Universe Today
Welcome back to our ongoing series, A Brief-ish History of SETI, where we examine the ideas and milestones that have come to define the Search for Extraterrestrial Intelligence (SETI). In Part I, we looked at the purpose and motivations for this field of study, some of the earliest experiments, and how they reflected a growing sense of curiosity about the cosmos and our place in it. In Part II, we examined the first SETI survey (Project Ozma) and its enduring legacy.
We also saw how the project's leader, Professor Frank Drake (the "father of SETI"), proposed the famous Drake Equation, which remains a foundational principle of the field. In addition to summarizing the challenges facing SETI astronomers, this equation also reflected humanity's growing sense of ambition and existential anxiety, characterized by bold plans for space exploration and the fear of nuclear annihilation.
Today, we will examine two more proposals that (like Fermi's Paradox and the Drake Equation) remain foundational to SETI research. This includes Freeman Dyson's concept of megastructures that could enclose entire star systems, as well as Kardashev's classification scheme for characterizing advanced extraterrestrial civilizations (ETCs). These ideas would explore the very real possibility that any civilizations in our galaxy that humanity could find someday would likely be far more advanced than ourselves.
Dyson & Megastructures
The famed British-American theoretical physicist and mathematician Freeman Dyson (1923-2020) is well known for his contributions to quantum mechanics, astrophysics, condensed matter physics, nuclear physics, and engineering. But in 1960, the same year that Frank Drake led the world's first true SETI experiment (Project Ozma), he published a seminal paper that would have a profound influence on the emerging field of SETI research.
In this paper, titled "Search for Artificial Stellar Sources of Infrared Radiation," Dyson addressed the recent paper by Cocconi and Morrison regarding monitoring space for signs of radio transmissions and suggested another possible technosignature to search for. In short, Dyson explained how ETIs could eventually use some or all of their planets to build spheres that encompass their entire solar system, which would appear as sources of radiated heat to infrared (IR) telescopes.
This would allow them to harness all of the energy of their sun while also providing virtually limitless space for their growing population. As he wrote:
We start from the notion that the time scale for industrial and technical development of these beings is likely to be very short in comparison with the time scale of stellar evolution. It is therefore overwhelmingly probable that any such beings observed by us will have been in existence for millions of years, and will have already reached a technological level surpassing ours by many orders of magnitude. It is then a reasonable working hypothesis that their habitat will have been expanded to the limits set by Malthusian principles.
This structure, known henceforth as a "Dyson Sphere," was the first in a long line of proposed megastructures collectively known as "Dyson Structures" today. Some examples include the Dyson Swarm, the Niven Ring (after Larry Niven, author of the Ringworld series), the Matrioshka Brain, and the Stellar Engine (or Shkadov Thruster). Dyson's proposal became another foundational principle for modern SETI and influenced another major contributor to SETI - famed Soviet-Russian astrophysicist Nikolai Kardashev.
The Kardashev Scale
When it comes to SETI and questions about what we might find out there, the name Nikolai Semenovich Kardashev (1932 - 2019) stands at the front of the pack. Coming of age in the postwar era, Kardashev studied under Iosef Samuilovich Shklovskii (1916-1985), the famed Soviet astronomer, astrophysicist, and author of Intelligent Life in the Universe (which was translated and added to by the late and great Carl Sagan).
After graduating from the Sternberg Astronomical Institute in Moscow in 1962, where his studies focused on radio astronomy, Kardashev turned his attention to how radio antennas could be used to detect extraterrestrial transmissions. One of his earliest contributions came in 1963, while Kardashev was studying CTA-102, a radio source 8 billion light-years from Earth that varied in intensity and emitted irregular bursts. Based on previous studies, Kardashev suggested that this might be a deliberate attempt by extraterrestrials to send out coded messages.
This theory would be disproven when astronomers learned that CTA-102 is an example of an Active Galactic Nucleus (AGN), aka. a quasar. Nevertheless, Kardashev shared this theory and other ideas in a paper titled " Transmission of Information by Extraterrestrial Civilizations," which was published in 1964. The purpose of this paper was to suggest what types of radio frequencies (and at what energies) SETI researchers should be looking for.
In keeping with the idea that civilizations could be billions of years older than humanity, Kardashev theorized that ETCs would be able to harness levels of energy far greater than anything humanity could fathom. To characterize the potential level of a civilization's development, Kardashev proposed a scale based on the amount of energy it could harness. The Kardashev Scale, as it came to be known, consisted of three civilization types:
- 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 4×1019 erg/sec, consisting of fusion power, antimatter, and renewable energy on a global scale.
- Type II - Stellar Civilizations: These civilizations 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, resulting in 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 amount to 4× 1044 erg/sec.
Kardashev also described the odds of detecting these types of civilizations and the amount of information they could transmit. Based on estimates from Project Ozma and similar SETI experiments at the time, he claimed that a Type I civilization would be extremely difficult to detect (although they would still be able to receive communications). In contrast, the odds of detecting a Type II or Type III civilization would be much higher due to the amount of information they could transmit and the distances over which they could transmit it (100,000 to 10 billion light-years, respectively).
Between these two major contributions, there is one major takeaway that would remain a permanent part of SETI research. Essentially, both Dyson and Kardashev acknowledged that intelligent life in our galaxy would likely have a head start on humanity. This is a perfectly rational assumption given the age of the Milky Way (ca. 13 billion years old) and the official estimates that most star systems had planets roughly 10 to 12 billion years ago.
These theories are a reminder of the "Cosmic Mirror" principle, which essentially states that SETI tells us more about our civilization than it does about extraterrestrials. In other words, speculations about megastructures and Type-I to Type-III civilizations were also an attempt to predict humanity's future development. This, in turn, reflects another enduring aspect of SETI: the understanding that humanity and alien civilizations are likely to follow similar evolutionary paths.
Whether this will prove to be the case or not, only time will tell! Stay tuned for our next installment, where we will take a look at the first attempts at Messaging Extraterrestrial Intelligence, plus a curious event considered the best candidate for a possible extraterrestrial transmission!
