Abstract
"If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know." This is the answer given to Augustine of Hipona when asked about what is time. This is the answer given by many, 15 centuries later, but this cannot be the answer given by modern physicists, who do not spend a single day without talking of time. This question has a well-defined answer in the cl…