June 1, 2006 At several hilly locations around the U.S., know as "gravity hills," objects such as cars left on neutral supposedly roll uphill, driven by unknown forces and against the force of gravity. Physicists say -- and GPS measurements confirm -- that the effects are illusions caused by the landscape. The position of trees and slopes of nearby scenery, or a curvy horizon line, can blend to trick the eye so that what looks uphill is actually downhill.
BUCKS COUNTY, Pa.--Mystery spots of land, often known as gravity hills, are found throughout the United States and seemingly pull objects uphill.
"I was surprised at how fast I was moving," says sightseer Andrew Carr.
The mysterious spot in Bucks County, Pa., he speaks about is called Gravity Hill. Put your car in neutral, and it will look and feel like it's rolling uphill. Rolling a ball or pouring water also appears to flow uphill.
Why did physics choose this hill to go berserk?
Carr says, "I thought it was some kind of odd magnetic pull or something like that here that, that pulled your car along."
"Some people think maybe there's a warp in the earth's gravity," Dennis Tice, directory of the Bedford County Visitors Bureau, tells DBIS.
Don't believe everything you hear or see. The truth behind this phenomenon is it's an optical illusion.
"The embankment is sloped in a way that gives you the effect that you are going uphill," materials physicist Brock Weiss, of Pennsylvania State University in University Park, tells DBIS.
The illusion is caused by the landscape. The position of trees and slopes of nearby scenery and a curvy horizon line have blended to trick the eye. GPS measurements by physicists show the elevation at the start is higher than at the end. The road actually slopes downhill!
Weiss says, "You are, indeed, going downhill even though your brain gives you the impression that you're going uphill."
Revealing the secrets behind Gravity Hill proves gravity hasn't gone haywire; it's just playing tricks on your mind.
"Oh, I think it's still fun," Carr says. "I would still bring people here to show them." A fun, quirky science site worth seeing.
Here you can find a gravity hill in your state.
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.