Dear Colleagues,Science & Ultimate Reality
Few disputes in theoretical physics have been as long-running or as vexed as
the problem of how the quantum and classical worlds join together. Quantum
mechanics is an enormously successful theory of the microworld, but the
conceptual structure of the theory is alien to the realm of everyday
experience. Since the everyday world of big things is made up of little
things, quantum mechanics ought to apply consistently to the world as a
whole. So why don't we see physical objects in superpositions of states, or
doing weird things like tunnelling through walls?
Melding the madhouse world of quantum uncertainty with the orderly operation
of cause and effect that characterizes the classical domain is a challenge
that has engaged many theorists. Among them is Wojciech Zurek from the Los
Alamos National Laboratory, whose paper is summarised below. In a nutshell,
Zurek's thesis is that quantum systems do not exist in isolation: they
couple to a noisy environment. Since it is central to the quantum
description of nature that matter has a wave aspect, then quantum weirdness
requires the various parts of the waves to retain their relative phases. If
the environment scrambles these phases up, then specifically quantum
qualities of the system are suppressed. This so-called 'decoherence' seems
to be crucial in generating a quasi-classical world from its quantum
components, and Zurek has played a leading role in establishing the
theoretical credibility of this explanation.
There is, however, a snag. Decoherence well explains how, for example,
Schrödinger's cat will always be seen either alive or dead, never in a
ghostly amalgam of the two conditions. But if quantum mechanics really is a
valid description of the universe as a whole, then by definition there is no
'external environment' to bring about cosmic decoherence. Many cosmologists
believe the universe was born in a quantum process, so we need a mechanism
to account for how the familiar classical universe we see today emerged from
the quantum fuzziness of the big bang. Theorists have proposed various
devices to confront this conundrum using ideas drawn from the decoherence
theory, but I think it is fairly true to say that in spite of this the
subject of quantum cosmology remains ill-understood and incomplete.
John Wheeler has asked: How come the quantum? The flip-side of this question
is: How come the classical? Only when we see both sides of this conceptual
coin will our understanding of reality be complete.
Paul Davies
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Title:Quantum Theory of the Classical
Author:Wojciech Hubert Zurek
Summary:
Quantum theory has been spectacularly successful in all respects but one:
Its very foundation -- the principle of superposition-seems to be at odds
with the "familiar classical reality." I explore behavior of quantum systems
that are open-that interact with their environments. This often leads to the
effective monitoring of the state of the system by the environment and
results in decoherence and environment-induced superselection
(einselection). Quantum theory of classical reality emerges from the
information-theoretic analysis of the correlations between systems, their
environments, and the observers. Effective classicality of states or of
events can be quantified either by their predictability, or by the
redundancy of their records imprinted in the environment.
The tension between the quantum formalism on the one hand and the
classicality of the macroscopic Universe on the other has been such a
constant and reliable concern throughout much of the past century that is
seems almost impolite to suggest that progress in resolving this issue is
being made. The interpretation problem can be traced to the superposition
principle. It states that a combination of any two legal quantum states is a
valid quantum state. This rule, extensively verified over the years in the
microscopic seems to be also valid in the experiments involving isolated
mesoscopic objects, and is expected to extrapolate to the macroscopic
domain. Yet, in the familiar classical realm, all the states we seem to
perceive come from a very small subset of a much more diverse quantum menu.
Somehow, their arbitrary combinations allowed and even mandated by the
superposition principle never seem to appear. Even in the situations when
either the correlation with the quantum system or the dynamics per se are
supposed to induce a non-local quantum superposition, macroscopic systems
remain boringly confined to the classical corner of their vast Hilbert
spaces. Thus, a pointer state of the measuring device or memory of a
record-keeping device are always in well-defined states (and never in their
superposition). And a macroscopic chaotic system (such as Hyperion, the
chaotically tumbling moon of Saturn or for that matter, solar system as a
whole) do not appear to be coherently spread over the dynamically accessible
range of the phase space, as Schroedinger equation would seem to demand.
Rather, they always appear to be well-represented by the classical point in
the phase space, in accord with the traditional view reaching back to
Descartes and Newton.
An alternative view of the quantum-classical conflict focuses on
information. This line of argument leads to the conclusion that it is
impossible to find out the initially unknown state of the quantum system
without putting its existence in peril. This is because in quantum physics
acquisition of information is typically associated with the collapse of the
wavepacket -with re-preparation of the system in a new state, one of the
eigenstates of the measured observable. In absence of any a priori
information about the state the observer will, in general, choose an
observable which will destroy the pre-existing state, and prepare some new
state.
The contrast between the quantum and the classical resides in the
accessibility and sustained validity of the acquired classical information,
or, perhaps, in the independence of the classical states from the
measurements carried out on them. The elusive nature of the quantum and the
reality of the classical realm is dramatically demonstrated by the
subjectivity of quantum states, including Bell's inequalities and the
no-cloning theorem. More recently, the contrast between the "private
quantum" and 'public classical' states is emphasized in the applications of
quantum physics to information processing. Thus, quantum cryptography relies
on the untouchability of quantum states to guarantee privacy of
communications. Quantum computing, on the other hand, depends on carefully
isolating state of the computer from the environment so that the there is
essentially no decoherence, and, thus, one can coherently interfere various
classical computations (which allows for quantum parallelism, and is the key
to the advantage quantum computers offer in some situations).
The resolution of the quantum-classical difficulties I shall advocate is
based on a fully quantum foundation. In effect, I shall use the principles
of quantum physics for systems immersed in quantum environments to explain
emergence of the effective classicality. The first view shall be based on
the damage selectively inflicted on the states of the system by the
environment. This by now better known side of the story focuses on
decoherence and einselection as means of restricting the possible set of
quantum states so that the principle of superposition loses its validity in
a manner that favors effectively classical pointer states. We shall see how
in open quantum systems-that is, in systems that entangle with, and are thus
continuously monitored by their environments-only a small subset of states
(or of the subspaces) in the Hilbert space of the system is relatively
immune from the consequences of the openness. These special pointer states
do not entangle. They are, therefore, predictable, and become obvious
candidates for the "classical realm." This view of the environment-induced
classicality was dismissed until the relatively recent demonstrations that
it leads to einselection-to the selection of preferred effectively classical
sets of states. Over the past decade it has been, nevertheless, slowly
making its way to the "standard lore," and may be by now known to many of
the readers of this paper. Its focus is on the stability of the states of
the open system. Classicality viewed in this manner is then synonymous with
the stable existence or predictable evolution of selected (einselected)
pointer states, singled out as a result of the "negative selection" imposed
by decoherence.
Complementary point of view looks at the effects of the system on the
environment. Its focus is the information about the system that has been
dispersed throughout-and is therefore available from -- the environment. It
has come to the fore only very recently and defines the classical set of
states of the open quantum system through their indirect accessibility-by
quantifying the ``advertising'' they have received throughout the Universe.
That is, the einselected states that are stable in spite of decoherence also
turn out to be the ones most redundantly recorded in the environment, and
can be relatively easily found out without being perturbed. Redundancy
implies that only a small fraction of the environment needs to be
intercepted to obtain all the information about them. This approach is best
motivated by the contrast between the reliable "objective existence" of the
classical states and the ephemeral nature and subjective malleability under
observation of the quantum states. Classicality of an observable can be then
quantified by the redundancy of its record in the environment. Redundancy
also explains easy accessibility of the information about the effectively
classical states. This resolves the apparent conflict between the "objective
existence" (which is the essence of the classical reality) and the
subjective nature of the qu antum states that are easily perturbed by
measurements. The ability to find out (and, therefore, to "clone" or
communicate a previously unknown state motivates this measure of effective
classicality in a completely quantum Universe.
While I shall be impolite enough to suggest that progress on the emergence
of "the classical" from "the quantum" has been made, I shall also note that
much more remains to be done. In particular, the overarching role of the
information in the quantum physics seems to point to a still deeper
understanding-perhaps even the raison d'être of the quantum based on the
role of information we may have already glimpsed-that
remains to be fully uncovered and explored.
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