The fundamental principle at the base of Einstein’s Theory of General Relativity – that all objects in freefall accelerate identically – has been verified on a stellar scale.
Einstein predicted that all objects behave identically when falling in an external gravitational field. For his theory to remain unchallenged it has long been thought critical to establish that the principle holds at all scales within the universe.
In particular, it was felt important to test whether the prediction plays out even in systems involving objects with strong “self-gravity” – that is, where the combined gravity of the constituents within an object, which thus hold it together, is extremely powerful. Neutron stars are examples of stellar objects with strong self-gravity.
Interestingly enough, despite their huge size, most objects in the universe – the majority of planets, stars and even galaxies, for instance – do not exert powerful enough self-gravity to able to be used as test subjects for the envelope-pushing extremes of Einstein’s theory.