I have a regular planet with regular gravity that pulls all on the surface towards the center. In almost all ways, it is identical to our own Earth.

The major difference is that, beneath the south pole, a couple dozen kilometers below sea level is the "demon core" and this attracts "demons". These creatures interact with matter in a completely mundane manner, the only exception being that they are not affected by gravity, but rather pulled towards the "demon core" as if it were the center of the Earth with a pseudo-gravitational force. The difference is that unlike the Earth where gravity is net zero at the center, this "demon core" is assumed to be a point-source. Here a quick diagram:

diagram of gravity

Effectively, this means that almost everywhere on Earth "demons" seem to stand in a slanted way or at an angle when they walk around, and the only place they appear to interact with gravity correctly (albeit slightly weaker) is at the north pole because the gravitational vectors line up. For demons, areas near the south pole are incredibly dangerous because the ocean's surface is "steep" enough that they can fall down it and skip across the water until they come to a (usually violent) stop. No other matter besides demonflesh is pulled towards this demon core, so if they were to wear mundane clothing, for example, it would look quite bizarre as it would still be under the influence of regular gravity.

The Question:

Is there any physics "gotcha" I need to watch out for with such a setup? For example, would it be possible to make a perpetual motion machine somehow by putting a demon on a properly positioned Ferris wheel or something? I suspect not, as this demon-gravity is still a conservative force, like gravity, but I'm not sure.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Jun 3, 2022 at 19:34

10 Answers 10


Is there any physics "gotcha" I need to watch out for with such a setup? For example, would it be possible to make a perpetual motion machine somehow by putting a demon on a properly positioned Ferris wheel or something?

Nope, you're good

This is not much different from magnets and ferromagnetic metals - supposing you have a huge magnet or an Oscar Isaac-amped Michael Fassbender at the south pole.

We can't do it with magnets, we won't be able to do it with demons.

Now of course, being close enough it would seem that your demon core would work like a magnetic monopole, since it is a point source. Still it wouldn't allow for perpetual machines. I saw someone commenting somewhere else in the internet that with a monopole you could have a DC motor without a commutator, which is an interesting application, but still has nothing to do with perpetual machines.

What you need to watch out for is that there might be a huge area of exclusion for demons. It is exact as you mention for one approaching the south pole, but it would extend far beyond it. A round'ish, not so dense demon left on the equator might reach the south pole in finite time by a combination of rolling and floating, and the closer they get, the faster they'll go and the harder it'll be to reverse their motion.

Edit: it just came, this opens up a new branch of physics called

Orbital demonics

I wish I could say this is not rocket science, but it is. I'll spare you the math but any demon accelerating towards the demon core from hundreds of kilometers or miles away and bypassing it - or being deflected by it - will go really really far past the horizon, possibly exiting the atmosphere.

Those who don't hit anything along the way will either eventually fall back and impact against the ground, possibly restarting the cycle, or given enough extra push from other sources will escape the planet (actually the core) altogether.

Those over escape velocity who hit something on the way after going past the demon core may be slowed down to orbital velocity. They will orbit the demon core rather than the planet's center of mass, and this is where things get interesting and bizarre.

Regular satellites will always cross the plane which contains their parent body's equator. This is not the planet for your demons, so if they have zero or little north-south movement when they start orbiting they will circle around the south pole in space, but never going too far away from it. This is just not possible for a satellite in real life and may have interesting applications. In real life when we want something over the poles we need a polar orbit, which means always visiting both poles in an orbital period, which is wasteful if you are interested in only one.

If their orbit has a considerable angle relative to the equator, though... If is it much larger than the planet they may be safe, but most likely it will intersect the planet (that is, it will be a pagh that passes through the planet). This means they will eventually hit the ground at bat out of hell speeds. We're talking about multiple miles or kilometers per second at the very least. Whether they bounce back, become a huge puddle or something else entirely is up to you.

With a caveat though

This is considering ideal conditions. Most likely, since your demons interact with matter, they will be slowed down by air. They will eventually reach a terminal velocity orders of magnitude less than escape velocity. The visible effect would be demons skidding towards the south pole at speeds comparable to that of an airplane. It would still hurt a lot to stay on their path.

