Tractor beam can pull as well as push

Posted by Emma Woollacott

Scientists have for the first time created a 'tractor beam' that can pull objects as well as push them.

While the words 'tractor beam' have been bandied about in the past - and we have to put up our own hands here - it hasn't until now been possible to reel objects in with a single beam as the Enterprise does so effortlessly.

"Our work demonstrates a tractor beam based only on a single laser to pull or push an object of interest toward the light source," says Haifeng Wang of the A Star Data Storage Institute.

Thanks to Albert Einstein and Max Planck, we know that light carries momentum that pushes objects away. In addition, the variation in intensity across a laser beam can be used to push objects sideways, for example to move cells in biotechnology applications.

But the nearest anyone's come before to a true tractor beam was in  2011, when researchers theoretically demonstrated a mechanism where light movement could be controlled using two opposing light beams.

Wang's team based their work on lasers with a particular type of distribution of light intensity across the beam, or so-called Bessel beams.

Usually, if a laser beam hits a small particle in its path, the light is scattered backwards, which in turn pushes the particle forward.

But Wang and his co-workers have now shown theoretically that for particles that are sufficiently small, the light scatters off the particle in a forward direction, meaning that the particle itself is pulled backwards towards the observer. The power of the beam depends on the electrical and magnetic properties of the particles.

Although the forces are small, such tractor beams do have real applications, says Wang.

"These beams are not very likely to pull a human or a car, as this would require a huge laser intensity that may damage the object," he says.

"However, they could manipulate biological cells, because the force needed for these doesn't have to be large."