Reversible tractor beam moves objects 100 times farther

Concept of photophoretic light–particle interaction in gases.

Credit: Nature Photonics (2014) doi:10.1038/nphoton.2014.242

A team of researchers working in Australia has built a tractor beam that bests the distance of other efforts a hundred fold, and it can both push and pull objects.

In their paper published in the journal Nature Photonics, the team describes their tractor beam project, how it works and what purpose it might serve.

A tractor beam is of course a beam emitted by a source that is used to hold and/or move another object.

The idea was popularized by the Star Trek series and other science fiction works, and was used typically by one starship to trap and move another starship.

In real life, physicists have made some inroads into developing a tractor beam (mostly based on the movement of photons to propel particles) but thus far, the objects trapped and moved have been extremely small and the distance moved even smaller.

In this new effort the researchers used a different technique to move an object that was bigger and to move it much farther, a hundred times as far.

Dr. Vladlen Shvedov (L) and Dr. Cyril Hnatovsky adjust the hollow laser beam in their lab at the Australian National University. 

Credit: Stuart Hay, ANU

The new tractor beam is based on heat, a laser that shines a doughnut-shaped beam (it has a cold center) was fired at a gold covered tiny (0.2mm diameter) glass bead that was small enough to just fit inside the beam, where it was cold.

The heat from the surrounding beam caused the surface of the bead to heat, creating hotspots.

When the hotspots came into contact with air particles, those particles were repelled, which in turn caused an opposing force against the glass bead, pushing it (up to a distance of 20cm).

The researchers found they could change the movement of the beads by adjusting the polarisation of the laser, causing changes in the hotspots on the beads.

That meant the beads could be pushed forward, stopped, pulled back, or held in place.

The team suggests their tractor beam could be useful in real world applications because of its versatility and because it requires just a single beam.

They believe it might be used for removing pollutants from the air, or for pulling undesirable particles from samples of materials.

They also note that it could very easily be made much larger, noting they were prevented from doing so by the small size of their lab.

More information: 
Economic tools for evaluating liabilities in environmental justice struggles, Nature Photonics (2014) DOI: 10.1038/nphoton.2014.242

Star Trek-style 'tractor beam' created at Scottish university

Scientists at Dundee University have created a working "tractor beam".

Normally the stuff of science fiction in Star Trek or Star Wars, physicists at the university used an ultrasound array to exert force on an object and pull it towards the energy source.

They say it is the first time such a beam has been used to move anything bigger than microscopic targets.

The technology could be put to use in medicine, helping to develop ultrasound-based clinical techniques.

Dundee researchers worked alongside colleagues in Southampton and Illinois on the project, the results of which have been published in the scientific journal Physical Review Letters.

In another sci-fi inspired project, the same team from the university's Institute for Medical Science and Technology (Imsat) created a Doctor Who-style "sonic screwdriver", also using ultrasound.

Ultrasound device
"This is the first time anyone has demonstrated a working acoustic tractor beam and the first time such a beam has been used to move anything bigger than microscopic targets," said Dr Christine Demore of Imsat.

"We were able to show that you could exert sufficient force on an object around one centimetre in size to hold or move it, by directing twin beams of energy from the ultrasound array towards the back of the object."

The team used an ultrasound device that is already clinically approved for use in MRI-guided surgery.

The team's work was carried out as part of a £3.6m programme initiated by the Engineering and Physical Sciences Research Council, combining expertise at four UK universities in Bristol, Dundee, Glasgow and Southampton with industrial firms.

Professor Sandy Cochran, of the University of Dundee, said: "Our partnership with industry has been vital to developing devices and capabilities that are delivering unprecedented sophistication in the field of ultrasound."

Scottish Researchers build acoustic tractor beam

(a) Nonconservative pushing force exerted on an object by a plane wave as a result of strong backscattering. 

(b) Decreasing of the pushing force due to an enhanced forward scattering in a nonparaxial beam. 

(c) The authors used a target designed to maximize the forward scattering of acoustic radiation, leading to a pulling nonconservative force towards the source: an acoustic tractor beam. 

Credit: APS/Alan Stonebraker

A team of researchers with members from the U.K., Scotland and the U.S. has built a functioning acoustic tractor beam in a lab, one that is able to pull objects of centimeter size.

In their paper published in the journal Physical Review Letters, the team describes how they built their device, why it works and to what applications it might be put.

Tractor beams, as we all know are a staple of science fiction, a beam is emitted from a spaceship that can be used to lock on to other objects, such as another space ship, and then used to move that other object in any direction, most interestingly, in the same direction from which the beam is being emitted, pulling it in.

Tractor beams seem counterintuitive as beams of light tend to push objects away, rather than attract them—but, as prior research has shown, optical tractor beams can be created at the nanoparticle level, e.g. optical tweezers.

In this new effort, the research team has extended the abilities of a tractor beam by using one based on acoustics, rather than optics.

Sending a beam (wave) at an object and having it pull the object closer rather than push it can work because of the scattering of the wave that occurs when it collides with the object and if the wave is sent at an angle to the object.

If the scattering and angle are controlled just right, a low pressure zone can be created in front of the object, in effect, pushing it back towards the origin of the beam.

In the lab, the researchers used ultrasonic sound waves in a tank of water.

They put an array of ultrasound emitters at the bottom of the tank and used a hollow isosceles triangular prism as the object to be pulled.

Using an array of emitters allowed for very precisely controlling the wave, which allowed for directing energy onto the outer surface of the object, causing backscattering that led to the frontal low pressure zone, which in turn led to pushing the object back towards the wave source.

An analogy would be squeezing a chocolate chip with your fingers, forcing it to move in whatever direction you choose.

Experimental configuration to demonstrate negative radiation forces with a planar ultrasonic array.

(a) Scaled cross-sectional geometry of the 550 kHz planar matrix array source and hollow, prism-shaped targets suspended above the array. Linear phase gradients applied to the array elements produce wave fronts steered at θ=50.6° towards the array center line.

Active subapertures, forming a hollow core with diameter Δxn, are stepped towards the center line by the array element pitch, with a corresponding lateral (±x) shift in the transmitted local wave fronts and an axial (−z) shift of the intersection with the axis.

(b), (c) Normalised maps of simulated instantaneous pressure field and

(d),(e) measured magnitude of the pressure field produced by the transmitting subapertures illustrated under the field maps. 

Credit: (c) PRL, DOI: 10.1103/PhysRevLett.112.174302

Because of the stipulations required to make it work, applications that could make use of such a tractor beam are clearly limited, though the researchers suggest it might prove useful in some medical situations.

More information: Acoustic Tractor Beam, Phys. Rev. Lett. 112, 174302 – Published 30 April 2014. dx.doi.org/10.1103/PhysRevLett.112.174302