Nuclear Clock Technology Ushers in Unprecedented Precision to Probe Fine-Structure Constant Stability
October 27, 2025 — Vienna University of Technology
In a groundbreaking advancement in precision measurement and fundamental physics, researchers at the Vienna University of Technology (TU Wien), supported by international collaborators, have demonstrated that nuclear clock technology can be applied to probe the stability of one of nature’s most important constants — the fine-structure constant, denoted by α.
The Thorium Nuclear Clock Breakthrough
Building upon their pioneering work in 2024, when TU Wien unveiled the world’s first nuclear clock based on thorium-229, the research team has now taken the technology further as a tool for exploring profound questions in physics. This success hinges on an extraordinary nuclear transition discovered within the thorium-229 atom’s nucleus — a state change that subtly alters the nucleus’s shape and the distribution of protons, thereby shifting its electric field configuration.
The crux of this innovation lies in the nuclear transition’s exceptional sensitivity to variations in the fine-structure constant, a fundamental parameter dictating the strength of electromagnetic interactions. Through precise laser spectroscopy experiments conducted on thorium-containing crystals, produced at TU Wien and measured in Boulder, Colorado, the team achieved measurement sensitivity three orders of magnitude greater than prior techniques — a factor improvement of about 6,000 in detecting minute shifts.
Why the Fine-Structure Constant Matters
The fine-structure constant α, approximately equal to 1/137, governs how charged particles interact electromagnetically, influencing everything from atomic spectra and chemical bonding to the behavior of light and matter. Traditionally, α has been treated as a universal constant, immutable across time and space.
However, certain advanced theoretical models suggest that α may vary slowly over cosmic time scales or oscillate periodically, potentially reshaping our fundamental understanding of the universe’s laws.
Professor Thorsten Schumm of TU Wien’s Institute of Atomic and Subatomic Physics explained, “If the fine-structure constant changes even slightly, it would imply new physics beyond our current framework. Our thorium nuclear clock now provides the unparalleled precision needed to experimentally test these hypotheses.”
How the Thorium Nuclear Clock Works
Thorium-229 nuclei can exist in two distinct states: a low-energy ground state and an excited state with slightly higher energy. The transition between these states is characterized by a tiny shift in the nuclear shape from more spherical to slightly elliptical (ellipticity), altering the quadrupole component of the nuclear electric field — essentially how much the shape is elongated versus flattened.
Since the quadrupole shift depends on the electromagnetic interaction strength, any variation in α would manifest as measurable changes in this nuclear transition energy. By monitoring this transition with extreme precision, scientists can detect whether α remains constant or exhibits subtle fluctuations.
Implications for Fundamental Physics and Beyond
This latest achievement, published in Nature Communications (DOI: 10.1038/s41467-025-64191-7), highlights the thorium nuclear clock’s dual utility: not only advancing ultraprecise timekeeping but also opening a new experimental window into testing the invariance of fundamental constants that underpin physical law.
“This is a major milestone,” says Schumm. “It ushers in a new era where we can scrutinize the fabric of physical constants with a sensitivity previously thought impossible. It paves the way to potentially discover new physics phenomena that have eluded observation so far.”
Context and Future Prospects
Fundamental forces — gravity, electromagnetism, and the strong and weak nuclear forces — each have associated constants defining their strengths. Verifying their consistency is critical for validating existing physics theories like Quantum Electrodynamics and the Standard Model.
The thorium nuclear clock system’s ability to detect changes in α with unprecedented sensitivity provides a robust experimental platform for ongoing and future investigations into cosmic evolution, dark matter interactions, and possible violations of fundamental symmetries.
As the research community harnesses this technology, the quest to understand whether the constants of nature are truly constant may soon yield transformative insights, reshaping our cosmic perspective.
Reference:
Beeks, K., et al. (2025). Fine-structure constant sensitivity of the Th-229 nuclear clock transition. Nature Communications, DOI: 10.1038/s41467-025-64191-7. —
Credit: Vienna University of Technology
Edited by Gaby Clark; Reviewed by Robert Egan
For further information, visit phys.org/news/2025-10-nuclear-clock-technology-enables-unprecedented.html.
