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Is gravity a force?

  1. Jan 3, 2008 #1
    Is gravity considered a force in modern physics? Im familiar with Einsteins model of gravity as curves in space-time, and i do realize its not a force in the classical sense, but is it still considered a force?

    I want to clear up a debate im having with a friend :)

    My argument is that force is defined as being something that causes a body of mass to accelerate. He argues that the object is not actually accelerating, but appears to be accelerating in a curved space-time.
  2. jcsd
  3. Jan 3, 2008 #2


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    Depends on your point of reference. For instance, centrifugal force looks as if it doesn't exist to an outside observer who isn't rotating, but to an object that is doing the rotating, it's very real.

    So to us, surely gravity would be a force, but not necessarily in the grand scheme of things. And yes your friend would be right in the last sentence. In any case, translating from one reference point to another might turn virtual forces into real forces and vice versa.
  4. Jan 3, 2008 #3
    I believe the following is the current understanding but I sure the experts will correct any mistakes I make.

    In classical physics an object moves at constant velocity in a straight line (or remains motionless) unless acted upon by an external force. For an object to orbit a massive body, a force (gravity) is required to make the object deviate from a straight line and follow the curved path of the orbit.

    In General Relativity an object follows a geodesic path unless acted upon by a force. In the vicinity of the massive object the geodesic is a curved path (in this case the orbital path) and no force is required to maintain a curved path or orbit. The gravity of the massive object simply sets up the curved space that defines the geodesics. To make the orbiting object go in a straight line a force is required to make it maintain a straight line.

    The geodesic for every point particle in a gravitational space is determined by its velocity. For a very large orbiting object, like a moon we can draw 2 geodesics for the parts of the moon nearest and furthest from the massive body. It turns out that points at the extremes of the moon cannot both exactly follow their natural geodesics because their velocities are locked to each other by the rigidity of the moon, so the moon experiences tidal forces. Within a radius of the massive body, called the Roche limit the tidal forces can be enough to tear the moon apart, and the individual pieces of debris from the moon settle down into their own individual geodesic paths.

    For a stationary object the geodesic is straight down towards the the massive body. The stationary object feels a force because it is not following it's natural geodesic and this has to be countered by a rocket engine or the reactionary force of a surface in order for the object to remain stationary. When the object is released it free falls, following its natural geodesic and no longer feels (or requires) a force.
    Last edited: Jan 3, 2008
  5. Jan 3, 2008 #4
    I suspect what you and your friend are in fact debating is the difference between :
    1) Particle Physics, the Standard Model, & Quantum Mechanics where gravity is expected to be explained by force particle exchanges, just as they for the most part are for electric, magnetic, weak, and strong forces.
    2) General Relativity with Space-Time curves using at least 4 or 5 dimensions to eliminate the phenomena of gravity as being caused by ‘traditional’ force particle interactions.

    These are two distinct and different fields of physics with at least the fundamental understand of what gravity is, being irreconcilable between them. Many are straddling the fence between the two in an attempt to reconcile them. But to be successful most expect at least one of these two Major Theories of Physics will require a large enough change in its fundamental foundation to consider the current Theory flat wrong.

    In order to answer your question, is gravity a “force” or a result of “Curvature of Space”, requires deciding which of the two theories is “correct”.

    Astrophysicists and Particle Physicists base their work on two entirely different answers to that question. All still wait for something or someone to resolve the issue. So don’t expect to find a final solution in this thread. Although, you may gain info to choose what you like best as most must do in order to define their career path.
  6. Jan 3, 2008 #5
    Thanks that was very helpful...

    Which one has more momentum/backing in physics, the first or second? I don't know too much, but string theory falls under the particle physics right?
  7. Jan 3, 2008 #6
    IMO which one has “more backing” depends on what you are. Even as an amateur, are you an Astrophysicist or a Particle Physicist?

    The one I figure has the least chance are the dreamers that hope to combine both without declaring ether one as “wrong”.

    Other than that, I don’t think there is any saying which one is “winning” until one is accepted by its current proponents as being wrong.
    For now, each side kind of ignores the implications of the other, at least on this issue.
  8. Jan 3, 2008 #7
    Is it not weight that causes something to accelerate towards a mass, weight being caused by gravity. w=mg
  9. Jan 3, 2008 #8


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    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.
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