Physicists have discovered new evidence of a quantum phenomenon predicted in 1931 by physicist Hans Bethe: the formation of clusters of atoms that behave as a single entity, known as Bethe strings.
To study this phenomenon, the team cooled cesium atoms to temperatures close to absolute zero and confined them in extremely narrow tubes. Measurements showed the presence of clusters consisting of at least six atoms, although they do not yet definitively prove the existence of these structures.
Bethe's prediction arose from research into the quantum behavior of physical systems and their connection to magnetism. His calculations suggested that particles could form groups with collective properties, which later became a subject of study in atomic systems.
To test this hypothesis, researchers worked with tens of thousands of cesium atoms cooled almost to absolute zero. Under these conditions, they created a Bose-Einstein condensate—a state where particles share the same quantum state.
Laser beams divided the material into thousands of narrow tubes, in which the atoms lined up in a row. The team also regulated the interaction between particles using a magnetic field, creating conditions favorable for the formation of bound clusters.
This configuration allowed the study of atom behavior in one dimension, preventing them from passing through each other. Hans-Christoph Nägerl, a physicist from the University of Innsbruck and co-author of the study, compared this arrangement to a string of beads: the atoms line up like beads on a thread.
The researchers did not observe Bethe strings directly. Instead, they studied signs of their presence by comparing the energy released by the atoms during expansion under different conditions.
When movement was restricted to the tubes, the clusters remained bound. However, expansion in three dimensions led to the decay of these structures, releasing energy in the form of motion. The difference between these two situations provided clues about the strength of the bonds between the atoms.
Measurements showed an amount of energy approximately three times greater during three-dimensional expansion. The data indicated clusters of at least six atoms and were consistent with the predictions of the theoretical model. Nevertheless, this is indirect evidence: the result is not equivalent to a definitive observation of the structures.
The experiment also faced limitations. Clusters may exist for a short time before decaying into ordinary cesium molecules. In attempts conducted at the lowest temperatures, the team lost about 40% of the atoms.
In an interview with Live Science, Nägerl emphasized the need for caution in interpreting the results: 'The fact that the theory matches its data is not proof that something exists.'
Among the central aspects of the research: the study published in the journal Nature Communications adds evidence to the study of Bethe strings, but leaves open the fundamental question: to demonstrate directly that the clusters predicted by the theory form and persist under observable conditions.