So I wish to make a sci fi setting that uses gravity manipulation tech and inertial dampeners.

I have a number of ideas for how such a system works, and about how it should go. At the moment I'm fielding an idea I've borrowed from the game Mass Effect.

However for various reasons, I strictly speaking don't want to accidentally create a perpetual motion machine or a planet busting superweapon a car mechanic could build in his garage.

To be clear this isn't hard sci fi, how precisely this work doesn't entirely matter, so the technobabble goes something.

The Aetheric Field Generator

There exists a technology capable of generating what are called aetheric fields. Which are basically artificially metastable regions of space time, which can be tweaked to have somewhat different properties than the vacuum of our own universe. The generators and fields also have a capacitive property and have to be actively maintained and kept metastable.

Firstly, it does not free the ship from the rocket equation. You cannot, for example, make the ship weigh less and therefore expend less fuel to accelerate it.

The aetheric field, particularly when used to spin a paragravity field, works to alleviate the affects of acceleration for the crew inside a ship.

They use as well in inertial manipulation of projectiles. Through mass manipulation, they can impart velocity to a projectile coming out a gun, but the projectile doesn't gain energy from nothing, it takes it from the aetheric field.

I also have an idea of how it could be used as a force field. Though I'm not sure how it would work out.

At its core, the aetheric field generator is obviously fantastical. It may cheat, bribe, negotiate, or pass on the debt to any sucker willing to take it on. But the one thing consistent about it, is that it can never outright escape paying the bill in the end.

This is just the rough draft for the concept of course. Assuming this is workeable, What problems are there that I need to address or patch over for this work?

Or am I better off inventing something else?

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    $\begingroup$ You want the crew to experience inertial dampening (making the crew accelerate/weigh less), but the ship cannot experience the same effects? I can think of one way that might happen (The Sojourn's gravitoelectric plating, like needing two poles to pass a unidirectional field), but looks contradictory on the surface. Making the ship less massive while packing the same thrust/ISP doesn't liberate it from the rocket tyranny. It just alleviates it somewhat. $\endgroup$
    – BMF
    Commented Mar 30, 2023 at 20:07
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    $\begingroup$ To be clear this isn't hard sci fi, how precisely this work doesn't entirely matter no, it doesn't matter at all. You are wasting time on completely unnecessary detail instead of focusing on what makes the story, which is its characters. $\endgroup$
    – Ian Kemp
    Commented Mar 31, 2023 at 11:56
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    $\begingroup$ You've got more than conservation laws to worry about if you've successfully made a physical concept wet. (damping != dampening) $\endgroup$ Commented Mar 31, 2023 at 13:50
  • $\begingroup$ You might be interested in this article. The physics textbook just got thicker. home.cern/news/news/physics/where-does-higgs-boson-come Unfortunately, most of the new text in the thicker book is all punctuated by question marks. $\endgroup$ Commented Mar 31, 2023 at 15:51
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    $\begingroup$ If all you want is a sci-fi setting using gravity-manipulation tech and inertial dampeners, why not simply go with Star Trek? $\endgroup$ Commented Apr 1, 2023 at 21:05

7 Answers 7


The main use of inertial dampening fields and inertial compensators in scifi stories appears to be to stop the contents of your ship from being reduced to a thin layer of paint on the aft bulkhead when you try to accelerate at 100Gs or something. The concept is so vague that lots of inconvenient things get swept under this particular carpet, but compensating for variations in inertia transfer (you know, acceleration) throughout the volume of your ship is more or less the whole point.

The sciencey solution is to suspend your delicate cargo in a neutral buoyancy environment that applies acceleration force evenly across the entire body. It's not a perfect solution unfortunately since the body isn't perfectly homogenous and blood (and internal organs, and... so on) tend to accelerate slower due to different densities and how force propagates through the body. You can get a little further using pressure wraps to hold things in place a little better and squeeze the blood up against the pull of the acceleration force, but 100G is still going to kill you pretty quickly.

But what if you could generate a field that applied the acceleration force directly to the entire volume of your ship? Instead of the engine pushing the back of the ship and that force propagating through the structure and contents, imagine that acceleration being applied equally across every part of the ship. If everything in the volume - including the fluids in your circulatory system, your organs, everything - is accelerating in the same direction at the same rate then it would be indistinguishable from free-fall. You might be accelerating at the equivalent of a few thousand Gs, but it would feel like you're in microgravity. As long as all acceleration is delivered via the field you wouldn't even be able to tell the difference.

