Why does time pass?

The equations of physics suggest time should be able to go backwards as well as forwards. Experience suggests, though, that it cannot. Why? And is time travel really possible? Click here to subscribe to The Economist on YouTube: Why does time pass? It is a question so profound that few people would even think to ask it. Yet its effects are all around. Human beings live in a perpetual present, inexorably sealed off from the past, but moving relentlessly into the future. For most people, time seems to be something that is just out there. A thing ticking away in the background - fixed, immutable. Time seems to go in one direction and in one direction only. But physicists see it much differently. One of the great minds who changed the way science thinks about time was Albert Einstein. In 1905 he published his special theory of relativity. In it he demonstrated that time passes differently in different places depending on how those places are moving with respect to