Searching for alien super civilisations with G-HAT
Over the last couple of weeks you may well have seen headlines like 'Search for alien life in 100,000 galaxies draws a blank' . The search these articles are talking about was conducted by a group of astronomers called G-HAT (Glimpsing Heat From Alien Technologies). Now, I've been out of the research game for a while, but I don't remember my research ever giving me such a wonderfully yes/no result* - usually it just gave me three more, more complicated, questions to answer. To be fair most news outlets gave the story more detail in the actual article, but I thought I'd get some more information– and luckily for me Professor Wright from G-HAT was happy to answer a few questions over e-mail. Before we start, a bit of background for those of you who aren't au-fait with the hunt for alien super civilisations**:
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| Above: A visulaisation of a Dyson energy collecting structure (more about those below) courtesy of the Smithsonian Air and Space Magazine. |
The idea behind G-HAT is that any energy used to do work will eventually be emitted as waste heat, so any technology being used, anywhere, must emit heat - this isn't just an imperfection in our technology, it's actually the result of fundamental law of physics (the 2nd law of thermodynamics). Since even a very advanced civilisation shouldn't*** be able to ignore basic physical laws they should put out a heat signature. It's this that G-HAT is searching for, in various ways. This general approach is called 'artefact SETI' or sometimes 'passive SETI' – searching for aliens by way of the unintentional traces they might leave.
One way a powerful civilisation might collect energy for use is to surround a star in energy collectors – this is called a Dyson structure, after the person who dreamed up the idea. This would make the star black in visible light but, thanks to the second law, that energy would still be emitted as infrared once it had been used. A very, very powerful civilisation might do this to many, or even all, the stars in their home galaxy****. This would make that galaxy visible only in the infrared. So a galaxy or star cluster that was a strong emitter of waste heat but dim or invisible in visible light might – just might – have be modified by an extra terrestrial intelligence.
The results from G-HAT that have generated the headlines were the result of winnowing through data on 100,000 galaxies from the WISE (Wide-field Infrared Survey Explorer) space telescope, looking for such signatures.
John: Hi Professor, thanks for taking the time to answer some questions. Collecting the energy output of an entire galaxy sounds like an extreme bit of engineering – like a terrestrial government damming every river in their country for power. Do you think it is possible to say whether the ‘maximum energy collection’ route is an inevitable one for a technological race, or might it just be one of many possible routes?
Professor Wright: I agree it is extreme, but extremes happen. Humanity has driven entire species extinct with hunting; humanity taps over 99% of the Colorado river; humanity could very plausibly use every drop of easily extracted oil (exhausting our coal would take millennia, but it’s not inconceivable). The US has dammed many major rivers, and if it needed the energy and environmental concerns were not an issue, it would be quite reasonable to dam them all.
This route for maximum collection of stellar energy is, of course, just one of many possible routes. We checked it first because it was the easiest one to check. But until we checked we didn’t know that it never happened, even once, in 100,000 galaxies.
John: The main object of the search of the WISE catalogue you recently published was Dyson structures that absorb starlight, but any civilisation using large amounts of energy in it’s home galaxy would, in principle, also been detectable: Their galaxy would give out higher than usual amounts of infra red. I’ve heard it suggested that they might engage in other mega engineering projects, such as collecting power from their galaxy’s central black hole or turning star forming nebula into many red dwarf stars to provide a more long lived stellar population. Would it be possible to look for the heat signature of such an, advanced but non Dyson structure building, civilisation?
Professor Wright: Sure. Collecting energy from a supermassive black hole instead of a star might look a bit like an obscured active galactic nucleus (a natural phenomenon) and we do see those in the mid-infrared. It might be very hard to distinguish an artificial one from a natural one, though. Altering the stellar population should also be detectable, though this one wouldn’t necessarily generate waste heat. We looked for exactly that possibility on a galactic scale in our “red spiral” search (see my blog posts and my answer below). All of this that you describe is part of “artifact” or “Dysonian” SETI.
John: Are there any kinds of plausible mega engineering that wouldn't have been detectable by comparing the IR wavelengths you’ve used in the most recent study, and visible outputs?
Professor Wright: Well, collecting only 10% of starlight would have escaped our notice. It’s also physically possible that the radiation temperature of the collectors could be too high or too low for us to notice. This would require the radiators to be many orders of time larger than necessary (at distances of 100 AU instead of 1 AU, so 10,000 times more surface area) or else so hot that they should not be solid. Of course, we don’t know what advanced engineering might be capable of.
