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Mass curves spacetime like a heavy stone pressing into an asphalt road. Light passing by skirts around this dent, its path lengthens, and it arrives late. Astronomers have observed this Shapiro effect many times near the Sun and planets.

A new plan proposes to recreate this delay on a lab bench. Two beams are simultaneously launched into a closed fiber-optic loop: one clockwise, the other counterclockwise. A massive object placed nearby curves space within the loop, and one of the beams arrives with a delay of billionths of a second. The technology is well-honed: similar devices already detect vibrations of space.

Instead of satellites and telescopes, Einstein's theory of gravity could be tested in an ordinary room. The precision is enough to spot deviations hinting at new physics. Remarkably, the light delay near bodies was originally proposed by Einstein himself in 1916—long before the first spaceflights.