Our speculations about advanced civilizations invariably invoke Nikolai Kardashev’s scale, on which a Type III civilization is the most advanced, using the energy output of its entire galaxy. Given the age of our universe, a Type III has seemingly had time to emerge somewhere, yet a recent extensive survey shows no signs of them. All of this leads Keith Cooper to consider possible reasons for the lack, including societies that use their energies in ways other than we are imagining and cultures whose greatest interest is less in stars than in their galaxy’s black holes. Keith is an old friend of Centauri Dreams, with whom I’ve conducted published dialogues on interstellar issues in the past (look for these to begin again). A freelance science journalist and contributing editor to Astronomy Now, Keith’s ideas in the essay below help to illuminate the new forms of SETI now emerging as we try to puzzle out the enigma of Kardashev Type III.

By Keith Cooper


It’s not often that SETI turns up with a result that can be considered far-reaching, but the initial results from the Glimpsing Heat from Alien Technologies (G-HAT, or ‘?’ for short) survey, which Paul wrote about in April (see G-HAT: Searching for Kardashev Type III), fit the bill. Using publicly-available data from NASA’s Wide-field Infrared Survey Explorer (WISE), astronomers have searched 100,000 galaxies for anomalous infrared emission that could be an indication of heat emitted from vast energy collectors and their consumers encircling myriad stars.

The idea is that as a civilisation grows more technologically advanced, its hunger for energy increases. Civilisations could build Dyson spheres to capture all the energy from their star; as they spread to other stars, they may build Dyson spheres around them too. After perhaps a few million years, they spread amongst all the stars in their galaxy, building Dyson spheres around every one of them. The Dyson spheres grow hot and re-radiate some of that thermal energy away as mid-infrared radiation. Consequently, a galaxy that has been completely filled with intelligent, technological life should completely alter the light coming from that galaxy, pushing it more towards the infrared.

Yet the search of 100,000 galaxies has not turned up even one single galaxy that has the signature of a civilisation harvesting the energy of an entire galaxy of stars. This would be analogous to a Kardashev Type III civilisation, referring to the scale developed by Soviet astrophysicist Nikolai Kardashev to measure a civilisation’s energy usage. He based his scale on the Milky Way, so a Type III civilisation resident in our own Galaxy would have a total output of 1036 watts; an analogous civilisation in another galaxy may have a higher or lesser energy output as a consequence of the differences in the number of stars between galaxies, but for the purpose of this article we’ll describe them as Type III too.

Going down the scale, there are Type II civilisations, which harness the energy of a single star, which in the case of the Sun would be 1026 watts; again, for other stars, this will vary. Meanwhile a Type I civilisation is able to collect all the energy available to it on its home planet, which for the case of Earth is about 1016 watts. Carl Sagan further developed the scale, adding graduations between the types. Human civilisation comes in at just 0.7 on the Kardashev–Sagan scale.


Image: A Kardashev Type III civilization would be able to exploit the energy of all the stars in its galaxy.

The point of all this is that the G-HAT result throws a spanner in the works, by finding no Type III civilisations anywhere. It demands that we look again at the Kardashev scale and the assumptions that it makes.

Indeed, at first glance it may seem like bad news for SETI. After all, the Universe is very old, as are the galaxies that inhabit it. There should have been plenty of time for a civilisation, or more than one civilisation, to colonise and collect the energy from every star they come across in their galaxy, so why haven’t they?

There are a couple of reasons why the apparent absence of Type III civilisations might not be bad news for SETI. First, although there may be no Type III civilisations out there, Jason Wright of Penn State University, who founded the G-HAT project, says we shouldn’t yet discount civilisations below that level.

“This search would have only found the most extreme case of advanced civilisation, one that had spread throughout its entire galaxy and was capturing and harnessing one hundred percent of the starlight for its own purpose,” he told me when asked about G-HAT’s findings. “Kardashev 3.0 is the most extreme possible case, but there could still be a Kardashev 2.9, where only ten percent of the starlight is being used, or 2.8 where only one percent of the starlight is being used. So we’ve ruled out 3.0, but we’ve not even gotten down to 2.9 percent yet, much less something smaller like 2.5, that could be very hard [to detect].”

So far, the G-HAT analysis has found no galaxies with an infrared emission signature suggesting more than 85 percent of the starlight is being converted into thermal radiation. Fifty galaxies in the survey did stand out as having greater than 50 percent of the starlight being transformed into infrared emission, and follow up work on these is the next step, but to confuse matters there are also natural phenomena that can mimic this infrared emission, chiefly interstellar dust. Starburst galaxies, which are experiencing a severe bout of star formation, produce substantial amounts of dust. This dust absorbs starlight, heats up, and re-emits at mid-infrared wavelengths. The fifty galaxies with high infrared emission are quite possibly starburst galaxies (one of them, Arp 220, certainly is).


