Gravity attracts. Anyone who has ever dropped a cup knows that. But in the quantum world, where a particle can be in several places at once, even this truth begins to crack. Physicists have proposed an experiment in which a source of gravity, smeared across two trajectories, forces a test mass to move away after a clever measurement. An invisible conductor making the orchestra play in reverse: that is one way to describe the effect.

The setup is elegant. Two nanocrystals, one a thousand times heavier than the other, are separated by a distance of 20 micrometers (one fifth the thickness of a human hair). The heavy source passes through a Stern-Gerlach interferometer, a device that splits particle trajectories according to spin, and ends up in a quantum superposition: it is simultaneously closer to the test mass and farther away. The source is then postselected, meaning events are filtered by the outcome of a subsequent measurement. A rare outcome is chosen in which the weak value of the projection onto the near trajectory becomes anomalously large and negative: the particle seems to be in a place where it is "less than absent." It is precisely this paradoxical effect that flips the sign of the gravitational impulse.

The probability of such an outcome is small: only 2.5×10⁻⁵ per attempt. But the payoff is enormous. The test mass receives an impulse directed away from the source and over 10 seconds shifts by 1 angstrom, the size of an atom. To catch a repulsion event, about 4×10⁷ repetitions of the experiment will be needed. The numbers sound daunting, but modern techniques for nanoparticle levitation and precision measurement are already approaching such milestones.

Until now, the quantum nature of gravity has been sought through entanglement: if two massive bodies exchange gravitons, their states should become entangled. Here the path is different. There is no need to create an entangled state between the masses; interference of amplitudes within the source itself is enough. The success of the experiment would be direct evidence that the gravitational field can exist in a superposition of states, which means spacetime obeys the laws of quantum mechanics. This is a step toward solving one of the great puzzles of physics: how to reconcile general relativity with quantum theory.

The invisible conductor making the orchestra play in reverse is a metaphor that is now taking on flesh. If the experiment succeeds, we will see for the first time how the quantum nature of gravity manifests itself not in equations but in the motion of real particles. And then, perhaps, it will become clear why our classical world, with its inexorable attraction, is only one facet of quantum reality.