Einstein's theory of relativity envisions time as a spatial dimension, like height, width, and depth. But unlike those other dimensions, time seems to permit motion in only one direction: forward.

One of the enormous conceptual ideas that came along with Einstein's theory of relativity was the surprise that time itself, long considered fundamental and universal, is actually relative. Different ...

Measurements showing that a quantum system follows the forward-pointing trajectory of the arrow of time have been done by Kater Murch and colleagues at Washington University in St Louis, US. The team ...