Going further and beyond

I specially love this comment from Shufflepants:

I however imagine, because only demons are attracted by the demon core, there might be a potential for reducing the fuel cost of launching regular satellites into orbit by including some demons inside your launch craft, accelerating up and towards the south pole, and then jettisoning the demons as you pass over the south pole.

  • 3
    $\begingroup$ But hey, you can make deals with them that do not involve selling your soul: "I'll carry you away from the core and help you avoid a horrendous death at terminal velocity if you..." $\endgroup$
    – PipperChip
    May 31, 2022 at 15:37
  • 2
    $\begingroup$ I hope you take this answer as permission to put demons on Ferris wheels. $\endgroup$
    – Willk
    May 31, 2022 at 15:59
  • 10
    $\begingroup$ It would also be like an electric monopole, which is a totally normal thing that exists and creates no problems whatsoever. $\endgroup$
    – user253751
    Jun 1, 2022 at 9:49
  • 10
    $\begingroup$ I however imagine, because only demons are attracted by the demon core, there might be a potential for reducing the fuel cost of launching regular satellites into orbit by including some demons inside your launch craft, accelerating up and towards the south pole, and then jettisoning the demons as you pass over the south pole. $\endgroup$ Jun 1, 2022 at 16:05
  • 4
    $\begingroup$ The interesting thing about orbital demonics is, that any spacecraft with demon flesh on board will see an effective gravitational field that is not spherical. For instance, you can have a satellite orbit above any fixed southern latitude by selecting the correct ratio of metal to demon flesh onboard the satellite. Excellent for espionage satellites against Australia. On the other hand, polar orbits would grant demon flesh enriched satellites a lower speed above the northern hemisphere, while zipping quickly across the (uninteresting) southern hemisphere. The possibilities are endless... $\endgroup$ Jun 1, 2022 at 23:53

Be aware that the demon core must be attracted towards demons with an equal and opposite force. Imagine two demons standing at the equator on opposite sides. They each are pulled (locally) down and south. The down forces cancel; the south forces do not. For everything to balance there must be a northward force exerted on the demon core. Depending on the number of demons and where they are, this could be significant.

This puts a lower bound on how large the demon sphere is, lest it get pulled down through the rock. The compressive strength of rock varies, but let's ballpark it as 200MPa.

And let's say the sphere pulls demons with the same force as Earth, were it a point source (so 1g at 1 Earth radius, and inverse-square from there).

If there's a million demons - ballpark as 100kg each nominal - standing on the North pole... Each one is 2r away from the core, so 1/4g, so the core is pulled North with a force of about 250 mega newtons.

Dividing this through, we require - in decent rock - a cross-sectional area of the sphere of $\ge1.2m^2$, lest the sphere get pulled down through the rock.

Ditto, consider a hapless demon at the South pole. It's ~24km away from the core itself, so it now weighs, what, 7000 tons or so? Something silly like that. Anyway, the minimum cross-sectional area of rock required to hold said force is a 58cm square. This means that it's entirely possible that said demon would be pulled through the ground and rock towards the core. (Releasing an awful lot of energy in the process, of course.)

There's also the observation that if a demon ever managed to make it to said demon core, it'd release an "infinite" amount of energy. This tends to make physicists sad. You can alleviate this to an extent by having demons attracted to the entirety of said sphere - although be aware that even with a ten-meter sphere the energy release would be... significant. Something like a megaton of TNT equivalent per demon. Which could easily damage enough to allow other demons (previously squashed against nearby bedrock) to fall to said sphere... it's the iron catastrophe in demon form!


Likely Fine

Yes, having an effective "second" gravity opens up a lot of possibilities, but our current understanding of the universe should not raise any objections. The demons need to work against their gravity to gain more potential energy, just like we do.

There may be some ramifications when we get into deeper physics, especially Relativity. Everything seems to respect curvature of space, even light: so why not demons? Are there demon black holes, and what does that look like? Do they have the same fundamental limits, like the speed of light? (All of this is likely irrelevant to your story, though.)

Why are demons subject to this force, EM, strong nuclear, and weak nuclear forces but not gravity? Can we force matter to switch from one to another? If yes, then we really have some grounds for "physics breaking" shenanigans.

