When we lose a loved one, there's an impulse to hope that in some version of reality, in some version of our universe, that person is still alive.
Hugh Everett was born in 1930 and attended Catholic University in Washington, D.C., where he received a degree in chemical engineering. But, Everett had also taken enough mathematics courses to qualify for a degree in mathematics as well, and following his graduation, he entered graduate school at the Princeton University mathematics department.
At Princeton, Everett drifted into physics under John Archibald Wheeler. It is Wheeler who envisioned the fabric of the universe as being a churning sub-atomic realm of quantum fluctuations, which he called "quantum foam". Wheeler also coined the concept and the word wormhole to describe possible tunnels in space-time, and he may have coined the term blackhole to describe a "gravitationally completely collapsed object."
It was under Wheeler that Everett wrote his dissertation entitled, The Theory of the Universal Wave Function. In his thesis, Everett wrote: "Since the universal validity of the state function description is asserted, one can regard the state functions themselves as the fundamental entities, and one can even consider the state function of the entire universe. In this sense, this theory can be called the theory of the 'universal wave function,' since all of physics is presumed to follow from this function alone."
The universal wave function is the quantum state of the totality of existence, the basic physical entity or the fundamental entity, and it obeys at all times a deterministic wave equation.
Everett's contention was that the universal wave function is real and doesn't collapse. The implication of this is that every possible outcome of a quantum measurement is realized in some "world" or universe. By that logic, there must be a very large, or infinite, number of universes.
That means that once we open up the box, Schrödinger's cat might be dead in our universe, but alive in another universe. In Austrian physicist Erwin Schrodinger's famous thought experiment, a cat is put into a box, and as a result of a random radioactive decay, poison is either released or it is not.
While the box remains closed, we don't know if the radioactive decay has occurred or not, and the cat is said to be in a state of superposition, where it is both alive and dead. However, when the box is opened, and the cat’s state is observed or measured, it is that observation that causes the cat to either be alive or dead. The observer is said to "collapse the probability wave function" and this is called the Copenhagen interpretation of quantum mechanics.
Everett questioned how just looking at something can affect the very behavior of a physical object. His theory makes the system independent of the observer.
In the Spring of 1959, Everett traveled to Copenhagen, Denmark to discuss his theory with the father of quantum mechanics Niels Bohr, but the two men found no common ground.
Everett's idea was met with ... nothing, it was literally ignored for a decade until American physicist Bryce DeWitt took it up, and it was DeWitt who coined the phrase "many-worlds interpretation."
Everett splits off from the academic universe
With little interest in his ideas, after receiving his Ph.D., Everett went to work at the Sandia National Laboratories in Albuquerque, New Mexico, where he worked on the Minuteman missile project. He later moved to Arlington, Virginia, where he did mathematical analyses of U.S. government programs.
It took 20 years before Everett's ideas began to be resurrected. In 1977, he was invited by John Wheeler to give a talk at the University of Texas at Austin. There, Wheeler’s graduate student, British physicist David Deutsch began promoting the many worlds interpretation to a wider audience. Wheeler, however, never fully backed the MWI publicly.
Everett splits off from our universe
Possibly due to his strong belief in the MWI, Everett ate whatever he liked, smoked three packs of cigarettes a day, drank to excess and refused to exercise. On the night of July 18 - 19, 1982, he died suddenly of a heart attack at age 51.
Everett's strange request to his wife was that he be cremated and that his ashes be disposed of in the garbage. In 1996, Everett's daughter Elizabeth killed herself, with her suicide note saying that she wanted her ashes also thrown in the garbage so that she might "end up in the correct parallel universe to meet up w[ith] Daddy."
Everett's son, Mark Oliver Everett went on to form the rock group "The Eels" whose music is often filled with themes of family, death, and lost love. In 2007, Mark Everett filmed a BBC documentary about his father entitled Parallel Worlds, Parallel Lives, and it was broadcast in 2008 on the PBS program Nova.
Of his father's death, Mark Everett has said, "I realize that there is a certain value in my father's way of life. He ate, smoked and drank as he pleased, and one day he just suddenly and quickly died. Given some of the other choices I'd witnessed, it turns out that enjoying yourself and then dying quickly is not such a hard way to go."
If Hugh Everett is indeed correct, then our loved ones are both alive and dead, and the only things we can count on are our memories of those people and the love we felt for them.