In my world, interstellar vessels use a drive whose design mankind learned from an extinct elder civilization. While construction of the drives is possible, the physical principles making them work are beyond mankind understanding. The drives have a number of curious properties, which I need them to have in order to prevent too many secondary applications of the technology. Additionally, there are a number of properties I gave them because since handwavium based technology is essentially magic, I might as well apply Brandon Sanderson´s Laws of Magic to it in order to keep stuff interesting. These limitations are beginning with the first category:

  • no real exhaust plume This does not mean there is no exhaust plume, but that it consists of particles that either decay quickly after leaving the engine or won´t normally interact with matter. (I´m still somewhat undecided here, so any particle fitting this description can be chosen to explain the drive.) This condition exists in order to avoid relativistic matter beams or death ray photon rockets capable of punching holes through gas giants.

  • the faster the vessel, the higher the power output The drives need to be supplied with energy in order to work, but unless they move faster than $0.15 c$ relative to their point of production no thrust will be produced. The trust increases with the speed roughly following $F = 1\,000\,000\,000 \times (23.81 \times v - 3.571)$ with $F$ thrust in newton and $v$ being velocity given as a fraction of lightspeed, meaning the maximum thrust that can be gained is about $2\times10^{10}\ \mathrm N$. So an average interstellar vessel of $1\times10^6\ \mathrm t$ moving near the speed of light can achieve an acceleration of about $20\ \mathrm{m/s^2}$. Vessels get up to a speed where the drive works using laser pushed sails. This is meant to prevent the drives from making other types of energy production obsolete and to be used in systems for energy production.

  • the alignment of the drive to the vector of motion matters If the drive structure is aligned to the vector the vessel moves along orthogonally the energy output is at its maximum if it is aligned parallel to the movement vector it reaches zero. This is meant to show that a specific volume of space is harvested for energy and to limit the maneuverability of interstellar vessels. The idea of harvesting a specific volume of space is also related to the point mentioned before, as the velocity the vessel moves at would increase the volume the machinery can cover per second. However, the harvesting aspect isn´t essential and can be swapped out for a more plausible explanation of these features.

These three are crucial and must be considered in the answer. The second category contains limitations based on

The limitations of a magic system are more interesting than its capabilities. What the magic can't do is more interesting than what it can.

These limitations fix minor issues the technology could create and should create interesting plot points.

  • drives hate being active near (meaning ca. 100 AU) objects above Jovian mass or close to each other Hate means that they tend to get unstable in these situations and can explode.

  • while drives are active movement near the drive should be avoided There is no fixed distance, just the guideline of the further away the better. This is the second reason why interstellar vessels are very long and arrowlike and why crews tend to dwell as high up in the vessel as possible. Bonus points if there is a reason why gas and fluid movement in pipes are fine but a human or a machine moving through a corridor is not. Additionally, a very slow movement of about a millimeter per second is fine.

  • weird accidents and failure modes The drive blowing up and vaporizing the vessel isn´t the worst thing that can happen. At least if one is willing to trust blurry telescope observations and spacers yarn. People merging with bulkheads, teleportation, visions, the works. I mostly will mostly explain this with the arcane nature of the technology, but if there is any physical explanation for these effects mentioning them would be appreciated.

Assume you are a physicist who is tasked with explaining this alien piece of technology within the realms of known physics. Known physics means you can use any theory currently proposed or accepted. The first three points must be explained and the last three would just be a bonus.

Any remarks about his being impossible to answer are not relevant since even within the setting the question has not been answered. I´m just interested in how one would attempt to explain the stardrive.

EDIT1: I created this question to solve the rest frame issue.

