Good morning all. I would like to try my hand at replying to Rob's Christmas message:
I don't get how gravity works when there is a vacum (outer-space) between the two bodies of mass that attract eachother. Can someone please explain.
I saw DA's reply and I believe this answer is correct. However, it will do me some good to try and refresh my notions of gravity in a more general sense.
According to the things I've read, the classic basic answer is that the force we call gravity is attributable to the warping of spacetime around massive objects, like a planet. The force you feel is actually due to the curvature. I believe the force of gravity is inversly propotional to the square of the distance between the two objects.
The somewhat simplified metaphor I keep in my head is the way water wraps around any object (that sinks) when you toss it into an aquarium. The 'warping' of the water is like the warping of spacetime around a massive object.
I can appreciate the classic bed-sheet or trampoline analogy but the aquarium works better for me because it completely surronds the object, whereas the bed sheet is two-dimensional.
Rob, to be honest I do not know if when physicists speak of the universe being flat they really do mean like a bed sheet or this is only a simplified analogy that is used for explanation. I do know there is constant debate about the shape of the universe and I try to take in as much as I can about the various arguements (positive curvature = spherical, negative curvature = modified saddle).
As DA alluded to, this flatness on a large scale may hide a very different view when the physics of the small comes into play. There are theories which suggest virtual particles poppping in and out of existence and all of this contributing to a seething bath of uncalculable variability at the quantum scale. So far, the two have not been able to meet. But this is the Gordian knot for a certain group of physicists and lay people who like to think about these things.
My personal belief is, like with M-theory, someone with special vision and talent (like Wheeler) will come along and show us how the big and the small are really the same thing and we have just been looking at them as different, assuming they are different. The math takes the theorists in different, somewhat opposing directions. My hope is an optimistic one that the two are the same and we just need some mathematical vision to look at gravity the correct way.
Please see my original post and reference that Scientific American article for someone who might answer you more in depth.
The answer is waiting.
Sorry, Rob, I got up on the soapbox there for a minute. My first paragraph is hopefully helpful.
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