# Naive questions on the Principle of Equivalence

The Principle of Equivalence which is at the heart of the conceptual foundation of General theory of Relativity, says that, the physics in a local gravitational field cannot be distinguished from the physics in a suitably chosen non-inertial frame in a gravity-free space.

1. Isn't the term spatially uniform or spatially constant gravitational field more accurate and sufficient than a using the term local gravitational field?

2. When we say physics in the two situations are indistingusable, do we only refer to gravitational experiments or both gravitational and non-gravitational experiments?

• Suppose, Alice stands on the Earth's surface in a closed box. He's not allowed to look outside. He's subjected to the local gravitational field of the Earth $\textbf{g}$ which is spatially constant or uniform. His friend Bob is in a similar box which accelerates in outer space with an uniform acceleration $\textbf{g}$, driven by an artificial engine. He can't look outside either. Then just by performing experiments inside their respective boxes, neither of them can tell which of the two situations he or she is in. – SRS Apr 21 '17 at 14:01
• For example, two identical balls each of mass $m$, when dropped from the same height at rest, both falls towards the ground with same acceleration as detected by both of them. This example seems to imply that a spatially uniform gravitational field throughout the boxes is enough. Isn't it so? – SRS Apr 21 '17 at 14:03