# Heisenberg's Uncertainty Principle May NOT Be U...

At the foundation of quantum mechanics is the Heisenberg uncertainty principle. Simply put, the principle states that there is a fundamental limit to what one can know about a quantum system. For example, the more precisely one knows a particle's position, the less one can know about its momentum, and vice versa. The limit is expressed as a simple equation that is straightforward to prove mathematically.

## Heisenberg's Uncertainty Principle may NOT be U...

Heisenberg sometimes explained the uncertainty principle as a problem of making measurements. His most well-known thought experiment involved photographing an electron. To take the picture, a scientist might bounce a light particle off the electron's surface. That would reveal its position, but it would also impart energy to the electron, causing it to move. Learning about the electron's position would create uncertainty in its velocity; and the act of measurement would produce the uncertainty needed to satisfy the principle.

When the researchers did the experiment multiple times, they found that measurement of one polarization did not always disturb the other state as much as the uncertainty principle predicted. In the strongest case, the induced fuzziness was as little as half of what would be predicted by the uncertainty principle.

On the other hand, if the physician is uncertain about which treatment is best for a patient, offering the patient randomisation to equally preferred treatments is acceptable and does not violate his or her duty. The uncertainty principle has been successfully used as a key eligibility criterion for large, simple trials.1-1,1-5,1-6

Now, physicists Jacques Pienaar, Tim Ralph, and Casey Myers at The University of Queensland in Australia have theoretically shown that OTCs can allow scientists to measure a pair of conjugate variables of a quantum state to an arbitrary degree of accuracy forbidden by the uncertainty principle. The finding could have implications for quantum gravity and change the way that scientists view quantum uncertainty.

"There is some speculation that the Heisenberg uncertainty principle might be different in a future theory of quantum gravity," Pienaar told __Phys.org__. "However, most of these studies suggest that quantum gravity will introduce more uncertainty. Our model suggests the complete opposite: that a theory of quantum gravity might actually remove the uncertainty of quantum mechanics."

"Deutsch's model describes the strange quantum effects that we might see in the presence of CTCs, within a future theory of quantum gravity," Pienaar said. "However, if there are no CTCs in the universe, then we would not expect to see the effects. But since the slowing of time due to gravity looks just like the effect of an OTC from the outside, and since OTCs still lead to strange effects (as we have shown), we suggested that these effects might turn up in strong gravitational fields, even without any closed loops in time. If so, then they would allow us to violate the Heisenberg uncertainty principle and clone coherent states of light without needing a full-blown time machine.

Werner Heisenberg's uncertainty principle, formulated by the theoretical physicist in 1927, is one of the cornerstones of quantum mechanics. In its most familiar form, it says that it is impossible to measure anything without disturbing it. For instance, any attempt to measure a particle's position must randomly change its speed. googletag.cmd.push(function() googletag.display('div-gpt-ad-1449240174198-2'); ); The principle has bedeviled quantum physicists for nearly a century, until recently, when researchers at the University of Toronto demonstrated the ability to directly measure the disturbance and confirm that Heisenberg was too pessimistic.

The findings build on recent challenges to Heisenberg's principle by scientists the world over. Nagoya University physicist Masanao Ozawa suggested in 2003 that Heisenberg's uncertainty principle does not apply to measurement, but could only suggest an indirect way to confirm his predictions. A validation of the sort he proposed was carried out last year by Yuji Hasegawa's group at the Vienna University of Technology. In 2010, Griffith University scientists Austin Lund and Howard Wiseman showed that weak measurements could be used to characterize the process of measuring a quantum system. However, there were still hurdles to clear as their idea effectively required a small quantum computer, which is difficult to build.

It is often assumed that Heisenberg's uncertainty principle applies to both the intrinsic uncertainty that a quantum system must possess, as well as to measurements. These results show that this is not the case and demonstrate the degree of precision that can be achieved with weak-measurement techniques.

