Although Napier has offered a definition of entropy, I think the concept needs to be illustrated more explicitly. The energy we call “heat” is simple randomized kinetic energy; that is, it is a bulk measure of the kinetic energy of all the little particles that are bouncing around in a fluid or vibrating in a solid continuum. Unlike mechanical kinetic energy–in which we can identify a body with a certain mass moving at such-and-such a speed–a body with heat energy may not be going anywhere at all. We mesure the heat in a body in terms of its temperature (relative to an “absolute zero” state of minimum atomic motion) and density, and calculate the movement of heat-energy in a system via its difference in temperature from one location, portion, or cycle, to another via the laws of thermodynamics. These are the basic rules of thermodynamic behavior, analgous to Newton’s laws in mechanics, Maxwell’s equations in electrodynamics, and so forth.
The zeroth law–so named because it was so obvious that no one really thought it needed to be stated until it was realized that you have to make things painfully clear to undergraduate students in the hope that they’ll learn anything–is familiar to algebra students as the associative principle; if two states are in equilibrium, a third state in equilibrium to one is in equilibrium to the other. Duh. The second law merely states that energy can’t be created or destroyed, i.e. the principle of conservation of energy that is a known factoid even to people who paid no attention in basic eighth level science. Pretty boring stuff, really, and not even worth cracking open a book to read about.
However, the second and third laws get far more interesting, in that they really get into the guts of thermodynamics, which is all about entropy. What is entropy? Entroopy is a measure of the disorder of a system, or alternatively, how homogenous it is. We’d prefer systems in which energy is all tucked away into one high temperature corner (thermodynamicists call this a “reservoir”) and then free to flow–by a process we direct–to a low temperature corner, doing for us valuable work in the process, like making an engine go. And rules #2 and #3 get into how this business works.
The second can be stated in a large number of different ways, but in essence, it says that no real world process is genuinely irreversable; you always “lose” energy. Of course, the smartacres in the upper rows will now crow about this being a contradiction to the previous rule, but in fact the supposed loss that occurs is in the form of energy no longer available for use; it’s still in the system, but it is now just random energy, incapable of being put to a direction. We quantify by the means of entropy, about which I’ll say more in a second. Rule three is even more interesting; it says that you can’t even stop the total level of system entropy from increasing; even if you manage to separate out the hot particles from the cool particles in one region–Maxwell’s famous daemon–you’ll get a corresponding increase in the disorder of the system somewhere else, such at overall disorder is always greater than what you started out with.
Eventually, you end up with this system which is which is cooler than your high temperature corner was and hotter than your low temperature corner was, and all the same temperature throughout, thereby giving you no flow from which to get any work done. Imagine if all those monkeys in the room with the typewriters kept punching the “F” key over and over; you’re never going to get the collected works of Shakespeare, or even a few lines from Hamlet, no matter how long you wait. This is what is meant by thermodynamics use of the term heat-death. It’s more significant than merely the inability to make your system do work, incidentially; it also means that ultimately the system can’t store information.
Now I’ve been talking in terms of closed systems, and in engineerirg idealized heat engines and steam cycles are used, like assuming that the horse is a sphere to make the math easier, for the sake of being able to get something done. In the real world, however, most systems are nonequilibrium thermodynamic (NET) systems, and although the also follow these rules implicity, the systems are so complex that you can’t just draw a box and call them adiabatic and in “pseudo-equilibrium”. Nonetheless, entropy occurs in those systems, too. Oh, you can make your house cooler by turning on the air conditioner, but only by making the outside hotter, and using a lot of energy in the process to reverse the normal flow of heat. You’re just lucky that there’s enough mass outside to accept this additional heat-energy, plus what you create in generating electricity for the compressor in the air conditioner.
So that’s entropy in a nutshell. What about gravity? As Chronos notes, gravity, like all fundamental forces, is conservative; that is to say, in absence of any interactions with a system involving heat, you’ll gain the same amount of energy going down as you did climbing up. Our Lady of Perpetual Gravity knows nothing of the crass prolitariat of Thermodynamics, working as they do at an intermediate stage of complex interactions between atoms and molecules (intermolecular forces) and pissing away their time bouncing from one covalent bond to another. Gravity has nothing to do with this; she maintains her stoic, unbending, highly directionalized and ordered grip between bodies of distinguished mass. The reason water doesn’t naturally pour up into a glass has nothing to do with thermodynamics (and indeed if that coarse rabblerouser Entropy had anything to say about it, water would pour every which way without regard to the attractive grace of Lady Gravity) but rather energy; when you pour water out of a glass, you’re giving away the potential energy required to lift the liquid up to a certain level, and giving it away to the stream of liquid flowing downhill in the form of highly directed kinetic energy.
However, review of the third law of thermodynamics tells us that no real world process, even one involving a pure fundamental force, is every without entropy on the macro scale, and this is true with pouring water as well, for the water we pour on the ground doesn’t say in a neat coil to be elevated back to the glass with the same amount of energy it gained from being dropped, but rather splashes all about in a big mess that will take more energy to collect it than it is worth. Similarly, while we can carefully direct a stream of water back up to the glass, doing so requires the generation of more energy, which inevitably loses some of the applied work to random heat. In the end, the Morelocks must eat in order to keep the machinery working, and their diet is whatever is wasted in the process of doing work in the real world.
Gravity appears to have only one pole–an attractive one–although there’s no clear reason why this should be so. There are various theories that posit a number of different ways in which gravity could have positive (repulsive) components, but there is no accepted notion of “anti-gravity” and no practical demonstration of any repulsive gravity effect that has stood up to investigation.
Stranger