A long time ago, in a galaxy far, far away …
And also in a parallel galaxy, sitting right alongside that first galaxy …
There were two identical ribbons of scrolling yellow text.
The scrolls, like the one that forms the distinct opening text for the "Star Wars" sagas, were exactly the same in every way and disappeared into the horizon at exactly the same angle. And yet, the scrolls appeared to split away from each other like two forking streams of the same river — one moving left, and the other moving right.
How could this be? Were the scrolls caught in the pull of an Imperial tractor beam or the gravity of a sinister moon? Or was it simply the observer's mind that was doing the pulling? ['Star Wars' Tech: 8 Sci-Fi Inventions and Their Real-Life Counterparts]
This phenomenon — known as the "Star Wars" scroll illusion — was first described by Arthur Shapiro, a visual illusions expert and professor at American University in Washington, D.C., in a 2015 issue of the journal i-Perception. According to Shapiro, the "Star Wars" scroll illusion is a more dynamic version of the leaning-tower illusion, in which two identical photos of Pisa's famous tower appear to slant in different directions, despite being identical, side-by-side copies of each other.
The same illusion.— Akiyoshi Kitaoka (@AkiyoshiKitaoka) February 11, 2018
Kingdom F A A, Yoonessi A, Gheorghiu E, 2007, "The Leaning Tower illusion: a new illusion of perspective" Perception 36(3) 475-477 pic.twitter.com/2UtBMrKKlN
Similarly, in the "Star Wars" scroll illusion, two identical planes of scrolling text appear to move toward the horizon at remarkably different angles, even though they are the same in every way.
"Part of the reason the scrolls seem to diverge has to do with how we interpret perspective in a scene," Shapiro said in a video accompanying his research. "The phenomenon works because it creates two ways of interpreting the scene."
The first way involves what Shapiro calls the picture plane, which refers to the invisible lines that define the space around the moving text. (As Shapiro demonstrates in the video above, drawing these lines around both blocks of text creates two identical triangles that point toward the top of the screen — proving the text scrolls are actually identical and oriented in the same direction.) The other way of interpreting the image is your own perspective interpretation, which is how you actually see what's going on.
"With two vanishing points, the lines in the picture plane are parallel to each other," Shapiro wrote, referring to the spots on the screen where the text from each scroll appears to vanish. "But our perspective interpretation is that the scrolling texts are aimed at different vanishing points and therefore appear to diverge and are not parallel to each other."
Why your mind is inclined to interpret the two vanishing points this way isn't known for sure. According to Shapiro's paper, "a major question for understanding how we see and interpret images concerns how the visual system can simultaneously maintain both of these types of representations."
Understanding that concept could help explain why your brain sometimes interprets circular shapes as squares, or why the image of an arrow can seemingly point right no matter how many times you rotate it. Until scientists provide a better answer, you'll have to make do with the sage words of Obi-Wan Kenobi:"Your eyes can deceive you. Don't trust them."
Originally published on Live Science.