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It's the future. The setting is about as "hard-ish sci-fi" as it is possible to be, with the one big exception of a viable FTL engine. Humans have began spreading out into the galaxy, colonising nearby star-systems that have compatible exo-planets. There are currently about 100 colonies, spread over a roughly 1000ly wide region. Travel times between stars frequently take multiple months. Humans still appear to be alone in the galaxy, but some questionable ruins of more primitive civilisations have been found.

The plucky humans are about to have a surprise run-in with a vastly more powerful civilisation, who instead of setting out into the galaxy, decided to stay at home and construct a Dyson swarm. The star hosting this swarm is only a hundred light years or so outside human controlled space. They built it a long time ago, so nobody on earth would have noticed it under construction. Never-the-less, I would have thought that the spectral lines and luminosity of a star with a few billion giant discs orbiting it would be different enough to be visible by future astronomers.

How and why did my future astronomers manage to miss this feat of stellar engineering on their (relative) doorstep?

I would expect an answer to fall into one of the following categories:

  1. Human astronomers saw the unusual characteristics of the star, but dismissed it as a natural phenomenon for some reason.
  2. The Dyson swarm builders constructed their swarm in such a way as to hide the fact that it is an artificial structure when seen from interstellar distances.
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    $\begingroup$ I strongly doubt the stay-at-home-part. Since they are technologically so advanced, they must know that eventually their sun will destroy their home planet, even if they have the tech to fend off planet-killer asteroids. I'm very sure that any civilisation that can leave their planet will do so. After all, it was curiosity that made them advanced. And that, if nothing else, will make them spread out. $\endgroup$ – Burki Feb 19 at 14:52
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    $\begingroup$ @MadScientist out of interest: if they aren't trying to leave, and they are happy living on their stable Dyson swarm, what possible reason could they have to need that amount of energy? Surely this is more than their life support systems need. $\endgroup$ – Plutian Feb 19 at 15:40
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    $\begingroup$ If the aliens never figured out FTL, departing their home star might be really daunting. $\endgroup$ – SRM Feb 19 at 19:43
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    $\begingroup$ @Burki If they are advanced enough, they can actually manage their star by lifting out excess hydrogen and most of the fusion byproducts (helium, etc) and end up with a very, very long lived star. $\endgroup$ – Michael Richardson Feb 19 at 20:55
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    $\begingroup$ If the reason for using a Dyson Swam instead of a Dyson Sphere is because you want to go for hard sci-fi, you might consider a Dyson Sphere around a white dwarf. Depending on the mass and the age, it's possible for a white dwarf to have a distance where the gravity and luminosity are both comparable to Earth normals. In this case, you wouldn't have to invoke artificial gravity for the Dyson Sphere to work. That system would also be pretty invisible to anyone not looking exactly for it, so that could solve your plot problem. arxiv.org/abs/1503.04376 $\endgroup$ – David Elm Feb 19 at 23:07

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Simplest explanation: until recently, humans simply didn't see the star directly as it was hidden behind a small, but thick star-forming nebula between it and Earth.

(One doesn't really exist, but one could.)

The nebula has some young, hot stars which are in the way of the star you're interested in, obscuring it. Only recently have humans expanded "laterally" to the point that they can clearly see around the back of the nebula to get a clear look at the star, and surprise!

If it's a dark nebula, then it doesn't have to be star forming or have any stars. The star might be detected in the infrared, but without visible spectroscopy it might not seem anomalous and just be considered a red dwarf or something that isn't very luminous.

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    $\begingroup$ I'd initially discounted this because I had assumed that nebula were far too large and diffuse order to occlude a star a few hundred light years away. Therefore, i was most surprised to read up on dark nebulas.(en.wikipedia.org/wiki/Dark_nebula). $\endgroup$ – MadScientist Feb 19 at 19:55
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    $\begingroup$ As I understand it, a Dyson sphere would still need to cool itself radiatively if it were to try to produce useful "work" (Thermodynamics in a vacuum). This would mean that it would be an incredibly bright long wave length EM source (really really bright in the Infra-Red, you couldn't miss it, if you were semi-looking for it). $\endgroup$ – Aron Feb 20 at 3:47
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    $\begingroup$ @Aron - True, but if the nebula scattered/absorbed enough of the IR, then the star would simply appear less luminous. Perhaps the nebula has an unusually high amount of CO and OH molecules. Without an unfitted spectrum to compare it against it still seems plausible that astronomers would not realise there is an artificial structure around the star. $\endgroup$ – MadScientist Feb 20 at 12:57
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    $\begingroup$ @TomášZato-ReinstateMonica - Of course they would still be able to see the star (or rather some part of it's spectrum). The idea that a nebula would block out every part of the spectrum just doesn't make sense. That's not how scattering and absorption work. They know the star is there. The key point of the nebula is that it can plausibly obscure enough of the star's luminosity and spectrum to hide the fact there is a mega-structure. $\endgroup$ – MadScientist Feb 20 at 13:12
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    $\begingroup$ @DavyM, we'd have a pretty good idea by now about close star-forming nebula. The closest one with massive stars is the Orion Nebula (1300 ly), and the closest with lower-mass stars is the Rho Ophiuchi cloud complex at 460 ly. $\endgroup$ – Keith Morrison Feb 21 at 21:14
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They’ve been actively trying to hide.

