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Thread: The notion of gravity as a dimension?

  1. #1 The notion of gravity as a dimension? 
    Forum Professor Daecon's Avatar
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    I was thinking about the relationship between spacetime and gravity, and how time as a dimension is completely unlike the three spacial dimensions, and I had a thought.

    Speed through the spacial dimensions influences speed through the time dimension, but gravity also influences the time dimension, doesn't it?

    Could it be feasible, or even sensible, to regard gravity of "5th" dimension of sorts? Would such a viewpoint even be compatible with current theories of the Universe, like Relativity and such?

    From what I understand, the search for a theory of everything is incomplete due to gravity, including the debate as to whether it's caused by the curvature of spacetime itself or is the result of force carrier particles? Could treating gravity as a dimension overly complicate matters, or would it be practical for removing gravity as a factor in this search?

    Or is the idea making me sound slightly cranky?


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  3. #2  
    ...matter and pixie dust wegs's Avatar
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    Intuitively, I "get" it...but how would it actually work. Why is gravity such a mystery?


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  4. #3  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by Daecon View Post
    Speed through the spacial dimensions influences speed through the time dimension
    Good insight. (It's a rotation.)

    but gravity also influences the time dimension, doesn't it?
    Gravity is the curvature of space-time. We perceive gravity affecting time but actually they are both the same thing.

    Or is the idea making me sound slightly cranky?
    Well, ....
    ei incumbit probatio qui dicit, non qui negat
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  5. #4  
    Forum Radioactive Isotope MagiMaster's Avatar
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    The biggest problem would be finding a consistent way to model gravity as a dimension as opposed to a field (Newtonian or graviton) or a geometric property of space(time) (GR). The issue is with the definition of a dimension.

    One way of defining a dimension is as a coordinate necessary to uniquely specify something. For example, space is 3D because we need 3 coordinates to uniquely specify a point in space. Spacetime is 4D because we have to add when an event occured (you can't get that info from just the 3 spacial dimensions).

    What would defining gravity as a 5th dimension mean? There are two approaches I know of. In one, gravity would be a fully separate dimension and our universe would exist as a 4D slice across this extra dimension. I'm not really sure where to go from there though as that doesn't really seem to have anything to do with the relation between mass and gravity.

    The other approach would be to make gravity a tiny 5th dimension in the vein of string theory. In that case, our universe includes the whole of this 5th dimension, but it's wrapped up so tightly that big things like atoms and photons don't notice the extra space. Again though, that doesn't really seem to have much to do with the observed nature of gravity.

    I'm no expert in such things though, so maybe there are other approaches that might make more sense, but GR describes gravity extremely accurately (read up on GPS) so any new theory would have to be even more accurate to be at all useful.
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  6. #5  
    Forum Professor Daecon's Avatar
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Daecon View Post
    Speed through the spacial dimensions influences speed through the time dimension
    Good insight. (It's a rotation.)

    but gravity also influences the time dimension, doesn't it?
    Gravity is the curvature of space-time. We perceive gravity affecting time but actually they are both the same thing.

    Or is the idea making me sound slightly cranky?
    Well, ....
    I was thinking about time dilation in a strong gravitational field, similar to time dilation at near-light speeds. That is, if they actually are similar or am I misunderstanding the subtleties?

    Quote Originally Posted by MagiMaster View Post
    The biggest problem would be finding a consistent way to model gravity as a dimension as opposed to a field (Newtonian or graviton) or a geometric property of space(time) (GR). The issue is with the definition of a dimension.

    One way of defining a dimension is as a coordinate necessary to uniquely specify something. For example, space is 3D because we need 3 coordinates to uniquely specify a point in space. Spacetime is 4D because we have to add when an event occured (you can't get that info from just the 3 spacial dimensions).
    Ah, yes. I didn't think about that. Perhaps a 5th variable could be the mass of the event or object being located with the other four dimensional co-ordinates, although that does seem rather redundant.

    I suppose that's related in a way to the idea of dimensions have directions such as left/right, up/down, forwards/backwards and I guess future/past?

    I expect it would be silly to give gravity a direction of "towards mass/away from mass" wouldn't it.
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  7. #6  
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    Your question involves the most pressing challenge in physics. The unknown question isn't just the placement of gravity, but also the reverse, the place of quantum theory in General Relativity.One starts to spit out "infinities' and thus one of the reasons for the increased acceptance of multiverse theory. Sort of deductive logic...it's the only theory that can make sense (or at least begin to) of gravity in a quantum universe.

    Lawrence Krauss ( and buddies)has some fascinating lectures on YouTube
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  8. #7  
    Forum Radioactive Isotope MagiMaster's Avatar
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    @Daecon, directions are one way of looking at dimensions, but it's worth pointing out that they're not unique. A simple example is x-y-z vs latitude-longitude-altitude. You can transform coordinates between those though, so it's somewhat just a matter of bookkeeping. That said, either giving gravity a direction nor rearranging the other four directions to accommodate gravity seems like it'd work very well.

    Mass as a dimension is at least a little easier to imagine, but suffers from a slightly different problem. What would it mean for there to be two points with the same time-space coordinates and different mass coordinates? Or, if the universe was a slice across the mass dimension, why would the universe bend through particular values in that dimension? (Out of all the options though, that last one is the closest to getting somewhere, but then it's also getting pretty close to just devolving into a field instead of a dimension.)
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  9. #8  
    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by Daecon View Post
    Ah, yes. I didn't think about that. Perhaps a 5th variable could be the mass of the event or object being located with the other four dimensional co-ordinates, although that does seem rather redundant.
    The source term of gravity is not mass, but the full stress-energy-momentum tensor; this is related in non-linear ways to the metric tensor via the Einstein field equations. The basic issue so far as your idea is concerned is that gravity cannot be consistently described by just a single scalar quantity ( as your 5th dimension implies ), hence this won't work as you suggested.

    It is, however, possible to generalise GR to five dimensions ( with the 5th being compactified ) - this model is called Kaluza-Klein gravity. It can be thought of as an attempt to unify gravity and electromagnetism.
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