I have not as yet been able to discover the reason for these properties of gravity from phenomena, and I do not frame hypotheses. For whatever is not deduced from the phenomena must be called a hypothesis; and hypotheses, whether metaphysical or physical, or based on occult qualities, or mechanical, have no place in experimental philosophy. In this philosophy particular propositions are inferred from the phenomena, and afterwards rendered general by induction.
[Isaac Newton, Mathematical Principles of Natural Philosophy, 1713]
We really know very little about matter, but we’re not quite as continent as Newton was when it comes to framing hypotheses. Descriptions of behavior, however precisely measurable, do not explain what really happens. Two bodies will exert a force on one another based on their masses and distance, but why that force is operating is not thereby explained. Quantum physics offers the notion that the force of gravity is mediated by a mass-less particle called the graviton (a hypothetical particle). In Relativity, Einstein offered as his hypothesis that mass causes time and space, the two combined in a single continuum called spacetime, to bend or to deform in some way thus creating a groove or slope in what (presumably) is an otherwise unchanging “environment.” This deformation is then used to explain gravitational effects. But what exactly are space and time? For Newton they were absolute and entirely unchangeable let’s call them realities; mass could not change either one or the other. For Immanuel Kant they were, at the same time, immaterial and thus insubstantial, in the human mind as a priori organizing principles enabling us to understand sensory experience and therefore ideas. But because they were linked to sensory experience, they were also empirically real. In other words we don’t really have an explanation—hence framing no hypotheses is indicated—at least in “experimental philosophy.”
Space and time are inherently involved in any kind of explanation of gravity, be it by gravitons or spacetime deformations—or invisible, intangible dark matter used by most astronomers to explain gravitational anomalies in star rotations around galaxies far away. That hypothesis, incidentally, has the small problem of increasing the total mass of the universe by huge gob. Visible matter is supposedly 4 and dark matter is 96 percent of the universe. Now supposing we did detect a graviton finally. Where would that leave us? It would leave us wondering what makes gravitons detach from one kind of mass to rush at the speed of light, and presumably no faster, to another essentially same kind of mass. And why then would this exchange become a “force”?
The description of the behavior of bodies must suffice us, I presume, because breakthroughs in the future will not provide any more knowledge than we have at present—only yet other hypotheses. Going down to have lunch now, I wonder if my mass will bend spacetime ever so, ever so faintly. Presumably so, if Einstein is correct.