Muons live for about 2.2 microseconds. That’s the entire window a team at ETH Zurich and the Paul Scherrer Institute has to build an atom out of one, launch it upward, and watch Earth’s gravity nudge it off course.
If they pull it off, it’ll be the first time anyone has checked whether gravity treats a second-generation particle the same way it treats the stuff you and I are made of.
“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.”
The generations problem nobody can explain
Protons, neutrons and electrons make up people, planets and nearly everything you’ve ever touched. Physicists file those under the first generation of matter. Two more generations exist, built from heavier particles, and the muon is one of them: a heavier cousin of the electron sitting in the second generation.
The Standard Model describes all three. It doesn’t explain why there are three.
“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?”
Which leads somewhere uncomfortable. If we can’t say why the heavier generations are there, we also can’t say with confidence that they fall the way lighter particles do.
Galileo’s law has never been checked on this
Drop two objects at the same spot in a gravitational field and they fall at the same rate. Galileo Galilei and Isaac Newton worked that out centuries ago, and Albert Einstein built it into his theory of gravity as the equivalence principle, tying gravitational mass to inertial mass.
Every demonstration so far has used ordinary matter or first-generation antimatter. That’s the gap. Measuring how muonium falls would be the first test on a second-generation particle, and muonium is the vehicle because it has no charge.
At PSI, a large particle accelerator produces muons and their antiparticles. Pair a positively charged antimuon with a negatively charged electron and you get 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.”
That neutrality isn’t a nicety. Gravity is feeble next to electromagnetism, and a charged particle would have any gravitational signal buried under stray electromagnetic fields long before the instrument saw it.
The part that kept stalling
Short lifetime is one obstacle. The other is that older production methods spat out muonium atoms at assorted speeds heading in assorted directions, which is close to useless when you’re hunting an effect this small.
The PSI group’s answer was to cool the atoms down.
“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 made out of chemical potential
The method appears in Nature Physics, and the working fluid is helium cooled to near 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 get fired into a thin layer of that helium and slow down inside it. When one meets a free electron, they form a muonium atom with positive chemical potential, and the liquid promptly rejects it. At the surface that chemical potential turns into kinetic energy and throws the atom straight up.
“So we’re using the chemical potential as an atomic cannon,” Zhang said.
The atoms have to cross the quantum liquid without collisions and at a speed you can predict. Any meaningful delay and they decay before they ever reach the surface.
“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.”
Reading gravity off an interference pattern
The measuring device is an interferometer, currently under construction. It uses the wave properties of atoms to produce an interference pattern, and Earth’s pull should shift that pattern by a tiny amount. Measure the displacement and you’ve measured how gravity acts on the muon.
The schedule is modest, and Soter says so.
“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.
There’s a side benefit worth flagging. The same beam could support far more precise laser spectroscopy on muonium, which would sharpen our numbers for the muon’s mass and for fundamental physical constants. That’s a separate long-term goal for the group.
What a null result would actually mean
Say muonium falls differently. Then things get interesting fast.
“That would indeed be surprising, and, in addition to other theories, it could point to the existence of a fifth force,” Soter said.
Physics currently counts four fundamental interactions: gravity, electromagnetism, the strong interaction and the weak interaction. A fifth has been proposed plenty of times. None has ever been confirmed, and hunting one isn’t what this experiment is for.
“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.”
The work is supported by the National Centre of Competence in Research Muoniverse. If you want a date to watch, it’s this year’s first beam test, because nothing else in the plan happens until the atomic cannon proves it can fire.
