How can something appear to roll uphill?

03 October 2010

Question

When I was on holiday in Barbados there was a road that seemed to defy the laws of gravity. If you placed a football at the end of the road which was quite steep downhill, the ball moved up the hill. I thought it was some kind of optical illusion at first but tried it myself and the ball did indeed move uphill against gravity! how is this possible?

Answer

We put this question to Amos Storkey, a lecturer in the School of Informatics in the University of Edinburgh...

Amos - From the description, I would have guessed that this happened on Morgan Lewis Hill which is in the St. Andrews District, Barbados. In fact, there are quite a few places like this in the world. There are often known by names like magnetic hill or gravity hill, and they are basically places where your eyes deceive you into believing a downhill slope is in fact a slope uphill.

So what makes us think that down is up?

Well first of all, we expect things like trees, lamp posts, buildings, cliffs, and so on to go straight up. If something is almost vertical, the brain may prefer to believe that it is vertical if it can explain the difference by some other means. The thing is that light, wind, and subsidence can affect tree growth and cause trees to slope. Ground movement can also affect things like lamp posts, or telegraph poles often in a consistent way. The overall landscape also makes a huge difference. Sloped or obscured horizons and widening roads can give a perspective effect that a road that curves around is actually headed up. A slight downward slope that goes over an edge can easily be interpreted as a summit. Finally, once the natural environment starts to look like a gravity hill, people come along and deliberately add things like wonky signs to enhance the illusion, and bring in tourists. Well, wouldn't you? The brain sees all this. If there was one sloping a tree, the brain will just put it down to a sloping tree. But if there are a number of sloping trees and nonvertical cliff, and it looks like the road is headed up due to a sloping horizon, that's it. Your brain just says "it's too much of a coincidence" and interprets it as the road going up instead. And because this is all unconscious processing, there is nothing much you can do about it.

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