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Gravitational Theory, Don´t grasp it.

  1. Aug 12, 2008 #1
    If matter distortions the spacetime structure and this structure keeps, let´s say, matter in orbit, i can´t think of this structure as inmaterial.
    How space time hold matter in orbit? by contact???
    Hasn´t something to be made of matter to curv?
    I understand ¨structure¨ of spacetime as matter??? HEEEELLLLPPP!!!

    Is spacetime something we can´t experience?

    Sorry about the english, i´m from argentina.
  2. jcsd
  3. Aug 13, 2008 #2


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    I think it's fine to think of spacetime as "material" in that case. Rindler describes geodesics (spacetime trajectories of particles under the influence of gravity and no other forces) as a unique set of rails!

    "In 1854 the railroad reached Goettingen, but Bernhard Riemann unveiled there an even more important system of rails, the geodesics of curved spaces."

    From Rindler, General Relativity before Special Relativity, American Journal of Physics 62:887-893, 1994.
  4. Aug 15, 2008 #3
    Space-time that we not only can't experience but also we can't imagine. But it can be understood only through noneucludean geometry/tensor calculus.
  5. Aug 15, 2008 #4


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    This is true, but Moeller in 'General Relativity' shows that space-time can be modelled as a dispersive medium with a refractive index. The book was published in 1950 and it is not a popular view now.
  6. Aug 16, 2008 #5
    "How space time hold matter in orbit? by contact???"
    This by no means any body can guess even now
    - Einstein said "There is no sense in regarding matter and field astwo qualities quite different from each other ... Could we notreject the concept of matter andbuild apure field physics? We could regardmatter as the regions in space where the field is extremely strong. A thrownstoneis,fromthis point of view, a changing field in which the states of the greatest field intensity travel through space with the velocity of the stone" -
    mystery lies in the fecund soil of quantum electro/chromo area but Einstein from 1930 till his death tried to establish his belief through only math without any result.
  7. Aug 18, 2008 #6
    Spacetime doesn't have to "hold" anything in orbit. The orbital trajectories are the natural paths a body will follow when NO forces are applied.

    We just have to restate Netwon's fisrt law:

    Original says: "Every body remains in its state of rest or uniform motion in a straight line unless acted upon by a force".

    After General Relativity, it has to be re-phrased to say: " Every body traces out a geodesic in space-time unless acted upon by a force."

    If there were no other masses around, the geodesics would be the same as Newton's
    "straight lines". But when there is another mass, Schwartzschild found the only spherically-symmetric solution for the spacetime around a single point mass, and its geodesics were not straight lines, but Kepler-like orbits (with a small correction). So a body's orbit is really a straight line in spherically symmetric space.

    Think of drawing the straightest possible line on the surface of the earth; what do you get? A Great Circle.
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