A muon survives for roughly 2.2 microseconds. That sliver of time is the entire window a team at ETH Zurich and the Paul Scherrer Institute has to assemble an atom around one, fire it upward, and record how Earth’s gravity bends its path.
Should the experiment work, it would mark the first occasion on which anyone has checked whether gravity treats a second-generation particle exactly the same way it treats the ordinary matter that you and I are built from.
“We have taken an important step towards carrying out an exciting experiment on this topic,” said Anna Soter, professor of physics at ETH Zurich. “We want to measure the gravitational interaction of the muon.”
Three generations, and nobody can say why
People, planets and very nearly every object you have ever handled are assembled from protons, neutrons and electrons. Physicists file those under the first generation of matter. Two further generations exist, made from heavier particles, and the muon sits in one of them: a heavyweight relative of the electron, parked in the second generation.
All three are described by the Standard Model. What the Standard Model does not do is explain why three of them exist.
“But we physicists do not yet understand why these additional generations exist at all in the first place,” Soter said. “And why are there three in total?”
That opens an uncomfortable door. If nobody can account for why the heavier generations are there, nobody can state with confidence that they fall the way the lighter ones do.
A law of falling that has never been tested here
Release two objects at the same point in a gravitational field and they descend at an identical rate. Galileo Galilei and Isaac Newton established that centuries ago, and Albert Einstein folded it into his theory of gravity as the equivalence principle, which binds gravitational mass to inertial mass.
But every demonstration to date has relied on ordinary matter or on first-generation antimatter. That is the hole in the record. Watching how muonium falls would be the first such test carried out on a second-generation particle, and muonium is the chosen vehicle precisely because it carries no charge.
PSI’s large particle accelerator churns out muons along with their antiparticles. Bind a positively charged antimuon to a negatively charged electron and the result is muonium, a neutral atom.
“The exotic muonium is very well suited to this because it is a neutral atom,” Soter said. “After all, to make something fall, you need something neutral.”
The neutrality is not a stylistic preference. Set against electromagnetism, gravity is a weakling, and with a charged particle any gravitational signal would be swamped by stray electromagnetic fields well before the apparatus could register it.
The obstacle that kept holding things up
The brief lifetime is only the first problem. The second is that earlier production techniques churned out muonium atoms travelling at a jumble of speeds in a jumble of directions — close to worthless when the effect you are chasing is this faint.
The PSI group’s solution was to chill the atoms.
“We have managed to produce the muonium atoms in a ‘cold’ state, which is what makes the gravity experiment possible in the first place,” Soter said. “In this case, ‘cold’ means that the atoms propagate at similar speeds, almost parallel to one another.”
A cannon built out of chemical potential
The technique is published in Nature Physics, and the working medium is helium chilled to just above absolute zero.
“In order to achieve this, we used superfluid helium that had been cooled close to absolute zero at minus 273 degrees Celsius,” said Jesse Zhang, lead author of the study. “Superfluid helium is what is known as a quantum fluid, in which the individual helium atoms lose their identity, and which does not tolerate any impurities within it.”
Antimuons from the accelerator are shot into a thin film of that helium, where they lose speed. When one encounters a free electron, the pair forms a muonium atom carrying positive chemical potential, and the liquid immediately expels it. At the surface that chemical potential converts into kinetic energy and hurls the atom vertically upward.
“So we’re using the chemical potential as an atomic cannon,” Zhang said.
The atoms must traverse the quantum liquid without colliding and at a velocity that can be predicted in advance. Any appreciable hold-up and they decay before the surface is ever in reach.
“For our experiments, we also rely on PSI’s particle accelerator, which generates the world’s most intense, continuous muon beams,” Soter said. “Thanks to this high-quality source, a great many muonium atoms can be produced.”
Gravity, read off an interference pattern
The detector is an interferometer, still being assembled. It exploits the wave-like behaviour of atoms to generate an interference pattern, and the planet’s pull ought to nudge that pattern by a minuscule amount. Quantify the shift and you have quantified how gravity acts on the muon.
The timeline is unassuming, and Soter makes no attempt to dress it up.
“We hope to be able to test the method for the first time with the atomic beam this year, and if all goes well, the actual gravity experiment should follow in two or three years’ time,” Soter said.
One secondary payoff deserves a mention. That same beam could underpin far more precise laser spectroscopy of muonium, tightening the figures for the muon’s mass and for fundamental physical constants — a separate long-term ambition for the group.
What an unexpected result would actually mean
Suppose muonium turns out to fall differently. Things would get interesting in a hurry.
“That would indeed be surprising, and, in addition to other theories, it could point to the existence of a fifth force,” Soter said.
The current tally of fundamental interactions stands at four: gravity, electromagnetism, the strong interaction and the weak interaction. A fifth has been floated on numerous occasions. Not one candidate has ever been confirmed, and chasing one is not the purpose of this experiment.
“I am completely open-minded,” Soter said. “I simply want to measure, for the first time, whether the equivalence between gravitational and inertial mass also applies to the second generation of particles — this alone is quite an inspiring piece of work.”
Funding comes from the National Centre of Competence in Research Muoniverse. For anyone looking for a milestone to mark, it is this year’s first beam test — nothing else on the schedule moves until the atomic cannon shows it can fire.