Not only does this resolve any little issues with physics (apart from being impossible as far as we can tell, but this is science fiction after all) it shouldn't be particularly costly in terms of energy. The same amount of thrust is being generated to produce the acceleration, it's just applied directly to the ship's contents instead of indirectly.

There are a few problems with just using it as a drive system, so how about having the field generate a cohesive, rigid inertial volume. Any force applied to the volume is distributed across the whole thing, which then reacts to the force as if it were a rigid object. You could bounce your ship off a moon and not disturb anything inside the field. Missile explodes on the side of your ship? Nobody noticed except the engineering team monitoring the inertial compensators and the navigator who has to fix your course. And the crew on that battleship you just side-swiped are going to notice the impact energy since they're outside the field.

Of course the generators are the weak point of this. They're dealing with a lot of energy all the time, and inefficiencies in the machines (because there always are) will produce some sort of byproducts, probably significant amounts of heat. Lots of fun to be had with the details of that.

Just don't try to justify how it works, or how it allows things to keep moving inside relative to the field itself. Maybe write some "bad things happened during development" paragraphs to let the reader know how hard it was to get this tech to work and move on. If they don't accept that then they're never going to accept the field itself.

(New week, clear[er] head. Realised I didn't explicitly address the question in the title.)

So... now that you have a field generator that is working hard (possibly at ruinous losses and with lots of heat buildup, but you can deal with that later) to create a zone of distributed force, how does that interact with conservation laws?

On the macro scale there are no problems. The field and its content is treated as a rigid body for all intents and purposes. The total mass of the body is applied to collisions (sorry Nosajimiki), acceleration and so on. The energy applied to create acceleration is conserved as momentum, the ship still falls in a gravity field, still bleeds energy as heat when it passes through an atmosphere at high speed, etc. At any scale where "the ship" can be treated as a single object everything balances as you'd expect.

At the micro scale things could get a little interesting - squishy crew need to be able to move around, breathe, pump blood and so on - but there's no need for violations of conservation. We may be violating causality at every turn, but conservation is well and truly intact. The field itself takes constant energy input to maintain, and perhaps the energy cost scales with acceleration or something. Likely the field is rated for some maximum power draw that equates to neutralisation of a specific max acceleration force, with some manageable ratio. Try to do too much and you could end up with a heap of glowing slag where your field generators used to sit.

So it's not a space opera inertialess drive (ala E.E. Smith's Lensman series), it's the kind of inertial compensator that has been used in SF shows for several decades now. It won't help you go faster or beat the light speed limit, but it will let you mount much larger drives and zip around the battlefield far more nimbly than ships without it.

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    $\begingroup$ here's an even more sciency solution: the field is the gravitational field of a black hole at the front of the ship. Don't ask how heavy the black hole is. $\endgroup$ Commented Mar 31, 2023 at 8:53
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    $\begingroup$ @user253751 The problem with singularity gravity fields is a little thing called spagettification. Not ideal for a ship full of squishy crew. You'd have to project your singularity tens of thousands of miles ahead of the ship - which means it needs to be quite massive - otherwise tidal forces would rip the ship apart. Oh, and the crew. $\endgroup$
    – Corey
    Commented Mar 31, 2023 at 12:11
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    $\begingroup$ There is that tricky little thing called 'spooky action at a distance'. Information transfer across the entire ship would have to be instantaneous, The same information applied to both ends of the ship at exactly the same time. 'OOOhhh, never thought of that!!!!' $\endgroup$ Commented Mar 31, 2023 at 14:56
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    $\begingroup$ The other thing that inertial dampeners do is solve for the Kzinti Lesson. Reducing actual inertia as opposed to uniform acceleration means that you can have have a relativistic ship impact a plant with non-planet busting force. $\endgroup$
    – Nosajimiki
    Commented Mar 31, 2023 at 15:15
  • $\begingroup$ @JustinThymetheSecond I know, right? That's one of the reasons I said it's impossible. But the lack of a "science" tag means we can get away with it 😁 $\endgroup$
    – Corey
    Commented Mar 31, 2023 at 16:16

Something that "turns off" inertia is no good. "Inertial dampeners", on the other hand, as depicted in science fiction, are just fine.

All you need to do is transfer any forces the accelerate the ship to distribute them uniformly across all of the particles inside the field. If every part of you accelerates uniformly, you won't feel it--that's why you feel weightless in free-fall. And thus, the inconvenient effects of inertia on squishy bodies inside ships undergoing acceleration are successfully dampened! We don't know of any way to do that with known physics, but it doesn't violate any conservation laws. An aetheric field that changes the properties of the vacuum inside the ship is as good an excuse for how that is accomplished as any.

