Thermodynamics is the science that deals with energy, its transmission from one body to another, and its transformation from one form to another, according to the book "The Refrigerator and the Universe," by Martin Goldstein and Inge Goldstein. The name betrays the origins of the science, which began with investigations regarding heat and its relationship to motion, notes Hans Christian von Baeyer in his book "Maxwell's Demon."
The First Law of thermodynamics recognizes that energy comes in different forms and undergoes various transformations. For example, heat can be transformed into motion, as when boiling water runs a steam engine, and motion can be transformed into heat, as when rubbing two sticks together generates heat through friction. Despite its transformations, however, energy, according to the First Law, remains constant in net quantity, note Goldstein and Goldstein.
The Second Law of thermodynamics states that, although energy cannot be destroyed, it can become unavailable for further transformation, observe Goldstein and Goldstein. Consider a log burning. Eventually it is reduced to ashes. This transformation involves no destruction of energy, but it does involve a dispersion of energy---energy in the form of heat. The Second Law says that there is no way to re-concentrate that heat, no way to reconstitute that log, except by the input of more energy, suggest Goldstein and Goldstein.
Applied to the universe as a whole, the Second Law implies that, eventually, all energy will dissipate completely. This is what the 19th-century German scientist Rudolf Clausius believed, notes von Baeyer. Clausius' contemporary, British scientist Lord Kelvin, agreed, von Baeyer observes. Once all energy was dissipated, Clausius and Kelvin claimed, no more energy transformations could occur ever again, state Goldstein and Goldstein.
At the level of individual molecules, however, the ultimate dispersion of energy implied by the Second Law appears uncertain. This is because the velocity and direction of molecules are never certain but only more or less probable, like the tosses of a coin, note Goldstein and Goldstein. Hence, at the molecular level, the dissipation of energy appears only highly probable, not inevitable. Given enough time, molecules may behave in improbable ways, apparently contradicting the Second Law, according to Goldstein and Goldstein.
The nature of time is somehow linked to the unfolding dissipation of energy. Only because of energy's dissipation can people exist and, thereby, experience the passage of time, note Goldstein and Goldstein. Yet time remains a stubborn mystery. Although many, including Einstein, have tried, no one has yet succeeded in reconciling what appear to be the reversible movements of molecules with time's apparently inexorable, one-way flow, according to von Baeyer.