Professor Susskind develops the equation for the probability that all molecules of a gas will converge in one half of a room, and concludes that this event is possible, but that the time scale for it to occur is incredibly long. This line of reasoning leads to the resolution of the paradox between the reversibility of classical mechanics and the apparent lack of time reversibility of the second law of thermodynamics by demonstrating that statistical mechanics processes are in fact time reversible if the system is known precisely enough and the observer waits long enough.
He then moves on to magnetism and begins to introduce the concepts of ferromagnetic phase transitions and spontaneous symmetry breaking. Spontaneous symmetry breaking occurs when magnets in a lattice begin to cool. With no external magnetic field, they may end up in one of two symmetrical states - e.g. all up or all down. But a very small magnetic field affecting just one of the magnets will break this symmetry and bias the system toward one of the ground states.