The discovery of entanglement: The concept, but not the name of entanglement, was first put under the scientific spotlight on May 15, 1935, when a paper by Einstein and two younger associates, Nathen Rosen and Boris Podolosky, appeared in the journal Physical Review.1 Its title – Can a Quantum-Mechanical Description of Physical Reality Be Considered Complete? – leaves no doubt that the paper was a challenged to Niels Bohr and his vision of the subatomic world. On June 7, Erwin Schrödinger, himself no lover of quantum weirdness, wrote to Einstein, congratulating him on the paper and using in his letter the word entanglement – or, rather, its German equivalent verschränkung – for the first time. This new term soon found its way into print in an article – sent to the Cambridge Philosophical Society on August 14 that was published a couple of months later.2 In it he wrote: When two systems ... enter into temporary physical interaction ... and when after a time of mutual influence the systems separate again, then they can no longer be described in the same way as before, viz. by endowing each of them with a representative of its own. I would not call that one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought. By the interaction the two representatives [the quantum states] have become entangled. The characteristic trait of quantum mechanics ... the one that enforces its entire departure from classical lines of thought – here was an early sign of the importance attached to this remarkable effect. Entanglement lay at the very heart of quantum reality – its most startling and defining feature. And Einstein would have none of it. For the best part of a decade, the man who revealed the particle nature of light (see Einstein and the photoelectric effect) had been trying to undermine Bohrs interpretation of quantum theory. Einstein couldnt stomach the notion that particles didnt have properties, such as momentum and position, with real, determinable (if only we knew how), preexisting values. Yet that notion was spelled out in a relationship discovered in 1927 by Werner Heisenberg. Known as the uncertainty principle, it stems from the rule that the result of multiplying together two matrices representing certain pairs of quantum properties, such as position and momentum, depends on the order of multiplication. The same oddball math that says X times Y doesnt have to equal Y times X implies that we can never know simultaneously the exact values of both position and momentum. Heisenberg proved that the uncertainty in momentum can never be smaller than a particular number that involves Plancks constant. In one sense, this relationship quantifies wave-particle duality. Momentum is a property that waves can have (related to their wavelength); position is a particlelike property because it refers to a localization in space. Heisenbergs formula reveals the extent to which one of these aspects fades out as the other becomes the focus of attention. In a different but related sense, the uncertainty principle tells how much the complementary descriptions of a quantum object overlap. Position and momentum are complimentary properties because to pin down one is to lose track of the other; they coexist but are mutually exclusive, like the opposite sides of the same object. Heisenbergs formula quantifies the extent to which knowledge of one limits knowledge of the other. Einstein didnt buy this. He believed that a particle does have a definite position and momentum all the time, whether were watching it or not, despite what quantum theory says. From his point of view, the Heisenberg uncertainty principle isnt a basic rule of nature; its just an artifact of our inadequate understanding of the subatomic realm. In the same way, he though, wave-particle duality isnt grounded in reality but instead arises from a statistical description of how large numbers of particles behave. Given a better theory, thered be no wave-particle duality or uncertainty principle to worry about. The problem was, as Einstein saw it, that quantum mechanics wasnt telling the whole story: it was incomplete. SOURCE: daviddarling.info/encyclopedia/Q/quantum_entanglement.html
Posted on: Wed, 29 Jan 2014 11:36:11 +0000
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