The Many-Worlds Interpretation of Quantum Mechanics

- a brief description for the lay reader, some philosophical considerations, and links to more rigorous treatments


In 1957, Hugh Everett III proposed a radical new way of dealing with some of the more perplexing aspects of quantum mechanics. It became known as the Many-Worlds Interpretation.

According to this interpretation, whenever numerous viable possibilities exist, the world splits into many worlds, one world for each different possibility (in this context, the term "worlds" refers to what most people call "universes"). In each of these worlds, everything is identical, except for that one different choice; from that point on, they develop independently, and no communication is possible between them, so the people living in those worlds (and splitting along with them) may have no idea that this is going on.

In this way, the world branches endlessly. What is "the present" to us, lies in the pasts of an uncountably huge number of different futures. Everything that can happen, does, somewhere.

Until Many-Worlds appeared, the generally accepted interpretation of quantum mechanics was (and perhaps still is) the Copenhagen Interpretation. The Copenhagen Interpretation makes a distinction between the observer and the observed; when no one is watching, a system evolves deterministically according to a wave equation, but when someone is watching, the wavefunction of the system "collapses" to the observed state, which is why the act of observing changes the system. The Copenhagen Interpretation gives the observer special status, not accorded to any other object in quantum theory, and cannot explain the observer itself, while Many-Worlds models the entire observer-observee system.

The Many-Worlds Interpretation is an interpretation of quantum mechanics, and pertains to quantum events. But it also has implications for macroscopic systems like you and me. Although you may think that there are certain alternatives you would never choose, can you really be sure of that? There are a practically infinite number of versions of you, who have all split off at some time in the past from the path you are now following. There may be versions of you that split off five or ten years ago, or perhaps five minutes after you were born, to whom those choices may not seem unthinkable. But in a very real sense, those people are still "you" (but it can be argued that we should not use the word "are", or even "were"; we need to invent a new kind of tense...)

Many people find the Many-Worlds Interpretation, and the consequences that flow from it, deeply disturbing. This includes a great many physicists. It is also apparent that many physicists, including many who teach physics, do not have a good understanding of Many-Worlds.

However, polls have been taken among theorists who study such things, and have revealed that most of them believe that the Many-Worlds Interpretation represents, in some sense, an accurate description of the way the world really is. The polls also show that many of them would rather not discuss the subject.

It's not hard to see why so many people find these ideas disturbing. For if they are correct, they have profound implications for our understanding of the nature of the Soul, because the Soul (if there is such a thing) must branch along with the worlds that contain it. It would appear that the writings on which many contemporary religions are based make no mention of such an idea.

It is commonly thought that Many-Worlds is an unprovable hypothesis, experimentally indistinguishable from the Copenhagen Interpretation, but this may not be the case. It may be possible to observe experimentally one of the predicted effects of Many-Worlds: quantum interference between adjacent worlds. It has even been suggested that the Heisenberg Uncertainty Principle derives from this quantum interference; after you make a measurement (which of course splits the world), you can't be sure about the subsequent state of the observed system, because you can't be sure which world you are in.

This brief description is not very rigorous, in a technical sense, and is intended for the lay reader. Others, far more qualified than I, have written much better on the subject; you can find some of their works by following the links below.

The Everett Interpretation

Many Worlds FAQ

Tony Smith: Many-Worlds Quantum Theory

Quantum Future Physics

Anders Sandberg: Thoughts and Comments of the Omega Point Theory

Henry P. Stapp: The Basis Problem in Many-Worlds Theories

Meta-Technology: An Analytical Sketch

James P. Hogan: Paths to Otherwhere

Many Worlds

William D. Eshleman: Bill's Many-Worlds Page!

Stephen Paul King: Outlaw Science

Anders Sandberg: Anders Transhuman Page

John G. Cramer: Alternate View Column AV-16

The Anderson/Savage Symposium: Poul Anderson and Marshall T. Savage

Nick Bostrom: Observational selection effects and probability

Jürgen Schmidhuber: A Computer Scientist's View of Life, the Universe, and Everything (download the file from here)

Max Tegmark: The Interpretation of Quantum Mechanics: Many Worlds or Many Words?

Max Tegmark: Which mathematical structure is isomorphic to our Universe?

David Deutsch: The Fabric Of Reality

Jacques Mallah: The Computationalist Wavefunction Interpretation Agenda (CWIA)
and The Observer as a Classical Computer in a Quantum Universe

Don N. Page: Observational Consequences of Many-Worlds Quantum Theory

James Higgo: Quantum Theory of Immortality

Bruno Marchal: Calculabilité, Physique et Cognition

Roy Frieden: Fisher information and the laws of physics

Wei Dai: "It may not be true, but it fits on half a page..."

Philip Gibbs: Event-Symmetric Space-Time

There is a substantial amount of discussion on related subjects in this conversation.


Please note that this list is not exhaustive, and many more resources exist. If you know of any sites of importance that you feel should be on this list, please feel free to send the URLs.

I have not read all of the material at all of these sites, and do not necessarily endorse any one over any other. They merely contain material related to the Many-Worlds Interpretation in some way. However, it appears that the Many-Worlds FAQ by Michael Clive Price, which appears in a number of the pages linked above, may be the most comprehensive (and popular) Many-Worlds document on the Web.

Note that some of the papers listed above are .ps files, and some of these have been compressed into .gz files. If you don't have the tools to open these files, look here.

Those who have read David Deutsch's recent book (see below) may wish to join ongoing discussions about Many-Worlds and related topics at the FoR (Fabric of Reality) internet discussion list (see the instructions near the bottom of the page).

There is also another list for discussion of the idea that all possible universes exist, started by Wei Dai; see the directions near the bottom of this page, or here. Some of the material from this list can be found here. The list archive can be found here.


Books and other dead-tree products:

The Many-Worlds Interpretation of Quantum Mechanics, edited by B. S. DeWitt and N. Graham (Princeton University Press, Princeton, 1973). Contains Hugh Everett's original paper.

The Mind's I, by Douglas R. Hofstadter and Daniel C. Dennett (Basic Books, New York, 1981).

Other Worlds, by Paul Davies (Simon and Schuster, New York, 1981; or Penguin, 1990).

Labyrinths, by Jorge Luis Borges (New Directions, New York, 1964).

The Fabric of Reality, by David Deutsch (Penguin Books, 1997).

The cover story in the September 2001 issue of Discover Magazine is about Deutsch and Many-Worlds.
Deutsch also appeared in the recent "Time Travel" episode of the PBS program NOVA, discussing Many-Worlds. It appears that Many-Worlds has graduated from an arcane idea discussed only by specialists to an idea at least worth mentioning in popular media.


This page was last modified on September 11, 2001.


Also at this site:

Beyond the Drake Equation

Are We Alone?




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