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http://arxiv.org/abs/quant-ph/0101077

100 Years of the Quantum

-Max Tegmark

Dept. of Physics, Univ. of Pennsylvania

-John Archibald Wheeler

Princeton University, Department of Physics

[snip]

The discovery of decoherence, combined with the ever

more elaborate experimental demonstrations of quantum

weirdness, has caused a noticeable shift in the views of

physicists. The main motivations for introducing the notions

of randomness and wave function collapse in the first place

had been to explain why we perceived probabilities and not strange

macrosuperpositions. After Everett had shown that things would

appear random anyway and decoherence had been found to explain

why we never perceived anything strange, much of this motivation

was gone. Moreover, it was embarrassing that nobody had managed

to provide a testable deterministic equation specifying precisely when

this mysterious collapse was supposed to occur. Even though the wave

function technically never collapses in the Everett view, it is generally

agreed that decoherence produces an effect that looks like a collapse

and smells like a collapse.

An informal poll taken at a conference on quantum computation at the Isaac

Newton Institute in Cambridge in July 1999 gave the following results:

1. Do you believe that new physics violating the Schroedinger equation

will make large quantum computers impossible?

1 yes, 71 no, 24 undecided

2. Do you believe that all isolated systems obey the Schroedinger equation

(evolve unitarily)?

59 yes, 6 no, 31 undecided

3. Which interpretation of quantum mechanics is closest to your own?

(a) Copenhagen or consistent histories (including postulate of explicit collapse): 4

(b) Modified dynamics (Schroedinger equation modified to give explicit collapse): 4

(c) Many worlds/consistent histories (no collapse): 30

(d) Bohm (an ontological interpretation where an auxiliary "pilot wave" allows

particles to have well-defined positions and velocities): 2

(e) None of the above/undecided: 50

The reader is warned of rampant linguistic confusion in this area. It is not uncommon

that two physicists who say that they subscribe to the Copenhagen interpretation

and themselves disagreeing about what they mean by this. Similarly, some view

the "consistent histories" interpretation (in which the fundamental objects are

consistent sets of classical histories) as a fundamentally random theory where

God plays dice (as in the recent "Physics Today" article by Omnes & Griffith), whereas

others view it more as a way of identifying what is classical within the deterministic

"many worlds" context. Such issues undoubtedly contributed to the large "undecided"

vote on the last question. This said, the poll clearly suggests that it is time to

update the quantum textbooks: although these infallibly list explicit non-unitary

collapse as a fundamental postulate in one of the early chapters, the poll indicates

that many physicists - at least in the burgeoning field of quantum computation -

no longer take this seriously. The notion of collapse will undoubtedly retain great utility

as a calculational recipe, but an added caveat clarifying that it is probably an not a

fundamental process violating the Schroedinger equation could save astute students

many hours of frustrated confusion.

The Austrian animal behaviorist Konrad Lorenz mused that important scientific

discoveries go though three phases: first they are completely ignored, then they are

violently attacked, and finally they are brushed aside as well-known. Although more

quantitative experimental study of decoherence is clearly needed, it is safe to say that

decoherence has now reached the third phase among quantum physicists | indeed,

a large part of current quantum computing research is about finding ways to minimize

decoherence. The poll suggests that after spending the sixties in phase 1, Everett's idea

that physics is unitary (that there is no wave function collapse) is now shifting from

phase 2 to phase 3, replacing the collapse interpretation as the dominant paradigm.

[snip]

--------------------------------------------------------------------------------------------------

"I've said it before, I'll say it again: Can a dog collapse a state vector?

Dogs don't use state vectors. I myself didn't collapse a state vector

until I was 20 years old."

- David Mermin

"To that view, actualities seem to oat in a wider sea of possibilities

from out of which they are chosen; and, somewhere, indeterminism says,

such possibilities exist, and form a part of the truth."

- James, 1884 (address to the Harvard Divinity Students)

"Indeed one of the standard research projects for the Everettistas

over the years has been to somehow derive the standard Born probability

rule from the relative-state formalism itself. Everett tried his hand at it in the

original paper . . . then Graham in his PhD thesis tried to fix that up . . . then

Benioff in several papers tried to refine that . . . and then, finally, Deutsch

in his papers tried to argue that the project needed to be given up and that

the Born rule should be taken as a postulate for describing the fraction of worlds

with one property or other. The main point is that in simple Everett,

there are no "correlations" in the classical, statistical sense because there are

no joint probability functions lying around with which to work."

- David Mermin (?)

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