Physicists at ETH Zurich and the Paul Scherrer Institute in Switzerland have created a controlled beam of muonium, an exotic atom made of a muon and an electron. It removes a major technical barrier to testing whether gravity acts on second-generation particles exactly as Einstein's theory predicts. The work was published in Nature Physics.
The team produced muonium inside superfluid helium cooled to about 0.2 kelvin, close to absolute zero. The quantum liquid expels the atoms vertically into vacuum at similar speeds, which gives the narrow beam an atom-interference experiment needs. The challenge is speed: a muon lives only about 2.2 microseconds.
Initial tests of the beam are expected this year, with the actual gravity experiment planned in two to three years. Any unexpected difference from Einstein's predictions would be a major surprise and could point to new physics, including a hypothetical fifth force. The beam could also enable more precise laser spectroscopy of muonium, improving measurements of the muon's mass and other fundamental constants.