Special Relativity predicts that moving clocks run slow, mass and length are variable and simultaneity is relative. These are 'real' phenomena and leads me to think that the reality of one observer (the passing of time, all physical properties and space-time events) can be regarded as 'his world'. Should this observer change to a different inertial frame he has then entered a new 'world' where physical properties will be seen to change. Are there not then an infinite number of potential realities or worlds? Is reality relative?