Penrose Stairs

The Penrose stairs, also called the impossible staircase, is a two-dimensional optical illusion depicting a staircase that appears to make four ninety-degree turns as it rises or descends while still forming a single, continuous, closed loop, so that a person following the stairs upward or downward would seem to be able to climb or descend forever without ever gaining or losing height, a result that is geometrically impossible in ordinary three-dimensional space, though theoretically possible in certain non-Euclidean geometries. The Swedish artist Oscar Reutersvard created the design in 1937, and the psychiatrist Lionel Penrose together with his mathematician son Roger Penrose independently discovered and popularized the figure, publishing their findings in 1958; the Penroses described the illusion by noting that each individual part of the structure is acceptable as representing an ordinary flight of steps, but the way the parts are connected makes the picture as a whole inconsistent. The Dutch artist M. C. Escher, after learning of the Penroses' work, created his famous 1960 lithograph Ascending and Descending, which used the impossible staircase to depict monks endlessly climbing and descending the same flight of stairs.