All spacecraft in existence today work based on Newton's third law — for every action there is an equal and opposite reaction. Most toss stuff out the spacecraft's back, creating a reaction force that accelerates the spacecraft forward.note The rest rely on stuff thrown at the spacecraft and driving it forward as it's hit, even if that "stuff" is just sunlight.
This means that current, real-life spaceships either—
- Are mostly fuel tanks with a controlled explosion chamber in the back, with comparatively little volume for basic life support and human habitation, or...
- Rely on an even more massive installation back home, and can't go anywhere out of that installation's range, because of Space Friction, or in harder science fiction, you can travel indefinitely, but can't come back, or...
- Have excruciatingly slow acceleration because they rely on very subtle forces, but are very fuel efficient or use fuel in the space around them. (This allows them to reach speeds greater than conventional rockets, since they are able to accelerate for a longer period of time, given adequate fuel.)
Most Science Fiction authors, however, are writing about spaceships because they want to write about humans going places in them. They don't want to write about reaction mass, so Science Fiction writers would prefer spacecraft to have much more of their volume dedicated to human activities. They typically want their spaceships to be able to operate within convenient distance of someplace interesting, rather than turning half the town into slag as they land and the other half as they leave. They want their spacecraft to flit from one Planetville to another, not just tool around wherever an earthbound laser can be shined on them. They definitely want it to go fast enough to get someplace interesting before everyone aboard dies of boredom. (Yes, even in hard sci-fi where they're all going slower than light; you still want to be able to at least reach other planets in the same star system.)
A reactionless drive or reactionless engine is a piece of advanced technology invented to make life easy for those Science Fiction writers.
For our purposes it is sufficient to define the drive as follows: "any form of self-contained propulsion not based around expulsion of fuel or reaction mass". In other words the drive will propel a vehicle, almost always a starship, anywhere it wants to go without having to waste space carrying propellant. Sometimes authors will try to control these drives by requiring a power plant to make it keep working but more than a few will keep working forever.
Obviously this would be an awesome invention! So why don't we have them? Well, naively it would shatter the fundamental basis of all physics since Isaac Newton, as detailed here, and modern theories predict effects too small to be useful. Thoughtful Speculative Fiction writers have also noted that any sort of reactionless drive would provide those who possess it with an infinitely powerful weapon (compare Weaponized Exhaust, which is the use of a reaction drive as a weapon). Note that this would not be a problem if they required truly massive amounts of power,note but many examples don't.
Some writers try to sidestep this potential danger by setting a maximum speed that the drive can go. Unfortunately while this eliminates the possibility of an infinitely powerful missile it still leaves the developer with an infinitely powerful energy source leaving the writer with most of the same problems. Other times, it may be limited to Higher-Tech Species or Sufficiently Advanced Aliens who presumably have some scientific understanding beyond the ken of "lesser races".
Super-Trope of the Alcubierre Drive; Sub-Trope of No Conservation of Energy. The Ramscoop and Solar Sail are sister tropes; each is a plain old reaction drive, but one that uses a clever trick to react with the ambient environment instead of needing to haul extra mass or stick to a base station. It is also very common for an incautious (or sneaky!) author to write something that's allegedly one of those two, but from the way it's described working is really a reactionless drive in disguise. The energy used to power one is sometimes derived from Antimatter annihilation, or the ship can be Powered by a Black Hole — and sometimes the black hole is the drive.
- Gunbuster features the Eltreum, a 70-km-long carrier craft that uses mathematical computations and a load of other stuff to somehow warp the fabric of spacetime itself in order to move.
- The Minovsky Drives used throughout the Universal Century setting are a subversion. They don't have any visible exhaust, but are still using Minovsky particles from a fusion reactor for propulsion, thus avoiding the need for dedicated reaction mass. It is more akin to a photon drive hybridized with an exotic fusion torch. For in-series decades, their size made them a feature exclusive to battleships, but by Mobile Suit Victory Gundam its been scaled down to fit in the Victory Gundam and its successor, the V2, granting thruster-less levitation within atmospheres and in the latter case enough surplus power to sustain the enormous "Wings of Light".
- Mobile Suit Gundam Narrative has the Unicorn Gundam Unit 03, better known by its moniker of Phenex. Due to being made almost entirely out of psychoframe, a metal that reacts to human emotions and thought and is less understood than any other material humanity has ever discovered, the Phenex can move through space at truly ridiculous speeds in perpetuity so long as a human consciousness is strong and present enough to will it to do so, with no observable physical reaction occurring at any point of its operation.
