xkcd.WTF!?

Explanation ➲

This comic is a pun on two uses of the term "space":

• in the everyday sense as a place for a person or thing to exist in; and
• as mathematical jargon used in topology (a branch of mathematics usually studied at graduate or postgraduate level).

Willem de Sitter was a notable Dutch mathematician and physicist in the late 19th and early 20th centuries, known for his work using topological models to describe the overall structure of the universe. He is best known for formulating the definition of a de Sitter space, which has constant positive (elliptic) curvature, where the sum of the internal angles of any triangle is more than 180°, and lines that initially appear parallel will always meet; the surface of a sphere is an example of a two-dimensional de Sitter space.

In contrast, an anti-de Sitter space is a space that has constant negative (hyperbolic) spatial curvature, where the sum of the internal angles of any triangle is less than 180°, and two parallel lines will diverge when extended. An anti-de Sitter space is a theoretical space with strange curves and extra dimensions which is used in studying the space-time structure of the universe.

The caption "My house is an anti-de Sitter space" refers instead to being against the actual physicist Willem de Sitter, after whom both spatial geometries are named, because the speaker forbids de Sitter from entering their house. This prohibition is pointless since he died in 1934.

The title text refers to conformal field theory, a set of rules for how tiny particles behave on boundaries. The ideas of conformal field theory and anti-de Sitter space are closely linked: everything in an anti-de Sitter space has a parallel in conformal field theory, a property which is usually called the anti-de Sitter space/conformal field theory correspondence or AdS/CFT correspondence for short. The extra pun here is "correspondence" also meaning "mutual communication or discourse", typically communication by letter or email.