Year Outside, Hour Inside: Thanks to relativity, truth in television for lighthugger crews, who slowly build up a hefty time differential (and cultural delta) with the parts of the universe operating on the empire-time frame.
(Hint: if there’s a family member or social grouping you just can’t live without, make sure you all sign up together.
…also, give up on playing MMOs or multiplayer games with anyone off the ship, no matter how much tangle you brought. Timeslip is much worse than netlag.)
Time Travel: Present in very limited forms: the kind you can do with wormhole shortcuts and relativistic travel (whose primary usage is permitting physics grad students to actually observe local causality violations), and the kind that lets transcendent AIs whisper to themselves from the future via acausal logic. In both cases, subject to Chronological Consistency Protection, as they’re operating in a block universe. (See Temporal Paradox.)
(To illustrate some of the issues.)
As the classic example of the effects of physics upon interstellar relations, consider the short and inconclusive conflict known as the Odeln Extrality Incident (from the Imperial perspective), or the First Border War (from the Republican perspective). It is generally agreed that the cause and first engagement of the conflict was the mutual destruction, a short distance outside the Odeln system volume, of the Imperial destroyer CS Joyful Dirge, and the Voniensan battlecruiser VNS Deliberative.
The causes of the incident remain obscure, beyond the rising post-contact tensions in the Seam, and even the details of it are disputed. Many of the ensuing skirmishes and inconclusive negotiations can be attributed to the relativistic nature of the engagement; the consensus of the Imperial observers, computed according to the empire time reference frame, was universally that Deliberative fired first upon Joyful Dirge, while the consensus of Voniensan observers in-system, operating in reference frames tied to Republic Universal, was that Joyful Dirge fired first upon Deliberative. It is the unfortunate nature of relativistic space-time that all of these observations can be simultaneously truthful and true.
Such are the difficulties of the strategos, the diplomat, and the historian in a non-Callaneth-compliant universe.
Today’s cross-infection between professions: since I’m mucking about with date/time libraries in .NET as the moment in my day job, wondering what a relatively (heh) simple type like, say, System.DateTime would look like in the Eldraeverse, where convenient assumptions like sharing the same concept of “now”, rate of passage of time, and event ordering with the other systems on the network are trivially false.
(For everyone, not just people coding for NASA or GPS systems.)
“The Academy of Relativistics and Temporal Mechanics proposed that relevant precedents from cases yet to be tried should be admissible in court. After consultation with the emissary incarnation of Esseldár, the Curia ruled by vote of three to two that the principle of local causality should be enforceable at law unless effects whose causes themselves lay in the future should be the subject of the suit.”
– Proceedings of the 42nd Great Clarification