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Space Time Fabric

  1. Jul 25, 2008 #1
    I recently read that space time is curved by and object with mass. The example given was of a trampoline with a bowling ball and pool balls moving across the surface. I can understand or at least visualize this easily.

    However, Since all objects do not exist on the same plane or do they, I would think that there are many surfaces. So what I picture is layer upon layer of space time running from under the object using the same bowling ball visualization, upward in layers till the a layer is above the ball and each layer passes through the ball.

    This raised a few questions about this visualization. My first question is based solely on the very first image of the ball on a trampoline. When the ball distorts the fabric the surface area of the trampoline increases as the material stretches. Alternately, if the material cannot stretch it must pull in from the sides and the surface area is distorted but remains constant. So how does space time distort? If it stretches that to me says that the surface area expands and the fabric in that location is thinner than it would be farther out away from the stretch. If it merely distorts all of the fabric would be forced to shift. Where would it shift from? In one of the examples, it said that the time fabric wrapped around in a big ball so it is finite but appears to be infinite. If that is so, would it not mean that the surface area of this ball has grown?

    If the fabric is multilayer and by saying that I might be removing all doubt about my lack of knowledge on this subject matter. But I will proceed on the concept that the only dumb question is one that is not asked and not the opposite position of better to be quiet and thought to be a fool than to open your mouth and remove all doubt.

    OK multilayers, each bending by the force of the ball. My first thought is that each layer that bends must also bend the layer next to it or say lower down. If there is no compression of the fabric then I have to think that this bend would continue down through every layer at exactly the same amount. I know this is not the case so does that mean that there also is some compression in each layer as well as stretch. I know I am using the wrong language but hope you can understand, so in my mind this would translate to space time fabric being thinner in spots, more compressed in other spots. Does this impact the density of the fabric? And if it does what explains why it has no impact.

    I also had some questions about each layer as you move up through the bowling ball but as I think about it my question about the layer that passed through the middle or on either side of the middle is resolved at least in my head by the forces of mass on either side of it.

    I thank you in advance for your patience and ask for your forgiveness for using language that needs to be translated to the correct scientific terminology.
  2. jcsd
  3. Jul 25, 2008 #2


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  4. Jul 25, 2008 #3
    i wouldnt say its in layers. its more like.....jello, with objects in the middle.
  5. Jul 25, 2008 #4
    Thanks for the link, there is a lot of information there, it will take a good while to not only read it but to look at all the links provided.

    But I have one more question. What is the force or the name of the force that pulls space time back into a straight line? It seems to me that everything that is bent stays that way unless there a force that is pulling on it so that it returns to the original shape.

    Also does the bending of space time distort it and then allow it to return to its original condition or does it cause some oscillation like you would see when you bend a tight string and release it. Even running your finger up a string on a guitar causes some oscillation to occur.
  6. Jul 25, 2008 #5


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    The analogy with the trampoline is not very close to GR. Space-time isn't like a material.
    It could be that people are taking the idea of ST curvature too literally. It's in the maths and it's called curvature, but it isn't necessarily a physical thing.

    There are (allegedly) oscillations in space-time, called gravitational waves, but like electromgnetic waves, they propagate without the need of a mddium.
  7. Jul 25, 2008 #6


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    The "spacetime fabric" is an analogy, which works specifically because it reduces the number of dimensions for the sake of visualization. It has limits. Don't waste your time trying to extend the analogy to situations where it does not apply.

    - Warren
  8. Nov 22, 2008 #7
    I'm new here, know nothing about physics, but have a question RE: space-time fabric. What I understand is that the notion of "fabric" is an analogy in 2D. I also understand that the analogy cannot be carried far. So, from a complete, but interested, ignoramus comes the setup and question.

    From everything I've seen and read over the years, what I can visualize is that space-time is n-dimensional and, metaphorically, gelatinous.

    The question is: What, if any, are the consequences of displacement by an object moving through?

  9. Nov 23, 2008 #8


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    The "gelatinous explanation" could go like this: The gelatin is more dense in some regions, so it slows down things moving there more (gravitational time dilation). It also changes the direction the things move, by slowing one side more then the other (newtons gravitational force).

    For the more common curved sheet explanation see this recent thread:
  10. Nov 23, 2008 #9
    Thanks, A.T. I looked at the other thread. The balloon-in-water analogy there was helpful in refining my question. What makes it helpful is that the water cannot be displaced and it pushes back. From that, I can imagine something moving right along with compaction (if you will) along its route rather than displacement. What I'm having trouble with is the notion of space-time (or just space?) being a box.

    If there's no box, is there no displacement?
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