All bodies in vacuum fall with the same acceleration: this principle, formulated by Einstein in general relativity, underpins our understanding of gravity. Yet so far it has been experimentally confirmed only for particles of ordinary matter. Now, muonium takes the stage — an exotic atom in which the nucleus is replaced by an antimuon (the antiparticle of the muon), while the electron shell is made of an ordinary electron. This hybrid of matter and antimatter is extremely unstable: its lifetime is only about two microseconds. Forming a dense and uniform beam from such atoms was long considered science fiction.

The key to the solution was superfluid helium. At temperatures close to absolute zero, liquid helium transitions into a superfluid state — a quantum liquid devoid of internal friction. The experiment uses a thin film of such helium, which acts as a factory-catapult. When high-energy muons pierce this film, they strip electrons from helium atoms, and these electrons fly out as a single compact group — a "plug." The speed of the resulting particle swarm is about 2180 m/s, and all particles move at nearly the same velocity, like a well-coordinated shot.

The main goal of these efforts is to measure, for the first time, the gravitational acceleration of muonium in Earth's gravitational field with a precision of one percent. If the result deviated from the predictions of general relativity, it would point to a fundamental crack in modern physics. Along the way, the cold, well-collimated muonium beam will allow a precise determination of the muon mass and tests of theoretical models using spectroscopy.