Scientists at ETH Zurich and the Paul Scherrer Institute are developing an experiment to measure how gravity interacts with muons, heavier relatives of electrons that belong to the second generation of matter. The research addresses a fundamental question in physics: does gravity affect all particle types identically, or do heavier particles respond differently to gravitational forces?
The experiment is designed to test Einstein’s equivalence principle, which states that objects fall at the same rate in a gravitational field regardless of their composition. While this principle has been verified with ordinary matter and first-generation antimatter, it has never been tested with particles from higher generations. Muonium, a neutral atom formed when a positively charged antimuon combines with a negatively charged electron, provides an ideal candidate for such testing. Neutrality is crucial because gravity is extremely weak compared with electromagnetism; charged particles would be overwhelmed by stray electromagnetic fields that would mask gravitational effects.
A major technical challenge is that muons decay within approximately 2.2 microseconds, requiring researchers to create and measure them rapidly. The team has overcome this obstacle by developing a novel method using superfluid helium cooled near absolute zero. Antimuons are directed into this quantum fluid, where they slow down and form muonium atoms with positive chemical potential. This chemical potential acts as an “atomic cannon,” propelling the newly formed muonium atoms vertically upward with consistent speeds and directions—essential for precise gravitational measurements.
The researchers plan to use an interferometer to detect minute shifts in atomic wave patterns caused by Earth’s gravitational pull. Initial testing with the atomic beam is expected this year, with the full gravity experiment potentially beginning within two to three years. If muonium responds to gravity differently from ordinary matter, the findings could suggest the existence of a fifth fundamental force beyond the four currently recognized interactions. Regardless of the outcome, the research promises to advance understanding of muon properties and fundamental physical constants.
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