Researchers at ETH Zurich and the Paul Scherrer Institute (PSI) in Villigen, Switzerland, have announced a significant milestone in a groundbreaking experiment designed to test one of the most fundamental tenets of modern physics: the universality of free fall. By focusing on the muon, a second-generation particle often described as the heavier cousin of the electron, the team aims to determine whether gravity interacts with all types of matter in an identical fashion. This research, led by Professor Anna Soter and her colleagues, could potentially challenge the Standard Model of particle physics or provide the first direct evidence of a "fifth force" of nature.
The experiment focuses on an exotic form of matter known as muonium—a neutral atom consisting of a positively charged antimuon and a negatively charged electron. While the Weak Equivalence Principle has been tested extensively with ordinary matter and, more recently, with first-generation antimatter, the behavior of second-generation particles in a gravitational field remains one of the great unobserved frontiers of science. The results of this study, expected to yield preliminary data within the next few years, could either solidify Albert Einstein’s legacy or necessitate a radical revision of how we understand the fundamental forces of the universe.
The Standard Model and the Mystery of Generations
To understand the significance of the ETH Zurich and PSI experiment, one must first look at the landscape of particle physics. The Standard Model, the prevailing theoretical framework for the subatomic world, classifies matter into three distinct "generations." The first generation consists of the particles that form the stable universe we interact with daily: the up and down quarks (which make up protons and neutrons) and the electron.
The second and third generations consist of particles that are essentially heavier replicas of the first. The muon, discovered in 1936, belongs to the second generation. It possesses the same charge and spin as an electron but is approximately 207 times more massive. These heavier generations are unstable and decay into first-generation particles almost instantly.
"We physicists do not yet understand why these additional generations exist at all in the first place," explains Professor Soter. "And why are there three in total?" This lack of clarity regarding the purpose of higher-generation particles leads to a deeper question: Do these heavier particles respond to gravity in the same way as the light ones? While the Standard Model assumes they do, this has never been empirically verified for the muon.
Einstein’s Equivalence Principle Under Scrutiny
The core of the experiment is the Weak Equivalence Principle (WEP), which posits that the trajectory of any point mass in a gravitational field depends only on its initial position and velocity, and is independent of its composition and structure. This concept traces its lineage back to Galileo Galilei’s legendary experiments at the Leaning Tower of Pisa and was later formalized by Isaac Newton.
In the early 20th century, Albert Einstein elevated this principle to a cornerstone of General Relativity, linking gravitational mass (the "charge" of gravity) with inertial mass (an object’s resistance to acceleration). If the equivalence principle holds, a muonium atom should fall at exactly the same rate as a hydrogen atom or a lead weight. However, if even a minute discrepancy is detected, it would imply that gravity is not as universal as Einstein predicted.
Until now, high-precision tests of the equivalence principle have been limited to first-generation matter—specifically atoms like rubidium or macroscopic objects like the Earth and Moon. Recent experiments at CERN have begun testing the principle with antihydrogen (first-generation antimatter), but muonium offers a unique opportunity to probe the second generation.
Technical Challenges: Neutrality and the Race Against Time
Testing gravity on subatomic particles is notoriously difficult because gravity is the weakest of the four fundamental forces—the others being electromagnetism, the strong nuclear force, and the weak nuclear force. For a single particle, the electromagnetic pull from a stray static charge can be trillions of times stronger than the Earth’s gravitational pull.
"The exotic muonium is very well suited to this because it is a neutral atom," Soter explains. By pairing a positive muon with a negative electron, the researchers create a system with zero net charge, effectively "shielding" the particle from electromagnetic interference and allowing the faint signal of gravity to be isolated.
However, muonium presents a significant temporal obstacle. The muon is notoriously short-lived, with a mean lifetime of only 2.2 microseconds (2.2 millionths of a second) before it decays into an electron and two neutrinos. To measure gravity, the researchers must create the atoms, launch them into a controlled beam, and measure their descent before they vanish.
