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String Theory Demystified

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CHAPTER 2 Equations of Motion 37<br />

homomorphic—the donut has a hole but a sphere does not. The bottom line is there<br />

is no way to transform the donut into the sphere.<br />

If a geometric shape is homeomorphic to a sphere, then the Euler characteristic is<br />

χ = V − E+ F = 2 (2.29)<br />

Many shapes have an Euler characteristic which vanishes. Some examples of this<br />

include a torus, a möbius strip, and a Klein bottle. Another example is a cylinder,<br />

which also has χ = 0 (see Fig. 2.2). Why is this interesting for us? If the worldsheet<br />

of a string has a vanishing Euler characteristic, then it is possible to write the<br />

auxiliary fi eld hαβ as a two-dimensional fl at space metric. That is, we take [using<br />

the choice of coordinates for the worldsheet as ( τ, σ ) ]<br />

Klein bottle<br />

h = αβ − ⎛ 1 0⎞<br />

⎝<br />

⎜<br />

0 1 ⎠<br />

⎟<br />

(2.30)<br />

Tours Mobius strip<br />

Cylinder<br />

Figure 2.2 Some surfaces with a vanishing Euler characteristic. When the Euler<br />

characteristic vanishes, we can defi ne the auxiliary fi eld such that it has<br />

a representation of the fl at space Minkowski metric.

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