Researchers from Ben-Gurion University of the Negev, Germany's University of Ulm and the University of Oxford have directly observed a long-predicted gravitational effect on a freely falling quantum object. The experiment does not solve the century-old conflict between quantum mechanics and gravity, but it provides a remarkable experimental bridge between the two theories.

Modern physics rests on two extraordinarily successful foundations. Einstein's general theory of relativity describes gravity, planets, stars, black holes and the evolution of the Universe. Quantum mechanics explains atoms, electrons and the microscopic world.

The problem is that physicists still do not possess a fully consistent theory describing situations in which gravity and quantum behaviour must operate simultaneously.

“Both theories work with extraordinary precision.”

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That is exactly the frontier the new experiment attempts to probe.

Galileo's falling object — but quantum

At the centre of the experiment is an apparatus the researchers call the Quantum Galileo Interferometer, a reference to Galileo's famous investigations of falling objects.

Instead of dropping cannonballs from a tower, however, the team used clouds of rubidium atoms cooled to temperatures just above absolute zero.

At such temperatures, atoms can display their quantum nature extremely clearly. Using microwave pulses, the researchers placed the atoms into a state known as quantum superposition — effectively allowing the quantum wave associated with an atom to follow two different paths simultaneously.

One part of the atomic wave was held stationary using carefully controlled magnetic fields. The other was pushed upwards and then released, allowing it to fall freely under Earth's gravity much like a ball tossed into the air.

The two parts were later brought together again.

Because quantum waves interfere with one another, the researchers could detect a tiny difference in their quantum phase caused by the period of free fall. That measured change agreed with the prediction obtained by applying Einstein's equivalence principle to the quantum system. (

It is the first direct measurement, the researchers say, of this predicted quantum phase produced by free fall.

Einstein enters the quantum world

Einstein's equivalence principle lies at the heart of general relativity.

In its simplest form, it says that someone falling freely under gravity should locally experience weightlessness. Imagine standing inside a lift whose cable suddenly disappeared: both you and the lift would accelerate downwards together, making you temporarily feel as though gravity had vanished.

This principle has been tested to extraordinary accuracy using ordinary matter.

Quantum objects are different. A quantum particle can behave like a wave, exist in superposition and effectively take more than one path at the same time. The question therefore becomes considerably stranger: what does Einstein's free fall mean when an object is simultaneously following different quantum histories?

The new experiment indicates that, at least under the conditions tested, Einstein's principle and quantum mechanics remain compatible.

Professor Ron Folman of Ben-Gurion University, the study's lead author, describes the significance as bringing a difficult experiment together with one of physics' most fundamental theoretical questions: how gravity and quantum theory might eventually be incorporated into a single understanding of nature.

What the experiment did not prove

The result needs an important qualification.

It does not prove that gravity itself is quantum.

Nor has the team produced the long-sought theory of quantum gravity that could reconcile general relativity with quantum mechanics.

What it demonstrates is narrower but still important: the behaviour predicted when Einstein's equivalence principle is applied to a quantum wave was observed experimentally.

This distinction matters because some of the biggest mysteries in physics exist precisely where the two theories collide.

General relativity predicts phenomena such as black holes, where enormous concentrations of matter curve space and time. Quantum mechanics dominates at extremely small scales. Near the centre of a black hole — and perhaps during the first moments after the Big Bang — both should matter simultaneously, yet our current theories cannot comfortably describe what happens.

Penrose and the next experiment

One of the study's co-authors is Oxford mathematician and physicist Sir Roger Penrose, winner of the 2020 Nobel Prize in Physics.

Penrose has long argued that sufficiently massive objects placed into quantum superpositions might eventually reveal a breakdown of conventional quantum mechanics — potentially because gravity itself causes the superposition to collapse.

The rubidium atoms in the present experiment were far too light, and their quantum states lasted for too little time, to test that idea.

But this may be only the beginning.

The researchers say their methods could eventually be extended to much heavier objects. Experiments involving nanodiamonds are already being pursued at Ben-Gurion University to explore precisely this frontier. (Oxford University)

That is where the story becomes particularly intriguing.

Physics has spent more than 100 years living with two different descriptions of reality: Einstein's smooth, curved spacetime and the uncertain, probabilistic world of quantum mechanics.

The Quantum Galileo experiment has not united them.

But for one tiny atom falling toward Earth, Einstein and quantum physics have now been placed in the same experiment — and, for the moment at least, they agree.