# Q & A: Local Equivalence of Gravity and Accelerating Frames

Q:
This is how to find out if you are inside a box resting over the Earth or submitted to an acceleration of 1G by a smooth and very quiet rocket. Just need simple but highly precise instruments: To make thing not very hard let´s imagine a very large box. Hung two weights with a piece of string in each side of the box to determine the direction of the force. If it is generated by a gravitational field, the string will point to a precise point at the center of the Earth and therefore will not be parallel. If the box is being accelerated, free of any gravitational field, the strings will be parallel.
- roberto patella (age 71)
rio de janeiro - brazil
A:

Hi Robert,

Your experiment (and other measurements of tidal forces) could indeed differentiate between an accelerating reference frame and the gravitational field of a planet.

For this reason, the equivalence principle () states that gravitational fields are locally equivalent to the pseudo-forces found in accelerating reference frames. The key word here is local. If the gravitational field isn't exactly constant in any finite region of space, then you could in principle detect the variations in strength (and so know you weren't in a spaceship). But for small enough distances, the equivalence is exact.

Cheers,

David Schmid

(published on 01/08/2014)