Scientists Observe Einstein's Gravity Effect in a Falling Quantum Object for First Time

An international team including Nobel Prize-winning physicist Sir Roger Penrose has directly observed a long-predicted gravitational effect in a falling quantum object for the first time. Led by Ben-Gurion University of the Negev, the University of Ulm and the University of Oxford, the peer-reviewed study appeared September 2 in Science Advances.

Modern physics rests on two frameworks never fully unified: quantum mechanics, describing the unusual behavior of atoms and other tiny objects, and Einstein's theory of gravity, explaining falling bodies and the universe's large-scale structure. The experiment tested Einstein's equivalence principle -- gravity effectively vanishes for an observer in free fall, so someone in a falling lift feels weightless -- by extending it to a quantum object, harder to test than ordinary matter since quantum objects behave like waves following more than one path at once.

The team built an instrument called the Quantum Galileo Interferometer, splitting an atom's quantum wave into two paths. At Ben-Gurion University, researchers including PhD student Or Dobkowski cooled rubidium atom clouds near absolute zero on an atom chip, used magnetic pulses to hold one part of the wave still while the other fell freely, then recombined both to measure the quantum-phase difference.

The measured phase matched the value predicted when Einstein's equivalence principle is applied to a quantum wave -- the first direct measurement of this predicted quantum phase from a freely falling object. Lead author Professor Ron Folman of Ben-Gurion University said the experiment offers hints toward unifying gravity and quantum theory; co-author Professor Vlatko Vedral of Oxford said it shows quantum mechanics' predictions hold.

The result does not unify gravity and quantum mechanics, nor show gravity itself is quantum, and does not disprove co-author Sir Roger Penrose's proposal that quantum mechanics could break down in sufficiently massive, long-lived superpositions, since the experiment used smaller objects. Researchers hope to extend the technique to heavier objects including nanodiamonds, already under way at Ben-Gurion University.