In 1964 the Soviet astronomer Nikolai Kardashev was thinking about how to find alien civilisations by their radio signals. In a paper titled “Transmission of Information by Extraterrestrial Civilizations” he sorted possible civilisations into three types by the amount of energy they command. The idea became the Kardashev scale, one of the most quoted ideas in the search for extraterrestrial intelligence.
The three types
- Type I uses energy on a planetary scale. Kardashev set it at about 4 × 10¹² watts, close to what humanity used in 1964. Today the definition usually quoted is harnessing all the energy that reaches a planet from its star, which for Earth is about 2 × 10¹⁷ watts.
- Type II uses the energy of its own star, about 4 × 10²⁶ watts. That is the level of a civilisation that has built a Dyson swarm.
- Type III commands the energy of an entire galaxy, about 4 × 10³⁷ watts.
The gaps between the types are vast: a star gives off billions of times more energy than reaches any one of its planets, and a galaxy holds hundreds of billions of stars. Kardashev’s point was practical: a Type II or Type III civilisation would be so powerful that we might detect it across great distances, so the search should include nearby galaxies.
Where are we?
In 1973 Carl Sagan suggested a way to place civilisations between the types. His formula, K = (log₁₀ W − 6) / 10, where W is power in watts, gives K = 1 at 10¹⁶ watts and K = 2 at 10²⁶ watts. Sagan wrote that our civilisation would be “something like Type 0.7”. Using world energy use in 2021, about 18.87 terawatts, the formula gives about 0.73.
The scale is logarithmic, so the remaining gap is larger than it looks. To reach Type I by Sagan’s formula, humanity would need to use about 500 times more power than it does today.
Beyond Type III
Later writers have added more rungs. The physicist Michio Kaku has described a Type IV civilisation drawing energy from outside its galaxy, and others have imagined civilisations that command whole universes. The cosmologist John Barrow turned the idea around in 1998 with a scale measured by how small a civilisation can engineer things, not how large. Sagan proposed a second scale too, ranking civilisations by how much information they hold.
None of these is a measurement. We know of exactly one civilisation, and it sits below the first rung. The Kardashev scale is a way of asking what the universe might contain, and of guessing how we might notice.