- Robotech has the Robotech Masters' motherships, that move through an unique reactionless drive. This actually backfires on them once, as they did not properly protect the drives and, once that was discovered, a small mecha was able to down their flagship with a single well-placed shot. Knowing well their own technology and not being stupid, the Masters were able to track down the flaw and a few similar others and fix them in time for the rescue attempt on the flagship, during which the rescuing mothership was hit in another supposed weak point to no effect.
- In a Shout-Out to H. G. Wells, one plot in The League of Extraordinary Gentlemen involves the substance Cavorite, which produces thrust so long as it isn't properly contained.
- Planetary has a ship belonging to aliens called angels that uses an "informational drive". Running on the principle that reality isn't truly 3D and composed of matter and energy but is instead a 2D plane of information emulating a 3D universe, it absorbs the information around it to eject information that it's accelerating. Essentially, it's a ship that warps reality to manipulate its own acceleration, reaching the edge of the solar system in moments.
- Most superheroes who can fly under their own power in space have some form of reactionless drive, even if the comic's authors don't call it this. Superman, for example, can accelerate, decelerate, and turn in Earth orbit, with no (obvious) rocket exhaust coming out.
- Rocketship Voyager: B'Elanna Torres dismisses the Dean Drive as crank science, but then goes on to explain several theoretical methods of Faster-Than-Light Travel, which turn out to be not-so-theoretical with the aliens who inhabit the far side of the galaxy. In the final chapter, she's drawn up a plan for a modified Voyager using what appears to be an Alcubierre Drive.
- In Alliance/Union, the Faster-Than-Light Travel drives that are used to enter Hyperspace can, while in normal space, be used to make instantaneous changes in velocity (piling on a second impossibility on top of normal Reactionless Drives).
- The eponymous technology in Anti-Grav Unlimited is rods that act like "gravity magnets". Through experimentation, he not only manages to create a perpetual motion engine for his van (by welding two rods perpendicular to each other so that they're always being pulled up on one side and down the other), but also manages to rig rods such that he can make the van fly.
- Arrivals from the Dark: The Bino Faata starship in Invasion uses gravity drives, mounted on the outside of its enormous cylindrical hull, to move through space. Aboard the ship, Artificial Gravity is ubiquitous and is under local control (i.e., it can be altered in a specific room). The smaller combat modules used by the Faata as fighters and/or bombers also use gravity propulsion, as evidenced when dozens of them hover over the major Earth cities. After the starship's destruction, this is one of the many pieces of technology recovered and reverse-engineered by humans.
- The Cities in Flight series has the Dillon-Wagoner Graviton Polarity Generator or "spindizzy", which gets more efficiency when it moves greater amounts of mass.
- Ender's Game:
- In Ender in Exile, the starship engines are still reaction drives, just not carrying the mass; they work with a directional forcefield, dissolving space debris in front of the ship and propelling it out the back, essentially being ramscoops. Of course, it's the same dissolving technology that created the Little Doctor Device, a weapon that rips molecules apart, increasing by proximity of mass, meaning that if someone drove the ship's engine into a sizable mass (say, a planet), the entire structure would unweave.
- The "Park Shift" drives in use by Speaker for the Dead seem to be true reactionless drives, somehow manipulating reference frames to spin the universe past your ship (at relativistic but subluminal speeds), but Card doesn't go into much detail. (The Park Shift drive is also an inertialess drive of sorts; a spacecraft can instantly switch from a dead stop to going 99% of the speed of light without having to spend time accelerating.)
- Unfortunately, Orson Scott Card can't seem to decide on how interstellar travel works in this 'verse. The instantaneous nature of the Park Shift is forgotten in a later novel when an admiral refuses to spend years decelerating his ships to quarantine a planet and instead opts to blow up said planet. The prequel novels remove the Park Shift completely and just have the Formic ship using a typical Ramscoop to travel between stars.
- In The Engines Of Dawn, FTL is usually acheived by hyperspace Engine. However, Eos University's Engine is destroyed during a terrorist attack. One student mentions that they can use the machine they're using to study black holes to create one a certain distance from the ship and use it to limp over to the nearby habitable planet so the Engine can be replaced. It doesn't need to be turned on and off, either; the singularity stays a fixed distance from the ship while the ship is pulled closer to it. Kind of like pulling yourself out of a swamp by your own bootstraps.
