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Gravity and Strings

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3.5 Final Comments 113<br />

theory as well as other alternative theories of gravity. Some of these elements are more<br />

or less trivial extensions of GR (for instance, its reformulation in the Vielbein formalism<br />

which allows the coupling to spinors) <strong>and</strong>, in fact, it is always (or usually) possible to see<br />

the new theories as GR coupled to different fields. In the next few chapters we are going<br />

to review these elements <strong>and</strong> theories that contain them: the Cartan–Sciama–Kibble theory,<br />

non-symmetric theories of gravity, theories of teleparallelism, <strong>and</strong> supergravity theories.<br />

The simplest way to introduce most of them is through a minimal action principle <strong>and</strong> the<br />

formulation of the minimal action principle for GR will be the first step in this direction.

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