When worldbuilding a science fiction or especially space opera setting, it is often desired for the scope to encompass more than one solar system, spanning part or all of one galaxy or even beyond. There is just one big problem: no object with mass can travel through space faster than the speed of light, and space is so big it takes light years to even get to from Earth to the nearest star, hundreds of thousands of years to cross our galaxy. Clearly, something must be done to have a galaxy-spanning story.
These intimidating facts are often cited as an argument against practical interstellar travel, but that’s not really the case. With near-future technology velocities of 10% or even 23% of light speed might be achieved, permitting the nearest stars to be reached within a few decades. Colonization of and migration between the stars may then proceed; at these sort of speeds our whole galaxy could be colonized within a few million years, an almost inconceivably long time in human terms but in geological terms quite short. Nevertheless, no story with the same characters (if they’re unmodified humans) with a setting much larger than a few star systems can be told under such conditions.
This changes somewhat even without (effectively) faster-than-light travel. We are often told that the speed of light limit imposed by relativity means we cannot travel to destinations beyond our galactic neighborhood within our lifetimes, but this view doesn’t take into account time dilation. An interesting feature of relativity is that when your ship is approaching light speed it looks to an outside observer like it’s getting harder and harder to accelerate (i.e. the next, say, 0.01c of speed takes more energy than the last), but from your point of view you’re continuing to accelerate more linearly almost as if you’re in a Newtonian universe. The destinations you’re approaching appear to come closer to you as you approach light speed, and time runs slower for you than it does for an outside observer.
An effect of this is that, assuming a sufficiently powerful energy source could be found, a constant 1g acceleration, the highest that is comfortable for humans and one that conveniently would provide artificial gravity with no rotation needed, could carry you from Earth to Alpha Centauri in 3.58 years, the Pleiades cluster in 11.88 years, the center of the Milky Way in 19.89 years, and the Andromeda Galaxy in a mere 28.62 years. 10 billion light-years could be traversed in only 44.67 years. The catch is that while traversing those 10 billion light-years about 10 billion years will have elapsed on Earth and at your destination; so much will have changed it might be best not to come back. In principle a whole civilization could be accelerated in this fashion and thus effectively contract the distances they need to traverse; short of that, however, the story you can tell will be limited to one ship or possibly a fleet of ships traveling together. Fascinating perhaps, but not material a space-operatic civilization can be built from.
As an aside, if higher accelerations can be achieved, perhaps through submerging the passengers in a liquid with the same density as the human body, travel times can be greatly shortened. Alpha Centauri may be reached in only 9 months at 10g, and 6 weeks at 100g. At 100g acceleration, even the Andromeda Galaxy may be reached in 4 months ship time.
Nevertheless, in order to have an interstellar culture you can tell space-operatic stories in, some form of travel that enables distant stars to be reached without the time dilation imposed by relativity must be found. This is an active area of research in real-life physics, and the answers arrived at so far revolve around moving space instead of the ship. Unlike an object with mass there is no fundamental physical limit to how fast space can move; indeed, cosmic inflation, thought to have occurred right after the Big Bang, necessarily involved space expanding faster than light, meaning that faster-than-light movement of space can occur naturally. If it can occur naturally it can in principle be harnessed artificially, even if, much like Leonardo da Vinci envisioning tanks and airplanes, we have no idea how to actually go about it.
Warp Drive: moving Space faster than Light
There are two ways to do this, the first being the famous Alcubierre drive, a metric discovered by Miguel Alcubierre that enables faster-than-light travel by contracting space in front of the ship and expanding space behind the ship, driven by a configurable energy field lower than that of the vacuum. This is a solution of Einstein’s field equations in general relativity and is thus consistent with the known laws of physics in the real world, because at no point does the spacecraft travel through the space it’s in faster than light would. Indeed, no movement of the physical object would be required at all, meaning no time dilation or even acceleration would be experienced by the travelers. The details are still being hotly debated and worked out, and by now there are many different version of the real-science-based warp drive, but at least the general idea seems to be scientifically valid.
While unknown quantum gravitational effects might render this solution invalid, they might on the other hand make it easier, or have no effect at all. While some physicists believe that such exotic solutions would be ruled out by future discoveries, the history of science is littered with physical phenomena implied by theoretical mathematics that were scoffed at only to be proven true by later theories, black holes and (in many ways) special relativity itself being among them. A region of negative energy density being required, which would need to be created by negative-mass “exotic matter”, is the biggest stumbling block for all these space-warp metrics, including wormholes. While this might need to be created artificially through some advanced process, the quite common and easily-generated Casimir effect is an instance of naturally-occurring negative energy density at the quantum scale and might fulfill this requirement.
