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In 2024, TU Wien presented the world's first nuclear clock. Now it has been demonstrated that the technology can also be used to investigate unresolved questions in fundamental physics.

Thorium atomic nuclei can be used for very specific precision measurements. This had been suspected for decades, and the search for suitable atomic nucleus states has been ongoing worldwide. In 2024, a team from TU Wien, with the support of international partners, achieved the decisive breakthrough: the long-discussed thorium nuclear transition was found. Shortly afterward, it was demonstrated that thorium can indeed be used to build high-precision nuclear clocks.

Now, the next major success in high-precision research on thorium nuclei has been achieved: When the thorium nucleus changes between different states, it slightly alters its elliptical shape.

This also changes the distribution of protons in the nucleus, which in turn alters its electric field. This can be measured so precisely that it allows for better investigation than ever before of the fine-structure constant, one of the most important natural constants in physics. This now makes it possible to investigate the question of how constant the fundamental constants of nature really are.

The study is in the journal Nature Communications.

The strength of the electromagnetic force

"As far as we know, there are only four fundamental forces in nature: gravity, electromagnetism, and the strong and weak nuclear forces," says Prof. Thorsten Schumm from Institute of Atomic and Subatomic ÌÇÐÄÊÓƵics at TU Wien. "Each fundamental force is assigned a fundamental constant that describes its strength in comparison to the others."

The fine-structure constant, with a value of approximately 1/137, determines the strength of electromagnetic interaction. If it were different, charged particles would behave differently, chemical bonds would function differently, and light and matter would interact in a different way.

"Normally, we assume that such constants are universal—that they have the same value at all times and everywhere in the universe," says Schumm.

"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 revolutionize 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."

Different atomic nucleus states—different electric fields

Thorium atomic nuclei can assume two different states—a ground state with little energy and an excited state with slightly higher energy. The difference between these two energy values can be measured with extremely high accuracy, which is also the basis for the nuclear clock.

"When the atomic nucleus changes its state, its shape also changes, and with it its electric field," explains Schumm. "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."

How much this value changes depends on the fine-structure constant. By precisely observing this thorium transition, it is therefore possible to measure whether the fine-structure constant is actually a constant or whether it varies slightly.

The thorium-containing crystals for the experiment were produced at TU Wien (Vienna), and the laser spectroscopy measurements were then carried out in Boulder, Colorado. "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," says Schumm.

"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."