# An anomalous propulsion mechanism

Noether’s theorem relates symmetries to conservation laws. In a classical theory, a symmetry of the action implies an associated conserved charge. This relation does not necessarily hold in a quantum theory. Indeed, a the violation of what would seem like a symmetry of the theory due to quantum effects is referred to as an anomaly. In a recent paper, E. Shaverin and A. Yarom have argued that such a quantum mechanical effect should be made visible at a macroscopic level. In particular, they consider a gas of particles whose dynamics are governed by an anomalous theory, and which is trapped inside a spherical rotating shell. If the gas is instantaneously released from the shell then the anomaly would force the emission of the gas to be directional which implies, via momentum conservation that the shell will be propelled forward. This is an instance were a subtle quantum mechanical effect has a large scale manifestation.