A blue guy and a red guy are moving at the same speed in opposite directions with respect to some rest system as depicted in the sketch below. Both guys are moving into their respective 4th dimensions at light speed. The same proper time has lapsed for blue and red at their respective events labled "B" (for blue) and "R2" (for red). But, when Blue is at his "B" event, his 3-dimensional "Blue World" includes Red at red's "R1" event. Question: Which 3-D world (Red World1 or Red World 2) is the Red guy physically really living in at the time blue is living in his "Blue World" containing event "B"?