Of course, when worldbuilding a science fiction setting you can deviate from known science for the good of the world or the story, but the point is the idea of a warp drive fits into not only a soft science fiction setting but also a harder science fiction setting. Indeed, Star Trek’s famous warp drive, although of course not exactly the same as the drives under scientific study, provided the inspiration for Alcubierre’s discovery. One feature of using warp drives as the faster-than-light (FTL) drive of choice is that ships when using it will appear to be traveling through open space at an impossibly fast speed. This means that ships cannot simply disappear to flee an enemy but may be pursued through space, and can in principle be detected and tracked as they travel. Warp drives may also be engaged anywhere in space, providing much more flexibility by eliminating the possibility of “choke points” for escaping or traveling.
Hyperdrive: faster than Light through a higher Dimension
Another type of faster-than-light travel is what is usually called a “hyperdrive”. This is where a starship enters another dimension, most commonly called “hyperspace”, where the laws of our universe don’t apply and the ship can go much faster. This is usually accomplished by the ship being able to go faster than light in this realm or distances being extremely compressed in hyperspace compared to normal space. Often hyperspace has its own topography, which opens up the interesting possibility of some routes being faster than others; sometimes it even has its own currents which aid or hinder travel much like the wind does for a sailing ship.
The topography or currents being hazardous in some areas of hyperspace but not others means hyperspace routes, most commonly known as “lanes” need to be charted before they can be deemed safe for travel. This has the intrigue-enhancing effect of making it faster to get to some places with the FTL drive than other places, possibly (if we add on the currents idea) at a somewhat unpredictable velocity. This enables travel times for a given distance to be highly variable, which might be desirable if e.g. you want the Orion Nebula or the Galactic Core to be easily accessible but not certain destinations that are closer to us. If you want one journey to take much shorter or longer than another journey even on the same route for storytelling purposes, “the speed of plot”, variable currents or topography makes such a development internally consistent and believable.
Another feature of hyperdrives as the FTL method of choice is that when engaged they make a ship disappear into another dimension, enabling instant flight from danger without danger of pursuit outside tracking, except perhaps, as in Star Wars, by calculating their last known trajectory at the time they jumped into hyperspace.
While hyperspace isn’t as far as I know being scientifically studied as a method of faster-than-light propulsion, the general idea of a higher dimension where the physical laws might be different is scientific enough; M theory, a variant of string theory that is under serious study as a theory of quantum gravity, famously posits no less than 11 dimensions. The related hypothesis of brane cosmology proposes that at least one of these dimensions, called the “bulk”, is large or perhaps even infinite, and that our universe is a “brane” within this bulk, analogous to a flower petal floating on water.
This idea is attractive to many scientists because, aside from explaining much else, being mathematically elegant, and being compatible with M theory, if gravity propagated across all dimensions but the other forces were constrained to our brane it would explain why gravity is so weak (in our universe) compared to the other forces. This implies that gravitational forces might be able to be exerted upon our universe from the bulk without regard for speed of light limitations in our universe. This idea was used quite effectively in the 2014 film Interstellar, and the whole concept does bear a resemblance to the science-fictional concept of hyperspace. Unfortunately I could find no reliable resources concerning the possibility of transmitting matter, like ships and people, through the bulk as opposed to gravitational forces, but for science-fictional worldbuilding purposes you can assume some way was found. One interesting feature is that the bulk can contain multiple universes, opening the door to inter-universal travel through hyperspace. Of course, this being worldbuilding you can make it any way you want it to; perhaps other universes can be reached but are more distant in hyperspace on routes that have not yet been charted.
Wormholes as a Jump Drive
Another type of FTL drive is the “jump drive”, which enables a ship to disappear instantly like a hyperdrive, but unlike a hyperdrive it appears at its destination instantly as well, requiring little or no travel time. This is certainly the fastest method of FTL travel, and if you want to for story purposes the range could be unlimited, permitting instantaneous travel anywhere in the universe if not beyond. Of course, you could also posit that greater distances require greater power, and that the distance a ship can jump is limited. These limits could come from power reserves, imposing a range limit, or the capacity of whatever mechanism drives the jump, imposing a speed limit.
The version of a jump drive that is based on real science is the famous wormhole; like the warp drive wormholes are a distortion of spacetime that greatly shortens the distance between two points. Also like the warp drive it violates no known laws of physics, since a light beam sent through the wormhole will always arrive at the destination faster than a starship. Unlike the warp drive, however, it is a fixed structure in spacetime that shortens the distance to an arbitrarily small length. This resembles the idea of a portal or gateway.
Wormholes would become strategic locations as natural choke points for long-distance traffic, permitting intrigue and space battles concentrated upon a specific point, which would be very useful for telling stories. Like the hyperdrive, it also has the potential for variable travel times across a given distance, though unlike some versions of a hyperdrive the same route would always take the same amount of time (almost instantly). Distance to the nearest wormhole would be the relevant variable. Wormholes only connect two points in spacetime, so for a more complex route passage through multiple wormholes would be required, much like the network seen in the 1997 film Contact. A much more realistic depiction of what a wormhole would actually look like is seen in the 2014 film Interstellar.
