Philosophically, the idea of forces is kind of shady. While we don't want to outright deny the notion, we also don't want to ignore the danger of possibly believing in what could turn out to be the scientific equivalent of leprechauns. The first problem is that 'forces' are not things experienced directly; rather, they are inferences produced on the basis of other experiences, and as inferences, these could turn out to be misinterpretations of what might be entirely different phenomena.Take the case of gravity. Before Einstein, it was believed to be a force in the classical sense, but post-Einstein, our understanding is that gravity is not so much a force as an illusion of a force produced by the curvature of space-time.
So, while Brian Cox introduces forces as the 'agents of change" of the universe in the following video clip (and does a nice job of explaining the basic history behind their discovery, understanding an unification), one should still be aware that he's treading in conceptually dangerous territory, and that just like philosophy devoid of science is likely to get itself in trouble, science would be equally foolish to eschew the importance of philosophical analysis. Let's make love, not war :)
I'm kind of curious... whatever dark energy turns out to be, is it possible it might provide evidence of a hitherto unknown (and possibly even unsuspected) new force?
And if you want a fuller treatment of these ideas, check out the Elegant Universe tag, and especially the documentary Atoms, with Jim Al-Khalili.
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