- The Expanse: The Eros Space Station from Leviathan Wakes becomes a Planet Spaceship of sorts as it becomes overran by the protomolecule, capable of changing its trajectory without ejecting any mass. This is a problem for the protagonists as they have to chase the station to keep it within the visual range. The chase becomes all the more difficult by the station being able to accelerate at rates no human is used to.
- Cavorite from H. G. Wells' The First Men in the Moon created anti-gravitational thrust. It blocks the earth's gravity in the same way lead blocks electromagnetic fields, allowing the moon's weaker gravity to pull the vessel up. Jules Verne famously called Wells out for falling back on such a device, though the one he used in his own space travel story — a giant cannon — hasn't held up much better in hindsight.
- In Foundation's Edge, the (First) Foundation produces a few top-secret starships that use a "gravitic drive" for maneuvering in normal space. These ships use no reaction mass. According to the next book, Foundation and Earth, the gravitic drive draws its power from the combined gravitational field of the entire galaxy.
- In the Giants Series, Ganymean starships such as the Shapieron achieve high-velocity (though not faster-than-light) travel by using a powerful gravity field to create a 'hole' in space in front of itself, into which it 'falls' continuously. The result is described by one Terran physicist as "like a four-dimensional tank track." While the Shapieron's drive is not FTL, the time-dilation produced by high sublight velocity, combined with the time-dilation produced by the drive's own mass, means that time moves much slower for the ship and its crew than for the Universe at large, allowing a single crew to make interstellar journeys.
- For The Gods Themselves, Isaac Asimov managed to come up with a fully thought through mechanism for this that doesn't involve abandoning conservation of momentum. There might not be anything to push against where you are in this universe, but what about the one next door?
- In the Great Ship series, the eponymous Planet Spaceship's true propulsion is a reactionless drive. The massive array of 14 world-sized engines on its rear face are merely for adjustment. When the Great Ship is hijacked in the finale to The Well of Stars, the hijackers ignite the reactionless drive that the human captains were completely unaware of.
- Honor Harrington:
- The Impeller Drive generates a pair of bands of extremely high gravitational distortion that allow a ship to go forward in a method that is likened to surfing. Top speed for unmanned items (such as missiles) is in excess of 99% of lightspeed under the right conditions. Manned vessels are generally restricted to 0.8c for military vessels and 0.6c for commercial, but that's a function of particle shielding not the drive. The real limiting factor is how great an acceleration that your crew can withstand, something that is increased by inertial compensators. One of the major disadvantages of the impeller drive is the fact that, even at low power, it's very difficult to hide from the enemy's gravitic sensors. As a side effect of the impeller drive, the top and bottom of any impeller-driven ship are virtually impervious to damage, as the wedges distort space to such an extent that no physical or energy weapon can penetrate it. The bow, aft, and sides are still vulnerable, though. Sidewalls are, basically, Deflector Shields that are also generated by the impeller nodes but are substantially weaker than wedges. The bow and aft remain vulnerable until the invention of bow walls.
- One star nation manages to develop a completely different type of propulsion named the spider drive, which uses powerful Tractor Beams to pull the ship towards a spot in hyperspace, while remaining in normal space. While significantly slower and leaving the ship defenseless (no wedges, sidewalls, or Inertial Dampeners), it does allow the ship to be virtually undetected to most sensors. However, technically, it's still a reaction drive, as it pulls against something, even if it's not a physical object.
- In Humanx Commonwealth, faster-than-light travel is accomplished using the "KK drive". Invented by two human scientists named Kurita and Kinoshita, the KK drive projects a powerful source of gravity just ahead of the ship. This pulls the ship forward, which pulls the gravity projector forward, so the gravity source is still ahead of the ship... Given sufficient power, a KK drive ship can exceed lightspeed, at which point it moves into "space-plus", a version of hyperspace.
- The Killing Star is all about why reactionless drives are a very, very bad idea. When humans develop near-lightspeed travel through reactionless antimatter engines, an alien race dubbed the Intruders destroy Earth with near-lightspeed missiles, annihilating the planet so thoroughly that only a deep-sea submersible was able to survive. The reason? Because the Intruders couldn't even risk having the same thing happening to them.
- The trope namer is the Known Space universe stories. The utility of this technology is made clear in the Ringworld books as it allows the ships to remain stationary relative to the Ringworld for extended periods.
