A Dyson sphere is a hypothetical megastructure that encompasses a star and captures a large percentage of its solar power output. The concept is a thought experiment that attempts to imagine how a spacefaring civilization would meet its energy requirements once those requirements exceed what can be generated from the home planet's resources alone. Because only a tiny fraction of a star's energy emissions reaches the surface of any orbiting planet, building structures encircling a star would enable a civilization to harvest far more energy.
The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high likelihood of its existence. As a 2015 article put it, "If life is so easy, someone from somewhere must have come calling by now."
Interstellar travel is the hypothetical travel of spacecraft from one star system, solitary star, or planetary system to another. Interstellar travel is expected to prove much more difficult than interplanetary spaceflight due to the vast difference in the scale of the involved distances. Whereas the distance between any two planets in the Solar System is less than 55 astronomical units (AU), stars are typically separated by hundreds of thousands of AU, causing these distances to typically be expressed instead in light-years. Because of the vastness of these distances, non-generational interstellar travel based on known physics would need to occur at a high percentage of the speed of light; even so, travel times would be long, at least decades and perhaps millennia or longer.
The Kardashev scale is a method of measuring a civilization's level of technological advancement based on the amount of energy it is capable of using. The measure was proposed by Soviet astronomer Nikolai Kardashev (1932–2019) in 1964 and was named after him.
Extraterrestrial intelligence, or non-human intelligence, refers to hypothetical intelligent extraterrestrial life. No such life has ever been proven to exist in the Solar System except for humans on Earth, and its existence on other star systems is still speculative. The question of whether other inhabited worlds might exist has been debated since ancient times. The modern form of the concept emerged when the Copernican Revolution demonstrated that the Earth was a planet revolving around the Sun, and other planets were, conversely, other worlds. The question of whether other inhabited planets or moons exist was a natural consequence of this new understanding. It has become one of the most speculative questions in science and is a central theme of science fiction and popular culture.
Nikolai Semyonovich Kardashev was a Soviet and Russian astrophysicist, Doctor of Physical and Mathematical Sciences, and the deputy director of the Astro Space Center of PN Lebedev Physical Institute of the Russian Academy of Sciences in Moscow.
Michio Kaku is an American theoretical physicist, activist, futurologist, and popular-science writer. He is a professor of theoretical physics in the City College of New York and CUNY Graduate Center. Kaku is the author of several books about physics and related topics and has made frequent appearances on radio, television, and film. He is also a regular contributor to his own blog, as well as other popular media outlets. For his efforts to bridge science and science fiction, he is a 2021 Sir Arthur Clarke Lifetime Achievement Awardee.
The concept of self-replicating spacecraft, as envisioned by mathematician John von Neumann, has been described by futurists including physicist Michio Kaku and has been discussed across a wide breadth of hard science fiction novels and stories. Self-replicating probes are sometimes referred to as von Neumann probes. Self-replicating spacecraft would in some ways either mimic or echo the features of living organisms or viruses.
Megascale engineering is a form of exploratory engineering concerned with the construction of structures on an enormous scale. Typically these structures are at least 1,000 km (620 mi) in length—in other words, at least one megameter, hence the name. Such large-scale structures are termed megastructures.
The zoo hypothesis speculates on the assumed behavior and existence of technologically advanced extraterrestrial life and the reasons they refrain from contacting Earth. It is one of many theoretical explanations for the Fermi paradox. The hypothesis states that alien life intentionally avoids communication with Earth to allow for natural evolution and sociocultural development, and avoiding interplanetary contamination, similar to people observing animals at a zoo. The hypothesis seeks to explain the apparent absence of extraterrestrial life despite its generally accepted plausibility and hence the reasonable expectation of its existence. A variant on the zoo hypothesis suggested by the former MIT Haystack Observatory scientist John Allen Ball is the "laboratory" hypothesis, in which humanity is being subjected to experiments, with Earth serving as a giant laboratory.
An interstellar war is a hypothetical space war between combatants from different planetary systems. The concept provides a common plot device in science fiction, especially in the space opera subgenre. In contrast, the term intergalactic war refers to war between combatants from different galaxies, and interplanetary war refers to war between combatants from different planets of the same planetary system.
The Xeelee Sequence is a series of hard science fiction novels, novellas, and short stories written by British science fiction author Stephen Baxter. The series spans billions of years of fictional history, centering on humanity's future expansion into the universe, its intergalactic war with an enigmatic and supremely powerful Kardashev Type V alien civilization called the Xeelee, and the Xeelee's own cosmos-spanning war with dark matter entities called Photino Birds. The series features many other species and civilizations that play a prominent role, including the Squeem, the Qax, and the Silver Ghosts. Several stories in the Sequence also deal with humans and posthumans living in extreme conditions, such as at the heart of a neutron star (Flux), in a separate universe with considerably stronger gravity (Raft), and within eusocial hive societies (Coalescent).
Physics of the Impossible: A Scientific Exploration Into the World of Phasers, Force Fields, Teleportation, and Time Travel is a book by theoretical physicist Michio Kaku. Kaku uses discussion of speculative technologies to introduce topics of fundamental physics to the reader.
Engineering on an astronomical scale, or astronomical engineering, i.e., engineering involving operations with whole astronomical objects, is a known theme in science fiction, as well as a matter of recent scientific research and exploratory engineering.
Physics of the Future: How Science Will Shape Human Destiny and Our Daily Lives by the Year 2100 is a 2011 book by theoretical physicist Michio Kaku, author of Hyperspace and Physics of the Impossible. In it Kaku speculates about possible future technological development over the next 100 years. He interviews notable scientists about their fields of research and lays out his vision of coming developments in medicine, computing, artificial intelligence, nanotechnology, and energy production. The book was on the New York Times Bestseller List for five weeks.
The cultural impact of extraterrestrial contact is the corpus of changes to terrestrial science, technology, religion, politics, and ecosystems resulting from contact with an extraterrestrial civilization. This concept is closely related to the search for extraterrestrial intelligence (SETI), which attempts to locate intelligent life as opposed to analyzing the implications of contact with that life.
An isotropic beacon is a hypothetical type of transmission beacon that emits a uniform EM signal in all directions for the purposes of communication with extraterrestrial intelligence.
The following outline is provided as an overview of and topical guide to extraterrestrial life:
Rendezvous with the Future is a 2022 television series commissioned by Bilibili and produced by BBC Studios which explores the science behind the science fiction of the award-winning writer Liu Cixin. The series premiered in China on 16 November 2022 and has been watched by a combined audience of more than 70 million. It was awarded 'China Story: Best Documentary Series' at the Guangzhou International Documentary Film Festival.
The Byurakan Conference was held at the Byurakan Astrophysical Observatory in Armenia in 1964, at the initiative of the young scientist Nikolai Kardashev. It brought together a large number of Soviet astronomers and astrophysicists to take stock of knowledge and results in the search for traces of extraterrestrial life. The possibility of detecting extraterrestrial civilizations with the instruments available at the time was also discussed, as well as the criteria for communication with extraterrestrial intelligence.
