Scallop theorem

The Scallop theorem states that to achieve propulsion at low Reynolds number in Newtonian fluids a swimmer must deform in a way that is not invariant under time-reversal. Edward Mills Purcell stated this theorem in his 1977 paper Life at Low Reynolds Number explaining physical principles of aquatic locomotion. The theorem is named for the scallop, the only bivalve able to actively swim distances. However, its technique is not applicable to viscous fluids, thus the name of the theorem.