- In the Nomes Trilogy, the Nomes (who are the descendants of alien visitors who have long forgotten where they came from) have figured out how human aircraft work, and that spacecraft work the same way only more so. So when they encounter their own ship, it confuses the heck out of them, because it doesn't work on those principles.
The Thing: Flames and smoke are not required.
- Averted early on in Perry Rhodan: the classic "impulse engine" was basically just a fusion torch drive using proper reaction mass (if still impossibly little of it for the implied thrust values given, which led to a later retcon explaining that the drive had used primitive hyperspace technology to "supercharge" its exhaust all along). The occasional Higher-Tech Species of generally extragalactic origin (to say nothing of the really Sufficiently Advanced Aliens) could play the trope rather more straight even then; post issue-1000 time skip, so did "standard" Galactic starship drives.
- Rendezvous with Rama gives one to the eponymous mysterious alien spacecraft, and acknowledges its impossibility in order to add to the mystery. "There goes Newton's Third Law."
- The Skylark in the Skylark Series is powered by the direct conversion of matter into energy. This energy isn't jetted out the back; the Skylark simply accelerates.
- Solar Warden: All races use the Alcubierre Drive for transportation. It's why Flying Saucers can hover and maneuver so easily. The drive can also allow FTL and Time Travel, since the two are linked.
- Star Carrier:
- Ships bigger than 80 meters use an Alcubierre Drive to accelerate to speeds fast enough to cross solar systems in hours. Ships smaller than 80 meters can generate a singularity ahead of the ship that allows far greater accelerations: whereas it takes the battle group's capital ships the better part of an Earth day to cross to Mufrid from the Eta Boötis Kuiper Belt, the fighter squadron Admiral Koenig sends ahead for a surprise attack crosses in about an hour and a half. On the other hand, gravitic acceleration is still limited by the speed of light, whereas the Alcubierre drive completely cuts off the ship from the normal space-time, folding space in front of the "bubble" and stretching it out behind it, and can accelerate the "bubble" (which is a non-event in space-time) to translight speeds (e.g., a ship using this drive can get to Alpha Centauri in less than 3 days).
- The author also convincingly explains the reason why ships don't need Inertial Dampening in this 'verse, despite enormous changes in acceleration (something like 5000 g's is common) and direction. Since ships pulled by a singularity as, effectively, perpetually falling into it (provided the ship continuously "extinguishes" it and reforms it farther ahead), both the ship and the people aboard are experiencing the same rate of acceleration, so no inertial compensation is required. Additionally, turning is achieved by flipping the singularity to the side and simply traveling along the curved space-time in a straight line (from the ship's point-of-view).
- In Stone by Adam Roberts, reactionless propulsion is achieved by extremely rapid teleportation in infinitesimal steps. This can even be applied to an individual, who can be wrapped in a protective shell, with life-support equipment and a teleportation device, and then sent off to their destination through interstellar space. The speed of this mechanism is affected by gravitational fields, where a stronger field requires more complex calculation (and thus less rapid steps). The reader may notice this sounds exactly like the Stutterdrive mentioned for Sword of the Stars in the Video Games folder. As Stone was published in 2003, and the game in 2006, one can only assume it was half-inched. Larry Niven proposed more-or-less this design in his "The Theory and Practice of Teleportation", originally a speech at Boskone in 1969.
- The Stormlight Archive features a magical version of this. Windrunner Radiants can redefine what direction "down" is for them (or anything else they want), allowing them to fly at upwards of 80 mph without any effort on their part.
- In Terre en fuite, the second civilization of humanity (after we mostly die out in another Ice Age) is conquered by aliens. When the aliens are defeated using a genetically engineered virus, they leave behind some of their technology, including their primary means of propulsion in space called "space magnets". Apparently, there are certain energy lines between nearby stars that can be used for space travel by using these "space magnets" to allow a ship to be "pulled" towards a specific star. A ship with a "space magnet" can accelerate to close to 80% of the speed of light. Ships can also maneuver with these drives similar to how sailing ships can be still pushed by the wind even going in a perpendicular direction. There are limitations, however. It is discovered that there is a barrier of sorts at midpoint between the two stars that prevents any physical object from moving farther. Exceptions include a planetoid-sized object traveling at a high percentage of the speed of light. This comes into play when the Sun is about to explode, forcing humans to build giant "space magnets" that allow them to move planets, such as Earth and Venus. Note that the Moon remains in Earth's orbit despite Earth itself leaving. There is even a scare halfway through the journey that the Moon might not have enough mass to pass through the barrier, but everything works out.
