“If your theory is found to be against the second law of thermodynamics, I can give you no hope.”

The Second Law of Thermodynamics describes the natural direction of physical processes: in an isolated system, entropy tends to increase rather than decrease. Energy remains conserved, but it becomes progressively less available for useful work as it spreads into more disordered forms. Heat flows spontaneously from hotter regions to colder ones, never the reverse without outside intervention. The law explains why perpetual-motion machines fail, why engines cannot be perfectly efficient, and why many processes are fundamentally irreversible. It gives time a preferred direction in an otherwise symmetric set of physical laws. It is used primarily in thermodynamics, mechanical engineering, and statistical physics.