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Mon 4 Mar, 2013 11:33 am

Physicists Discover a Way Around Heisenberg's Uncertainty Principle
Quote:Science Daily Headlines reports that researchers have applied a recently developed technique to directly measure the polarization states of light overcoming some important challenges of Heisenberg's famous Uncertainty Principle and demonstrating that it is possible to measure key related variables, known as 'conjugate' variables, of a quantum particle or state directly. Such direct measurements of the wave-function had long seemed impossible because of a key tenet of the uncertainty principle — the idea that certain properties of a quantum system could be known only poorly if certain other related properties were known with precision. 'The reason it wasn't thought possible to measure two conjugate variables directly was because measuring one would destroy the wave-function before the other one could be measured,' says co-author Jonathan Leach. The direct measurement technique employs a 'trick' to measure the first property in such a way that the system is not disturbed significantly and information about the second property can still be obtained. This careful measurement relies on the 'weak measurement' of the first property followed by a 'strong measurement' of the second property. First described 25 years ago, weak measurement requires that the coupling between the system and what is used to measure it be, as its name suggests, 'weak,' which means that the system is barely disturbed in the measurement process. The downside of this type of measurement is that a single measurement only provides a small amount of information, and to get an accurate readout, the process has to be repeated multiple times and the average taken. Researchers passed polarized light through two crystals of differing thicknesses: the first, a very thin crystal that 'weakly' measures the horizontal and vertical polarization state; the second, a much thicker crystal that 'strongly' measures the diagonal and anti-diagonal polarization state. As the first measurement was performed weakly, the system is not significantly disturbed, and therefore, information gained from the second measurement was still valid. This process is repeated several times to build up accurate statistics. Putting all of this together gives a full, direct characterization of the polarization states of the light."

Links in original article.

Why do you keep spelling his name with a zee? That technique does not exactly break Heisenberg's principle. Like many non-rigorous descriptions, the summary makes the mistake of describing the uncertainty principle as if it is a measurement problem, where the lack of precision somehow arises from inadequate measurement technology. This is not a correct statement of the uncertainty principle. The fundamental issue is that the conjugate variable values are linked on a quantum level, such that there is a certain amount of natural, inherent uncertainty in their collective values due to the statistical/wavelike nature of the quantum particle. With perfect measurement, there is still uncertainty in the pair of values for any conjugate variables because the uncertainty lies in the actual values themselves. Position and momentum are the quintessential conjugate pair. The Heisenberg uncertainty principle is sometimes framed as the idea that you cannot know the speed and position of a particle at the same time. But it's more correct to say that a particle does not HAVE an exact speed and position at the same time. This weak measurement technique is certainly useful and interesting since it allows some observations of wavefunctions without collapse, but it does not actually allow the measurement of conjugate variables more precisely than the uncertainty principle allows - because the values themselves do not exist more precisely than that.

@contrex,

Not a violation of the uncertainty principle (Score:5, Informative)

by mpoulton (689851) on Monday March 04, @11:38AM (#43069105)

Quote:

Like many non-rigorous descriptions, the summary makes the mistake of describing the uncertainty principle as if it is a measurement problem, where the lack of precision somehow arises from inadequate measurement technology. This is not a correct statement of the uncertainty principle. The fundamental issue is that the conjugate variable values are linked on a quantum level, such that there is a certain amount of natural, inherent uncertainty in their collective values due to the statistical/wavelike nature of the quantum particle. With perfect measurement, there is still uncertainty in the pair of values for any conjugate variables because the uncertainty lies in the actual values themselves. Position and momentum are the quintessential conjugate pair. The Heisenberg uncertainty principle is sometimes framed as the idea that you cannot know the speed and position of a particle at the same time. But it's more correct to say that a particle does not HAVE an exact speed and position at the same time. This weak measurement technique is certainly useful and interesting since it allows some observations of wavefunctions without collapse, but it does not actually allow the measurement of conjugate variables more precisely than the uncertainty principle allows - because the values themselves do not exist more precisely than that.

Why would you steal a comment from slashdot and present it as your own?

@DrewDad,

DrewDad wrote:Why would you steal a comment from slashdot and present it as your own?

Because it represents my own thoughts and is a hell of a lot quicker than writing them out. However: The article describes the uncertainty principle as if it is a measurement problem. It isn't. The uncertainty is inherent in the thing being measured. Have you anything to say about this?

@contrex,

I don't have much to say to someone who copies other people's stuff and tries to pass it off as their original work, no.

If I wanted to argue about it, I'd go find mpoulton on slashdot and talk to him (or her) directly.