In five minutes
The Kardashev scale ranks a civilization by the energy it can put to work. A Type I civilization uses all the energy that reaches its home planet. A Type II uses the output of its whole star. A Type III uses the output of an entire galaxy. It is a thought experiment, and nobody has found a civilization to place on it.
Nikolai Kardashev, a Soviet radio astronomer, proposed the idea in a 1964 paper about how extraterrestrial civilizations might send messages across space. His question was practical: how strong a signal could a society afford to send? That depends on how much power it controls, so he sorted possible civilizations by power. The whole scale follows from that one choice.
The idea has since left astronomy behind. Science fiction writers use it as a ruler for how big a future can be, and popular science uses it to ask how far humanity still has to go. It also fits into the history of science fiction, where stories about star-sized machines and galaxy-spanning empires came before anyone attached numbers to them.
The Kardashev scale: three types
Kardashev described his levels in terms of power, which is energy per second, measured in watts.
- Type I, a planetary civilization. It controls roughly the power available on its own world. Kardashev put this near 4 × 10¹² watts, about where humanity stood in the early 1960s. Later writers, Carl Sagan among them, rounded it to 10¹⁶ watts, which is in the range of the sunlight a planet like Earth intercepts.
- Type II, a stellar civilization. It uses the output of its whole star. Kardashev's figure was about 4 × 10²⁶ watts, close to the Sun's luminosity of roughly 3.8 × 10²⁶ watts. Sagan's rounded version is 10²⁶.
- Type III, a galactic civilization. It uses the energy of a whole galaxy. Kardashev gave about 4 × 10³⁷ watts, and Sagan's rounding is 10³⁶. The Milky Way's total light output is of the same order, roughly 10³⁷ watts.
Notice the jumps. In Sagan's rounded version, each type commands ten billion times more power than the one before it. Kardashev's own figures are spread even further apart: about 10¹⁴ times from Type I to Type II, and about 10¹¹ times from Type II to Type III. The scale is built to cover huge differences, not to split hairs between neighbours.
Sagan's decimal version and where we stand
In his 1973 book The Cosmic Connection, Carl Sagan suggested a formula that lets a civilization fall between the types. If P is the power in watts, then K = (log₁₀ P − 6) / 10. At 10¹⁶ watts, K is 1. At 10²⁶ watts it is 2, and at 10³⁶ watts it is 3.
Humanity uses about 20 terawatts of primary energy, which is 2 × 10¹³ watts. Put that in Sagan's formula and you get a K of about 0.73. We are not Type I yet. We would need roughly 500 times more power. Because the scale is logarithmic, every 0.1 step means ten times more power, so the last stretch is the steep one.
The real science
What is established. The numbers behind the scale are measurable. The Sun's output, the sunlight reaching Earth (about 1.7 × 10¹⁷ watts) and humanity's energy use are all well known. It is also solid physics that any civilization using energy has to get rid of waste heat.
That second point gave astronomers something to look for. In 1960, physicist Freeman Dyson suggested that a civilization capturing most of its star's light would glow in the infrared, and we could search for that glow. Our page on the Dyson sphere covers the idea in detail.
What has been searched. A 2015 survey of about 100,000 galaxies, using data from NASA's WISE infrared telescope, looked for galaxies that glow strangely in the mid-infrared, as a Type III civilization might. It found nothing that clearly pointed to a galaxy-spanning civilization. In the same year, the odd dimming of the star KIC 8462852, nicknamed Tabby's star, made headlines. Later studies favoured ordinary dust as the cause, and there is no evidence of an alien structure.
What is speculation. Everything about whether civilizations actually grow this way is a guess. Perhaps energy use does not keep rising for ever. Perhaps advanced societies choose to become more efficient rather than bigger. The empty skies connect directly to the Fermi paradox: if Type II and III civilizations were common, we might expect to see their waste heat, and so far we do not. The scale does not explain that silence. It only sharpens the question.
Where it shows up in fiction
Stories about huge civilizations came well before the scale. Olaf Stapledon's novel Star Maker (1937) imagines civilizations wrapping their stars in artificial shells, an image Dyson later credited as an inspiration for his own idea. Isaac Asimov's short story "The Last Question" (1956) follows humanity as its energy use and its computers grow across the ages, until they reach the scale of the universe.
Liu Cixin's The Three-Body Problem works in the same territory without naming the scale. In the first book, humanity meets signs of a civilization far ahead of it in technology, and the story asks what a gap that large means for both sides. The book is a good way to feel the distance between the types, and the plot of this opening volume can be read without knowing the rest.
In everyday talk, fans often use the types as a shorthand. "A Type II civilization" in a story or game tends to mean a society that builds things at the scale of stars, even if the author never gives a wattage.
Common misconceptions
- "It is a law of nature." It is not. It is a classification that Kardashev proposed to organize a question about communication. Nothing says civilizations must climb it.
- "Humanity is Type 0." Kardashev's original paper had only three types. The idea of a Type 0 and the figure of 0.73 come from Sagan's later formula.
- "Type I means covering a planet in machines." The type is about energy that could be used, not a particular look. A Type I society might use many energy sources and leave large areas wild.
- "Higher type means wiser." The scale measures power only. It says nothing about ethics, happiness or intelligence.
- "There are official Type IV and V." Some writers have added higher types, for example a civilization that controls a whole universe. These are informal extensions and not part of Kardashev's paper.
Read next
- Dyson sphere: the star-wrapping megastructure that Type II civilizations are usually imagined building, and how we might spot one.
- Fermi paradox: why the lack of any visible civilization makes the scale a puzzle as well as a ruler.
- The Three-Body Problem: a novel about meeting a civilization far ahead of our own.
