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# A dimension for gravity?

ocalhoun
Whenever you have an explanation for the general theory of relativity, you inevitably see a diagram like this one:

This diagram ignores the third spatial dimension, and time, but represents three dimensions anyway. I'm thinking that the third dimension in this picture actually represents gravity itself, or the dimension in which gravity acts.

What I'm wondering is if this really is another dimension, could that be why we haven't been able to get quantum gravity figured out? (Because we can't find the quanta of any dimension yet, if such things exist.) Could it also be why we haven't found any gravitons, even though they should be very common?
Indi
 ocalhoun wrote: This diagram ignores the third spatial dimension, and time, but represents three dimensions anyway. I'm thinking that the third dimension in this picture actually represents gravity itself, or the dimension in which gravity acts. What I'm wondering is if this really is another dimension, could that be why we haven't been able to get quantum gravity figured out? (Because we can't find the quanta of any dimension yet, if such things exist.) Could it also be why we haven't found any gravitons, even though they should be very common?

Ah, you're overcomplicating the diagram. It represents space as two dimensional, and shows the warping of space in those two dimensions using a third dimension. They could have done it with colour, or numbers - they didn't need to use a third dimension. That just makes it easier to see.

They could also theoretically show a 3-dimensional image of space, and show the distortion by gravity using colour, numbers or a fourth spatial dimension... but who'd understand it?
ocalhoun
Indi wrote:
 ocalhoun wrote: This diagram ignores the third spatial dimension, and time, but represents three dimensions anyway. I'm thinking that the third dimension in this picture actually represents gravity itself, or the dimension in which gravity acts. What I'm wondering is if this really is another dimension, could that be why we haven't been able to get quantum gravity figured out? (Because we can't find the quanta of any dimension yet, if such things exist.) Could it also be why we haven't found any gravitons, even though they should be very common?

Ah, you're overcomplicating the diagram. It represents space as two dimensional, and shows the warping of space in those two dimensions using a third dimension. They could have done it with colour, or numbers - they didn't need to use a third dimension. That just makes it easier to see.

They could also theoretically show a 3-dimensional image of space, and show the distortion by gravity using colour, numbers or a fourth spatial dimension... but who'd understand it?

Well, yes, I was just using that as an example though, and to show where the inspiration for the thought came from. I certainly don't think that the people who make such diagrams think along these lines while doing so.
DoctorBeaver
There are theories (notably Randall-Sundrum) that say gravity originates in a higher dimension. It is weaker than the other forces because this extra dimension dilutes it.
nilsmo
We understand our view of three dimensions and not much more. Distortions in three dimensions are too hard for us to understand, so this picture helps us visualize these distortions by showing how "curving" two dimensions changes its geometry (thereby distorting it). A third dimension is used to represent this curve, and the third dimension is akin to "our" third dimension because when we fold a paper it does distort the coordinate system on the paper. Thus it is a better way of representing distortion than colors or something else.
Indi
 DoctorBeaver wrote: There are theories (notably Randall-Sundrum) that say gravity originates in a higher dimension. It is weaker than the other forces because this extra dimension dilutes it.

i don't know Randall-Sundrum, but M-theory holds that gravity diffuses over higher dimensions (whereas the other forces don't)... but not that it originates in a higher dimension.
DoctorBeaver
Indi - Have a read of this

Or this if you like maths.
Indi
 DoctorBeaver wrote: Indi - Have a read of this Or this if you like maths.

That's a really neat model - and simple to grok, too. Or, at least, i think i've grokked it. ^_^; Because i'm still a little confused about how gravity originates in a higher-level brane. What i'm getting is that gravity can originate anywhere in the bulk, depending on brane "tension". It could originate within our own brane (and diffuse into the bulk) - but again, it doesn't have to originate in our brane.

Am i reading that right? When you said gravity originates in a higher dimension, did you just mean it could, not it must?
DoctorBeaver
Indi - I'm not entirely sure whether gravity must or just could originate in a higher dimension. My understanding of RS1 & RS2 is very limited (far brighter people than I have had trouble understanding the implications).

I know they include localised gravity and locally localised gravity. If you can get hold of a book called "Warped Passages" by Lisa Randall, that explains it all.
Indi
 DoctorBeaver wrote: Indi - I'm not entirely sure whether gravity must or just could originate in a higher dimension. My understanding of RS1 & RS2 is very limited (far brighter people than I have had trouble understanding the implications). I know they include localised gravity and locally localised gravity. If you can get hold of a book called "Warped Passages" by Lisa Randall, that explains it all.

i will! Thank you for the tip! My entire understanding of gravity in modern physics may be wrong. ^_^; i won't be satisfied until i know whether it is or not.
DoctorBeaver
 Indi wrote: My entire understanding of gravity in modern physics may be wrong.

That makes 2 of us. There seems to be a mutitude of current theories of gravity and I can't understand a single 1 of them