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Contrary to expectations, the tin-116 nucleus turned out not to be a sphere but a host to three different shapes at once.
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An atomic nucleus is not just a ball of protons and neutrons. It has a shape, and that shape can change. The tin-116 nucleus was long considered almost perfectly spherical: it has 50 protons — a 'magic' number that usually gives a nucleus spherical symmetry. But a new ultra-precise measurement showed otherwise.
Physicists used the method of Coulomb excitation. They accelerated a beam of tin-116 nuclei and directed it at a target, making the nuclei 'shiver' under the influence of an electric field. By observing how the nucleus returned to its initial state, they reconstructed its shape. It turned out that in the ground state, the lowest-energy state, the nucleus is not a sphere but a slightly elongated ellipsoid, like a melon rather than a watermelon.
The big surprise: three different shapes coexist in the tin-116 nucleus. The ground state is weakly deformed, the first excited state is more elongated, and the second is even more so. This is like a balloon that can be round, oval, or pear-shaped, depending on how much energy it contains.
Previously, such multiple shape coexistence was only hypothesized, but now it has been directly proven. The precise values of the quadrupole moments, quantities describing the deviation from a sphere, are the first unambiguous evidence. This matters for all of nuclear physics: the shape of a nucleus affects how it interacts with other particles and even the stability of the atom.
Tin-116 is one of the most stable nuclei: its half-life exceeds the age of the universe, so it is called 'practically stable.'
The idea of multiple forms of a single object echoes the quantum entanglement in Hannu Rajaniemi's novel 'The Quantum Thief,' where a personality can exist in several states simultaneously.