In 1919, the first experimental test of Einstein’s general theory of relativity took place; measuring the mass of the Sun by the way it bends light around it. Almost 100 years later, the theory is finally being used to measure the mass of other stars.
Relativity, in 1914, had predicted a large body would distort space and time around it, meaning light from another object behind that mass would deflect the path of light around it.
Just like a lens in front of a light source, the gravity from a large enough object would bend the light around it. But the only object big enough to measure this distortion was the Sun.
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