Why does time move in only one direction? This still-unsolved question was posed in 1927 by the British astrophysicist Arthur Eddington, and the concept came to be known as the arrow of time. As theoretical physicist @SabineHossenfelder explains, there's a longstanding mystery in the foundations of physics: If we look at the laws for microscopic constituents, like elementary particles, they work the same way forward in time as they do backward in time. But the same does not hold true on macroscopic scales. In this Big Think video, Hossenfelder dives into this mystery and explores how it has captivated the minds of so many scientists and science fiction writers.