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    $\begingroup$ That's a great idea. I wish I'd thought of... oh, wait. $\endgroup$
    – Corey
    Commented Mar 31, 2023 at 12:12
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    $\begingroup$ @Corey No such thing as 'respecting IP rights' around here. $\endgroup$ Commented Mar 31, 2023 at 14:59
  • $\begingroup$ @Corey Hah! You beat me by an hour! Guess I should've refreshed the page before posting my answer. $\endgroup$ Commented Mar 31, 2023 at 16:04
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    $\begingroup$ Not salty, just poking some fun. $\endgroup$
    – Corey
    Commented Mar 31, 2023 at 16:15

Sorta, but not really.

I myself have been looking for a way to justify inertia manipulation in my worldbuilding for a while. This is the closest I've come.

Why it breaks conservation laws Changing inertial mass alone violates conservation of momentum & energy.

$$p=mv$$ $$KE=\frac{1}{2}mv^2$$

If $m$, say, goes down, and everything else stays the same, then momentum, $p$, and kinetic energy, $KE$, seemingly vanish from the system. If $m$ goes up then both are seemingly added to the system out of nowhere.

Let it happen

You could say it "goes/comes from somewhere else", relegate the problem to "undiscovered physics". If this okay for you, then spacecraft projecting an "inertia reduction field" lose some apparent mass but continue at the same speed. This might be useful if the rocket part of the ship pokes out of the field and the reaction mass is allowed to regain and exchange its normal mass-momentum.

The effect is like having a hammerspace to "hide" some of the fuel mass in which helps to increase the efficiencies of normal rockets.

Conserving 4-momentum

You can attempt to conserve both momentum and energy by retaining momentum and paying an "energy debt". To conserve momentum, $v$, goes up when $m$ goes down, such that $p$ is always constant. Switching on an inertia reduction field causes the ship to immediately accelerate to retain a certain quantity of momentum.

However, kinetic energy, $\frac{1}{2}mv^2$, is not conserved. It's easy to see why: kinetic energy increases with the square of velocity while momentum increases linearly. If mass is reduced by half and velocity is doubled (as per the rule of momentum conservation above) then kinetic energy is also doubled.

Where did that energy come from?

Well, we can pay for that energy through an "energy debt". To establish an inertia reduction field the energy debt in Joules must be paid, which is the difference in kinetic energies between start and end states:

$$KE_{debt}=\frac{1}{2}\left(m_{0}v_{0}^{2}-m_{1}v_{1}^{2}\right)$$ $$\text{or,}$$ $$KE_{debt}=KE_0\left(1-\frac{1}{u}\right)$$

Where $r$ is the factor by which inertial mass is reduced, such that $m_1=um_0$ and $v_1=\frac{1}{u}v_0$.

The inertial reduction device converts electricity into kinetic energy (by mystical means), thereby conserving energy and momentum. By the same principle, the device returns the same energy once the field is switched off.

The device receives power from an onboard reactor and then later returns it once the field is dropped, the field acting something like a battery. This allows the ship to establish a field and move whenever it has a store of electrical energy by paying the debt/discharging the capacitor banks, and then recharging the capacitors with the returned energy once the ship stops.

(If we're increasing inertial mass, rather than decreasing, the device actually dishes out energy at first, and then later demands it back when the field is to be dropped. Make of that what you will...)

Problem with that...

There's a big glaring problem with all that. Measuring the momentum of an object requires choosing a frame of reference. There are unlimited frames to choose from, and none are better than any other. So, when the field is established, in which direction does the ship gain velocity? Any change in velocity violates momentum conservation in all but the chosen frame.

You could assert that the field effect only works in one particular frame, maybe a frame that's important to everyone, such as the solar barycenter. Something, something, local space-time curvature gradients.

Unfortunately, this is where it falls apart, and as far as I can tell there's no reconciling it.