"The results force us to adjust our view of exactly what limits quantum mechanics places on measurement," says Rozema. "These limits are important to fundamental quantum mechanics and also central in developing 'quantum cryptography' technology, which relies on the uncertainty principle to guarantee that any eavesdropper would be detected due to the disturbance caused by her measurements."

In scientific circles we are perhaps used to thinking of the word "principle" as "order", "certainty", or "a law of the universe". So the term "uncertainty principle" may strike us as something akin to the terms "jumbo shrimp" or "guest host" in the sense of juxtaposing opposites. However, the uncertainty principle is a fundamental property of quantum physics initially discovered through somewhat classical reasoning -- a classically based logic that is still used by many physics teachers to explain the uncertainty principle today. This classical approach is that if one looks at an elementary particle using light to see it, the very act of hitting the particle with light (even just one photon) should knock it out of the way so that one can no longer tell where the particle actually is located -- just that it is no longer where it was.

Smaller wavelength light (blue, for example, which is more energetic) imparts more energy to the particle than longer wavelength light (red, for example, which is less energetic). So using a smaller (more precise) "yardstick" of light to measure position means that one "messes up" the possible position of the particle more by "hitting" it with more energy. While his sponsor, Nehls Bohr (who successfully argued with Einstein on many of these matters), was on travel, Werner Heisenberg first published his Uncertainty Principle Paper using this more-or-less classical reasoning just given. (The deviation from classical notion was the idea of light comes in little packets or quantities, known as "quanta," as discussed in article one). However the uncertainty principle was to turn out to be much more fundamental than even Heisenberg imagined in his first paper.

Momentum is a fundamental concept in physics. It is classically defined as the mass of a particle multiplied by its velocity. We can picture a baseball thrown at us at 100 miles per hour having a similar effect as a bat being thrown at us at ten miles per hour; they would both have about the same momentum although they have quite different masses. The Heisenberg Uncertainty Principle basically stated that if one starts to know the change in the momentum of an elementary particle very well (that is usually, what the change in a particle's velocity is) then one begins to lose knowledge of the change in the position of the particle, that is, where the particle is actually located. Another way of stating this principle, using relativity in the formulation, turns out to be that one gets another version of the uncertainty principle. This relativistic version states that as one gets to know the energy of an elementary particle very well, one cannot at the same time know (i.e., measure) very accurately at what time it actually had that energy. So we have, in quantum physics, what are called "complimentary pairs." (If you'd really like to impress your friends, you can also call them "non-commuting observables.")

One can illustrate the basic results of the uncertainty principle with a not-quite-filled balloon. On one side we could write "delta-E" to represent our uncertainty in the value of the energy of a particle, and on the other side of the balloon write "delta-t" which would stands for our uncertainty in the time the particle had that energy. If we squeeze the delta-E side (constrain the energy so that it fits into our hand, for example) we can see that the delta-t side of the balloon would get larger. Similarly, if we decide to make the delta-t side fit within our hand, the delta-E side would get larger. But the total value of air in the balloon would not change; it would just shift. The total value of air in the balloon in our analogy is one quantity, or one "quanta," the smallest unit of energy possible in quantum physics. You can add more quanta-air to the balloon (making all the values larger, both in delta-E and delta-t) but you can never take more than one quanta-air out of the balloon in our analogy. Thus "quantum balloons" do not come in packets any smaller than one quanta, or photon. (It is interesting that the term "quantum leap" has come to mean a large, rather than the smallest possible, change in something, and the order of the dictionary definitions of "quantum leap" have now switched, with the popular usage first and the opposite, physics usage second. If you say to your boss, "We've made a quantum leap in progress today" this can still, however, be considered an honest statement of making absolutely no progress at all.)

In the next essay we will combine the uncertainty principle with the results of Bell's Theorem and increase the scale of the double slit experiment to cosmic proportions with what Einstein's colleague, John Wheeler, has called "The Participatory Universe." This will involve juggling what is knowable and what is unknowable in the universe at the same time. 041b061a72