Only the succession of causally unrelated event, what Hans Reichenbach called «unreal
sequences» (PST 147 - 49) can by an appropriate choice of the frame of reference be reduced to apparent simultaneity or even inverted; but the real concrete
causal links — the «world - lines,» in the language of relativity — are objectively
irreversible since the order of succession of the events of which they consist remains the same in every frame of reference.