These aliens don’t want to be found. As such they actively keep track of nearby astronomical phenomena that indicate younger civilisations and, upon detecting them, engage in active surveillance/espionage to track their expansion. Their Dyson swarm is actually very efficient, and will capture almost all energy unless the energy collectors are folded/rotated/made translucent. In this way the aliens can selectively leave ‘holes’ in the swarm to make it appear to any given nearby star that they are just another normal star. This also has the effect of turning their star into a shkadov thruster that constantly moves on a vector away from other races.

But every hole is less energy being collected, and humans have spread so far so fast (galactically speaking) that these aliens can no longer capture as much energy as they want to and maintain the illusion of normalcy. As such they have abandoned any pretence and just set their swarm to gather as much energy as possible as quickly as possible, effectively ‘turning down’ the star overnight (an event sure to get people’s attention!)

Humanity just has to wait and find out what they’re going to do with all the extra power...

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“Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.”

- Douglas Adams, The Hitchiker's Guide to the Galaxy

There are two reasons why your civilization doesn't spot the Dyson Swarm ("DS"), the combining of the two making the DS almost invisible.

  1. Space is really big. We already have problems even finding other countries' military satellites, and that's just a planetary-orbit scale.

  2. Your hypothetical civilization has FTL. Due to the above, one of the only ways for your civilization to find the DS is if one of their ships does a fly-by. Unfortunately, your civilization figured out how to use FTL quite early on, so they can just go through the outer reaches of the system at ~3c. This saves them a huge amount of time, but it also prevents them from getting close enough to the swarm to notice it.


So, your civilization goes for a few hundred years without noticing the DS, and everything's fine. However, they still think that 3c is pretty slow, so they put a bunch of money into developing a faster drive. Eventually they this investment pays off, and they come up with a Asimovian hyperspace drive. This allows them to get from point A to point B in literally no time at all, skipping the years spent in transit.

However, this new hyperspace drive has one glaring problem. Although the hyperspatial drive can get you from point A to point B, there are some "this giving me a headache, I need a supercomputer"--level computations involved, as otherwise you will end up hundreds of lightyears from your destination. They are already hard to do in flat space; they are next to impossible to solve when you factor in one or more gravity wells.

As a result of this complication, ships with the new hyperspatial drive use it for the part of the journey which is outside star systems, and then do a in-system approach/exit flight on (decidedly non-FTL) thrusters.

Now, space may be vast, but so is a Dyson Swarm. As a result, it is noticed almost immediately by ships with the new hyperspace drive, as they are forced to fly right through it on their way to the inner system.

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    $\begingroup$ You seem to be describing a diffuse ring of orbital habitats, not a Dyson Swarm. A Dyson Swarm by definition is constructed at least in part to harvest energy from a star, and would occlude most of the light from the star. A Dyson Swarm will be in close orbit about the star, you won't be flying past it to reach the "inner system". $\endgroup$ – Harabeck Feb 19 at 22:55
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    $\begingroup$ Part of "space is vast" is that any dyson swarm of reasonable size would not need all the light produced by the sun; most would get through. Relative to the "closer in" part, you're right: I didn't think of that. $\endgroup$ – Marvin the Paranoid Android Feb 19 at 23:21
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    $\begingroup$ @Harabeck No, a Dyson Swarm covers anything from a single shared orbit of solar satellites. Even if it captures 0.01% of the solar output, it's still a Dyson Swarm. A Dyson shell would necessarily capture 100% of the output (and shift the spectrum of the star like crazy to IR), but a Dyson swarm can be built up over time to suit the needs of the civilization. A swarm large enough to power 100x the power requirements of our civilization would still be essentially invisible to us at 100 ly. If you're not doing interstellar trips or such, it's hard to see why you'd need anywhere near to 100%. $\endgroup$ – Luaan Feb 21 at 8:45
  • $\begingroup$ @Luaan Alright, but the OP's question makes no sense if we're talking about such a diffuse swarm. $\endgroup$ – Harabeck Feb 21 at 15:16
  • $\begingroup$ We already have problems even finding other countries' military satellites, and that's just a planetary-orbit scale. Those satellites don't perform fusion on their surfaces, emitting every frequency of electromagnetic radiation. Astronomers are very good at detecting stars, despite the vast distances. $\endgroup$ – cowlinator Feb 21 at 18:40
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I'm not too familiar with star characteristics, so wouldn't know what humans could or would dismiss as natural. But I have some ideas on how to hide such a megastructure:

