Probability drives directly manipulate quantum structure across an infinite number of alternate universes in order to move a ship faster than light. If these devices are possible at all, they would be on the very edge of understandable future technology, able to alter the fundamental structure of reality itself.
FTL travel is impossible according to our current understanding of the universe and the way it works. In all likelihood it will never be a reality unless the laws of physics turn out to be much different from what we expect. And if an FTL probability drive does someday become a reality, it raises the unpleasant specter of both time travel and violating causality. However, for this article, such implications will be largely ignored, much as they are in science fiction. Time travel and all its implications will be addressed in a future section dedicated to it.
Because probability drives are mostly pure conjecture with only a tangential connection to real world science, their placement on the Tech Level scale is somewhat arbitrary. They are placed at the high end because of the extreme sophistication needed to fundamentally alter reality at its mots basic level.
Though the primary fictional sources for these drives are mostly played for humor, they still represent intriguing takes on FTL travel.
This drive is the main feature of the webcomic Starslip (formerly Starslip Crisis.) A version of it is also mentioned in the novel Old Man’s War by John Scalzi.
This drive takes advantage of the Many Worlds Interpretation of quantum mechanics, which states that there are an infinite number of alternate realities, each branching off from the others in infinite directions at every moment in history. For any one action or event, all possible outcomes happen, each splitting off into its own universe or reality.
Its unknown for sure if the Many Worlds Interpretation is the most accurate model fo the quantum world, but it has gained a great deal of acceptance and weight in the scientific community since it was first proposed by Hugh Everett in 1957. Links to more detailed explanations of the theory are provided at the end of this article.
In the context of the Starslip drive, what this means is that there are an infinite subset of realities that are almost exactly identical to our own. The degree of difference in some may be so tiny as to be unnoticeable, involving perhaps nothing more than the position of a single hydrogen atom by a few microns many galaxies away. Others may have differences that may be more profound, such as one person being elected president instead of another, a near-miss by an asteroid turning into an actual collision, or the sun following a slightly different path through the galaxy. So on and so forth.
When a ship with a Starslip drive wants to get to a destination, it shifts through the endless probabilities of parallel universes to find its counterpart in a reality where it is already there. It tries to get as close a match to itself as possible. In other words, the same kind of universe with the same star and planet positions, the same known history, the same kind of ship, with the same exact cargo and with the same crew manifest. If it gets a close enough match, it will replace the alternate reality ship at the desired destination with itself.
For the large majority of such ‘starslips,’ the differences between the originating reality and the destination reality are so small that the crew never notices them. Occasionally, a minor change is noted, such as different entertainment shows or different fashions being popular, but this is considered just part of the routine of Starslip travel. Only very rarely do travelers notice profound differences between the universes, and these usually pertain to alterations in either a crewmember’s personal history. In one instance in comic, where a crewmember found that a fiance who was dead in her originating universe was alive in her new one. The changes could also be in human history as a whole; in the comic, the crew entered a universe where Starslip travel was being abandoned, and had squeaked in on the very last starslip jump there.
The ship which is displaced is assumed to have undergone a starslip jump at the exact same time as the ship that takes its place. This ship in turn takes the place of another ship that’s starslipping into yet another reality, and so on. Across infinite universes, these all eventually even out, with a ship starslipping in to replace the original ship in our chain. There is no mention of a ship ever being ‘hijacked’ away by an incoming starslip, so its assumed that one starslip jump simply replaces another ship also undergoing a jump at the same time.
How exactly the ship is able to scan infinite realities for its near-exact duplicate is unknown. One conjecture is that it uses very advanced quantum computers to build up virtual models of every pertinent alternate universe from the Big Bang onward. When one evolves along a desired line of events, its followed through a nigh-infinite amount of branching world lines until the desired outcome is found. When this desired universe is properly simulated, the starslip drive engages what basically amounts to a very advanced Interdimensional Drive to insert itself into the target spacetime at the proper relative coordinates. It uses its virtual model of the new universe to adjust its own quantum properties/superstring vibrations/dimensional phase (or however interuniversal travel would supposedly work) to match those of its destination cosmos, allowing it to slip from one dimension to the other.
This all speaks of extremely sophisticated technology across many disciplines, from quantum computing to interdimensional travel to precision quantum manipulation; Hence the Starslip Drive’s very high ranking on the Tech Level scale.
In the comic, other means of FTL travel are available, but Starslip was preferred because it allowed instantaneous travel form one point in the universe to nearly any other, but without the huge energy expenditures needed for alternatives, such as wormholes or stargates. In the comic, the more identical a target ship is to the original, the less energy the starslip displacement will take. However, finding a very exact match (99.99999+% match) that would require minimal energy expenditure can take days even with the advanced computers of the comics’ mainstream civilizations.
Despite its tongue-in-cheek approach, the webcomic did occasionally explore the human consequences of such a drive. For example, there is the psychological effect of realizing that by taking to the stars, the traveler can never really go home again; his originating universe—along with his original, true family, schoolmates, etc—is forever lost behind him in an infinite shuffle of potential existences. The shock of inter reality changes can at times also be devastating; already mentioned was one crewmember’s suddenly-alive-again fiancé. The fact that all possible outcomes not only occur but can be experienced can also prey on the human psyche. In the comic, one ongoing subplot is of one crewmember’s obsessive efforts to find a cosmos where a lost love never died in a tragic accident.
