"Let one member of the gang evade capture and the whole process could be undone if the one who remained at large struck back by creating a causal loop that would leave Ben-Eytan and his team hung up in some limbo of unreality." (p. 239)
Limbo of unreality? What does that mean? Poul Anderson's Time Patrol series uses no such terminology. The Patrol, trying to preserve a linear timeline, must prevent both paradoxes, not only causality violation (certainly that) but also circular causality - except when closing a causal circle is the only way to prevent a causality violation.
(i) Stane steals a time machine and tries to change history. Time Patrolmen arrive, kill Stane and retrieve the stolen machine but leave behind the shielded box containing its radioactive fuel because that box, buried with Stane, will alert the Patrol to his extratemporal activities in post-Roman Britain.
(ii) Carl Farness, mistaken for Odin while visiting the Goths, must either enact their account of Odin's betrayal of his followers or return to a changed twentieth century.
In both these cases, the paradox of circular causality is preferable to the paradox of causality violation but how could the former leave anyone in a limbo of unreality? Anyone involved in such a circle lives through it just once and continues to live in linear time afterwards. Carl is not forever betraying his followers in some unreal realm. When discussing odd events in an sf novel, one fan seemed to think that the phrase "causal loop" was sufficient explanation. This could mean either circular causality as in (i) and (ii) above or a closed circular timeline, which might constitute a "limbo" by contrast with the open-ended universe but how would it be possible to enter such a timeline? And no such limited timeline is suggested anywhere in the Time Patrol series.
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