The search for extraterrestrial intelligence (SETI) has traditionally focused on looking for messages, encoded in radio waves or laser pulses, that aliens might have sent our way. There is, however, another approach to SETI: we could look for evidence of activity that would allow us to deduce the presence of intelligence. The detection of an alien message aimed directly at Earth’s inhabitants would undoubtedly be more exciting than the discovery of, say, exhaust from a distant antimatter rocket – but the key conclusion would be the same: we would know that we were not alone.
In 1960, Freeman Dyson pointed out that the development of technologically advanced civilisations would be limited by energy considerations: growing civilisations require access to increasing amounts of energy. Dyson suggested that a sufficiently advanced civilisation would eventually need to harvest the entire energy output of its home star – which it could achieve by dismantling a planet and using the resulting material to create a host of solar collectors. Such a star-enshrouding swarm came to be known as a Dyson sphere. A galaxy-spanning civilisation – a so-called Kardashev type-3 civilisation – would do this for all the stars in its galaxy.
It would be difficult to hide such astroengineering activity. As Dyson himself pointed out, the laws of thermodynamics mean that a Dyson sphere could be detected by the glow of its radiated waste heat. Visible starlight would be replaced by radiation that peaked in the mid-infrared. If the creation of Dyson spheres is indeed a common activity then the signature of a type-3 civilisation would be a galaxy dim in the visible part of the spectrum but bright in the mid-infrared part of the spectrum. In other words, we can do SETI by looking for the absence of light combined with the presence of waste heat.
Jason Wright, an astronomer at Pennsylvania State Universe, and his colleagues have recently taken precisely this approach to SETI. They wrote software to assess the 100 million objects contained within the catalogue generated by NASA’s WISE (Wide-field Infrared Survey Explorer) mission. Catalogue objects that were merely mission artefacts, or were clearly not galaxies, were rejected by hand. The team then began looking for optically dim and infrared bright objects. They ended up with a target group of about 100,000 galaxies emitting large amounts of radiation in the mid-infrared. None of those 100,000 galaxies contained a civilisation that was reprocessing more than 85% of its starlight into the mid-infrared: there are no galaxy-spanning type-3 civilisations in our cosmic neighbourhood. Only 50 of those 100,000 galaxies possessed a spectrum that was consistent with the reprocessing of 50% of its starlight into the mid-infrared; but of those few galaxies the infrared brightness can almost certainly be explained by natural processes. So although Wright and his colleagues found a few objects that might be of interest to SETI scientists (and are certainly of interest to traditional astronomers), in essence they registered a null result.
Now, you could argue that we have no idea of the choices that technologically advanced civilisations might make. Perhaps they won’t build Dyson spheres. Well, that’s surely the case. Perhaps they use rotating black holes or antimatter annihilation or even something we don’t yet understand for power generation. Nevertheless, any astroengineering that takes place on a galaxy-wide scale is going to generate waste heat. Wright and his team were looking for signs of that waste heat and they found nothing.
Traditional SETI has so far failed to find a civilisation in our Galaxy at the type-1 or type-2 stage. This recent result from Wright and his colleagues suggests that type-3 civilisations anywhere are rare or non-existent. What are we to make of these null results? Well, perhaps our understanding of the development of technological civilisation is flawed; perhaps aliens are “green” and their long-term survival requires a more sustainable approach to energy use. Perhaps. But there’s another, more obvious explanation: we are alone in the universe.