What is the Kardashev Scale?

Are we alone in the Universe? Could there be countless sentient life forms out there just waiting to be found? Will we meet them someday and be able to exchange knowledge? Will we even recognize them as intelligent life forms if/when we meet them, and them us? When it comes to astrobiology, the search for life in the Universe, we don’t know what to expect. Hence why all the speculation and theoretical studies into these questions are so rich and varied!

One such study was conducted by famed Soviet and Russian astrophysicist and radio astronomer Nikolai Kardashev (1932 – 2019). While considering an important question related to the Search for Extraterrestrial Intelligence (SETI) in 1964, Kardashev proposed a classification scheme for ranking a civilization’s development. This would come to be known as the Kardashev Scale, which remains one of the most influential concepts in SETI to this day.

In 1960, the first modern attempt to detect extraterrestrial communications (“Project Ozma“) was made in the United States. Led by Frank Drake, creator of the Drake Equation and hailed as the “Father of SETI,” this experiment was conducted using the radio telescope at the National Radio Astronomy Observatory in Green Bank, West Virginia.

This experiment monitored two nearby Sun-like stars – Tau Ceti and Epsilon Eridani – for radio signals in the 1420 MHz range, corresponding to the frequency of cold hydrogen gas in interstellar space. While the project failed to detect anything beyond radio static, Ozma paved the way for far more ambitious SETI (and METI) efforts. It also inspired astrophysicists worldwide to consider what future surveys should be on the lookout for.

At a time when a search for extraterrestrial intelligence was more a thought experiment than an actual discipline, physicists in the Soviet Union formulated many of the seminal ideas that would endure until this day. Of these, Nikolai Kardashev was one of the earliest and most consequential thinkers, and his ideas remain foundational to the field of SETI.

Nikolai Kardashev

Coming of age in the postwar era, Kardashev studied under Josef Shklovskii (1916-1985), the celebrated author of Intelligent Life in the Universe. This book was the first general treatise on how humans could search for technologically capable beings in the cosmos. Shklovskii’s book was translated (and added to) by Carl Sagan and became the inspiration for many scientists after it was published in 1966 in the West.

For much of his formative years, Kardashev was on his own. His mother and father were both victims of the Great Purge in 1937 and 38 (respectively), and he was sent to an orphanage. He was eventually released into the custody of his aunt (on his mother’s side), who died in 1948 when Kardashev was 16. His father was eventually shot while his mother worked in a prison labor camp until she was released in 1956.

A year before, Kadashev graduated from Moscow State University, where his studies focused on radio astronomy, an emerging field at the time. He then worked at the Sternberg Astronomical Institute in Moscow, earning his Ph.D. in 1962. Kardashev’s early exposure to the fundamental premises of SETI led him to turn his inventive mind to how radio astronomy could be used to detect transmissions that were not terrestrial in origin.

One of his earliest contributions to SETI came in 1963 while Kardashev was studying CTA-102, a radio source located 8 billion light-years from Earth. Based on previous studies, this object was known to vary in intensity and send out irregular emissions. Kardashev famously suggested that this might be a deliberate attempt by extraterrestrials to send out coded messages.

This theory would eventually be discarded as astronomers learned that CTA-102 is an Active Galactic Nucleus (AGN), aka. a quasar, which astronomers know to be variable in nature. Nevertheless, Kardashev shared this theory and other imaginative ideas in a paper that would establish him among the astronomical community.

Birth of the Scale

This paper, titled “Transmission of Information by Extraterrestrial Civilizations,” was published in 1964. The paper’s purpose was to suggest what types of radio frequencies (and at what energies) SETI researchers should be looking for. In keeping with the idea that there may be some civilizations billions of years older than humanity, it stood to reason that these civilizations would be able to harness levels of energy that were not humanly possible.

