The concept of entropy is still being actively debated by philosophers of science and is difficult to convey, so what follows is my own working definition. I find it useful to define an increase or decrease in entropy as a loss or gain in any one, two, or all three of these properties of a system: order, information, and available energy. The Law of Entropy then states that, in any real-world situation, entropy irreversibly increases for an isolated system.
Consider an ordinary piece of photocopy paper. There is a certain amount of order to it (its geometric shape, uniform thickness, and so on). It also contains information, since all of its particles reside within its clearly defined form and have definite locations within it. It also has some available energy, since we can burn it to produce heat and light. Suppose we now do ignite this piece of paper and let it burn completely. Order has been lost because there is no longer a nice rectangular shape to the material, and the particles have dispersed. Information is lost because we no longer know where a given particle is; most have in fact broken up into smoke and ashes. And available energy is lost too, because the heat and light have dissipated into the environment and the burnt remains possess far less available energy than the paper did. Basically, entropy has increased.
However, if we can "recombine" the fire, smoke, and ashes by reversing every microscopic process involved in the combustion, could we reconstitute the paper? In theory, yes-but only through external efforts; one consequence of the Law of Entropy is that the paper (like any isolated system) will not spontaneously regenerate itself. In practice, of course, this would be an unfeasible task, so the burning of the paper remains an irreversible process. The same holds, for example, for the death of any living being.
All living creatures take in energy from their surroundings to offset the natural tendency toward increasing entropy (and its ultimate consequences, death and total decomposition). But while this allows for small-scale, individual growth in size and complexity (increasing order, information, and available energy, meaning a local decrease in entropy), the entropy of the ambient as a whole increases. As the Sun emits energy into space, its entropy increases irreversibly. A plant uses a tiny fraction of this energy, and chemicals from its environment, to decrease its own entropy as it grows. Put the plant in an airtight, lightproof container, though, and this now-isolated system will quickly succumb to the Law of Entropy: It will die and decompose as it approaches its maximum entropy state.
Another consequence of the Law of Entropy is that all real-world processes, biological or otherwise, must produce some waste in the form of cast-off energy (and, often, matter also). However small this waste may be, it is never zero-that is, no natural or man-made process can ever be 100 percent efficient. The human metabolism, for example, is only about 50 percent efficient; half of the energy we derive from food and oxygen intake becomes waste heat. Clearly, the Law of Entropy rules out practical perpetual-motion machines whose efficiency is by definition 100 percent, not to mention those miraculous "free-energy" machines that, on their own, produce more energy than they consume (thus exceeding 100 percent efficiency).
We conclude by noting that the Law of Entropy can be stated in terms of the Law of Extremes: All natural processes act to maximize the entropy of a system. As we have seen, any such system can temporarily sustain itself from the energy cast off by another system as it progresses towards its own state of maximum entropy, but ultimately the entropy of the entire ambient must irreversibly increase. So even if something in the human body was able to survive death, that thing would not be eternal. Eventually it too would undergo the effects of entropy.
Sources:
- Kline, M. 1964. Mathematics in Western Culture. New York: Oxford University Press.
- Sachs, M. 1988. Einstein versus Bohr: The continuing controversies in physics. La Salle, Ill.: Open Court.
- http://www.encyclopedia.com
- http://en.wikipedia.org/