What happens when quantum mechanics and relativity meet?

Measuring the same thing in free fall adds another layer of complexity. “Drop a stone and after one second it is 5 meters below you,” Folman says. The question asked by the team was how to bring a wave packet that has fallen back into contact with the one that stayed behind—the same atom on the other path—so that the two can interfere. Finding the solution took his team years, even though in hindsight it sounds rather obvious. Grabbing a falling particle and somehow bringing it back to where it started was out of the question. “Even if you could do it with some tweezer or some trap, this very violent action would create so much noise that we wouldn’t have known what is fake news and what is really the effect that we want to measure,” Folman joked. So the team figured the right way to do it would be to put an atom into a superposition of paths, where one path sees the atom thrown upward and has gravity return it to its original location. The other path isn’t really a path at all; the atom just stays where it started. “You have to shoot one with a cannon, into a ballistic motion like artillery,” Folman says. But that was just the start of the problems. The challenge, obviously, is designing a cannon that may or may not affect an atom (technically, it only affects one wave packet of the atom without affecting the other one). Then there is the issue of keeping the system in a quantum state. Interference only occurs when there is no way, even in principle, to tell which path the particle took—every measurement destroys the quantum superposition and forces the particle to behave like a localized classical object. Unfortunately, a particle returning from a ballistic flight is moving fast, while its stationary twin is not. That difference in speed is itself information about its path, so no interference pattern would appear. Atom interferometers have existed for 30 years, but none could accomplish what Folman and his colleagues wanted to do. So, they built a new one called the Quantum Galileo Interferometer (QGI). “We were able to show that the behavior of a quantum system still follows the principle of equivalence,” Folman says. “This has never been shown before—that the equivalence principle can work in a quantum system in a superposition.” In the end, the measured phase of the quantum wave sits 2.5 percent from the theoretical prediction. The result, the team argues, is a checkpoint on the border between Einstein’s relativity and quantum mechanics—two great theories in physics that stubbornly refuse to work together. “This [incompatibility] is such a problem for physicists that very smart people say that it hints that at least one of the two theories is fundamentally wrong,” Folman says. Unfortunately, the QGI did not bring us much closer to figuring out which one it might be. But Folman and his lab already have a road map they hope will get us there one day. The place to look for the answers, Folman argues, is in heavier masses. Today, his lab is building a larger version of the QGI experiment that puts nanodiamonds into the same superposition—objects 10 orders of magnitude heavier than atoms, massive enough to warp spacetime around themselves. “If we put the same nanodiamond in two places, we actually have a superposition of two different curvatures of spacetime,” Folman says. And this, according to Penrose’s hypothesis, developed independently by the Hungarian physicist Lajos Diósi, is where quantum mechanics should break down. “Roger Penrose says that the Universe cannot sustain a superposition of curved spacetime. That this should collapse,” Folman says. Watching that collapse happen, or fail to, he says, would be the most direct test yet of whether gravity is quantum. The team hopes that putting nanodiamonds in superposition will become possible within the next four or five years. “It’s a very, very challenging project,” Folman says. Science Advances, 2026. DOI: 10.1126/sciadv.aec8045