On some natural climbs, such as the famous Bom Jesus do Monte hill in Braga, cars in neutral seem to defy the laws of gravity, “climbing” by themselves. It is known that these phenomena are just illusions, but realizing this is only the first step to explaining them properly.
Countless hills on the planet, spread across different continents, have attracted the attention of thousands of people due to a mysterious phenomenon: in these, a car in neutral “ascends alone” the hill, apparently counteracting the force of gravity. Known as "anti-gravity slopes", "mysterious slopes", "magic slopes" or "magnetic slopes", these have become true tourist spots.
In Portugal, there are at least two – one is on the national road N247, near Malveira da Serra; the other, perhaps more famous, is located on the Sameiro road in Bom Jesus do Monte, in Braga (represented in the image).
Relatively obviously, the mere designations these slopes have been given immediately foreshadow the most popular explanations, which are often the only ones offered to visitors.
One of these claims that a strong magnetic field, caused by the presence of ores at the site, is responsible for the strange phenomena. That magnetism contributes nothing to the observed phenomena can be easily proven by the fact that also water and rubber balls, which should not be affected by magnetism, roll “upwards” in a direction apparently contrary to gravity.
There are others who claim that, in these places, the Earth's gravity is altered, causing objects (including automobiles) to move in a direction contrary to normal.
However, this claim too falls short of a satisfactory explanation. It is true that gravity on the Earth's surface is not entirely uniform: its magnitude varies with latitude, Earth's rotation, altitude and local geographic topology.
Taken as a whole, however, and comparing extreme locations, the force of gravity could vary up to a maximum of 0.7%, negligible enough to be considered a relevant factor.
Finally, the direction of gravity can vary very slightly with geographic topology, but the differences are tiny and only detectable with high-precision instruments.
More importantly, an explanation based on variations in gravity tends to ignore that any change in the force/direction of gravity would affect not only cars, water and balls, but also the very people who observe these phenomena – this means that if we were in a place subject to such a variation in gravity, the direction in which objects "descend" would be congruent with our sense of gravity, as well as the direction we would identify as being "downward" and therefore nothing mysterious would be observed - everything would seem absolutely normal to us.
Thus, the mere fact that, on these slopes, some objects seem to have "strange" or "mysterious" behavior reveals, by itself, that there is a conflict between what we take as the downward direction and the one where the force actually acts. of gravity.
In other words, the phenomena we observe on these slopes result from a failure of our brain to correctly identify the direction of gravity – they are perceptual illusions.
Although the illusory character is duly recognized by countless people, and it may not be a novelty for you, it is one thing to say that these slopes are an “illusion”, another is to detail the neuronal processes that give rise to it.
For that, it will matter first of all a brief description of how our brain concludes that a given direction corresponds to that of gravity or, better, where the “down” direction is.
Our body has a series of neurophysiological sensors dedicated to detecting gravity. For example, we have specialized sensors in our inner ears – the vestibular apparatus.
This is made up of three semicircular canals, more or less orthogonal to each other, which signal rotations of the head to our brain, and a small structure made up of calcium carbonate deposits attached to small filaments that swing more or less freely - called otoliths.
The latter, which play an important role in our postural balance, work in a way not unlike the tilt sensors on our mobile phones (which change the direction of the viewfinder image to match the vertical) or as a plumb line: any inclination of our head in relation to the vertical axis translates into the orientation of the filaments that activate nerve endings, and whose signals are later processed by specialized nerve areas.
Despite their apparent complexity, otoliths are not entirely reliable: imagine having to estimate the direction of gravity by holding a pendulum in your hand while running or while moving your arm (this comparison is very simplified and not entirely correct for a series of reasons, but for the sake of brevity will come to the purpose).
Our brain finds itself in a similar situation – among the signals sent by the otoliths, it must estimate which ones are due to an inclination of our body, which to neck movements, which to sudden movements of our head, which to tremors of our steps, when we walk, what are the variations in the speed at which we move, etc.
In order to get a more reliable estimate, especially when the otolith signals are relatively small (when they signal a very slight slope), our brain – in particular, an area at the junction between the temporal and parietal lobes – makes use of additional information , mostly coming from vision (that this sense provides more accurate information is intuitively known to all of us, who do not hesitate to say when we saw something “with our own eyes”): buildings are usually constructed with vertical lines and therefore provide a track for the “up-down” axis; people, being bipeds, tend to align their body axis with gravity, just as trees tend to have vertical trunks; when an object is dropped, it falls “downwards”; the horizon line, where the parallel horizontal lines converge, is at the height of our eyes and is orthogonal to the vertical; an inclination is estimated as such in relation to what we take to be the ground (which tends to be horizontal); etc.
