By Weizmann Institute of Science August 10, 2025
Collected at: https://scitechdaily.com/scientists-may-have-found-the-ultimate-dark-matter-detector/
Physicists are harnessing thorium-229’s unusual nuclear properties to develop an ultra-precise “nuclear clock” capable of detecting forces 10 trillion times weaker than gravity.
Such sensitivity could make it the ultimate tool for spotting the elusive influence of dark matter, which subtly distorts the properties of ordinary matter.
The Long Quest for Dark Matter
For nearly 100 years, researchers worldwide have been attempting to uncover the nature of dark matter, an invisible substance believed to comprise roughly 80 percent of the universe’s total mass. This mysterious substance is essential for explaining many observed cosmic phenomena, yet it remains undetected in any direct experiment.
Scientists have explored a wide range of approaches to find it, from attempting to create dark matter particles in high-energy particle accelerators to searching for faint cosmic radiation it might emit. Despite these efforts, its core characteristics are still largely unknown. While it does not interact with light, dark matter is believed to subtly affect the behavior of visible matter, but in ways so delicate that current instruments cannot measure them directly.
Nuclear Clocks: A New Frontier in Detection
Experts suggest that building a nuclear clock, a device that measures time based on the oscillations of an atomic nucleus, could make it possible to detect dark matter’s influence. Such a clock would be so precise that even the smallest fluctuations in its timing could signal the presence of dark matter. In a significant step forward, research teams in Germany and Colorado achieved an important milestone last year by using the radioactive isotope thorium-229 in the early stages of constructing such a clock.
When scientists in Professor Gilad Perez’s theoretical physics group at the Weizmann Institute of Science learned about this progress, they saw a chance to contribute to the search for dark matter without waiting for a fully operational nuclear clock. Working together with German researchers, they developed and published a new strategy for detecting how dark matter might subtly alter the properties of the thorium-229 nucleus in Physical Review X.
The Unique Properties of Thorium-229
Much as pushing a child on a swing requires the right timing to maintain a smooth, consistent motion, an atomic nucleus also has an optimal oscillation frequency, known in physics as its resonance frequency. Radiation at precisely this frequency can cause the nucleus to “swing” like a pendulum between two quantum states: a ground state and a high-energy state.
In most materials, this resonance frequency is high, requiring strong radiation to excite the nucleus. But in 1976, scientists discovered that thorium-229, a byproduct of the US nuclear program, was a rare exception. Its natural resonance frequency is low enough to be manipulated by standard laser technology using the relatively weak ultraviolet radiation. This made thorium-229 a promising candidate for the development of a nuclear clock, in which time is measured by the nucleus “swinging” between quantum states like a pendulum in a traditional clock.
“A nuclear clock would be the ultimate detector – capable of sensing forces 10 trillion times weaker than gravity, with 100,000 times the resolution of today’s dark matter searches.”
Overcoming Decades of Measurement Challenges
However, progress on the nuclear clock stalled at the very first stage, when scientists tried to measure the resonance frequency of thorium-229 with the utmost precision. To determine a nucleus’s resonance frequency, physicists shine a laser on it at varying frequencies and observe how much energy it absorbs or emits while transitioning between quantum states. From these results, they construct an absorption spectrum, and the frequency that causes peak absorption is taken as the nucleus’s resonance frequency.
For nearly five decades, scientists were unable to measure thorium-229’s resonance frequency with enough precision to build a nuclear clock, but last year brought two major advances. First, a group at the National Metrology Institute of Germany (PTB) published relatively accurate measurements. A few months later, a team from the University of Colorado released results that were several million times more precise.
Dark Matter’s Subtle Fingerprints
“We still need even greater precision to develop a nuclear clock,” says Perez, “but we’ve already identified an opportunity to study dark matter.” He explains: “In a universe made up only of visible matter, the physical conditions and the absorption spectrum of any material would remain constant. But because dark matter surrounds us, its wave-like nature can subtly change the mass of atomic nuclei and cause temporary shifts in their absorption spectrum. We hypothesized that the ability to detect minute deviations in the absorption spectrum of thorium-229 with great precision could reveal dark matter’s influence and help us study its properties.”
Theoretical calculations made by the team – led by Dr. Wolfram Ratzinger from Perez’s group and other postdoctoral fellows – showed that the new measurements could detect dark matter’s influence even if it were 100 million times weaker than gravity, a force that is itself weak and rarely crosses our minds in daily life.
“This is a region where no one has yet looked for dark matter,” says Ratzinger. “Our calculations show that it’s not enough to search for shifts in the resonance frequency alone. We need to identify changes across the entire absorption spectrum to detect dark matter’s effect. Although we haven’t found those changes yet, we’ve laid the groundwork to understand them when they do appear. Once we detect a deviation, we’ll be able to use its intensity and the frequency at which it appears to calculate the mass of the dark matter particle responsible.
“Later in the study, we also calculated how different dark matter models would affect thorium-229’s absorption spectrum. We hope this will ultimately help determine which models are accurate and what dark matter is actually made of.”
Potential Beyond Dark Matter Research
Meanwhile, laboratories around the world are continuing to refine the measurement of thorium-229’s resonance frequency, a process expected to take years. If a nuclear clock is eventually developed, it could revolutionize many fields, including Earth and space navigation, communications, power grid management, and scientific research.
Today’s most accurate timekeeping devices are atomic clocks, which rely on the oscillation of electrons between two quantum states. These are highly precise, but they have one significant drawback: They are vulnerable to electrical interference from the environment, which can affect their consistency. Nuclei of atoms, by contrast, are far less sensitive to such disturbances.
According to a leading model of dark matter, the mysterious substance is made up of countless particles, each of which has a mass at least 1,000,000 times smaller than that of a single electron.
The Ultimate Detector Potential
“When it comes to dark matter,” says Perez, “a thorium-229-based nuclear clock would be the ultimate detector. Right now, electrical interference limits our ability to use atomic clocks in the search. But a nuclear clock would let us detect incredibly slight deviations in its ticking – that is, tiny shifts in resonance frequency – which could reveal dark matter’s influence. We estimate it will enable us to detect forces 10 trillion times weaker than gravity, providing a resolution 100,000 times better than what we currently have in our search for dark matter.”
Reference: “Searching for Dark Matter with the 229Th Nuclear Lineshape from Laser Spectroscopy” by Elina Fuchs, Fiona Kirk, Eric Madge, Chaitanya Paranjape, Ekkehard Peik, Gilad Perez, Wolfram Ratzinger and Johannes Tiedau, 15 May 2025, Physical Review X.
DOI: 10.1103/PhysRevX.15.021055
The European Research Council (ERC) recently awarded an ERC Advanced Grant to Prof. Perez’s group to support the continued development of this line of research. Also participating in the study were Prof. Elina Fuchs and Dr. Fiona Kirk from the National Metrology Institute of Germany (PTB), Braunschweig, Germany, and Leibniz University Hannover, Germany; Dr. Eric Madge and Chaitanya Paranjape from Perez’s group in Weizmann’s Particle Physics and Astrophysics Department; and Prof. Ekkehard Peik and Dr. Johannes Tiedau from the National Metrology Institute of Germany (PTB), Braunschweig, Germany.
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