What is a force, exactly?

This is not so much a question as a philosophical discussion of physics. Being neither physicist nor philosopher, I’ll do my best, but you would do better to work your way through some of the books such as Penrose’s Road to Reality.

>Human beings have identified four fundamental forces of nature (the strong nuclear force, the weak nuclear force, the electromagnetic force, and gravity), and it appears that these four fundamental forces enable us to account for everything we observe in the universe (including all matter – i.e. all particles and subatomic particles are understood to be stable manifestations of these four fundamental forces). It appears that all other forces observed in nature can be derived from these four fundamental forces.<

Well, for a start, weak and electromagnetic forces have been shown to be fundamentally the same, aspects of the “electroweak” interaction, though their actual unification is only obvious at very high energies, at which they become indistinguishable. By way of analogy, at supercritical temperatures and pressures there is no way to distinguish between liquids and gases.

Furthermore, there is a suspicion that there are similar grounds for unifying the electroweak and strong forces, though there is no hard evidence for that as yet. There also are some nagging difficulties to be overcome if this is to be shown to be true. For example, some “laws” of conservation, such as baryon number would have be violated or circumvented. That does not prove that the idea is wrong of course; it has been done before in the violation of the conservation of parity, as long as charge and time can adapt to make up for the violation. What had seemed to be a simple conservation turned out to be part of a more complex conservation, which last I heard, was still intact.

>If it is correct that Energy is defined not by what it is but by what it can do, then is it correct to assume that energy must be some measure of the fundamental forces of nature acting upon one another?<

Correct? It seems reasonable, but a little vague. Reasonable is about as good as things seem to get in these matters.

>The fact that mass (such as matter) can be converted into energy (such as in a nuclear explosion) in accordance with Einstein’s equation suggests that everything we observe in the universe is simply the various stable manifestations of the four fundamental forces acting upon one another. If this is correct, then what is a force, exactly (i.e. what is a force, not what does it do)?<

Again, that is a bit vague. Do we have words for what you are asking? Could you give us a reasonable basis for the distinction between what a hammer or a bench “IS” and what it “DOES”? What is the “HAMMERNESS” of an object on my bench, as opposed to the same striking a nail or a billet of hot iron? When does my bench cease being a bench and become a barrier or a seat? We can define force as a term in Newton’s F=MA, but how do we tell from the figure that we calculate, the we were referring to potential, kinetic, or electromagnetic force, say?

>According to the second law of thermodynamics, the energy of a closed system always tends towards equilibrium (because an equilibrium state has a higher probability than any other) and the overall entropy always increases. Can energy (all forms including heat, potential energy, kinetic energy, chemical energy etc.) be explained and defined by the fundamental forces of nature tending towards equilibrium?<

In so far as this might be meaningful, I suspect that you might be confusing two concepts: the cardinalities of the states that we see as being “in equilibrium” relative to the far smaller cardinalities of the states we see as being “out of equilibrium”; and the actions of forces that alter states. To answer your question concerning the relationship between the two, in particular their possible identity, you would have to show something very significant about what forces have to do with equilibria, and possibly what both have to do with time. Certainly at the quantum level there does not seem to be a clear connection. Good luck with that one!!!

Cheers,

Jon