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.

These shape changes modify the nucleus's electric field in ways that can be measured with extreme precision. The thorium transition allows scientists to detect variations in the fine structure constant with accuracy 6,000 times greater than existing methods.

"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.

"Normally, we assume that such constants are universal - that they have the same value at all times and everywhere in the universe," Schumm stated. "However, there are also theories that predict that the fine-structure constant changes slowly by a small amount or even oscillates periodically. That would completely revolutionise our understanding of physics - but to find out, we need to be able to measure changes in the fine-structure constant with extreme precision. Our thorium atomic clock now makes this possible for the first time."

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.

"We were able to show that our method can detect variations in the fine-structure constant three orders of magnitude more precisely than previous methods, i.e. by a factor of six thousand," Schumm noted. "This shows that the thorium transition we discovered can not only be used to build a new generation of high-precision clocks, but also allows research into new physics that was previously inaccessible experimentally."

Research Report:Fine-structure constant sensitivity of the Th-229 nuclear clock transition

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