How Mercury and Starlight Overthrew Newton
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Gravity isn’t a force pulling across empty space. It’s the bending of space and time by matter and energy. Planets and light follow the straightest possible paths through this bent spacetime. Starlight passing the edge of the Sun is bent by about 1.7 arcseconds.
General relativity can’t be the whole truth about gravity. Together with quantum theory, it implies that gravity can’t be measured precisely in tiny regions of space. So the smooth spacetime of general relativity must break down there.
General relativity has to explain the extra movement of Mercury’s perihelion, which Newton’s law can’t.
General relativity explains it. It makes Mercury’s perihelion move 43 arcseconds per century more than Newton’s law does, with no hidden matter needed.
Einstein’s prediction that the Sun bends starlight by 1.7 arcseconds hasn’t been tested yet. The eclipse of May 29, 1919, can test it, since the Sun will be among the many bright stars of the Hyades.
The eclipse photos of 1919 show starlight bent by between about 1.6 and 2.0 arcseconds at the edge of the Sun, as general relativity predicts.