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Gravity, sinking, floating

  1. Jul 27, 2007 #1
    Imagine bubles rising or weights sinking in water. They sink or rise perpindicular to the sphere of the earth. In space however a tiny object would be falling or rising omnidirectionally. The question i ask is, does matter float or sink in space.

    People use the bowling ball in the sheet metaphor to visualize how space time is destorted by matter. But this is only representative of the plane that our senses percieve. The sheet sinking in all directions doesn't seem to make sense when visualizing the gravitational force in the dimensions of space. If matter was sinking in all dirctions, then it seems that it would not be forced together, but would be forced apart from the center out. The conclusion would seem to me that matter is not sinking into space, but is suspended in space like a bubble in water.

    The problem is that to make sense of a suspended sphere in space time. Mass would need to be less dense than what surounds it. that would meen that gravity must be a phenomia which excludes physical weight as a factor in the densities. It would seem then that the force which is linked to gravity is within absolute space itself and that the density of that force is greater when void of mass.
  2. jcsd
  3. Jul 28, 2007 #2
    The reason why an object is floating vs sinking into water is because in the first case (floating) it is in equilibrium, ie the gravitational force of the earth (which depends only on the mass) is compensated by the opposite buoyancy force produced by the surrounding water (which depends on the mass AND the relative mass densities of the object vs that of water) .

    In empty space, there is no such buoyancy as in water, the reason why the earth, for example, is rotating around the sun in its particular orbit, is because the gravitational energy of the sun is balances the kinetic energy of the earth's rotation around the sun. This calculation does not take into account the density of the earth, nor that of the sun, but only their masses.

    So I guess you could say that the earth appears to be "floating" in space around the sun, despite the fact that the mass density of the earth is of course much larger than the density of the vacuum.

    Sinking and floating are used to refer to a body's movement in a liquid (or gaz) and are not appropriate verbs to describe a body's movement in empty space as it is for example rotating around another body.

    So when you say : "The problem is that to make sense of a suspended sphere in space time. Mass would need to be less dense than what surounds it.", it doesn't make sense because you are thinking of "space time" as "water".

    The sheet analogy you are refering to is used in the context of General relativity to give the idea that gravity around a given mass curves the "fabric" of space-time, ie impacts on the actual metric itself. There is no "sinking" in the sheet.

    Hope it helps, just let me know.
  4. Jul 28, 2007 #3
    Just to add a little to what chrisina said, you have to be careful with that "rubber sheet" analogy that is so popular for describing the effects of gravity in the context of General Relativity. That analogy is limited not only by the fact that it's representing space-time as a two-dimensional surface, but also because it assumes that this two-dimensional sheet is embedded inside our normal three-dimensional world, complete with an external gravitational field pointing in a direction perpendicular to the sheet (thus the marble or what-have-you rolls into the well created by the bowling ball). This means that there are two gravitational fields to be aware of: the one that you are modeling with the distorted rubber sheet, i.e. the field of the bowling ball, and the external one that causes the bowling ball to sink into the sheet in the first place (the source of which is generally not specified since we all have an intuitive feel for things "sinking" into an elastic sheet).

    This is all just to say that you have to be careful when talking about this stuff. As chrisina said, the Earth doesn't "sink" into the local space-time, since there's no "external" field to pull on it. It just distorts space-time by its very presence.
  5. Jul 30, 2007 #4
    I know that there is a big difference between bodies of water and space.
    Mainly I was just using that example for imagery. I was using the terms floating and sinking to refer to the two opposit forces that create the phenomina of gravity.

    The most interesting property of mass, to me, is that its attraction is so far reaching. Isn't it thought that gravitational fields attract all other mass at some level depending on its mass and distance. The only way I can think to make this realistic to me is to insert some kind of existence throughout space that links all matter together.

    When trying to understand through einsteins theory of reletivity. I had to picture space as something that could be distorted. That there was some sort of facric of space time that could be streched by the presence of mass. All this doesn't make any sense to me though because space is said to be vacuum containing no connecting medium between matter. For general reletivity to work there must be a connection between space and matter. Matter attract other matter through it's interaction with space. Space therefore must be something that can interact. What is the nature of how space can interact?

    I thought it interesting to just picture a more simple alternative that is probably incredibly full of contradictory an inaccuracy but still interesting
    from a philisophical standpoint. What if space wasn't void at all. Maby space is full; A sea of energy. What if this energy occupys and creates this structure of space time itself. What if in space this energy was without frequency, just existing and occupying space in its least resistent way. What if mass is a less pure form of this energy. The only way to see and measure the energy is through it's imperfections(disturbancs). All we are measuring is the energy of the disturbance, and the disturbance is reletively the energy. The amount of this energy that causes these disturbances is like the power of a ripple in a pond. What can be achived by its power is its energy. It is not the disturbance which is a physically tangible thing. It is not the waver that truly exists, it is the water. Perhaps we are observing only the waves and have no way of seeing the ocean so we take what we can see and call that what is real when really it is in-perrminent, everchangin and phyisically imaginary.

    Photons are massless as are electrons. However protons and nutrons have mass yet they can be broken down into massles forms of energy. Mass which meens that it creates a gravitational effect within the structure of space time. What I'm leading up to is; what if the attraction of mass was actually an un attraction. What if space was actually atracted to itself more densely than matter. What if matter has an effect which disturbs the abilty of the energy field, or sea, by not allowing it to occupy area where protons and nutrons exist.

    Perhaps matter contains more energy than space and the result is that we are sinking. Or is it that we are floating. Since mass has mass, an in having it, it is of weight. Yet when atoms split they no langer retain mass. Because of the law of conservation, what constituted the mass must still exist. Therefore mass must be made of that which is massless. So what does it meen to have mass?
  6. Jul 30, 2007 #5


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    Staff: Mentor

    There is only one force of gravity and both floating and sinking are a result of it.
    Space can have structure and properties and still be completely unlike matter. Things get a little mushy from there. A "thing" can only be described by its properties.
    Electrons are not massless.

    Most of the rest of that is just wild speculation. There isn't much to it and we don't entertain freeform discussions like that.
    [edit: missing a quote tag]
    Last edited: Jul 30, 2007
  7. Jul 30, 2007 #6


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    Staff Emeritus
    Science Advisor

    I'd also like to add a few things to what chrisina said in her excellent post. What makes thinks float, "buoyancy", is not "gravity" or some sort of anti-gravity. What makes things float is pressure. If you have a boat floating on water, for instance, the pressure of the water is higher the deeper you go. There is a difference in pressure between the bottom of the boat and the top of the boat (the top of the boat only has air pressure. It is thi difference in pressure is what causes a net upwards force on the boat to balance out the downwards force of gravity. This finds a formal expression in the equations of hydrodynamic equilibrium.

    I'd suggest reading


    or a standard physics book for more about buyoancy and pressure.
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