One of the great debates in the history of science concerned the reversibility of time. Classical dynamicists such as Isaac Newton viewed time as essentially reversible. Simultaneously invert the velocities of all the parts of a system and the system goes backwards in time, much like playing a movie in reverse creates the visual impression of backward time travel. The science of heat, thermodynamics, told a different story. According to the second law, the entropy of the universe was always increasing. The universe as a whole was aging irreversibly, and much like the constituents of a burnt piece of paper could never be reassembled into a clean, white sheet, time cannot go backwards either in practice or in principle.