Image: Messier 82 (top of image), seen here with the spiral Messier 81, is a starburst galaxy, meaning it is currently forming stars at an exceptionally high rate. This huge burst of activity was caused by its close encounter with Messier 81, whose gravitational influence caused gas near the center of Messier 82 to rapidly compress. This compression triggered an explosion of star formation, concentrated near the core. The intense radiation from all of the newly formed massive stars creates a galactic “superwind” that is blowing massive amounts of gas and dust out perpendicular to the plane of the galaxy. This ejected material (seen as the orange/yellow areas extending up and down) is made mostly of polycyclic aromatic hydrocarbons, which are common products of combustion here on Earth. It can literally be thought of as the smoke from the cigar. Credit: NASA/JPL-Caltech/UCLA.

However, the analysis has not yet looked at galaxy type. “That would be an excellent next step, to separate out the galaxies that have a lot of dust and which we would expect to be giving out a lot of heat, from the ones that have hardly any dust and shouldn’t be giving out any mid-infrared radiation at all,” says Wright. He is referring here in particular to dust-free elliptical galaxies; if one was found to have infrared emission that might be relatively low compared to a starburst galaxy, but was high for an elliptical galaxy, it might signal something unusual.

It would seem then that there could still be life in these galaxies, life that could be technological, star-faring and energy consuming – we’ve barely scratched the surface. And yet, one pertinent question still remains unanswered: where are all the Type III civilisations?

The G-HAT results tell us that Type III civilisations do not exist (or, at best, have a frequency of less than one Type III civilisation per 100,000 galaxies). This is why I suggested at the top of this article that this result is far-reaching – we now know something that we didn’t know before, namely that civilisations do not seem to reach Type III status. This, though, is the second reason why the result is not necessarily bad for SETI. Think of it this way: the Kardashev scale has become part of the SETI furniture since it was first proposed in 1964. The G-HAT result forces us to question our assumptions about the Kardashev scale and broaden our thinking about extraterrestrial civilisations to encompass other ideas.

Of course, any model has assumptions inherent in it. So let’s assume that technological extraterrestrial civilisations do exist in the Universe and that they are far older than we are (dictated by the fact that the Universe is very old, and there has been plenty of time for civilisations to have gotten well ahead of us before there was even life on Earth); these seem fairly safe assumptions for this kind of discussion. Somewhere along the line they are falling off the Kardashev trajectory. Why?

I want to flag up three possibilities. They may not be the only possibilities. We’ll discount for now the notion that civilisations could destroy themselves – once they become interstellar the task of destroying themselves becomes inordinately more difficult, so for our purposes we’ll assume they at least reach the stage of interstellar flight. On what alternate trajectories away from the Type III destination could their evolution take them?

1. They fail to colonise all the stars

This hypothesis would to an extent fit with the G-HAT observations – extraterrestrial civilisations haven’t built Dyson spheres around 100 percent of the stars in any of 100,000 galaxies, but the result leaves room for them to have done so around a smaller percentage of stars. Perhaps the best reasoning as to why an advanced civilisation possessing the ability for interstellar travel would fail to colonise an entire galaxy is Geoffrey Landis’ percolation theory.

Landis makes the assumption that interstellar travel is short haul only. We might be able to make direct flights to alpha Centauri or epsilon Eridani, but anything much beyond that, moving at just a small fraction of the speed of light – let’s say between 5 and 10 percent – is going to take far too long. So instead, civilisations will hop across the cosmos via the stepping stones of the colonies they set up along the way. For example, imagine three worldships leaving the Solar System for pastures new: let’s say alpha Centauri, epsilon Eridani and Barnard’s Star, all of which are relatively nearby. They set up colonies there, begin building Dyson spheres and perhaps, after a few centuries, those colonies are ready to send out their own pilgrims to new stars further afield, which then found new colonies and, after a few centuries, they too head out on voyages of colonisation, and so on. Over the millennia, humankind’s reach gradually telescopes outwards.

What Landis realised was that not all colonies will seed daughter colonies. The drive to go further will not exist in every colony; cut-off from their mother-world, Earth, by time and space, they build their own cultures, their own histories, and face their own, perhaps unique, challenges. Some will be content to not explore further. Others may destroy themselves, or exhaust their resources before they can build a Dyson sphere. In some cases, there may be no worlds in nearby systems suitable for colonisation. The consequence of any of these possibilities is that some colonies will become dead ends and will fail to colonise further.

