Thursday, March 3, 2011
'Tractor beam' is possible with lasers, say scientists
The idea hinges on the use of a laser whose beam is of a specific shape
A laser can act as a "tractor beam", drawing small objects back toward the laser's source, scientists have said.
It is known that light can provide a "push", for example in solar sails that propel spacecraft on a "wind of light".
Now, in a paper on the Arxiv server, researchers from Hong Kong and China have calculated the conditions required to create a laser-based "pull".
Rather than a science fiction-style weapon, however, the approach would only work over small distances.
The effect is different from that employed in "optical tweezers" approaches, in which tiny objects can be trapped in the focus of a laser beam and moved around; this new force, the authors propose, would be one continuous pull toward the source.
And it relies on directly impinging on an object, making it distinct from an approach demonstrated in 2010 by Australian researchers whose trapping worked by heating air around a trapped particle.
The trick is not to use a standard laser beam, but rather one known as a Bessel beam, that has a precise pattern of peaks and troughs in its intensity.
Seen straight-on, a Bessel beam would look like the ripples surrounding a pebble dropped in a pond.
If such a Bessel beam were to encounter an object not head-on but at a glancing angle, the backward force can be stimulated.
As the atoms or molecules of the target absorb and re-radiate the incoming light, the fraction re-radiated forward along the beam direction can interfere and give the object a "push" back toward the source.
"Light can indeed pull a particle," the authors wrote, "...and 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."
Ortwin Hess at Imperial College London called the work - which has not yet been peer-reviewed - as "fascinating", saying that it "takes a radical idea forward".
READ THE REST OF THE STORY HERE AT THE BBC
Posted by Steve Douglass at 6:41 AM
Washington (CNN) -- The U.S. Army Wednesday notified Pfc. Bradley Manning, a prime suspect in the WikiLeaks case, that he now faces 22 more charges in connection with allegedly downloading secret information from computers in Iraq.
most serious new charge alleges that he aided the enemy by making this information public. That charge is punishable by death. A news release from the Army said the prosecution team "has notified the defense that the prosecution will not recommend the death penalty," but technically it is up to the commander overseeing the case to make the final decision about the death penalty.
All told, Manning, a military intelligence analyst from Oklahoma, now faces a total of 34 charges in the case, including:
-- Wrongfully causing intelligence to be published on the internet
-- Theft of public records
-- Transmitting defense information
-- Transferring classified data onto his personal computer
-- Disclosing classified information concerning the national defense.
Manning's lawyer, David Coombs, would not comment on the new charges, but posted a statement on his blog Wednesday evening:
"Over the past few weeks, the defense has been preparing for the possibility of additional charges in this case."
U.S. military officials have said that Manning is the prime suspect in the leak of many thousands of classified documents that ended up on the WikiLeaks website. However, WikiLeaks is not mentioned in the charge sheets.
Last August, Coombs said he'd seen no evidence tying Manning to the WikiLeaks case.
Even though the investigators filed the new charges, there are still several legal steps that would be taken before any decision will be made on which charges, if any, Manning would actually face in a court-martial.
Posted by Steve Douglass at 5:09 AM