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At some level, the majority of physicists believe that Einsteins theory of gravity, called general relativity, is correct. However, that belief is mainly based on observations of phenomena taking place in regions of space with weak gravity, while Einsteins theory of gravity is meant to explain phenomena taking place near really strong gravitational fields, neutron stars and black holes are the objects that have the strongest known gravitational fields, so any test of gravity that involves these objects really test the heart of Einsteins gravity theory.