# Is gravity a force?

## Main Question or Discussion Point

Gravity, as defined by Einstienien Theory, is a warping of space. And any mass traveling through space will take the path of least inertial resistance, ie not necessarily a straight line, dependent on that warping of space. So, is gravity a force, or just an expression of warped space, that happens to influence matter passing through it?

## Answers and Replies

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Drakkith
Staff Emeritus
Science Advisor
See this thread: https://www.physicsforums.com/showthread.php?t=207078

I'll quote the last post.

The best short answer is that both views are basically correct. Speaking rather loosely, given a particular coordinate system, the curvature of space-time can be reduced to a "curvature of time" and a "curvature of space". (For the more expert here, we are using the popular term "curvature" to describe the Christoffel symbols, and not the Riemann curvature tensor, and the above classification is slightly oversimplified).

The "time curvature" part of GR introduces both gravitational time dilation, and acts mathematically in the equations of motion (the geodesic equation) just as if it were a force.

The "space curvature" part of GR cannot, however, be directly modeled as a force.

So the space-time curvature model is a more complex model than a force model, because it includes ideas that can be interpreted as forces, and ideas that cannot be directly modeled only by forces.

A.T.
Science Advisor
Gravity, as defined by Einstienien Theory, is a warping of space.
Space-time not just space.

And any mass traveling through space will take the path of least inertial resistance, ie not necessarily a straight line, dependent on that warping of space.
A free falling object travels on straight paths (geodesics) in wrapped space-time, which might look curved when projected onto space.

So, is gravity a force, or just an expression of warped space, that happens to influence matter passing through it?
Both models can be used to describe the common effect of mass attraction. But Einstein's model predicts some effects like light bending better than Newton's model, and gravitational time dilation which Newton doesn't predict at all.

Also note that even in Einsteins model you can treat gravity as a force, but unlike in Newton's model it is an inertial force due to an accelerated reference frame, not an interaction force:
http://en.wikipedia.org/wiki/Fictitious_force

But if you model the accelerated reference frame with distorted coordinates, instead of inertial forces, then indeed there is no "force of gravity". And free falling a objects follow a straight path as shown in this animation:

https://www.youtube.com/watch?v=DdC0QN6f3G4