For decades, physicists have described the universe’s interactions through four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. But recent research suggests there might be more lurking within the heart of atoms.
A team of physicists from Germany, Switzerland, and Australia has placed new constraints on a potential “fifth force” that could subtly influence interactions between electrons and neutrons inside atoms. Their work focused on precise measurements of atomic transitions in five isotopes of calcium, each differing in neutron number.
Using a technique called a King plot, which maps how these transitions should behave according to the Standard Model, the researchers detected small deviations that might hint at an unknown force. This force could be mediated by a hypothetical particle, sometimes referred to as a Yukawa particle, with a mass somewhere between 10 and 10 million electronvolts.
While these findings are tantalizing, they are far from definitive. The deviations could stem from unknown factors within established physics or truly signal new physics beyond the Standard Model. Further experiments and improved theoretical models will be essential to unravel this mystery.
If confirmed, the discovery of a fifth fundamental force could revolutionize our understanding of the universe, shedding light on elusive phenomena like dark matter and the quantum nature of gravity.
The study was published in Physical Review Letters in June 2025.