stingc

How else do you define an object? I don't know what anything 'is,' nor do I even know what an answer to that sort of question would look like. Give an example of what you mean.

Gravity is understood classically (at least the fundamentals are), but it has been very hard to quantize the theory. Your comment about extra dimensions is referring to string theory, which is one of the postulated extensions of gravity into the quantum world (it actually does quite a few other things as well). Despite being in fashion right now, those ideas are very far from representing anything very coherent so far.

A fundamental problem is that the gravitational field is actually much more complicated than the one I described in my previous post. That field is for Newton's theory. It is a simply a vector (3 numbers) defined at each point in space for all time. Einstein's version has 10 numbers at each point in spacetime, but more importantly, the meanings of those points change depending on the values the field takes on. It is describing geometry. Before general relativity, space and time were considered immutable concepts, but it turns out that they are not. It is much less difficult to come up with a theory describing a field existing on a fixed geometry versus one which changes the very nature of space and time as it 'propagates.' If you want to confuse yourself, try to imagine what it means for something to move when the meanings of distance and time change as it goes along. Now try to find a quantum theory describing that.