Description
Whereas the usual Wigner-In\"on\"u contraction $c\to\infty$ of the Poincar\'e group yields the Galilei group, another $c\to0$ contraction yields the ``Carroll group" of L\'evy-Leblond. Both boost-invariant theories are conveniently unified within the ``Eisenhart-Duval'' framework. Plane gravitational waves carry a non-trivially implemented Carroll symmetry with broken rotations.
