text: If the trajectory of a photon travelling in a material possesses this property, the photon can constructively self-interfere via coherent backscattering.
full_question: If the trajectory of a photon travelling in a material possesses this property, the photon can constructively self-interfere via coherent backscattering. For Stokes flow, a swimmer whose swimming motion possesses this property will not achieve propulsion according to the scallop theorem. The surface states of topological insulators are protected by conservation of particle number and this property. All spin-1/2 eigenstates of a Hamiltonian with it must be at least doubly-degenerate according to Kramers' theorem. Applying the antiunitary operator associated with this property to the neutron causes a change in its spin, but no change in its electric dipole moment. The incompatibility between systems whose microscopic dynamics possess it and the second law of thermodynamics is described by Loschmidt's paradox. The combination of charge conjugation and parity inversion with this symmetry is an exact symmetry of all particle interactions according to the CPT theorem. For 10 points, name this symmetry in which the temporal order of events is flipped.
first_sentence: If the trajectory of a photon travelling in a material possesses this property, the photon can constructively self-interfere via coherent backscattering.
raw_answer: time reversal symmetry [or T-symmetry until "CPT" is read and prompt thereafter; prompt on "reversibility"]