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    $\begingroup$ "How would a physicist explain this starship engine?" I would call it [expletive in the gerund] magic. $\endgroup$ May 18, 2019 at 17:01
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    $\begingroup$ No thrust unless moving 0.15c with respect to what, there is no such thing as a fixed frame. This statement makes any physics explanation impossible. $\endgroup$ May 18, 2019 at 17:08
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    $\begingroup$ @conman The distance from the engines matters so my wording might have been somewhat ambiguous. Gonna fix that. thanks for pointing it out. $\endgroup$ May 18, 2019 at 18:28
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    $\begingroup$ @conman You can experience 1g (or any constant acceleration) relative to your own frame of reference indefinitely. Relative to the rest of the Universe, well that's quite different. $\endgroup$
    – Spencer
    May 18, 2019 at 19:01
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    $\begingroup$ @TheDyingOfLight No, maybe I'm just completely wrong, and it's perfectly fine. en.wikipedia.org/wiki/Space_travel_using_constant_acceleration $\endgroup$
    – conman
    May 18, 2019 at 19:01

5 Answers 5


EDIT: In retrospect, I think it's possible there is some confusion about what you're asking for.

If you're asking us to help fit your story requirements to real science, that's what I've tried to do below. It's a loose fit, to be sure.

But, if you don't actually care what the purported mechanism is, and you're just asking us to help invent diegetic technobabble -- such as inventing names for the different parts of the engine -- then I don't know how much use you'll find my post. The same applies if you're asking us to help anticipate how a fictional physicist might speculate as to the workings of an engine given only the observations you've outlined.

As I re-read your post, I get the impression this is maybe what you're asking for.

I think the easiest solution here would be to assert that the drive works by interacting with dark matter and/or energy.

For one thing, no real exhaust plume is readily handwaved: the exhaust consists entirely of dark matter, and thus doesn't interact with everyday objects or physical systems.

Second, the faster the vessel, the higher the power output seems to come naturally if we assume the drive has some kind of ramjet-like mechanic. Whatever the process is that the drive uses to generate thrust, it depends on an influx of dark matter or energy. In reality, there is some interesting speculation about the way dark matter is distributed in the real world, but for your purposes the advantage is that you can play as fast and loose as you want -- since it's not directly observable, you can make any assertions you want about whether there's currently too much or too little dark matter present to get the drive going. There will be precious few opportunities for other facts to constrain your choices.

I'm a little less clear on your third rule, but I think a vague ramjet gestalt sets you up to talk about "harvesting" volumes of space.

Why do drives hate being active near (meaning ca. 100 AU) objects above Jovian mass or close to each other? Because dark matter responds to gravity, and is thus likely to pool in very large wells. And while this drive depends on the presence of dark matter, it can't tolerate being drowned in the stuff -- especially when not operating at speed. If you just turn the thing on and then submerge it in a well full of dark matter, it's gonna clog. How does a device behave that's built with exotic materials, when you literally submerge it in one of its key fuels? I certainly wouldn't want to find out. But I bet it's unpredictable and bad. Imagine starting a car engine -- which requires a constant influx of vaporized gasoline -- and then lowering it into a tank of liquid gas. I assume there will be a fire, and the engine will stop operating. I also assume some really weird stuff will happen internally before the fire occurs.

While drives are active movement on the lower decks should be avoided this is a little harder to explain. I think you're looking for a story where there are bad physical consequences for breaking this rule. Since the effect presumably permeates floors and walls, we can reach for some kind of radiation-like effect, or we can reach for localized disturbance of space-time. However, you might consider that the effects are not physical. Perhaps the drive has strange effects on the functioning of the human brain. If space-time is being rapidly perturbed, perhaps it has really strange effects on the human brain: imagine a neuron firing in response to the alternatives of the quantum wavefunction before it actually collapses. Perhaps it's actually pretty tolerable as long as you're basically stationary, but if you start moving around it's like "scrubbing" your mind against a neurochemical washboard, leading to temporary (or permanent) psychosis.

Another interesting possibility is that the "can't go near it" effect is not a necessary consequence of the drive's operation, but a kind of anti-tampering feature added by the designers. Perhaps the elder civilization wants to prevent the engine from being studied closely, to prevent lesser races from figuring out how to weaponize the technology. So they added a entirely separate system that generates this harmful effect. The lesser civilization lacks the scientific understanding to realize that this anti-tampering device is not a critical part of the engine -- so whether you've got us building from blueprints or maintaining a fixed inventory of engines gifted to us by the aliens, we are ignorant of the fact that this nasty effect is actually an add-on.

