Does gravity affect every type of particle in exactly the same way? Researchers at ETH Zurich and the Paul Scherrer Institute (PSI) in Villigen are preparing an experiment that could test that fundamental assumption using one of nature’s more unusual particles.
“We have taken an important step towards carrying out an exciting experiment on this topic,” says the professor of physics: “We want to measure the gravitational interaction of the muon.”
The familiar matter that makes up people, planets, and nearly everything around us consists of protons, neutrons, and electrons. Physicists classify these particles as belonging to the first generation of matter. Two additional generations also exist, made up of heavier particles. One of them is the muon, a heavier relative of the electron that belongs to the second generation.
Researchers at PSI can create muons and their antiparticles using a large particle accelerator. When a positively charged antimuon combines with a negatively charged electron, the pair forms a neutral atom known as muonium.
Why Physicists Want to Test Muons
The Standard Model of particle physics describes these different generations of particles, but it does not explain why nature has multiple generations in the first place.
“But we physicists do not yet understand why these additional generations exist at all in the first place,” says Soter. “And why are there three in total?”
That mystery raises another important question. Do the heavier particles of the second and third generations respond to gravity in exactly the same way as the lighter particles found in the first generation?
Testing Einstein’s Equivalence Principle
For ordinary matter, objects at the same location in a gravitational field fall at the same rate. Galileo Galilei and Isaac Newton recognized this universality of free fall centuries ago. It later became central to Albert Einstein’s theory of gravity through the equivalence principle, which connects gravitational mass with inertial mass.
So far, however, researchers have demonstrated this principle only with ordinary matter or first-generation antimatter. Measuring how muonium behaves under gravity would provide the first test involving a second-generation particle.
“The exotic muonium is very well suited to this because it is a neutral atom,” explains Soter: “After all, to make something fall, you need something neutral.”
Neutrality is essential because gravity is extremely weak compared with electromagnetism. If researchers tried to perform the experiment with a charged particle, stray electromagnetic fields could overwhelm the gravitational effect they are attempting to detect.
Muonium presents another major obstacle. Muons survive for only about 2.2 microseconds before decaying. Earlier methods also produced muonium atoms traveling at different speeds and in many directions, making them poorly suited for extremely precise gravity measurements.
Researchers at PSI have now found a way around that problem.
“We have managed to produce the muonium atoms in a ‘cold’ state, which is what makes the gravity experiment possible in the first place,” says Soter. “In this case, ‘cold’ means that the atoms propagate at similar speeds, almost parallel to one another.”
Superfluid Helium Creates a Controlled Muonium Beam
The team describes its new method for producing muonium in Nature Physics.
“In order to achieve this, we used superfluid helium that had been cooled close to absolute zero at minus 273 degrees Celsius,” explains 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.”
The process begins by directing antimuons from PSI’s accelerator into a thin layer of superfluid helium. Inside the helium, the particles slow down. When an antimuon encounters a free electron, the two form a muonium atom with positive chemical potential.
That chemical potential effectively drives the newly formed atom out of the liquid. When it reaches the surface, the chemical potential is converted into kinetic energy, giving the muonium atom a boost that sends it vertically upward.
“So we’re using the chemical potential as an atomic cannon,” explains Zhang.
The muonium atoms must pass through the quantum liquid without collisions and at a predictable speed. Their lifetime is so short that any significant delay would prevent them from reaching the surface before they decay.
“For our experiments, we also rely on PSI’s particle accelerator, which generates the world’s most intense, continuous muon beams,” says Soter. “Thanks to this high-quality source, a great many muonium atoms can be produced.”
Watching Gravity Shift an Atomic Pattern
The researchers are now building an instrument known as an interferometer to measure how Earth’s gravity affects the muonium beam.
The device takes advantage of the wave properties of atoms to produce an interference pattern. Earth’s gravitational pull should cause an extremely small shift in that pattern. Measuring the displacement would allow the researchers to determine how gravity acts on the muon.
“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,” as Soter relates.
The new beam could also make much more precise laser spectroscopy experiments with muonium possible. Those measurements could improve scientists’ understanding of the muon’s mass and fundamental physical constants, another long-term goal of the research group.
Could the Experiment Reveal a Fifth Force?
If muonium responds to gravity differently from ordinary matter, the implications could be profound.
“That would indeed be surprising, and, in addition to other theories, it could point to the existence of a fifth force,” as Soter outlines.
Modern physics recognizes four fundamental interactions: gravity, electromagnetism, the strong interaction, and the weak interaction. Scientists have repeatedly proposed the possibility of an additional fifth force, but no such force has ever been confirmed.
Discovering one is not the main objective of Soter’s experiment. The immediate goal is more fundamental: determining whether one of Einstein’s central principles also holds for a different generation of particles.
“I am completely open-minded,” she says. “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.”
This research is supported by the National Centre of Competence in Research Muoniverse.
