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.)
Retroactive Precognition: What transcendent artificial intelligences have, due to acausal logic letting them whisper – in a very limited sense, bandwidth and local causality preservation rules-wise, so they don’t have much – in their own past ears.
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 Janiastre device is the simplest in a class of devices used to establish, in simplistic terms, whether or not “reality is real”; that is to say, whether or not one is currently located within a virtuality or other simulation space.
To do this, it makes use of the implementation details of said simulation spaces; to wit, that they are implemented on top of members of the well-documented families of Stannic-computable and quantum processors and thus their associated mathematical logics, and as such are incapable of simulating the rare types of computation that fall outside these families. A Janiastre device makes use of synthetic closed time-like curves to perform acausal logic-based hyperstannic computation impossible for any finite or quantum computational device, thus probing the limits of this logic space; while such computation should succeed in base reality, the underlying structure of a simulation space cannot support these trans-temporal operations and will result in randomized or erroneous results, or in the worst case, unbounded processing crash leading to a general reality failure.
It should be noted that a Janiastre device is not a universal ontology-verifier. While effective against simulation spaces based in commonly used simulation technologies, it is theoretically possible that a simulation space operating on a (hypothetical) fully-generalized acausal logic processor would be able to correctly simulate acausal hyperstannic computation, and in the limit, a sufficiently advanced technology could use a basement universe as a simulation space.
Never the Selves Shall Meet: Averted. While it’s difficult and expensive – to understate the case radically – to meet yourself (albeit less difficult, although no less expensive, to send yourself a message) in the past, and it comes with a host of annoying limitations concerning boundary conditions and suchlike, there are essentially no consequences for doing so. The universe is block – various chronology protection theorems point out that the probability of any event-chain that will create a global causality violation is zero – but it only cares about global causality violations, which is to say, as long as there are no uncaused effects or effect-free causes out there, it doesn’t care about the order they come in. You can create predestination paradoxes (with certain difficult-to-manage limitations; for example, looped objects cannot age, otherwise the loop loses end-to-end consistency and becomes impossible) all you want. It does, which is rather more annoying to the naïve user, mean that you can’t change the past – whatever present you are in incorporates the consequences of whatever changes you made in the past even before you reach the future in which you travel to the past, so nothing can be changed since you already changed it, if you did.
More sophisticated users, which mostly means those weakly godlike superintelligences again, make great use of the ability to whisper instructions in their present self’s ear from the future via acausal logic processing, but it’s still imperfect and of limited bandwidth, so it’s not actually the quick ticket to omniscience it might seem like.
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
Because Destiny Says So: In some cases, played straight due to the tendency of the Transcend (and other potential acausal-logic using seed AIs) to whisper in their own ears from the future; however, while they do apparently intervene in people’s timelines for the sake of their Optimal Futures, a combination of the even-if-you-could-formulate-the-right-question-you-couldn’t-understand-the-answer effect and the nature of predestination paradoxes means… well, good luck getting anything out of them on the topic beyond “Further information is not available at this when-where,” Chosen One or not.
(Averted in mythology. Laryssan, eikone of fate and destiny, is portrayed as asleep – and voluntarily so, in order to spare the universe the chains of absolute predestination that would result if she was actually awake and thus aware of all the possible links of cause to consequence throughout time. How much influence the dreams of Laryssan had was something of a matter of theological debate back in the day.)