Another possibility is that they are using most of the energy in some way, such as matter-to-energy conversion, or for laser, radio, or other forms of communication. In this case, only a small fraction would be radiated as thermal photons. They could also radiate the energy away in another form, for instance, neutrinos, which we would not detect, using physics we do not understand.
John: The search has returned around 50 objects that might (as one of many possible explanations) be a galaxies with up to 85% of their light being collected for use, and over 90 galaxies that are consistent with a civilisation collecting up to 25% of their energy and poorly studied. Are there any of the relatively unknown ones that stand out in your mind (for SETI or general astrophysics)?
Professor Wright: None of them look particularly strange from a SETI perspective. More interesting are 5 “red spiral” galaxies that seem to lack the UV emission that comes with star formation [John's note: Here's Prof Wrights blog entry on these] . How one could form a spiral without star formation is a puzzle, especially since these galaxies have rather large amounts of MIR emission (as would be expected for star formation). I’d like to know what’s going on in these galaxies.
John: G-HAT could apply to less massive things than entire galaxies – a star system in the Milky Way, for example, might be modified with a Dyson structure. Given our current level of technology, what is the smallest waste heat signature it would make sense for us to look for?
Professor Wright: We can get down to a few percent waste heat before it’s just too hard to notice anything. This is the level we often see from what are called “debris disks” — the leftover pieces from planet formation (so, sort if like gigantic versions of the asteroid belt).
John: I’ve read you referencing ‘Romulan warp-core singularities’ on your blog while on the subject of what unknown power sources an alien civilisation might use – I actually know where that reference is from as I’m a bit of a Sci-Fi addict (Star Trek the Next Generation, a show I grew up with). Although most science fiction is usually far from scientifically rigorous, do you feel that science fiction plays a role in helping us imagine what we expect to see from a possible alien civilisation?
Professor Wright: I think some science fiction, particularly the “hard” variety championed by the likes of Asimov, Sagan, Niven, Reynolds, and Robinson, is where some of the best ideas about advanced civilizations and space exploration come from. This is an important role, because it helps frame the problem for scientists, who can then collect data to further guide the conversation. We can rule out some of the more exotic scenarios in these works of fiction.
But you’re right that these are works of fiction and it’s important to separate the tropes of the genre from the ideas they put forward. Scientists with similar ideas might write in their genre, an article in the likes of the Journal of the British Interplanetary Society; authors have a more artistic (and slightly more lucrative) route in their genre, science fiction.
But my use of the ST:TNG reference wasn’t about that; it was an arch way to acknowledge that advanced civilizations will have access to engineering, and probably physics, that we can’t imagine yet. As scientists, need to keep that in mind when designing our experiments. We can’t let it paralyze us, though.
John: Thank you very much for taking the time to answer Professor.
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| Above: Courtesy of the Galaxy Zoo project, four of the mysterious red spiral galaxy's compared to their normal counterparts. |
More information on G-HAT can be found on professor Wrights blog - which goes into the future of G-HAT and some of the more mysterious finds like red spiral galaxies in more detail. Here's the link.
Elsewhere on the internet:
*Admittedly mine was a lot duller (the abstract of one of my research posters is here if you're curious just how dull).
** What's up with you? All the cool kids are doing it.
*** It's not beyond all imagining that some quirk or loophole we've yet to discover allows a way around the 2nd law, but since we can't know what that might be unless we discover it ourselves we can't use that idea to learn anything useful - so we'll assume for the moment the 2nd law is true for every civilisation.
****There's a scale of how energy hungry a civilsation can get, called the Kardashev scale. On that scale turning your whole galaxy into a power station makes you a type 3 civilisation, the highest level on the scale.
** What's up with you? All the cool kids are doing it.
*** It's not beyond all imagining that some quirk or loophole we've yet to discover allows a way around the 2nd law, but since we can't know what that might be unless we discover it ourselves we can't use that idea to learn anything useful - so we'll assume for the moment the 2nd law is true for every civilisation.
****There's a scale of how energy hungry a civilsation can get, called the Kardashev scale. On that scale turning your whole galaxy into a power station makes you a type 3 civilisation, the highest level on the scale.
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