Full coverage
If this civilisation is as advanced and powerful as you state, and they have the resources to actually construct a Dyson swarm effectively, they might have had enough resources and/or time to fully cover their star (potentially with large screens). This is vastly dependent on which distance from their star they can survive at, being much shorter than our distance from the sun. Full coverage would effectively hide the star altogether, and there won't be anything to investigate. Alternatively the coverage could be semi-transparent, making the star appear much weaker or smaller, and less interesting.

Little/thin coverage
Another option is the opposite of above, the strips or structures are so thin and spread out, or form such a perfect raster, that it can't be easily spotted from earth with conventional methods. Only if a star is of particular interest to Humans, they will investigate further. This star didn't look too noteworthy, so they didn't look too close.

Asteroid belt
Our sun has a huge asteroid belt around it, at a large distance. However this doesn't have to be the same for each star. Stars might have asteroid belts at closer proximity to their sun, and such won't be an uncommon sight. An option would be that this particular Dyson sphere was constructed to closely resemble an asteroid belt, and as such won't be easily spotted as artificial.

Glass
The entire Dyson sphere could be constructed of a resource abundant to this race, which just happens to be fully transparent. This would mean they would probably struggle with privacy, but by large their structures would be hard to spot at intergalactic distances. Simply because there isn't a construction large enough to spot which you can't see straight through. Structural integrity issues handwaved for the sake of this option.

Up in the crowd
All in all, humans might have spotted something slightly off with it, but it wasn't too noteworthy, as the star was of little interest to begin with. Due to the vast numbers of stars in the universe with potentially "Something off" about them, it was thrown on a pile of "Perhaps we should monitor this" and simply forgotten about, or there simply wasn't enough funding or time to efficiently investigate.

Stolen thunder
The star might be in (semi-) close proximity to a huge unexplainable celestial anomaly, which is of massive interest to us, and thus is largely overlooked because anyone looking that way is quickly distracted and only pays attention to the anomaly. This option might backfire though as they might investigate the effect of the anomaly on nearby stars, depending on what it is.

Down low, too slow
The star is only properly visible from territory humans only recently expanded to (note, expansion can work linear as well as spherical). They did notice the anomaly, but were too busy developing their settlement that they didn't pay enough attention quick enough. They have their run-in with the other race before they know what's going on. Alternatively, we only just might have stumbled into what they consider their territory, and they are rushing to squish the new menace.

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    $\begingroup$ Making the swarm sparse or transparent would reduce the collected energy from the star, which is the whole point that anyone might build a Dyson swarm in the first place. I'm afraid it's also not plausible that a star with "something off" would not be investigated further. Those are precisely the kind of stars that astronomers would be the most interested in, and would seek funding to investigate over "boring" stars. $\endgroup$ – MadScientist Feb 19 at 13:29
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    $\begingroup$ Could you give an example of how "off" the "something" could be before astronomers wouldn't want to look at it? Bearing in mind that modern astronomers are already highly motivated to investigate the most minute variances in stellar luminosity and spectra in order to detect transiting exoplanets. It doesn't really matter how efficient their energy collection mechanisms are. If they are building a Dyson swarm, they are doing so to collect unimaginably vast amounts of energy for whatever purpose they choose. No matter how efficient, they are always going to want to expand the swarm. $\endgroup$ – MadScientist Feb 19 at 13:44
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    $\begingroup$ "Full coverage" would not completely hide the star - at least according to our knowledge of physics. It would, however, produce a very interesting anomaly: A body radiating heat (infrared) only, roughly on the level of a star's output! $\endgroup$ – Syndic Feb 19 at 14:23
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    $\begingroup$ The infrared problem is very much part of the issue. Infrared astronomy is a very common tool, with a number of automated star surveys using infrared telescopes. If there was a star within 1000 light years that was only radiating in the infrared, I'm pretty confident we would pick it up in short order with modern technology. Let alone what could be achieved by an interstellar civilisation. $\endgroup$ – MadScientist Feb 19 at 14:36
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    $\begingroup$ @Plutian That's an interesting find, but wouldn't make sense for this case. The reason we call them OH/IR stars is that the hydroxyl spectral lines are very sharp. The IR output of a civilisation using stellar energy for some sort of useful work would be much more likely to resemble a black-body spectra. The difference would be immediate and obvious. $\endgroup$ – MadScientist Feb 19 at 15:29
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It was never deemed worth investigating closely.

