# String theory-connecting strings

#### pforeman

I take it strings are too small to have "connectors" on their ends, there is no glue etc. so how could strings combine to become larger particles like bosons.
I have never heard anyone describe the reason for the extra dimensions that string theory says are there.
Could it be that these strings are forced together because they are trapped in a universe barely large enough to hold the strings (these tiny extra dimensions)
In other words, one of these extra dimensions contain 3 strings which become a boson perhaps another dimension can contain several bosons and becomes the strong nuclear force.
Each tiny dimension is a universe unto itself, from which obviously nothing can escape.

Thanks
Paul

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#### bapowell

I take it strings are too small to have "connectors" on their ends, there is no glue etc. so how could strings combine to become larger particles like bosons.
Why do you suppose that strings "combine" to form bosons? Bosons are elementary particles and so correspond to the excitations of single strings.
I have never heard anyone describe the reason for the extra dimensions that string theory says are there.
There are important technical reasons; in particular, something called anomaly cancellation that only occurs in set numbers of dimensions.

#### Chalnoth

There are important technical reasons; in particular, something called anomaly cancellation that only occurs in set numbers of dimensions.
To explain a little bit about what this means, imagine we want to describe the physics of a rubber band. What we're going to do, first, is write some numbers down to describe the rubber band. This shouldn't be an important operation: what numbers we use to describe the band should have no impact on how the band behaves. They're just our description, after all.

This is a rather important feature of any reasonable theory of our universe: the representation used to describe it shouldn't have any impact on how the theory behaves.

But in string theory, when you write down how the behavior of objects changes when you change the numbers used to describe them, you get these extra terms called, "anomalies." These anomalies make no physical sense. But they do disappear neatly in certain specific numbers of dimensions (either 10 or 26 depending upon the underlying description).

#### Korybut

But in string theory, when you write down how the behavior of objects changes when you change the numbers used to describe them, you get these extra terms called, "anomalies." These anomalies make no physical sense. But they do disappear neatly in certain specific numbers of dimensions (either 10 or 26 depending upon the underlying description).
In string theory Lorentz invariance preserved only in 26 dimension.

#### bapowell

In string theory Lorentz invariance preserved only in 26 dimension.
In bosonic string theory, yes, the critical dimension is D=26. In superstring theory, the critical dimension is D=10. But string theorists also explore non-critical strings in other dimensions.

#### Chalnoth

In bosonic string theory, yes, the critical dimension is D=26. In superstring theory, the critical dimension is D=10. But string theorists also explore non-critical strings in other dimensions.
Bosonic string theory isn't a workable theory, however, as it contains tachyons (and has an unstable vacuum as a result). But heterotic string theory works just fine, and it has 26 dimensions.

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