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Gravity and curvature in spacetime

  1. Jun 11, 2008 #1
    I'm am new to the forums and have a quick question. When I see diagrams of objects in the universe on the spacetime fabric, the bottom of the object seems to be touching the fabric. My question is, does the fabric touch Earth for example on the south pole like presented in the pictures, or on all 360 degrees of the Earth. Another question is, is the concept of gravity and the orbits of sattelites different than the concept of gravity that keeps us on the ground.
  2. jcsd
  3. Jun 11, 2008 #2


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    Welcome to PF!

    The diagram you would have seen do not represent any physical theory. They are just a simplified metaphor to try and convey the idea that gravity is caused by curvature. For starters, the 'fabric' in the pictures is just 2D, when it is representing 4D spacetime.

    As for the second question, yes, it is the same concept of gravity that does both of those things.
  4. Jun 11, 2008 #3

    a) The "space-time fabric" touches the Earth everywhere. It is more like an all encompassing "blanket" or "web" that has four (maybe more) dimensions. As the earth rotates it drags this "space-time" web with it also. Those pictures in books are only simplified two-dimensional models but are not totally accurate.

    b) The answer to your second question is yes.... gravity is the same no matter what. As long as any object has "mass" it will be affected by gravity's "force". Gravity's "force" is due to space-time curvatures that cause inertially moving objects to accelerate towards each other.

    The reason satellites don't come crashing down directly into Earth due to this "curvature" is because the satellites are rotating at a certain speed that allows them to escape accelerating directly into the Earth. Thus they will always be rotating but never "touch" the earth.
  5. Jun 11, 2008 #4
    Thanks for the replies. I had trouble trying to understand how there could be a flat sheet spread across the universe if all the objects are in so many directions. So is the tension between Earth and the fabric, the force that makes objects fall to to the center of the Earth, for instance an apple falling from a tree?
  6. Jun 12, 2008 #5


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    Bear in mind that gravity is not a force [in the classical sense]. Einstein viewed gravity as a topological anomaly. Many scientists have misunderstood this point. It looks, smells, and behaves like a classical force, until you push it to the quantum level.
  7. Jun 21, 2008 #6
    Well you may not be able to tell, but the space around is bent. It's bent because of how large and massive the earth is. So yes, as you try to "escape it" (by jumping), you can only get so high up before you come back down.

    It's similar to trying to run up a wall. You can only go up it so far before you fall on your head.
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