This exception has vexed physicists since Albert Einstein’s heyday. Electromagnetism the “strong” nuclear force that binds atomic nuclei and the “weak” nuclear force that causes radioactive decay-they are all quantum to the core, leaving gravity as a sole, mysterious outlier. Of the four fundamental forces in the universe, gravity is the only one that cannot be described by the laws of quantum mechanics, the theory that applies to all other forces and particles known to physics. Like Cavendish, he has plans for an ambitious, seemingly impossible experiment, one that might transform our understanding of gravity: he wants to use a small-scale setup-literally on a tabletop in his lab-to find evidence that gravity might be a quantum phenomenon. ” Cavendish’s 220-year-old tour de force, though not actually conducted on a tabletop, is a source of inspiration for Aspelmeyer, a physicist at the University of Vienna in Austria. “It was the first precision tabletop experiment. “It’s an incredible story,” says Markus Aspelmeyer, who has been recounting the Cavendish experiment during a Skype call. Researchers are also trying to measure the gravitational fields of millimeter-wide gold spheres ( right) to observe gravity closer to the quantum realm. Credit: Mattia Balsamini Superconducting circuits ( left) aid the levitation experiment. Over the course of nine months he repeated the experiment 17 times and found that Earth weighed 13 million billion billion pounds, a result essentially identical to the best modern estimates. Because the gravitational force is directly proportional to the masses being measured, he could use that ratio to solve for Earth’s unknown mass. Because he already knew that Earth exerted a gravitational force of 1.6 pounds on each of the small spheres (in the English system of units, a pound is by definition a measure of force), Cavendish could set up a simple ratio: the gravitational force between the small sphere and the large sphere compared with the gravitational force between the small sphere and Earth. This allowed him to directly measure the gravitational force exerted by each of the larger spheres on the smaller ones. Cavendish expected that the gravitational pull of the heavy spheres on the smaller ones would make the wood rod rotate ever so slightly, and he was right-it moved just over a tenth of an inch. Two much heavier lead spheres, each weighing nearly 350 pounds, were suspended separately about nine inches away from the lighter balls. The device he had built in a shed on his estate in southwest London consisted of two 1.6-pound lead balls attached to opposite ends of a six-foot-long wood rod, which hung from a wire fastened to an overhead beam. On an August day more than a century later Cavendish proved Newton wrong. “Nay, whole mountains will not be sufficient to produce any sensible effect,” he wrote in his masterpiece, the Principia, which laid out his laws of motion and gravitation. But Newton summarily dismissed his own idea-he thought the attraction between the spheres would be too small to detect, even with impractically large masses. He had even suggested a way to determine Earth’s absolute mass: measure the gravitational attraction between two small spherical masses with great accuracy, then extrapolate Earth’s own mass from the result. Isaac Newton had compared Earth’s mass with that of other bodies in the solar system and knew, for example, that Earth was more massive than the moon. Was it mostly solid rock? Did it vary with depth? Astronomer Edmond Halley even suggested that Earth might be hollow. In 1797 Henry Cavendish, one of Great Britain’s leading scientists, built a contraption to weigh the world.Īt the time, Earth’s mass was unknown, as was its composition.
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