In what way do we still not understand gravity?

[dydxforsn]

I've always thought that Einstein's theory of general relativity was quite beautiful, it explained gravity as merely an intertial tendency of objects to move in straight lines (albeit different kinds of straight lines defined to be paths of greatest proper time) in a curved space-time. This explains why it's so distant from other forces in the standard model, because it's really a result of geometry.

In what way do we still not understand the mechanism of gravity? It would seem that this is quite explanatory. I do not doubt the claims that we don't quite understand it, as our knowledge of something is never entirely certain, but objects travel in straight lines seems to be a good enough reasons for the effects of the so called "gravity". Is it just a stagnant philosophical struggle that we do not quite understand gravity, or is there more to this claim?

 

[1mmorta1]

Are you familiar with gravitons?

 

[Drakkith]

See here: http://en.wikipedia.org/wiki/Gravity#Anomalies_and_discrepancies

That has a decent list of a few discrepencies or unknowns.

 

[1mmorta1]

There are two ways of approaching physics that are proven: Relativity and Quantum Mechanics...both of which can be compatible with the standard model. According to relativity, spacetime is smooth and bending spacetime is an inherent property of matter...much like inertia.
According to QM, spacetime is much different. It isn't smooth, and gravity is projected by a massless, spin 2 boson called a graviton.
The reason we don't understand gravity is that quantum mechanics and general relativity are incompatible at the energies and distances on which gravity is a factor in both theories. Infinite energies and other odd things pop up.
Remember, gravity, the weak force, the strong force, and the electromagnetic force were all unified at the universes birth, so it is necessary to calculate massive velocities and densities(problems suited for general relativity) as well as very small time scales and distances(problems suited for QM) when studying the origin of the universe. One mystery of gravity is why it has turned out to be so much weaker than the other three forces.
If you find a way to reconcile the two in a quantum theory of gravity, you'll win the nobel prize, and should probably look at the Riemann Hypothesis next :)