To characterize the potential level of a civilization’s development, Kardashev proposed a three-level scale based on the amount of energy they could harness. This Scale included:

  • Type I – Planetary Civilizations: This refers to those that have developed the means to harness and store all of their home planet’s energy. According to Kardashev, this would amount to the consumption of 4×1019 erg/sec, which would likely be in the forms of fusion power, antimatter, and renewable energy on a global scale.
  • Type II – Stellar Civilizations: These are civilizations that have evolved to the point where they could harvest all the energy emitted by their star — which Kardashev speculated would likely involve a structure like a Dyson Sphere. In this case, this would work out to a consumption of 4×10³³ erg/sec.
  • Type III – Galactic Civilizations: These are civilizations that would be able to harness the energy of an entire galaxy, which would work out to energy consumption on the order of 4×1044 erg/sec.
The three types of civilizations and their estimated energy consumption as defined by the Kardashev Scale. Credit: Wikimedia Commons

Kardashev also described the odds of detecting a Type I, II, or III civilization and how much information they would be able to transmit. Referencing estimates from Project Ozma and related SETI experiments in the U.S., he claimed that detecting a Type I civilization was extremely unlikely (although they would still be able to receive communications). In contrast, a Type II or Type III would be able to transmit considerable amounts of information over vast distances.

Based on the estimated power transmission levels of a Type II Civilization, Kardashev estimated that a Stellar Civilization would be able to transmit considerable amounts of information within our galaxy (which measures 200,000 light-years in diameter). This ranged from a rate of 3×109 bits/sec within a radius of 100,000 light-years from their star system, 3×105 bits/sec within a radius of 1 million light-years, and would be incapable of transmitting a large quantity of information beyond that.

Meanwhile, a Type III civilization would be capable of intergalactic transmission, broadcasting at a rate of 2.4×1015 and 2.4×1013 bits/sec over a radius of 100,000 and 10 million light-years, and 3×1010 bits/sec over a radius of 10 billion light-years.

Implications of the Scale

For SETI researchers, the Kardashev Scale established some general constraints on the types of technosignatures (and associated power levels) they should be looking for. For example, based on the power estimates established by the Scale, scientists have since come to theorize what types of technologies a Type I, II, and III Civilization would be capable of utilizing and what the resulting technosignatures would be.

For a Type I Civilization, the ability to harness 4×1019 erg/sec of energy would imply access to fusion power and matter-antimatter annihilations. However, a discernible technosignature would likely take the form of renewable energy infrastructure. For example, a constellation of space-based solar satellites (aka. a “Clarke Belt“) would be detectable using our current instruments, according to the NASA Technosignature Report (released in 2018).

A Type II Civilization would use the same techniques employed by a Type, I but applied to a larger scale. In 1960, physicist Freeman Dyson famously suggested that an advanced civilization could harness the energy of its star by building a massive system of satellites that enclosed it. These megastructures (commonly referred to as Dyson Spheres or Dyson Structures), said Dyson, could be searched for by looking for large infrared signatures – the resulting heat radiated by these megastructures.

It has also been suggested that a Type II Civilization would also be capable of “Star Lifting,” where they can reposition stars. A possible method for doing this is a variation on a Dyson structure known as a Shkadov Thruster (or Stellar Engine) that partially encloses a star and uses its radiative force and gravitational attraction to achieve momentum and even relativistic velocities (which could be an explanation for some hypervelocity stars).

Type III Civilizations would similarly be capable of these activities but on a galactic scale. It’s also been suggested that Galactic Civilizations could create structures that would allow for “Galaxy Lifting,” which could be done to counteract cosmic expansion and keep certain clusters closely together. Another idea is to monitor quasars, the active galactic nuclei (AGN) of large galaxies, for indications of structures that could harness the power of supermassive black holes (SMBH).

This could include feeding matter onto an SMBH and harnessing the resulting radiation they emit or simply harnessing the energy they already put out. Two possibilities for this latter scenario involve harnessing the angular momentum of their accretions disks (the “Penrose Process“) or capturing the heat and energy generated by their hypervelocity jets.

Kardashev himself suggested criteria for how the estimated power of broadcasts could be differentiated from natural phenomena, such as the synchrotron radiation from nebulae formed in supernova explosions, from radio galaxies, or discreet radio sources. As he indicated:

“The artificial sources would evidently 1) have to have very small angular dimensions (at least in the case of Type II civilizations);… 2) they would have to possess circular polarization, so that the effect of the Faraday rotation of the plane of polarization in the interstellar medium would not distort the information received; 3) they would have to exhibit variability in time without leading to statistical fluctuations… 4) finally, it is to be anticipated that certain details would be present in the spectrum of the source suspected of artificiality which would have been designed for the express purpose of emphasizing its artificial origin.”

