Hugh Everett III (; November 11, 1930 – July 19, 1982) was an American physicist who proposed the relative state interpretation of quantum mechanics. This influential approach later became the basis of the many-worlds interpretation (MWI). Everett's theory dropped the wave function collapse postulate of quantum measurement theory, incorporating the observer in the same quantum state as the observation result. The quantum statistic becomes a measure of the branching of the universal wave function.
Although largely disregarded until near the end of his life, Everett's work received more credibility with the discovery of quantum decoherence in the 1970s and has received increased attention in recent decades, with MWI becoming one of the important interpretations of quantum mechanics.
Everett also helped found small companies specializing in contracts with the U.S. government.
Hugh Everett III was born in 1930 and raised in the Washington, D.C. area. His parents separated when he was young. Initially raised by his mother (Katherine Lucille Everett, née Kennedy), he was raised by his father (Hugh Everett, Jr.) and stepmother (Sarah Everett, née Thrift) from the age of seven.
At age 12, Everett wrote a letter to Albert Einstein asking him whether that which maintained the universe was something random or unifying. Einstein responded as follows:
Dear Hugh: There is no such thing like an irresistible force and immovable body. But there seems to be a very stubborn boy who has forced his way victoriously through strange difficulties created by himself for this purpose. Sincerely yours, A. Einstein
Everett won a half scholarship to St. John's College High School in Washington, D.C. From there, he moved to the nearby Catholic University of America to study chemical engineering as an undergraduate. There, he read about Dianetics in Astounding Science Fiction. Although he never exhibited interest in Scientology (which Dianetics became), he retained a distrust of conventional medicine throughout his life.
During World War II, Everett's father was fighting in Europe as a lieutenant colonel on the general staff. After the war, Everett's father was stationed in West Germany, and Everett joined him in 1949, taking a year off from his undergraduate studies. Father and son were both keen photographers and took hundreds of pictures of West Germany being rebuilt. Reflecting their technical interests, the pictures were "almost devoid of people".
Everett graduated from the Catholic University of America in 1953 with a degree in chemical engineering, although he had completed sufficient courses for a mathematics degree as well.
Everett received a National Science Foundation fellowship that allowed him to attend Princeton University for graduate studies. He started his studies at Princeton in the mathematics department, where he worked on the nascent field of game theory under Albert W. Tucker, but slowly drifted into physics. In 1953 he took his first physics courses, notably Introductory Quantum Mechanics with Robert Dicke.
In 1954, Everett took Methods of Mathematical Physics with Eugene Wigner, although he remained active in mathematics and presented a paper on military game theory in December. He passed his general examinations in the spring of 1955, thereby gaining his master's degree, and then started work on his dissertation that would (much) later make him famous. He switched thesis advisor to John Archibald Wheeler sometime in 1955, wrote a couple of short papers on quantum theory, and completed his long paper "Wave Mechanics Without Probability" in April 1956.
In his third year at Princeton, Everett moved into an apartment he shared with three friends he had made during his first year, Hale Trotter, Harvey Arnold and Charles Misner. Arnold later described Everett as follows:
He was smart in a very broad way. I mean, to go from chemical engineering to mathematics to physics and spending most of the time buried in a science fiction book, I mean, this is talent.
During this time, Everett met Nancy Gore, who typed up his paper "Wave Mechanics Without Probability". He married her the next year. The paper was later retitled "The Theory of the Universal Wave Function".
Wheeler traveled to Copenhagen in May 1956 with the goal of getting a favorable reception for at least part of Everett's work, but in vain.
In danger of losing his draft deferment, Everett took a research job with the Pentagon the year before completing the oral exam for his PhD and did not continue research in theoretical physics after his graduation. He started defense work in the Weapons Systems Evaluation Group (WSEG) in June 1956. Completing his PhD within a year of starting at WSEG was a job requirement, and in April 1957 he returned briefly to Princeton to defend his thesis. The oral examination took place on April 23. The principal examiners—Wheeler, Valentine Bargmann, H. W. Wyld, and Dicke—concluded: "The candidate passed a very good examination. He dealt with a very difficult subject and defended his conclusions firmly, clearly, and logically. He shows marked mathematical ability, keenness in logic analyses, and a high ability to express himself well." With this Everett completed his PhD in physics from Princeton, his doctoral dissertation titled "On the foundations of quantum mechanics".
A short article, which was a compromise between Everett and Wheeler about how to present the many-worlds concept and almost identical to the final version of his thesis, was published in Reviews of Modern Physics, accompanied by a favorable review by Wheeler. Everett was not happy with the article's final form.
On October 23–26, 1956, Everett attended a weapons orientation course managed by Sandia National Laboratories at Albuquerque, New Mexico, to learn about nuclear weapons, and became a fan of computer modeling while there. In 1957, he became director of the WSEG's Department of Physical and Mathematical Sciences. Much, but not all, of Everett's research at WSEG remains classified. He worked on various studies of the Minuteman missile project, which was then starting, as well as the influential study The Distribution and Effects of Fallout in Large Nuclear Weapon Campaigns.