Ice-nine is a fictional material that appears in Kurt Vonnegut's 1963 novel Cat's Cradle. Ice-nine is described as a polymorph of water which instead of melting at 0 °C (32 °F), melts at 45.8 °C (114.4 °F). When ice-nine comes into contact with liquid water below 45.8 °C, it acts as a seed crystal and causes the solidification of the entire body of water, which quickly crystallizes as more ice-nine. As people are mostly water, ice-nine kills nearly instantly when ingested or brought into contact with soft tissues exposed to the bloodstream, such as the eyes or tongue. It wasn't explained why the same doesn't happen by mere skin contact or inhalation of dust generated from mechanical impacts.
In the story, it is invented by Dr. Felix Hoenikker and developed by the Manhattan Project in order for the Marines to no longer need to deal with mud. The project is abandoned when it becomes clear that any quantity of it would have the power to destroy all life on Earth. In the novel's climax, the Earth's oceans are accidentally frozen solid by ice-nine, prompting a doomsday scenario.
Vonnegut encountered the idea of ice-nine while working at General Electric. He attributes the idea of ice-nine to his brother Bernard, who was researching the formation of ice crystals in the atmosphere. A later account of the events attributes the idea to the chemist Irving Langmuir, who devised the concept while helping H.G. Wells conceive ideas for stories. Vonnegut decided to adapt the idea into a story after Langmuir's death in 1957.
The phase transitions without any thermal effect and into the state of lower entropy described in the book are purely fictional and impossible according to the current mainstream physics. In the assumption that the phase transition was described inaccurately and has thermal effects, which were forgot to be described, it would have happened without any outside intervention, the initial seed being nucleated spontaneously due to fluctuations that are always present, the same way that the ordinary liquid-solid transitions happen. One also must take into account that supercooling phenomena, that can be tried to be used to describe existence of liquid water in the world of the book under standard conditions, happens in pure liquids free of nucleation centers, while natural water, containing particles, bubbles and organisms is not such. Due to the fact body fluids are solutions rather than pure water and freezing-point depression phenomena it could be possible that the ice-nine transition temperature got below human body temperature, making the described phenomena impossible.
- While multiple polymorphs of ice do exist (they can be created under pressure), none have the properties described in this book, and none are stable at standard temperature and pressure, that is above the ordinary melting point of ice. The real Ice IX has none of the properties of Vonnegut's creation, and can exist only at extremely low temperatures and high pressures.
- The ice-nine-like phenomenon has occurred with a few other kinds of crystals, called "disappearing polymorphs". In these cases, a new variant of a crystal has been introduced into an environment, replacing many of the older form crystals with its own form. One example is the anti-AIDS medicine ritonavir, where the newer polymorph destroyed the effectiveness of the drug until improved manufacturing and distribution was developed.
- Ice-nine has been used as a model to explain the infective mechanism of mis-folded proteins called prions which are thought to catalyze the mis-folding of the corresponding normal protein leading to a variety of spongiform encephalopathies such as kuru, scrapie and Creutzfeldt–Jakob disease.
- Ice VII, stable at room temperature but only under very high pressures.
In Posthumanism in the Novels of Kurt Vonnegut, ice-nine is described as an example of a wampeter in the fictional Bokonon religion, the pivot around which a karass, or a group of randomly interrelated people, revolves. Calling it both the cause of the apocalypse and one of the book's main sources of humor, the book argues that ice-nine sets the entire tone for the novel, which is "rendered inert" by its fragmentary structure and fundamental tension. The book also states that ice-nine reverses the normal hierarchy in which living organisms use water as a resource, becoming the "successor of organic life on planet Earth".
The Cosmic Landscape calls Cat's Cradle and its use of ice-nine a "cautionary tale about madness and instability in a world full of nuclear weapons", as well as being based on the real scientific principle of metastability. Saying that, while in the real world, liquid water at room temperature is stable, it explains that in Vonnegut's universe, normal water is only metastable, and since ice-nine is more stable than water, it will form naturally "sooner or later" even without the introduction of the seed crystal. It describes the fact that supercooled water takes on similar traits, and will completely freeze over if a normal chunk of ice is introduced. The book also relates this to the metastability of vacuums in string theory and their ability to create a diverse universe.
Dr. Strangelove's America states that ice-nine represents a collaboration between science and the military that, like with the atomic bomb, proves their "indifference to the fate of the human race", and the "inhuman and immoral results of [...] pure research". The book notes that the pure search for knowledge without "living human principles" as depicted in Cat's Cradle ultimately tarnishes science.
- Esther Inglis-Arkell (Oct 18, 2013). "The Real-Life Scientist Who Inspired Kurt Vonnegut's Cat's Cradle".
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- Liberko, Charles A. (2004). "Using Science Fiction To Teach Thermodynamics: Vonnegut, Ice-nine, and Global Warming". Journal of Chemical Education. 81 (4): 509. Bibcode:2004JChEd..81..509L. doi:10.1021/ed081p509.
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- Lansbury PT, Jr; Caughey, B (1995). "The chemistry of scrapie infection: implications of the 'ice 9' metaphor". Chemistry & Biology. 2 (1): 1–5. doi:10.1016/1074-5521(95)90074-8. PMID 9383397.
- Chang, Kenneth (5 February 2018). "New Form of Water, Both Liquid and Solid, is 'Really Strange'". The New York Times.
- Hicks, Andrew (2020). "Ice-Nine: A Wampeter". Posthumanism in the Novels of Kurt Vonnegut. Taylor & Francis. ISBN 9781000092820.
- Susskind, Leonard (2008). "Stability and Metastability". The Cosmic Landscape. Little, Brown. ISBN 9780316055581.
- Henriksen, Margot A. Dr. Strangelove's America. University of California Press. p. 313. ISBN 9780520340909.