So actually, we know it's location, and have for a hundred years or more. Every 10 years or so some astronomer gets excited, gets a grant and does research on it; usually the conclusion ends up that it's a star that's being mostly occluded by an asteroid belt. Because all of the planets in the system were used in the construction of the Swarm, the stars wobble is non-existent (this is how we can tell which stars have planets in the current day), despite the odd interference it's obvious there are no planets, making it a curiosity at best.

Once someone even proposed that it was a Dyson Swarm, but a competing astronomer got better grants and suggested an alternate theory that got the first guy discredited. Like obviously something is weird about it, but ultimately there are plenty of better prospects for potentially habitable worlds, and the expense of sending probes (or a manned expedition) is prohibitive compared to the potential benefits of going to a star that apparently has a big asteroid belt and little else. So until recently aside from a study here or there every 10 or 20 years it's been basically ignored.

Finally someone got enough money to send a probe (or expedition) and the truth of the matter is discovered. Or maybe a new colony looks at it from sufficiently parallax view to see that the swarm isn't just a band of asteroid and a closer look is finally called for.

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    $\begingroup$ This is exactly what I was thinking. If you look at the neighborhood around Earth you'll see that when you get to a distance of 100 light years you're dealing with tens or even hundreds of thousands of stars. It's way down there on the list of stars worth visiting to learn more about it. $\endgroup$ – Rob Watts Feb 20 at 18:12
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The Dyson swarm isn't actually between the star and us

The alien's Dyson swarm isn't a full shell - instead, it is more of a ring, with some objects being perhaps a few tens of millions of miles of what used to be the planentary ecliptic. However, the view of the star from the poles is largely unobscured. Of course, one of the poles points almost directly at the human empire.

Because the view of the star (from the human perspective) is unobscured, observations of the stellar spectra and intensity wouldn't show anything interesting. And, as the objects of the ring are relatively balanced, the star wouldn't shift much (with is the other current method used to find exo-planets). It's possible that future technology would be able to detect the ring itself; however if the star is relatively uninteresting (long in metals if the aliens really went hog-wild in mining stuff from their sun), it might be put fairly low on the list of things for astronomers to check out...

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  • $\begingroup$ That would help, but the blackbody radiation from the swarm would still be adding a noticeable amount of infrared to the spectrum of the star even if nothing is being blocked. $\endgroup$ – smithkm Feb 20 at 1:12
  • $\begingroup$ @smithkm: yes, there would be some infrared. However, I do believe that the star would emit significantly more, and perhaps the aliens might take some steps (for some reason) to reduce the amount of emitted radiation in the polar direction.. $\endgroup$ – poncho Feb 20 at 4:10
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    $\begingroup$ The question posits that the swarm is capturing at least 20% of the energy of the star and all of that needs to be re-emitted as waste heat. That is going to be a very noticeable change in the spectrum of a star. Controlling the direction that waste heat is emitted is very difficult and wasteful so the aliens need a really good reason to do it. Also, a 1000 ly wide human controlled space (assuming it's roughly spherical) at a distance of 100 ly has an angular diameter of 113 degrees within which they would need to not emit waste heat. $\endgroup$ – smithkm Feb 20 at 18:04
  • $\begingroup$ Alternatively, the ring could be oriented in such a way that it is edge-on to Earth, and dense enough to completely block our view of it - we might not even notice there's a star there. This is close to being a Dyson sphere, but only in a narrow band that is by chance or by design directly between the star and us. $\endgroup$ – Darrel Hoffman Feb 20 at 22:16
  • $\begingroup$ @DarrelHoffman: the IR kills you; I was thinking that the IR re-radiated by the ring would be relatively small compared to the IR generated by the star (and so it might not be noticeable unless you could see the spacial separation) - of course, if the ring is absorbing 20% of the star's output, well, stars generally don't generate that much of their spectrum in IR. However, if the ring was blocking the star, well, the IR output would be there (and there's nothing to mask it). Yes, you couldn't see it - it would stand out to anyone scanning the IR spectra... $\endgroup$ – poncho Feb 20 at 22:21
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The swarm are really small objects, enough to be disguised as interstellar dust

If each unit is comprised of elements in the order of centimeters and surround the star, it can easily be confused. Only a probe sent specifically to study the star would find something off, and even then a second one would be needed to certify the findings.

