How might my futuristic interstellar civilisation have missed a Dyson swarm on their doorstep?

Question

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.

Answer 1

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.

Answer 2

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...

Answer 3

“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"), which together make the DS functionally invisible.

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

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. Your civilization figured out how to use FTL quite early on, so they tend to go through the outer reaches of the system at superluminal speeds. Such high velocities are good for getting places quickly, but make sightseeing practically impossible. Anyway, why would people want to look at that area of space anyway? It's just a bunch of boring old asteroids.

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So, your civilization goes for a few hundred years without noticing the swarm, and everything's fine. However, they still think that 150 times the speed of light is pretty slow, so they put a bunch of money into developing a faster drive. Eventually this investment pays off, and they come up with an 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.

There's only one catch. Although the hyperspatial drive is very good at getting you from point A to point B without having to go through the rest of the alphabet, there are some complex calculations involved. So complex, in fact, that you can't do them without the newest in supercomputing technology. Already extremely difficult, these computations become almost impossibly hard when you toss in a gravity well.

Despite having multiple space drives, your society still hasn't managed to prove that P = NP, so hyperspatial drives are only used for the interstellar portion of journeys. The new drive makes 99.99% of a trip go by in the blink of an eye, but the remaining 0.01% has to be made with other means of propulsion.

Unfortunately for aspiring day-trippers, traditional FTL drives interact explosively^* with hyperspatial drives. The only other option is ionic thrusters, which are decidedly non-FTL. This forces ships to go through systems at sightseeing speeds. If only there was something out there to sightsee...

Space may be vast, but so is a Dyson Swarm. Without anything else to do, bored passengers soon spot the swarm.

^{* Actually, it's a topographically complex n-dimensional spatial fracture, but the end result is the same.}

Answer 4

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.

Answer 5

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.

Answer 6

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...

Answer 7

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.

Answer 8

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.

Answer 9

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

Answer 10

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.

Answer 11

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.

Answer 12

Two words: Tabby's Star

This is a well-publicized stellar observation that was thought, briefly, to be potential evidence of a megastructure similar to a Dyson swarm, perhaps one still under construction. It's now believed that this star's gradual and irregular (but quite deep) dimming is due to dense clouds of dust, likely created by planetary formation collisions (big pieces stick together, little ones are ejected temporarily).

Since this was determined (right or wrong) any star we see with similar light curves will be thought to simply be young enough to still be forming its planets. Astronomers might well put it on a list of anomalous stars if the spectral data seems to contradict this young age -- but with the number of stars to be seen even in a mere couple hundred light year sphere, it might be a long time before it seems important enough to go look -- and until someone goes and looks, the swarm builders either won't know we're out here, or won't consider us a likely threat.

First time a ship drops out of FTL close to their system (say, without a fraction of a light year), that may change...

Answer 13

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.