Tuesday, July 12, 2016

SF Setting - Momentum Exchange Devices

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.

Artificial Gravity
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.

Fusion Reactors

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.

Inertial Compensators
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.

Levitation Devices
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.

Mass Drivers
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.

Plasma Shields
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.


  1. It's a technological version of Force Magic, I'm loving the cross over. I don't think the hovercraft would work though. They would fly fine but they would still crush anything underneath them with the same force as if they were sitting on them. If the fields could be expanded to a much larger footprint than the vehicle and pushed on the air as well as the ground this could be mitigated to a large extent but it would cause a powerful wind similar to a helicopter with the same noise, disruption, and potential instability problems. The plasma shield will also be tricky because any disruption in the fields or maneuvering to fast for the fields to compensate would end with the ship getting burned by it's own plasma.
    This is a good bit of "magical tech". It only bends the laws of physics rater than ignore them outright while still giving you alot of room to play around with large range of cool tech. I'm still missing the next Daniel Black release but these looks into the world building are getting me interested in the new series.

    1. It seemed to me that the Field could be tuned to interact mostly with a specific range of densities. A field tuned to matter with a density of rock or better would solve most of the weight/crushing issues.

    2. You're right that violent maneuvers could disrupt the plasma shield, but there's not enough material in the field to really be dangerous to the ship. It only needs to hold enough plasma to keep the field opaque, so we're talking about a few centimeters of moderately hot gas at substantially less than normal atmospheric density.

  2. A Tractor beam is a possibility. Also, if you are going to use reaction mass, FTL is not happening. But I think you had that covered in a prior post.

    I like the inertial compensator idea. Plus since it is explained as tech, and not magic. Having it fail at the wrong time adds a weakness that can be exploited. Just like, say shields.

    1. One reason I put such a short range limit on the effect was to make sure tractor and repulsor fields aren't useful as weapons. But yes, I could see that kind of thing being used for docking or other close-range maneuvering.

    2. oooh, I imagine Tractor beams could be used to create devastating kinetic energy weapons out of a nearby asteroid field.

  3. Remember your comment that it will only effect things with mass photons (several basic components of the standard model dont have mass) do not have mass so red shifting lasers doesnt make sense. Also if you are goimg to make it follow newton's laws of motion simulating gravity on a ship would be no different to having any form of engines running at 1G as such this would not work on any ship moving at a constant velocity or stationary. Sorry to burst the bubble the excepted limit on the magical tech in scifi is that it has to be bullet proof internally consistent. Try to avoid thinking of how your special tech will create all the scifi normallity and think of how it could create something original. My take space lanes freight moving in one direction acts on freight moving in the other direction and there is no problem decelerating freight at planets due to the mass relative to a freight container.

    1. You might want to make sure you actually have your physics right before you post comments like this.

      On the first point you are incorrect - photons do have mass. They don't have rest mass, but that simply means that there's no such thing as a stationary photon.

      On the second item, I already pointed out how they get around the problem. The artificial gravity field doesn't cover the whole ship, it's just projected into the empty spaces of the crew quarters. So the crew, furniture, air and other contents of the habitat get pulled in one direction, the rest of the ship gets pulled in the opposite direction, and the two forces cancel out.

      As for your closing suggestion, that would require ships to pass dangerously close to each other (because of the short range of the momentum transfer effect) while moving in opposite directions, not to mention that you;d have to somehow schedule things so that there's always a ship in the right place to exchange momentum with when you need to change course. That sounds incredibly impractical for most applications.

    2. I apologise for any offence caused, my concern about the artificial gravity is that you said you would leave the mechanical spaces in zero G.

      The comment on the mass of photons is that any reference to photons having mass is meaningless they don't have rest mass correct but you have suggested in your answer that they have relativistic mass which is also not the case it is fair to say photons have energy but this also means you can not apply Newtonian relations to them.


      Mass as a concept can not be applied to photons.

  4. This comment has been removed by a blog administrator.

  5. So, current technology on Earth IRL has frequent use of photovoltaic power cells (solar power), however one of the currently impractical but theoretically exciting corollaries of this technology is that it can act in a similar manner when exposed to heat. Meaning direct heat-to-electricity conversion. This would be an excellent futuristic development to utilize some of the massive waste heat generated in some of those field applications.

