100 Years of the Quantum
Dept. of Physics, Univ. of Pennsylvania
-John Archibald Wheeler
Princeton University, Department of Physics
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
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.
"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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