So, it's kind of ramjet that facilitates ultra-high-speed collisions of dark matter to produce thrust. It accomplishes this by the controlled generation of microgravity waves. Its reactants and exhaust are both weakly-interacting, so it can theoretically be mounted anywhere within the ship. However, the effects of the microgravity on the human mind are extremely dangerous, so it's wise to keep this thing -- filled with dangerous, exotic materials -- at arm's length.

Oh, and it was designed to traverse the distances between stars, not for local travel, so it operates best out in the open at "highway speed." It's usually safe for planet-hopping, but it's literally unusable for station-keeping and extremely dangerous in close range to planets and other large bodies. To say nothing of space cancer.

From a storytelling perspective, it's a decent shorthand to treat dark matter as a kind of physical manifestation of gravity: you are more likely to find dark matter in big gravity wells where it can be held "captive," and less likely to find it between planets and stars. My understanding is that this is not actually true, but it's consistent with some generally-believed and easily understood ideas about dark matter.

Taking your 100-AU no-fly-zone into account, we might want to posit that the drive leverages environmental dark energy -- vacuum energy that only exists in the vast spaces between stars -- to create the microgravity waves that are used to coerce particles of dark matter to collide inside the drive and produce thrust. While none of this is really hard science, if you make the drive dependent upon dark energy, you're effectively saying it can't operate within a planetary system. So, that's an option to consider.

EDIT: if you want to prevent robots from approaching the drive while it operates, best go with an effect that interferes with electrical signals: that will keep away animals, and robots. It could also make it tricky to deactivate the drive: an electrical signal might have a really hard time penetrating the region of the disturbance. Maybe it's controlled entirely with a couple of giant pushbuttons, and we usually attach long metal rods with actuators on the end so we can push those buttons mechanically.

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    $\begingroup$ I would go with dark matter over dark energy. The energy density of dark energy is so low as to be effectively negligible on small scales. Also, dark matter does not "clump" at all near gravity wells. While this may be easy to sell in a book, there are no generally-believe (by scientists) ideas about dark matter that cause it to clump near stars and therefore be more rarefied elsewhere. $\endgroup$
    – conman
    May 18, 2019 at 17:58
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    $\begingroup$ @conman, well, it doesn’t tend to clump around star-sized gravity wells. It does however tend to clump around galaxy-sized gravity wells. $\endgroup$ May 18, 2019 at 20:33
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    $\begingroup$ "neuron firing in response to the alternatives of the quantum wavefunction before it actually collapses" - that's not how quantum physics works. Reacting to a quantum state = observing it = wavefunction collapse $\endgroup$ May 18, 2019 at 20:38
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    $\begingroup$ I like the anti-tampering idea though $\endgroup$ May 18, 2019 at 20:39
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    $\begingroup$ A few jovian-sized objects are absolutely not going to be able to collect dark matter. It would be easier to explain it as sensitivity to gravitational distortions. $\endgroup$
    – forest
    May 19, 2019 at 6:18

As an engineer I’d explain it by showing you the schematics. As a Priest of Our Elder Saviours I’d explain it as Divine Power.

As a physicist? I wouldn’t explain it. Until we’ve developed suitable theoretical frameworks to explain it’s operation any attempt at explanation is the creation of a hypothesis at best (which requires much more detailed knowledge of the construction of/measurements of the device, which obviously I can’t get right now) and wild conjecture at worst. Hypotheses can be tested and models updated until we do understand it, at which point the physics is no longer beyond us.

But you stated in the OP that the physics of it is beyond our understanding. Therefore any physicist claiming they can definitively explain the workings of the drive is either a liar or mistaken. I personally fail to see how the physics would be beyond us for long (We’re pretty good at making models to explain things, at the worst we’ll develop a new branch of science), but with the problem as stated no physicist can give you an honest answer.

Now, any engineer explaining the device will point at the schematics and say ‘Here’s what the instruction manual says it can do’, and so the drives will be usable.

And any Priest explaining the drives will be able to say ‘Divine will’ and so the arcane and terrifying side effects will be explained away in a haze of Faith.

But if the workings of the drive are truly inscrutable then no physicist worth the title will pretend to understand how the damn things work.

