Science News from research organizations Particles may not follow Einstein’s paths after all A team of physicists has developed a new way to explore one of science’s biggest challenges: combining quantum physics with gravity. Date: March 9, 2026 Source: Vienna University of Technology Summary: Physicists have long struggled to unite quantum mechanics—the theory governing tiny particles—with Einstein’s theory of gravity, which explains the behavior of stars, planets, and the structure of the universe. Researchers at TU Wien have now taken a new step toward that goal by rethinking one of relativity’s core ideas: the paths particles follow through curved spacetime, known as geodesics. By creating a quantum version of these paths—called the q-desic equation—the team showed that particles moving through a “quantum” spacetime may deviate slightly from the paths predicted by classical relativity. Share: Facebook Twitter Pinterest LinkedIN Email FULL STORY Large masses – such as a galaxy – curve spacetime. Objects move along a geodesic. If we take into account that space-time itself has quantum properties, deviations arise (dashed line vs. solid line). Credit: Oliver Diekmann, TU Wien One of the biggest unsolved challenges in modern physics is bringing together two powerful theories that describe very different parts of reality. Quantum theory explains the behavior of extremely small particles with remarkable precision. Einstein's general theory of relativity, on the other hand, describes gravity and the motion of planets, stars, and galaxies. Yet despite their success, these two frameworks still do not fully align. Physicists have proposed several possible ways to merge them into a single theory. Ideas such as string theory, loop quantum gravity, canonical quantum gravity, and asymptotically safe gravity all attempt to bridge the gap. Each approach has advantages and limitations. What researchers have lacked so far is a clear observable effect that experiments could measure to determine which theory best reflects how nature actually works. A new study from TU Wien may represent a step toward solving that problem. Searching for the "Slipper" of Quantum Gravity "It's a bit like the Cinderella fairy tale," says Benjamin Koch from the Institute for Theoretical Physics at TU Wien. "There are several candidates, but only one of them can be the princess we are looking for. Only when the prince finds the slipper can he identify the real Cinderella. In quantum gravity, we have unfortunately not yet found such a slipper -- an observable that clearly tells us which theory is the right one." To identify the right "shoe size," meaning a measurable way to test different theories, the researchers focused on a central concept in relativity called geodesics. "Practically everything we know about general relativity relies on the interpretation of geodesics," explains Benjamin Koch. A geodesic describes the shortest path between two points. On a flat surface, that path is simply a straight line. On curved surfaces, the situation becomes more complicated. For instance, traveling from the North Pole to the South Pole along Earth's surface follows a semicircle, which represents the shortest possible route on a sphere. Einstein's theory connects space and time into a single four dimensional structure called spacetime. Massive objects such as stars and planets curve this spacetime. According to general relativity, the Earth circles the Sun because the Sun's mass bends spacetime and shapes the path the Earth follows into an orbit. Creating a Quantum Version of Spacetime Paths The exact shape of these paths depends on something called the metric, which measures how strongly spacetime is curved. "We can now try to apply the rules of quantum physics to this metric," says Benjamin Koch. "In quantum physics, particles have neither a precisely defined position nor a precisely defined momentum. Instead, both are described by probability distributions. The more precisely you know one of them, the more fuzzy and uncertain the other becomes." Quantum theory replaces precise particle properties with mathematical objects known as wave functions. In a similar way, physicists can attempt to replace the classical metric of relativity with a quantum version. If this happens, spacetime curvature is no longer perfectly defined at every point. Instead, it becomes subject to quantum uncertainty. This idea creates extremely difficult mathematical problems. Benjamin Koch, working with his PhD student Ali Riahinia and Angel Rincón (Czech Republic), managed to quantize the metric using a new method for a specific but important case: a spherically symmetric gravitational field that remains constant over time. Such a model can describe systems like the gravitational field of the Sun. The researchers then calculated how a small object would move in this field when the metric itself is trea