    1. Yes, that's one of the reasons I'm assuming such high efficiency numbers for most of the power-hungry systems on a spaceship. But those devices require a substantial temperature difference to work - you have a hot side and a cold side, and you get power when heat flows from one to the other. So you can never recapture more than a modest fraction of the waste heat this way

    2. Unless you use a material that can vent large amounts of heat to space via infrared or some other electromagnetic radiation, then you can use all of space as a heat sink and make that your cold side. Almost anything is Hot compared to that.

  6. The versatility of such a concept would hinge on the scalability, physical size, and the power source. It would make for some awesome nano-bots, hulk hand would be pretty sweet, a directional gravity suit would be neat. A sword with a cutting edge smaller than an electron would awesome (but I think you've already done that). I believe Asimov tended to use micro-miniature cold fusion batteries in all his work's gizmos.

  7. Magic tech. End of story lol. Any issue you can think of, remember ftl is supposed to be impossible, and most space based science fiction movies, books, comics, use ftl. Dont complain, roll with it and accept that its not your universe, the physics may be different.

    1. FTL isn't impossible. It was possible but the maths couldn't support powering such a device when Einstein proposed the concept of bending space-time to go FTL. It was revisited in the 70's and they came up with a weird the answer "All of the sun's out put for a thousand years" or something ridiculous like that. Which was at least a mathematically measurable amount. They got it down to "All electrical power produced on earth for a hundred years" back in the 90's. For scale from one number to the next, that's like a trillionth of a trillionth of a trillionth of the first number. In twenty years. If that scale holds true, my watch will be capable of propelling me to alpha centauri sometime in the next couple of decades.

  8. Magic tech. End of story lol. Any issue you can think of, remember ftl is supposed to be impossible, and most space based science fiction movies, books, comics, use ftl. Dont complain, roll with it and accept that its not your universe, the physics may be different.

    1. Yeah, this is the attitude that makes so much modern entertainment so bad. Even my fantasy stories at lest have internally consistent rules. If I'm going to write SF I want my setting to actually be consistent with the known laws of physics, or at least close enough that any remaining problems are only apparent to a specialist.

      For instance, you'll note that the 'FTL' in this setting doesn't violate relativity. Nothing ever moves faster than light, it just turns out that the stars aren't as far away as they look if you take a hop through the right parallel universe to get there. Similarly, I want to make sure the momentum exchange tech works in a way that doesn't violate any conservation laws or other fundamental principles.

  9. I truly love the idea of your momentum exchange generators in fusion reactors to deal with the biggest issue that fusion reactors of any decent size have in that the magnetic fields that are used in current modern fusion reactors tend to not only eat more energy than the reactor produces but also don't provide enough containment for prolonged use. IE Wendelstein 7-X

  10. A series that echoes SciFi's "Golden Age" MagiTec?
    My concern would be the low-end devices, such as hand-held mass drivers. I can see developments in capacitor technology to power the device, but have difficulties in the resulting heat generation via inertia. Definitely a bit of "tap dancing" to avoid destroying devices and burning user's hands off...

  11. In regards to heat to electricity methods read H.Beam Pipers book Junkyard Planet AKA Galactic Computer

    1. Read it on your recommendation... thank you for that. Tech is a little out of date but still a great book.

    2. Read it on your recommendation... thank you for that. Tech is a little out of date but still a great book.

  12. William, you don't need magic. You can accomplish everything you listed here without it, or you can't do it with it. As a sci-fi fan, I think rules lawyering to do everything you describe WITHOUT needing to cheat is much more interesting than a setting where cheats are used.

    Artificial Gravity - you don't need this. A ship's habitation section can just be a hollow can full of air, and inside that can, which is non rotating, is this merry go round spinning wheel, with open struts and mesh floors to save weight. It would look kind of like the crew quarters on Skylab, but mounted into a wheel. The floors would tilt slightly on bearings so the net direction of gravity, including engine thrust, is down. 2 separate wheels, of course, or a counterweight wheel.

    Deflectors - you don't need magic force fields. Rapid manufacturing of new components and robots to repair the ship after it takes hits, or evading the incoming fire and using active defenses is the realistic way to do it.

