# Brane world with finite thickness

1. Aug 4, 2011

### mersecske

In most of the brane world models:
our world is supposed to be an infinite thick mathematical surface
in the higher dimensional world.
Which is very curious.
Are there any model in which the thickness of the "surface" is finite?
Please, could anyone submit a good paper to me in this topic?

2. Aug 5, 2011

### Chalnoth

It's, um, the other way around. It's an infinitessimally thin surface.

3. Aug 5, 2011

### mersecske

NOT. Its a lower dimensional surface! You are wrong. I've talked a lot with an expert.

4. Aug 5, 2011

### Chronos

Discussion with one expert is insufficient to confer credibiltiy. The whole brane thing is purely speculative to begin with, so, who has the moral authority to judge what may be right or wrong when it's all just toy models?

5. Aug 5, 2011

### mersecske

I say that:
Most of the models assume a lower dimensional surface (=infinitely thin),
and not a same dimensional but thin physical surface.

If you know something belongs to the latter one

6. Aug 5, 2011

### Chalnoth

Branes are boundary conditions for open strings. That is to say, in string theory, when you have a string that doesn't wrap back on itself, it's called 'open'. The tips of the string can be confined onto a surface of any number of dimensions. And if our universe is a brane, it would be a 3+1 dimensional surface to which the tips of these strings are confined (the tips of these open strings would be seen as particles). This brane would have no extent in any of the other directions besides the 3 spatial and 1 time dimensions we experience (i.e. no thickness).

7. Aug 5, 2011

### mersecske

I am talking about brane worlds, and not branes in string theory. Branes in string theory is totally different. Brane in brane-world theories represent the whole Universe and not a particle.

Yes, most of the brane-world theories the Universe is a 3+1 dimensional infinitesimal surface in a higher dimensional "world". But its very unphysical. An other possibility is that the brane is a higher dimensional object, but very thin, and practically is like a surface, but with finite thickness (maybe Planck scale). Thats what I am talking about.

8. Aug 5, 2011

### Chalnoth

The brane world is string theory.

I don't think finite extent in this manner makes sense. The way this is done, instead, is that the brane is wrapped back on itself tightly in these other dimensions. This wouldn't be seen as a thickness. However many dimensions it has, it's still infinitessimally thin. Even if some of those dimensions are small and wrapped up.

9. Aug 5, 2011

### bapowell

Dude, you're totally confused. Brane-worlds were originally based on M-theory branes, and have since been generalized to accommodate branes from the 10-dimensional string theories. Branes in string theory are extended, non-perturbative objects; 3-branes are popular candidates for our universe in brane-world models.

Not sure what you mean by 3+1 dimensional infinitesimal object. Let's focus just on the spatial dimensions here for simplicity. A 2-brane, for example, is a 2D sheet. In 3D space, it has no thickness. When embedded in an even higher dimensional space, it likewise has no thickness in any of the higher dimensions. You seem to be suggesting the following: a 3-brane with 2 infinite dimensions and a 3rd finite dimension -- giving the sheet some tiny thickness (like a 2-brane with some small thickness in 3rd dimension.)

There are theories in which the brane is given a finite thickness in directions orthogonal to those dimensions that are space filling -- I believe DGP branes have a finite thickness.

10. Aug 5, 2011

### mersecske

I dont understand how it is possible that you dont understand me :)
It is very hard to imagine (if you are a physicist)
that there exist a higher dimensional physical World,
but our world is only a surface in that World.
We can never prove its physical existance.
That higher dimension can be only a mathematical imagination
for a nice theory. However I can imagine that the real physical world
has more dimension than 3+1, and we are localized
on a real surface with finite thickness,
and our measurements are somehow restricted also,
therefore we can observe 3+1 dimension SEEMINGLY.

11. Aug 5, 2011

### bapowell

Why not?

12. Aug 7, 2011

### mersecske

Because you are in an other dimension.
Its like being outside the event horizon you can never prove the existence of the event horizon.

13. Aug 7, 2011

### bapowell

It's actually nothing at all like being outside an event horizon. In some string theories, gravity probes the whole extra-dimensional space. Since we interact with gravity, in principle, we interact with the extra dimensions.