BACKGROUND: Visit Gravity Hill in New Paris, Pa., and you will see cars that appear to roll uphill, while water flows the wrong way. Don't worry -- gravity hasn't really gone haywire. It's simply an optical illusion, one that can be found in hundreds of similar areas around the world.
HOW IT WORKS: The human eye and brain can be easily fooled into thinking the laws of physics are being defied -- but it's all distortions in perspective and odd angles. What such "mystery sites" all have in common is a completely or mostly obstructed horizon, which makes it difficult for human beings to judge the slope of a surface. They lack a reliable reference point, and this can override the body's sense of balance, especially if the slope is shallow. In the case of Gravity Hill, the layout of the surrounding landscape produces the illusion of a very slight downhill slope, when in fact it is a slight uphill slope. So a car left out of gear will appear to be rolling uphill.
WHAT IS GRAVITY? In the 17th century, Isaac Newton came up with the law of universal gravitation, but he couldn't explain the underlying mechanism behind gravity. In 1917, Albert Einstein proposed his theory of general relativity. This attributes the force of gravity to the unseen warping of the fabric of spacetime, caused by the presence of mass (or energy). The earth always travels in a straight line. The presence of the sun curves space and thus the earth appears to be moving in an elliptical orbit. Imagine a rubber sheet stretched out tightly. If a bowling ball is placed in the center, the ball's mass will cause a depression in the sheet. If you then place an apple on the edge of the sheet, it will roll down the slope towards the center. The depression can't be seen by someone looking straight down at the sheet from above, so it appears that the apple was pulled by an invisible force.