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

NASA Cassini Image: Prometheus practices its pull

NASA Cassini Image shows Shepherd moon Prometheus hovering just inside the reflective F ring. 

Credit: NASA /JPL-Caltech /Space Science Institute.

Lit by eerie, reflected light from Saturn's F ring (and a casting a faint shadow through a haze of icy "mist") Saturn's moon Prometheus can be seen in the raw image above, captured by Cassini's narrow-angle camera on Feb. 5 from a distance of 667,596 miles (1,074,392 km).

It's also receiving some light reflected off Saturn, which is off frame at the top (where the outermost edge of the A ring and the Keeler gap can be seen.)

As the potato-shaped Prometheus approaches the ring it yanks fine, icy material in towards itself, temporarily stretching the bright particles into long streamers and gaps and even kicking up bright clumps in the ring. It's a visual demonstration of gravity at work.

At its longest Prometheus is about 92 miles (148 km) across, but only 42 miles (68 km) in width.

It circles Saturn in a wave-shaped, scalloping orbit once every 14.7 hours.

Earth's Gravitational Pull Threatens Near-Earth Asteroids

Asteroids, such as Itokawa, pictured here, are thought to be more like piles of rubble loosely clung together, than solid chunks of rock. Credit: ISAS/JAXA

steroids may want to think twice before they swing too close to Earth. A new study has found that our planet's gravity can cause seismic tremors, or asteroid-quakes, if the space rocks stray too close.

This process could explain why many space rocks orbiting nearby appear pristine, as if they were covered in a new and clean surface, researchers said.

Normally, asteroids are weather-beaten, their top coats of rock made dirty and reddened by the onslaught of charged particles streaming off the sun during up to 4 billion years or more of wandering the solar system.

"Any part of the surface that's facing into the sun is hit by the solar wind, which damages the mineral grains and turns them red," said the study's lead researcher Richard Binzel of MIT. "An analogy is a sunburn."

Like a sunburn on your skin, the reddening of an asteroid is only skin deep, with fresher material lurking just beneath the sun-drenched surface of the space rock, he added.

But when asteroids approach the Earth, our planet's gravity may induce small quakes that shake up the space rocks, causing the weathered pebbles on their surface to turn over, revealing their cleaner undersides. Asteroids are thought to be more like piles of rubble loosely clung together, than solid chunks of rock, which means even a small tremble could displace surface material.

"All of the particles that got reddened are going to flip over and you're going to have new material that's fresh now out facing the sun," Binzel told SPACE.com. "So it's going to change the color of the asteroid from red to a brighter gray."

The idea has been suggested before, but now Binzel and his colleagues have finally found observational evidence that it's happening.