  • 1
    $\begingroup$ If force matter switching from one to another uses energy equal to the difference in attraction, it's fine. See: electrically neutral objects can become electrically charged (and therefore subject to "electric gravity") but you have to do the work to move the charges. $\endgroup$
    – user253751
    Jun 1, 2022 at 9:51
  • 3
    $\begingroup$ Dark matter is actually demons! $\endgroup$ Jun 1, 2022 at 18:19

Assuming that the demon core has a similar pull to that of earth (ie its "demon mass" is the same as earth). Then there may be some issues.

Firstly you must realize that the 'demon gravity' experienced by demons is going to be vastly different depending on location. At the north pole about 13000km away from the core, their experienced gravity will be about 2.4 m/s^2, At the equator, about 9000km away from the core, about 5m/s^2 and at the south pole about 24km away from the core, about 700000 m/s^2.

All of this has major implications as if you have demons living on the north pole or anywhere north of the equator, they will be incredible weak physically compared to humans who have to live in much stronger gravity. It also means there is no chance of demons ever being able to go to the south pole, they and their mode of transportation would be crushed under their weight as they approached the south pole.

The next problem is if a demon, or parts of one, were to approach the point of the demon core, gravity would essentially be infinite. What that does to demon matter, I don't know. What if you had normal matter wrapped in demon matter?

  • $\begingroup$ I suspect the demon core isn't normal matter. $\endgroup$ Jun 1, 2022 at 15:33

Perpetual motion

You probably have a Perpetual motion machine. This is because demons have to eat, defecate or die. At a certain point this requires a change from normal to demon, or from demon to normal.

Let's imagine no change happens. If the demon eats, it'll replace parts of itself with the sustenance. It can't only eat demon, so it would replace itself over time with earthly material. This removes the demon idea you have going.

If it does change, you can gain or remove energy from the system. Say one is close to the south pole and eats something. It becomes demon, suddenly gaining a lot of attraction to the demon core. You let this energy out by moving to it, gaining friction, and thus heat and motion energy. Then it defecates. The result is normal material again, which can be transported up with low energy cost.

The easiest to imagine it is if the demon is an astronaut above the south pole. It eats a lot, after which it crashes down with the high gravity. After it has done it's business you travel back up the lower normal Earth gravity with food, repeating the process. As the food goes up with 9m/s gravity and comes down at (as some estimate) 7000m/s gravity, you're making energy.

You can solve this by having the demons never eat, drink or otherwise gain Earthly materials, but this would be strange if they weren't at least breathing. The other is to have them eat, drink and breathe, but always expell whatever they take up. This has many more implications, like demons not regenerating. That means they can't even take the slow sanding of the skin over time. If they magically regenerate, you have a Perpetual motion again. As mass can be added and removed for extra energy.

Even in death this is a problem. If a demon is killed and doesn't revert, you'll get more and more demon essence. If it disappears or something, you can create extra energy by having demons spawn in, pushed and killed at the south pole and disappear again.

Not as bad as you think

Might not all be bad. Even if such Perpetual motion is realised by some people, it might be hard or impossible to utilise. In addition, it might be impractical to gain useful energy from it.

Though it breaks the laws of physics, so does having demons and a demon core. The impractical nature prevents useful usage of the energy. Even if you did find a practical way to harness such power in large quantities, how would you put such plans into motion?

  • $\begingroup$ You do need to go faster than 9.8m/s to get liftoff, but the idea of dropping to the correct area while eating food to gain (or lose) energy in the system seems sound $\endgroup$
    – Demigan
    Jun 2, 2022 at 8:14
  • $\begingroup$ @Demigan you need 9.8m/s for food, 7000m/s for the demon. On the way back it's all 7000m/s, so a big net gain on the food. $\endgroup$
    – Trioxidane
    Jun 2, 2022 at 12:02
  • $\begingroup$ Good catch on the eating thing! I'll need to think of something... $\endgroup$
    – Dragongeek
    Jun 3, 2022 at 16:01

One thing missed by others is that it breaks General Relativity. If demons interact with matter they need to have some inertial mass. If they are not affected by gravity they don't have gravitational mass.

