Researchers at TU Wien and partner institutions have used precision measurements of thorium-229 to estimate how strongly a nuclear clock’s frequency would respond to possible changes in the fine-structure constant.
Correction, October 4, 2026: The earlier title and text confused estimated sensitivity to changes in the fine-structure constant with an achieved improvement in measuring the constant itself.
Researchers built upon their 2024 achievement of creating the world’s first nuclear clock by utilizing thorium atomic nuclei to investigate fundamental physics questions. The team discovered that when thorium nuclei transition between different energy states, they undergo subtle changes in shape that alter the distribution of protons within the nucleus.
A model informed by these measurements gives a sensitivity factor of K = 5900 ± 2300. This describes the fractional frequency response to a fractional change in the constant. It is not a measurement of the constant with 6,000-fold greater accuracy, and it does not show that the constant has changed.
“As far as we know, there are only four fundamental forces in nature: gravity, electromagnetism, and the strong and weak nuclear forces,” said Prof. Thorsten Schumm from Institute of Atomic and Subatomic Physics at TU Wien. “Each fundamental force is assigned a fundamental constant that describes its strength in comparison to the others.”
The fine structure constant, valued at approximately 1/137, determines electromagnetic interaction strength. Changes to this constant would alter how charged particles behave, modify chemical bond formation, and transform light-matter interactions.
Some theories predict that fundamental constants could vary over time. Comparing nuclear and atomic clock frequencies would provide a way to test those ideas.
Thorium nuclei exist in two states: a ground state with minimal energy and an excited state with higher energy. The energy difference between these states can be measured with extreme accuracy, forming the foundation for nuclear clock technology.
“When the atomic nucleus changes its state, its shape also changes, and with it its electric field,” Schumm explained. “In particular, the quadrupole component of the field changes – this is a number that describes whether the shape of the electric field is more elongated, like a cigar, or more squashed, like a lentil.”
The magnitude of this change depends directly on the fine structure constant. Precise observation of thorium transitions enables researchers to determine whether the constant remains stable or varies over time.
The research team produced thorium-containing crystals at TU Wien in Vienna, then conducted laser spectroscopy measurements in Boulder, Colorado. The combined approach validated that thorium transitions can power both next-generation precision clocks and experimental investigations into previously inaccessible physics phenomena.
The result supports future searches for varying constants. The authors stress that better nuclear models and additional measurements are needed; the quoted uncertainty does not include every uncertainty in the theoretical model.
Research Report:Fine-structure constant sensitivity of the Th-229 nuclear clock transition