All of these are clues that our brains don't overlook, when available, and that help to fine-tune vestibular information to improve our estimate of which direction is “up” or “down” and hence the direction of gravity.
Relevantly, there are some of these clues which, in turn, are taken as more accurate than others (people are not always standing, nor do trees have their trunks always aligned with the vertical), and the importance of our brain assigns to each of them reflects the precision assigned to it (how invariable is its relationship to the vertical direction).
Some of the most informative visual cues in this regard and, consequently, the ones our brain uses the most, are the horizon line and the ground surface - there is some redundancy in these, as under normal conditions the latter visually converges to the former , but this only accentuates its relevance (after all, the horizon is not always visible).
The "mysterious slopes" are generally slopes of small magnitude (only a few degrees), have no buildings visible nearby, the horizon line is either hidden (eg, by vegetation) or does not correspond with the horizontal plane (eg, it is given by an elevation farther away from which we can only see the upper part) and in the vicinity are other slopes of different magnitude.
These are ideal situations to create ambiguities in the way we estimate what the “downward” direction is, as even the cues that are usually more reliable provide misinformation here or are simply not visually available – better than ours. brain can do, in these cases, is to try to “guess”.
In science, it is not enough to point to a set of factors and to affirm that it is to them that any phenomenon is owed, however well justified it may be.
Furthermore, it is necessary to ensure that these factors really explain the phenomenon, verifying that the latter varies according to changes in the causal factors – in short, it is necessary to test any explanation experimentally.
Obviously, it is not possible (or at least it would be extremely difficult and expensive) to vary the physical characteristics of these “mysterious slopes” to our liking.
Yes, it is possible, and this has already been done, to try to reproduce the phenomenon in a controlled context where we can vary all aspects of the situation.
For example, recreating the “mysterious slopes” in a simple model. This is precisely what researchers at the University of Padua in Italy did in 2003: the magnitude of slopes and the visibility or elevation of a horizon on scale models were carefully varied.
The model was observed by participants, who were unaware of its characteristics, and who were asked to estimate the magnitude of the visible slopes.
It was found that, for example, and when the horizon line is hidden, an ascending segment of a road (a climb with a 1.5% incline) is erroneously perceived as a descent when preceded or preceded by an also ascending segment, but with a higher slope (between 3% to 9%).
Furthermore, the illusion increases the more the greater the difference in slopes between the two segments.
In another case, when two ascending or descending slopes are seen side by side (as in the case of the Bom Jesus do Monte road in Braga), the one with the smallest slope is perceived in the opposite direction (an ascent looks like a descent and vice versa. -versa).
This effect is greatly amplified when an artificial horizon line is presented to suggest that the steepest road is horizontal: for example, a descent with a 3% incline that appears to converge to the horizon (due to the perspective with which it is viewed ), tends to be perceived as horizontal and an adjacent descent with a slope of only 1.5% is consequently mistakenly taken as an ascent.
This is exactly the case for the Bom Jesus hill: both roads, in the vicinity of their intersection, are effectively descended; but the one on the right, with a greater inclination, is seen as almost horizontal (its inclination is underestimated) due to an erroneous judgment about the horizon (what we see in the distance are some hills that hide what would be the geometric horizon, and the visual estimate of this is lower than it should be). As a result, the road on the left side is mistakenly perceived as an uphill climb.
Finally, let it be said that all participants in the model experiment, without exception, showed great surprise (and in one case, according to the authors, fear) when they were shown that a marble "went alone" what they perceived to be clearly a climb, and even this simple demonstration did not make them see that the slope was actually a descent.
The comparison with what visitors to the “mysterious slopes” report is all too obvious.
Author Nuno de Sa Teixeira
Science in the Regional Press – Ciência Viva
Nuno Alexandre de Sa Teixeira he graduated in Psychology from the Faculty of Psychology and Educational Sciences of the University of Coimbra, and a Ph.D. in Experimental Psychology from the same institution.
He worked as a PhD researcher at the Department of General Experimental Psychology at the University Johannes-Gutenberg, Mainz, Germany, and later at the Institute of Cognitive Psychology at the University of Coimbra.
He is currently a PhD researcher at the Center for Space Biomedicine at the University of Rome 'Tor Vergata', Italy.
His scientific work has focused on the study of how physical variables (in particular, gravity) are instantiated by the brain, as “internal models”, to support perceptual and motor functions in the interaction with the world.
Thus, his interests depart from the hinge between thematic areas such as the Psychology of Perception, Psychophysics and Neurosciences.
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