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    $\begingroup$ This only works if the "dust swarm" is as transparent to stellar radiation as an interstellar dust cloud. If that is the case, it's not really doing it's job as a Dyson swarm, which is to collect as much energy as possible from a star. $\endgroup$ – MadScientist Feb 19 at 13:31
  • $\begingroup$ maybe they never needed that much energy, and so their swarm doesn't consist of too many satellites. $\endgroup$ – Burki Feb 19 at 14:55
  • $\begingroup$ @madscientist: Depends on the distance. If it’s far enough away humans might take the reduced brightness of the star as evidence of a patch of unusually dense dust stretching between us and the star, when in fact it’s just the swarm at work. $\endgroup$ – Joe Bloggs Feb 19 at 15:14
  • $\begingroup$ @Joe bloggs - It's a nice idea, but I don't think it would work. The humans in this scenario are spread out over a wide area, which means the hypothetical dust cloud would have to occlude a sizeable portion of the star's circumference for people to arrive at the dust conclusion. If that was the case, astronomers would expect to see other stars in the same arc of sky to also show signs of occlusion. They would get suspicious when it was only one star. $\endgroup$ – MadScientist Feb 19 at 15:33
  • $\begingroup$ @Madscientist True enough. Depends on the exact geometries and distances I suppose. $\endgroup$ – Joe Bloggs Feb 19 at 16:02
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You don't need an explanation - there could be Dyson swarms all around us and we might not notice

Let's think about what scientists will see if they look at the star.

They might see it dimming and brightening a bit as the distribution of solar panels around it shifts, but stars dimming and brightening every so often are nothing new.

Its spectrum would also have a slight dip along a wide range of frequencies, but this kind of feature is unlikely to be seen as abnormal. Every star has its own spectral distribution, with slight variations. Unless the effect of the Dyson swarm is a sharp peak or trough in a few very specific frequencies, nobody would look at it twice. And we'd expect that the solar collectors in the swarm would absorb a wide range of frequencies, like our solar panels today do.

So in the first place, it might not have any unusual features. But let's say for argument's sake that it does have some unusual features. We still might not notice. There are so many dang stars that nobody looks at most spectra by eye - everything's done by algorithm, which is why a feature that nobody's looking for will be unlikely to be noticed. Sure, maybe some studies will be looking for alien structures specifically, but the star's spectrum might be weird in a way even they don't expect, and these studies might not single it out.

Okay, but let's assume that we get lucky and some study happens to pick out this one star and examine it and it happens to be pretty strange. Well, we have a bunch of stars like that now, and the best funding we throw at them is a few hours of telescope time a year. They could try to get together an expedition to travel to the weird star, but unless they have some other evidence that it's aliens, it's completely plausible that nobody cares enough to do that. After all, it's 100 ly outside of human-occupied space. That's several decades even with your FTL, followed by a several-decade return trip. If your study can't be finished within the span of a single student's second degree, then nobody will take it on unless it's unquestionably Very Interesting.

Basically, unless this Dyson swarm causes the star to freak out like Tabby's Star, we won't notice it until someone basically runs into it. And to quote Dr. Boyajian herself,

Extraordinary claims require extraordinary evidence, and it is my job, my responsibility, as an astronomer to remind people that alien hypotheses should always be a last resort.

So I wouldn't worry about any serious researcher singling out the star and getting a full mission together just because it might be a Dyson swarm.

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    $\begingroup$ I don't think an actively space faring civilization would just miss the star because there are so many of them. Computerized telescopes are a thing, and the star in this scenario is close to their border. Also, you cannot block all light with a Dyson Swarm, it only downshifts into the infra-red. And if the swarm isn't blocking most of the light, then it's doing its job of capturing that energy. $\endgroup$ – Harabeck Feb 19 at 23:00
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Taking advantage of the fact that FTL implies time travel, every time they get discovered, the aliens fly back in time, violate causality, and destroy whatever human noticed them (or at least the records of the discovery). The fact that they have been discovered now just means humans have finally learned to patrol their own backstory.