- In Tom Swift and the Race to the Moon, the plucky hero's spaceship is driven by "repellatrons." While there is no exhaust, these don't violate the Conservation of Momentum, because they work by pushing remotely against the Earth.
- In We Are Legion (We Are Bob), the SURGE drive (Subspace Reactionless Geotactic Emulation) is a relatively recent invention. Not many people actually know exactly how it works, only that it seems to push against space-time itself. Conventional drives are slowly being replaced on existing ships with SURGE drives. A SURGE drive's acceleration depends on available power and shielding (to protect against any high-speed collisions). Bob's original probe is capable of going at an acceleration of 2.5g. During his trip to Epsilon Eridani, he spends a part of it at a very high percentage of the speed of light, before flipping around and starting the deceleration. Later on, he builds better probes, which can push up to 10g. The Others encountered later can move at even greater accelerations using their equivalent of the drive.
- In the Xeelee Sequence, Xeelee craft are propelled by discontinuity drives, which use planes of distorted spacetime (similar to cosmic strings) to accelerate the craft to near lightspeed. Humanity eventually learns how to replicate the drives, and in one of their countless assaults on the Xeelee Ring they sink a discontinuity drive into the heart of a neutron star and fire it at the Ring.
- Babylon 5:
- The First Ones and the Minbari use gravity drives to propel their ships without expelling matter out the back. Given that the Minbari provide Artificial Gravity technology to humans after the Earth Civil War, it can be assumed that future EarthForce ships will be using gravity drives as well.
- Subverted by the Centauri and most other races with Artificial Gravity tech: while they theoretically could do it, they apparently find it too difficult with their current technology, and the artificial gravity merely facilitates the work of the thrusters. EarthForce Warlock-class destroyers, the first Earth-made ship with artificial gravity, uses the same trick, as the Minbari-designed White Stars and the Excalibur.
- One of the many ways Cylons in Battlestar Galactica (2003) have outstripped their creators technologically is by giving all of their Baseships reactionless drives. In fact, due to the star shape and lack of outward features, it's sometimes unclear which way is their up, down, or forward. Their fighters still use normal engines, presumably due to power/size constraints.
- The Firefly class of merchant spaceships does spew a little bit of exhaust when it goes to full burn, but this exhaust is extremely rarefied and appears static against the backdrop of interplanetary space. Given the spacecraft's lack of internal space for storing propellant, the exhaust may merely be the (unaccelerated) fuel expended to power the Reactionless Drive.
- In Salvation, in order to divert an incoming extinction-level asteroid, tech billionaire Darius Tanz enlists the help of a brilliant student and his professor to develop the previously-thought-impossible EM Drive (see Real Life below), which appears to violate Newtonian physics. Indeed, in order to develop it, they first have to get the help of an AI to go through all the possible frequencies and then use an extremely rare crystal that only shows up on meteorites to build the drive — and they manage to do it in a matter of days!
- Star Trek:
- The nacelles may appear to be this at first, except they house the warp coils (for FTL travel, closer to Alcubierre Drives than anything else), and not the sublight impulse engines. The impulse engines are the red thruster-looking parts (e.g., the one on the back of the Enterprise-D's engineering section), which are a type of fusion rocket.
- Also, ships are definitely the "power plant inside" version. It takes a lot to make that little blue (or red) light push the ship forward.
- Some of the more advanced races probably feature these. Borg cubes seem to be able to move any which way they please, implying they're not using conventional thrust.
- The starships of 2300 AD use a stutterwarp drive: the ship essentially teleports a short distance, and the "speed" one travels is dependent on how fast the engine cycles to make the individual jumps. While this is their primary method of interstellar FTL travel, they also use it within systems to move at the equivalent of sublight velocities.
- The various reactionless drives in GURPS: Spaceships are most obviously useful in that they save a tremendous amount of space because even the best reaction engines require a large fraction of the ship's mass in order to reach useful speeds. All of them require a great deal of power to operate, but not nearly enough to explain the thrust through anything but superscience.
- Rifts: Mutants in Orbit introduces the Traction Drive, which basically grabs the very fabric of space and pulls the ship along, like the caterpillar tracks of a bulldozer of a tank. It's very slow (to start) but has near-infinite acceleration.
- The "ether propeller" of Space 1889.
