Although dark matter makes up around 80 percent of the observed mass of the universe, physicists still don’t know what it actually is. Part of the problem is that dark matter doesn’t interact with light, which most of our telescopes use to collect information about the universe.
To overcome this challenge, many researchers are working to leverage the fundamental laws of physics and develop new methods to detect dark matter. One way to do this is to use atomic clocks, one of the most precise instruments we have today.
In a recent Physical Review Letters paper, researchers from the Physikalisch-Technische Bundesanstalt (PTB) institute in Germany compared two atomic clocks to try to find the smallest differences in their “ticking” — which may be a signature of dark matter.
What Is an Atomic Clock?
Atomic clocks work by carefully measuring the energy of atoms as they transition from a higher energy state (typically triggered by a laser pulse) to a lower energy state. In some instances, this transition results in the emittance of a photon, or light particle. This can also be used to measure the energy gap between an atoms’ lower and higher states.
“The idea is that you look at all of your different atomic transitions, and you find out the most stable one in terms of transition,” says Yu-Dai Tsai, a postdoctoral researcher at the University of California, Irvine who was not involved with the recent project. “You need to be able to observe it precisely to ensure your time is permanently kept.”
Because atomic clocks rely on the energy transition of atoms like cesium, which are stable across massive time scales, their timekeeping is incredibly precise. They lose one second of time only every 100 million years.
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Can Atomic Clocks Detect Dark Matter?
Because of this accuracy, physicists are looking at the atomic clock as a way to detect dark matter. Specifically, they’re searching for “ultralight” dark matter.
“We use the term ‘ultralight’ to refer to dark matter models where the particles have much lighter masses than those we are familiar with,” says Joshua Eby, a postdoctoral researcher at the Kavli Institute for Physics and Mathematics of the Universe in Tokyo Japan who was not involved in the research project.
“One incredible consequence of the small masses of these ultralight particles is that their quantum wave nature can manifest on macroscopic distance scales,” he continues. “Dark matter ‘waves’ moving through space with sizes comparable to planets, solar systems or, in the extreme case, approaching entire galaxies.”
As these waves pass through dark matter detectors, researchers can observe their ebb and flow in predictable ways.
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The Fine-Structure Constant
The researchers at PTB hoped to observe whether these waves affect a natural timekeeping constant known as the fine-structure constant. This constant affects the energy transition of the atomic clock, which means that any oscillations in this constant create differences in the “ticking” of an atomic clock.
To measure these oscillations, the researchers compared two atomic clocks in the same experimental setup, something that had not been done previously.
“By interrogating two optical clock transitions in a single ion in an interleaved fashion, we demonstrated that one experimental setup is sufficient to provide the data for a competitive [ultralight dark matter] search,” says Melina Filzinger, a Ph.D. student at PTB and the paper’s lead author.
The researchers studied the clocks for more than 26 months, taking careful measurements, but did not see any oscillations in the fine-structure constant — meaning there was no difference in the “ticking” between the two atomic clocks.
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Dark Matter Evades Detection
While their results found no differences between the two atomic clocks at all, they did improve their measurements’ boundaries; dark matter signatures may still exist, in other words, just outside the current limits of atomic clock measurements.
And though the results revealed no dark matter detection in that particular location, the team hopes to continue improving their experiment and pushing their measurements to smaller scales, where dark matter may be hiding.
As many other researchers worldwide are working to improve atomic clocks’ accuracies for other uses, such as GPS navigation, these improvements could also significantly help with the future of dark matter detection.
“Today,” says Eby, “atomic clocks are already some of the most sensitive dark matter probes in the world and have a great capacity to improve in the coming years.”
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