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    $\begingroup$ Some of this is similar to Smith's concept of the inertialess drive in the Lensman series: at some point, you had to pay the piper. A ship retained the inertia it had when it switched on the field ("went free") when it was turned off, which meant that if you were transferring personnel (or large masses) between two ships, they had to turn off their fields, match their velocities in Newtonian realspace, transfer, and then go their separate ways. Otherwise, chunky salsa for the transferee because ships might have had differences of kilometers per second in relative velocity. $\endgroup$ Commented Mar 30, 2023 at 20:05
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    $\begingroup$ @KeithMorrison Sounds similar to a "warp field" metric I once read about. Space inside a warp field (causally disconnected) preserves its original frame. A warp field hovering in a gravitational field, opposing its acceleration, "accelerates" the frame it contains. Something, something, conservation laws. When the field shuts down, anything that was situated inside suddenly exhibits the velocity of its initial frame plus the time spent hovering in gravity as actual velocity. In short, put a boulder in a warp field for 1 day, switch it off, and get the kinetic impact that killed the dinosaurs. $\endgroup$
    – BMF
    Commented Mar 30, 2023 at 20:16
  • $\begingroup$ @BMF In regards to your comment on 'hammerspace' I do have an idea in that regard, since for FTL I'm using the 'gateway to another dimension' kind of travel. So that could probably be my hammer space. $\endgroup$ Commented Mar 30, 2023 at 20:20
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    $\begingroup$ If we're increasing inertial mass, rather than decreasing, the device actually dishes out energy at first, and then later demands it back when the field is to be dropped. <- If modern banking as told us anything, some wanker will just keep turning it up and up and up and never bother to turn it off. "Don't touch that button" "Why not?" "Well... it will cause all the nearby stars to collapse in on themselves for starters"... $\endgroup$
    – Nosajimiki
    Commented Mar 30, 2023 at 20:21
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    $\begingroup$ @JustinThymetheSecond IIRC, the Higgs is only responsible for 1% of the mass of baryons. The rest comes from the binding energy of the strong force. $\endgroup$
    – BMF
    Commented Mar 31, 2023 at 17:42

There is more than one way to balance a momentum equation

The kinds of engines that we know how to make today work by adding energy to a system which increases the velocity and momentum, but there is one very often overlooked variable here because we don't know how to manipulate it: the speed of light.

Momentum = Mass * Velocity = Energy / (Speed-of-light^2) * Velocity.

If your field generator increases the speed of light without actually adding any energy to the system, then your Velocity will increase, but your momentum will stay the same. This means that a ship moving at 5km/sec could be sped up to a 500km/sec without adding any energy to the system. In this sense, you're not so much "dampening" your inertia so much as maintaining it while moving faster.

This solves a few very important problems with your idea:

I don't want to accidentally create a perpetual motion machine.

It multiplies how fast you move, but you still need to invest in moving and overcoming resistance. As long as this "Speed-of-light" stuff is in some way finite or requires some input to harness, you can't just accelerate forever. But more importantly, the system does not create any more energy. Each interaction with the outside world reduces your momentum by the same % as it would if you had a lower speed of light. So, if you tried to turn a wheel with an accelerated motor for example, it would not put more energy into the system, just alter the "mechanical advantage" in more or less the same way you do with levers, gears, or pullies

... or a planet busting superweapon

This is the real problem with most FLT/relativistic drives. But with this, your 500km/sec ship will NOT actually hit any harder with its Aetheric Field Generator turned on than it would moving at 5km/sec with it turned off because its momentum is unchanged.

  • $\begingroup$ Thanks, though to clarify the aetheric fields aren't used for FTL drives in the way Mass Effect uses. The FTL is more the kind where you open a gateway to hyperspace like in Halo or Star Wars. $\endgroup$ Commented Mar 30, 2023 at 20:16
  • $\begingroup$ @AllSeeingEye33 If you have that kind of hyperdrive, do you actually need inertial dampeners then? $\endgroup$
    – Nosajimiki
    Commented Mar 31, 2023 at 15:23
  • $\begingroup$ Sort of, for interplanetary travel, space battles, and other assorted uses. Hyperspace travel exists, but isn't casual and exists for separate reasons. $\endgroup$ Commented Mar 31, 2023 at 16:01
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    $\begingroup$ @AllSeeingEye33 In that case, this should still apply. While you could theoretically go FTL with a system like this, as long as '"Speed-of-light" stuff is in some way finite', you can still use fuel storage, scarcity, and/or diminishing returns to limit this to whatever you consider a reasonable speed limit. $\endgroup$
    – Nosajimiki
    Commented Mar 31, 2023 at 18:53

When implementing technologies that manipulate properties and interactions around mass, even when mass can seem to be changed, an equal but opposite change appears elsewhere and can even cancel out the effects.

This will be an issue for ships carrying a generator. Imagine coming to a near immediate stop, using the field as a damper. If the field absorbs that inertia, the field itself may gain too much energy and the whole of the ship and generator will experience the full impulse of the break.

