FOCUS ON CORONAVIRUS Can you relax after a first vaccine shot? How deadly is the UK variant? ALIEN MEGASTRUCTURES Inside the hunt for Dyson spheres

WEEKLY January 30 – February 5, 2021 THE OTHER HUMANS The extraordinary story of the Denisovans, the lost people who once shared our world

GASLIGHTING No3319 US$6.99 CAN$9.99 How to stop people messing with your mind PLUS QUANTUM HYPERCHAOS / SEXTUPLE STAR SYSTEM / ROBOSQUID / SCIENCE OF COMPOST / NAKED AMMONITE S cience and technology news www.newscientist.com R E N O C L A F

M A S

44 | New Scientist | 30 January 2021 Features

How to spot an alien megastructure

Intelligent extraterrestrials may have built vast power plants, known as Dyson spheres, around stars. The search for their telltale glow is hotting up, finds Mordechai Rorvig

HERE comes a point at which any Dyson’s original proposal was quite broad. effectively reprocess incoming starlight into advanced alien civilisation worth its In a one-page paper, he suggested simply outgoing infrared light, creating a substantial T salt has to dismantle a neighbouring that advanced extraterrestrial civilisations, excess of infrared light compared with what planet for spare parts. This isn’t an act of should they exist, would be likely to convert you would see from the same star uncovered. vandalism, you understand, but rather a the light from their stars into energy on an In 1960, Dyson thought these precursor to building an enormous solar epic scale, leaving observational clues for megastructures would stick out because power plant that surrounds its entire host those who cared to look. Since then, others strong sources of infrared light seemed star. What else would an ambitious alien have developed the argument. Jason Wright, rare in space. The first proper surveys of the society do to continue its expansion? an astrophysicist at Pennsylvania State cosmos at this wavelength began with the How else would it meet its ever-increasing University, for instance, concluded in a Infrared Astronomical Satellite (IRAS), a demand for energy? 2014 paper that “long-lived civilizations space telescope launched in 1983. The This scenario, or something like it, is the with large energy supplies might… be problem was that it revealed a teeming founding principle of the search for alien expected to rely almost entirely upon multitude of objects radiating in the infrared. megastructures, which in this case would look starlight for their energy needs”. Some were stars that are bigger and brighter something like dark embers when viewed In terms of engineering, theorists say than our sun. Others were stars surrounded through infrared telescopes. The search began there are no serious obstacles to building by clouds of gas and dust, which become in 1960, when physicist Freeman Dyson giant solar power plants around stars. “There heated and radiate infrared light – just like a proposed it as a way of finding alien life. is nothing really weird about the physics of Dyson sphere would. The implications for the More than 60 years later, the hunt for Dyson a Dyson sphere,” says Anders Sandberg at search for Dyson spheres were confounding. spheres, as they are now known, remains a the Future of Humanity Institute at the “The huge and unexpected abundance of minority sport among those involved in the University of Oxford. Any such structure infrared sources made the search harder than search for extraterrestrial intelligence (SETI), probably wouldn’t be a simple monolithic people thought it would be,” says Wright. an enterprise that has focused primarily on sphere. More likely, a Dyson sphere would listening for radio signals from other worlds. consist of a collection of orbiting solar But astronomers are still prospecting for panels that only partially cover the star. Spectral signatures evidence of alien engineering. In particular, In any case, there would be clear Richard Carrigan was among the first to they have been working to put the pursuit observational signatures for astronomers try to sort through the clutter, completing of Dyson spheres on a rigorous scientific here on Earth. Dyson spheres would a landmark search in 2009. A particle footing. Now, they are poring over the most inevitably give off heat and energy that physicist at the Fermi National Accelerator precise cosmic cartography ever produced to would make them extremely difficult to Laboratory near Chicago, Carrigan pored try to find stars that could be surrounded by conceal. Indeed, Dyson sphere hunters need over the spectroscopic data from IRAS. Like swarms of solar panels and distinguish them only make one key supposition – that the a prism, spectroscopy splits the light from from naturally occurring infrared herrings. structure would get warmed by starlight to a source into its constituent wavelengths, They are already narrowing down candidates. a moderate temperature, somewhere above telling a remarkably full tale about the They have even begun to think seriously the background temperature of space. All nature of the object that emitted the light. about the final hurdle: how to tell the warm matter produces an infrared glow, and He identified a handful of stars with the sorts difference between an infrared herring a massive megastructure would produce a of spectral signatures one would expect if and a genuine alien megastructure. great deal of it. The structure would therefore they were surrounded by Dyson spheres. >

