© unknown
After examining some 100,000 nearby large galaxies in 2015 a team of researchers lead by The Pennsylvania State University astronomer Jason Wright concluded that none of them contain any obvious signs of highly advanced technological civilizations. Turning his focus closer to home this past spring of 2017, Wright proposed that an advanced civilization-an indigenous technological species could have arisen in the solar system before Earth-bound life did. Wright suggests that traces of its technology-"technosignatures"-may have survived, provided they were made of material not easily degraded by erosion or time and may remain hidden awaiting discovery under the surface of Venus and Mars.

"As we improve our understanding of ancient Earth and the history of our solar system, perhaps we may someday uncover evidence that suggests the activity of another technological civilization right here in our neighborhood," said Andrew Siemion, the director of Berkeley's SETI Research Center.

Wright suggests there could have been an explosion in life around the time of or after the Cambrian period, when complex animals first appeared, according to fossil records. A cosmic catastrophe may have destroyed this early species, Wright suggests, erasing signs that it ever existed and "forcing the biosphere to 'start over' with the few single-celled species that survived." We may have already seen technosignatures in geological record, but mistaken them for natural phenomena, Wright said. Or, the evidence may be long gone, erased from the surface by shifting tectonic plates. "The Earth is quite efficient, on cosmic timescales, at destroying evidence of technology on its surface," he concludes in the paper.

Wright's 2015 study --by far the largest of study of its kind to date-earlier research had only cursorily investigated about a hundred galaxies-- looked for the thermodynamic consequences of galactic-scale colonization, based on an idea put forth in 1960 by the physicist Freeman Dyson who postulated that a growing technological culture would ultimately be limited by access to energy, and that advanced, energy-hungry civilizations would be driven to harvest all the available light from their stars.

Wright's team searched for type 3 civilizations in an all-sky catalogue from NASA's Wide-field Infrared Survey Explorer (WISE). They looked for objects that were optically dim but bright in the mid-infrared-the expected signature of a galaxy filled with starlight-absorbing, heat-emitting Dyson spheres. After using software to automatically sift through some 100 million objects in the WISE catalogue, Wright's student Roger Griffith examined the remaining candidates by hand, culling those that weren't galaxies or that were obvious instrumental artifacts without success.

"Looking for the absence of light as well as the waste heat like Wright and his colleagues have done is really cool," says James Annis, an astrophysicist at the Fermi National Accelerator Laboratory who in the late 1990s used different methods to survey more than a hundred nearby galaxies for type 3s. "In some sense it doesn't matter how a galactic civilization gets or uses its power because the second law of thermodynamics makes energy use hard to hide. They could construct Dyson spheres, they could get power from rotating black holes, they could build giant computer networks in the cold outskirts of galaxies, and all of that would produce waste heat. Wright's team went right to the peak of the curve for where you'd expect to see any sort of waste heat, and they're just not seeing anything obvious."

"Life, once it becomes spacefaring, looks like it could cross a galaxy in as little as 50 million years," Annis says. "And 50 million years is a very short time compared to the billion-year timescales of planets and galaxies. You would expect life to crisscross a galaxy many times in the nearly 14 billion years the universe has been around. Maybe spacefaring civilizations are rare and isolated, but it only takes just one to want and be able to modify its galaxy for you to be able to see it. If you look at enough galaxies, you should eventually see something obviously artificial. That's why it's so uncomfortable that the more we look, the more natural everything appears."

Annis suspects that fast-gamma-ray bursts which were more frequent in the cosmic past, until recently suppressed the rise of advanced civilizations and that we inhabit "the beginning of history."

"If there are any real aliens, they are likely to behave in ways that we never imagined," said Freeman Dyson. "The WISE result shows that the aliens did not follow one particular path. That is good to know. But it still leaves a huge variety of other paths open. The failure of one guess does not mean that we should stop looking for aliens."