In a pot of boiling water, bubbles of steam are born and pop, but the water itself remains a liquid. Something similar happens in an ultracold atomic gas: atoms constantly form pairs and then fly apart again, even though superfluidity has not yet set in. As a result, a 'dead zone' appears in the particle spectrum — a pseudogap where energy cannot take certain values.

Fluctuations of pairs are the key to understanding. Atoms in the gas interact, and sometimes two atoms stick together into a pair for a short time. The pair does not live long, but its birth and decay change the energy of neighboring atoms. Previously, scientists simplified calculations by treating pairs as long-lived, but now, for the first time, they accounted for their finite lifetime and collisions between pairs and single atoms. It is as if an interpreter at negotiations not only translated phrases but also occasionally got distracted by conversations in the hall — the meaning gets distorted, but in a different way.

The calculations were compared with experiments using radio-frequency spectroscopy: atoms are irradiated with radio waves, and the energies they absorb are observed. It turned out that the pseudogap grows smoothly as the system transitions from weak to strong interaction, and pairs at energies above twice the gap behave as diffusing: they constantly break and are born anew. This confirms that the pseudogap arises precisely from pair fluctuations, not from other effects.

Now the picture is clearer: the pseudogap is not an exotic phenomenon but a natural consequence of pairs constantly forming and decaying in a strongly interacting gas. This helps understand not only ultracold atoms but also high-temperature superconductors, where a similar effect hinders lossless current flow.