The Second Law of Thermodynamics, also known as Entropy, states that in a closed system energy flows from order to disorder. Common expressions of Entropy in day-to-day life include: “things fall apart”, “ashes to ashes”, “rust never sleeps” and “shit happens.” Entropy is a key to understanding our universe.

An egg sitting on a table can fall and break into many pieces. Thus, order (the egg) to disorder (broken egg). Also, if you tear the pages out of a book and throw them up in the air they likely will not settle into numerical page order. This is interesting, because the laws of physics are such that they should be able to move in either direction, yet with rare exception we see the laws of physics move only in one direction.

Thus,  Entropy gives time its arrow; it’s why the future flows into the past.

A perfect example is if you take a container that is divided into two parts with a door in the divider. On one side there is Oxygen and the other Argon gas. If you open the door the Argon will move and disburse into the Oxygen side and vice versa. After a short period of time you will have the same concentration of molecules of each gas on each side. So, the move is from order (Argon on one side and Oxygen on the other) to disorder (the gases are mixed). Yet, the laws of physics would suggest that it is equally likely that you could move from mixed gas to each type of gas separating into one side or the other. I know this doesn’t sound correct so as support I give you a quote by the famous physicist Roger Penrose:

“All the successful equations of physics are symmetrical in time. They can be used equally well in one direction in time as in the other. The future and the past seem physically to be on a completely equal footing. Newton`s laws, Hamilton`s equation`s, Maxwell`s equations, Einstein`s general relativity, Dirac`s ecuations, the Schroedinger equation—all remain effectively unaltered if we reserve the direction of time. (Replace the coordinate t which presents time, by –t.) The whole of classical mechanics, is entirely reversible in time.”
-Roger Penrose

But, we (almost) never see the laws of physics go in any direction but the one we are used to. From order to disorder. Another good quote: “My cup of coffee cools in the direction of the future and becomes hotter in the direction of the past. But the behaviour of heat is not reversible in time and characterizes according to the second law of thermodynamics a direction in time. That the coffee cools off ought to astound everyone. For ultimately according to classic mechanics the movement of each single particle of which the coffee consists is reversible in time.”
-Dr. Heinz-Peter Breuer.

Of course, things can go from disorder to order. We don’t always see order decay to disorder. Roads are built. Trees grow. Streams are dammed into lakes. My daughter Claire cleans her room (usually) by Saturday nights otherwise she can’t leave the house. Thus, it gets messy (disorder) and then it gets clean (order). The difference in those examples is that energy has been applied. The conclusion of the law of Entropy applies to closed systems. So, the Earth, as a system, does experience Entropy, but it is not a closed system. The Earth is barraged with energy from the Sun. This energy overcomes Entropy. As humans, we eat food that gets energy from the Sun which in turn gives our bodies the energy to stave off the effects of Entropy for awhile. Of course, death visits us all and at some point we are returned to a disordered bunch of the base minerals of which we are constituted.

Another interesting thing to note is that the law of Entropy is really statistical in nature and not absolute. It is possible, though highly improbable, that the pages of a book torn from its cover would randomly reorder themselves into numerical order. Also, it is possible, though highly improbable, that a mixture of two gasses would separate so that all of one gas would be on one side and all of the other gas on the other. So, Entropy is a “tendency” not an absolute rule.

Are you still reading?  If so, a final quote by physicist Arthur Eddington: “Why the awe for the Second Law? The Second Law defines the ultimate purpose of life, mind, and human striving: to deploy energy and information to fight back the tide of entropy and carve out refuges of beneficial order. An underappreciation of the inherent tendency toward disorder, and a failure to appreciate the precious niches of order we carve out, are a major source of human folly.”