02The Scale
Energy as a yardstick.
In 1964 the astrophysicist Nikolai Kardashev proposed sorting hypothetical civilizations into three types by the power they command. Climb the scale in the window below, then see what he actually wrote and how the idea changed.
2.1Scale window
From here to a galaxy.
Humanity today
You are here
We run on a small share of the sunlight that reaches Earth. Published estimates put humanity near 0.73 on Carl Sagan’s decimal version of the scale.
Power compared with humanity today
Humanity today
On Sagan’s decimal scale, each step of 0.1 means ten times more power.
- Humanity todayYou are here
- We run on a small share of the sunlight that reaches Earth. Published estimates put humanity near 0.73 on Carl Sagan’s decimal version of the scale.
- Type I · planetaryA planet’s worth
- Power on the scale of a whole planet. Today this is usually read as using about as much energy as all the sunlight reaching it. In Kardashev’s 1964 paper, Type I was simply Earth’s own level at the time.
- Type II · stellarA star’s worth
- A civilization able to harness the energy radiated by its own star. In 1960 Freeman Dyson suggested that such a civilization might give itself away by glowing in infrared.
- Type III · galacticA galaxy’s worth
- Power on the scale of its own galaxy: roughly the light of all its stars.
An illustration, not a simulation. Distances and sizes are not to scale. Levels use Carl Sagan’s decimal version of Kardashev’s 1964 scale.
2.2Origin
A paper about radio signals.
The scale comes from Kardashev’s paper Transmission of Information by Extraterrestrial Civilizations, published in Russian in Astronomicheskii Zhurnal in 1964 and in English translation in Soviet Astronomy the same year.
The paper was not mainly a ranking exercise. It asked how much power a civilization would need to send signals that could be detected across interstellar or intergalactic distances, and which radio frequencies would suit such signals. The three types grouped possible senders by the power they could command.
Kardashev presented the work at the First All-Union Conference on Extraterrestrial Civilizations, held on 20–23 May 1964 at the Byurakan Astrophysical Observatory in Soviet Armenia.
The scale is qualitative. Its types are rough bands meant to frame a question, not measurements of any real civilization.
2.3Type explorer
Pick a type.
Type I: a planet’s worth
Kardashev defined Type I as a “technological level close to the level presently attained on the earth”. In the original paper, Earth in the early 1960s was the reference point.
Most popular accounts today use a different, later definition: a “planetary” civilization that can use all the energy its planet receives from its star. That is tens of thousands of times more than Kardashev’s own figure, so on that reading humanity is still well short of Type I.
Type II: a star’s worth
A civilization capable of harnessing the energy radiated by its own star. Kardashev’s figure for it is close to the Sun’s entire output.
The idea sits next to Freeman Dyson’s 1960 paper in Science, Search for Artificial Stellar Sources of Infrared Radiation, which argued that a technological civilization might capture much of its star’s light and re-radiate it as infrared, and proposed looking for that glow.
Type III: a galaxy’s worth
A civilization in possession of energy on the scale of its own galaxy: about the light of all the Milky Way’s stars together.
A civilization this powerful could, he argued, be heard across intergalactic distances. In the same paper he pointed to two compact radio sources, CTA-21 and CTA-102, as possible artificial signals from a Type II or III civilization. In 1965 Maarten Schmidt measured how far away CTA-102 is, and it turned out to be a distant quasar.
Sagan’s decimal scale
In The Cosmic Connection (1973), Carl Sagan suggested replacing Kardashev’s three steps with a decimal number, so a civilization could sit between types: 0.5, 0.7, 1.4 and so on. On his version, every step of 0.1 means ten times more power.
Sagan rated the Earth of 1973 at about 0.7. Published estimates put humanity today at about 0.73: Robert Gray put terrestrial civilization at about 0.72 around 2015, and a 2023 study in Scientific Reports gave 0.7276.
Sagan’s anchor points are not the same as Kardashev’s own figures, which is one reason different sources disagree about where humanity stands.
Type 0 and IV: later additions
Neither is in Kardashev’s 1964 paper. They are later proposals by other authors.
- Type 0. Running Sagan’s decimals below Type I gives a bottom rung at zero. In 2020 Robert Gray formalised an Extended Kardashev Scale with whole types from 0 to 4, each ten billion times more powerful than the one before.
- Type IV. Zoltán Galántai (2004) proposed it for a civilization controlling the energy of the visible universe, and argued its activity might be indistinguishable from nature. Gray’s Type 4 sits at the scale of the observable universe.
Barrow’s inward scale
In Impossibility: The Limits of Science and the Science of Limits (1998), the cosmologist John D. Barrow turned the idea around. Instead of ranking civilizations by how much energy they use, he ranked them by how small the things they can control are, arguing that mastery of ever smaller scales may pay off better than ever larger ones.
2.4Sort them
Which type is it?
Six civilizations, real and imagined. Pick a type for each and see why.
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01Uses about as much power as all the sunlight reaching its home planet.
That is the popular modern reading of Type I.
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02Surrounds its star with collectors and captures most of its light.
Harnessing the energy radiated by its own star is Type II.
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03Draws on power comparable to the light of every star in its galaxy.
Power on the scale of a whole galaxy is Type III.
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04Humanity today.
Not yet, on the modern reading: published estimates put humanity near 0.73 on Sagan’s decimal scale.
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05Earth’s technology in 1964, as Kardashev described it in his paper.
In the original paper, Type I was pegged to Earth’s own level at the time.
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06The kind of civilization Kardashev wondered might be behind the radio source CTA-102.
He suggested it as a possible signal from a Type II or III civilization. It turned out to be a quasar.
0 of 6 answered
2.5Two readings
Same names, different meanings.
| Type | Kardashev, 1964 | Popular reading today |
|---|---|---|
| Type I | Close to Earth’s own level at the time. | All the sunlight reaching its home planet. |
| Type II | The energy radiated by its own star. | The full output of its star. |
| Type III | Energy on the scale of its own galaxy. | The full output of its galaxy. |
2.6Sources
Sources and further reading
- N. S. Kardashev, Transmission of Information by Extraterrestrial Civilizations, Astronomicheskii Zhurnal 41, 282 (1964); English translation in Soviet Astronomy 8, 217 (1964). NASA ADS
- C. Sagan, The Cosmic Connection: An Extraterrestrial Perspective, Anchor Press / Doubleday (1973).
- R. H. Gray, The Extended Kardashev Scale, The Astronomical Journal 159, 228 (2020). doi:10.3847/1538-3881/ab792b
- Z. Galántai, Long Futures and Type IV Civilizations, Periodica Polytechnica Social and Management Sciences 12(1), 83–89 (2004).
- M. M. Ćirković, Kardashev’s Classification at 50+: A Fine Vehicle with Room for Improvement, Serbian Astronomical Journal 191, 1–15 (2015). arXiv:1601.05112
- J. D. Barrow, Impossibility: The Limits of Science and the Science of Limits, Oxford University Press (1998).
- F. J. Dyson, Search for Artificial Stellar Sources of Infrared Radiation, Science 131, 1667–1668 (1960).
- Zhang et al., estimate of humanity’s Kardashev rating, Scientific Reports (2023). doi:10.1038/s41598-023-38351-y
- A. Prša et al., Nominal Values for Selected Solar and Planetary Quantities: IAU 2015 Resolution B3, The Astronomical Journal 152, 41 (2016): the Sun’s nominal output and the sunlight reaching Earth. arXiv:1510.07674
- L. M. Gindilis and L. I. Gurvits, history of SETI in the USSR and Russia, Acta Astronautica (2019). arXiv:1905.03225