The arrow of time, or time’s arrow, is a term coined in 1927 by the British astronomer Arthur Eddington to describe the "one-way direction" or "asymmetry" of time. This direction, which can be determined, according to Eddington, by studying the organization of atoms, molecules and bodies, might be drawn upon a four-dimensional relativistic map of the world ("a solid block of paper").

Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true. Yet at the macroscopic level it often appears that this is not the case: there is an obvious direction (or flow) of time.

In the 1928 book The Nature of the Physical World, which helped to popularize the term, Eddington stated:

Let us draw an arrow arbitrarily. If as we follow the arrow we find more and more of the random element in the state of the world, then the arrow is pointing towards the future; if the random element decreases the arrow points towards the past. That is the only distinction known to physics. This follows at once if our fundamental contention is admitted that the introduction of randomness is the only thing which cannot be undone. I shall use the phrase ‘time’s arrow’ to express this one-way property of time which has no analogue in space.

Sean Michael Carroll, PhD (born 5 October 1966 in Philadelphia, Pennsylvania, USA) is a senior research associate in the Department of Physics at the California Institute of Technology. He is a theoretical cosmologist specializing in dark energy and general relativity. He is also a contributor to the physics blog Cosmic Variance, and has published in scientific journals and magazines such as Nature, Seed, Sky & Telescope, and New Scientist.

Described as a gifted science communicator, he has appeared on the History Channel's The Universe, Science Channel's Through the Wormhole with Morgan Freeman, and Comedy Central's The Colbert Report. Carroll is the author of Spacetime And Geometry, a graduate-level textbook in general relativity, and has also recorded a set of lectures for the Teaching Company entitled Dark Matter, Dark Energy: The Dark Side of the Universe. His most recent book, released in January 2010, is on the arrow of time entitled From Eternity to Here.

Carroll received his Ph.D. in Astronomy and Astrophysics in 1993 from Harvard University, where his advisor was George B. Field. His dissertation's title is "Cosmological Consequences of Topological and Geometric Phenomena in Field Theories". He worked as a postdoctoral researcher at the Massachusetts Institute of Technology and Kavli Institute for Theoretical Physics at the University of California, Santa Barbara, and as an assistant professor at the University of Chicago until 2006 when he was denied tenure.

Leonard Susskind (born 1940) is the Felix Bloch Professor of Theoretical Physics at Stanford University. His research interests include string theory, quantum field theory, quantum statistical mechanics and quantum cosmology. He is a member of the National Academy of Sciences, and the American Academy of Arts and Sciences, an associate member of the faculty of Canada's Perimeter Institute for Theoretical Physics, and a distinguished professor of the Korea Institute for Advanced Study.

Susskind is widely regarded as one of the fathers of string theory, having, with Yoichiro Nambu and Holger Bech Nielsen, independently introduced the idea that particles could in fact be states of excitation of a relativistic string. He was the first to introduce the idea of the string theory landscape in 2003.

In 1997, Susskind was awarded the J.J. Sakurai Prize for his "pioneering contributions to hadronic string models, lattice gauge theories, quantum chromodynamics, and dynamical symmetry breaking." Susskind's hallmark, according to colleagues, has been the application of "brilliant imagination and originality to the theoretical study of the nature of the elementary particles and forces that make up the physical world."