Nor public flame, nor private, dares to shine;
Nor human spark is left, nor glimpse divine!Lo! thy dread empire, Chaos! is restored;Light dies before thy uncreating word:Thy hand, great Anarch! lets the curtain fall;And universal darkness buries all.
~ Alexander Pope, The Dunciad
Coined by a German physicist in 1865, the term 'entropy' is commonly used, though not commonly understood. You may have even casually heard it thrown about with the words 'chaos' and 'disorder'. But what is the significance of entropy? Basically, it is what gives time direction. You can call it "the arrow of time". So, while it's super important, it isn't the elusive hard-to-grasp concept it seems to be.
The second law of thermodynamics (the science of the relations between heat and other energy forms) tells us about entropy, by claiming that the universe is constantly moving towards a state of randomness. That is, the quantity called 'entropy' can never decrease, it can only increase. Processes end when maximum randomness and disorder are achieved. Spilled milk remains spilled -- you cannot reverse the effect somehow, and watch the milk molecules fly back into the cup, exactly the way they were before.
Humpty Dumpty was in a state of low entropy when he sat on the wall. When he had a great fall, he moved into a state of higher entropy, or disorder. All the kings horses and all the kings men couldn't put Humpty together again... They couldn't reverse the process, or in other words: decrease the entropy!
The second law of thermodynamics must not be confused with that of the first law, which talks about the conservation of energy. While the first law says that energy can neither be created or destroyed -- only converted from one form to another -- the second law says that this conversion cannot take place with perfect efficiency. Some energy is always lost during the conversion process, thereby increasing the entropy. For instance, a running car does not convert all of the burning gas into motion; quite a bit is lost as heat or noise. During this process, entropy has to increase as per the second law. The process moves from less disorder to more disorder, and cannot be reversed because we can never gather all of the dispersed energy and turn it back into gas for the car. (This is, by the way, why we can't simply renew our resources. We can't, say, harness pollution. Resources will run out one day, and entropy will just point and laugh at us when they do!)
Now, here's the thing about the word 'disorder'. In English, it has no set definition. Disorder is actually pretty relative, and it is often misused or misunderstood when linked with entropy. In the context of entropy, disorder is basically the state any irreversible process tends to reach. Water flows from top to bottom in a waterfall, not the other way around. Cream added to coffee spreads and spontaneously mixes with the drink; it doesn't clump back together to reach the state it was originally in when added to the cup. The motion or kinetic energy of an intact car disperses into sound and fast moving broken pieces if the car slams into a brick wall. Therefore, the entropy can be said to point in the direction of energy dispersal, or chaos. Which, in turn, gives time direction. Any spontaneous process that happens in the material world can be attributed to entropy, as an example of the second law of thermodynamics. This means that it is very much prevalent in our everyday lives.
You know, I think the next time I feel the spontaneous urge to indulge in a three scoop double chocolate nutty sundae, thereby setting my tummy and metabolism into a flurry of disarray, I should just blame entropy!
Entropy is Simple -- If We Avoid the Briar Patches! (A nice explanation behind common misconceptions)