Gravity: an Introduction

Gravity is one of four fundamental forces in nature. Everyone knows that it is a mutual force of attraction between two bodies that have mass. Yes, there is a gravitational attraction between all macroscopic bodies, including you and the chair you're sitting in, or your computer and its monitor. For such small objects, however, the force of gravity between them is so weak that it produces no noticeable movement. Though much stronger, the gravitational force of the Earth on you and everything in sight is still incredibly weak. If we compare the strength of the gravitational force to one of the other three fundamental forces, say electromagnetism, we find the electromagnetic force to be some 1,000,000,000,000,000,000,000,000,000,000,000,000,000 times stronger. If you lined up this many atoms, end on end, they would stretch to the edge of the universe and back a thousand times. Gravity is so weak that a small kitchen magnet can hold a photo to the refrigerator against the gravitational pull of the entire Earth. (The gravitational force between two objects is proportional to the product of the masses of the two objects and inversely proportional to the distance, squared, between them.)

Gravity's relative weakness, compared to the other fundamental forces, baffles scientists. Some speculate that some of the gravitational force must leak out into other dimensions beyond the 4 (including time) that we are familiar with. There must be 6 or 7 other dimensions out there, that we are unable to perceive, into which gravity leaks. No one knows for sure if there are indeed this many dimensions to our universe, but it's one of the theories currently circulating. (String theory incorporates this view.)

Actually, perhaps we shouldn't call gravity a force. Einstein explained gravity as the effect of warped space-time. Think of the universe as a giant rubber sheet, with different-sized balls (representing planets or massive bodies) sitting on it. These balls, especially the heavy ones, sink and create depressions in the rubber sheet. Picture a bowling ball on the rubber sheet. This could represent the sun. Now as a marble, the Earth, passes by, if it gets close enough to the bowling ball (sun), it falls into the depression created by the heavier ball. If the marble stopped moving forward all of a sudden, it would fall right into the bowling ball. But it has inertia, so that it continues to move forward, and therefore it can attempt to escape the "pull" of the bowling ball. Moving forward at the right speed, it can fall into an orbit around the bowling ball. Think of a ball spinning around the perimeter of a roulette table. It's initially going fast enough to "orbit" the center of the table. After it loses energy and slows down, it falls toward the center of the table and into a numbered compartment. But if it didn't slow down, it would continue "orbiting" indefinitely. This describes the motion of the Earth around the sun. The Earth is moving fast enough to maintain an orbit around the top of the bowl-like depression in space-time created by the massive sun. If it was slowed down enough, it could fall into the sun. If it was sped up enough, it could swing out of the depression and escape the "gravity" of the sun. This explanation of gravity is known as general relativity.

Is the "force" of gravity between two objects instantaneously transmitted? That is, if the moon were quickly moved (by some alien) to a location twice as distant from the Earth, would the force of gravity between the Earth and moon instantaneously decrease or would it take some period of time before the planets "realized" they were now farther apart and stopped pulling on each other so forcefully? Einstein said instantaneous transmission of gravity (or anything) was impossible. It violated one of the most important findings in all of physics: that nothing travels faster than the speed of light. Therefore, gravity can't travel faster than light, which travels at about 186,000 miles per second. (Since gravity is itself without mass, it can and does travel at the speed of light.) If the sun were to disappear, the Earth wouldn't notice for about 8 minutes, because it takes that long for light (and gravity) to travel here from the sun. For those 8 minutes, the Earth would continue in its orbit, completely oblivious to what was about to happen.