Einstein
established two rules: (1) the laws of physics must have the
same form in all frames of reference, inertial or
non-inertial, and (2) the effects of acceleration are locally
completely indistinguishable from the effects of gravity.

Consequences: light is
affected by gravity, processes run slower if they experience
more gravitational acceleration, light leaving a massive body
loses KE so its frequency decreases, etc.

Einstein found that the
simplest way to include all effects is to replace
gravity by space-time curvature. Thus gravity is reduced
to geometry. Objects follow a geodesic (shortest
possible) path in curved space-time.