Newton, Einstein and Gravity?

Question

if an object attracted by another object actually has no force exerted on itself but is an 'illusion' from the space around it being bent like gravity, why do the newtonian laws of motion seem to fit this phenomenon?

Answer 1

Drs. N and E simply use a different coordinate systems. Dr N's F=MA includes an acceleration A, which is defined as the second derivative of position wrt time. Here, "position" refers to coordinates in a given coordinate system, or reference frame. Dr. N assumed space could represented by a Euclidean metric, so the surface of an isolated mass (far from other masses) can be considered stationary. That means if you release something above its surface, it accelerates downward.

Dr. E pointed out space cannot exactly be represented with a Euclidean metric. Instead, he defined a different coordinate system (a dynamical one that moves with time) consistent with his principle of equivalence of acceleration and gravity This is the axiom of General Relativity that separates it from Special. In this coordinate system, a person on the surface of Earth is actually *accelerating upward* at 9.8 m/s^2 due to the pressure of the ground on his feet. In order to be "stationary" one must be in free fall. So, it's the *ground* that actually applied a force, not gravity.

So you see, it's all relative :-)

Answer 2

Newton's laws don't tell us what causes physical phenomena, they merely describe them. His formulas for gravitational attraction work quite well. Einstein took it step farther, explaining that gravity worked because of the curvature of space. There is really no conflict between the two.