The clock in your head that tells you when reality exists 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?
(First shown on BBC1)