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Universal speed of gravity attraction?

  1. Jan 11, 2005 #1
    In theory:

    If mass is created all the sudden, how long does it take for another pre-existing mass to "feel" the new one? is this dependant on the medium through which the "pulse" travels? would it be instant? would it be light speed?
  2. jcsd
  3. Jan 11, 2005 #2
    According to the General Theory of Relativity (GRT), the speed of gravity is the speed of light. So if (for instance) the Sun were to disappear right now, we wouldn't know it for the eight minutes it would take both the change in the light and the change in the gravity to reach us.

    This is one of the major differences between GRT and Newton's Theory of Universal Gravitation (TUG); the TUG says that the speed of gravity is instantaneous. This introduces a difference in the way that the planets are predicted to orbit the Sun; these differences are most notable in the closest planet, Mercury. In fact, one of the known problems with the TUG that was being studied at the time of the discovery of Neptune (during the mid-nineteenth century) was the orbit of Mercury.

    Mercury, like all the planets, orbits the Sun in an ellipse. The Sun is at one focus of this ellipse. The major axis (the line connecting the foci of the ellipse) of this ellipse rotates around the Sun. After accounting for all of the influences of the other planets on Mercury, astronomers found that there was a remaining unexplained discrepancy in the rate of rotation of the major axis of Mercury's orbit, an error of about 43.03 seconds of arc per century. This meant that Mercury's orbit's major axis was making a complete extra turn, and an unexplained one, every 3 million years.

    In 1915, Einstein, using the equations of GRT, showed that it accounted exactly for this discrepancy. This was the first astronomical proof of GRT.

    Thanks to Dr. Isaac Asimov, for the explanation in his History of Physics, Walker, 1984, which I have used as a source.
  4. Jan 11, 2005 #3
    can you do a little bit explanation here?
  5. Jan 12, 2005 #4


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    The answer is very simple. You can't create mass "all of a sudden". So you get paradoxes when you assume you can. The solution to the paradoxes - realize you can't "create mass", because it is conserved.
  6. Jan 13, 2005 #5
    Given that I explained the problem with the orbit of Mercury and the resolution of it by GRT, I guess I'm not quite clear on what you'd like explained.
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