment.