Thursday, March 3, 2011

Tractor beam is possible with lasers.

Wake me up when we have holodecks. From BBC:
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.

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