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What is gravity?

  1. Jul 8, 2010 #1


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    I know gravity is a force, along with electromagnetism, strong-weak nuclear. I can understand how they work. I also know that a huge mass attracts other masses, but how?
  2. jcsd
  3. Jul 8, 2010 #2
    You're gonna have to specify a little
  4. Jul 8, 2010 #3


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    I dont know if it will make it easier, but how does gravity work?
  5. Jul 8, 2010 #4


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    Staff: Mentor

    I'm not sure those questions are really answerable.
  6. Jul 8, 2010 #5


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    Science Advisor

    Mass produces the gravitational field, just as charge produces the electric field.

    The difference between Newton and Einstein is that the source of the gravitational field is energy (which includes mass), and the gravitational field is not a scalar, but a tensor (a scalar is a tensor, but there are tensors that are not scalars).
  7. Jul 8, 2010 #6
    As russ said, that question may not be answerable.

    There are basically two models of gravity. One is from general relativity, and it is very well developed and has very strong predictive and descriptive ability. In general relativity gravity is a consequence of geometry. The geometry of spacetime bends such that objects move not in what we think is a straight line, but curved towards mass. In the other model gravity is manifested by particles, and these particles 'carry' the force from the mass exerting the force to the mass feeling the force. This model is not as well developed and does not have as strong of predictive ability. Of course we would expect that these models should be different sides of the same phenomenon and thats what alot of theorists work on, trying to reconcile these two models.
  8. Jul 9, 2010 #7
    We've theories which answer 'how much' , not 'what'.
  9. Jul 9, 2010 #8
    One of the things Einstein dispelled was the notion that masses act directly upon one another - Einstein postulated that matter affects space - and that the conditioning of space is what causes the local affect we observe when objects are attracted to one another.
  10. Jul 9, 2010 #9
    Mass acts to change the shape of spacetime. In turn, the shape of spacetime determines the trajectories of particles in such a way that the spacetime interval between any two points on a trajectory is maximal.

    This is the spacetime extension of the notion that the shortest distance between any two points is a straight line. We might see the path of an orbiting planet as having an elliptical orbit, but for two points on that orbit, the spacetime displacement between two points on the orbital is longest.

    You might be thinking "why longest?" In space it's the shortest distance. In spacetime intervals, it's the longest, at least among the contenders of neighbouring paths.
    Last edited: Jul 9, 2010
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