A galaxy-spanning empire that is millions, perhaps even billions of years old, and which has conquered every star in its domain and now harnesses mind-blowing amounts of energy. It’s acclaimed as the pinnacle of technological achievement and the ultimate trajectory of technologically intelligent life. It’s not necessarily fiction, however. Such civilizations are one of the main targets of SETI, the Search for Extraterrestrial Life.

It’s a vision both exciting and terrifying in equal measure. Exciting because our ideas about technologically developed civilizations are essentially extrapolating into our own possible future. And its terrifying, because nothing seems to be out there. Are we truly alone?

Such civilizations, if they do exist, are described as Kardashev type III civilizations. Let’s explain. Sixty years ago, the Soviet astrophysicist Nikolai Kardashev of the Sternberg Astronomical Institute in Moscow developed a system for characterising civilizations based on their energy consumption. His genius was in realizing that there was a cost to sending interstellar transmissions of the kind that we hope SETI will one day eavesdrop. The energy required to broadcast interstellar distances and be heard is immense (which is why our television signals are unlikely to be detected beyond our Solar System).

“The Kardashev scale was the obvious thing to do, because Kardashev wanted to think about how powerful a radio signal would need to be in order to be detectable, and what that implied in terms of energy consumption,” says Michael Garrett, who is the Director of the Jodrell Bank Centre for Astrophysics at the University of Manchester, and Vice-Chair of the International Academy of Astronautic’s SETI Permanent Committee.

Kardashev invented a three-tier scale of energy consumption. A type I civilization is able to harness and utilize the entire energy available to it on a planet, estimated to be between 10^16 to 10^17 watts. Compare that with our current energy expenditure, which in 2018 was measured by the International Energy Agency to be 1.9 x 10^13 watts.

A type II civilization is a big step up. Far from being content with just the energy available to it on a single planet, a type II civilization would collect all the entire radiated by its star. This is popularly envisaged as a spherical shell, called a Dyson sphere, encapsulating the star. One of the most notable renditions of such a structure was seen in the Star Trek: The Next Generation episode ‘Relics’, where it was depicted as a solid sphere with a habitable inner surface. Alas, a solid sphere would not be stable, for even a small gravitational nudge, perhaps from a passing star, could ultimately cause the sphere to collide with the star at its centre. Instead, a vast swarm of individual energy collectors, each with maneuvering thrusters for station keeping, is the more idealized solution. Yet there are still hazards. An accident could create a cascading wave of collisions, like Kessler syndrome only on a much larger scale. However, if a ‘Dyson swarm’ could be carefully constructed and maintained, then the amount of energy available to a civilization would be huge, although the exact amount would depend on the luminosity of the star, since not all suns shine equally. A Dyson swarm around our Sun could collect on the order of 10^26 watts.

Then there are the type III civilizations that achieve dominion over all the starlight in a single galaxy, perhaps by traveling to all the stars and constructing Dyson swarms around each. The total energy they would have available for consumption is estimated to be between 10^36 and 10^37 watts, though again it depends on the size, star-formation history and luminosity of the galaxy in question.

The scale has been finessed over the years — Carl Sagan introduced decimalized graduations, and human civilization today rates at 0.728 on the Kardashev scale — but overall it continues to remain prominent in our thinking about extraterrestrial life.

Yet there’s something paradoxical about The Kardashev Scale. And it’s questionable whether it would have been proposed today, in an era when we are all too aware of the damage that rampant industrialization and energy consumption inflicts upon the environment.

In 1972 the Club of Rome published their Limits to Growth report, which predicted that if the global population continued to grow and if energy use kept increasing unabated, human civilization would collapse by the mid 21st century.

Indeed, Brendan Mullan and Jacob Haqq-Misra of the Blue Marble Space Institute of Science in Seattle have expressed skepticism that the Kardashev scale is feasible given how climate change and population growth is stymying our efforts to grow towards becoming a type I civilization.

On the other hand, critics of the Limits to Growth report argue that we can protect the environment and continue to enjoy growth. In his vision of how humanity might reach type I status, Caltech’s Jonathan Jiang argues that good stewardship of a planet is all part of becoming a type I civilization. If we are able to transition from fossil fuels to nuclear and renewable power over the next 20 years, while adhering to the internationally agreed target of keeping warming to less than 2 degrees, Jiang suggests that we could become a type I civilization in another 349 years. Mark the year AD 2371 on your calendar. Moving energy production into space, perhaps as the beginning of building a Dyson swarm, or at least part of one, would also remove some of the environmental effects.

