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        Physically, this principle (sometimes referred as to EEP = Einstein's Equivalence Principle) postulates that there is no experiment done in a small confined space which can tell the difference between a uniform gravitational field and an equivalent acceleration.

Mathematically, this important observation states that in presence of a gravitational field, small enough free falling frames will be inertial, and that in these Local Inertial Frame (LIF), where the metric gμν reduces to ημν, the laws of physics from Special Relativity will hold true.

Refer to  the article The Equivalence Principle for more details.

Refer to the below youtube tutorial to get a good overview of the mathematical expression of the principle, i.e. how a local inertial frame (black colour) can be obtained by a general coordinate transformation at any point P of a manifold (blue colour)

 

Refer to the article 1907 Equivalence Principle first published expression to read the Einstein's first formulation of this principle.