Darren New <dne### [at] sanrrcom> wrote:
> It *is* a force, depending on your definition of "force". :) As in 
> "F=ma". If you're going to argue gravity isn't a force, then magnetism 
> isn't a force either, nor is pushing on something.

  According to GR gravity is not a force. A force causes acceleration and
gravity doesn't.

  An object in free fall does not accelerate, it's simply travelling along
its geodesic. It might *look* to us like it was accelerating, but that's
only because we are seeing a 3D slice of space-time. (It's a similar effect
that railtracks in a photograph look like they are converging even though
in reality they are parallel and never converge.)

  An object "resting" on the surface of the Earth, for example, is in
constant acceleration, caused by the force exerted by the surface of
the Earth. This force makes it constantly deviate from its geodesic.

  What gravity is, is a deformation of space-time. Masses deform space-time
in such way that geodesics are not linear anymore (in the cartesian sense),
and thus objects in free fall which travel forward in time will travel
along these geodesics and will look like they are accelerating in our 3D
slice view of the world.

  It's not even a question of "definition" of "force". According to GR
gravity is *not* a force because it doesn't cause an acceleration, and
a free-falling object is *not* accelerating.

-- 
                                                          - Warp

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