Wormholes, like warp drives, require a region of negative energy density to keep them stable, but much like warp drives that might be accomplished by naturally-occurring quantum effects. A wide variety of wormhole metrics consistent with the known laws of physics, both traversable and non-traversable, have been proposed by scientists, including some that require no exotic negative-mass matter have been proposed. Some speculative theories of quantum gravity, including those studied in the context of brane cosmology, permit a wormhole to be stable with no mass, exotic or otherwise, needed at all. One feature of stable wormholes is that many metrics exert no gravitational force at all, meaning passage through them would be, unlike the usual depiction in science fiction, very gentle.
Constructed, Pre-Constructed, or natural Wormholes?
The construction of wormholes would require the two destinations to be reached through normal spacetime first, in real life and many science fiction settings implying slower-than-light travel. This means that the range of destinations reachable by wormholes would be limited by the speed of light from the time wormholes were discovered. Therefore from Earth’s point of view it would take 100,000 years to connect the whole galaxy with wormholes; once constructed, however, nearly instantaneous travel to any connected destination would be possible. This limitation is used to interesting effect in the Orion’s Arm worldbuilding project.
Worldbuilders who don’t want such a limitation often posit the existence of wormhole networks that were laid out by aliens the hard way a long time ago, thus providing plenty of time to connect wildly disparate destinations; such aliens are usually vanished precursors who left behind the wormholes for species like near-future humans to use without interference. This enables such humans and even other species to get across the galaxy easily. If such networks prove durable later species might expand the network the hard way or even connect them with other alien precursor networks.
There is also the possibility of naturally occurring wormholes. The ER=EPR conjecture, currently under serious scientific study as a pathway to a theory of quantum gravity, posits that quantumly entangled particles are connected by extremely small wormholes, and that this might determine the structure of spacetime. It’s a heady and fascinating theory. Quantum foam is another hypothesis that suggests that wormholes are ubiquitous at quantum scales. If naturally-occurring wormholes at this scale, either from the quantum foam or entangled particles, could be stabilized and greatly enlarged, this might permit travel through wormholes to any point in the universe and perhaps beyond without first having to travel the distance conventionally.
If incorporated into a science fiction setting, a drive harnessing these ubiquitous wormholes would function as a true jump drive, enabling instantaneous travel to any point in space without a pre-built gate or portal. The only limitation might be the power required to harness a wormhole, perhaps limiting the number of jumps in any given time period rather than the range or speed. This would be the ultimate FTL drive.
How much faster than Light?
It is worth noting that there are degrees of FTL travel; a slow FTL drive (e.g. 2c) has very different effects on society compared to a casual jump drive. A ship that can reach Alpha Centauri in a day (1500 times the speed of light) would take 65 years to reach the other end of our galaxy, and 1600 years to reach the Andromeda Galaxy. By the same token, a ship that can reach the Andromeda Galaxy in a day would be capable of reaching Alpha Centauri in a tenth of a second, at that distance scale having the same effect as a jump drive. Which speed scale is most appropriate will depend on the setting you are trying to create.
A fixed speed scale mostly applies to warp drives. Hyperdrives need not have any fixed speed scale due to factors previously mentioned, and for a wide range, perhaps the majority, of science fiction worlds might be the best option. Wormholes and other portals have rather fixed travel times but they do not scale linearly with increasing distance. A wormhole to another galaxy takes as much time to traverse as one to the nearest planet; the relevant variable with this method is the distance to the nearest wormhole and where the wormholes go.
These methods can be combined, of course; hyperdrives may be restricted to activating at certain points in space, yielding some of the same choke-point-inducing storytelling benefit as wormholes. One setting could easily have multiple methods of FTL travel; Star Trek, for example, has both warp drives and wormholes. There is nothing stopping you even from having warp drives, hyperdrives, jump drives, and wormholes in the same setting.
Faster-than-light travel is often characterized a necessary break from reality in order to have a galaxy-spanning science fiction or space opera story. Faster-than-light travel, no matter how unrealistic it is, is indeed a necessary foundation of space opera, and many such settings don’t even try to hew close to known science when it comes to faster-than-light travel; contrary to protestations from parts of the hard-science-fiction crowd that doesn’t mean it’s magic like in fantasy, since most of these settings treat FTL as scientific in-universe and often keep it internally consistent.
Effective or apparent faster-than-light travel isn’t even unrealistic anyway; as mentioned earlier warp drives and especially wormholes are subjects of active scientific research and do not violate any known laws of physics and in the case of wormholes might even occur naturally. As far as we know there is no scientific obstacle to a spacefaring civilization eventually warping their way through the cosmos and constructing wormhole networks. Hyperspace and hyperdrives are far more speculative and aren’t even based on any known science, but even then the principle is sound enough.
There are many plausible ways faster-than-light drives could work, and thus many different effects on the setting could be arranged. Exploring such effects and working them out is an exciting task for anyone worldbuilding a science fiction or space opera setting, be they an artist, an author, or even someone worldbuilding for the fun of it.