A tachyon (from the Greek takhús, meaning "swift") is any hypothetical particle that travels at superluminal velocity. The first description of tachyons is attributed to German physicist Arnold Sommerfeld, but it was George Sudarshan in the 1960s who advanced a theoretical framework for their study. Tachyons have recurred in a variety of contexts, such as string theory, and in science fiction. In the language of special relativity, a tachyon is a particle with space-like four-momentum and imaginary proper time. A tachyon is constrained to the space-like portion of the energy-momentum graph. Therefore, it can never slow to light speed or below.