Believe it or not, environmental concerns could also affect type III civilizations. To build a Dyson swarm you need a lot of raw material – asteroids and moons wouldn’t be nearly enough. Instead, Freeman Dyson casually suggested that we could one day dismantle Jupiter.

As well as how this might affect any native life in those planetary systems, it also uses a bunch of raw material.

“What if extraterrestrial intelligence tries to become a type III and then fails?” muses Brian Lacki, a researcher with the Breakthrough Listen SETI project at the University of California, Berkeley. “If they have consumed all their resources trying to become one, and then they self-destruct and turn to dust, will there be anything left in the galaxy for anyone else to use in the future?”

Admittedly, there is something arrogant in our belief that we can predict what a civilization with technology millions or billions of years ahead of us might do.

“We’re not even a type I civilization, so for us to guess the characteristics of a type III civilization sometimes feels a stretch,” says Garrett.

Something is certainly amiss. Our careful searches for type III civilizations have so far found nothing. A Dyson swarm absorbs sunlight, but left to its own devices, the temperature of the solar collectors would increase until they melted. So, they must radiate excess thermal infrared radiation, which should in principal make them visible to some of our astronomical satellites. The problem is, there are also natural sources in space of thermal infrared, specifically interstellar dust. Evolved stars shrouded by dust have an infrared signature every bit like a Dyson swarm. Initial surveys of initially anomalous infrared sources by Fermilab’s Richard Carrigan found no evidence for Dyson swarms, while in 2015 the Glimpsing Heat from Alien Technologies (G-HAT) project led by Penn State’s Jason Wright surveyed 100,000 infrared-strong galaxies, but found none containing signs of a type III civilization. More recently, Garret and Hong-Ying Chen of the Chinese Academy of Sciences drew a similar conclusion when searching for type III civilizations by taking advantage of a correlation between infrared emission and radio waves produced by star formation in a galaxy. In an unmodified galaxy, there should be a very strong correlation, whereas a galaxy modified by a type III civilization would deviate from that correlation quite significantly. Cross-matching radio data to 16,367 galaxies with strong infrared emission found only four that had more infrared than expected relative to their radio waves. Two of these are already known to astronomers as being purely natural phenomena, but the other two remain unidentified. The likelihood is that they too are natural galaxies, probably with more dust than usual. Garret says that he’s tried to get time on other telescopes to follow-up on these two candidates, but with no luck so far.

Does this mean that such civilizations don’t exist? Not necessarily, says Lacki. “Even if they do spread throughout their galaxy and start doing things like building Dyson swarms, there might still be limits which prevent them from being noticed.”

In 2018, Lacki wrote a paper in which he posited that building Dyson swarms around the most massive, most luminous stars might be cost prohibitive. Take the star Betelgeuse in our galaxy as an example. Its diameter is 1.23 billion kilometers – 881 times wider than our Sun. Building a Dyson swarm around a star so enormous is going to take a lot more material than building one around our Sun — thousands of Jupiters’ worth – so much more that there’s not nearly enough material in such star systems to do it. Furthermore, such stars only last a few million years before exploding as supernovae. A civilization aiming to survive into the deep future would be best staying away from such stars. Instead, Lacki suggests, civilizations will find it easier to build Dyson swarms around Sun-like stars, or smaller and longer-lived red dwarfs. If this is the case, the near-infrared signature of galaxies where this is happening will not change enough to be noticed from afar, because most of a galaxy’s light is produced by massive stars.

Recently, both Garrett and Lacki have started to wonder if this paradigm of alien civilizations with interstellar capabilities building Dyson swarms around stars is the correct idea at all.

“I just wonder if there is a trick we’re missing in the sense that we’re extrapolating from our physics and current status as a sub-type I civilization and trying to imagine what a really advanced civilization would be like,” says Garrett.

Lacki supports this, pointing out that “Being a type III is not necessarily this very specific thing, there might be several ways to get there.”

What we need now is outside-the-box thinking, and indeed Lacki has developed an alternative to Dyson swarms that is startlingly different to previous ideas.

Lacki’s idea is based around dust. When we look up at the Milky Way arcing across a dark sky, we see that it is split by dark abysses of dust absorbing the light of stars beyond. Perhaps a civilization could take advantage of that to not only become a type III civilization, but to potentially evolve into a quite different life-form.

