Article examines rare quantum physics effect

quantum physics
Electron waves being shot past a tiny toroidal magnet.

( -- There's nothing University of Nebraska-Lincoln physicist Herman Batelaan likes more than a challenge. And there are few areas of science more challenging than working at the sub-atomic, or quantum, world, where the laws of physics are different from those of our macro world.

For the last four years, a little-known area within that field has captured his attention -- the Aharonov-Bohm effect. Predicted by theorists Yakir Aharonov and David Bohm in 1959, the effect is a phenomenon where electrons can be affected by electromagnetic potentials without coming in contact with actual force fields.

"It's what is called 'action-at-distance,'" said Batelaan, an associate professor of physics and astronomy. "An example of action-at-a-distance would be that you push with your finger in mid-air one foot above your table and a book that is lying on the desk moves. That would be weird -- and so is any action-at-a-distance phenomenon."

Aharanov and Bohm's prediction was originally received with skepticism, but many questions were answered by experiments in Japan and at UNL in which electron waves were shot past tiny toroidal magnets (wires rolled up in long spirals with current running through them). A 1986 experiment led by Akira Tonomura at Hitachi Ltd. and the RIKEN Frontier Research System in Japan, showed the action-at-a-distance effect is real; and a 2007 experiment by Batelaan's group at UNL showed that there is no force acting on the electrons.

That work is highlighted on the cover of the September issue of Physics Today, the journal of the American Institute of Physics, along with a feature article by Batelaan and Tonomura reviewing research into the little-understood phenomenon that among other things shows that some aspects of classical physics such as Newton's third law -- every action has an equal and opposite reaction -- don't always apply at the quantum level. But Batelaan and Tonomura are in good company. Even Nobel laureate Richard Feynman (1918-88), thought by many to be the greatest American physicist, pointed out in his "Lectures on Physics" that Newton's third law does not always hold.

"I feel that I'm lucky that we can tinker with physical realizations of the theoretical systems that Feyman was considering," Batelaan said.

Another puzzling thing about the Aharonov-Bohm effect, Batelaan said, is that while knowledge of the effect has been around for a half-century and is a modern cornerstone of quantum mechanics, it has not led to technological progress in developing nanometer-size devices.

"On the one hand, I have the feeling that there's something that's not quite understood there, and on the other hand, I feel there must be something we can do with this," Batelaan said. "But isn't that the heart of science, that you're at the limit of your knowledge and you're wondering what's going one there? If we knew where it was going, then it wouldn't be for me. I'm looking for fundamental stuff. I would rather open the route to something new. To me, that's very exciting and with this (Physics Today) article, Tonomura and I wanted to send a message to young researchers that a lot of things are still unknown and mysterious."

Provided by University of Nebraska-Lincoln (news : web)

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Sep 24, 2009
This is not logical. If the electron waves are affected by the magnets but measurements show no force acting, would you not be looking for a force that is not detectable with you measuring equipment?

Is science not grounded by cause and effect anymore? Or are we now to accept tales of magic power?

Sep 24, 2009
Is this suggesting that an effect can exist without a cause?

That is not QM. It is nonsense.

Sep 24, 2009
RE: above comment.

Yeah,especially in the picture. I believe in understanding this effect its better to picture a "perfect" coil in which the magnetic field is confined to the inside of the coil. The field still acts on the electron even though the electron doesn't pass through the coil. This has been shown experimentally with diffraction patterns.

Sep 24, 2009
It's a potential/phase effect. In Sweden undergrads do Aharonov-Bohm experiments.

Sep 24, 2009
Not heard of this effect before and it seems very interesting.

In QM an electron is supposed to be everywhere with varying amounts of probability. So to say that an electron, or whatever, is in an area where the field is zero is a classical description. Is the mixing up of classical descriptions with QM in the same statement what is causing the oddity?

The electron in QM will come into contact with the non-zero field in some of its superimposed states with some non-zero probabilities. So the effect has a cause after all.

A similar oddity is in the recent random walk article in these web pages, where a solitary electron, or whatever, seems to interect with itself and position itself in a bimodal situation. Kind of repelling itself across an enclosure. No other force present except the quantum vacuum.

(Hope this is OK though I am not a physicist.)

Sep 25, 2009
If the electron waves are affected by the magnets but measurements show no force acting, would you not be looking for a force that is not detectable with you measuring equipment?
The sentence "electrons can be affected by electromagnetic potentials without coming in contact with actual force fields" is incorrect - they come in contact with electromagnetic field, just in less or more delocalized form. Aharamov-Bohm effect does work for long solenoid, it doesn't work for toroidal solenoid, where outer electromagnetic field really doesn't exist, because all magnetic field lines of force are trapped inside of toroid spiral.

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