Popular Science - USA (2020 - Spring)

POPSCI.COM / SPRING 2020 113

stairs

that start

nowhere

BYSARA KILEY WATSON

GLANCE AT THE STAIRS ABOVE.

Find the base...rather, spot the top. Upon

closer examination, you’ll realize that

there is no beginning or end. There’s no

way that’s feasible, right?

These familiar steps, called the

Penrose stairs, are a type of “impossi-

ble object”—a construction that could

not exist in reality even though its indi-

vidual pieces look totally valid, says Erez

Freud, a cognitive neuroscientist at York

University in Toronto. The paradoxical

item takes form when the brain attempts

to turn a 2D image into a 3D object.

From years of trusted experience,

our noggins assume lines are always

straight and corners precisely 90 de-

grees. But those facts can’t be true

and still create this eternal four-way

staircase. You can’t walk upstairs for-

ever, especially in a loop. Normally, two

regions in the brain’s visual cortex—the

ventral visual pathway and the dorsal vi-

sual pathway—communicate to identify

objects and place them in space. With

impossible objects like the stairs above,

those two pathways in your gray mat-

ter fire back and forth more than usual,

trying to come to a conclusion. But they

can’t, resulting in an uneasy feeling.

The key to this scene is perspective—it

works only in 2D. If you saw these steps in

real life, and viewed them from any other

angle, you’d uncover what you always

suspected: a gap between two sections.

In other words, a staircase like any other.

ILLUSTRATION BY
STUART PATIENCE