The clock in your head that tells you when reality exists, it only does so for you - not anybody else. And it runs fast or slow depending on your mood. So - what time is it? And why is this writing in your past? In the third of a series of six programmes, James Burke spends 30 of his minutes (not necessarily yours) looking at why the world around you can't exist ' now ' for everybody. This programme will be short, or long, depending on you. Its real length? Who knows?