  • $\begingroup$ If you aren't going to explain it, what are you doing on this stack exchange posting "answers"? $\endgroup$
    – Innovine
    May 20, 2019 at 10:23
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    $\begingroup$ @innovine: This question does not ask for a model that explains the drive. It explicitly asks for how a physicist would attempt to explain this drive. The answer to the question "How would a physicist explain this thing which is beyond science" is "They shouldn't, unless they can build a model that explains it, at which point it ceases to be beyond science". That's the answer I've given. $\endgroup$
    – Joe Bloggs
    May 20, 2019 at 13:21
  • $\begingroup$ Oh ye of little faith. There are enough information to be scrutable for devising various hypotheses for this drive. Those hypotheses wouldn't be full-blown explanations, but when has that ever stopped a strong-minded physicist? Hardly ever! What is a worry is its safety, reliability and stability (or lack thereof). $\endgroup$
    – a4android
    May 21, 2019 at 12:38
  • $\begingroup$ @a4android: A hypothesis without experimental results is conjecture. Conjecture is useful, and if you can back it up with a combination of proven models then you can start to use it as an prediction (black holes being a good example) but if the drive defies analysis or it simply hasn’t been properly analysed yet (which is what I think ‘beyond current science’ means) then those strong-minded physicists are liars or charlatans. Or possibly after more grant money. Either way: it’s either beyond science and currently being analysed (so not explicable) or it’s not beyond science! $\endgroup$
    – Joe Bloggs
    May 21, 2019 at 15:16
  • $\begingroup$ @JoeBloggs Thank you for dissertation on the philosophy of science. Hypotheses are usually based on a combination of existing models and conjectures. The OP said the physical principles of the drive are unknown. Theoretical physicists will work hard to devise putative models to explain it, which may not be possible to verify experimentally. Like string theory or whatever is its equivalent in the OP's world. It might simply take a long time to study the alien drives to understand them. I agree with your final sentence. We are not far apart after all. $\endgroup$
    – a4android
    May 22, 2019 at 9:28

The drives works in more dimensions then we are used to

We use to think of the universe as a 4 dimension space, 3 axis (up\down, left\right, forward\backward) and 1 for time (which moves forward only), but just because that's all we can see doesn't mean that that's all there is, what if this engine works with the other dimensions?

Let's say that this engine works in 5D space just to keep things as simple as possible, the 5 dimension will help the engine get up to speed as maybe moving in is a shorter distance between the two points (so you get to the destination faster?)

Let's go over the checklist:

  • no real exhaust plume - there is one, but it goes on the fifth dimension only so for "normal" space there isn't really any
  • ** the faster the vessel, the higher the power output** - the distance might be shorter in the fifth dimension but you still have to cover that shorter distance, and of course the faster you move in in it the more power you need
  • Hate being near other engines of this type or larger mass objects - this is less to do with the engine and more with our limited understanding of the physics involved, as both the engine & large mass objects affect this 5D space which we don't really understand we run the risk of getting into their gravity affect so it's safer to not use them close to either of them.
  • movement should be kept as far away as possible while the drive is active - rotational momentum is a bitch even in 3D, gyroscopes act very strangely on 3 axis only, can you imagine what will happen with 4 axis? and as already established humanity doesn't really know a lot about this extra dimension so once again we error to the side of safety and try to keep any movement in "normal" 3D space as far away as possible, a pipe of water has less mass then a grown man\woman so they tend to cause less angular velocity which is why they tend to have lower "safety distance limits" then the crewmembers.
  • The alignment of the drive to the vector of motion matters - it needs to push the ship forward in the fifth dimension, this may create very weird facing in the "normal" space but for the fifth dimension it will point directly behind the ship and it's target.
  • Weird failures & accidents - you might move in the 5D space in a way where your body ends up in two room at the same time in "normal" space... That counts as weird in my book.

