Traditionally every hard SF setting gets to have one piece of ‘magical’ tech that current physics says should be impossible, in addition to whatever you use for FTL. For this setting, I’ve chosen an exotic quantum mechanical effect that allows the transfer of momentum between objects that aren’t in physical contact. The momentum exchange effect obeys all the same conservation laws as more conventional ways of moving things around, but even so it makes possible quite a few traditional space opera technologies that otherwise would never happen.
Momentum exchange fields can be projected only over short distances (typically up to about 2x the diameter of the emitter), and the efficiency of the interaction falls off rapidly with distance. In theory it can affect anything with mass, but to get good coupling (i.e. fast and efficient momentum transfer) practical devices have to be tuned to affect a particular class of targets (i.e. baryonic matter, photons, neutrinos). A momentum exchange device that actually encloses the target gets extremely good coupling, making it a highly efficient way to manipulate matter and energy.
Interactions obey Newton’s laws, so accelerating an object in one direction produces a reaction in the opposite direction. They also obey conservation of energy, so large velocity changes require a lot of power. Interactions that decrease the kinetic energy of the target produce enough waste heat to make the equations balance, just like a physical impact.
A momentum exchange field applies an acceleration in a uniform direction to anything that enters it, so using the technology for anything more sophisticated than simple push/pull effects is complicated. It’s possible to create overlapping fields oriented in different directions, and the shape of the field can be manipulated fairly well. But typically you can only get sophisticated telekinesis-like effects by surrounding an enclosed space with arrays of manipulators, which usually isn’t practical outside of industrial applications.
A final important constraint is that the momentum exchange effect isn’t instant. Any particular field will only transfer energy at a finite rate, which has major implications in weapon design.
This one technology has so many applications that it radically changes what the setting looks like. Some of the more common applications are listed below.
Momentum exchange fields can easily be used to simulate gravity for a ship’s crew. Normally this is only done inside the inhabited parts of a ship, while the much larger machinery spaces are left in zero gravity.
A repulsive momentum exchange field wrapped around a ship’s hull makes an effective defense against many forms of attack, so these deflector shields are a standard feature of all warships. A warship’s deflectors won’t necessarily stop mass driver rounds, but they greatly reduce their effectiveness by slowing down and deflecting projectiles. They also prevent more diffuse threats like plasma clouds or nanite swarms from reaching the ship at all.
Lasers are a major weakness - while a deflector can red-shift incoming light, the interaction tends to be too weak to protect against heavy weapons firing beams at x-ray or gamma ray wavelengths. The field can also be momentarily overloaded by too many impacts in a short time frame, and under sustained attack cooling the system can become a serious problem.
Achieving plasma confinement with momentum exchange fields is far easier than with magnetic fields, making compact fusion reactors relatively easy to build. Practically all starships run on fusion power, as do stations and planetary power grids. Reactors with a volume of less than a few hundred cubic meters quickly become less efficient, but tanks and the larger warbots usually use them anyway.
A system similar to artificial gravity, but designed to protect passengers from acceleration stress when a ship is maneuvering. A ship’s inertial compensators normally only cover the spaces where crew and passengers are expected to be, and leaving these areas during a hard burn can easily be fatal to humans. The same system can also protect against the shock of impacts as long as the ship’s computer can see them coming, so you aren’t going to see crewmen getting tossed around like the extras on a Star Trek set.
Systems designed to interact with the ground can easily support hovering vehicles in a way that looks just like classic space opera antigravity, and the strong coupling makes levitation devices efficient enough that they’ve replaced wheels or treads for many applications. These devices perform a lot like hovercraft - they can cross flat ground or water with no need for roads or bridges, and tend to be quite fast.
Once you get too high to get good coupling with the ground you need a completely different kind of device. Lightweight vehicles can use a system that pushes all the surrounding air down to generate lift, producing an effect similar to a helicopter but with a lot less noise. Heavier or faster vehicles often use a system more like a starship thruster instead, sucking in air at one end of a tube and accelerating it out the back. These kinds of systems have largely replaced propellers and jets because they’re more efficient, more reliable and don’t generate as much noise pollution.
One twist that deserves special mention is the effect of field emitter scaling on levitation devices. A hovercar 4 meters long with a lift system on the underside will have a maximum altitude of maybe 4-6 meters, high enough to pass over people and avoid a lot of ground clutter. A 12-meter truck will be able to cruise at ~16 meters, flying over trees and other obstacles. The bigger the vehicle is the higher it can fly, and the less it has to worry about terrain. On densely populated worlds this leads to phenomena like 200-meter cargo ships cruising the skies, or giant resort hotels floating half a kilometer above scenic locations.
A railgun-like device that uses a momentum exchange field to accelerate a projectile to high speeds. Weapons of this type are frequently used as small arms, or as the primary armament of ground vehicles or small spacecraft. Guns designed for use in an atmosphere will have a muzzle velocity of several thousand meters per second, while those mounted on spaceships will frequently reach thousands of kilometers per second.
A much larger variety of mass driver, with a muzzle velocity in excess of 0.98C, is used as a spinal mount weapon on some large warships. At these velocities point defense systems generally can’t intercept the projectiles, making them a highly effective way to deliver energy to a target. The impact energy of these weapons is limited primarily by waste heat generation - if you want to fire shells with hundreds of megatons of kinetic energy you’re going to be generating tens of megatons of waste heat inside the gun, so you’d better have a truly massive heatsink or cooling system.
If you’re worried about people shooting at you with lasers, using a momentum exchange field to trap a cloud of ionized gas in a bubble around your ship can be an effective defense. Of course, the cloud will also interfere with your own sensors, and if it absorbs too much laser fire it will get hot enough to leak out of the confinement field. Layering both deflectors and a plasma shield around the same ship provides an excellent defense against most weapons.
A momentum exchange thruster is simply a mass driver optimized to handle large amounts of liquid reaction mass instead of small projectiles. The exhaust velocity is limited by both the length of the drive tube and the amount of available power, but obviously ship designers strive to make it as high as possible.
Large starships normally have drive tubes several hundred meters long, with an exhaust velocity of ten thousand kps or more. With fuel tanks making up 5% - 20% of the ship’s mass, this gives ships a total delta vee in the range of 3,000 - 4,500 kps. Small ships have a lower exhaust velocity due to their shorter drive tubes, and will typically have larger fuel tanks and only 1,000 - 3,000 kps of total delta vee as a result. With a typical acceleration in the range of 20 - 60 gravities, ships can do quite a bit of maneuvering and frequently cruise at 500 - 1,000 kps on interstellar trips.
However, these numbers also imply that a ship’s drive is an immense heat source when it’s operating. Even at 90% efficiency, burning terawatts of power will quickly melt your ship if you don’t get rid of the heat somehow. Most drive systems are designed to dump as much heat as possible into the reaction mass as it’s being fired out of the ship, making engine exhaust a brilliant plume of hot gas even though it isn’t being combusted. Turbulence in the exhaust plume and collisions with the interplanetary medium add to this effect, making it impossible to miss a maneuvering starship under normal conditions.
Stealth thrusters do exist, but they’re far less powerful. Usually a stealth thruster would fire a stream of dense beads of cold solid matter at a velocity of a few hundred kps, and dump its waste heat into an internal heat sink. Total delta vee is generally less than a hundred kps, and even then the fuel tanks and heat sink will take up most of the ship. So this sort of system is normally installed only on dedicated recon or espionage platforms, which don’t need to carry weapons or cargo.