The Superfluid Helium Innovation: An Atomic Cannon
The breakthrough that has made this experiment feasible involves the use of superfluid helium. Traditionally, muonium atoms were produced with a wide range of speeds and directions, making them impossible to use for precise gravitational measurements. The team at PSI, led by study author Jesse Zhang, developed a method to produce a "cold" and highly directed beam of muonium.
The process takes place in a cryostat cooled to near absolute zero—minus 273 degrees Celsius. At these temperatures, helium becomes a superfluid, a quantum state of matter with zero viscosity. When antimuons from PSI’s massive particle accelerator are injected into a thin layer of this superfluid, they slow down and capture electrons to form muonium.
Because superfluid helium does not tolerate impurities, the newly formed muonium atoms are pushed toward the surface by their chemical potential. "We’re using the chemical potential as an atomic cannon," says Zhang. As the atoms reach the surface, they are ejected vertically upward in a nearly parallel beam at predictable speeds. This "cold" state is essential for interferometry, as it ensures the atoms remain coherent long enough for their path to be measured.
The Interferometer: Measuring the Invisible Shift
To detect the influence of gravity, the ETH Zurich and PSI team is constructing a specialized matter-wave interferometer. This device utilizes the wave-particle duality of matter. As the muonium atoms travel through the vacuum, they pass through a series of gratings that split and then recombine the atomic "waves," creating an interference pattern.
If gravity is acting on the muonium, it will cause the atoms to fall slightly as they travel, leading to a vertical shift in the interference pattern. Because the distance the atoms fall in 2.2 microseconds is incredibly small—on the order of picometers—the interferometer must be extraordinarily sensitive.
"We hope to be able to test the method for the first time with the atomic beam this year," Soter notes. If the initial phase is successful, the full-scale gravity measurement is slated for two to three years from now. This timeline reflects the complexity of the instrumentation required to observe a force as weak as gravity on a timescale as short as a muon’s life.
Seeking the Fifth Force and Beyond
While the primary goal is to confirm Einstein’s equivalence principle, the physics community is keenly aware of the alternative: discovery. If the muonium atoms fall at a rate different from that predicted by General Relativity, it would be a "smoking gun" for physics beyond the Standard Model.
One possibility is the existence of a "fifth force." Currently, we recognize gravity, electromagnetism, and the two nuclear forces. However, many theories in high-energy physics, including string theory and various models of dark matter, suggest the existence of additional long-range forces that might interact differently with different generations of matter.
"That would indeed be surprising, and, in addition to other theories, it could point to the existence of a fifth force," Soter outlines. Even if no fifth force is found, the experiment will provide the most precise measurements to date of the muon’s mass and other fundamental constants through laser spectroscopy, which the new beam also enables.
A Global Effort in Fundamental Physics
The research at PSI is part of a broader global effort to probe the limits of our physical laws. At CERN in Geneva, experiments like ALPHA-g and GBAR are performing similar tests with antihydrogen to see if antimatter falls "up" or at a different rate than matter. The PSI experiment is complementary but unique, as it is the only one targeting second-generation particles.
The Paul Scherrer Institute is uniquely positioned for this work, housing the world’s most intense continuous muon source. This high-intensity beam allows the researchers to produce enough muonium atoms to achieve statistical significance despite the particles’ rapid decay.
As the team prepares for the first beam tests later this year, the scientific community remains watchful. Whether the experiment confirms that the second generation of matter obeys the same gravitational laws as the first, or reveals a hidden complexity in the fabric of spacetime, the results will mark a milestone in our quest to understand the universe.
For Professor Soter, the motivation remains purely scientific: "I am completely open-minded. 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."
Supported by the National Centre of Competence in Research (NCCR) "Muoniverse," this project stands at the intersection of cryogenic engineering, particle physics, and general relativity. If successful, it will provide a definitive answer to a question that has lingered since the discovery of the muon nearly a century ago: Does gravity truly see all matter as equal?