General Relativity does away with the notion of gravity as a force. Things are not attracted by other objects - the objects curve the spacetime and objects 'falling' just move along straight lines in curved space. Because it is not a force but a consequence of spacetime you can say that inertial mass and gravitational mass must be the same (to oversimplify a bit).

But in your example system they are not. So General Relativity cannot be true in your universe (or rather it cannot explain all known phenomena). That raises the question if gravity behaves classically in your universe (just plug in a 19th century understanding of gravity and mechanics and you should be good, maybe with quantum physics - but no GPS for you) or we still get predictions of General Relativity but we know it cannot be correct as it doesn't work on Demons.

The good news is that we already know that Quantum Physics and General Relativity cannot be both correct. So you can just handwave the question of 'what happens if you accelerate a demon to relativistic speeds' or 'how does spacetime work' with 'good question - physicists would love to put a demon in a particle accelerator but it is not there yet'. If demons were discovered by physicists recently you can comment that they know that General Relativity is not correct but they have no idea exactly how.

  • 4
    $\begingroup$ "but no GPS for you" - as long as radio propagation speed stays the same, GPS would be easier. All the work we have to do right now for time dilation adjustments and so on mostly goes away (except Doppler, he gets a pass). Trilateration is still hard, but totally doable. $\endgroup$
    – Corey
    Jun 1, 2022 at 23:19

Not locally, but it may have affects on the cosmos.

Gravity, matter and energy are all connected by the medium of spacetime. Matter and energy are the same stuff, just more and less organized respectively. Gravity is a side effect of matter - the more that densely organized matter is collected, the more gravity surrounds it - and time is a side effect of gravity. This part is weird, but basically, the more gravity something is exposed to, the more time passes for it.

For demons to not be affected by gravity suggests that they exist in a different, overlapping system, made up of an entirely different sort of material - lets call it demon matter. Demon matter also exists in the medium of spacetime, and I think it's okay to say that it can overlap with or interact with matter and energy as we know it while still mostly ignoring gravity. Light hits them and bounces off, people can touch them or be touched by them, but gravity doesn't really care about them. Probably what you would find is that demons are effected by it on a very small scale that ends up rounding down to zero in all but the theoretical sense.

So, collections of demon matter create demon gravity that affects other demon matter. So far, so good. The thing is, that gravity should also create its own independent time. Demons would have their own sense of time relative to how much demon gravity affects them. Locally, this is still fine. You can just say that the concentration of demon gravity is close enough to the concentration of regular gravity around the planet to exude a similar field, thus leading demons to have about the same experience of time as everything else on the planet (though, allowing for small differences, letting them experience things slower or faster, could add to their demonic feel. it would make it harder for them to recognize or communicate with humans as equals, and could be the basis for powers they have that humans don't)

The biggest question mark for me is the cosmos. What does the rest of the solar system, the universe, look like if this is true? If it's happening here, it must be happening in the rest of the universe as well. If there is Demon matter and demon gravity, is there demon energy - demon light, with its own independent speed of light? Demon radiation, with its own variety of wavelengths? Having its own gravity, does demon matter have other properties - magnetic feilds, ionic bonds, etc.? Are there Demon Black Holes, Demon Stars, Demon Galaxies? If Demon matter can experience time differently from other matter, while taking up the same space, what does that look like? If demon matter can move faster than the speed of light for regular matter, does it break laws of causality?

I don't think you need to answer all of these questions to write your story. But especially if some characters in your story are science literate, having some suggestion of demon matter outside of earth will help readers to suspend disbelief. Considering how much we don't know about the cosmos already, I think its perfectly fair to leave many of these questions as mysteries or with half answers that serve a practical purpose but aren't fully satisfying. Lots of what we currently know about the cosmos is exactly like that - unsatisfying. That's why we keep pushing to know more.


This works fairly well, but there are some details.

To make the demons stand up consistently under gravity at any point on the planet, we want the force to not be inverse square. There is a precedent for this with strong nuclear force. This is a very different situation, but unknown physics are ... unknowable. If you go this way, the demons fall straight down toward your core even from planets around other stars.

