The latest attempt to bring quantum physics and gravitation into the same theoretical framework comes from a team led by Dr. Benjamin Koch of the Vienna University of Technology. For decades physicists have worked with two models that are both highly successful yet fundamentally incompatible. Quantum theory describes the behaviour of particles with remarkable precision, while general relativity continues to explain gravity through spacetime curvature on planetary and cosmic scales. The difficulty is that the two systems do not merge seamlessly when pushed to extremes such as the early universe or conditions near a black hole. Dr. Koch and his collaborators have introduced a new approach that may help clarify where the theories diverge and how they might be reconciled.
Koch, B., Riahinia, A., & Rincon, A. (2025). Geodesics in quantum gravity. Physical Review D, 112(8), 084056. https://doi.org/10.1103/w1sd-v69d
Their study focuses on geodesics, which in traditional general relativity describe the paths followed by freely moving objects. These paths are the straightest possible lines within curved spacetime. When the mass of a star or planet bends the geometry around it, objects move along curves that reflect these distortions. The team sought to examine what happens when the geometry itself shows quantum behaviour. In quantum mechanics a particle does not have a single definite position or momentum, but a distribution of possible values. By analogy, the researchers explored how a spacetime metric might behave if treated as a quantum object with its own uncertainty rather than a fixed background field.
Dr. Benjamin Koch of the Vienna University of Technology stated,
“We now need to analyze this in more detail, of course, but it gives us hope that by further developing this approach we can gain a new, and observationally well testable, insight into important cosmic phenomen such as the still unsolved puzzle of the rotation speeds of spiral galaxies.”
The process of quantizing the metric is mathematically challenging, but the group developed a new method tailored to the well known case of a static, spherically symmetric gravitational field. This idealised setting often stands in for objects like the Sun. With the metric treated quantum mechanically, they then calculated how a test particle would move within this environment. This required determining when they were allowed to replace the quantum operator with its expectation value and when doing so would erase important quantum structure. The outcome of this work is what the researchers call the q-desic equation, a quantum corrected version of the classical geodesic equation.
The q-desic equation predicts that particle trajectories in a quantum spacetime will differ slightly from those expected under classical general relativity. Under ordinary gravitational conditions these differences are extremely small and far beyond experimental reach. For typical situations, such as the Earth orbiting the Sun, the motion remains effectively unchanged. However, the picture shifts when the cosmological constant is included. This constant is associated with the energy driving the expansion of the universe, often labelled dark energy. With this term added into the quantum corrected framework, the deviations grow markedly at extremely large distance scales.
These discrepancies become noticeable at distances on the order of ten to the power of twenty one metres, where classical general relativity already faces unresolved questions related to galaxy rotation curves and large scale cosmic behaviour. Although these corrections would not influence everyday astronomical systems, they may become relevant in regions where gravitational models struggle to explain observed motion. This opens the possibility that quantum gravity effects, long assumed to be hidden at the smallest scales, might instead leave subtle signatures on the largest structures in the universe.
The broader significance of the work lies in its potential to supply a measurable criterion for comparing different theories of quantum gravity. Researchers often describe the search for quantum gravity as a situation with many competing ideas but no clear observational handle to distinguish among them. Dr. Koch likens it to the Cinderella story in which the true identity is uncovered only when the correct slipper is found. The q-desic framework may serve as one such candidate, offering a way to check how various theoretical proposals align with the behaviour of matter under quantum corrected curvature.
The authors emphasise that the results are preliminary and confined to simplified conditions. Real astrophysical systems involve rotation, non static fields and interactions that complicate the picture. Even so, the mathematical structure of the q-desic equation provides a new direction for examining how quantum fluctuations of spacetime might influence motion. It encourages rethinking how classical assumptions are applied in semiclassical gravity and highlights where expectation values may erase important physics.
As observational astronomy becomes increasingly sensitive, with surveys mapping the universe at unprecedented resolution, frameworks like this one provide guidance for where to look for possible quantum gravitational effects. While the study does not yet unite the quantum and gravitational worlds, it offers a clearer bridge between them and supplies tools that may eventually help determine which theories capture nature most accurately. For a field seeking even modest experimental footholds, the appearance of a potentially testable signature marks meaningful progress.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
