Conservation of energy

One rather little known fact about the law of conservation of energy is that this law is equivalent to saying that time is symmetric when reversed. A full understanding of this requires advanced physics and it does assume there are no energy losses during the process (such as heating or an increase in disorder). It is only when including the useless heat energy lost in converting one form of energy into another that the true nature of energy is realized to be a conserved quantity. You always have the same amount of energy before and after any time or process. The energy is simply converted into one or more different forms through any sequence of operations or events.

Źródło: Conservation of energy