This involves the Law of Entropy-the observed behavior that all the universe progresses from order to disorder as a net function. The Law of Entropy is, of course, also known as the Second Law of Thermodynamics. One current debate in cosmology is whether the thermodynamic arrow arose from boundary conditions prior to the Big Bang, or whether it developed early in the Big Bang expansion. Time's Arrow is an important but mysterious property of the world we live in. Theoretically, in the situations covered by Newton's Laws, forward time and backward time are indistinguishable.
Heat flow is involved when ice melts and the thermodynamic arrow is immediately evident.
The kettle boils only after we have set it on the stove, never before. The apple ripens only after the blossom falls, and all of us grow older with time, not younger. This is not the case in quantum or atomic physics, where time reversal seems to be entirely possible and even reasonable to imagine. However, even in atomic physics, though some processes can appear to run in reverse, time is not one of them-some very recent research has confirmed this. But the arrow of time is apparently still intact at the subatomic level.