Is anybody out there? THOSE who look for intelligent life on other planets usually confine their efforts to listening for radio signals from stars that are, on the cosmic scale of things, fairly close by. The resounding silence so far does not deter enthusiasts, who point out that there are 100 billion stars in Earth’s home galaxy, the Milky Way, almost all of which have not yet been examined. He who lives by the billion, however, may also die by it. For another number measured in billions is the Milky Way’s age in years. Habitable planets have probably been present in it for at least 10 billion years, which is more than twice the span of Earth’s existence. And a spacefaring civilisation, even one relying on craft travelling at far below the speed of light, would be able to colonise the entire galaxy in a few hundred million years. It therefore follows that if intelligent, technologically capable life forms had emerged elsewhere in the Milky Way, they would probably have done so long enough ago that they would, by now, be everywhere—which evidently they are not. This line of reasoning suggests humans really are the only intelligent life in this particular galaxy.

Perhaps, therefore, the search for aliens is looking in the wrong place. The calculation that intelligent life will rapidly colonise its entire home galaxy—first made by Michael Hart, an American astrophysicist, in 1975—suggests it is not other solar systems which should be scoured for little green men, but other galaxies. And this is just what Roger Griffith, an astronomer at Pennsylvania State University, has done.

Appropriately for an enterprise which sits on the cusp of science and science fiction, Dr Griffith’s search is based on an idea that came from a story. It was written in 1937 by Olaf Stapledon and proposed that an advanced civilisation with a huge demand for energy might sate that demand by building a sphere enclosing its home star. It could thus capture and use every photon emitted.

Outside the sphere, the star in question would be invisible. But, as Freeman Dyson, a polymathic physicist, pointed out in 1960, the sphere itself would radiate in the infrared, since whatever the advanced civilisation was actually doing with the starlight, the result would be heat which had to be dumped into outer space. Rather unfairly on Stapledon, such putative pieces of spaceborne civil engineering are thus known as Dyson spheres.

Putting Mr Dyson’s and Dr Hart’s ideas together, Dr Griffith reasoned that a galaxy inhabited by Dyson-sphere-constructing aliens would have an unusual, infrared-rich and visible-light-poor spectrum. With the aid of an American space telescope called the Wide-field Infrared Survey Explorer, he searched 100,000 galaxies for such spectra. What he found, as he reports in the Astrophysical Journal, is tantalising.

No galaxies appeared to host civilisations that were using more than 85% of the available starlight as a power source. Fifty, however, were red enough to be hosting aliens gobbling up half or more of their starlight. Since even the most enthusiastic colonists would not, presumably, set up shop around every single star, and also because realistic versions of Dyson spheres would not totally enclose a star, these galaxies might indeed be the empires of individual alien species.

Power-hungry aliens are not, sadly, the only explanation for the spectra Dr Griffith has found. More prosaic things, such as vast clouds of interstellar dust, might produce a similar signal. Nevertheless, these 50 unusual galaxies (and also 95 more which had spectra that were weird in other ways) might repay further study. The odds are that Dr Griffith’s discovery will have a humdrum explanation. But it is just possible he has answered the age-old question of whether humanity is alone.