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Questions about gravity and curved space

  1. Apr 13, 2008 #1
    Hi, i just found this forum when trying to find an answer to a question. I am not a physicist, not even close, I'm in highschool still. Anyway, i will try to explain my question the best i can. It would be a lot easier if i could draw it but alas i can not...

    I am trying to conceptualize curved space in my head. To me its the equivalent of trying to make a globe of the earth into a flat map of the earth.

    From what i understand, from our perspective, space isflat, however if looked at from an outside perspective space folds on itself, hence where the possibilty of wormholes come from, opening short cuts across these folds of space. When i think about this, and try to combine it with my understanding of gravity, which is that, as analogy, gravity wells are like the effect an object placed on a stretched sheet has. If you were to put a marble on a sheet, it will create a depression. A bowling ball will create a bigger depression. Also variations of denisity will create deeper depressions. The depressions affect other objects as they roll along the sheet, and fall into the depression.

    Now, if I say that the sheet is space, and space is curved, if two depressions on either side of a sideways "U" shaped space are so "deep" that at some point they would either interact with either other and combine, or pass right by each other, creating some kind of anamoly in the middle, because from the wells' perspective they are far apart and not interacting, they will continue moving "down" relative to their perspective.

    Another way i thought to describe it is this.

    Situation 1: Picture a cross section a curved space, with an object like he earth, on opposite sides. The shape of the cross section would look like a peanut, with the two depressions creating a fat figure 8 shape.

    Situation 2: Now imagine at a certain point, the two depressions merge, creating a perfect figure 8 shape. This to me would be something possibly like a black hole, but i don't know. I would think that the cross over would be when a neutron star becomes a black hole, but thats assuming black holes are special. For all i know they could just be very very dense, and the fact that light can't escape makes us believe they are special. Anyway...

    Situation 3: Imagine now that the objects are so dense, perhaps, black holes, perhaps not, that they go beyong merging, and continue on in their respective "down" directions. This would create some "overlap" between, where on each side, the object on the left is being pulled left by gravity but is actually below the object that started on the left, which is now on the right. The area between would be up or down relative to which side you started from?

    Also, is there a maximum size a gravity well can be? At some point would the "sheet" "rip".

    I am also now realizing that i am assuming the "space" in the middle has dimensions, which i don't know either. As you can see, i am very confused XD IF you can decipher my question and idea, could you enlighten me to what the reality is?
  2. jcsd
  3. Apr 13, 2008 #2
    Gonna bump this before i go to bed, so in the morning hopefully i will be enlightened :P
  4. Apr 13, 2008 #3

    Vanadium 50

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    Um, why do you think this is beyond the standard model? (One could also ask why you think a bump after 45 minutes would make a difference, but I think more clarity would be added if you answered the first question)
  5. Apr 13, 2008 #4


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    This is a misleading analogy of gravitation in GR. The sheet represents curved space only. But the cause of gravity is curved spacetime. Here a much better visualization, of curved space vs. curved spacetime and their effects:
    more links to curved spacetime visualizations:

    Spacetime can appear "infinitely stretched" to a distant observer. But locally everything can look normal. So it is not really a ripped.
  6. Apr 13, 2008 #5

    Why are you showing the same links again and again?
  7. Apr 13, 2008 #6


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    Because some people don't search the forums and ask questions, that have been already discussed multiple times.
  8. Apr 13, 2008 #7
    Alright thanks A.T. that is a very helpful read.
  9. Apr 13, 2008 #8
    I beg to differ. This has been discussed in detail in another thread and the reason that is not true is explained there. Basically it is tidal gravity that is curved spacetime, not gravity itself (tidal gravity = non-uniform gravity = tidal gradients).

  10. Apr 13, 2008 #9
    Can you link to that thread so i can check it out?
  11. Apr 13, 2008 #10
  12. Apr 14, 2008 #11


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    What pmb_phy is saying is that, in a region of spacetime where you would say, in Newtonian language, "the acceleration due to gravity has a constant magnitude and direction", in GR language you would say there was gravity but no curvature in spacetime.

    (And in a free-falling GR frame there would be neither gravity nor curvature. Gravity depends on which coordinate system you choose, but curvature does not. In a sense, curvature is the variation in gravity rather than gravity itself.)

    Nevertheless, this is a technicality, and in practice, in the real universe, finite lumps of matter cause both gravity and curvature.
  13. Apr 14, 2008 #12
    Another thing that i'm not quite clear on. Now by now i bet your rolling your eyes, lol, but does this conflict with string theory? I was watching a documentary like last year and it was talking about gravitons, and how they are attracted to the far end of the "loaf" of bread where our universe is a "slice". Now are they saying in that that gravity is a force, or that the gravitons are the reason mass creates curvature in spacetime, or something else that i have overlooked?
  14. Apr 15, 2008 #13


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    When a mass accelerates, it causes a change in the curvature, which travels as ripples or waves in spacetime. The ripples travel at the speed of light. In quantum theory it is thought that these waves consist of gravitons. So gravitons are the mechanism by which a change in curvature travels from one place to another.
  15. Dec 21, 2008 #14
    Just want to say that this is a very nice thread :)
    And I like your links.

    Btw: Corvetteguy, are you satisfied with the answers?
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