New Thorium Measurements May Reveal Variations in Fine Structure Constant

Physicists at TU Wien in Austria have conducted high-precision laser spectroscopy measurements on thorium-229 nuclei, which may provide new insights into the fine structure constant. This constant is critical for understanding the strength of the electromagnetic interaction, one of the four fundamental forces in nature, alongside gravity, and the strong and weak nuclear forces. The fine structure constant, denoted as α, has a value of approximately 1/137. Any variation in this value could lead to significant changes in the behavior of charged particles, chemical bonding, and light-matter interactions.

Study leader Thorsten Schumm from the Institute of Atomic and Subatomic Physics at TU Wien emphasizes the universality of these constants, stating, “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.” Yet, modern theories, particularly those addressing the nature of dark matter, suggest potential fluctuations in these constants. Proving that the fine structure constant is not a fixed value could fundamentally alter our understanding of the universe.

Advancements in Thorium Spectroscopy

The recent research builds on a previous project that resulted in the world’s first nuclear clock. It involves precisely measuring the shape changes of the thorium-229 (229Th) nucleus when one of its neutrons transitions from a ground state to a higher-energy state. “When excited, the 229Th nucleus becomes slightly more elliptic,” Schumm explains. Although this shape alteration is minimal, at approximately 2%, it significantly impacts the Coulomb interactions between protons within the nucleus, leading to changes in the geometry of the nucleus’ electric field. The degree of this change is highly sensitive to the fine structure constant’s value.

Researchers created crystals of 229Th doped in a CaF2 matrix at TU Wien before moving to the next phase of the experiment at JILA in the University of Colorado Boulder, where they directed ultrashort laser pulses at the crystals. While they did not detect any changes in the fine structure constant, they successfully determined how any potential changes could affect the energy of the first nuclear excited state of 229Th. Schumm noted that the impact of this modification is significant: “This change is huge, a factor of 6000 larger than in any atomic or molecular system, thanks to the high energy governing the processes inside nuclei.”

This high enhancement factor, confirmed through their measurements, has sparked renewed interest among researchers, who have debated its likelihood for decades. Theoretical predictions concerning this enhancement have varied widely, ranging from zero to 10,000. Schumm’s team aims to leverage this confirmation to initiate a systematic search for variations in the fine structure constant.

Theoretical Implications and Future Research

Andrea Caputo from CERN, who was not involved in this study, has lauded the experimental results as “truly remarkable,” highlighting the unprecedented precision achieved in probing nuclear structure. Nevertheless, he pointed out that the theoretical framework still requires substantial development. “In a recent work published shortly before this study, my collaborators and I showed that the nuclear-clock enhancement factor K is still subject to substantial theoretical uncertainties,” Caputo noted. “Much progress is therefore still required on the theory side to model the nuclear structure reliably.”

Looking ahead, Schumm and his colleagues are focused on increasing the spectroscopic accuracy of their 229Th transition measurement by an additional one to two orders of magnitude. “We will then start hunting for fluctuations in the transition energy,” he reveals, “tracing it over time and – through the Earth’s movement around the Sun – space.” The findings of this research are detailed in the journal Nature Communications.

As scientists continue to explore the fundamental forces that govern the universe, the implications of this research could reshape our understanding of physical laws and the very fabric of reality.