This drive come from Douglas Adam’s classic Hitchhiker’s Guide to the Galaxy series of novels, though variations are sometimes seen in other sources as well, such as David Brin’s Uplift universe.
Whereas the Starslip drive uses one aspect of quantum mechanics, namely the Many World Interpretation, the Infinite Improbability Drive (IID) takes it operating principle from another: the Heisenberg Uncertainty Principle.
To put it basically, the Heisenberg Uncertainty Principle states that the more certain you are of a particle’s position, the less you can know its momentum, and vice versa. In more simplistic terms, the more certain of where a particle is at one moment, the less certain you can be of where it is going to end up the next. The reverse is also true; if you know which direction and how fast a particle is going (its momentum), the less you can know about its current position.
It should be noted that this uncertainty is not caused by any fault of the observer or the observer’s instruments; the universe itself doesn’t know where that particle is going to end up. Uncertainty is a fundamental aspect of all existence.
There’s a fairly high probability that the particle will end up somewhere expected. For example, if an electron is orbiting an atom, there is a very high probability it will still be in orbit around that atom the next moment. However, there is a very small chance that it could simply end up a little ways outside the atom altogether. There’s also a very tiny, remote chance that it could end up a few feet away. And even more insanely infinitesimal chance that it could end up halfway across the galaxy, or even on the far side of the universe.
But even though these outcomes are highly improbable, the probability of their happening can’t be reduced to zero. There’s always a chance, however remote, that it could happen.
The particle also doesn’t ‘go’ anywhere in between. If it does move from its expected course, it will simply cease to exist in the one place and pop into existence in the other, like a variable in a cosmic equation. Links to more detailed explanations of the Heisenberg Uncertainty Principle are provided at the bottom of the page.
The IID manipulates these probabilities so that even the most unlikeliest quantum event becomes likely. It is unimaginably improbable that all the particles that make up a starship would just spontaneously appear at a desired location, but the IID plucks at the inner most quantum workings of the universe to make it not just possible, but the most likely outcome upon activation.
Thus, the Infinite Improbability Drive can move a ship instantaneously from one point to another, anywhere in the universe. It can easily transport across time as well and to points outside the universe proper.
The energy cost of such a drive is unknown. However, since its not actually ‘moving’ the ship, just shifting its existence from one point to another, it seems relatively low compared to most other FTL schemes. It would still probably be very high from the point of view of the modern, Tech Level 10 world, however.
The IID may produce secondary and tertiary probability effects. These were used to great humorous effect in the Hitchhiker’s books, but in reality to those experiencing them, they would probably be quite terrifying.
Basically, in an unspecified area surrounding the ship’s departure point and arrival point, probabilities go wild when the drive is activated, resulting in highly unlikely
events to occur spontaneously. Reality literally reorders itself around the ship.
For example, in the books, activating the drive has been known to cause: the creation, and spontaneous upending, of a million-gallon vat of custard; the transformation of a pair of guided nuclear missiles into a sperm whale and a bowl of petunias; redesigning the interior of the Heart of Gold; turning Ford Prefect into a penguin; transforming the desert world of Kakrafoon into an incredibly habitable oasis during a Disaster Area concert; ridding the people of Kakrafoon of their telepathy during the same concert; and allowing for the discovery of Magrathea by Zaphod Beeblebrox. Some of these effects were temporary (such as Ford’s penguinization;) others were permanent.
In a more ‘serious’ take on the drive, far more terrifying effects could occur. People could suddenly up and disappear, never to be seen again; some or all of person’s particles may be spontaneously replaced with their anti-particle; Van der Waals forces my be significantly weakened, causing all molecules to slide apart; human blood may be replaced with nitroglycerin; and so on. Its assumed that sane designers would do their best to try and minimize these effects as the drive was perfected; even in the books the IID was a new prototype and the side effects may have been an unforeseen flaw.
Infinite Improbability Drives would require a total mastery of how even the tiniest and fundamental levels of reality worked. How exactly it could manipulate probabilities the way it does would be a complete unknown, and would seem nigh-magical to those of us in the early Twenty First Century.
Mentioned occasionally in the Uplift series of novels by David Brin, these are essentially weaponized, single-use probability drives. They are maneuvered into an enemy fleet’s position (presumably by a high-tech missile, but at this Tech Level it could also be delivered by teleportation, wormhole, or another probability drive) and activated for a brief, intense pulse, affecting probabilities over as wide an area as possible. Unlike the IID, which usually has a directed purpose (moving a ship), these drives simply randomize reality as much as possible in the surrounding volume. Ships are likely to just disappear, suffer major system failures, the hull may spontaneously become radioactive, and many more bizarre and often debilitating effects. In one instance cited in the novel, portions of one ship’s hull was swapped out with an element from a presumed alternate dimension, which was unknown in our universe.
Force fields and other defenses are completely useless in defending against a probability weapon.
The Hitchhiker’s Guide To the Galaxy by Douglas Adams
Startide Rising by David Brin
On the Internet
Many World Interpretation of Quantum Mechanics
Heisenberg’s Uncertainty Principle