WHAT is “now”? It is an idea that physics treats as a mere illusion, yet it is something we are all familiar with. We tend to think of it as this current instant, a moment with no duration. But if now were timeless, we wouldn’t experience a succession of nows as time passing. Neither would we be able to perceive things like motion. We couldn’t operate in the world if the present had no duration. So how long is it…?

This article first appeared in New Scientist on 10 January 2015. To continue reading, click here (paywall).