One hint that there is more to physics than meets the Classical eye can be obtained by the following Gedankenexperiment credited to J.C. Maxwell [whom we shall meet again soon]:
We know that a system prepared initially in a highly ordered state - i.e. one whose gross macroscopic properties can only be achieved by a very small subset of all the possible fully specified microscopic states (e.g. a box full of marbles with all the white ones on one side and all the black ones on the other side) - is sure to drift toward more probable, less ordered (more random) states (e.g. all the marbles mixed up) as time goes on, if some "jiggling" is provided by the world around it. This intuitively obvious conclusion is translated by Physicists into the SECOND LAW OF THERMODYNAMICS, which states that entropy will always increase in any spontaneous process involving a highly complex system.16.1 When examined critically, this conclusion can be seen to contain virtually everything we know about the "arrow of time" - i.e. the only practical way to tell whether a movie of some process is being shown forward or backward. So it is a pretty basic idea.
Now suppose that we build a modern, microminiaturized robot16.2 that sits by a hole in a divider between the left and right sides of the box of marbles and opens the door only for white marbles heading toward the right side and for black marbles heading toward the left side. This action can presumably take far less energy than the marbles' kinetic energy; we simply substitute "will" (in this case, the programmer's will as translated into action by the robot) for "brute force" and avoid any "waste" of energy. Is it possible to reverse the SECOND LAW OF THERMODYNAMICS using a "Maxwell's Demon?"
The answer is not obvious. One can see why by examining the analogous example of keeping one's office or bedroom tidy: in this case a simple application of will should suffice to maintain Order (keeping Entropy at bay) by simply putting every article in its proper place every time the opportunity arises; however one is apt to notice some dissipation of energy as such good habits are put into practice. With the possible exception of a few "Saints of Order," we all think of "tidying up" as work; and the human machine is fuelled by a form of internal combustion which entails a massive increase of "global" entropy as food is consumed and digested. Therefore we may be able to suppress the SECOND LAW OF THERMODYNAMICS locally (e.g. in our office or bedroom), but only at the expense of a far greater increase in the entropy of our surroundings.16.3
Can we, however, beat this "entropy backlash" by building a much more efficient machine into which we program our will? Can we build a housekeeping robot that will keep our office/bedroom tidy without consuming more than a fraction of the energy it saves? Or, driving the analogy back to the microscopic level, can we build a "Maxwell's Demon" robot that will let only fast air molecules into our house and let only slow ones out, so that the average kinetic energy increases (i.e. the air warms up) and we can stop paying our heating bill? One problem is the cost (in energy or entropy increase) of building such a Demon-robot; but this can be disregarded if the robot is so well-constructed that it never wears out, since any such system that gains on the SECOND LAW will eventually gain back any finite initial outlay.16.4 If such a device is possible, then we can make as many of them as we please and use them to store up energy which we can use in even our less efficient machines to push back the tide of Entropy on all fronts. We can even picture self-replicating Maxwell's Demons that get sent out into the Universe to reverse the SECOND LAW everywhere - the ultimate Conservationist scheme! Never mind whether this sounds like a good idea; could it work?
The answer is still not obvious. We will have to come back to this question after we have a working knowledge of Quantum Mechanics -- and even then it will probably not be obvious, but at least we may be able to find an answer.