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1. The Relativity of Einstein's *General Relativity*

Einstein's theory of General Relativity (GR) is based on two postulates:

- the Principle of Equivalence (EP);
- the principle of relativity;

In addition, Einstein's field equation relates the space-time curvature to matter.

There have been some if not many interpretations of the principle of equivalence and relativity. These notes derive from the reading of J. Norton, "general covariance and the foundations of general relativity: eight decades of dispute",

*Rep. Proc. Phys.***56**, pp. 791-858 (1993).http://www.pitt.edu/~jdnorton/papers/decades.pdf

#### Special Relativity (SR)

Special or restricted theory of relativity.

- All natural laws must be covariant with respect to Lorentz transformations.
- Minkowski space-time description of the Special Relativity.

#### General Relativity (GR)

The general theory of relativity.- Einstein's principle of equivalence (EEP):
*the equivalence between gravitation field and the acceleration of the reference frame*.

Einstein observed that his Principle of Equivalence (EEP) led to an extension of the principle of relativity in SR: