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This article is about a hypothetical substance. For the musical composition, see Ylem (Stockhausen). For the musical album, see Ylem (album).

Ylem is a term that was used by George Gamow, Ralph Alpher, and their associates in the late 1940s for a hypothetical original substance or condensed state of matter, which became subatomic particles and elements as we understand them today. The term ylem was actually coined by Ralph Alpher.[1]

In modern understanding, the "ylem" described as by Gamow was the primordial plasma, formed in baryogenesis, which underwent Big Bang nucleosynthesis and was opaque to radiation. Recombination of the charged plasma into neutral atoms made the Universe transparent at the age of 380,000 years, and the radiation released is still observable as cosmic microwave background radiation.


It reportedly comes from an obsolete Middle English philosophical word that Gamow's assistant Ralph Alpher came across while thumbing through a dictionary, which means something along the lines of "primordial substance from which all matter is formed" (that in ancient mythology of many different cultures was called the cosmic egg) and ultimately derives from the Greek ὕλη (hūlē, hȳlē), "matter", probably through an accusative singular form in Latin hylen, hylem. Restated, the ylem is what Gamow and colleagues presumed to exist immediately after the Big Bang. Within the ylem, there were assumed to be a large number of high-energy photons present. Ralph Alpher and Robert Herman made a scientific prediction in 1948 that we should still be able to observe these red-shifted photons today as an ambient cosmic microwave background radiation (CMBR) pervading all space with a temperature of about 5 kelvins[2] (when the CMBR was actually first detected in 1965, its temperature was found to be 3 kelvins). It is now recognized that the CMBR originated at the transition from predominantly ionized hydrogen to non-ionized hydrogen at around 400,000 years after the Big Bang.


In a taped interview available from the American Institute of Physics, conducted by Charles Weiner at Professor Gamow's home in Boulder, Colorado, US, on April 25, 1968, Gamow talked about YLEM:[3]

...this is YLEM. All this photography was done when I was still in Washington. When this series of origin of elements, alpha, beta, and gamma worked, I remember I bought the bottle of Cointreau to celebrate and wrote YLEM on it and then photographed the bottle and put my head as a genie coming out of it.

Weiner: What is YLEM in this case?

Gamow: A mixture of protons, neutrons and electrons.

Weiner: But the letters stand for what?

Gamow: You can look in the Webster dictionary. This is a word—I think it's an old Hebrew word, but Aristotle was using it—in Webster dictionary it says "material from which elements were formed".

Weiner: The primordial substance?

Gamow: The primordial substance, yes—ylem.

Weiner: The thick soup or whatever you want to call it?

Gamow: I mean this is the old Hebrew word meaning something like "space between heaven and earth".

In popular culture[edit]

After the term ylem was coined by Gamow's assistant Ralph Alpher, it was used in the 1952 science fiction novel Jack of Eagles by James Blish. It was also used by John Brunner in his 1959 short story "Round Trip", reprinted in the collection Not Before Time. Keith Laumer in the novel "Dinosaur beach" introduces the ylem field 1969. The term is also used by British author Richard Calder in the 1990s to describe the quantum mechanical state of the "quantum magic" in the girls/robots in his "Dead" trilogy (Dead Girls, Dead Boys, Dead Things). John C. Wright used the term in his debut novel The Golden Age to describe a "pseudo-matter" that forms "temporary virtual particles". A German black metal band "Dark Fortress" also released an album titled "Ylem". The video game series Ultima uses "Ylem" as a Word of Power in its incantation and runic based spell casting system, its meaning being "matter".

See also[edit]


  1. ^ The Cosmos--Voyage Through the Universe series, New York:1988 Time-Life Books Page 75
  2. ^ The CosmosVoyage Through the Universe series, New York: 1988 Time-Life Books, Page 80.
  3. ^ George Gamow, Oral History Interviews at aip.org.