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Einsteins gravity theory (bending space) question

  1. Nov 13, 2009 #1
    Ok I don't know much about physics but have read a tiny bit for fun.

    Now einsteins theory that say for example the earth is bending space and that is why things go toward it.

    Two analogies I have heard about this one simple one is if you take a sheet stretch it taught but then put a heavy marble in the middle anything you put on the sheet will than move toward the center marble.

    Another example was of how forces do not exist and are something that is a part a higher demention. The example was of flatlanders on a sheet of paper supposedly if you fold and crumple this paper the flatlander would experience these folds and tears as invisible forces.

    But what I do not understand is how can the gravity force be bending of space when gravity is required for those bends and curves to be detectable as a force. So if for example we take those flatlanders on the bent sheet into space with no gravity the "force" disappears. Anything can be bent in any possible way but take away gravity and it will have no effect on the direction something on this bent sheet or peace of paper or anything will want to move.

    Is it maybe because the distant between two points increases in one direction and decreases in another due to the bending of space which causes you to take more time to get from one place to another is felt simply as a force by us and maybe we feel it as a force and see it as a force because all matter (like light) takes the easiest path to get somewhere. Which would be if the bent space does not change the shortest path.

    Or maybe does the bent space change that space turning it into a proverbial glass that is harder to move through. (just like the light wants to spend least time in the hard to move through glass matter does the same avoiding whatever space is hardest to move through which has been effected by other matter bending it/ changing it in some other way. Like the earth being that giant marble on the sheet that is space. Since there is no gravity pulling it down towards the marble of the sheet it is instead that either the space is curved in a way that makes the distance in a direction toward the center of the earth less than the distance going away from the center of the earth or it distorts the space in some other way to make travel toward the earth easier than away from it and matter then like light follows the fastest path.

    Hey maybe does it change space by a large mass of matter displaces space increaseing the density (it is space so there is no density) so I geuss "thickness" of space around it which makes it harder to move through this also would explain why gravity gets weaker the further you move away. Put that marble in pudding the pudding it displaces will mostly be right around the edge of it becoming less and less thick as you move away from it.

    Dylan Martinelli
  2. jcsd
  3. Nov 13, 2009 #2
    So my question is basically what is the force that makes the bending fell like a force with the absence of gravity since gravity is supposedly just bending of space?

    I just ended up trying to answer my own question.
  4. Nov 13, 2009 #3


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    Just a minor clarification - gravity is the bending of spacetime (not just space). If there are no other forces (such as electricity) acting on a particle, then it will just move in a "straight" line in curved spacetime.
  5. Nov 13, 2009 #4


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  6. Nov 13, 2009 #5
    Ok so that rules out matter finding the fastest or least resistance path like light. Makes it more confusing.

    So I heard about einsteins experiment with the sun effecting the travel of stars light. So this means the space in that area is definetly bent (by gravity) but then time is also bent so if I at where to go to that area of bent spacetime even if I am not moving time will be moving at a different rate or even not in the right direction (but I couldn't notice it but if someone saw me in a telescope.

    But I still do not understand what it is that holds me to the earth or the moon near the earth. Spacetime is bent and that is all that gravity is what is the force that makes me go in the direction towards the earth. The flatlanders on a flat but crumpled surface in the abscensce of gravity will not feel a force.

    Another example I read was of a ice skating rink and if it became hilly or shaped like a bowl and you would say in the bowl feel forces forcing you toward the center of it. But with out gravity why would you feel a force toward the center/bottam anymore than the edge of it.

    Is the curving of spacetime according to the experiments done with the sun's gravity effect the light of stars away from the sun ( if so this brings me to my next question can matter displace space time?) or is the bending toward it.
  7. Nov 13, 2009 #6
    O thanks I was writing and posted before I saw the link. Thanks a lot for helping out guys I'll researh this stuff. IF anyone else has any other links I'd appreciate it. So me being pulled to the earth has something to do with time curviture. So any other links would be great thanks alot. I am definetly a layman as these guys in the link are talking about.

    So what I am getting is just that this visualizations are not perfect and you need to possibly be able to visualize a higher demention to see a picture that would perfectly work.
  8. Nov 13, 2009 #7


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    Sure, they all omit some dimensions. But the sheet-marble-analogy omits the time dimension and therefore cannot show how space-time curvature causes gravity.
  9. Nov 13, 2009 #8


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    No, that's right. Matter finds the path of least "resistance" or, more precisely, of least "4D-acceleration" - the straightest line in curved spacetime - called a geodesic". When space is not curved, then this straight line in spacetime is what we call moving at constant 3D-velocity.
  10. Nov 14, 2009 #9
    O ok. I think I understand now. So space and time, like someone explained to me here, are one. So the space is curved and so is the time so since time is curved by moving toward that curve time is longer so the matter now has to move faster in space and/or time on this curved space time to appear to be the same as it was. And when something is moving faster it requires more energy now matter naturally goes to the lower energy path and that energy difference is the energy or "force" that we feel as gravity. If I am right I see the main thing needed to understand this was to realize space and time are one. Correct me if I was wrong.

    Now I have another question because of the bent space time if we were moving say with just enough force to move upward from earth at 1 mile per and hour would time slow down (still the level of time change would be to small to measure I am sure) by a lot more than that slow speed should have it change by. And if you were escaping a far more powerful gravity like the sun but your movement was measured as very slow but you are actually moving at the speed required to escape the suns gravitational field plus whatever speed you are moving so the time slowing would be large.

    Thanks for all explainations everyone this stuff is very interesting.
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