Lacki’s idea is to engineer the dust, with a swarm of self-replicating nano-probes that can transform the dust into tiny light collectors.

“My thought was that you might harvest the molecular clouds [of gas in the galaxy], where this material is relatively dense and easier to collect, and if you wait long enough most of the cold gas and dust in the galaxy will cycle through those molecular clouds,” Lacki says.

The molecular clouds could be laced with smart dust that would eventually percolate out to fill the galaxy, creating an effective shroud that absorbs starlight and results in a very different kind of type III civilization. In the Dyson swarm paradigm, the energy collection is very focused, making huge amounts available in a relatively small volume of space. In Lacki’s smart dust concept the energy collected is more diffuse, across the entire diameter of a galaxy that could measure tens to hundreds of thousand of light years across.

However — and this is where Lacki, by his own admission, gets really speculative — the smart dust could contain nano-electronics, with each grain networked together. Scientists including the SETI Institute’s Seth Shostak and the Astronomer Royal Martin Rees have speculated that technological life may ultimately upload itself into a digital realm. In Lacki’s proposal, life could upload itself to the network of self-replicating smart dust.

“You might imagine an ecosystem being established in interstellar space” says Lacki. “I don’t know if it could sustain consciousness at galactic levels, especially on cosmological timescales, but I draw an analogy with Gaia, where you have this self-regulating ecosystem. I pictured basically living galaxies, where the interstellar medium has these response mechanisms that are self-regulating. It’s definitely speculative.”

It might take tens of millions of years to accomplish this, but if the ambition of technological life is to survive into the deep future then it has to take on the challenge of such long-term projects.

Then we come to the question of what will life even do with the entire energy of a galaxy in the palm of its hand? Futurists imagine immense computing projects, generating virtual reality universes, or manipulating the very fabric of space-time. Michael Garrett quips that “People argue that they would do computing, but maybe they just sit by the beach and drink cocktails.” Although it’s a flippant comment, it highlights that we don’t really know what a civilization with technology millions or even billions of years ahead of us will aspire toward.

For Lacki, it’s very simple — they will want to eat, at least by consuming energy. In his smart dust concept, “The amount of energy per capita might not be too big,” he says, alluding to how energy collection will be more diffuse across the galaxy. “If these things are self-replicating and spreading out across the galaxy, then I think that’s like asking, why does life on Earth need to cover the entire Earth? And everything has got to eat, even if it’s in a very general sense.”

Becoming a type I civilization certainly seems to be within our reach, and presumably is achievable by other technological civilizations out there. We might detect them by their signals, as Nikolai Kardashev imagined when he originally developed his scale.

Type II civilizations are more ambitious. Theoretically they are plausible. Realistically there are time, technological, resource and budgetary constraints that could limit their development, although partial Dyson swarms may be more common. If many technological civilizations do exist, then it doesn’t seem unreasonable that some of them may have built a megastructure like a Dyson swarm around their star. Indeed, if humanity can use becoming a type I civilization as a springboard to building a robust spacefaring infrastructure in the Solar System, we too might progress along that trajectory. But finding Dyson swarms around individual stars is more of a challenge for SETI.

Type III civilizations should be much more obvious, yet as Garrett says, our experiments “sort of rule out the idea that there are civilizations that are extracting energy from all the stars in their galaxy, and then using it to do stuff, because there would be a lot of waste heat and I don’t think there’s much evidence for that kind of civilization.”

Interstellar travel, for one, has to be achieved. Theoretically, it is possible, if we’re willing to put up with extremely long journey times between stars. Then there’s the issue of not being able to build Dyson swarms around every star — some stars are just too big. Time is also working against prospective type III civilizations. Are they immortal? Do they eventually die off?

The lack of type III civilizations has some researchers worried that we are indeed alone in the Universe, and that spells bad news for us. The so-called ‘great filter’ must be lurking around the corner, ready to snuff us out, like all those other would-be civilizations. But it doesn’t have to be that way. Perhaps we just need to re-adjust our ideas about how to become a type III civilization. Or a type II. Or even question whether the Kardashev scale is the right way to think about technologically advanced civilizations. When it comes to the future of humanity, we still have the power of choice: we still get to decide whether we grow as a civilization, or stop.

Choose wisely, and the stars could very well be in our favor.