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worry about the mass of a space probe when they use a planet's gravity to adjust its flight path. A heavy spacecraft and a light one respond in exactly the same way.
It's hardly necessary to review why gravity is important in our lives. However, most people don't appreciate something surprising about it: Of the four basic forces of nature  --gravity, electromagnetism , and the "strong" and "weak" forces that operate within the nucleus of an atom--gravity is by far the weakest.  To see this in one way, just rub a balloon in your hair or on a wool sweater a few times and stick it on your sleeve. Despite the gravitational pull of the entire Earth, the balloon stays attached to your arm because of a tiny bit of static electricity. On a far smaller scale, the strong force that binds the nucleus of an atom together is more powerful than gravity by a factor of more than a trillion trillion trillion. And yet gravity dominates every facet of the universe, from the Big Bang to the creation of galaxies, stars, and our planet.
This seems strange, but the reasons are easy to comprehend. Two of the four basic forces work only over the tiny distances within atoms. Electrical attraction is powerful over larger distances, but matter contains positively and negatively charged particles (protons and electrons, respectively) that cancel each other out. The only force that has no negative counterpart yet still operates across vast reaches of space is gravity. In matters universal, gravity always wins.
Another way to look at the dominance of gravity is to consider what happens when Earth's gravity accelerates falling objects. When you jump from a bridge with a bungee cord around your feet, your speed reaches about 20 miles per hour after one second and about 40 miles per hour after two seconds. A powerful motorcycle also can accelerate that fast on a straight, smooth road. But the motorcycle's engine does not rival Earth. It can accelerate only one vehicle, whereas Earth could accelerate millions, even billions, of bungee jumpers at the same time, if that many people took leave of their senses and jumped at once. That is the power of gravity: On small scales it cannot compare with forces generated by other means, but on planetary, interstellar, and galactic scales, it has no equal.
We are still learning about gravity's effects on objects in the cosmos. For example, stars in the outer parts of spiral galaxies revolve surprisingly quickly around the galactic centers. We believe that an unseen extra source of gravity--some mysterious "dark matter "--exerts an additional tug on the stars as they orbit. This tug pulls the stars along faster than they would travel if the gravity came only from the glowing stars themselves. dark matter will play a key role in determining the fate of the universe billions of years from now.
The oddest thing about gravity is that it acts across the gulf of space with no apparent connection between objects. Even an apple lacks an obvious reason to fall down. There's no contact between it and the Earth, and no easily identified "field" like the electromagnetic fields around bar magnets and electrical transformers. When Albert Einstein considered this situation and other riddles about gravity, he devised his general theory of relativity --a profound idea that, once again, extended Newton's work.
One part of the theory, published in 1916, holds that there is no difference between the acceleration due to gravity and the force felt within an accelerating frame of reference. Imagine standing in an enclosed box and feeling a downward force that produces your normal weight. The box could be resting on Earth's surface, and the downward force you feel would be the planet's gravitational pull. However, the box also could be zipping on a straight line through space with a constant acceleration of 32 feet per second every second. That would produce a force toward the floor of the box identical to Earth's pull. The box could even be whirling at the end of a long cable in space or inside a giant rotating wheel, like the spacecraft in 2001: A Space Odyssey. Inside your box you could not distinguish among these situations. The next time you feel your weight "increase" as an elevator starts upward, remember that you'd feel the same sensation if Earth's mass suddenly increased to create a stronger gravitational pull--or if your spaceship's engines just turned on.
The most mind-bending impact of general relativity came from the way in which Einstein changed our conception of space and time. His theory held that throughout the cosmos, space and time are woven together in four dimensions, like the threads in a piece of cloth. Gravity, Einstein proposed, is the wrinkling of the fabric of this "space-time " by objects within the universe. That statement sounds unfathomable. However, it helps to think of a three-dimensional analog to Einstein's idea. Touch your fingertip to the surface of a bowl of Jell-O. The surface bends inward with the weight of your finger, the same effect that a mass creates in the weave of space-time .
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