However, much like the similar Drake Equation, the scientific value of the Scale is more qualitative than quantitative in that it summarizes the challenges SETI researchers face. At the same time, the Scale is encouraging since it shows that the existence of a single Type II Civilization in our galaxy would not go unnoticed forever. As Kardashev noted in the final section of his paper, which addressed the implications of his Scale:

“The estimates arrived at show that should there exist even one type II civilization within the confines of the local system of galaxies, there will be a realistic possibility of securing an enormous quantity of information. The same holds for the existence of even one single type III civilization in the portion of the universe accessible to observation…

“Finally, it is entirely reasonable to assume that type II and type III civilizations would be in possession of information many orders of magnitude in excess of what we have available at the present tie. For that reason, they would have to be broadcasting practically continually, and this would also be the case for increasing the possibility of reception by type I civilizations.”

In summary, Kardashev estimated that advanced civilizations would be capable of broadcasting signals that type I civilizations (comparable to our own level of development) would be able to receive. Moreover, these transmissions would be broadcast continuously to ensure that they would be picked up – and possibly looped so that recipients wouldn’t miss a beat!

Variations on the Scale

Since Kardashev’s time, many additions and extensions have been suggested for the Scale. Some recommended adding classifications that fall between the three Types, while others have recommended that additional Types be added. This includes a Type 0 rating (describing pre-industrial civilizations) and Type IV and V, which refer to Civilizations that can harness the energy of the entire Universe or even collections of Universes, respectively.

Others have suggested that the Scale needs to be redrawn using other metrics. In his book, Cosmic Connections: An Extraterrestrial Perspective, Carl Sagan suggested that civilizations could be classified based on their level of “information mastery.” That is to say, the more advanced the species, the more information they would have at their fingertips. Sagan’s Scale used the Alphabet as a classification system, with each letter representing 10 million (106) unique bits of information.

Robert Zubrin, famed science communicator and Mars Society founder, has suggested that a civilization could be measured by “planetary mastery.” In his book, Entering Space: Creating a Spacefaring Civilization, Zubrin described how a civilization’s level of development could be pegged to the number of planets (or star systems) they have successfully colonized:

“Adopting Kardashev’s scheme in slightly altered form, I define a Type I as a civilization that has achieved full mastery of all of its planet’s resources. A Type II civilization is one that has mastered its solar system, while a Type III civilization would be one that has access to the full potential of its galaxy.”

But it was perhaps John D. Barrow – an English cosmologist, theoretical physicist, mathematician, and Royal Society Fellow (FRS) – who introduced the most radical reinterpretation. In his 1998 book, Impossibility: The Limits of Science and the Science of Limits, Barrow showed how humanity’s technological progress has allowed us to extend our control over the environment to increasingly smaller scales.

Rather than characterizing a civilization by the amount of outer space it commands, Barrow ventured that more advanced species would grow to harness the full power of inner space. From this, he created what is known as the Barrow Scale, a reverse classification that consists of seven Types:

  • Type I-minus: capable of manipulating objects similar in size to themselves (building structures, extract ore, monuments, etc.)
  • Type II-minus: capable of manipulating and altering the development of living things (organ transplants, studying DNA, genetic engineering, etc.)
  • Type III-minus: capable of manipulating molecules and molecular bonds to create new materials
  • Type IV-minus: capable of manipulating individual atoms, creating nanotechnologies and complex forms of artificial life
  • Type V-minus: capable of manipulating atomic nuclei and engineering nucleons that compose them
  • Type VI-minus: capable of manipulating the elementary particles (quarks and leptons)
  • Type Omega-minus: capable of manipulating the basic structure of space and time

In accordance with the Barrow Scale, advanced civilizations would not focus on claiming more space and resources but would optimize the space they already occupy. Rather than exploring the galaxy and beyond, they would choose to remain within their solar system and harness the power of their sun. One scenario includes the possibility that they will convert all the matter in their system into computronium and arrange it in layers around the star – creating a megastructure known as a “Matrioshka Brain.”


Several subsequent studies have called into question many of the assumptions employed in the Kardashev Scale. Foremost among them is the belief that extraterrestrial intelligence will be compelled to colonize well beyond its homeworld and home star. This has been challenged based on the ideas raised by the Barrow Scale and other researchers who have emphasized “space optimization” over expansion.

Another argument against the Kardashev Scale is based on the lack of observable evidence for advanced civilizations, especially Type III. Since the Universe has been around for 13.8 billion years, and our Solar System has only existed for the last 4.6 billion years, it would seem likely that a few civilizations would have been able to achieve a Type III level of development by now.