To model this, Landis assigns a probability of being colonised to a given planetary system. If that probability is above a critical threshold, then it will be colonised. If it is below the threshold, colonisation of that system will not take place. Eventually, all colonies may result in dead ends, ultimately limiting the extent to which that species colonises the galaxy it exists in. Even if there is one line of colonisation that does continue for a time, there will be voids all around it, left empty by the dead end colonies. A civilisation would struggle to reach Type III status in this fashion.

Landis’ percolation theory is not without its critics. Robin Hanson of the University of California, Berkeley, points to economics and argues that the only way to survive would be to keep up with a colonising wave because the wave would consume all the resources, leaving little of value behind it, a kind of ‘burning of the cosmic commons’ as Hanson describes it. Jason Wright is also critical, arguing that the proper motion of stars would eventually allow active colonies to spread to other stars. For what it’s worth, Landis agrees that the percolation model is not without its problems.

Landis counters that the motion of stars is slow, at least compared to the lifetimes of civilisations in human history, although Wright points out that all a colony then has to do is wait for one of its neighbours to die off before moving in. Landis is unperturbed by the critics, however.

“A lot of people have commented saying they don’t think it is a sophisticated enough model and that they think it needs more work, and that’s fair,” he told me during an interview in 2013. “I just worry that a model that has too much sophistication into which you are putting data that has no validation is hard to really justify.”

Perhaps percolation theory as it stands isn’t therefore the best solution, but instead maybe it’s a good starting point for considering alternatives to how civilisations could migrate through a galaxy.

2. Their energy requirements are low

Another alternative may be that they never really begin to climb the Kardashev ladder at all, which could lead to two outcomes.

Serbian astrophysicist Milan ?ircovi? has described civilisations that are driven by optimisation, rather than expansion. The optimisation is focused primarily on computation (Jason Wright suspects that Type II and Type III civilisations would use large amounts of their energy for computing, which produces heat). An optimised society would not need to colonise other stars and capture their energy because they would lack the population or computing power that would otherwise soak up vast amounts of energy.

“An optimised society is intrinsically less likely to be observed because most of the things that we tend to associate with advanced technology and advanced societies actually consist of waste energy and the waste of resources,” ?ircovi?, referring to the Kardashev scale, told me in an interview around five years ago.

An optimised society need not be limited to one planetary system – they may still wish to explore, sending out probes to all corners of their galaxy, but colonising star systems to harvest their energy and resources is not on their list of ‘to do’ things. Rather than building galactic empires, optimised civilisations could be like the ancient Greek city states, which would send out scouts just to explore, says ?ircovi?.

Jason Wright acknowledges that a galaxy-spanning civilisation need not be a Type III civilisation; it could still be possible to colonise a galaxy without having to build Dyson spheres around every star. Such galaxy-spanning civilisations could be very hard to detect. However, if an advanced civilisation has had time to colonise a galaxy, why would they not build all those Dyson spheres? The distances involved would mean that colonies, or clusters of close colonies, would develop their own societies relatively independently of the others. Some may chose to become optimised, others may be expansionist and energy-hungry, but the result would be a galaxy-spanning civilisation that does not use all the energy of that galaxy.

3. Black holes are more interesting

I confess, I’m rather taken with this idea. It could still be wrong, but it strikes me as being more purposeful than percolating slowly and somewhat randomly through a galaxy, and more ambitious than an optimised city state.

Suppose Kardashev is right, and Milan ?ircovi? is wrong, and that civilisations actively seek energy. So let’s imagine that a civilisation reaches Type II status, after which it heads for the stars, perhaps even building Dyson spheres around some of them. Estimates suggest that there could be as many as 100 million stellar mass black holes in our Galaxy. Some of them remain dark, while a few are lit up in X-ray binary systems, feeding off a companion star. Sooner or later a star-faring civilisation is going to bump into a black hole. What then?


Image: Simulated view of a black hole in front of the Large Magellanic Cloud. The ratio between the black hole Schwarzschild radius and the observer distance to it is 1:9. Of note is the gravitational lensing effect known as an Einstein ring, which produces a set of two fairly bright and large but highly distorted images of the Cloud as compared to its actual angular size. Credit: Alain r (Own work) [CC BY-SA 2.5], via Wikimedia Commons.

Black holes seem to hold a special fascination for physicists: they create the most extreme gravitational conditions in the Universe, making them a great place for thought experiments. Numerous physicists including John Wheeler, Roger Penrose, George Unruh and Princeton’s Adam Brown have all speculated on methods by which, in principle, it might be possible to draw energy from a black hole. And my, so much energy! Paul Davies in his book The Eerie Silence suggests that a spinning black hole could power our present human levels of energy consumption for at least a trillion trillion years, long after the stars have gone out.

