Does Einstein's Theory of Gravity Hold Near Black Holes?

General relativity has never been tested in places where the effects of gravity become truly extreme—for example, at the edge of a black hole. That will soon change

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Scientists have been trying unsuccessfully to poke holes in Albert Einstein's general theory of relativity for a full century. So far, however, Einstein's theory has had it easy. Every assessment to date has been conducted in rather weak gravitational fields. To put general relativity to its greatest test, we need to see whether it holds up where gravity is extremely strong. And nowhere in the universe today is gravity stronger than at the edge of a black hole—at the event horizon, the boundary beyond which gravity is so overwhelming that light and matter that pass through can never escape.

The interior of a black hole is unobservable, but the gravitational field surrounding these objects causes matter close to the horizon to produce huge amounts of electromagnetic radiation that telescopes can detect. Near the black hole, the crushing force of gravity compresses inflowing matter, known as the accretion flow, into ever smaller volumes. This causes the infalling matter to reach temperatures of billions of degrees—which, ironically, makes the vicinity immediately surrounding a black hole one of the brightest spots in the cosmos.

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