New Laser Measurements Challenge Understanding of Fine Structure Constant

Physicists at TU Wien in Austria have made significant progress in understanding the fine structure constant through new high-precision laser spectroscopy measurements on thorium-229 nuclei. This constant, denoted as α, plays a crucial role in determining the strength of electromagnetic interactions, one of the four fundamental forces in nature alongside gravity, and the strong and weak nuclear forces.

The fine structure constant has a value of approximately 1/137. Its stability is pivotal; any variation could fundamentally alter the behavior of charged particles, the nature of chemical bonds, and light-matter interactions. “As the name ‘constant’ implies, we assume that these forces are universal and have the same values at all times and everywhere in the universe,” stated Thorsten Schumm, the study leader from the Institute of Atomic and Subatomic Physics at TU Wien. Yet, modern theories, particularly those addressing dark matter, suggest that these constants may fluctuate over time and space.

Measuring the Fine Structure Constant

To explore these potential variations, Schumm and his team utilized thorium spectroscopy, a sensitive method to detect changes in the fine structure constant. Their research builds on a previous initiative that resulted in the world’s first nuclear clock, allowing precise measurements of how the thorium-229 (229 Th) nucleus changes shape during neutron transitions from a ground state to a higher-energy state. “When excited, the 229 Th nucleus becomes slightly more elliptic,” Schumm explained. “This shape change, although small at the 2% level, significantly affects the Coulomb interactions among protons in the nucleus.”

This alteration influences the geometry of the electric field produced by the 229 Th nucleus, which is highly sensitive to the value of the fine structure constant. By closely observing this transition, researchers aim to determine whether the fine structure constant remains consistent or varies.

In collaboration with the JILA laboratory at the University of Colorado, Boulder, the team fired ultrashort laser pulses at crystals of 229 Th doped within a calcium fluoride matrix. Although their initial measurements did not indicate any changes in the fine structure constant, they successfully calculated how any potential variations would affect the energy of the first nuclear excited state of 229 Th.

“This change is substantial, a factor 6000 greater than any observed in atomic or molecular systems, due to the high energy involved in nuclear processes,” Schumm remarked. This finding underscores a significant increase in sensitivity to variations in the fine structure constant compared to earlier measurements, a topic that has sparked debate among researchers for decades.

Implications and Future Research

Andrea Caputo from CERN’s theoretical physics department, who was not associated with this study, characterized the experimental results as “truly remarkable,” noting the unprecedented precision achieved in probing nuclear structure. Nonetheless, he cautioned that theoretical models still need refinement, as existing frameworks exhibit substantial uncertainties regarding the nuclear-clock enhancement factor K.

Schumm and his colleagues are now focused on enhancing the spectroscopic accuracy of their measurements by one to two orders of magnitude. “We will then start hunting for fluctuations in the transition energy, tracing it over time and through the Earth’s movement around the Sun,” he stated. The findings of this research are detailed in the journal Nature Communications, marking a significant step forward in the quest to understand the fundamental constants of nature.