What is dark matter? An invisible substance thought to compose ~80% of the Universe’s mass and needed to explain various physical phenomena, it has nevertheless defied all attempts at detection since its existence was first postulated nearly 100 years ago.
Scientists have hypothesized that a nuclear clock, which uses the atomic nucleus to measure time with extreme precision, could do the trick, as even the tiniest irregularities in its ticking could reveal dark matter’s influence. Last year, physicists in Germany and Colorado took a significant step toward building such a clock, using the radioactive element thorium (Th)-229. Now, in collaboration with the German team, Prof. Gilad Perez and his group have published a study in Physical Review X proposing a novel method for detecting the influence of dark matter on the Th-229 nucleus.
Much as pushing a child on a swing requires the right timing to maintain consistent motion, an atomic nucleus 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 Th-229, a byproduct of the US nuclear program, is a rare exception. Its natural resonance frequency is low enough to be manipulated by standard laser technology using relatively weak ultraviolet radiation.
In theory, this makes Th-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. In practice, the precision required to build a nuclear clock around Th-229 resonance has remained an elusive dream for nearly five decades.
Now, a new approach led by Dr. Wolfram Ratzinger from the Perez group, together with other postdoctoral fellows, showed that an alternative scheme could detect dark matter’s influence even if it were 100 million times weaker than gravity. The researchers devised a way to use the resonance shifts (irregularities) of laser-excited Th-229 nuclei to detect particles that may constitute dark matter, such as quarks and gluons—probing a previously unexplored region of the light spectrum.
“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 the effect of dark matter,” Dr. Ratzinger explains.
Although the team hasn’t found those shifts yet, they have laid the groundwork to understand them when they do appear. Once they detect a deviation, they can use its intensity and the frequency at which it occurs to calculate the mass of the dark matter particle responsible.
If a nuclear clock is eventually developed, it could revolutionize many fields, including Earth and space navigation, communications, power grid management, and scientific research. Prof. Perez estimates such a clock could provide a resolution 100,000 times better than what is currently available for hunting dark matter.
