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

NASA MABEL: Laser Lidar technology reveals how ice measures up

NASA's Multiple Altimeter Beam Experimental Lidar flew over Southwest Greenland's glaciers and sea ice to test a new method of measuring the height of Earth from space. 

Credit: NASA/Tim Williams

New results from NASA's MABEL campaign demonstrated that a photon-counting technique will allow researchers to track the melt or growth of Earth's frozen regions.

When a high-altitude aircraft flew over the icy Arctic Ocean and the snow-covered terrain of Greenland in April 2012, it was the first polar test of a new laser-based technology to measure the height of Earth from space.

Aboard that aircraft flew the Multiple Altimeter Beam Experimental Lidar (MABEL), which is an airborne test bed instrument for NASA's ICESat-2 satellite mission slated to launch in 2017.

Both MABEL and ICESat-2's ATLAS instrument are photon counters – they send out pulses of green laser light and time how long it takes individual light photons to bounce off Earth's surface and return.

ICESat-2's ATLAS instrument

That time, along with ATLAS' exact position from an onboard GPS, will be plugged into computer programs to tell researchers the elevation of Earth's surface – measuring change to as little as the width of a pencil.

This kind of photon-counting technology is novel for satellites; from 2003 to 2009, ICESat-1's instrument looked at the intensity of a returned laser signal, which included many photons.

So getting individual photon data from MABEL helps scientists prepare for the vast amounts of elevation data they'll get from ICESat-2.

"Using the individual photons to measure surface elevation is a really new thing," said Ron Kwok, a senior research scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

"It's never been done from orbiting satellites, and it hasn't really been done much with airborne instruments, either."

Ron Kwok

ICESat-2 is tasked with measuring elevation across Earth's entire surface, including vegetation and oceans, but with a focus on change in the frozen areas of the planet, where scientists have observed dramatic impacts from climate change.

There, two types of ice – ice sheets and sea ice – reflect light photons in different patterns.

Ice sheets and glaciers are found on land, like Greenland and Antarctica, and are formed as frozen snow and rain accumulates.

Sea ice, on the other hand, is frozen seawater, found floating in the Arctic Ocean and offshore of Antarctica.

MABEL's 2012 Greenland campaign was designed to observe a range of interesting icy features, said Bill Cooke, MABEL's lead scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.

With the photon counts from different surfaces, other scientists could start analyzing the data to determine which methods of analyzing the data allow them to best measure the elevation of Earth's surface.

MABEL, short for "Multiple Altimeter Beam Experimental Lidar," serves as an ICESat-2 simulator. 

Credit: NASA /Kelly Brunt

"We wanted to get a wide variety of target types, so that the science team would have a lot of data to develop algorithms," Cooke said.

"This was our first real dedicated science mission."

The flights over the ocean near Greenland, for example, allowed researchers to demonstrate that they can measure the height difference between open water and sea ice, which is key to determining the ice thickness.

MABEL can detect enough of the laser light photons that bounce off Earth surface and return to the instrument, and programs can then make necessary elevation calculations, Cooke said.

Bill Cooke

"Part of what we're doing with MABEL is to demonstrate ICESat-2's instrument is going to have the right sensitivity to do the measurements," Cooke said. "You can do this photon counting if you have enough photons."

In an article recently published in the Journal of Atmospheric and Oceanic Technology, Kwok and his colleagues showed how to calculate elevation from MABEL data, and do so over different types of ice – from open water, to thin, glassy ice, to the snow-covered ice.