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.)
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.)
It is widely believed that time travel is useless.
After all, everyone knows the Block Universe Theory and its limitations: changing the past is impossible, and as such all grandfather paradoxes are banned. Predestination paradoxes are permitted, but obviously only create the already-known current state of affairs, rather than alter it; while this admits of certain limited applications in commerce (such as negative-frequency trading, although as a practice this quickly drives the market volatility operator to zero per the Market Chronology Protection Theorem, eliminating its own profit potential) and in military affairs (knight’s-move bypass transits, including optional proleptic tactical data transfer, which in practice rapidly become zero-sum effective between technologically matched opponents), these are special cases, few and far between. And while looped objects “borrowing” mass-energy from the substrate for the duration of their existence are theoretically possible, the nature of the loop requires that such objects exist in a synthetic or simulated nullentropic state, since the quantum state information at the earliest point of the loop must correspond exactly to that extant at the latest point of the loop.
It would seem, therefore, that the Chronological Consistency Protection Theorem would ban all interesting applications of closed time-like curves.
This, of course, is not the case. While it prevents the construction of technologies based upon its violation, the existence of a universal “paradox censor” that forces the probability of all causally inconsistent events to zero is of great application in several families of technologies.
The best known of these is hypercomputation. Acausal logic processors operate – to paraphrase a series of complex operations – by receiving an answer to a problem from the future, verifying its correctness, and transmitting it back to themselves in the past if and only if the answer is correct. Since the only causally consistent scenario is that in which the correct answer is received, such a device always produces correct answers to any PSPACE-definable problem. (The extraction of information without apparent computation inherent in the operation of acausal logic processors poses interesting problems at the intersection between information physics and ontotechnology, currently the province of ongoing research.)
As well-known is the so-called “probability kiln”, a class of manufacturing devices which utilize such hypercomputation for phase-space pruning; that is, to isolate and remove from future worldlines all those possibilities in which low-yield operations fail, ensuring that the only causally correct possibility is their success, thereby operationalizing even otherwise extremely impractical industrial processes.
Then there is the third class of device, a defense research project designated UNMOVED MONAD.
UNMOVED MONAD makes use of an extremely simple form of synthetic closed time-like curve, in the form of a tangle channel constructed and manipulated such that it links two points separated along the time-like axis, rather than two points separated along the space-like axis. To this extent, it is merely a trans-temporal communications facility. However, unlike trans-temporal communications performed via conventional means or space-like tangle channel, UNMOVED MONAD derives another unique property from its time-like separation: indestructability.
It is important to note that UNMOVED MONAD is a singular device: the tanglebits within are entangled with themselves across time, rather than with a matched set elsewhere. Upon activation, an UNMOVED MONAD device receives a complex, full-width verification signal from itself in the future. As such, it cannot be destroyed until this signal has been sent: such destruction would cause the tanglebits to decohere, ensuring that the trans-temporal signal never could have been received; a causally inconsistent state. Thus, per the Chronological Consistency Protection Theorem, the probability of the UNMOVED MONAD device being destroyed in the interim is forced to zero.
Properly packaged and placed, UNMOVED MONAD can even function as the mythical “synthetic luck machine” – while it is entirely possible for a bearer of an UNMOVED MONAD or the local environment to be destroyed so long as the device itself remains intact, ensuring an event phase-space with plentiful higher-order probability events resulting in the survival of the device can avoid such low-likelihood outcomes; the CCPT worldline shifts tend to go through the highest probability alternative regions. The universe evidently prefers not to work any harder than it has to.
– Temporal Mechanics: The State of the Art, “Popular Physics”, Cailmaen 6722
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