Science fiction is the medium of many a fantastical future civilisation, some of them spanning not just this solar system, but far beyond. Compared to these vast galactic empires, our primitive Earth doesn't seem so impressive any more, though it may still be a prediction of what may be. Sci-fi often also speaks of advanced alien civilisations, sometimes benign, other times intent to conquer the Earth, or just blow it up because it's in the way of a super galactic highway.
It is not just writers that keep themselves busy with imagining how an advanced civilisation may look. Even though humanity spreading through the galaxies is still centuries away, thinking about how we will develop can still help us answer questions today.
In 1964, the scientist Nikolai Kardashev realised that we can measure how advanced a civilisation is by a power consumption. If you look at our very own history, this becomes apparent, as our power consumption has grown exponentially over the past few decades. He came up with three categories of civilisation. A Type I civilisation is a civilisation that consumes the energy that falls upon an entire planet. A Type II civilisation is a civilisation that consumes all the energy from a star. Finally, a Type III civilisation is a civilisation that consumes all the energy in a galaxy.
Even a Type I civilisation would look vastly different from ours today. It is almost physically impossible to capture all the energy that falls upon a planet, and so the best bet for humanity to reach Type I status is to expand to other planets. Still, Type I civilisations are pretty easy to imagine.
From Type II onwards, things start to get out of hand. How would a civilisation ever collect all the energy from a star? The most popular idea for this - and one of my favourite concepts to come out of science in general - is the Dyson Sphere. The concept of a Dyson Sphere is a ball completely surrounding a star. This means that all the heat and light of the star falls upon the surface on the inside of the Sphere, where the energy can be collected. There is no known material - existing or theoretical - that could support such a large structure, so an alternative has been proposed, known as the Dyson Swarm. This is a massive constellation of satellites surrounding the star in multiple layers, together capturing all the energy. Even this - however - is well beyond our capabilities, as surrounding our own sun would take more material than is known to exist in our solar system.
A Type III galaxy would have to build a Dyson Swarm or Dyson Sphere around each star in a galaxy, or start using alternate matters, such as extracting power from (supermassive) black holes, antimatter reactions, or quasars. The means of a Type III civilisation are probably as exotic to us now, as electricity is to people several centuries ago. Type IV and V civilisations have been proposed, as those who control the entire universe, and collections of universes respectively.
As humanity, we haven't even attained Type I status yet. As a matter of fact, to attain Type I status, humanity needs to consume about a million times as much energy as it does today! Since the so-called Kardashev scale isn't very granular, Carl Sagan made a suggestion to allow for values between the types. As you can see, the step from a Type I to a Type II civilisation is much smaller than the step from Type II to Type III. This is an example of an exponential scale (also very common in epidemiology, so you've probably heard this word a lot), so by mapping an exponential curve to the three data points we have for the three types of civilisation, we can start figuring out where we are on the Kardashev Scale. As of 2018, humanity was estimated to score about 0.73. It sounds like we're getting close, but generating a million times more energy as we do today... that's no easy feat, and we are estimated to be at least 100-200 years off, and that's optimistic.
So if we're so far off reaching Type I status, why are we even thinking about how a Type I civilisation may look, let alone Types II and III? This all comes back to that one simple question: "are we alone in the universe?" Based on the predictions of how the energy may be generated, we know that for a Type II or Type III civilisation, we can look for stars that are surrounded by a Dyson Swarm or Dyson Sphere. As recent as 2015, we found fluctuations around a star that could have been attributed to a Dyson Swarm. Sadly, it turned out to be false alarm.
It can also teach us something about our own future. If a Type III civilisation really needs to control the vast majority of the stars in a galaxy, then we can be pretty certain that our own Milky Way doesn't have one, or we would surely have noticed. Are we the first, do civilisations tend to disappear and crumble before they reach Type III status, or is there some other reason we don't see evidence of our potential galactic neighbours?
By imagining how Type II and Type III civilisations may look, we know what life outside of our solar system might look like, teach us more about our own origins and future, and ultimately answer that one important question: are we alone?