There are numerous options for deriving energy from black holes. Hawking radiation is not the best option, because it leaks out at a trickle, is very low temperature and is difficult to bottle. Small black holes that evaporate relatively quickly would be more efficient for this, but they would not last long. Hawking radiation would make the perfect waste disposal system though – drop your rubbish into the black hole, wait a little while and get energy from Hawking radiation back out.

Then there is the energy radiated by the hot plasma in an accretion disc around a black hole, which is often funneled away in a magnetically collimated jet. This could be created artificially – perhaps by sending a steady stream of asteroids and comets, perhaps even planets and stars themselves using Shkadov thrusters (giant mirrors larger than a star, which act as immense solar sails, the mirror’s huge gravity pulling the star along with it) to nudge the star towards the black hole. Alternatively, there are instances in nature whereby a star naturally exists next to a black hole – the aforementioned X-ray binaries (though in many X-ray binaries the black hole is substituted for a neutron star). Jason Wright suggests that the energy efficiency of such a system would be 10 percent, making it the most efficient sustainable method of converting mass to energy.

Then there is the rotational energy of a spinning black hole. To illustrate the concept, in their book Gravitation, Charles Misner, Kip Thorne and John Wheeler imagined some form of cosmic dump truck swooping down through a black hole’s ergosphere – a region just outside a rotating black hole where an observer is forced to rotate with the black hole, but at the same time can also extract energy from the black hole. The dump trucks, each packing a million tonnes of rubbish, take a particular trajectory through the ergosphere and are able to tip out their industrial waste into the black hole. The dump trucks recoil from the ejection of the rubbish and are catapulted back the way they came, stealing away some of the black hole’s rotational energy in the process. Because the mass of the black hole has increased by the mass of the garbage dumped into it, the mass-energy of the black hole is higher than before the dump truck entered it, allowing the truck to leave with more energy than it started with. To put this in terms of the amount of energy available, up to 29 percent of the mass of the black hole is expressed in terms of its rotational energy, according to Paul Davies – this is leagues above the one percent of a star’s mass that is radiated away over a stellar lifetime.


Image: Artist”s impression of a black hole and a normal star separated by a few million kilometres. That’s less than 10 percent of the distance between Mercury and our Sun. Because the two objects are so close to each other, a stream of matter spills from the normal star toward the black hole and forms a disc of hot gas around it. As matter collides in this so-called accretion disc, it heats up to millions of degrees. Near the black hole, intense magnetic fields in the disc accelerate some of this hot gas into tight jets that flow in opposite directions away from the black hole. Credit: ESO/L. Calçada.

The difference between collecting energy from stars and latching onto black holes is that you can do more with black holes than simply generating power, and it is these extra factors that could make them more attractive than colonising the stars. For one, black holes could potentially make the most powerful computers in the Universe. A computer’s computational power is a function of both its computational efficiency and its mass. Black holes have great mass, but computational efficiency? That would take a bit of organising. The trick is to use Hawking radiation, which is formed of pairs of virtual particles that appear close to the black hole’s event horizon.

For each pair, one particle heads inwards towards the black hole’s singularity, while the other quantum tunnels its way through the event horizon and escapes. However, both particles are forever connected via quantum entanglement. Now, send matter into the black hole – perhaps the waste on the dump trucks – in a specific fashion to ‘program’ the black hole, and it will interact with the infalling Hawking radiation particles. This interaction, specifically fine tuned, will then change the state of the outgoing Hawking radiation particle via entanglement, hence producing an ‘output’. Of course, all the Hawking radiation would have to be gathered, sorted through for the relevant bits of data and processed using knowledge of quantum gravity, a theory that remains stubbornly beyond our limits for the time being.

Then there is the possibility proposed by Sir Roger Penrose that black holes are the birth-sites of new universes; an advanced civilisation may choose to somehow enter one of these universes in a black hole, therefore disappearing from our Universe.

Pressing black holes into service could possibly be within reach of an advanced civilisation; black holes provide astoundingly attractive destinations for intelligence. Clément Vidal, in his book The Beginning and the End, points out that there is a surprising over-abundance of X-ray binaries within three or four light years of the galactic centre – maybe advanced civilisations around their stellar-mass black holes migrating towards the supermassive black hole at the centre of our Milky Way galaxy?

Perhaps. The stars still have their attraction, but as the G-HAT result shows, we need to start looking for alternatives to Type III civilisations. These are just three ideas – your own ideas may well be better!