You seem pretty well grounded in science and philosophy to me. But it's not my intention to lead the
innocent astray, if that is the case. I tend to unconventional interpretations and personal theories, so what I say should be taken as food for thought.
AllShips wrote:We're commonly told there are four fundamental forces of nature. We're also commonly told ... that gravity is not a force... rather, simply the curvature of spacetime...Is gravity a force or not?
I think you are best off considering gravity as a force. I certainly do. What distinguishes force expressed as gravitation from expression as electromagnetism(EM) is the
directness of interaction.
You are doubtless aware of
Newton's third law (
N3), which I interpret (
watch out) as,
Forces are always observed in pairs. Well, two particles interacting electrically are thought to do so by each acting upon the other, a fairly direct mechanism. By contrast, two particles interacting gravitationally are considered to do so by the effect each has on the continuum separating them, a less direct mechanism.
So you will find electric interaction modeled by particles emitting so called "force carriers" (virtual photons) at each other. And while the standard model also predicts a force carrier for gravity (the graviton), it has so far, not been forthcoming.
Now to me, there is no reason the same force can't do both. Generally speaking, what a force, as an object, does to
space is one thing (gravitation) and what it does to a
particle is another (EM). The other two "forces" can be similarly imagined but that would be asking a bit much in this discussion. It can be said that this model needs no force carriers, since a force may be seen as an object in itself.
I think it would be fair to say that if pressed, Einstein would admit that
how a mass causes space to curve may be construed as a force. He would have preferred the term "field" as would all those espousing quantum field theory today. Nevertheless, if you press them as to:
A field of what?, the answer will boil down to a field of
potential force. That is, a set of locations where a test object would experience a force (and in doing so become half of a Newtonian pair).
In my view, a reasonable corollary to
N3 would be,
Unpaired forces are unobserved. This has the decency to allow for
Newton's own impression that there is
conservation of force (conventionally supplanted by conservation of energy). I find this much more appealing than expecting forces to simply
disappear every time they are not paired. We can't observe what forces do when they are
off duty but we can easily surmise that it is generating fields.