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Do objects float or rest?

  1. Dec 16, 2008 #1
    This is probably a very elementary question so I apologize in advance.

    Relativity says that objects of great mass warp the fabric space, hence why we experience gravity.

    Do objects of great mass simply warp and float ABOVE the fabric of space, or do they rest ON the fabric like marbles on a bed?

    Thank you!
  2. jcsd
  3. Dec 16, 2008 #2


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    Spacetime not just space
    They are neither on nor above but rather inside. The 4-dimensional spacetime contains 3-dimensional objects. And they never rest, but always advance trough spacetime. But keep in mind that it is just a mathematical model.
  4. Dec 16, 2008 #3
    A.T. thank you for your response, but I'm trying to think of things a bit more physically.

    I have a ball, and place it in a body of water. I can say that the ball is floating on or floating in the water. Both statements are correct.

    Gravity is trying to pull the ball down, but the air inside the ball is the force keeping it afloat.

    What keeps the Sun "afloat"?

    I know this might sound dumb, but I'm working my mind around the idea that all observable objects are just falling at a terminal velocity so "relatively" everything is "floating"
  5. Dec 16, 2008 #4


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    What would it mean for the Sun not to be "afloat" in 3D space or 4D spacetime? Where would it go if it "sank"?
  6. Dec 16, 2008 #5
    That is the root of my question.

    Is everything in the universe simply falling at a terminal velocity with no bottom?

    Are we really AFLOAT on the fabric of space, because it would seem to me that if objects of significant mass can warp space, then they must have the weight to make it bend.
  7. Dec 16, 2008 #6
    Objects do not displace spacetime the way a ball displaces a little water and air as it floats. Analogies are called that because they are not perfect representations. Another way to view this is that mass requires spacetime for its existance.

    Even some "dumb" ideas that may not work out sometimes provide valuable insights and even if they don't, you'll end up with perspectives you likely did not have originally. All scientists have more wrong ideas, than right ones....its picking among concepts that is the first test..those which "smell" right.

    I have no idea what "falling", "terminal velocity", "relatively" nor "floating" means in your statement and likely you have not defined them either, so I'm not even going to comment further. And what does your idea have do with your posted question??

    How can you prove or disprove your own idea? Does it make any new predictions? Are those predictions testable? Do the tests confirm or deny your ideas?....superficial ideas are a dime a dozen; in this economy you won't even get 5 cents for them...it's the downstream work that is most difficult and that's why so many "idea" posts here don't have consequences stated nor thought through.

    I think it was Richard Feynmann who said essentially that only after an idea has been subjected to every test you can think of and proven itself, should you begin to think it worthwhile.
  8. Dec 16, 2008 #7
    That is incorrect, 4-dimensional spacetime contains 4-dimensional objects. A point in space is a line in spacetime, a surface in space is a 3d object in spacetime and a 3d object in space is a 4d object in spacetime.
  9. Dec 16, 2008 #8


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    A 4D manifold can contain 4D, 3D, 2D and 1D objects, so my statement is not wrong. But I see how it can be misinterpreted.
  10. Dec 16, 2008 #9
    OK, let me see if I can describe what it is I mean as I seem to have created some confusion.

    Lets say I have some marbles (representing objects of great mass) and I have the sheet (representing the fabric of space). When I place the marbles on my sheet they make indentations because gravity pulls them down into the bed.

    The next part is going to sound bad for no other reason than I really don't know how to scientifically express my thinking.

    Now say I take my sheet and marbles to the top of a building, and pull the sheet taunt over empty space (over the edge), then pull the fabric out from under the marbles without putting any force or spin on them. The marbles SHOULD fall straight down and will all reach a terminal velocity (no matter the mass of each object) before hitting the ground.

    In space there is no "ground" but can all heavenly bodies fall like the marbles?

    I'm wondering if the "fabric" of space is necessary, because if all things are falling at the same speed and there is no "ground" then we would still see the same thing.

    I don't know...weird thinking isn't it?
  11. Dec 16, 2008 #10


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    That's why I wrote that the curved space time is just a mathematical model. You are taking misleading analogies like "balls on a fabric" much to literally. Words like "above" or "afloat" don't really make sense in the context of spacetime.

    This is using gravity to explain gravity. This is not how gravity is described in relativity. Read this post on that:
    Last edited: Dec 16, 2008
  12. Dec 16, 2008 #11


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    Where would they fall? There's no up or down in space, nor is there any universal gravity field.
    The "rubber-sheet" analogy is just that, an analogy. In the analogy, a curved 2D sheet stands for curved 4D spacetime (and you should really picture objects as being completely contained in the 2D surface rather than sitting on top of it, as in the story Flatland). Also, it isn't important to the analogy that masses cause depressions in the sheet, you could equally well picture them causing raised bumps in it--all that's relevant is that it changes the curvature of the sheet, the orientation of the curved surface is irrelevant. Are you familiar with the notion of "geodesics" on a curved surface? On a curved 2D surface, they basically just mean the shortest path between two points--for example, on a globe, if you draw two points on the globe the shortest path between them will always be a section of a great circle (like the equator or a line of longitude) that passes through both points, which might not look like a straight line if you plotted the path on a map of the globe (it would depend what map projection scheme you used). In curved 4D spacetime, instead of minimizing the spatial distance, a geodesic is a path through space and time that maximizes the "proper time" (time as measured by a clock moving along that path), and the geodesics will look different depending on the curvature just as they do on a curved 2D surface.

    You might also find this helpful: http://www.bun.kyoto-u.ac.jp/~suchii/apple.html [Broken]
    Last edited by a moderator: May 3, 2017
  13. Dec 16, 2008 #12
    I appreciate the feedback. I'm not a physicist. I was horrible in high school math and I'm really just starting to get interested in physics so the question I'm asking isn't an educated one...just a curious one.
  14. Dec 17, 2008 #13
    Hey Chris

    The link to the parable of the apple by JesseM is extremely good, but if you are still struggling to picture what is going on here, that is your mistake.

    Consider many centuries ago, the world was thought to be flat. There was no apparent reason to argue this fact. When you looked out at the ocean, there sea seemed to make a "flat line" where it met the sky. It was easy to view the earth as a flat disk, but only when more carefully considered, could someone say that the earth was a "ball".

    General relativity does a similar thing. Intuitively we wish to impose a “flatness” to our universe, because it is what we are used to working with. Only when observed much more carefully does one realise that this condition of flatness does not give as good a description of nature.

    With the sheet analogy, you need to limit your imagination. You are putting this curved sheet into a "flat" space so that you can visualise it. What you should imagine instead is that you live "in" the sheet. The only directions along which you may look are along the sheet. The concepts of above or below the sheet do not exist. Everything that can be perceived by you is in the sheet.

    The value of the analogy is that when considering curved spaces or space-time, we need a clever way of measuring distances, angles and time (ever hear of time dilation or length contraction). This curvature just tells us how to make these measurements.

    I hope this helped a little
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