This is an alternate worlds / time travel idea I might use in the future.
The Basic Model
The “Many Worlds” interpretation of quantum mechanics posits that a new universe springs into existence to account for all possibilities. The massive number of quantum events each second would spawn billions of universes. For all practical purposes, there are four categories of alternate timelines:
“The fuzz” consists of universes which differ unnoticeably or trivially from each other. A few extremely powerful beings can manipulate the fuzz to their advantage, but for most travelers it simply makes crossing timelines harder. The “fuzz” ends once one gets far enough into interstellar space; with less matter fewer alternate universes spawn. Consequently, universe jumping becomes much easier in interstellar space. Faster-than-light travel between planets or star systems can also dump the unaware into an alternate version of that planet.
A hysteresis arises when an event creates two timelines resolve to the same end result. The alternate timeline essentially merges back into the branch, creating a single timeline again. Many time travelers try to “set history right” by converting a major divergence into a hysteresis.
At major decision points, different versions of the same event create distinct alternate universes called “branches” or “timelines”. New branches arise naturally at points where a small change can have huge consequences, although travelers from other timelines can, unwittingly or deliberately, interfere with history and spawn another branch. Ethicists debate whether “cloning” a universe in this way is ethical because it creates life, or unethical because these new residents suffer the consequences of an altered time line.
Beyond the branching alternate worlds lie the Anomalies: worlds of fantasy, worlds where sapient reptiles rule, worlds with different physical constants and natural laws. The further one travels from one’s home universe, the more variations accumulate, until universes can no longer sustain carbon-based sapient life.
Time Travel Paradoxes Resolved
The preceding hypothesis solves some classic time travel paradoxes, if we assume the following rules.
- When a person travels backward in time, he removes himself from the time stream.
- A traveler is not “cloned” when a major event creates a branch. Rather, he follows the branch that results from his presence.
- When a person travels forward in time, he follows the timestream he’s currently in.
- A traveler retains all his memories and physical possessions, even if they no longer line up with current history.
The Grandfather Paradox
Shooting one’s grandfather creates a decision point. In one branch, the traveler disappears – or is retroactively erased – from the point of view of observers within the time stream and time proceeds as it always did. In another branch, where the traveler ends up, the grandfather is dead, history changes (perhaps dramatically), and the traveler is now an anomaly, a person without a past.
Ontological Paradoxes, a.k.a. the Bootstrap Paradox
Information or objects “bootstrap” from a parent universe to a new branch. For example, a time traveler jumps backward to give something to his past self. This creates a branching point: along one timeline, the past self never received the object, and in the other he did. The traveler will remember the original timeline in which his past self never got the object.
This gets trickier with information. For example, someone travels backward in time to dictate Hamlet to Shakespeare. Who, then, wrote Hamlet? The time traveler may say the Shakespeare of the original timeline who never received the book. However, this presumes Shakespeare (or someone using his name) would have written Hamlet without interference.
The Predestination Paradox, a.k.a. Causality Loops
Since all possibilities happen simultaneously, a traveler creates a new decision point and a new timeline when and where he travels into the past. In one timeline, he disappeared into a time machine and history proceeds as if he ceased to exist; in the other, he arrives to begin the causal loop.
Consequences of Time Travel
Time travel, as described, moves a traveler back and forth along timelines. From the perspective of a naive time traveler, there’s only one timeline that changes every time the traveler changes the “past”.
Some other consequences of this model:
Travel to the absolute past is impossible. Every trip backwards forks a new timeline; the original past still exists.
The traveler enters a world that started identically to a particular moment, but will drift based on travelers' actions. Depending on a chain of events resulting from their actions, this timeline will be lost in the “fuzz”, form a hysteresis, or fork off into a brand new timeline.
The “fuzz” demonstrates a tendency for timelines to drift back toward their parent. Stepping on a butterfly will go unnoticed. In most cases a human being, even with advanced technology, must expend considerable effort to convert their parallel into a major timeline … although accidents do happen.
If travelers want to “fix” an altered timeline, they must do so right then. Travelling forward and then back will put them in yet another copy of that timeline; even if they convert this copy into a hysteresis the branch they created on their last trip will still exist.
A second trip backward will put would-be rescuers in yet another parallel timeline.
Some travelers attempt to aid their fellows by traveling to an earlier time. Even if travelers make no changes to the timeline that aren’t lost in the “fuzz”, their aid has at best a 50% of reaching the intended target. Odds approach 1 out of 2 to the Nth power, where N is the number of branches forking off the timeline between the time the aid arrives and the time the original travelers receive it. (“Interesting” time periods might have thousands of branches.) An intervening hysteresis has a 50% chance of delaying help until its two loops meet.
Travelers can reliably receive support from their home time through a continuous gateway between two times. Advanced time travel technology can “lock” onto a particular timeline without maintaining a continuous connection, as long as no further branches emerge from that timeline.
The observant might note that the “fuzz” consists of hystereses which are too brief or too similar to the major timeline for most travelers to notice.
Other Modes of Travel
Many worlds believe theirs is the only timeline. Even “time travelers” believe in only one real timeline; the others cease to exist when the past changes. Previous timelines become inaccessible through linear time travel, so nothing in their science disproves their theory.
Sufficiently advanced travelers have means beyond these simple “time machines”. World Jumpers can identify a parallel time line in infinite-dimensional space and “jump” to it directly. Jumpers typically arrive at a point in space-time equivalent or at least analogous to the point they left. Consequently, jump does not create a cloned timeline, since the jumper is not native to that time line, nor did he move backwards along it.
Infinity Unlimited perceives the multiverse as a series of parallels and echoes arranged in “Quantum Levels”. According to their theories, manipulating the timeline moves it among Quantum Levels, making it more or less accessible to “homeline”. A rival organization from another timeline, called Quantum, holds to similar beliefs. In fact, their flawed method of locating timeline causes timelines to become less or more accessible as new branches appear.
The near-godlike Probability Walkers unconsciously shift themselves among actual and potential timelines depending on what they desire. One group called “Amberites” believe that only their home timeline is “real”, and all others are “shadows”. Sightings of another group of reality-warping “Lords of Gossamer and Shadow” have yet to be confirmed.
A civilization of quasi-crustacean fungoid creatures have spread throughout spacetime in multiple universes, including those normally hostile to organic life. These beings are notorious for infiltrating less advanced planets and stripping them of whatever resources they need.
Other people have already used some or all of these ideas. E.g. Ryan North’s How To Invent Everything posits time travel forks off a new copy of the universe, so stranded time travelers can “change history” to their heart’s content, assuming they survive.