[annalex]

Wrong. Thirty-five years ago you could still find a physicist talking about a cetrifugal force. But that force doesn't exist; it only appears to exist; it is a fictitious force. A gravity field does not emerge to push you into your seat.

I was referring to the General Relativity Equivalence Principle:

Experiments performed in a uniformly accelerating reference frame with acceleration a are indistinguishable from the same experiments performed in a non-accelerating reference frame which is situated in a gravitational field where the acceleration of gravity = g = -a = intensity of gravity field. One way of stating this fundamental principle of general relativity is to say that gravitational mass is identical to inertial mass.

(Principle of Equivalence)

Given our laser beam focus on the free will and the errors of Luther, it is a bit offtopic. The classic example is that the gravity one experiences in a static elevator at the earth surface is undistingushable from the inertial force in an accelerating elevator in the outer space. Gravity is acceleraion; the underlying physical reality is that either gravity or acceleration are different labels we put on the curvature of timespace. Likewise, in an accelerating car the observer experiences one curvature that pushes him and his coffee downward, formed by the mass of the earth, and he normally refers to that one as the earth gravitational pull; and the other curvature pushing him to the backrest of the seat and the coffee to the edge of the cup, created by the energy of the engine, and he normally refers to that one as inertial force. If instead of a car accelerating along the surface of the earth we had a car-shaped spaceship accelerating upward (toward the roof of the car) and pulled by a planet behind its hood, with a proper adjustment of the energy of that spaceship providing the upward thrust, and the size of the planet providing the rearward pull, we would have the exact same observations, even though the directions of gravity and acceleration have been swapped, gravity threatening to spill the coffee and acceleration securing it inside the cup.

The famous ontological equivalence of mass (source of gravity) and energy (source of acceleration) follows from this example.

[stripes1776]

Experiments performed in a uniformly accelerating reference frame with acceleration a are indistinguishable from the same experiments performed in a non-accelerating reference frame which is situated in a gravitational field where the acceleration of gravity = g = -a = intensity of gravity field. One way of stating this fundamental principle of general relativity is to say that gravitational mass is identical to inertial mass.M

The key is uniformly accelerating. That wasn't my example at all. A car traveling at constant velocity that undergoes a sudden acceleration is not traveling at uniform acceleration. The coffee just continues at the same velocity in the horizontal direction because the friction forces between it and the dashboard are not great enough to create a differential in the volocity of the cup, i.e. to accelerate it. That is why the cup slides off the dashboard and you have to invent a mysterious fictitious gravity force that emerges out of nowhere because you forgot to notice that the original acceleration was zero.

Non of this changes Einstein's observation of a uniformly accelerating reference frame. You have to meet the condition before the principle applies. If an object in space is undergoing nonconstant rates of change of acceleration with repect to time and you don't know what force is causing the chanes in acceleration and you have no inertial reference frame, then you will be inventing all sorts of fictitious forces to explain these "jerks" in motion.