For all our telescopes and colliders, dark matter has remained an elusive ghost for the better part of a century. It outweighs everything we see by a factor of five, yet it slips past every detector built to catch it. Now, a team led by the Weizmann Institute of Science, with collaborators in Germany and Colorado, has turned to a nuclear clock with the hope of revealing the faint fingerprints of this hidden matter.
At their hearts, current nuclear clocks rely on thorium-229, an isotope with the lowest-known excitation energy in any atomic nucleus. First identified in 1976, thorium-229 has a resonance so unusually low it makes the nucleus accessible to laser spectroscopy, something no other element has offered thus far. That unusually low resonance makes nuclear clocks uniquely suited to timekeeping at the nuclear scale, and sets them apart from their atomic cousins.
Where atomic clocks measure time by tracking electrons shifting between quantum states excited by a microwave oscillator, nuclear clocks turn inward, measuring transitions within the nucleus itself. Because protons and neutrons are shielded from many environmental disturbances, those transitions may carry the subtle imprint of dark matter.
By pairing atomic and nuclear clocks side by side, physicists can monitor tiny variations in their time signatures that could betray dark matter’s presence.
“In a universe made up only of visible matter, the absorption spectrum of any material would remain constant,” says Prof. Gilad Perez of the Weizmann Institute of Science. “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.”
Turning that idea into a test required a rare, cross-lab effort.
To probe it, the Weizmann team joined colleagues at the University of Colorado and Germany’s national metrology institute, PTB. They analyzed thorium-229 spectra from both labs, modeling how dark matter could distort the nuclear lineshape – essentially, the detailed profile of a signal’s intensity across frequencies. It’s not just the peak’s position that matters, but the whole shape, which can carry subtle signatures of hidden forces.
By comparing what they measured against theoretical models, the team was able to place the first experimental limits via lineshape analysis on how ultralight dark matter might couple to nuclear matter. Those limits set the stage. But pushing further comes down to sharpening the signal itself.
The thorium-229 spectra used in the current study had a linewidth of about 20 gigahertz, considered broad compared to the razor-thin signals needed to probe dark matter. Recent experiments have already narrowed that to 300 kilohertz, and upcoming work could shrink it further to hundreds of hertz.
That’s a leap of eight orders of magnitude, from a mountain-wide blur down to something closer to a laser pointer. With that kind of narrowing, nuclear clocks are projected to surpass atomic clocks as the most sensitive instruments for probing ultralight dark matter.
The potential reach is staggering. A fully developed nuclear clock would be “capable of sensing forces 10 trillion times weaker than gravity, with 100,000 times the resolution of today’s dark matter searches,” the team reports. In practice, that means opening a vast new window on unexplored physics, probing interactions previously far beyond the grasp of any laboratory tool.
While not yet as precise as current atomic clocks, the experiment has already set the stage for one of the most sensitive dark matter detectors ever imagined. Reaching that sensitivity means looking beyond a lone resonance shift to the full nuclear lineshape.
“Our calculations show that it’s not enough to search for shifts in the resonance frequency alone,” notes Dr. Wolfram Ratzinger, a co-author on the study. “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.”
For now, dark matter remains elusive, a ghost against the backdrop of galaxies and laboratories alike. But the pursuit itself underscores the ingenuity of modern physics. One that could ultimately transform how humanity navigates Earth and space, strengthen global communication, and probe the hidden laws of reality.
This study was published in Physical Review X.
Source: Weizmann Institute of Science
