# Is it possible that strings are higher dimensional branes?

• Nav
In summary, strings are one dimensional objects that are compacted to look like they have more dimensions. They are made up of smaller 1d objects that are in a 2d space.
Nav
Are strings really just one dimensional? Is it possible that strings are 3 dimensional? Or even 10 dimensional?

If they had more than one dimension, they would not be called strings.

There is something called 'brane theory' (brane is short for membrane) that has big membranes wobbling around in a many-dimensional manifold, with exciting things happening when they touch. In common language, a membrane is two-dimensional. So I imagine Branes have two dimensions, although it could be 'two or more'.

andrewkirk said:
If they had more than one dimension, they would not be called strings.

There is something called 'brane theory' (brane is short for membrane) that has big membranes wobbling around in a many-dimensional manifold, with exciting things happening when they touch. In common language, a membrane is two-dimensional. So I imagine Branes have two dimensions, although it could be 'two or more'.
but what if they are like a string like a string on a guitar which has 3 dimensions

Strings as is are really one dimensional. By definition. The same as point particles being zero dimensional (point particles ##\neq## atoms/nuclei although we often approximate those as points) by definition
In string theory there are several objects of higher dimensions. The ones you encounter quite soon in modern treatments are D-branes.
And a first idea about why they exist (that I've encountered so far) have to do with 'fixing' the endpoints of the string.

Nav said:
but what if they are like a string like a string on a guitar which has 3 dimensions
A "string" on a guitar is not a string in that sense, BECAUSE it is 3D. It is a tube with length, breath, and height, not a string

Is it possible that strings are higher dimensional branes but are just more compacted to seem like they are one dimensional when in reality they are more?

atyy
Nav said:
Is it possible that strings are higher dimensional branes but are just more compacted to seem like they are one dimensional when in reality they are more?

It's the other way around, branes are like higher dimensional strings.

Or even 24 dimensional, that works too.
Have to remember that string theory is not as yet supported by evidence, though a lot of people like the idea.

Nav said:
Is it possible that strings are higher dimensional branes but are just more compacted to seem like they are one dimensional when in reality they are more?

There are models where higher-dimensional membranes can wrap a compact surface to give a string-like object in the noncompact dimensions. When the wrapped object is a so-called D-brane, these are sometimes called D-strings to distinguish them from the fundamental string. Also, in one version of M-theory, starting from 11D spacetime, a membrane (two-dimensional object) is wrapped on a circle to form the fundamental string of the so-called IIA string theory in 10D spacetime. There is some discussion at a very basic level at http://superstringtheory.com/basics/basic7.html

MacRudi and atyy
rootone said:
Or even 24 dimensional, that works too.
Have to remember that string theory is not as yet supported by evidence, though a lot of people like the idea.

I think you may be thinking of 26-D bosonic string theory, it is not the strings that are 26-D that is the space they live in.

You end up with branes, and as far as I know the action of such a brane is not renormalizable, and we don't know how to write a Polyakov-like action like we know for strings. I'm also not sure how you could quantize such an action.

haushofer said:
You end up with branes, and as far as I know the action of such a brane is not renormalizable, and we don't know how to write a Polyakov-like action like we know for strings. I'm also not sure how you could quantize such an action.

The Polyakov action of a p-brane (in D-dimensional spacetime) is just the (p+1)-dimensional Sigma model: $$S[X] \sim \int d^{p+1} \sigma \ \sqrt{-\gamma} \ \gamma^{ij}(\sigma) \ \partial_{i}X^{\mu}(\sigma) \ \partial_{j}X^{\nu}(\sigma) \ g_{\mu \nu}(X) ,$$ where $\sigma = (\sigma^{0} , \sigma^{1} , \cdots , \sigma^{p})$ are the world-volume coordinates and $X^{\mu}, \ \mu = 0, 1, \cdots , D-1$ are the spacetime coordinates of the brane: mapping of the (p+1)-dimensional parameter space of the p-brane into spacetime.
Notice that for $p = 0$, the above action becomes that of a point particle, i.e. 0-brane is point particle. For $p=1$, you get the Polyakov action for string, i.e. 1-brane is string, and 2-brane is a membrane, etc.
When a p-dimensional object moves in spacetime, it sweeps out a (p+1)-dimensional tube. This fact can be used to write a Polyakov-type action for any value of p.

Sam

MacRudi
I think if lower dimensional object can exist in higher dimensional space, then yes there is no reason (to me) that something can exists and have length but no height or width. Which any number of 1d objects and exist in a 2d space. So is a brane a 2d object composed of 1d strings? Our Universe is a 3d object in 4d space? Could those universes be separated by the 4th dimension?

## 1. What are strings and branes?

Strings and branes are theoretical objects in physics that are proposed to be the fundamental components of the universe. Strings are one-dimensional objects, while branes are higher-dimensional objects (such as 2D, 3D, etc.). They are believed to be the building blocks of particles and matter.

## 2. How are strings and branes related to higher dimensions?

Strings and branes are proposed to exist in higher dimensions beyond the three dimensions of space and one dimension of time that we experience. In string theory, it is suggested that the universe may have more than three spatial dimensions, and that strings and branes can exist and interact in these higher dimensions.

## 3. Is it possible that strings and branes are the same thing?

While both strings and branes are proposed to be fundamental components of the universe, they are distinct objects. Strings are one-dimensional, while branes are higher-dimensional. Additionally, strings vibrate and interact with each other, while branes can stretch and wrap around other objects.

## 4. What evidence supports the existence of strings and branes?

Currently, there is no direct experimental evidence for strings and branes. However, string theory and brane theory are mathematically consistent and have been able to unify different aspects of physics, such as gravity and quantum mechanics. Some scientists also believe that certain patterns in the cosmic microwave background may be evidence of branes in the early universe.

## 5. How does the concept of strings and branes relate to the search for a theory of everything?

The search for a theory of everything, or a unified theory that can explain all the fundamental forces and particles in the universe, is closely tied to the concept of strings and branes. String theory and brane theory are currently the leading candidates for a theory of everything, as they are able to unify gravity with the other fundamental forces, unlike traditional theories such as the Standard Model. However, the existence of strings and branes has not been proven, and the search for a theory of everything is ongoing.

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