My favorite write-up, with pictures, explain why FTL violates causality using just geometry. Straightforward even if you’re not a math geek: http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel

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    $\begingroup$ 1. The aliens actually never figured out FTL in this scenario, so that's not an option to them. 2. FTL implying time travel is only the case assuming that there are no privileged reference frames that all FTL occurs relative to. Although this is a postulate of special relativity, it is also the easiest one to dispense with in a fictitious world, while leaving the rest of physics as undisturbed as possible. $\endgroup$ – MadScientist Feb 19 at 21:10
  • $\begingroup$ @MadScientist True. But the question was tagged science-based, so I figured this was at least an option. :-) $\endgroup$ – SRM Feb 20 at 3:39
  • $\begingroup$ Don’t forget that relativity is a model, not the reality. It may be the best model yet found to describe what we’ve observed. But just like Newton’s model was eventually found inaccurate in some ways, relativity could be. The accidental discovery of FTL might require a new model. Or the discovery of an inaccuracy in relativity might give us a new model that leads to FTL. $\endgroup$ – WGroleau Feb 20 at 15:43
  • $\begingroup$ @WGroleau That reminds me of a novel I read where the secret to FTL was discovered to be that Time Travel was possible but unstable - after less than a second in the past, you (and everything else sent back) were thrown back to the future. But, since the universe was so much smaller in the past, you could travel quite a large "present distance" in that fraction of a second, and arrive almost the moment you left. $\endgroup$ – Chronocidal Feb 20 at 16:33
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It was mis-identified as a planet

A Dyson Swarm is a beefy beefy project, for efficiency, the individual satellites are constructed near one another in a spreading region rather than deployed evenly around the star.

For whatever reason, they stopped building, perhaps they ran out of resources, or they simply got far enough to feel they had enough.
Maybe they're still building when they need to, but progress has slowed.

The swarm is a dark stain that orbits the star, incomplete, but still incredibly large.
But to the telescopes of the various planet-finding agencies it's just a large object transiting the star and looks exactly like a planet.
Nothing important, most stars have planets of some size.
They log that there is a super-earth type planet in the habitable zone of the star and move on. Perhaps noting some unusual spectral data about it as they go.

The truth is only revealed when someone finally goes to visit the system and discovers that what they thought was a large planet is in fact so much more.

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Humans use Portals and Hyperspace to travel, and ignore normal-space most of the time

In Peter F Hamilton's Pandora's Star, humans create portals to far-off stars. Normal-space ships are mostly not used. So most star systems are given only a cursory glance, and left behind. So a secret-in-plain-sight military base is built around Alpha Centauri after public interest in the closest star wanes. No one builds a portal to it. No one travels to it via ship. It's just present, able to portal out or normal-space travel wherever it needs to go, and the public is unaware.

Similarly, your invaders can be patient normal-space builders using thousands of years to spread between stars. Humans flit about with FTL, unaware of the huge number of ships approaching until the decelerating drive plumes turn on.

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The Dyson sphere is complete, and cooled to ~2.7 Kelvin

Basically the star is completely invisible because all the star's radiation is captured by the Dyson sphere, and the sphere itself is kept cool, so it's blackbody radiation is the same as the background.

They manage the cooling feat by a giant laser directed into empty space, or dumping it into an artificial black hole. The linked question asks about a portable heat sink, which an answer shows that it requires a black hole the size of the moon. But your aliens don't need to be mobile at all.

For the motivation, I'm not actually sure if this is more efficient than uniformly radiating heat. I suspect there have to be losses somewhere, but black holes seem pretty magical anyway. However if the civilisation is smart enough to build a Dyson sphere, they'd be smart enough to see humans too, or at least predict that some alien intelligence might be a threat. And because they decided to keep all their eggs in one solar system, they'd treat security seriously. With the logical conclusion that only way to win a war against a galaxy spanning civilisation is to not fight at all. Thus being perfectly stealthy was a design requirement.

Finally they were discovered by sheer luck, when an exploration ship crashed directly into the the solar panels without seeing them. Because the solar panels are incredibly thin and light, the ship survives and uses it's backup engine to return to base and tell of their discovery.

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  • $\begingroup$ Spheres aren't swarms. $\endgroup$ – RonJohn Feb 20 at 16:58

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