One option is to reimagine this tech to scale. Perhaps the universe is filled with field, but ships tap into it using some sort of engine. A downside to this could be that, like antennas tap into the EM field, ships can experience interference from waves/particles in this field coming from other massive objects or ships braking nearby. An extra boon would be that more portable versions if this technology can be used in weapons or personal vehicles.

You should also consider the reverse. If using the technology for inertial damping, what are the applications for acceleration?

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    $\begingroup$ "Perhaps the universe is filled with field" A universal 'everywhere' field implies that there is a fundamental relativistic viewpoint that pervades the universe - the static all-present-everywhere field that all other relativistic viewpoints are mapped on to. This field would now be relative to everything. Velocity and acceleration would be fundamentally relative to this field, if it were involved with inertia. $\endgroup$ Commented Mar 31, 2023 at 15:14

Interacting with an as-yet-undiscovered field sounds like a viable option. It's neat in that you can handwave away all the seemingly physically impossible phenomena but you can do even better: you can make up the rules of your field.

And I don't mean "conjure up all the equations", nobody would be interested in that anyway, but treat it as some kind of semi-logical magic.

So for example:

  1. Interacting with the field always requires prior input of energy. Think of it like "activation energy" in chemistry. This helps with "infinite free energy" type questions. Your car mechanic in their garage won't be able to produce enough activation energy to significantly affect the mass of the planet for example. (But expect a spate of newspaper stories about bored, middle-aged men thinking it'd be a good idea to get themselves and a crate of beer to hover and getting it wrong, this time without helium balloons.)

  2. You can even make the energy requirement scale non-linearly: you can still make a reactor that produces enough energy to manipulate a spaceship's inertia but not for planets. Weapons are eminently possible, as the generator is in the weapon, not the projectile.

  3. Drawing energy from the field depletes it locally. Again, this protects against infinite free energy but it also means that interacting with the field makes waves. Waves that can be detected.

  4. If you leave an inertially manipulated system alone, it will return to equilibrium over time, as the energy levels equalise between the coupled fields.

These are only a few examples of plausible-sounding limiting effects, but how many, if any you want to adopt (and then how many of those you want to reveal!) can be dictated by what kind of narrative you want to weave around it.


First, there is nothing wrong with perpetual motion. What IS controversial is a Perpetual Motion Machine - something that will do work without any energy input (Zero Point Energy notwithstanding). And here we get into the quantum mechanics decoherence conundrum - how does one know that something is happening perpetually, without observing it? And as soon as one observes it. you have the properties of a machine. Energy out (whatever is used to observe it) requires energy in. There are many hypothetical quantum systems that indeed can be perpetual until decoherence.

Second, as for a solution/answer, the first step is to recognize exactly what 'inertia' is. Not an easy task, since the physics mechanisms behind it were not known until a few decades ago. We knew what inertia is in terms of it's effects and application, just not what creates it. This was solved with the discovery of the Higgs Field and Higgs Boson. Without these, there would be no inertia. There is nothing else in any other physics concept that demands 'inertia' be a 'thing'.

Thus, any 'inertial dampening' would, methinks, have to be somehow related to either the Higgs Boson or the Higgs Field, and/or the interaction between them.

It seems to me that if the Higgs Boson and the Higgs Field were 'frozen' together such that, as the Higgs Boson speed or velocity were changed, so too the Higgs Field altered synchronously. Much like electromagnetism - a wire moving through a magnetic field causes an induced current to flow. But if the magnetic field and the wire movements are synchronized, there is no induced current. If the Higgs Field and Higgs Boson are similarly synchronized, there will be no inertial effects,

So if there were some mechanism for completely containing the Higgs Field within the confines of the space/time of the spaceship, such that all of the Higgs Bosons within the spaceship were synchronized with the displacement of the field, no inertial effects.

In broad terms, it would be like the Alcubierre warp drive, in that a local bubble of space/time would be contained in an 'inertial field bubble' such that all Higgs Boson-Field interactions would be completely contained and localized in its own relativistic bubble.

This does not really violate any principles of physics as documented in the newest thicker physics textbook, as long as there is sufficient energy being put into the system. There would hypothetically be a LOT of energy required to contain the Higgs Field (for your purposes, 'exotic energy' as a word salad choice?) and manipulate it appropriately.

If the Alcubierre drive is acceptable as a science fiction trope, so too should this Higgs inertial dampening system be acceptable.

See, for instance, this article on the recent musings over the Higgs Boson/Field.


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