Past / Future Incursions : This type of case is the most common one encountered by the Temporal Investigations Agency. A Future Incursion involves an object moving along the time axis at a rate, from the point of view of the present in the preferred timeline, greater than one second per second; hence, the objects and/or personnel involved will move into the future. A Past Incursion involves an object moving along the time axis at any negative rate, hence travelling into the past. Such incursions, more commonly called 'time jumps' or more simply 'time travel', unusually involve movement at a rate vastly higher than normal, so that the journey itself can be virtually instant. Temporal Investigations has dealt with many past and future incursions, and is authorized to make such incursions itself when circumstances require it.
All wormholes have two endpoints, which can be located anywhere in space-time and are connected by a tunnel of finite length. The remarkable aspect of the wormhole is that its internal length Di seems to be entirely independent of the external distance De between its endpoints. Thus Di can be very much greater or less than De. The large majority of known wormholes have a value for Di which is very much less than De - for example the Bajoran wormhole has a value for De of 70,000 light years but Di is only some 200,000 km, or 3.3 trillion times less. This would make wormholes ideal for long range travel - but unfortunately the endpoints of a wormhole are notoriously prone to sudden and massive changes in their position. The Bajoran wormhole remains unique in that both ends are stable.
While wormhole endpoints can be separated by huge distances, it is common for both ends to be located at the same point in time. However, some examples have been discovered in which the endpoints are widely spaced in time, with little or no spatial displacement. Wormholes of this sort are commonly responsible for past/future incursions encountered by Temporal Investigations.
The rarest of all wormholes are those demonstrate significant time and space displacement. Such a one was discovered by USS Voyager in 2371. The Bajoran wormhole, which is unique in many respects, has also demonstrated time displacement on occasion.
Time Loops : These occur when a person or object involved in a Temporal Incursion initiates an event which results in that person or object becoming a causal factor in the original incursion. Loops of this kind rarely reset themselves perfectly, and as a result there is a slight departure from the original sequence of events within each repetition. The cumulative nature of the alterations makes it certain that, after a sufficient number of cycles, conditions within any time loop will alter so much that the initiating event would be avoided. Such loops must therefore be of limited life span.
Alternate Timelines : The universe we see around us is the end result of an apparently infinite number of events interacting with each other in a particular order. Each one of these events generates alternate timelines - 'what might have been' universes in which every possible outcome is played out. Any temporal incursion which alters any past event will therefore generate an alternate timeline; and since an 'event' in this context constitutes any degree of interference, even the disturbing of a single atom, then all incursions generate not one but many alternates.
This has led to the concept of the 'preferred timeline'. Temporal Investigations originally defined this as the timeline which would exist if temporal technology had never been created, but this has since been updated and the preferred timeline is now said to be the timeline which would exist if no further incursions occurred. Temporal scanning has recorded the signature of the preferred timeline, and this is used as the basis for comparison between timelines. The Temporal Prime Directive states that in cases involving temporal phenomena, Starfleet must act so as to minimize disruption to the preferred timeline. TI generally accepts changes to the preferred timeline if such changes are judged to be within certain limits - the actions of Captain Benjamin Sisko in deliberately meeting with Captain James Kirk during a recent temporal incursion is one example. Many people - both outside TI and within it - have complained that this means that the preferred timeline has thus become 'the timeline that the Federation wants to exist in', and this criticism has some justification. Nevertheless, repeated attempts to correct the timeline would likely generate even worse distortion. Strictly speaking this invalidates the concept of the preferred timeline, but some standard is required if Temporal Investigations is to fulfil its function and so the agency continues to act on this basis.