    Fusion Reactors - the method talked about these days is a hybrid, where an outer cloud of ionized gas is used to keep the inner core hot. Electromagnets and electrostatic fields act on the outer cloud. This has been demonstrated to work.

    Inertial Compensators - you don't need this because no realistic rocket is both high efficiency and gives you more than 1 G.

    Levitation Devices - you can also use VTOLs, or manufacture a levitation track on the ground.

    Mass Drivers - existing methods are fine. Don't forget neutral particle beam weapons, another practical weapon that you can use instead.

    Shields : see above about a regenerating ship or outshooting the enemy instead

    Thrusters : this will not work. If you work out the energy needed per unit of thrust at the ISPs you mentioned, you end up needing power densities measured in megawatts or gigawatts per kilogram. Your magic cheat does not solve this in any way. That fusion reactor will still emit enough heat to melt the ship to slag. There is no way to transfer energy to the propellant like you describe in a way that gets rid of all the heat, it's a violation of laws regarding entropy. (TLDR, the stream of hot plasma in 1 direction has too low an entropy to dispose of heat which is high entropy, it's the same reason you cannot cool a spaceship by laser cannon file)

    Also stealth thrusters won't work either.

    If you want a cheat solution that WILL work, think access to extra dimensions. If you could dump your waste heat into a convenient nearby dimension of negative energy, or tap into another dimension for an infinite source of fuel, you could do all the things you described.

    1. I actually have worked out the energy consumption for the thrusters, and it isn't nearly as bad as you think. For instance, a typical military scout ship with a mass of a million metric tons, a max acceleration of 60 gravities, and an exhaust velocity of 10,000 kps ends up needing about a terawatt of power for the drive. That's a big number, but if the drive makes up 5% of the ship's mass it only works out to 20 megawatts per ton. For far-future technology that has a large flow of coolant that seems doable, especially since it isn't normally going to run for more than a few minutes at a time.

      As for the energy transfer, that's something real rockets already do. You just design the engine to run hot, and circulate the cold reaction mass through the mechanism as coolant before dumping it into the acceleration chamber. If we assume operating temperatures comparable to modern jet engines the reaction mass will easily end up absorbing several hundred gigawatts worth of thermal energy, and that's without assuming any additional tricks to increse the energy transfer. Then you accelerate the resulting hot gas out the back of the ship to generate thrust, and it carries off that heat without any entropy problems.

      Most of your other comments basically amount to complaining that you want me to write a different kid of story. Well, maybe someday I'll try out an ultra-hard setting with no FTL or physics surprises at all, but that day is not today.

    2. You can't do what you describe because there is no way to transfer the heat to the engine reaction mass without melting the material doing the transfer. Highest melting point material found melts at 4k C, and fundamental limits on solid matter found on the known periodic table (and remember, the bottom right edge, a common sci fi trope, is not realistic because the half lives of the elements discovered so far are measured in picoseconds and the "island" of stability is relative) mean that limit can only be raised so much.

      You have to either touch the plasma, hot as the core of the sun, with some other material, to transfer heat, or you can do it slowly by radiating but the equilibrium temperature of the radiator device ends up equalling the temperature of the plasma.

      Either way, this is not compatible with the physics of our universe.

    3. Well if the momentum exchange devices work as advertised then one could simply remove some of the momentum of the atoms that make up the reaction mass of a the engines and transfer it wherever you want the heat to go with no need for materials that would melt in order to transfer heat. Heat is simply the movment of atoms. Atoms have mass and therefore they have momentum which can be removed and transferred. Which would have a byproduct of dropping the temperature. But even baring all of that it doesn't really matter if it works according to conventional physics it's called science fiction for a reason. And even baring that I would say that most of us are here to enjoy the latest product of an imagination we respect not to dither over a psuedo scientific explanation wouldn't you say?

  13. Hm, right after I read the introduction to momentum exchange device, somehow my mind jumped straight to an idea inspired by novel Rowan by McCaffrey. There, telekines throw starships around with help of gigantic generators. In your setting I would imagine it as the device takes the momentum of incoming ship, just to transwer it straight away to a ship waiting for launch - only with your limitations, it probably wouldn't fly due to the range and unability to store said energy(?).