  Nuclides sharing the same mass number (isobars) are observed ubiquitously
along the stability line. While having nearly identical radii, stable isobars
can differ in shape, and present in particular different quadrupole
deformations. We show that even small differences in these deformations can be
probed by relativistic nuclear collisions experiments, where they manifest as
deviations from unity in the ratios of elliptic flow coefficients taken between
isobaric systems. Collider experiments with isobars represent, thus, a unique
means to obtain quantitative information about the geometric shape of atomic
nuclei.
