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How does gravity tend to propagate?

  1. Apr 30, 2009 #1


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    I was wondering, how does gravity tend to propagate? Across multiple examples, such as from a planet, a star, a galaxy, a black hole etc. I've always thought of gravity as something that propagates outwards in all directions, but the more I learn the less I'm starting to think that's the case.

    The main thing that confuses me, is how galaxies and black holes appear to have a shape or structure which is disk like, having jets of matter spewing out from the top and bottom. How is this that gravity causes things to form in disks? How does the gravitational force compare at the poles than it does around the equator of the object?
  2. jcsd
  3. Apr 30, 2009 #2
    too many complex questions..but here are a few brief explanations:

    In classical Newtonian physics gravity is a force directed along a line between the centers of gravity of two masses....in General Relativity gravity is viewed as the result of curvature of space time..hence objects "fall" towards each other...like the bowling ball on a trampoline analogy.

    many galaxies are disk like, many are not...Black Holes are not disk like, but point like with a singularity at the center with a spherical event horizon (when the black hole is rather stationary) but is often represented as a disk in a two space dimensional and a time dimension along the vertical axis...so it appears disk like in the simplified form...

    The jets emanating from black holes are actually a complex phenomena with an observer free falling toward the absolute event horizon seeing virtual particles emitted while an accelerating observer, stationary above the horizon, observes REAL particles being emitted. Matter is not emitted from a black hole, but energy does radiate.....oddly making the black hole hotter!!!!
  4. Apr 30, 2009 #3


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    I see, thanks. So the analogy of a sphere is a pretty solid way to imagine the propagation of gravity for a body like that?
  5. Apr 30, 2009 #4


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    Staff: Mentor

    You'll note that Newton's gravity equation doesn't say anything about direction, so it acts the same in all directions. The natural result of such action is spherical bodies.
  6. Apr 30, 2009 #5


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    Thanks for the input. I'm probably moving further into astrophysics here, but why do some galaxies develop discs, or even simpler, why do the planets orbit roughly the same path around the sun, when gravity propagates in all directions? Has that got something to do with the magnetic field?
  7. Apr 30, 2009 #6


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    The disc formations are the effect of gravity, but not gravity from the main body in the system - gravity from the secondary bodies on each other.

    Particles or bodies that are in randomly-oriented orbits around a central body are not stable. They push and pull on each other, knocking each other into slightly different paths as time passes. Many will end up falling into the central body or being ejected from the system. A whole pile of different things happen but the net effect is that particles in orbits that same the same orientation are more stable.

    The solar system, as with other star systems, probably started off spheroidal and only flattened as it aged and stabilized.

    Generally, disc-like systems are an indication of a system that has been around for a longer time without being disturbed than a spheroid system.
  8. May 1, 2009 #7
    Good point...I knew I missed something!!!
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