Since beginning its experiments, the Eöt-Wash group has delivered an impressive array of data indicating that light and heavy objects accelerate exactly the same way—with a maximum discrepancy of approximately one out of 10 trillion. Gravity, the team has found so far, behaves in an identical fashion, whether it is twirling stars around a galactic core or lowering a speck of dust toward the ground. With these successes in hand, the team is pushing its equipment to its absolute limit, hoping to map out every facet of gravity’s terrain.

Today, not all tests of the equivalence principle involve nimble balances twisting and turning in labs. Some of the newer experiments have forsaken Chubby Checker moves for Obi-Wan Kenobi maneuvers. With lasers and space probes now used to make ultraprecise measurements, tests of general relativity have entered the Star Wars age.

THE PROOF IN SPACE’S PUDDING

In the 1960s and 1970s, space agencies such as NASA (National Aeronautics and Space Administration) captivated the public through unprecedented manned missions, like the Apollo Moon landings. These days such centers have broadened their scope to include a wealth of scientific satellites and other instrumentation designed to investigate the nature of space itself. The Hubble Space Telescope, the most famous of these instruments, has been joined by numerous other devices probing the deep structure of the cosmos.

Witness a new APOLLO (Apache Point Observatory Lunar Laser-ranging Operation) mission, one that sends laser beams instead of people to the Moon. It makes use of five retro-reflectors—banks of special prisms left behind by the astronauts on the lunar surface. These mirrored surfaces reflect incident light back to Earth, enabling precise measurements of the distance to the Moon. By shining a laser from Earth onto one of these and timing how long it takes for the



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