Sunday, 19 May 2013

Advances in quantum theory




Interpretation of quantum theory has seemed largely moribund for most of the past 50 years. Other than extensions of the theory explaining the three types of quantum interactions or forces little of fundamental importance has occurred. 

While the practical application of quantum theory has probably been the most successful in the history of science the theoretical problems with quantum theory remain monumental. These problems revolve around the quantum wave function, the mathematical object which the axioms underlying quantum theory tell us is the source of all knowledge concerning quantum systems.

Most theorists who have attempted an interpretation of the wavefunction have concluded it to be a statistical tool for calculating probabilities having no physical existence. This spooky interpretation of quantum theory was decried by Einstein but 70 years on little in the way of clarification has been offered.

The belief that the central mechanism in a branch of science might have no physical existence has an interesting history. In the early 1900s while developing the work of Gregor Mendel, William Bateson coined the term ‘genetics’ within the study of biological heredity. However Bateson considered the ratios which Mendelian genetics conferred to be mere mathematical calculation devices. At the time it was the consensus view (Hull, 1988):

Similarly, as much as Bateson might disagree with Pearson and Weldon about the value of Mendelian genetics, he agreed with them that it was unscientific to postulate the existence of genes as material bodies. They were merely calculation devices.

When the mechanisms involved lie beneath the resolution that current technology is able to detect, when there is no direct physical evidence, we might expect that weird and spooky notions might come to prevail.

The major exception to a lack of progress within quantum theory has been, I believe, the research program of Wojciech Zurek. Zurek has largely solved, the ‘measurement problem’ which was a large thorn in Einstein’s side. His development of ‘decoherence’ has detailed the mechanism of wave function collapse and shown it to be the expected mechanism of information transfer between a quantum system and its environment. This is the unique method of information transfer from quantum systems to the environment and the information which is transferred is purged of quantum weirdness and composes our familiar classical reality.

Zurek describes this purging as a selection mechanism he has named Quantum Darwinism. A subtle implication of his theory is given that something described by the mathematics of the wave function transfers information to its environment and further given that information is always physical we can conclude that the wave function must have a physical analogue.

Although Zurek’s work on decoherence is now perhaps the consensus view his theory of Quantum Darwinism has been largely ignored. This is unfortunate as I believe his theory is key to removing the weirdness and spookiness from quantum theory and placing it within the naturalistic theories of science.

Last year the physics community was roiled by a paper, On the reality of quantum states, which argued that the wave function is physically real, that it has an ontological existence. As reported on the journal Nature’s website:

At the heart of the weirdness for which the field of quantum mechanics is famous is the wavefunction, a powerful but mysterious entity that is used to determine the probabilities that quantum particles will have certain properties. Now, a preprint posted online on 14 November1 reopens the question of what the wavefunction represents — with an answer that could rock quantum theory to its core. Whereas many physicists have generally interpreted the wavefunction as a statistical tool that reflects our ignorance of the particles being measured, the authors of the latest paper argue that, instead, it is physically real. 

Last week a paper was published connecting the reality of the wave function with the theory of Quantum Darwinism (Korbicz, Horodecki, & Horodecki, 2013).  From the paper’s abstract:

Quantum mechanics is one of the most successful theories, correctly predicting huge class of physical phenomena. Ironically, in spite of all its successes, there is a notorious problem: how does Nature create a ‘bridge’ from fragile quanta to the robust, objective world of everyday experience? It is now commonly accepted that the most promising approach is the Decoherence Theory, based on the system-environment paradigm. To explain the observed objectivity of information in the classical realm, Zurek proposed to divide the environment into a number of independent fractions and argued that each of them carries a nearly complete classical information about the system. Here we prove that the necessary and sufficient condition for objective existence of a state is the spectrum broadcasting process, which, in particular, implies Quantum Darwinism.

The information transfer mechanism of Quantum Darwinism, the ‘bridge’ between quantum and classical reality, is described more precisely as information transfer in the form of ‘spectrum broadcasting’. Spectrum broadcasting conveys information about the quantum system to its environment in the form of a probability distribution. In this form the information is robust and many independent and redundant copies are preserved in the classical environment. Due to the existence of many copies of this information numerous observers in classical reality will all be able to agree on the information. The authors of this paper accept Zurek’s definition of objectivity:

Definition 1 (Objectivity)
A state of the system S exists objectively if ”[...]many observers can find out the state of S independently, and without perturbing it.”

While an objectively known entity may not have a guaranteed ontological existence this latest result does add support to that notion:

As a final touch, we quote the results of Refs. [29] ( On the reality of quantum states) on the epistemological versus ontological interpretation of a quantum state itself: under suitable assumptions, a state of a quantum system is a property of the system rather than a state of knowledge about it. This somewhat strengthens our result and justifies the use of quantum states for studying objective existence: the latter gains a certain ontological status, as it intuitively should.

It appears, at least to this observer that advances in quantum theory are now accumulating at an accelerating rate. Further, it appears that these advances are lending support to Zurek’s initial vision of quantum processes as mechanisms within physical reality which operate as Darwinian processes. I am hopeful that if these trends holds true our interpretation of quantum processes may soon lose its exotic nature and join the rest of science as a naturalistic process.

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