Updated: November 16, 2012
Entropy is a man-made concept designed to calm you down. It tells us the world works as it should and that things happening around us are happening the way they are because that's how they are supposed to be. It's a bit of a paradox, but entropy has found good use in physics and chemistry somewhere in the 19th century and has remained lodged firmly there ever since.
Perhaps in the olden days, things were not so obvious like they are today, so the scientists needed some new way to baffle the plebes. But it mostly had to do with the ultimate chase after the perpetuum mobile. People were confused about why their stupid contraptions consisting of wheels and levers and pulleys did not work. So they discovered entropy and things have never been quite the same.
Entropy in an eggshell
Because a nut is too small to contain it all!
For normal people, entropy is intangible, so they decided that the best way to measure is through heat dissipation in thermodynamic processes where internal energy is converted into work. If you sweat, you entropize. If a cube of ice melts, it does the same thing. It's converting its internal thingie into heat. So there's energy, only one that you cannot use directly. However, you can measure the heat dissipation. And the intrinsic properties that determine how the thermodynamic process occur is entropy. Although, it's more like a confirmation of what you already know.
An alternative explanation for entropy is the level of disorder or randomness in a system. Much like the first definition, it's fancy and hard to comprehend and goes back to the fundamental obvious like-duh it's how things work. If you drip some ink into a glass of water, the ink will spread. We all know that. The question is why?
Well, if you use the order theory, then a drop of ink is very orderly. Entropy wants to make it disorderly, so it forces the drop to spread in the surrounding medium. If you approach the phenomenon from the energy perspective, it becomes quite obvious. All systems in nature tend to minimize their energy, hence a highly concentrated and energized drop of ink will try to reach a state of lower energy, which would be a uniform spread in water. Entropists will tell you that the processes want to be entropically flavored, so they will only happen in the direction that maximizes entropy. In other words, a higher state of entropy is one where particles of ink are randomly dispersed in the surrounding medium. This is quite simplistic, really.
Time, now it's time
But there's a much better explanation. It has to do with time. In physics, you can define a process and a reverse process. There's nothing to prevent that. However, while some processes occur spontaneously in nature, others do not. For example, at room temperature and standard pressure, ice will melt, but the water will not solidify. So it comes down to time and temperature.
Temperature is a definition of energy freedom. Zero temperature is the state where everything is in its minimal energy state and all of the energy has been freed. Time is the directional constraint for processes. Since you can't have negative time and negative temperature, so to speak, then you're left with a puzzle. So how do you cause things to do the opposite of what they've just done? The answer is - energy, converted to work, heat dissipation, entropy, boom.
And from here, you get some pretty obvious conclusions - time and entropy share the same direction and might in fact be synonymous. If there's an increase in entropy in a system, this means the system has been irreversibly changed; it cannot be placed into a previous state without an external input of energy. There's no machine with 100% efficiency, and definitely nothing above that. Unless, of course you could get something that works at 0 degrees Kelvin temperature. And by works, I mean emits information of any sort. And negative values would give us even more. Which isn't done really. Laws of thermodynamics can now work without fear.
Temperature, the philosophical question
One problem that normal people might have is with the temperature definition of the absolute zero. For people who use the Celsius scale, the figure stands at -273.15 degrees. This number sounds arbitrary, so you would assume you could go lower than that. But if you look at the problem from the other direction - the absolute zero expressed in Kelvins, then it's zero, and there's only one way up. The Celsius zero is the triple point of water, the thermodynamic state where ice, water and vapor can co-exist in an equilibrium. So our understanding of the world is misaligned to water, but in practice, things are a bit more complex.
Now, the scale could have been different. The scientists could have used a logarithmic scale, just for fun, and then you would have more difficulty trying to understand why things are the way they are. But people should not treat temperature as a value, more as the lower limit of information exchange. It's the same way the speed of light is not speed but the property of the medium.
Absolute zero is the limit where there's no exchange of information and the entropy change approaches zero. At this would-be temperature, all of the energy that could have been freed has been freed and all particles and systems are in their minimal state. It's a boring state.
And so, temperature is a measure of the willingness of a state to share its internal properties, which could be useful energy that we can convert to work, with some efficiency, while losing a battle to entropy. Don't confuse it with energy or heat. Closely related, but not quite. Energy exists even when fully isolated. Temperature is meaningless without interaction. Hence, the entropy; the medium to reducing the efficiency of the world while allowing it to go about its business. Finally, heat - the indicator of the process reaction.
I've written about this before, on time reversal, and how life is a manifest of negative entropy, fighting until it loses the battle, mostly when telomeres fray out and no more chromosome replication is done properly, so the DNA information is lost. And this is exactly the sum of entropy that we contain, until the nature claims its course.
But in theory, if we could replenish the telomeres and make them work perfectly forever, we could create the immortal man. It would be a sort of a manifestation of the perpetuum mobile, as we would still need to feed, to sustain our energy, but still, as close as it gets. And here's a nice picture of wobbly things, the treasure of entropy that masquerades as life.
Quite a lot on entropy on Wikipedia
Temperature, also on Wikipedia
Third Law of Thermodynamics
I hope this article makes sense. There's really no point repeating everything you can find in books anyway. But I wanted to give a little more humane perspective on something that's not quite so trivial to understand. People may intuitively understand heat, but not what role it plays. Apart from the actual IR spectrum that causes us to feel warmth and thus perceive heat, there's the information component, the work component that is not easy to grasp. Well, it should be now.
This article does not have a witty punch, as most of my hillbilly work does. Still, it sheds light on entropy. Just think of it as the missing link between why and how natural processes occur. And if something isn't happening by itself, then you need to input energy to get it done. Reverse the time, you might get negative temperatures and negative entropy, profit! Finally, there's the efficiency, for closed cycle systems, which comes down to the ratio between high-end and low-end temperatures in the cycle. The bigger the difference, the more efficient the system is. And life, a temporary flux in the world order. That would be all.
P.S. The image of the DNA strand is in public domain.