Einstein’s theories of relativity hold that space and time are woven together into a four-dimensional fabric, and that a weighty body like a planet or a star depresses that fabric, like someone sitting on a chair or a trampoline. Gravitational attraction is really just objects following the warped path.

What’s more, the rotation of a massive body would also affect the fabric, so that a distant observer would perceive objects close to a gravitational body as being dragged around. Think of Earth sitting in a vat of liquid — as the planet rotates, the liquid starts to swirl, too, and so does everything near the Earth.

If this is true, the axis of a gyroscope would change when compared to the light from a faraway star. This is what GP-B was designed to do.

Orbiting 400 miles above the Earth in a polar orbit, GP-B contains four gyroscopes made of quartz-silicon spheres that are considered nearly perfect — they’re in the Guinness Book of World Records. It has a telescope that stared at a single star, IM Pegasi, while the satellite made its rounds. If the Earth’s mass did not affect spacetime, the gyroscopes would point the same direction forever. But they didn’t, experiencing teeny but measurable changes in the direction of their spin. This is exactly what Einstein predicted back in 1916.