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by what means would/could cause a causality loop to occur? An open Rift used by an unexperienced Shifter? A TW teleporter? Experimental CS teleportation machine? I want to run a game in which the players get caught in one and have to figure out the correct sequence of events to stop it (and get all the appropriate rolls also).

Well this WILL be a long one.....so let's start with what is a causality loop....A causal loop, in the context of time travel or retrocausality, is a sequence of events (actions, information, objects, people) in which an event is among the causes of another event, which in turn is among the causes of the first-mentioned event. Such causally-looped events then exist in spacetime, but their origin cannot be determined. A really good example of this in recent comics (a few years old) is Marvel's Zombies or the origin of the Zombies in Marvel. It opens with Sentry falling from the sky as a Zombie, killing and turning others into Zombies. In the Marvel Zombies 2 series it is shown in the end (spoilers get over it) that when the Zombie Sentry tries to attack Uata the watcher; Uata sends the Zombie Sentry back in time to his own universe before the Zombie Apocalypse started, hence looping it to the beginning. The original cause no longer relevant and causing a causality loop in the Marvel Zombie's Universe. As for the cause of your Causality loop..... all of the above and more. Backwards time travel would allow for causal loops involving events, information, people or objects whose histories form a closed loop, and thus seem to "come from nowhere." The notion of objects or information which are "self-existing" in this way is often viewed as paradoxical,with several authors referring to a causal loop involving information or objects without origin as a bootstrap paradox, an information paradox, or an ontological paradox. The use of "bootstrap" in this context refers to the expression "pulling yourself up by your bootstraps" and to Robert A. Heinlein's time travel story "By His Bootstraps". The term "time loop" is sometimes used to refer to a causal loop, but although they appear similar, causal loops are unchanging and self-originating, whereas time loops are constantly resetting.

General relativity permits some exact solutions that allow for time travel. Some of these exact solutions describe universes that contain closed timelike curves, or world lines that lead back to the same point in space/time. Physicist Igor Dmitriyevich Novikov discussed the possibility of closed timelike curves in his books in 1975 and 1983, offering the opinion that only self-consistent trips back in time would be permitted. In a 1990 paper by Novikov and several others, "Cauchy problem in spacetimes with closed time-like curves", the authors suggested the principle of self-consistency, which states that the only solutions to the laws of physics that can occur locally in the real Universe are those which are globally self-consistent. The authors later concluded that time travel need not lead to unresolvable paradoxes, regardless of what type of object was sent to the past. In short as long as the group is shot back into time any one of your ideas can work. It just depends on the beginning of how your story begins....are they CS? Are they Mercs? Are they just a band of adventurers? Hope that helps. A good reference for such a game in which they must solve the loop is Star Trek Next Generation's episode "Cause and Effect." Look it up and get ideas from there.

the Star Trek episode and a recent episode of The Librarians and Stan vs Evil is what gave me the idea for a game plot. Star Trek was caused by a space/time anomaly, the other shows were supernatural sources.

Thyne O'Rouke wrote:General relativity permits some exact solutions that allow for time travel. Some of these exact solutions describe universes that contain closed timelike curves, or world lines that lead back to the same point in space/time.

The problem, however, is the amount of matter you need, which is basically an infinite amount of it.