### Equivalence principle

The equivalence principle, explored by a succession of researchers including Galileo, Loránd Eötvös, and Einstein, expresses the idea that all objects fall in the same way. The simplest way to test the weak equivalence principle is to drop two objects of different masses or compositions in a vacuum, and see if they hit the ground at the same time. These experiments demonstrate that all objects fall at the same rate when friction (including air resistance) is negligible. More sophisticated tests use a torsion balance of a type invented by Eötvös. Satellite experiments are planned for more accurate experiments in space.^{[4]}

Formulations of the equivalence principle include:

- The weak equivalence principle:
*The trajectory of a point mass in a gravitational field depends only on its initial position and velocity, and is independent of its composition.*^{[5]} - The Einsteinian equivalence principle:
*The outcome of any local non-gravitational experiment in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime.*^{[6]} - The strong equivalence principle requiring both of the above.

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