So on another post somewhere it was mentioned that while velocity is relative, acceleration is absolute, this might be true for the magnitude of acceleration but how are we sure of the direction, am I accelerating or is the rest of the world. Is there any design of accelerometer that doesn't work by measuring the relative motion, and therefore acceleration, of two components? Under the force of gravity the jar and cork are accelerated uniformly and so no measure of acceleration is given. I understand that in most cases we can tell which of two bodies is accelerating by comparing measurements on their accelerometers, but if two people are are constant velocity in space, and one nears a black-hole(with no stars around for gravitational lensing) their relative velocity increases, but which one is near the black hole? So if objects in relativity's gravity follows geodesics rather than accelerating how do their relative velocities increase?