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from the the-universe-is-stranger-than-you-imagine dept.
You may (or may not) have heard about the Heisenberg Uncertainty Principle:
In quantum mechanics, the uncertainty principle, also known as Heisenberg's uncertainty principle, is any of a variety of mathematical inequalities asserting a fundamental limit to the precision with which certain pairs of physical properties of a particle, known as complementary variables, such as position x and momentum p, can be known.
According to a report in ChemistryWorld, a new technique allows atomic spin properties to be measured simultaneously with greater accuracy — Atomic Spins Evade Heisenberg Uncertainty Principle:
Many seemingly unrelated scientific techniques, from NMR spectroscopy to medical MRI and timekeeping using atomic clocks, rely on measuring atomic spin – the way an atom's nucleus and electrons rotate around each other. The limit on how accurate these measurements can be is set by the inherent fuzziness of quantum mechanics. However, physicists in Spain have demonstrated that this limit is much less severe than previously believed, measuring two crucial quantities simultaneously with unprecedented precision.
Central to the limits of quantum mechanics is the Heisenberg uncertainty principle, which states that it is not possible to know a particle's position and momentum with absolute accuracy, and the more precisely you measure one quantity, the less you know about the other. This is because to measure its position you have to disturb its momentum by hitting it with another particle and observing how the momentum of this second particle changes. A similar principle applies to measuring a particle's spin angular momentum, which involves observing how the polarisation of incident light is changed by the interaction with the particle – every measurement disturbs the atom's spin slightly. To infer the spin precession rate, you need to measure the spin angle, as well as its overall amplitude, repeatedly. However, every measurement disturbs the spin slightly, creating a minimum possible uncertainty.
The alternative approach suggested by Morgan Mitchell's group at the Institute of Photonic Sciences in Barcelona, could circumvent this problem. The spin angle, they say, is in fact two angles: the azimuthal angle (like longitude on the Earth's surface) and the polar angle (like latitude). To measure the precession rate, you need only the azimuthal angle. Therefore, by loading as much uncertainty as possible into the polar angle, you can measure the two quantities you need – the azimuthal angle and amplitude of the spin – and therefore measure the spin precession rate much more accurately than previously thought possible.
Is this the harbinger of finer-grained and/or quicker MRIs?
References: G Colangelo et al, Nature, 2017, DOI: 10.1038/nature21434
(Score: 2) by Justin Case on Monday April 03 2017, @06:07PM (3 children)
I understand that the precise truth is reserved for those who can read the symbols of that foreign language called "math", and the English version is necessarily watered down.
to measure [a particle's] position you have to disturb its momentum by hitting it with another particle and observing how the momentum of this second particle changes
Is is really possible to work with individual particles like that? How the hell do you hit Particle One with Particle Two when you don't know the exact location of Particle One?
How do you then measure the allegedly unmeasurable properties of Particle Two, in particular, its changing momentum, given that you can't know where it is?
It sounds like blindfolded billiards.
(Score: 3, Interesting) by JoeMerchant on Monday April 03 2017, @09:37PM (2 children)
Most of these measurements are done with millions, if not billions, of particles and summing up the result into something observable.
The progress the French lab made is (obvious now that they've demonstrated it) to focus the uncertainty into one of two degrees of freedom, thereby reducing the product. Something like: 0.5 * 0.5 = 0.25, but 0.1 * 0.9 = 0.9, so measure the 0.1 * 0.9 case and you get almost 1/4 the uncertainty.
Grossly oversimplified, of course, but who really wants to read a bunch of French guys explaining quantum physics in English?
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(Score: 2) by Justin Case on Monday April 03 2017, @10:51PM (1 child)
measurements are done with millions, if not billions, of particles and summing up the result
Yeah, more or less as I suspected. So why don't they say so? It is like the dual-slot photon experiment, where they say that a single photon goes through both slots. I always wondered where do you get a single photon, and how do you see it? Then came the admission that they're testing with a beam of (probably billions? of) photons.
(Score: 2) by JoeMerchant on Tuesday April 04 2017, @12:38PM
The next thing you will hear is that it's not a particle when it goes through the slot, it's a wave - can't you see the interference patterns?
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