Extra dimensions

[ablprop]

My question on entropy was answered so satisfactorily that I thought I'd try again. As a science popularizer (I work in a museum, and also write for children's science magazines), I often find myself helping a learner explore some aspect of science, only to discover that my own understanding is incomplete. Such is the case with extra dimensions.

I know of course of the three obvious spacial dimensions. I also understand that time is a dimension. I know that string/M-theory posit extra dimensions, perhaps curled-up, perhaps not. But I recognize that in my (perhaps incorrect) understanding of cosmology, I assume another dimension. How does it fit?

That extra dimension is the one that makes expansion of the universe make sense. I know of the balloon analogy, in which galaxies are scattered on the surface of the balloon and the balloon expands, causing all the galaxies to seem to rush away from one another. But this is an analogy, in that the galaxies exist of necessity on a two-dimensional surface. We do not (as far as I experience, though I'm still trying to digest the holographic universe idea - but that's another post ) live on a two-dimensional surface. I can draw lines out from the Milky Way in all directions, more or less, and hit another galaxy. So that analogy breaks down, in that no matter how I try to arrange the galaxies, there would be some inside the balloon and some outside the expanding surface.

I've always understood this to mean that the universe is a sort of three-dimensional "surface" and that the expansion is into another, unseen dimension. But what is that dimension? It isn't time, I don't believe. Is doesn't seem to be one of the curled-up dimensions of string/M-theory. Is it an additional dimension on top of all the others? Perhaps this idea is completely wrong. Can anyone help me undo this tangle?

[Thumpalumpacus]

I suspect a Nobel Prize or MacArthur grant would await he who figured that out.  Having said that, your explanation seems comprehensible to me; I just have the normal difficulty of being unable to properly envision any other dimensions, no matter how much I understand them in the abstract.

[hackenslash]

Tricky. I'll have to give some thought to this, not least because the framing of the questions is rooted in an intuitive view, while what is being questioned is anything but intuitive.

[hackenslash]

I suspect a Nobel Prize or MacArthur grant would await he who figured that out.

Indeed. Anybody who can actually elucidate the topography of the dimensional manifold will go down in history alongside the greatest thinkers of all time. Some work is being done in this arena, but it is far from complete.

[McQ]

This is a subject for real subject matter experts. Why don't you ask an astronomer or cosmologist at the museum where you work? Until then, "I don't know." is an acceptable answer when people ask you about this. I'm not trying to be flippant, I mean this sincerely. Ask a person who deals with this subject to help you. Hell, for that matter, write to someone like Brian Cox, or Neil Tyson. I'm willing to wager that no one here can do much more than this and help try to find info from the web.

[ablprop]

I suspect a Nobel Prize or MacArthur grant would await he who figured that out.

Or she  

[ablprop]

This is a subject for real subject matter experts. Why don't you ask an astronomer or cosmologist at the museum where you work? Until then, "I don't know." is an acceptable answer when people ask you about this. I'm not trying to be flippant, I mean this sincerely. Ask a person who deals with this subject to help you. Hell, for that matter, write to someone like Brian Cox, or Neil Tyson. I'm willing to wager that no one here can do much more than this and help try to find info from the web.

Yeah, I suppose. I often interview scientists for various projects. This question hasn't come up yet. Maybe it will. What I'm looking for, though, is a usable idea, and I've found that those often come from folks explicitly trying to make things understandable to the layman (like me).

I'm reading "The Shape of Inner Space" by Yau (of Calabi-Yau fame). I don't know if this question will come up, I rather doubt it, but maybe that will give me a better handle.

Funny, I thought the entropy question was a lot harder.