Galileo Galilei observed it, Isaac Newton formulated it and Albert Einstein made it a cornerstone of his general theory of relativity: The equivalence principle states that gravity and accelerations have the same effect on all objects in the absence of other forces – regardless of their mass and composition. This means, for example, that a feather and a lead ball fall at the same speed in a vacuum.
Looking for a connection: Einstein and quantum mechanics
But does the equivalence principle also apply in the quantum world? “So far, the two cornerstones of physics – quantum mechanics and general relativity – have stood side by side in an incompatible way,” explain Or Dobkowski from Ben-Gurion University of the Negev and his colleagues. “Connecting them remains one of the most important open questions in physics.” Physicists are therefore intensively searching for interfaces between the two.
Theoretical models predict that free fall should influence a quantum wave packet in a certain way – provided that the equivalence principle also applies in the quantum world. Compared to a static, unmoving quantum particle, a quantum object falling under the influence of Earth’s gravity would have to show a certain phase shift afterwards. “Surprisingly, this phase has never been measured before,” report the physicists.
Entangled wave packets in free fall
Dobkowski and his colleagues have now repeated this experiment. To do this, they cooled a cloud of rubidium atoms to just above absolute zero and placed them in a vacuum chamber. Using microwaves, they then brought the atoms into a state of quantum physical entanglement. In quantum physics, the atoms can also be described as entangled wave packets.
Now the drop test followed, for which the physicists developed a new measuring instrument. This so-called Quantum Galileo Interferometer (QGI) uses an atom chip to generate precisely dosed magnetic pulses and magnetic gradients. At the same time, it can compare the phases of the quantum physics wave packets using interference – and thus check the theoretical predictions.
For the test, one of the entangled wave packets remained suspended in the magnetic field. This wave packet served as a reference. The other wave packet was first lifted up by a magnetic pulse and then dropped. This quantum object was in free fall for a short time before it returned to the level of the reference wave packet. The physicists now compared the phases of both wave packets.
Schematic setup of the experiment. Of two wave packets entangled in quantum physics, one will remain static, the other will go through a ballistic curve and in the second half a free fall. — © Dobkowski et al./Science Advances, CC-by-nc 4.0
Phase measurement confirms the equivalence principle in quantum
The measurements actually showed a tiny change, the value of which corresponded to what was theoretically predicted. “We have confirmed with high reliability the phase that speaks for the validity of the equivalence principle for a freely falling wave packet,” report Dobkowski and his colleagues. “Our experiment therefore acts as a fundamental test at the interface between quantum theory and gravity.”
The physicists conclude: “Nature seems to be able to reconcile quantum mechanics and the equivalence principle – at least at these low masses and energies,” they explain. However, it is still unclear whether this also applies when the entangled quantum objects are heavier and more energetic. Dobkowski and his team see their experiment as an important step towards testing this in the future.
A delicate link between separate worlds
Even after this experiment, quantum mechanics and Einstein’s general theory of relativity remain two separate “pillars” of our physical worldview. But the fact that the equivalence principle can also be transferred to the quantum world shows at least some points of contact – and possible starting points for the long-sought connection between the two, as the team explains.
“Our experiment provides evidence of how such a unification of these pillars of modern physics could be achieved in an overall theoretical concept,” says senior author Ron Folman from Ben-Gurion University.
Source: Or Dobkowski (Ben-Gurion University of the Negev, Be’er Sheva) et al., Science Advances, 2026; doi: 10.1126/sciadv.aec8045