The Wave Function: It or Bit?

Dear Colleagues,

Science & Ultimate Reality

In his excellent summary of John Wheeler's career, Kenneth Ford identified three phases in Wheeler's thinking:  the particle period, the field period, and the information period.  The concept of information is inextricably bound up with quantum mechanics, and Wheeler's incisive question "How come the quantum?" stands as a challenge for any attempt to provide a unified theoretical description of nature.  It is not enough that the rules of quantum mechanics are found to work.  To fully understand nature at the deepest level we need to know why the world obeys quantum rules. Part of the answer involves knowing how the quasi-classical world of observation emerges from the weird domain of quantum physics.

Wheeler's "It from bit" program proceeds from a well-known yet still mysterious fact.  The wave function describing a quantum particle expresses what is known about that particle, i.e., it represents information, or software, whereas the particle itself is an object, or hardware.  How do these fundamentally different concepts, associated with different levels of description, fit together, and how does this fit recover the usual notion of "reality"?  This dualism (particle-wave, hardware-software, it-bit) lies at the heart of quantum physics, yet its true character continues to tantalize.  As the wave function evolves and observations are made, hardware and software, its and bits, become entangled.  Whenever we encounter entanglements of different conceptual level, the possibilities for confusion and dissent are legion. (Consider that other bone of contention, the mind-body problem, with mind as software, body as hardware, somehow intermingling descriptively if not physically when thoughts beget actions and sense data beget thoughts; indeed, some people believe this problem is intimately related to the it/bit quantum problem.)

Dieter Zeh from theUniversityofHeidelbergis an expert on the conceptual foundations of quantum mechanics, time asymmetry, and the emergence of classicality through decoherence.

Paul Davies


The Wave Function: It or Bit?
by Dieter Zeh


Schroedinger's wave function shows many aspects of a state of incomplete knowledge, or INFORMATION ("bit"): (1) it is defined on a space of classical configurations, (2) its entanglement is, therefore, analogous to statistical correlations, and (3) it determines probabilites of measurement outcomes.  Nonetheless, quantum superpositions (such as represented by a wave function) define individual PHYSICAL states ("it"), as we know from many examples.

This dilemma may have its origin in the conventional OPERATIONAL foundation of physical concepts, successful in classical physics, but inappropriate in quantum theory because of the existence of mutually exclusive operations.  In contrast, a hypothetical realism, based on concepts that are justified only by their universal and consistent applicability, favors the wave function as a description of (nonlocal) physical reality.  The (conceptually local) classical world then appears as an illusion, facilitated by the phenomenon of decoherence, which is consistently EXPLAINED by the very entanglement that must dynamically arise in a universal wave function.  Decoherence is meaningful for observers that are ASSUMED to be local (for dynamical reasons).

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