Laz Gamio / Axios
Scientists announced Wednesday that they had observed starlight as it was warped by the gravity of another, more distant star - one of the key predictions of Einstein's general theory of relativity. It was the first deep-space detection of gravity's space-time bending effects — but it had been seen once before, closer to home. In 1919, a solar eclipse allowed physicists to watch the sun bend starlight.
The prediction: In Einsteinian physics, gravity is a force from massive objects that bend the fabric of space-time, like a bowling ball dropped in the middle of a tightly-held blanket. Because space-time is bent, any light passing through it would also be bent.
Einstein's idea: When he proposed his theory, he noted that if it was true, it should be possible to observe the light from stars shifting as our sun moved past them in the sky. But this was hard to observe, because the sun's brightness drowns out the surrounding stars, so he suggested testing it during an eclipse.
How they did it: In 1919, British physicist Sir Arthur Stanley Eddington went on an expedition to the island Príncipe off the coast of Africa to measure the positions of stars during the eclipse. It was a rainy day, but right as the moon moved in front of the sun, the clouds parted. Eddington took sixteen pictures of the stars, two of which yielded clear measurements. Sure enough, when compared to earlier photos, the light from the stars had shifted.
The legacy: Eddington's observations launched relativity into the spotlight and the resulting media storm turned Einstein into a household name. But in 1938, Einstein wrote that he didn't think we'd ever see another effect of gravity's ability to bend light: gravitational microlensing. If a distant star's light was bent by something outside of our solar system, Einstein hypothesized the light would be bent into a ring. That's the big result physicists reported this week. One can only imagine Einstein's reaction to seeing another one of his unprovable predictions pass the experimental test.