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.