Pervasive illusions
When I was young, I had been fascinated by an optical illusion that you have likely encountered yourself: a diagram where two identical parallel lines are made to appear unique lengths by the inclusion of two brief arrow-like fins at each of the ends, pointing inwards in the case of one line and outwards in the instance of another.
What was fascinating was that the persistence of this illusion in the face of critical knowledge it was not real. The error I made when I first saw the diagram — that I was looking at two lines of various lengths — was swiftly and easily adjusted. However, my sensory understanding of what I was looking at was not so easily swayed; and that was at once strange and marvellous.
Even though I understood the parallel lines were exactly the exact same length, and even though I repeatedly verified this using the ruler out of my school pencil case, I couldn’t make my mind see this actuality. However long I stared, the lineup with all the outward-facing arrows inexorably appeared more than the other. Unless, that was, I covered up the two sets of fins with my fingers, at which point the lines moved back into being nothing more than equal marks on a piece of a paper.
I did not know it at the time, but I had been looking at a version of a diagram devised in 1889 by the German sociologist Franz Carl Müller-Lyer, demonstrating what he called”ein optisches The paradox is that the lines are and are not the same. They are clearly and verifiably indistinguishable on the page. At the exact same time, they’re both clearly and verifiably perceived as distinct lengths by human observers; and these senses could be manipulated in predictable ways.
Plenty of research has been conducted investigating Müller-Lyer’s illustrations in the 130-odd years because his original paper. Adjusting the length and angle of the fins alters the strength of the illusion (the optimal fin/shaft ratio for maximum distortion is, seemingly, around 36 per cent, while the optimal angle for the fins lies between 40 and 60 degrees). Different cultures appear to experience the illusion to differing degrees, with some correlation between urban development and its durability, suggesting a potential root cause in a feeling of perspective inhabited by landscapes and architecture. In my pleasure, however, there’s no definitive arrangement as to how the illusion works. Besides their inherent interest, optical illusions are often used to impart the lesson that human perception cannot be trusted: it offers, at best, a shaky and restricted glimpse of truth. That is correct, obviously, but it’s also inadequate. It is not sufficient to say the twin lines in the illusion are’really’ indistinguishable, full stop, because this ignores most things worth understanding about how folks turn sensory information into expertise, and experience into understanding. All things considered, the lines are not really straight in strict mathematical terms, or identical, or parallel. They’re inked marks on paper: gestures towards common assumptions about meaning and representation whose central wonder isn’t so much the limitations of understanding as how much we can infer upon its basis.
Once it comes to perception, illusion is more pervasive than you might think. I look out of the window in my backyard and I see trees swaying in the breeze. I take a movie with my phone and watch it. Once more, I visit swaying branches. However what I’m watching isn’t, technically, a shifting image: it is a series of static pictures that, when half of these are shown each second in succession, create a compelling illusion of movement (thanks to some more imperfectly known phenomena bracketed under the heading”the persistence of vision”).
If I slow down the rate of video playback, this illusion is slowly replaced by an awareness of the gap between each frame. Based on what you are filming, things start to stutter at about three glasses per second. When I place my phone into slow-motion manner, I can picture at 120 or perhaps 300 frames per second. Playing these films back creates the illusion that time is moving four or four times slower than normal — yet this illusion is not any more or less real, in its own raw mechanics, compared to any other recording. Whatever I perceive, what is”actually” happening when I see my videos is that a human being is celebrating a rapidly shifting series of bright dense pixels onto a little display. The striking strangeness of slow or jerky movement simply marks the space induced between my perceptions and everyday experience: those ways illusions have opened my thoughts.
Inside his 2011 book The start of Infinity, the physicist David Deutsch describes how, as a graduate student, he pored over photographic negatives from the Palomar Sky Survey, which showed stars and galaxies as dark shapes on silvered glass plates, clustered so minutely that microscopes were needed to analyze them. At one point, a tiny flaw in the image led him briefly to imagine he’d discovered a new reef. Was he actually looking at unimaginably distant galaxies, or at specks of silver ?
Looking back from the present instant, Deutsch has no doubt: both things are simultaneously correct. “I was indeed looking at galaxies,” he writes. “Seeing a galaxy through specks of silver is no different… from observing a garden via images on a retina. In all cases, to state that we’ve genuinely observed any given thing is to say that we’ve correctly imputed our evidence (ultimately always evidence inside our brains) to this thing.” His eyes might have been concentrated on earthy artefacts, but his mind was concentrated on wonders half a universe away. And, being equipped with an adequate method for sifting delusions from illusions, he can grasp these as surely as anything else in the realm of perception.