Gravity still stumps scientists

In the deepest depths of space, gravity tugs on matter to form galaxies, stars, black holes and the like. In spite of its infinite reach, however, gravity is the wimpiest of all forces in the universe. This weakness also makes it the most mysterious, as scientists can't measure it in the laboratory as easily as they can detect its effects on planets and stars. The repulsion between two positively charged protons, for example, is 1036 times stronger than gravity's pull between them — that's 1 followed by 36 zeros less macho. Physicists want to squeeze little old gravity into the standard model — the crown-jewel theory of modern physics that explains three other fundamental forces in physics — but none has succeeded. Like a runt at a pool party, gravity just doesn't fit in when using Einstein's theory of relativity, which explains gravity only on large scales.

"Gravity is completely different from the other forces described by the standard model," said Mark Jackson, a theoretical physicist at Fermilab in Illinois. "When you do some calculations about small gravitational interactions, you get stupid answers. The math simply doesn't work." The numbers may not jive, but physicists have a hunch about gravity's unseen gremlins: Tiny, massless particles called gravitons that emanate gravitational fields. Each hypothetical bit tugs on every piece of matter in the universe, as fast as the speed of light permits. Yet if they are so common in the universe, why haven't physicists found them?