Sunday, March 27, 2005

Everett and many-worlds

I recently found this wonderful biography of Hugh Everett III, the discoverer of the many-worlds interpretation of quantum mechanics. While its name suggests science fiction, one is naturally led to this interpretation by requiring completeness and economy of quantum mechanics.

In the usual formulation, an isolated system is described by a wavefunction which evolves unitarily in time according to the Schrodinger equation. When an external observer makes a "measurement" on the system, its state "collapses" into a particular eigenstate associated with the quantity (operator) that is measured, such as position, momentum, spin, etc. However, an astute thinker (like Everett) is led to ask why one has to introduce the rather mysterious concepts of measurement and collapse. Shouldn't the universe as a whole be described by a single wavefunction, which evolves according to Schrodinger dynamics? In that case, a measurement is a process by which the state of the observer's brain (or apparatus) becomes correlated with the state of the observed system. All possible outcomes are described by different "branches" of the universal wavefunction (hence the many-worlds description). Each of our consciousnesses is a particular semi-classical branch of the part of the wavefunction describing the atoms and molecules in our brains.

Everett's interpretation doesn't (as far as I know) make any new experimental predictions - it predicts the same probabilities as the old Copenhagen measurement-collapse description. (There is some debate over whether it gives a more satisfying explanation of how probability arises in quantum mechanics.) However, I and many others find it more intellectually satisfying and complete. The real argument starts over interpretation of the "other" branches of the wavefunction. Are they real, or just a mathematical abstraction? Can we ever detect their existence?

The biography I linked to fills in a number of the historical questions I had about many-worlds. Why is it that so many younger physicists (perhaps without thinking too deeply about it) accept many-worlds, while many (but certainly not all) of our elders regard it as nutty? Apparently the idea languished in obscurity for 20 years before being re-introduced into the physics community by Bryce DeWitt and others. Everett himself, obviously a first-rate intellect, left physics and worked in the defense and software industries, where he made important contributions and became a wealthy man. This story should be a cautionary note to academic scientists about ignoring the many creative and original thinkers who continue doing interesting things outside the academy.*

See here for a nice FAQ on many-worlds. See here for a letter to Physics Today from Murray Gell-Mann in which both he and Feynman are identified as believers in many-worlds.

* Only a small fraction of our students manage to stay in academic research - a quick calculation confirms this: we have zero population growth in academic science, but each professor produces several if not many PhD students in their career. One might argue that it is the best and brightest among our students who stay in academia, but even if true on average, that leaves many talented former-academicians in the real world!


Anonymous said...

Nice find. I'm fascinated by stories of former academic physicists in the real world. They tend to be much more interesting than pure academic biographies.

Anonymous said...

Readers of this post might also be interested in Michael Nielson's new post Early and late adopters in research, this commentary by Ray Streater, and remarks in Carlton Caves' paper Resource Material for Promoting the Bayesian View of Everything.

Anonymous said...

For interested readers, Bryce's 'The Global Approach to QFT' (which for is well worth the effort in general) discusses Everett's interpretation in ch 8, 9 and 12. In the comments section of those ch. he left the following:

ch 8. p. 144
"E's interpretation has been adopted by the author out of a practical necessity: He knows of no other. At least he knows of no other that imposes no artificial limitations or fuzzy metaphysics while remaining able to serve the varied needs of quantum cosmology, mesoscopic quantum physics, and the looming discipline of quantum computation"

ch 9. p. 161
"In ch 8 and ch 9 an attempt has been made to present E's interpretation in as sharp and uncompromising a way as possible. The reader may not be sympathetic to it but at least he will know what it is. One conclusion that he may be unable to accept is that the probability interpretation is already contained in the formalism of state-vector spaces and operators and does not need to be externally imposed. A reader who takes this stance is obliged to answer the historical question: Why did the fraternity of physicists adopt the probability interpretation so rapidly and so unanimously after the formalism was invented? Can the reader honestly claim that it was primarily for experimental reasons?"

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