Time flies like an arrow. Fruit flies like a banana. – Groucho Marx
The second law of thermodynamics has long been a topic of discussion in the evolution/creation debate. What is the second law of thermodynamics? Let us start with the first law of thermodynamics — that energy can neither be created nor destroyed. While the total amount of energy is conserved, energy can be transferred and converted into different forms. We observe that in these changes, energy becomes less useful to us. To quantify this observation, physicists define a term, entropy, to describe how un-useful energy is. Thermodynamic entropy is defined by energy divided by temperature, expressed on an absolute scale. The preferred unit of energy is the Joule (J), and the preferred absolute temperature scale is Kelvin (K), so entropy is properly expressed in J/K. The second law of thermodynamics can be stated a number of different ways. The simplest form is that entropy never decreases. We normally use the letter S to represent entropy, and the Greek letter ∆ to represent a change, so mathematically we express the second law of thermodynamics as ∆S ≥ 0.
This expression shows that, while entropy can be increased, it cannot be decreased. This peculiarity introduces an asymmetry that makes the second law of thermodynamics fundamentally different from the first law, and from many other laws of physics. While other physical laws permit changes that can go either way in time, the second law works only one way. Any process that follows other physical laws is permitted, as long as entropy does not decrease. Thus, the second law imposes a direction to time, so some physicists and philosophers refer to the second law of thermodynamics as time's arrow.