Also if 5D is not enough there is noting that stopping you from saying the engine works in 39D space... (but not 40D as that's where the Glorfth lives & nobody likes how they smell)

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    $\begingroup$ Most versions of String theory call for 11 dimensions "compactified" at the Plank scale. Getting in and out of that sort of interdimensional space would be...interesting. $\endgroup$
    – Thucydides
    May 19, 2019 at 2:35
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    $\begingroup$ @Thucydides you'd have to pick a 3+1-brane theory then. As a neat side-effect, the same theory also gives you a place to store other universes. $\endgroup$ May 19, 2019 at 5:30
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    $\begingroup$ As for the 40D space - string theory requires the number of dimensions to be 12k-1. 11 dimensions is just the smallest possible, but if you want more, you have to go all the way to 23 and then 35 and 47. 39 doesn't work. $\endgroup$ May 19, 2019 at 5:33
  • $\begingroup$ Why would it skip the 4th dimension? I imagine you'll say that time is the 4th dimension, but it is not. It is the first temporal dimension. 3+1 spacetime has three spacial dimensions and one temporal dimension. $\endgroup$
    – forest
    May 19, 2019 at 6:20

This engine is a warp drive. It changes the shape of space in front of itself, and then gets pulled forward to try to catch up with the space distortion. It is "surfing a wave" in space-time of its own creation.

As a4android points out, this warp-drive "isn't necessarily a FTL drive. It warps the shape of space and that moves the spacecraft.… There's no reason why warp-drives should always be FTL drives. A sublight warp-drive makes more conceptual sense than a FTL one. Gravitational waves aren't gravity. So it isn't generating gravity."

The wave has a wake. This wake is a gravity wave. Gravity waves are effectively sound waves. Ordinary sound waves are the propagation of a density difference in a fluid or a solid. Gravity waves are the propagation of a change in the shape of space-time through space-time. They can be "heard" by ultra-sensitive pairs of eardrums; confirmation by a second ultra-sensitive pair of eardrums is required to rule out false positives. So far, the only gravity waves we have been able to hear have been 1/3 second long chirps as 30-solar mass black holes spin into each other at high speed. So it is possible for the wake to carry away a huge amount of power in a way that is hard for ordinary technologies to detect.

The huge amount of power can be generated by a Bussard ramjet. These ramjets need large, intense electromagnetic fields to pull in interstellar gases; they get more fuel the faster the ship goes.

The engine needs to be regularly adjusted to accommodate the motions of stars that are within a few light-years. Unfortunately, the adjustments are not fine enough to handle the motions of Jupiter-sized planets in Jupiter-scale orbits that are within 100 AU. These motions create disturbances in the gravitational field, which interfere with the engine's internal workings. (It is possible that the problem is the planet's acceleration, not the planet's velocity.)

The wake caused by another such engine within 100 AU is just as bad as the effect of a Jupiter-sized planet in a Jupiter-like orbit.

By the R-squared law, a too-fast (or too-jerky) motion of a small, nearby mass also causes uncompensated disturbances in the gravitational field inside the engine. Fluids moving within the ship are okay as long as the movement is steady.

  • $\begingroup$ While the suggestion is decent it doesn't fit my setting since no FTL is a core rule. Warp drives introduce the horrific specter of causality violation and that's something I'm unwilling to deal with. Additionally mankind has no other means of producing gravity than mass, spin and acceleration so the drive beeing able to produce gravity would cause trouble due to secondary applecations. $\endgroup$ May 20, 2019 at 7:32
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    $\begingroup$ @TheDyingOfLight While this answer postulates a warp-drive this isn't necessarily a FTL drive. It warps the shape of space and that moves the spacecraft. You're thinking Star Trek. There's no reason why warp-drives should always be FTL drives.A sublight warp-drive makes more conceptual sense than a FTL one. Gravitational waves aren't gravity. So it isn't generating gravity. $\endgroup$
    – a4android
    May 21, 2019 at 12:30

Tom's answer is spot-on in the physical backgrounds, the answer just lacks a good name for the drive. I would simply call it either

  • Dark-matter Jet Engine (for formal use)

  • Dark Jet (for colloquial use)

  • DJE (for engineering slang)

The first term really explains quite well what this thing does: It ingests some dark matter, does something with it, and then exhausts it in the form of a directed jet. Just like a normal jet engine does with air. The other terms are just abbreviations of the first.

For the details of how well this covers your different conditions, please refer to Tom's answer.


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