Arrow of Time


Actually, nothing was wrong with that picture. Newton's laws do not distinguish between running a movie forward in time or backward in time. With no preferred direction, going backwards to the past is the same as going forwards toward the future. In fact, suppose we could stop time, point the velocity of each particle in the opposite direction, and then start time again. Newton's laws say we return to the past, just as if we had traveled backwards in time. This reversal of velocity is exactly what happens in the video.

So why do we never see a pot unbreak in the real world? This is due to the Second Law of Thermodynamics, which states that the total entropy in the universe increases as time progresses towards the future. While entropy is commonly associated with the disorder of a system, this fuzzy picture is not strictly correct. We'll say more about entropy increases soon. For now, the Second Law defines an arrow of time that points towards the future.