30 January 2021 | New Scientist | 45 One star looked particularly promising. figured, we can identify middle-aged “main In the end, however, it was impossible to sequence” types of stars that are less likely distinguish it from an ordinary type of star to be mistaken for dusty impersonators. called a red giant. These are old, bright and The recent releases of data from the emit a great deal of infrared. Worse, they are European Space Agency’s Gaia space often shrouded by dust, meaning they mimic telescope give Dyson sphere prospectors a Dyson sphere’s optical dimness. Carrigan exactly what they need to narrow their didn’t see signs of dust in his candidate’s search. Gaia was launched in 2013. Its mission signature, but that only sufficed to make is to measure the distances to more than a it an unusually undusty red giant. “Not billion stars in the Milky Way and beyond. anything you’d say eureka about,” he says. In the process, it has identified precisely the The experience was instructive, though. It main sequence stars SETI researchers are R E I

demonstrated the difficulty in distinguishing looking for – the least dusty candidates. All N U R B

a Dyson sphere from a natural phenomenon of which explains why Zackrisson and his . S / O

that has a similar spectral signature. Many of team are so keen to make use of the Gaia data, S E

: D

these phenomena turn out to be associated which has been released in three tranches so N U O R

with stellar age: newborn stars form within a far, the latest coming in December 2020. G K C A

cocoon of dense gas and dust, for example, B

; B A

while old stars can blow out a dense shell L A I

Fresh search D of carbon dust that looks a bit like a E M

G T

megastructure. For Dyson sphere searchers, Zackrisson’s first study using Gaia results A / A S

the full list of mimics is long and sobering. came out in 2018. He and his colleagues E “Confirming whether something is really looked for stars that seemed too dim in due to extraterrestrial intelligence and not visible light for their distance, suggesting just some very unusual astrophysics, it’s they might be shrouded. The distance was Artist’s impression of the hard,” says Erik Zackrisson, an astronomer given by Gaia, with an independent estimate Gaia satellite, which helps at Uppsala University in Sweden who is of dimness acquired from ground-based us hunt alien engineering heading the largest ever Dyson sphere search. telescopes. The trouble is that spectroscopic The challenge is to weed out these mimics, data takes a long time to collect. Gaia won’t and we already have a few ideas of how it can be providing much of it, and that limits the sequence star from the Gaia/WISE data set be done. Though IRAS was revolutionary for number of stars you can scour for signs of and look at how its infrared emissions its time, it couldn’t tell how far away the Dyson spheres with this approach. would change if it was surrounded by a Dyson infrared sources it detected were: it only Now, Zackrisson, along with Wright and sphere that covers a given percentage of its measured brightness, not distance. A star others, is experimenting with a new method surface. A sphere with a more complete that seemed bright in infrared could just be a that allows them to scour many more stars. swarm of solar installations would build star that was nearby, rather than an unusually The researchers are combining the Gaia data up more heat and therefore generate more infrared bright Dyson sphere. Conversely, set with observations from the Wide-field infrared light. Zackrisson and his team then an optically dim-seeming star could just be Infrared Survey Explorer (WISE) space compare these emission signatures with the far away, rather than having its visible light telescope, launched in 2009 as a kind of actual emissions from stars in the Milky blocked by a megastructure. Carrigan supercharged successor of IRAS. They are Way to see how many match. This way, they realised that measuring the distance to a focusing on undusty main sequence stars, can calculate the prevalence of possible Dyson candidate would help pinpoint its identity. readily identifiable thanks to Gaia, and spheres with various covering fractions. Initial results, presented last year by Zackrisson’s colleague Matías Suazo showed “ The idea is to release a list of interesting that Dyson spheres covering 90 per cent of their star seem to occur around at most 1 in objects for the whole SETI community” 10,000 stars in the Milky Way. The results served as proof of the principle for this sort But distance can also help to identify stars looking exclusively for infrared brightness of analysis. Well, sort of. Closer attention that are less likely to have dust in the first over and above what you would typically revealed something of a hiccup in that place. The distance to a star can be used to expect, rather than combing through the the candidates that they identified weren’t deduce its true intrinsic brightness, or details of each star’s spectroscopy. “While main sequence stars after all, let alone Dyson luminosity. This in turn correlates with its they are on the main sequence, you don’t spheres: Gaia had been tricked by binary age; old stars like red giants burn bright, for expect them to have a very strong infrared star systems and other stellar objects like instance. Age then speaks to the presence of excess,” says Zackrisson. planetary nebulae, which can be closer or dust – which is more common around very Their first goal is to estimate the possible further than they appear. But Suazo is in young or very old stars. Through this chain prevalence of Dyson spheres in the galaxy. no doubt that, as the team analyses the data of reasoning, Zackrisson and his colleagues To do this, the researchers take every main more closely, such wrinkles will be ironed out.

