An alien civilisation is growing. As it evolves, it develops ever more complex and energy-consuming technologies. Soon the resources of its home planet no longer satisfy its energy needs, so it begins building megastructures to harvest the energy from its very own sun. Within a relatively short period of time these structures almost completely encase the star, forming a so-called Dyson sphere, providing energy in the order of yottawatts (1024 watts) – a sufficient amount to fuel a trip to a neighbouring star system.

So far, so sci-fi. Except that last year, astronomers discovered a star behaving so strangely that so-called alien megastructures were invoked as a possible explanation. The star is KIC 8462852 or ‘Tabby’s Star’ after Tabetha Boyajian, the astronomer who first brought it to the world’s attention. Situated around 1,500 light years away in the constellation Cygnus, KIC 8462852 was first observed by citizen scientists of the ‘Planet Hunters’ team looking for Earth-like planets using data on star dimming from the Kepler Space Telescope. KIC 8462852 was dimming by such large amounts – up to 20 per cent over short periods – that it couldn’t be explained by a passing planet or, for that matter, anything that scientists had observed before.

Boyajian, an astronomer at Yale and the head of the Planet Hunters team, posited the improbable explanation that the dimming was caused by a gigantic cloud of comets dragged along in the wake of a rogue star. Another astronomer, Jason Wright of Penn State University, was soon on hand to offer another seemingly improbable explanation – that the dimming was consistent with the kind of megastructures an alien civilisation might build.

Could Tabby’s Star be harbouring a Dyson sphere? It was strange, but so was the behaviour of Tabby’s Star and, as Wright pointed out, unusual events justify unusual explanations. “It’s actually very unusual for stars to get dim, even a little bit dim,” explains Seth Shostak, senior astronomer at the SETI institute, “so you have to come up with some method. It has to be said that building a giant megastructure, a Dyson sphere – we can’t do it but it doesn’t mean another civilisation couldn’t do it. It’s just engineering, so maybe it’s worth considering that that’s what you’ve found.”

In the year since the story sent the world’s press into overdrive, the mystery of Tabby’s Star has only deepened. Analysis of photographic plates of the star going back to the 1890s showed that it has dimmed by a huge 19 per cent over the last 100 years. A study of four years of the Kepler data seems to corroborate this, showing a one per cent dimming over three years, followed by an inexplicably sudden dip of two per cent over six months. Now there isn’t just one type of mysterious dimming occurring on Tabby’s Star, but three. All of which led Boyajian to dub it “the most mysterious star in the universe”.

How easy would it be for an alien civilisation to create a Dyson sphere? “It’s the sort of project that we could talk about how it might actually be done,” says Wright, “as opposed to warp drives or teleporters where you’re making stuff up. It seems like something that would require tens of thousands of years and we presume that alien civilisations have millions of years at their disposal.”

One thing these civilisations wouldn’t be making, it turns out, is a sphere. A full Dyson sphere, known as a Dyson shell, is a misconception of the idea put forward by physicist Freeman Dyson in the 1960s. A Dyson shell would be gravitationally neutral, floating freely in space with the danger of simply colliding with the star. Before that happened it could easily collapse or be ripped apart by huge internal forces. “You’ve basically built an arch the size of the solar system,” explains Jason Wright, the astronomer who first suggested the alien megastructures explanation, “and arches [can only] get so big before they buckle. The compressive forces are way beyond anything normal matter can handle.”

There is a possibility, says Wright, that a sufficiently advanced civilisation could use an ultra-dense form of matter such as material from a neutron star. Even then, the structure would be mechanically unstable. When pressure is put on the top of an arch, it collapses for a tiny fraction of a second, causing a pressure wave to travel down through the arch to the ground before bouncing back to the top and restoring the arch’s stability. The same would be true of a Dyson shell, except that its sheer size would cause problems with the restoration of the structure.

“With a Dyson shell, if you push on the top the restoring force has to go all around the shell and back,” says Wright. “But the fastest it can do that is the speed of light, which is going to take something like half an hour – half an hour of drift down before it can be restored.”

During this time the dip in the arch would create a convex, or negative, curvature, which would then be subject to tensile instead of compressive forces – forces seeking to tear it apart rather than squash it. “There are no forces in nature we know of that could withstand those kinds of tensile strengths so it will rip itself apart,” says Wright. “The whole thing would buckle the very first time you blew on it.”

Fortunately there is a more realistic form that such megastructures might take.

A Dyson swarm would be a cloud of objects surrounding the star in independent orbits, each one stable on its own and each collecting its own piece of solar energy. A civilisation could start small with just one or two structures and gradually build up the swarm as it developed, forming the kind of partial structure that could account for the dimming of Tabby’s Star.

Commencing work on a Dyson swarm could be so easy that one expert believes humanity could start building one now. It’s all about automation, according to Stuart Armstrong of the Future of Humanity Institute at Oxford University. “If you have the cycle of building factories that build solar panels that power factories that build mining equipment that extract resources that are made into more solar panels, then you can get this vast amount of production done,” he says. “If you can get this recursive manufacturing loop going then you can get there pretty quickly.”

Armstrong has developed a theoretical plan for how humanity could build a Dyson swarm. It involves the small matter of disassembling the planet Mercury, a task which, given the exponential increase provided by recursive manufacturing, would not take as much time as you might think.

