While technique would likely destroy starships, researchers envision using the technique on cells

Dr. Tomas Cizmar, a research fellow in School of Medicine at the University of St Andrews in Scotland, is hopeful that one day tiny tractor beams could be used to sort certain kinds of cells (say separate white blood cells from the blood stream) or arrange tiny circuit components.

Today a technique called optical tweezers, which relies on the electromagnetic characteristics of laser beams, is commonly used to position cells and other small microscopic or nanoscopic objects.  Other similar techniques billed as "tractor beams" commonly use heating of a gas or other phenomena to shuffle particles.  By contrast, the new "tractor beam" techniques directly rely on the momentum transfer of photons hitting an object to either "push" or "pull" it through space.

The momentum transfer, also known as "optical forces", was first observed by German astronomer Johannes Kepler in 1619, who observed that comet tails always pointed away from the sun and hypothesized that sunlight must confirm some force onto the particles in the comet wake.

Such forces typically pushed objects in the direction of the photon stream.  But recently theoretical studies hypothesized that under highly specific conditions objects could actually be pulled against the photon stream using a combination of two or more lasers.

Tractor BeamFiber optic needle
The two laser tractoring mechanism (left) could one day be used to sort cells, which Dr. Cizmar's group is currently performing micro-needle waveguide experiments on (right).
[Image Source: St. Andrews]

The U.S. National Aeronautics and Space Administration recently announced $100,000 USD in funding to study the phenomenon, which they hoped could be used to gather samples in space.

In the latest study, which is potentially the first to experimentally demonstrate a purely photonic tractoring technique, Dr. Cizmar's team paired with Prof. Pavel Zemánek at the Institute of Scientific Instruments (ISI) in the Czech Republic.  The authors use a two-laser setup to manipulate a series of spherical nanoparticles of various sizes.  A variety of kinds of motion range from attraction to 2D motion to 1D sorting are achieved, using different beam techniques.

Dr. Cizmar says that the ability to pull objects against the photon stream may seem particularly odd to the layman, remarking to BBC News, "It's surprising.  Only when we looked in detail at the process did we see the reversal. It's quite a narrow field it occurs at."

The new technique bears advantages over other kinds of "tractoring" in the sense that it could work both in liquids and in vacuum.  It also is highly selective, perhaps more so than the other forms of tractoring.  Dr. Cizmar warns that tractoring large objects would be undesirable, even if the approach could somehow be scaled up.  The beams transfer a small amount of energy to the object, which is in turn converted into heat.  On a microscopic scale, the amount transferred is very small, but for a person -- let alone a starship -- the heating would likely be very destructive.

It's possible such an approach could be weaponized, though, offering up another popular science fiction staple -- the "phaser".

But in the near term, researchers remain focused on smaller, more practical applications.  Their recent work was published in the prestigious peer-reviewed journal Nature Photonics.  They hope to follow the work with more sophisticated sorting demonstrations, possibly on cells.

Skylark of SpaceStar Trek tractor
While Star Trek (Right) popularized the tractor beam, it was first presented in EE Smith's 1920s "Skylark" space opera. [Image Source: Google Images]

For science fiction fans, note that while it was Star Trek who popularized the notion of the "tractor beam", the fictional device was first dreamt up by American science fiction serialist EE Smith.  Mr. Smith called the device an "attractor beam" and used it in his 1928 story The Skylark of Space.  Dubbed the "father of the space opera" by man, perhaps Mr. Smith knew something his peers at the time didn't -- he did have a Ph.D in Chemical Engineering, after all.

Sources: Nature Photonics, BBC News

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