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Saturday, 31 May 2014

A Meta-Law to Rule Them All: Physicists Devise a “Theory of Everything”

Scientific American

“Once you have eliminated the impossible,” the fictional detective Sherlock Holmes famously opined, “whatever remains, however improbable, must be the truth.” That adage forms the foundational principle of “constructor theory”—a candidate “theory of everything” first sketched out by David Deutsch, a quantum physicist at the University of Oxford, in 2012. His aim was to find a framework that could encompass all physical theories by determining a set of overarching “meta-laws” that describe what can happen in the universe and what is forbidden. In a May 23 paper posted to the physics preprint server, arXiv, constructor theory claims its first success toward that goal by unifying the two separate theories that are currently used to describe information processing in macroscopic, classical systems as well as in subatomic, quantum objects.

Computer scientists currently use a theory developed by the American mathematician and cryptographer Claude Shannon in the 1940s to describe how classical information can be encoded and transmitted across noisy channels efficiently—what, for instance, is the most data that can be streamed, in theory, down a fiber-optic cable without becoming irretrievably corrupted. At the same time, physicists are striving to build quantum computers that could, in principle, exploit peculiar aspects of the subatomic realm to perform certain tasks at a far faster rate than today’s classical machines.

But the principles defined by Shannon’s theory cannot be applied to information processing by quantum computers. In fact, Deutsch notes, physicists have no clear definition for what “quantum information” even is or how it relates to classical information. “If we want to make progress in finding new algorithms for quantum computers, we need to understand what quantum information actually is!” he says. “So far, the algorithms that have been discovered for quantum computers have been surprises that were discovered by blundering about because we have no underlying theory to guide us.”

In 2012 Deutsch outlined constructor theory, which, he believes, could provide the underlying foundation for a grand unification of all theories in both the classical and quantum domains. This latest paper is a first step toward that larger goal—a demonstration of how classical and quantum information can be used to unify two physical theories. According to constructor theory, the most fundamental components of reality are entities—“constructors”—that perform particular tasks, accompanied by a set of laws that define which tasks are actually possible for a constructor to carry out.

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