Space and Physics
PUBLISHED
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Across the world there are special places where the rules of nature seem to give up their ubiquity. Imagine this: you park your car on a hill and put it in neutral, but rather than rolling down the hill as you would expect, the reverse happens. In other instances, cyclists may find themselves pedaling hard to progress down an obvious slope, as if gravity is pushing them in the opposite direction.
These locations – sometimes referred to as gravity, mystery, or magnetic hills – have been recorded across the world and their strange effect is very entertaining. In fact, there are even roadside tourist sites across the US and Canada that have capitalized on the phenomenon, each offering a sci-fi-like or supernatural explanation for the strangeness. But there is nothing uncanny about these places. As with so many things, it’s all just a trick of our minds.
Illusory landscapes
In essence, these strange hills are the result of optical illusions caused by a combination of factors. The first, and potentially most important, is the lack of a visible horizon. If the horizon can’t be seen or if it’s not level, then it becomes difficult for our brains to assess the slope of the road. This becomes particularly tricky if the surrounding terrain is tilted.
Our sense of balance is, to a large extent, produced by the vestibular system in our inner ear, but we also rely on visual data as well. In some cases, this visual information can override what our bodies feel, which can mean we overestimate or completely misjudge a slope’s gradient.
In 2003, researchers from the Universities of Padova and Pavia in Italy studied how the horizon can mess around with our perspective by recreating the effects of these gravity hills in the lab. They first built tabletop models of several known gravity hills from across the world and then recruited volunteers to look at them through small holes that gave them the on-the-ground perspective. They also had the ability to change the horizon to see how this would impact the viewer.
The researchers found that, when the true horizon wasn’t in sight, objects like trees or road signs could actually trick people’s minds. The participants found it more difficult to judge a slant in relation to the horizontal plane. This was particularly true when a slight downhill stretch was preceded, followed, or flanked by a steeper downhill slope.
But for those who are still unsure about these more mundane explanations, you can easily check them yourself. A simple spirit level is enough to demonstrate that, contrary to how our eyes see it, the actual slope goes in the opposite direction.
Researchers have also used other professional surveying instruments – theodolites – to examine these roads from farther away (you know, just in case gravity is being silly in the location and screwing with the spirit level). As you can imagine, these instruments all indicate that the slopes are running in the opposite direction to our how eyes perceive them.
If you’re still unconvinced, you can take a look at GPS elevation data to confirm the literal lay of the land.