- The spelljamming helms of Spelljammer, which literally propel a ship by magic.
- The Ion Drive engines of Starfire are probably reactionless drives (they let a starship instantly switch back and forth between a dead stop and 10% of the speed of light at the flick of a switch, so they definitely ignore pesky details of physics like inertia), but we are never told outright whether they spew exhaust or not.
- Traveller: Manoeuvre drives are implied to be either this or extremely powerful rockets. The New Era (3rd edition) replaces these with HEPLAR (High Energy Plasma Recombination) rockets which are much more limited, with the classic manoeuvre drives only becoming available at high Technology Levels.
- In Warhammer 40,000, reactionless drives are unique to the Necrons, who are also the only race to possess inertics and 'true' FTL travel as opposed to using the Warp. In Battlefleet Gothic their unique access to these engines allow Necron ships to accelerate, decelerate, and turn at rates that are impossible for any other species. Their higher-tier ships have no true 'front' or 'back' and are capable of moving in reverse while still pointing all the big guns at their opponents, which is also something no other faction can do. To balance things out, Necron ships tend to be extremely expensive, and their ships cannot outrun top-speed-focused factions like the Eldar.
- Judges Guild's Wilderlands of High Fantasy (Issue N) has a plethora of relic high-tech items with little description, including anti-grav vehicles.
- One of the drive systems in Ascendancy is a gravity drive which works by projecting gravitons in front of the ship, which then pull the ship forward. Presumably, the gravitons are removed after the ship passes, otherwise, they would then pull it backwards. It should be noted that this is by far not the fastest engine in the game and has no advantages other than be slightly faster than the one preceding it.
- The Normandy (both of them) in the Mass Effect games uses a "Tantalus drive" that propels the ship by creating mass concentrations in front of the ship that it falls into through gravity, principally for stealth purposes to avoid the use of heat-emitting thrusters. This has several limitations, chiefly that it also involves storing the Normandy's waste heat in internal sinks instead of radiating it, which must eventually be vented or they'll cook the crew. It's also extremely expensive: The oversized element zero core required gave the Normandy SR-1, a frigate, the price tag of a cruiser, and the SR-2's is even bigger.
- Sword of the Stars:
- The Liir have what's called a Stutterdrive, which teleports their ships an infinitesimal amount millions of times a second. They use no thrust and have no inertia, and their ships are not affected by in-game techs that affect thrust. This is also out of necessity, as their ships are filled with water and trying to move them through normal means would be an engineering nightmare. However, they do have a disadvantage when maneuvering near large celestial bodies, as gravity wells make teleportation calculations more complex, slowing down the ship. They still use thrusters to rotate and for emergency propulsion if the stutterdrive is destroyed in battle. However, it's not explained why, when the drive is destroyed, they keep drifting, even though they should stop dead.
- The Tarka normally have reaction drives, but their hyperdrives can be enhanced to allow them to maneuver at subluminal speeds without the use of reaction drives. This leaves space in the aft section to add more and/or heavier turrets.
- The Morrigi use gravity manipulation for their propulsion. They are Precursors who used to have very advanced tech, though.
- In The Inexplicable Adventures of Bob!, the Nemesites have been shown using both Bussard Ramscoops and some sort of Reactionless Drive called a "Grav Drive." Much of their technology is based on gravity manipulation, and this is apparently just one more aspect of it.
- In Schlock Mercenary, almost all futuristic technology is either possible, though difficult, according to our current understanding of physics, or is based on gravitics. Their engines create a gravity well in front of the ship.
- The greater powers of Orion's Arm have access to three kinds of reactionless drives for their spaceships, all largely based on the Alcubierre Drive but limited to just below the speed of light (attempting to hit c allegedly causes void bubble collapse). Like an Alcubierre drive, they depend on negative mass, and as per the setting's guidelines, elaborate justification has been provided as to their plausibility. Thematically, the lesser gods' Displacement and Halo Drives (pictured above) have only disposable engines located inside void bubbles and magnetically or gravitationally coupled to an external ship, since at their level, the only way to take down said bubbles is to destroy them chaotically. However, the highest archailects' Void Drives are true warp craft, with entire ships or fleets being contained within a void bubble and re-entering normal space smoothly upon reaching their destination.
- Since finding a technology that could really do this would be of enormous military and economic benefit, people have tried to come up with various methods for making this work. The Dean drive is probably the most famous of these.