Even with our modest means, it would be very difficult for humans not to detect the signs of such a civilization. The fact that we haven’t found any yet would indicate that either we are:

  1. Not looking in the right places;
  2. Not looking for the right things;
  3. Haven’t been looking long enough;
  4. All of the above,

The only other possibilities are that none have emerged yet, or that nobody is out there. The former idea was raised by Carl Sagan and William I. Newman in their 1981 study, “Galactic civilizations: Population dynamics and interstellar diffusion,” who ventured that this could indicate that advanced life in our galaxy may have emerged in the more recent past.

Another criticism has to do with the idea that civilizations will expand over galactic distance. This possibility was explored in detail by Geoffrey A. Landis in his 1993 paper, titled “The Fermi paradox: an approach based on percolation theory,” where he argued that limitations imposed by a relativistic Universe would mean that exo-civilizations are only able to expand so far throughout the galaxy.

As Landis put it, civilizations would “percolate” outward rather than colonize consistently and rapidly outward. He also ventured that not all species would choose to go this route:

“Since it is possible, given a large enough number of extraterrestrial civilizations, one or more would have certainly undertaken to do so, possibly for motives unknowable to us. Colonization will take an extremely longtime, and will be very expensive.

“It is quite reasonable to suppose that not all civilizations will be interested in making such a large expenditure for a pay off far in the future. Human society consists of a mixture of cultures which explore and colonize, sometimes over extremely large distances, and cultures which have no interest in doing so.”

Considering the time and energy it would take to reach even the nearest stars and the delays in sending messages, it makes sense to assume that some species would choose to forgo interstellar expansion! A similar argument, which looks at the hazards of space travel and settling on alien worlds, was made in 2019 by Prof. Adam Frank and a team of exoplanet researchers from NASA’s Nexus for Exoplanetary Systems Science (NExSS).

In a study titled “The Fermi Paradox and the Aurora Effect: Exo-civilization Settlement, Expansion, and Steady States” – inspired by the book Aurora by Kim Stanley Robinson (released in 2015) – Frank and his colleagues argued that settlement of the galaxy would occur in clusters because not all potentially-habitable planets would be hospitable to humans or other alien life forms.

Another criticism is the other major assumption presented in Kardashev’s original study. This is the belief that advanced civilizations would be motivated to broadcast their existence to benefit other species. “First of all,” wrote Kardashev, “we assume here that one of the principal tasks of such communication efforts would be the transmission of information from a more highly developed civilization to a less highly developed one.”

The possibility that advanced intelligence would be compelled by the exact opposite motivations (eliminating potential competition, fear, xenophobia, etc.) has been explored at length by researchers and SETI scientists. Examples include the Berserker Hypothesis, the Dark Forest Hypothesis, and various interpretations of the “Great Filter,” where a desire for self-preservation or dominance drives interstellar communications or expansion.

Such concerns have been raised in the context of Messaging Extraterrestrial Intelligence (METI), which has emerged as a distinct and separate field from SETI. Given the possibility that even one hostile advanced civilization out there, is it wise for humanity to be broadcasting its existence to the cosmos? Moreover, could other intelligent life have concluded the same thing a long time ago, which is why the “Great Silence” persists?

Similar concerns have been raised by researchers who suggest that extraterrestrial transmission could be a threat. Here on Earth, governments regularly commit cyberattacks on other governments to obtain information or sabotage vital infrastructure. Similarly, hackers employ computer viruses phishing scams and encourage users to share personal information.

Is it possible that alien civilizations (or even groups and individual sentient beings) are sending out messages loaded with viruses designed to cripple our infrastructure or scam us somehow?

Decades after Kardashev first published his seminal paper, academics, scientists, and regular people alike are referencing, citing, or attempting to expand on his famous Scale. While some of the assumptions and estimates he offered have been criticized over the years, this is to be expected from something as foundational as the Kardashev Scale. As Zubrin stated, the theory was proposed in the “Sputnik Era,” when human space exploration was just out of its cradle and barely walking.

Like so many other things that are foundational to the field of SETI, like the Fermi Paradox and the Drake Equation, the Kardashev Scale remains with us because of the fresh insight it offers. To put it simply, Kardashev was one of the first people ever to offer serious thoughts on how humanity could find evidence of ETIs in the cosmos. Its influence can also be seen in how it has helped to frame every discussion that’s happened since about ETI and how we might achieve “First Contact” someday.

As legacies go, Nikolia Kardashev and the Scale that bears his name are virtually unrivaled in the annals of SETI and astrophysics.