Source: http://www.wired.com/2014/11/time-gravit...

We can’t avoid the passing of time, even at the DMV, where time seems to come to a standstill. And daylight savings notwithstanding, time always ticks forward. But why not backward? Why do we remember the past and not the future? For a group of physicists, the answers to these deep and complex questions may arise from a familiar source: gravity.

Even though time is such a fundamental part of our experience, the basic laws of physics don’t seem to care in which direction it goes. For example, the rules that govern the orbits of planets work the same whether you go forward or backward in time. You can play the motions of the solar system in reverse and they look completely normal; they don’t violate any laws of physics. So what distinguishes the future from the past?

“The problem of the arrow of time has been boggling minds forever,” said Flavio Mercati of the Perimeter Institute for Theoretical Physics in Waterloo, Canada.

Most people who’ve thought about this arrow of time say it’s determined by entropy, the amount of disorder in a system (like, say, a bowl of cereal, or the universe). According to the second law of thermodynamics, the overall entropy of a closed system must always increase. And time appears to travel in the same direction as rising entropy.