In his lithographs Relativity(1953) and Balcony (1945), M.C. Escher interrogates the spectator about gravity by representing paradoxical three-dimensional images in a two-dimensional medium.
In Relativity, 16 bulb-headed, identical humanoid figures in an architectural structure seem proceeding ordinary life. Three disparate gravity sources coexisting in the same picture produce the paradox. There are four stairways, and each stairway is subject to different gravity sources.
In Balcony, Escher distorts his perspective with a gravitational singularity. He depicts an apartment complex next to a river embankment. The image exaggerates the center of the building, and magnifies a fifth floor balcony, as if a powerful magnet placed just before the spectator were to thrust it outwards. Yet the distorted image remains captured in a two-dimensional lithograph.
In these two works, Escher successfully realizes his conception of impossible spaces by embedding vivid three-dimensional images in the flat surface of a gray scale print.
*The official M. C. Escher website: http://www.mcescher.com/
** The picture source of Relativity: http://en.wikipedia.org/wiki/Image:Escher%27s_Relativity.jpg
***The picture source of Balcony: http://www.globalgallery.com/enlarge/015-20758/