Monday, March 10, 2014

Relativity, Quantum Theory, and the EPR Paradox

I am not a physicist, but I do fit the technical definition of an "amateur:" I love learning what I can.

In November 2005, the APS News, a publication of the American Physical Society, published a brief article entitled "Einstein and the EPR Paradox" (Volume 14, No. 10, p.2; pdfweb). The article summarizes a significant paradox that currently exists in the scientific world.

On the one hand, it has been proven mathematically and experimentally that nothing can travel faster than the speed of light. This is part of Einstein's theory of relativity, consistent with classical Newtonian physics. On the other hand, there's quantum physics, which seems to be equally true but appears to violate the Newtonian understanding of physics in fundamental ways, casting doubt on the very nature of matter, space, and time.

This excerpt from APS article explains further (bolded emphasis mine):

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By the 1920s, it had become clear to most physicists that classical mechanics could not fully describe the world of atoms, especially the notion of “quanta” first proposed by Planck and further developed by Albert Einstein to explain the photoelectric effect. Physics had to be rebuilt, leading to the emergence of quantum theory.

Werner Heisenberg, Niels Bohr and others who helped create the theory insisted that there was no meaningful way in which to discuss certain details of an atom’s behavior: for example, one could never predict the precise moment when an atom would emit a quantum of light. But Einstein could never fully accept this innate uncertainty, once famously declaring, “God does not play dice.” He wasn’t alone in his discomfort: Erwin Schrödinger, inventor of the wave function, once declared of quantum mechanics, “I don’t like it, and I’m sorry I ever had anything to do with it.”

In a 1935 paper, Einstein, Boris Podolsky and Nathan Rosen introduced a thought experiment to argue that quantum mechanics was not a complete physical theory. Known today as the “EPR paradox,” the thought experiment was meant to demonstrate the innate conceptual difficulties of quantum theory. It said that the result of a measurement on one particle of an entangled quantum system can have an instantaneous effect on another particle, regardless of the distance of the two parts.

One of the principal features of quantum mechanics is the notion of uncertainty: not all the classical physical observable properties of a system can be simultaneously determined with exact precision, even in principle. Instead, there may be several sets of observable properties–position and momentum, for example–that cannot both be known at the same time. Another peculiar property of quantum mechanics is entanglement: if two photons, for example, become entangled –that is, they are allowed to interact initially so that they will subsequently be defined by a single wave function–then once they are separated, they will still share a wave function. So measuring one will determine the state of the other: for example, with a spin-zero entangled state, if one particle is measured to be in a spin-up state, the other is instantly forced to be in a spin-down state. 

This is known as “nonlocal behavior;” Einstein dubbed it “spooky action at a distance.” It appears to violate one of the central tenets of relativity: information can’t be transmitted faster than the speed of light, because this would violate causality. [...]

There have been numerous theoretical and experimental developments since Einstein and his colleagues published their original EPR paper, and most physicists today regard the so-called “paradox” more as an illustration of how quantum mechanics violates classical physics, rather than as evidence that quantum theory itself is fundamentally flawed, as Einstein had originally intended.

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You can read the rest of the American Physical Society article here: pdfweb.

If you want to learn more, other related topics worth exploring are the double-slit experiment, Schroedinger's cat, and string theory (which is an attempt to theoretically resolve the paradox discussed).

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