Editing Equivalence principle (section)

=== Strong equivalence principle ===
The strong equivalence principle applies the same constraints as the Einstein equivalence principle, but allows the freely falling bodies to be massive gravitating objects as well as test particles.<ref name=CliftonFerreiraPadilla/>
Thus this is a version of the equivalence principle that applies to objects that exert a gravitational force on themselves, such as stars, planets, black holes or [[Cavendish experiment]]s. It requires that the [[gravitational constant]] be the same everywhere in the universe<ref name=Will2014/>{{rp|49}} and is incompatible with a [[fifth force]]. It is much more restrictive than the Einstein equivalence principle.

Like the Einstein equivalence principle, the strong equivalence principle requires gravity to be geometrical by nature, but in addition it forbids any extra fields, so the [[metric tensor (general relativity)|metric]] alone determines all of the effects of gravity. If an observer measures a patch of space to be flat, then the strong equivalence principle suggests that it is absolutely equivalent to any other patch of flat space elsewhere in the universe. Einstein's theory of general relativity (including the [[cosmological constant]]) is thought to be the only theory of gravity that satisfies the strong equivalence principle. A number of alternative theories, such as [[Brans–Dicke theory]] and the [[Einstein-aether theory]] add additional fields.<ref name=CliftonFerreiraPadilla/>