Making the demons not be affected by normal gravity is the real sticking point. If the demon starts falling toward the demon core, you expect it to accelerate at some rate - the force being divided by its inertial mass to get acceleration. There is a big angels-on-a-pin discussion to be made about whether inertial mass is always the same as gravitational mass, and if your physicists see something like this, they will be forced to pay attention. You could avoid that if the demons are just really light, but batting demons around like a whiffle ball is not the way I'm expecting you to go with the story.

Don't sweat the conservation of energy. You can calculate exactly how much energy it takes to lift the demon to any point (it's just the distance times the constant force the way I pictured it), and that doesn't depend on the route, so no perpetual motion. If the demons have any gravitational mass, you'd have to calculate that also, but it would again be a gravitational potential not depending on the route taken.

  • $\begingroup$ Couldn't there be a middle ground of having the force decay linearly, or by some other exponent between 0 and 2? Linear decay would mean your weight (if you're a demon) would still differ by a factor sqrt(2) between the north pole and the equator and demons would still be killed at the south pole, but you avoid the most extreme effects of a force that increases quadratically as you approach its source. I still think it would work with exponent 2, but if you're going to change it, is really 0 the only other option? $\endgroup$
    – EdvinW
    Jun 3, 2022 at 7:57
  • $\begingroup$ I have to admit that I never truly grokked why the strong force is linear in the first place - if somebody wants to help me out, please feel free! But the strong force does become less than constant once multiple quarks are involved, because the ones nearer cancel out and shield the ones further away. $\endgroup$ Jun 3, 2022 at 23:38

Under the currently popular models of phsyics there are two forms of matter: normal (for us anyway) and dark. Normal matter is all the physical stuff that shows up in the Standard Model: electrons, protons, neutrinos and so on. Dark matter - if you believe in the stuff - is a form of matter that doesn't interact much with normal matter but still warps spacetime, thus affecting gravity.

What you have is a third class of matter that strongly interacts with baryonic matter but doesn't interact with our spacetime. Instead it warps and moves relative to some other spacetime matrix that exists alongside our own. Perhaps the particles of demonic matter don't interact with the Higgs field but with some other field that only demonic matter can interact with. The other basic fields work the same, which is why we can interact with demonic matter at all. In fact apart from gravity, everything else works just the same as normal matter interactions.

Which is a problem for your demon core because although it doesn't exert gravitational force, it still has all those other interactions. You have a ball of extremely dense demonic matter with about the same effective mass as the Earth, surrounded by normal matter that is constantly accelerating in the direction of the Sun. As the Earth proceeds around its' orbit it now has to provide enough force to push the demon core around with it, and all that force has to be applied on the very small surface area of the core... and thus on the very small amount of material pushing on the core.

I give it a couple of days before it rips a spiral channel through the Earth and wanders off into space.

To balance this we need a source of demonic gravity the same size as our Sun, but made out of stuff that doesn't interact with normal matter and only interacts gravitationally with demon matter. Now we have a fourth type of physical stuff in the universe: dark demon matter! OK, we still can't explain why it would just happen to follow the same path around the galaxy as our sun does, but let's hope like hell it keeps doing so otherwise Earth's getting an unexpected south polar reaming.

Under these conditions the conservation laws are preserved, so there's no free energy to be had. The fact that some matter has a skewed gravity vector compared to others is interesting, but only as long as it doesn't mix too heavily with normal matter.

Which is a real problem if organic demonic life is a thing.

Let's take a human and flip all her subatomic particles for the demonic equivalent. Assuming she doesn't die during the process, the only difference is the pull of gravity. She takes a breath of normal air, her body uses combines it with demonic carbon atoms and breathes out hybrid CO2... which immediately starts heading south for the (insert season here). Over time the south pole will be covered in a miles-thick high density cloud of CO2 composed of demon carbon bonded to normal oxygen. The oceans in the area turn to carbolic acid and all oxygen-dependent life in the south polar region dies horribly. Meanwhile the reduced availability of normal oxygen in the rest of the world causes problems for the other inhabitants of the planet.

Similar things start to happen with other metabolic products. High quantities of demonic matter in the body's waste products cause problems for waste processing systems, piling up on the side of sewers and treatment tanks or floating above (and to the side of) the heavier normal matter in northern latitudes.

