Physicists are exploiting the unusual nuclear properties of thorium-229 to develop an ultra-accurate "nuclear clock" capable of detecting forces 10 trillion times weaker than gravity.This sensitivity could make it the ultimate tool for discovering the elusive effects of dark matter, which subtly distorts the properties of ordinary matter.
For nearly a century, researchers around the world have been trying to understand the nature of dark matter. Dark matter is an invisible substance that is believed to make up about 80% of the total mass of the universe. This mysterious substance is crucial to explaining many observed cosmic phenomena, but so far it has been eluded by any direct experiment.
Scientists have explored a variety of ways to look for it, from trying to create dark matter particles in high-energy particle accelerators to looking for the faint cosmic radiation it might emit. Despite these efforts, its core properties remain largely unknown. Although dark matter does not interact with light, it is thought to subtly affect the behavior of visible matter, but the effects are so subtle that existing instruments cannot directly measure them.
Experts have suggested that building a nuclear clock - a device that measures time based on the oscillations of atomic nuclei - might be able to detect the effects of dark matter. This clock is so precise that even the smallest fluctuations in its time can indicate the presence of dark matter. Last year, a team of researchers in Germany and Colorado achieved a major milestone by using the radioactive isotope thorium-229 in the early stages of building such a clock.
When scientists in Professor Gilad Pérez's Theoretical Physics Group at the Weizmann Institute of Science learned about this development, they saw an opportunity to contribute to the search for dark matter without waiting for nuclear clocks to be fully operational. Working with researchers in Germany, they developed a new strategy for detecting how dark matter subtly changes the properties of thorium-229 nuclei and published it in Physical Review X.
The unique properties of Thorium-229
Just like pushing a child on a swing requires the right timing to maintain smooth, consistent motion, atomic nuclei also have an optimal oscillation frequency, known in physics as a resonant frequency. Radiation at exactly this frequency can cause atomic nuclei to "swing" like a pendulum between two quantum states: the ground state and the higher-energy state.
In most materials, this resonance frequency is high and requires intense radiation to excite the atomic nuclei. But in 1976, scientists discovered that thorium-229, a byproduct of the U.S. nuclear program, was a rare exception. Its natural resonant frequency is low enough that it can be manipulated using relatively weak ultraviolet light using standard laser technology. This makes thorium-229 a promising candidate for the development of nuclear clocks. In a nuclear clock, time is measured by the "oscillation" of atomic nuclei between quantum states, much like a pendulum in a conventional clock.
“Nuclear clocks will be the ultimate detectors—capable of sensing forces 10 trillion times weaker than gravity, with a resolution 100,000 times greater than today’s dark matter searches.”
However, progress on nuclear clocks stalled at the earliest stages, when scientists tried to measure the resonant frequency of thorium-229 with maximum precision. To determine the resonant frequency of an atomic nucleus, physicists illuminate the nucleus with laser light of different frequencies and observe the energy it absorbs or releases as it transitions between quantum states. From these results, they constructed an absorption spectrum and used the frequency that resulted in the peak absorption as the resonant frequency of the atomic nucleus.
For nearly fifty years, scientists have been unable to measure the resonant frequency of thorium-229 with high enough accuracy to build a nuclear clock, but two major advances were made last year. First, a team from the German National Metrology Institute (PTB) published relatively precise measurements. A few months later, a team at the University of Colorado published results that were millions of times more accurate.
The subtle fingerprint of dark matter
"We still need greater precision to develop nuclear clocks," Perez said, "but we have discovered an opportunity to study dark matter." He explained: "In a universe consisting only of visible matter, the physical conditions and absorption spectrum of any substance would remain unchanged. But because dark matter surrounds us, its volatility can subtly change the mass of atomic nuclei and cause temporary shifts in their absorption spectrum. We hypothesize that being able to detect small deviations in the absorption spectrum of thorium-229 with high accuracy could reveal the influence of dark matter and help us study its properties."
Theoretical calculations by a research team led by Dr. Wolfram Ratzinger and other postdoctoral fellows in Perez's team show that the new measurement method can detect the influence of dark matter even if it is 100 million times weaker than gravity, which is itself so weak that we rarely think about it in our daily lives.
"This is a region where no one has yet looked for dark matter," Ratzinger said. "Our calculations show that simply looking for changes in the resonance frequency is not enough. We need to identify changes in the entire absorption spectrum to detect the influence of dark matter. While we have not yet discovered these changes, we have laid the foundation for understanding when they occur. Once we detect a deviation, we can use its intensity and frequency of occurrence to calculate the mass of the dark matter particle responsible for the deviation."
Later in the study, we also calculated how different dark matter models would affect the absorption spectrum of thorium-229. We hope this will eventually help determine which models are accurate, and what exactly dark matter is made of.
Potential beyond dark matter research
Meanwhile, laboratories around the world are continuing to refine methods for measuring thorium-229's resonant frequency, a process expected to take several years. If a nuclear clock is ultimately developed, it could revolutionize many areas, including Earth and space navigation, communications, power grid management, and scientific research.
Today's most accurate timekeeping devices are atomic clocks, which rely on electrons oscillating between two quantum states. Atomic clocks are extremely accurate, but they have a significant drawback: They are susceptible to environmental electrical interference that affects their consistency. In contrast, atomic nuclei are much less sensitive to such disturbances.
According to leading models of dark matter, the mysterious substance is made up of countless particles, each at least 1,000,000 times less massive than a single electron.
“When it comes to dark matter,” Perez said, “nuclear clocks based on thorium-229 would be the ultimate detector. Currently, electronic interference limits our ability to search using atomic clocks. But nuclear clocks could allow us to detect extremely small deviations in its telltale signs—and It's a tiny change in the resonant frequency - which we estimate will allow us to detect forces 10 trillion times weaker than gravity, at a resolution that's 100,000 times better than what we currently use in dark matter searches."
Compiled from /scitechdaily