The speed of light gives astronomers a special trick when examining the tangled-up gravitational well around a black hole. Researchers have proposed that they can measure the spin of a black hole because of the bizarre path that photons take in their vicinity, released by accreting matter. Some photons blast straight out, while others are lensed indirectly, and some travel around the black hole twice before escaping, hinting at the black hole's rotation rate.

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