Meanwhile the demonic matter in our test subject's body is being replaced over time by normal matter, with similar problems. Intracellular structures start to differentiate because of the difference in gravity strength and direction between the different types of matter until the delicate balance of the cells fails and they die. Eventually all of the parts of the body that age out and get replaced - which is most of the body except for things like bones and a few other bits - will be replaced with a combination of demon and normal matter. Sadly the problems caused by the different gravitational forces each type of matter experiences will likely kill the subject long before her body reaches equilibrium unless she's luck enough to be in that sweet spot a couple thousand miles above the South Pole where the gravity vectors are equal.

On the plus side, the availability of materials with different gravitational alignment makes it possible to build taller structures at the north pole. Much taller. Given a few decades we can build up a few miles off the surface using a demonic alloy that thinks it's on a low-gravity planet. We can build launch vehicles with the same materials, allowing us to get a payload off the ground much cheaper than equatorial launch facilities, even with having to burn fuel to get up to orbital velocity. Orbital mechanics will get a bit screwy with having to worry about hybrid materials being pulled in different directions depending on their purity, but that's what computers are there for I guess.

So the best we can get is a bit of a break on launch costs and a few other similar gimmicks, at the cost of potentially ending a substantial amount of life on the planet.

I vote no.


When you compare normal gravity to demon gravity, the different gravitational origin point is going to impact two things: magnitude and direction. For both of these cases, your latitude is going to be important.

As far as magnitude goes, 45ºS latitude is the point where demon gravity is equal to regular gravity. Your distance from both center points is the same. Everywhere north of here demon gravity will be weaker, and everywhere south it will be stronger. A demon on the north pole would feel 25% of the gravitational pull as they would at 45ºS. Only the tip of South America and a wee bit of New Zealand extend below 45ºS, so a demon would generally feel less gravitational pull than a human at the same location. The most irritating part of this is the gravity gradients (the change in potential energy needed to travel between two points). A walk over flat terrain is easy for a human but might be a fairly steep hill for a demon, or vice versa. Humans can at least see the ground sloping away from them. Without those visual clues, demons would likely be on the clumsy side.

Regarding the orientation of gravity, the difference between the two gravitational vectors will be zero at the north pole, 45º at the equator, and nearly 90º at the south pole. Demons in Iceland would walk nearly vertically. In Italy they'd be leaning about 25º off the vertical (almost like the demons themselves were written in italics, ha!). By the time you got to the Tropic of Cancer, you'd be tilted at such an angle (>33º) that you'd start to have practical problems. The force of friction between you and the ground at that point would be $sin(90-33) = 54\%$ of what a human of the same weight would experience. Lower gravitational force means you'd weigh less than that human, making the problem even worse. Demons would have serious traction problems just walking around, and slippery surfaces would be downright impassable. Cleats would be standard footwear for demons, and they would greatly prefer being outdoors on soft dirt over indoors on hard floors. By the time you hit the equator you'd be slanted at 45º, and passing through a door would be less walking and more acrobatics. How far south a demon could realistically go would depend on their footwear, their athletic ability, and the local terrain. Practically speaking, however, demons in the southern hemisphere would be rare.

These two things together open up an interesting possibility. You can use Antarctica as a demon prison colony. Gravity has an inverse-square relationship to distance so it would be unbearably strong at the south pole. Gravity's vector relative to the terrain would be so severe that demons would have close to zero traction. Escaping would be like scaling a nearly-vertical, icy-slick cliff face that's more than 2000km tall. Escape would require some sort of powered vehicle, and that's just to get to the Antarctic coast. Gravity is still so strong (and in a bad direction) at that point that you wouldn't be buoyant, so you'd need some sort of ship to get you back to civilization.

If you want to make this scenario less extreme, move the center of demon gravity farther into the planet. Moving the demons' gravitational point 1/3 of the way to the surface instead of all the way will still have noticeable consequences but would avoid many of the extremes. Just whatever you do, don't move the center of gravity above the surface. If you somehow get the two centers of gravity working against each other (meaning the angle between the two gravity vectors is more than 90º), you start opening up the doors for all sorts of weirdness.


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