Physics professor Anna Soter and her team at ETH Zurich and the Paul Scherrer Institute (PSI) in Villigen are exploring whether gravity affects all particles in the universe the same way, including rare or exotic types of matter. Their research focuses on the muon, a particle similar to the electron but much heavier. The standard model of particle physics categorizes matter into generations, but scientists have yet to explain why there are three generations of particles. A key principle in Einstein's theory of gravity is the universality of free fall, meaning all objects fall at the same rate in a gravitational field, regardless of their composition. This has been tested with ordinary matter and first-generation antimatter. Now, the team is studying muonium, a neutral atom made from an antimuon and an electron. By observing how muonium falls, they aim to see if gravity behaves the same for second-generation particles as it does for ordinary matter. This is important because muonium is neutral, meaning it is less affected by electromagnetic forces and can be more easily influenced by gravity alone. Creating muonium atoms in a "cold" state—where they move at similar speeds—was a major breakthrough. The team used a new method involving superfluid helium cooled near absolute zero. Antimuons produced at PSI are shot into a thin layer of this superfluid, where they slow down. When an antimuon meets a free electron in the helium, a muonium atom forms and is expelled from the liquid. The energy from this chemical process turns into motion, allowing the atom to rise vertically out of the helium. The researchers are now building an interferometer to measure how gravity affects the muonium beam. By using the wave-like properties of the atoms, they can create an interference pattern. A slight shift in this pattern, caused by Earth's gravity, would reveal how gravity influences muons. They plan to test this setup with the atomic beam this year, with the full gravity experiment potentially starting in two to three years. The new muonium beam could also open the door to experiments in laser spectroscopy, helping scientists better understand the muon's mass and other fundamental constants. While the team is not primarily looking for a fifth force, if gravity behaves differently on muonium than on ordinary matter, it could hint at new physics. Soter's goal is to determine, for the first time, whether the equivalence between gravitational and inertial mass holds true for second-generation particles.