Temporal Paradox: It’s a block universe; the short form would be to say that predestination paradoxes are permitted (although the non-informational kind are tricky) and grandfather paradoxes are not, since even though you can violate local causality, global causality is always preserved.
And if you find yourself caught up in one of these, You Can’t Fight Fate. You may, however, be able to cheat, if there’s a chance – and there usually is a chance – that what you thought happened first time through might will be not have been what actually happened.
You Already Changed The Past: While extraordinarily limited, time travel does exist in the Eldraeverse – as it must, because so does FTL travel. This is all you can do with it, however, because the Chronological Consistency Protection Theorem is very firm about that, and the universe is even firmer.
(And if you’re going for an object paradox, you’d better be grabbing something that doesn’t age during the loop. Said universe will check things that your fleshy senses and quite possibly the technological sensors you’ve invented haven’t thought to look at and indeed can’t perceive.)
You Can’t Fight Fate: According to everything known about temporal mechanics, the universe is a block universe – which is to say, while local causality violations are possible (effects can, sometimes, precede causes), global causality violations are not (effects, nonetheless, always have causes). Or to put it another way, while predestination paradoxes are permitted – and enforced – grandfather paradoxes are not. The probability of any event-chain that might lead to a global causality violation is always zero, and anything which happened in the past, even if it involves the future of your personal timeline, will necessarily happen.
You can sometimes fiddle fate, because what you think you know about the past is not always what actually happened in the past; but you can’t fight it head on. Free will may be stronger than destiny, but it’s not stronger than causality.
The Second Guideline of Temporal Communication, should you happen to find yourself in the position (possible, albeit rare outside navigational errors, advanced relativistics classes, and other esoteric situations) of being the subject of a closed timelike curve formed by the appropriate combination of wormhole traversals and near-luminal travel, or alternatively should you find yourself in the much less likely position of having access to a trans-temporal ansible without being an acausal-logic-using temporally-transcendent seed AI, is traditionally given as follows:
“Listen to your future selves, and politely fulfill whatever requests they have of you. They’ve been you; by definition, they know everything you know, have experienced everything you’ve experienced, and then have learned more on top of that. They know better.”
In practice, it’s not as vital as this makes it sound; the Chronological Consistency Protection Theorem tells us that global causality is always preserved, and that while effects may precede their causes locally, nonetheless the causal graph is always complete. Even if you choose to ignore or defy your future self, you cannot damage the fabric of causality by doing so.
The corollary to this, of course, is that it doesn’t matter. Your future self knows exactly what you will do in response to any interactions you may have, because they were you when they had them the first time around. It follows, therefore, that they always say and do exactly the things required to cause you to do whatever you are going to do to cause the timeline that resulted in the encounter you are now having in the first place.
The Second Guideline, therefore, does not exist to protect the integrity of the temporal continuum; merely to prevent a lot of pointless and futile arguing with oneselves.
– Practical Temporal Mechanics for Amateurs