- #1
benbenny
- 42
- 0
Hello,
Im trying to learn about string theory in toroidal compactification on an undergraduate level. I am mostly using Zwiebach's "A first course in string Theory" but now I am trying to do something that doesn't seem to be covered in the book or any other literature explicitly. Perhaps because I am misunderstanding the issue. The periodicity condition for a closed string with winding number m, in a 1-D compact space is
[tex] X(\tau, \sigma) = X(\tau, \sigma +2\pi) +2 \pi R m \label{1} [/tex]
I can understand this if I imagine one extended dimension and one compactified dimension. Such a space looks like a cylinder and a closed string can be wrapped around the cylinder m number of times. To return to the same point on the string thus we have to travel a distance of 2\pi R m.
My confusion is when you generalize to arbitrary number of compactified dimensions:
[tex]X^{j}(\tau,\sigma)=X^{j}(\tau,\sigma+2\pi)+2\pi R_{j}m^{j} \label{2} [/tex]
I can understand this if I think about a particle in compact space and disregard the first term on the right hand side which pertains to a string: The distance a particle will travel to return to the same point is the sum over the lengths of all the compact dimensions in the space.
What I don't understand is why this equation holds for a string.
I have tried to think about the 2 compact dimensional case. In this case the compact space can be said to look like a 2-torus. Say you have a 2-D ring-torus with R the radius of the torus, and r the radius of the tube.
For a point on a string which is wrapped around the 2-torus, traveling around the r circle will bring us back to the same point after a distance of 2\pi r. But if the string travels along the torus, around the R circle it will have traveled a distance of 2\pi R only if r=0. Otherwise, as far as I can see, if the point is on the inwards side of the torus, it will have traveled a distance of [tex] 2 \pi (R-r) [/tex], and if the point on the string is on the outwards side of the torus it will have traveled [tex] 2 \pi (R+r) [/tex].
So as far as I can see the periodicity condition for a 2-D torus should be
[tex] X = X(\sigma+2\pi) +2\pi r m +2\pi (R+r\cos \theta) \label{3} [/tex]
where \theta is the parametrization angle of the r circle.
Then I have no idea how to proceed for a d-torus.
I would appreciate any help on this.
B
Im trying to learn about string theory in toroidal compactification on an undergraduate level. I am mostly using Zwiebach's "A first course in string Theory" but now I am trying to do something that doesn't seem to be covered in the book or any other literature explicitly. Perhaps because I am misunderstanding the issue. The periodicity condition for a closed string with winding number m, in a 1-D compact space is
[tex] X(\tau, \sigma) = X(\tau, \sigma +2\pi) +2 \pi R m \label{1} [/tex]
I can understand this if I imagine one extended dimension and one compactified dimension. Such a space looks like a cylinder and a closed string can be wrapped around the cylinder m number of times. To return to the same point on the string thus we have to travel a distance of 2\pi R m.
My confusion is when you generalize to arbitrary number of compactified dimensions:
[tex]X^{j}(\tau,\sigma)=X^{j}(\tau,\sigma+2\pi)+2\pi R_{j}m^{j} \label{2} [/tex]
I can understand this if I think about a particle in compact space and disregard the first term on the right hand side which pertains to a string: The distance a particle will travel to return to the same point is the sum over the lengths of all the compact dimensions in the space.
What I don't understand is why this equation holds for a string.
I have tried to think about the 2 compact dimensional case. In this case the compact space can be said to look like a 2-torus. Say you have a 2-D ring-torus with R the radius of the torus, and r the radius of the tube.
For a point on a string which is wrapped around the 2-torus, traveling around the r circle will bring us back to the same point after a distance of 2\pi r. But if the string travels along the torus, around the R circle it will have traveled a distance of 2\pi R only if r=0. Otherwise, as far as I can see, if the point is on the inwards side of the torus, it will have traveled a distance of [tex] 2 \pi (R-r) [/tex], and if the point on the string is on the outwards side of the torus it will have traveled [tex] 2 \pi (R+r) [/tex].
So as far as I can see the periodicity condition for a 2-D torus should be
[tex] X = X(\sigma+2\pi) +2\pi r m +2\pi (R+r\cos \theta) \label{3} [/tex]
where \theta is the parametrization angle of the r circle.
Then I have no idea how to proceed for a d-torus.
I would appreciate any help on this.
B