Heisenberg's uncertainty principle gets less certain

Univ. of Toronto's Dylan Mahler (l) and Lee Rozema (r) prepare pairs of entangled photons to study the disturbance caused by measuring them. Their work suggests some measurements don't wreak so much havoc on a quantum system. / University of Toronto,Dylan Mahler

(LiveScience) More than 80 years after the uncertainty principle was first proposed, scientists are ironing out some uncertainties about the famous physics notion.

The uncertainty principle, proposed in 1927 by German physicist Werner Heisenberg, states that the more precisely a particle's position is measured, the less precisely its momentum can be known, and vice versa. It has long been invoked to describe the way measuring an object disturbs that object.

But a new experiment shows this doesn't have to be true.

"You don't have to add more uncertainty to a quantum system by measuring it," said Lee Rozema, a graduate student at the University of Toronto who led a new study of the uncertainty principle.

Rozema and his colleagues found this aspect of the uncertainty principle is often misunderstood, and that quantum measurements don't wreak as much havoc on what they're measuring as many people, including physicists, assume. [Graphic: Nature's Tiniest Particles]

The researchers used the test case of a particle of light, called a photon. They wanted to measure the polarization, or orientation, of the photon. In order to avoid disturbing the photon any more than was absolutely necessary, they employed a method called weak measurement, which indirectly measures a quantum system by analyzing its interactions with a related quantum system.

"If you want to make a measurement without disturbing your system, then you can make the interaction very weak, but then you don't get very much information on the system," Rozema told LiveScience. "What we do instead is do it many, many times and build up statistics."

In the case of the photon, the physicists measured the interaction between the particle's polarization and its position in space. After repeated measurements, they arrived at an estimation of the photon's polarization. They then used an apparatus to directly measure the photon's polarization, and compared the results.

"The disturbance that we found is less than what you'd get if you naively applied the Heisenberg uncertainty principle to the measurements," Rozema said.

Previously, researchers have had a hard time studying how much a measurement disturbs a system, because they haven't been able to separate the intrinsic disturbance any measurement would make from the disturbance particular to the measuring apparatus. Weak measurement solves this problem.

The findings don't disprove Heisenberg's uncertainty principle, but they help clarify it, Rozema said. The uncertainty quantified in the principle isn't a result of measurement, but originates in the intrinsic uncertainty of all subatomic, quantum systems, due to the fact that particles exist in states of probability, rather than certainty.

"Your quantum system still has the uncertainty in it that Heisenberg's uncertainty principle says it does," Rozema said. "But you don't have to add more uncertainty to the quantum system by measuring it."

A paper detailing the study was published earlier this month in the journal Physical Review of Letters.

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[Dr_Tony_Fleming]

may come as a surprise to past and present adherents of Heisenberg's Uncertainty Principle (HUP) but recent mathematical progress means we can also look at uncertainty from a theoretical point of view. Quantum theory, and therefore on, is incomplete as Einstein thought. See book Self-field theory, a new mathematical description of physics, by A.H.J. Fleming, published by Pan-Stanford Press 2012; analytic solutions for the motions of the electron and the proton inside the hydrogen atom have been found obviating the need of the numerical and probabilistic quantum theory. The basis of this new formulation includes the magnetic currents of particles and not just the electric fields as in quantum theory. In this formulation, the photon is composite and hydrogenic-like.

It is well known the inequality relationship of HUP applies to any quantum system in general. The equations for the orbital and cyclotron motions of each electron in self-field theory (SFT) are given as two equality equations. Apart from the 'greater than' relationship compared with the exact relationship, the 3 equations are identical. Whereas there is one inexact relationship in HUP there are two equality relationships in SFT. SFT thus completes the Bohr Theory that did not include any magnetic effect on the electron.

In the light of this mathematics HUP can be seen as a theoretical error; in practice it appears as a numerical error in any computer calculations.

[Dr_Tony_Fleming]

Let me add that HUP will always be a good engineering approximation able to be used across domains from photon to universe in the same way that Newton's law of gravitation is still used today by those involved in gravitational research.

Let me add that the magnetic moments involved in this new mathematics (SFT) at the terrestrial domain may be able to give us much more quantitative information about the way techtonic plates, earthquakes and tsunamis develop over time.

[BrianFraser]

I think the article confuses uncertainty with the issue of measurement. (The investigators, for clarity, should also have explained the difference between uncertainty and indeterminancy.)

"The Uncertainty Principle states that if we wish to locate any particle to within a distance Dx, then we automatically introduce an uncertainty in the momentum of the particle. . . . It is important to realize that this uncertainty is not due to poor measurement or poor experimental technique but is a fundamental property of the act of measurement itself." (Quantum Chemistry, Donald A. McQuarrie, 1983, p. 36-37)

"It must not be supposed . . . that the quantum uncertainty is somehow purely the result of an attempt to effect a measurement, a sort of unavoidable clumsiness in probing delicate systems. The uncertainty is inherent in the microsystem; it is there all the time, whether or not we actually choose to measure x or p [position or momentum]." (Quantum Mechanics, P.C.W. Davies, 1984, p. 8)

"The position/momentum uncertainty is the archetypal example, first described by Werner Heisenberg in 1927. It means that no entity can have both a precisely determined momentum . . . and a precisely determined position at the same time. This is not the result of the deficiencies of our measuring apparatus; it is not just that we cannot measure both the position and momentum of, say, an electron at the same time, but that an electron does not have both a precise position and a precise momentum at the same time. . . . (Some reference books still tell you that quantum uncertainty is solely a result of the difficulty of measuring position and momentum at the same time; do not believe them!)" (Q is for Quantum: An Encyclopedia of particle physics, John Gribbin, 1998, under "Uncertainty")

Readers who want to make sense of quantum mechanics should read the on-line article "Intuitive Concepts in Quantum Mechanics" at:
http://scripturalphysics.org/qm/qmconcpt.htm

[Smegacool]

Huh...?