Armstrong’s vision begins with a structure just 1km2 and half a millimetre thick, made from haematite – an oxide of iron common on Mercury. It would consist of a mirror tethered to a simple heat pump. The energy generated could be sent back to Mercury to power the continued manufacturing of more mirrors, factories, mining equipment and so on. Automation would be crucial because, without the feedback loop, the energy required to take apart Mercury would be completely out of our range. “We get something like 4×1020 joules of energy every year on Earth,” explains Armstrong. “The gravitational binding energy of Mercury is about five billion times that so it would take five billion years, which is about the time we have left before the Sun boils away.” But with the feedback loop provided by recursive manufacturing, Armstrong predicts we could construct a Dyson swarm made up of half the mass of Mercury and generating 3.8×1026 watts in just four decades.

A less high-energy (and kinder to Mercury) alternative would be to manufacture the Dyson swarm from asteroids. However, this would seriously limit the amount of material available and would necessitate the use of a super material that, as yet, we haven’t discovered. “I’m looking at something like a relatively thick haematite, half a millimetre for the main reflecting surfaces,” says Armstrong. “With super materials you could get this down to a few atoms of thickness and the amount of material drops ridiculously.” However, Armstrong believes we are close to developing the kind of recursive manufacturing that would make such super materials unnecessary, either through nanotechnology, self-replicating 3D printers or some form of AI. “If you have full AI with a robot body, you have an intelligence that can build more versions of itself and copy itself into that.”

If Dyson swarms are so easy to build, why haven’t we seen any yet? Or perhaps we already have?

However alluring the idea, alien megastructures are most experts’ least favourite bet for the dimming of KIC 8462852. “The history of discovery in astronomy shows that every time you find something unusual, the first explanation very often is aliens,” says SETI’s Seth Shostak. SETI has already trained its Allen Telescope Array on the star for several days. It found no evidence of radio signals an alien civilisation might be emitting. Similarly, large objects orbiting close to the star ought to generate waste heat, which would be detected as extra infra-red radiation, an emission which hasn’t been found from Tabby’s Star. Shostak believes that the Tabby’s Star data may have been caused by a fault with the instrumentation of the Kepler Space Telescope. Even Jason Wright, the astronomer who first proposed the idea of megastructures, believes the effect is most likely caused by a cloud of dust somewhere in the interstellar medium between us and KIC 8462852.

We’re back to the question ‘Why aren’t we seeing Dyson swarms in the universe?’ There are two main conclusions. Either they are trickier than we assume, perhaps because of some unforeseen stumbling block on the path to recursive manufacturing, or intelligent life in the universe is rarer than we think. Armstrong believes the latter. “I would partly reduce the probability that Dyson swarms are doable,” he says, “but I would more strongly decrease the probability that there are other civilisations close. I am becoming more and more convinced by the argument that there is no other advanced life within our Hubble volume.”

Not everyone is so pessimistic. “Maybe there’s no life or maybe it doesn’t tend to build giant solar panels,” says Wright. “Maybe they’ve got better things to do with their time.”

Shostak agrees, pointing out how hard such megastructures are to detect. “There ought to be lots of Dyson swarms around but how would you find them?” he asks. “You can’t see them with the kinds of telescopes we have.”

Shostak remains upbeat, predicting that we fill find evidence of intelligent life in the next two decades. He points to the vanishingly small number of stars we have observed. “The number of stars looked at is in the thousands,” he says. “The number of stars in the Milky Way galaxy alone is 100 billion, so that’s not even one in a million. I usually liken it to saying, you’re going to Africa to look for megafauna that can use their noses to pick up peanuts. You look at one city block of Africa and don’t see any elephants and say, ‘Oh well, they’re not here’. That would be very premature. It’s still early days.”

Variations of Dyson Sphere

Dyson Sphere or Dyson Shell

Despite its allure to writers of science fiction a sphere enclosing a star would be almost impossible to build. A volume of material on the scale of the entire planet Jupiter would be needed to fully surround our Sun.

Dyson Swarm

This would be more possible and relatively easy to construct. Made up of individual structures on separate orbits around the star, a Dyson Swarm could start with a small number of structures at a low energy expenditure, then use the energy collected to build more. A Dyson Swarm harvesting the energy from our Sun could use up half the mass of Mercury or else would require the use of a so-far undiscovered super material.

Dyson Bubble

Instead of structures orbiting the star this is made up of lots of independent objects in static positions. These solar sails would be held in place by using radiation pressure from the star to counteract its gravity. This would still require a new super material as the density of the structures would need to be less than anything we can produce, while reflecting the required amount of radiation. Case study: The dimming of Tabby’s Star

Case study: The dimming of Tabby’s Star

Tabby’s Star is subject to three forms of strange dimming events. First there were the periodical dimming periods of up to 20 per cent flagged up by the Planet Hunters team. Then astronomer Bradley Schaefer of Louisiana State University found that the star had dimmed by 19 per cent between 1890 and 1989. A study of Kepler data from 2009 to 2013 by Ben Montet from Caltech and Joshua Simon of the Carnegie Institute found that the star had dimmed by 1 per cent over three years followed by another rapid dip of 2 per cent in just six months.

Many possible explanations have been offered, from interstellar dust to black holes. Alien megastructures is just one among many possible explanations. But for now, at least, it seems most options are still on the table.