What is gravity? We know Newton's formula but he did not answer what the true nature of gravity is. I hear talk about "gravitons" — is this real or just another elegant metaphor? — BC 

Newton's gravity has been superceded by Einstein's gravity; the gravity of general relativity. In this understanding of gravity, the accelerations associated with gravity result from a curvature of space/time around concentrations of mass & energy. The gravity of general relativity is responsible for such exotic effects as the bending of light by gravity and the existence of black holes.
But physicists are still not satisfied with the gravity of general relativity. General relativity is what's known as a "classical" theory of interactions—it does not include quantum physics and is thus considered to be incomplete. All the other classical theories of interactions have given way to quantum theories. For example, the classical theory of electromagnetic interactions, dating from the works of Oersted, Ampere, Maxwell and others in the 1800's, was replaced in the 1940's and 50's by quantum electrodynamics, through the works of Feynman, Schwinger, Tomonaga, and others. Each time that a classical theory is replaced by a quantum theory, the responsibility for the interactions themselves shifts from classical fields (e.g., the electric and magnetic fields) to quantized or particulate fields (e.g., photons). These sorts of quantum field theories, theories in which interactions between particles are mediated by the exchanges of other particles (the particles of the quantized fields) are the bases for all modern interaction theories except gravity itself. People are still trying to quantize gravity but so far without real success. The particles that mediate gravitational interactions have been named gravitons, but the full theory in which these particles operate is still uncertain.


