Best of 2011: How To Turn A Laser Into A Tractor Beam?

How To Turn A Laser Into A Tractor Beam

Physicists work out how to generate a  backward pulling force from a forward propagating beam

A photon has a small momentum which it can impart to anything it hits, as Arthur Compton and Peter Lebedev discovered at the beginning of the last century. We now know that photons can be used to push anything from electrons to solar sails.Today, Jun Chen from Fudan University in China and a few pals demonstrate the counterintuitive result that photons can pull things too. In other words, they’ve worked out how to generate a backward pulling force from a forward propagating beam.

Chen and buddies say this is possible when the system meets two conditions. First, it works only for beams in which the momentum in the direction of propagation is small, as is the case for beams that merely glance off an object. Second, the photons must simultaneously excite several multipoles within the particle, which scatter the beam.

If the scattering angle is just right, the total momentum in the direction of propagation can be negative, meaning the particle is pulled back towards the source and the light becomes a tractor beam.

This must not be confused with various “optical tweezer” type mechanisms in which particles trapped in a beam follow the intensity gradient of the light. In this case, the particles always reach some point of equilibrium where the intensity reaches a maximum.

Chen and co’s new force works when there is no gradient. Given the chance, their tractor beam will pull a particle all the way back to the source.

That’s a handy additional tool in the nanomanipulator’s box of tricks. “This may open up new avenues for optical micromanipulation, of which typical examples include transporting a particle backward over a long distance and particle sorting,” say Chen and co.

This is a theory paper so there’s one piece of the puzzle left to fit. All they have to do now is demonstrate that their tractor beam works.

Ref: Backward Pulling Force From A Forward Propagating Beam

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Electrons Are Near-Perfect Spheres

By Wired UK Email Author

By Duncan Geere, Wired UK

A 10-year study has revealed that the electron is very spherical indeed.

To be precise, the electron differs from being perfectly round by less than 0.000000000000000000000000001 cm. To put that in context; if an electron was the size of the solar system, it would be out from being perfectly round by less than the width of a human hair.

The Imperial College team behind the research, which was conducted on molecules of ytterbium flouride, used a laser to make measurements of the motion of electrons, and in particular the wobble they exhibit when spinning. They observed no such wobble, implying that the electron is perfectly round at the levels of precision available, reflected in the figure above.

The co-author of the report describing the research, Jony Hudson, said: “We’re really pleased that we’ve been able to improve our knowledge of one of the basic building blocks of matter. It’s been a very difficult measurement to make, but this knowledge will let us improve our theories of fundamental physics. People are often surprised to hear that our theories of physics aren’t ‘finished’, but in truth they get constantly refined and improved by making ever more accurate measurements like this one.”

The next step is to up that precision level even further, using new methods to cool the molecules to extremely low temperatures and control their motion. The results are important in the study of antimatter, and particularly the positron — which should behave identically to the electron but with an opposite electrical charge. If more differences can be found, it could help to explain why far less antimatter has been discovered in the universe than predicted by theory.

Image: Lawrence Rayner/Flickr


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The Quantum Physics Behind The Death Of Osama Bin Laden

Steve Kleinman

Is information technology outpacing the brain’s ability to process it?

The death of Osama bin Laden, while a geostrategic event of real importance, is also a prime example of how three principles from quantum physics might explain the reality and the potential of such occurrences in the context of international affairs.

The first principle would suggest this incident–or, more precisely, the news of this incident–is comprised of little more than energy and information, the same two fundamental building blocks that form the universe. The second principle describes how mere observation of an event will influence how it ultimately manifests. The third principle introduces nonlocality, how an event occurring in one physical space has the potential to profoundly alter events unfolding elsewhere. The combination of these principles offers a framework for eliciting more meaningful insights into the event’s real import.

As to the first principle, information includes the surface level details: bin Laden was killed during a unilateral raid by U.S. Special Forces at a residence located within Pakistan. Of greater importance are the deeper details: bin Laden was the leader of al Qaeda, the perpetrator of the Sept. 11 attacks, and the figurehead for a movement to restore the Caliphate in the Middle East. It is the emotional energy attached to this that animates it–from the euphoric celebrations in front of the White House to the angry reactions within Pakistan. This combination of energy and information proved sufficient to temporarily move the major stock exchanges, spur anti-terrorist programs into high gear and cue debate within American political circles as to who should rightfully take credit for the killing.

The second principle reminds us that human observation changes the nature of the event. In most of the Western world bin Laden’s death was heralded as a major success in the war on terror and evidence to support the intelligence-drives-kinetic-energy approach to counterterrorism. This perspective may also reframe the event as a culminating point of victory in the battle against Islamic extremism (the “cut off the head of the snake” philosophy).

In the Middle East, Horn of Africa and parts of Southeast Asia, human observation characterized it in a far different light. The killing of bin Laden, this perspective argued, provides additional evidence of America’s systematic disregard for the sovereign rights of nations with Muslim majorities and his death–at the hands of the crusaders–has turned a man surrounded by myth into a truly mythical icon for the ages.

Finally, there is the nonlocal element. A century ago the death of bin Laden would have been, at best, a regional story; with modern technology, a Jakarta housewife has access to–and is affected by–information rivaling that available to analysts at the National Counterterrorism Center. The rate and volume of information flow surrounding an event of this nature has the potential to inform national policy and alter vacation plans in equal measure. The concern here is that technology continues to evolve far more quickly than the brain’s ability to make sense of the flood of data. Hence, an Indonesian housewife’s rationale for changing her family’s plans for a holiday is as valid as that used to alter a nation’s approach to counterterrorism.

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