In his [Albert Einstein] theory,
the gravitational force arises as a consequence
of space and time (which together form “spacetime”)
being curved by the presence of matter. A
loose analogy is that of a bowling ball
placed on a rubber sheet along with a
marble that is rolling around nearby. The
balls could represent the sun and the
earth, and the sheet is space. The bowling
ball creates a deep indentation in the rubber
sheet, and the slope of this indentation
causes the marble to be deflected toward
the larger ball, as if some force—gravity—
were pulling it in that direction. Similarly,
any piece of matter or concentration of
energy distorts the geometry of spacetime,
causing other particles and light rays to be
deflected toward it, a phenomenon we
call gravity.

...

Because general relativity deals in the geometry of
spacetime, a quantum theory of gravity
will in addition be a quantum theory of
spacetime.