46 | New Scientist | 30 January 2021 trouble making an indisputable ruling. Nowhere in the wavelengths is it likely to Traces of state unequivocally ALIENS WERE HERE. The only way to know for sure if there is an technology artificial megastructure around a star would Glowing megastructures aren’t be to detect an alien radio signal coming from the only “technosignatures” alien it. Zackrisson’s team plans to hand its list of civilisations might leave behind candidates to radio SETI colleagues for follow up observations. “The idea is to release a list of interesting objects for the whole SETI POLLUTANTS community,” says Zackrisson. Intelligent extraterrestrials are likely Then again, there remains the possibility to have transformed their planet that we might actually see an alien structure, with industry. SETI researchers have rather than rely on infrared inferences. proposed that we could look for their Interferometers, which combine the light non-natural waste products such as from multiple telescopes to enhance chlorofluorocarbons (CFCs), which resolution, have already demonstrated the can persist in the atmosphere for ability to take startling images of faraway tens of thousands of years. solar systems. The Atacama Large Millimeter/ submillimeter Array (ALMA) interferometer PARTICLE COLLIDERS in northern Chile has revealed natural If intelligent aliens are curious megastructures around nearby stars with about the fundamental forces of high clarity, such as phenomena known as nature, they might have built a debris disks – left over from planet formation. The next stage is to whittle down however particle collider that makes our Using earlier instruments, “all the disks many possibilities the team ends up with to Large Hadron Collider look puny. would look like blobs”, says Mark Wyatt at the the most promising. One way to do this is by A black-hole-powered accelerator, for University of Cambridge. Now, with ALMA, scrutinising the spectroscopic information instance, would produce super-high- disks have been clearly observed as vast belts for each star of interest, which provides a energy neutrinos, particles that could of rocky debris, like the rings of Jupiter, but a wealth of insight on the presence and nature be detected from Earth. thousand times larger. Would a Dyson sphere of dust. If a star is surrounded by a common look truly distinctive? “It’s harder to say,” says form of dust known as polycyclic aromatic APOCALYPSE Wyatt. “We don’t know what those look like.” hydrocarbon (PAH) dust, for instance, then Any advanced alien civilisation runs Wright, Zackrisson and Lisse recently ultraviolet light is absorbed and re-emitted at the risk of destroying itself, and the argued that the search for Dyson spheres has specific infrared wavelengths, says planetary fallout might be visible to distant matured to the point that the biggest obstacle astronomer Carey Lisse at Johns Hopkins observers. Nuclear bombs would is now funding. There is precious little money University Applied Physics Laboratory release flashes of gamma rays, devoted to such pursuits. But there are signs in Maryland. See extra light at those but they would be fleeting and this is beginning to change. Last year, NASA wavelengths and you know there is PAH dust. the resulting dust would be hard awarded its first ever grant to search for non- Crucially, a smooth distribution of light to distinguish from that produced radio alien “technosignatures”, including over many wavelengths indicates an absence by an asteroid strike. solar arrays on the surface of exoplanets. of dust – and the possibility that the star in When it comes to finding Dyson question is surrounded by a Dyson sphere. spheres, no one is under any illusions “An optically thick Dyson sphere or swarm about the scale of the challenge. “It’s going should be spectrally featureless,” says Lisse. observations will be able to tease out the to continue to be hard for a long time,” says It is going to be extremely difficult to answers to many questions, including Zackrisson. And yet as they learn more about rule out every natural explanation for what a candidate’s shape, temperature and the natural phenomena that can shroud appears to be a vast solar plant around a star. material composition – including the stars, astronomers are building up a formal “This is a problem for many types of SETI,” presence or absence of dust. The precision framework by which they could, should the says Zackrisson. But he expects his highly with which these questions can be addressed opportunity ever present itself, objectively targeted search to generate at least 100 to will be enhanced by the upcoming James distinguish between a mere dusty shroud 1000 potential candidates, all of which will Webb Space Telescope (JWST) – that is, if and an alien megastructure. ❚ have to be further scrutinised to see if they Dyson sphere searchers can gain precious can be explained with natural phenomena. observing time. “If we find an interesting “We will have our list of weird objects at the candidate, we will need to argue that it is Mordechai Rorvig is a end of the day, and I think it will be interesting enough for JWST regardless science writer based in substantial,” says Zackrisson. of its nature,” says Wright. Massachusetts The idea is that follow-up spectroscopy But even the best spectroscopy will have

30 January 2021 | New Scientist | 47