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It’s a cosmic game of hide-and-seek. A team of astronomers say that the next search for advanced extraterrestrial civilisations should look for stars – or even galaxies – that have vanished without a trace, as anything so unexplainable could only be due to life far more intelligent than us.

at Uppsala University in Sweden says this crazy idea has been gnawing at her since her first year of graduate studies. Now she and two undergraduates have finally taken the plunge. They scoured multiple surveys of the night’s sky by eye in order to see if any of nearly 300,000 light sources disappeared from one survey to the next.

So far the results are mixed. The team found one interesting artefact that looks like it might have vanished, but they can’t be sure. “It was a depressing case in the sense that we neither could reject it and neither could we say that it was a real candidate,” says Villarroel. Although the team checked for so-called false positives, throwing out hundreds of similar disappearing objects, this one withstood all tests – but only just.

Even if the disappearance is real, there could still be an astrophysical explanation. Quasars – the bright centres of galaxies powered by supermassive black holes – can shut down in less than a decade and drop drastically in brightness. Stars, too, can be highly variable.

Tech vs magic

That’s why Villarroel and her colleagues plan to search for this missing object (and any others found in the future), on the largest telescopes. If it is still not visible, then they will be able to rule out most astrophysical phenomena and say with more likelihood that it has vanished. Only then will they begin speculating about extraterrestrial causes

“I think it’s a very reasonable thing to do,” says at Boise State University, Idaho. “It’s a deliberate search for something very unusual, which could be hiding in existing data across time. At this stage of the game, it’s a very limited search, but it illustrates well what can be accomplished.”

Unlike Tabby’s Star, a baffling star that dips in brightness and made headlines last year when astronomers suggested that “alien megastructures” could be the culprit, these objects would have no physical culprit behind their sudden disappearance – it’s just not possible for something to suddenly vanish from the universe without a trace. Instead, Villarroel and her colleagues invoke Arthur C. Clarke’s third law: “Any sufficiently advanced technology is indistinguishable from magic.”

So while other teams search for possible signatures of astro-engineering like the one suspected around Tabby’s star, Villarroel and her colleagues are searching for things that seem impossible. If they confirm a star that has vanished without an accompanying supernova explosion or a galaxy that has disappeared from view, there’s simply no physical explanation – save for aliens.

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