How long can an atomic nucleus remain standing ? -- a fundamental quantum question

Abstract

The shape of an object is of fundamental interest and high importance, but is not a straightforward subject if the object is on quantum scale. We here discuss how a shaped micro-object can be looked at within quantum mechanics. For this purpose, atomic nuclei are suitable, because they are tiny shaped objects. The majority of atomic nuclei are shaped like ellipsoids. Although an ellipsoid is oriented in a direction classically, such a nucleus is pointing in all directions with certain probabilities in quantum eigenstates, fulfilling rotational symmetry. This makes the direct observation of shapes formidably difficult. Here, we show, including examples, that the ellipsoidal nucleus is basically standing in a fixed direction for finite time \sim some 10^{-23} sec, as a robust consequence of time-dependent Schrodinger equation in quantum mechanics and a well-known rotational feature of nuclei. This consequence not only provides Relativistic Heavy-Ion Collisions9 with experimental feasibilities, but also leads to a deeper general understanding of stationary states with restored broken symmetry: time-dependent symmetry-breaking (e.g., ellipsoid shape) properties arise from stationary states with symmetry. This work depicts direct relevance to fusion, fission and $α$ decay/emission in terms of time evolution, including applications to the synthesis of superheavy elements.