How the Many-Worlds theory of Hugh Everett split the Universe | Aeon Essays

via How the Many-Worlds theory of Hugh Everett split the Universe | Aeon Essays

Splitting the Universe

Hugh Everett blew up quantum mechanics with his Many-Worlds theory in the 1950s. Physics is only just catching up

Photo courtesy ESA/Hubble/NASA, Fillipenko, Jansen

Sean Carroll

is a theoretical physicist at the California Institute of Technology. He specialises in quantum mechanics, gravitation, cosmology, statistical mechanics and foundations of physics. His latest book is Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime (2019). He lives in Los Angeles.

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Edited by Pam Weintraub

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One of the most radical and important ideas in the history of physics came from an unknown graduate student who wrote only one paper, got into arguments with physicists across the Atlantic as well as his own advisor, and left academia after graduating without even applying for a job as a professor. Hugh Everett’s story is one of many fascinating tales that add up to the astonishing history of quantum mechanics, the most fundamental physical theory we know of.

Everett’s work happened at Princeton in the 1950s, under the mentorship of John Archibald Wheeler, who in turn had been mentored by Niels Bohr, the godfather of quantum mechanics. More than 20 years earlier, Bohr and his compatriots had established what came to be called the ‘Copenhagen Interpretation’ of quantum theory. It was never a satisfying set of ideas, but Bohr’s personal charisma and the desire on the part of scientists to get on with the fun of understanding atoms and particles quickly established Copenhagen as the only way for right-thinking physicists to understand quantum theory.

In the Copenhagen view, we distinguish between microscopic quantum systems and macroscopic observers. Quantum systems exist in superpositions of different possible measurement outcomes, called ‘wave functions’. A spinning electron, for example, has a wave function describing a superposition of ‘spin-up’ and ‘spin-down’. It’s not merely that we don’t know the spin of the electron, but that the value of the spin does not exist until it is measured. An observer, by contrast, obeys all the rules of familiar classical physics. At the moment that an observer measures a quantum system, that system’s wave function suddenly and unpredictably collapses, revealing some definite spin or whatever has been measured.

There are apparently, therefore, two completely different ways in which quantum systems evolve. When we’re not looking at them, wave functions change smoothly according to the Schrödinger equation, written down by Erwin Schrödinger in 1926. But when we do look at them, wave functions act in a totally different way, collapsing onto some particular outcome.

If this seems unsatisfying, you’re not alone. What exactly counts as a measurement? And what makes observers so special? If I’m made up of atoms that obey the rules of quantum mechanics, shouldn’t I obey the rules of quantum mechanics myself? Nevertheless, the Copenhagen approach became enshrined as conventional wisdom, and by the 1950s it was considered somewhat ill-mannered to question it.

 

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