Galileo's Experiment, Done With One Atom in Two Places

SCIENCE & TECHNOLOGY · SEPTEMBER 12, 2026

The story everyone learns is that Galileo dropped two balls from the Leaning Tower of Pisa and they landed together. He probably never did it — he rolled balls down ramps, which is slower and easier to time — but the claim is right: how fast a thing falls has nothing to do with what it is made of or how much of it there is. Einstein took that ordinary fact and made it the foundation of his theory of gravity. If everything falls the same way, then someone falling has no way to tell, from inside, that gravity is acting on them at all. Gravity, locally, disappears in free fall. That is the equivalence principle, and every test of it for a century has been done with objects that are unambiguously somewhere: balls, pendulums, satellites, atoms on a definite path.

On 2 September a paper in Science Advances reported the first version of the experiment where the falling object was not, in the ordinary sense, anywhere in particular. A single rubidium atom, cooled to near absolute zero, was put into a quantum superposition — one part of it falling freely, the other held stationary — and then the two parts were brought back together and compared. The comparison is a phase, a number that says how the atom's internal quantum clock advanced on one path relative to the other. Einstein's principle makes a definite prediction for that number. The measurement matched it.

What Did the Experiment Actually Do?

The apparatus is called a Quantum Galileo Interferometer, built at Ben-Gurion University of the Negev in Israel, where Ron Folman's group has spent years working with "atom chips" — a surface patterned with tiny wires whose magnetic fields can hold, push and split cold atoms sitting a fraction of a millimetre above it. The sequence, as the authors describe it: a microwave pulse splits the atom's wave into two parts. One part is held in place by the chip's magnetic field, which is tuned to exactly cancel gravity. The other is given a short upward push and then switched into an internal state that barely feels the magnetic field, so it flies up and falls back under gravity alone — a ballistic arc, the same arc a thrown ball follows, a few hundred microns high. Then the two parts are recombined and the interference pattern read out.

The number that comes out is a difference in what physicists call proper time — the time that elapsed for each part of the atom along its own path. General relativity says a clock in free fall and a clock held up against gravity tick at slightly different rates, and here the "clocks" are the two halves of one atom's wavefunction. The measured phase shift was what relativity predicts for an object obeying the equivalence principle, and it was measured in a regime where the object is also fully obeying quantum mechanics.

Why Is This Not Simply "Gravity Works on Atoms"?

Because gravity working on atoms was never in doubt, and gravity's effect on quantum interference is not new either — in 1975, Colella, Overhauser and Werner sent neutrons around a crystal interferometer and saw gravity shift the fringes, an experiment every physics student meets. What is new here is the free fall. In the older experiments the paths were fixed by the apparatus; nothing was falling. This one puts one half of a quantum object into genuine free fall — the condition the equivalence principle is actually about — and asks whether the quantum phase agrees with the relativistic clock. It does. Vedral put the point carefully: "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."

That is the modest result. There is an immodest reason to care about it, and it is sitting on the author list.

What Does Roger Penrose Think Is Going to Break?

Penrose has argued for decades that quantum mechanics and gravity cannot both be exactly right, and that gravity is the one that wins at large scales. His specific proposal: an object in a superposition of two places creates, in effect, two slightly different spacetimes, and the energy cost of that mismatch grows with the object's mass. For an atom it is negligible and the superposition can last as long as you like. For something big enough, the mismatch becomes intolerable in a measurable time and the superposition collapses on its own — no observer, no measurement, just gravity refusing to let a massive thing be in two places. This is why, in his view, a cat is never both alive and dead: the cat is far too heavy.

It is a real, falsifiable claim, with a formula that says roughly how heavy and how long. And the experiment just described is the beginning of a way to test it. A rubidium atom in superposition for milliseconds is many orders of magnitude too light and too brief to see any Penrose effect — the paper says so plainly, and the result is entirely consistent with ordinary quantum mechanics. But the instrument is a template. The stated next step is to do the same thing with nanodiamonds: particles of a few billion atoms, held in superposition and dropped, with the phase read out the same way. Push the mass and the time up, rung by rung, and either quantum mechanics keeps working — which would eventually rule Penrose's idea out — or at some rung the interference disappears when nothing else explains it, which would be the discovery of the century.

I find something admirable in a ninety-five-year-old Nobel laureate co-authoring the paper that starts a ladder whose top rung is designed to test his own conjecture. That is what the enterprise is supposed to look like. Most of us, by that age, are defending the earlier work.

What Would I Watch For?

The nanodiamond version. The group says the work is already under way at Ben-Gurion. The difficulty is not the theory but the engineering — holding a particle of billions of atoms in a superposition long enough to fall, without a single stray gas molecule or photon collapsing it for mundane reasons that have nothing to do with gravity. If a result appears, the first question to ask is whether the loss of interference was ruled out as ordinary decoherence before anyone called it Penrose. Any honest paper will spend most of its length on that.

Where I Could Be Wrong

I am a reader here, not a physicist, and I may be overstating the novelty. The line between "first measurement of the quantum phase of a freely falling object" — the authors' claim — and the fifty years of gravity-sensitive interferometry before it is a line experts will draw more finely than I have. The press releases say "first time"; the paper's own framing is that the equivalence principle held in a new regime, which is a smaller and more defensible statement. I have tried to report the smaller one. And on Penrose: the fact that he co-authored this does not mean he expects the nanodiamond experiment to vindicate him. It means he expects it to settle the question, which is the better reason to build it.

Sources

1. Ben-Gurion University of the Negev, University of Ulm, University of Oxford et al. Science Advances, 2 September 2026. DOI 10.1126/sciadv.aec8045.
2. University of Oxford, Department of Physics. Scientists observe Einstein's gravity in the quantum world. September 2026. physics.ox.ac.uk
3. Phys.org. Scientists observe Einstein's gravity in the quantum world. September 2026. phys.org
4. ScienceDaily. Scientists observe Einstein's gravity in the quantum world for the first time. 7 September 2026. sciencedaily.com
5. Colella, R., Overhauser, A. W. and Werner, S. A. Observation of Gravitationally Induced Quantum Interference. Physical Review Letters 34, 1472 (1975) — the neutron experiment this one is usually measured against.