The Dean Machine, the Dean Machine
You stick it right in a submarine
And it flies so high that it can't be seen!
That wonderful, wonderful Dean Machine!
- There is one confirmed way to get many of the benefits of this without breaking physics. If you make it so that the ship is not a closed system, then it doesn't have to carry fuel or reaction mass. Of course this just means that the reaction happens somewhere else. A sail is the form most familiar to us.
- A sail is not a truly reactionless drive - indeed, such a thing may not actually be possible. In the case of a Solar Sail, for example, the ship doesn't carry fuel but it still has reaction mass, because it's still a Newtonian reaction generating the sail's thrust. In this case the reaction mass is photons from the sun. Or in the case of the Medusa, controlled nuclear explosions to push the sail.
- A Bussard Ramscoop, likewise, doesn't carry its reaction mass with it, but it still has to gather reaction mass from the interstellar medium.
- A photon drive fits our definition; photons do exert pressure, and they are massless. A photon drive only consumes power. Unfortunately, a photon drive is horribly inefficient.
- The Ambient Plasma Wave Drive would use ambient plasma as its reaction medium, generating waves in it to propel itself. Sort of like how a propeller uses ambient air (or water) to create thrust.
- The Alcubierre Drive doesn't move in a local sense, and thus doesn't require reaction mass. It's also capable of moving faster than light, and even back in time. Of course, the mass needed to stretch space enough to build one is far more than all the reaction mass you'd ever need.
- But since the inside of its "bubble" is causally unconnected to the past, present and and future of the outside universe, it's questionable whether it meaningfully counts as a form of transportation, or even as something that can be said to physically exist at all.
- If you could produce a negative mass you could produce a ships that accelerates without using reaction mass. This occurs without violating conversation of energy because as the negative mass increases in speed its energy decreases. Strictly speaking negative mass is not disallowed by any law of physics.
- In fact, most of current cosmological theories require our Universe as a whole to be of negative mass to match the currently known pattern of expanding. Well, they mostly express it as a negative energy, but E=mc^2, y'know...
- The Mach Effect (AKA "Woodward Effect", but Dr Woodward prefers the former), the claim is it can be derived fairly directly from Relativity, and so involves no new physics, plus we already have most of the technology to try it in a lab... The claim is it is reactionless without violating any conservation laws, cool stuff if it works. NOTE: we may have most of the tech to build a test device, but it won't be very efficient, and the effect will be small. It will probably take several years of work to develop capacitors and piezoelectric materials that will make a useful version. That's assuming that the thing works at all.
- It should be noted that the Mach Effect DOES conserve momentum, the "equal and opposite reaction" is spread out over a vast amount of mass outside the engine (possibly the entire universe).
- The Electrodynamic Tether also achieves transfer of momentum without using any propellent, it's basically a simple form of magnetic sail.
- Spacetime swimming is another viable alternative for propulsion in a curved spacetime. This is analogous to how you can change the orientation of an office chair without touching anything (try it, unless someone is watching). It is also a small effect, and it is unclear whether or not it would be useful. This device exploits quirks in general relativity that allows one to sidestep the issue of conservation laws — let's just say general relativity is a bit sticky on this subject, and that since this sort of drive would not work in the usual manner, i.e., it does not apply a continuous force, accelerating the craft, but instead needs to operate continuously in order for the craft to continue moving, ensuring that no conservation laws are harmed. Here is a more palatable description. This is the method of propulsion chosen by the alien Xeelee, in Stephen Baxter's novels.
- Effects of the quantum vacuum on magnetoelectric materials exhibit this, but again, it is unclear whether technological applications will ever be possible.
- Proponents of Heim theory claim you can get a reactionless drive with Faster-Than-Light Travel thrown in at no extra cost. Unfortunately, according to mainstream physics, the theory is most likely false.
- The EM Drive is purportedly verified by Chinese scientists who report having built a functional prototype, and also being examined by NASA to determine whether it's legit. It remains to be seen whether the effect has commercial applications, but if a sufficiently powerful device is developed using the principle, implications could be staggering. Allegedly it would make it possible to travel from Earth to Mars in a matter of weeks and even making it feasible to do a fly-by of Alpha Centauri and return to Earth without the astronauts spending most of their lifetimes on the trip. More recent experiments by German scientists seem to suggest a more mundane explanation: the test chamber wasn't properly shielding the drive against Earth's magnetic field. The only way to be sure is to build a properly-shielded chamber and try again.