dimensions in string theory

[michel123456]

From the wikipedia article about String Theory.

 

An intriguing feature of string theory is that it involves the prediction of extra dimensions. The number of dimensions is not fixed by any consistency criterion,[dubious discuss] but flat spacetime solutions do exist in the so-called "critical dimension". Cosmological solutions exist in a wider variety of dimensionalities, and these different dimensionsmore precisely different values of the "effective central charge", a count of degrees of freedom which reduces to dimensionality in weakly curved regimesare related by dynamical transitions.[14] One such theory is the 11-dimensional M-theory, which requires spacetime to have eleven dimensions,[15]

 

Evidence shows that the 4 known dimensions (3 of Space, 1 of Time), are only positive. It means: distance along X, Y, Z is only positive, time is only positive.

 

My questions are the following:

 

1.Is this "positive feature" appearing from the Theory or artificially inserted on the basis of evidence?

2. what about the remaining dimensions: are they positive? are they negative? or are they not constraint by anything being positive negative wathever it takes?

Edited January 24, 2011 by michel123456

[ajb]

You usually assume that there is only one time dimension of the bulk. This will allow you to have space and time coordinates on the worldsheet.

 

This can be weakened, and is essential in twistor string theory. Twistor theory requires 4D Minkowski space and the (2,2) metric signature.