100 years ago, an eclipse proved Einstein right - Today, black holes do too — for now | Eurasia Diary - ednews.net

20 June, Thursday

100 years ago, an eclipse proved Einstein right - Today, black holes do too — for now

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British astronomer Arthur Stanley Eddington and his colleagues photographed a solar eclipse a centry ago. The photos affirmed for the first time a key prediction of Albert Einstein’s general theory of relativity: Mass bends spacetime.

The expeditions marked a revolution in physics and made Einstein a celebrity. Today, physicists are at it again — on a much larger scale. In April, the Event Horizon Telescope (EHT) collaboration released the first picture of the edge of a black hole. That image again showed that massive objects, such as black holes or the sun, can change how light travels, just as Einstein predicted, Science News

So far, the new black hole data have confirmed general relativity. But future EHT images of the gravitational beasts — especially the one at the center of our own galaxy — could potentially poke holes in Einstein’s famous theory.

A hundred years ago, scientists didn’t have a black hole to test for cracks in general relativity —black holes were just the stuff of imagination back then — but they did have the 1919 total solar eclipse. At the time, the predominant theory of gravity was Newtonian, which says that gravity is a force. Forces can accelerate objects that have mass, but since light has no mass, gravity shouldn’t affect it, the thinking went. But a few years earlier, in 1915, Einstein had proposed his general theory of relativity, which says that gravity comes from matter and energy warping spacetime, generating curves that change objects’ motion or even the path of light itself.

In Eddington’s and his colleagues’ photographs of the eclipse, stars appeared in different positions in the sky during the eclipse, when their light had to pass the sun to reach earthly observers, than on an ordinary night. The sun’s gravity had changed the path that the starlight took. Einstein was right.

These days, the idea that gravity can curve light is so well understood that physicists use it to probe the properties of spacetime itself. Before the EHT started taking data in 2017, for example, scientists had used Einstein’s equations to get a precise idea of what a black hole should look like, if the theory didn’t break down in the extreme environment.

Black holes curve spacetime so extremely that light gets trapped inside them. So physicists can’t see light emitted by the black hole directly. But they can see the black hole’s shadow on bright material around it. Under general relativity, that shadow should have a specific size and shape: a circle whose width is directly related to its mass. “This all falls out of Einstein’s equations,” Johnson says, The Washington Post reported. “If you have a different theory of gravity, you can predict a different ring on the sky.”

The universe has not run out of surprises, and theoretical physicists remain in business. The questions don’t tend to get easier over time. When Emil Mottola, a theoretical physicist at the Los Alamos National Laboratory, is asked about what happened, exactly, at the beginning of the universe, he says, “Sometimes you have to say you don’t know.”

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