|Motion | Pages 40-41 ||
According to Einstein, living in the universe is like living on a huge piece of soft elastic rubber. Space-time is a medium that has shape and form. Objects within this medium can flex and twist it. Every object in the universe pulls on the space around it, drawing the fabric of space-time toward its center. The more massive the object, the more it pulls. The amount of pull exerted by an object on the universal fabric is its gravitational force. So the apple falls to Earth because Earth has warped space-time in such a way that the apple must move toward Earth's center. More massive planets create a deeper warp, imparting a faster acceleration to objects that wander past. The physicist John Wheeler captured these odd notions perfectly: "Matter tells space how to curve, and curved space tells matter how to move."Gravity and LIGHT
Just as remarkably, gravity works not only on objects but also on light. After all, like apples and cannonballs, light travels through space-time, too. A massive body, such as a star, bends a passing beam of light much as a subtle curve in a putting green bends a golf ball toward the hole. Einstein calculated exactly how much our Sun would deflect starlight in this way. He predicted that the effect should be noticeable during a solar eclipse. In 1919 the British astrophysicist Sir Arthur Eddington set up expeditions to Africa and South America to look for changes in the positions of stars near the Sun during a total eclipse. The displacements were extremely small, an angle less than the thickness of a dime as viewed across a football field. Even so, they matched Einstein's prediction. At that moment, Einstein became an international celebrity. Such a displacement of light is called "gravitational lensing." It occurs throughout the universe. Massive objects act like handheld glass lenses, magnifying and distorting light waves from the stars and galaxies behind them.
Other effects of gravity are considerably more down to Earth. One that everyone recognizes is the daily swelling and retreat of Earth's oceans, known as tides. Tides occur because the Moon's gravitational pull is strongest on the side of the planet nearest the Moon, less strong at Earth's core, and weakest on the far side of the planet. The differences in these forces are enough to distort Earth and its oceans into a slightly elongated shape, pointed in the direction of the Moon. It's hard to notice the bulging of the solid Earth, but oceanic flows produce high and low tides twice a day as Earth (continued)