Pull me in—a tractor beam in Singapore

Who hasn’t wanted a tractor beam at one time or another? The notion that beaming a ray of light at something would allow you to bring it closer is very appealing. And, if you’re willing to settle for a particle, you could have  a tractor beam in the near future according to scientists in Singapore. From the May 23, 2012 news item on Nanowerk,

Tractor beams are a well-known concept in science fiction. These rays of light are often shown pulling objects towards an observer, seemingly violating the laws of physics, and of course, such beams have yet to be realised in the real world. Haifeng Wang at the A*STAR Data Storage Institute and co-workers have now demonstrated how a tractor beam can in fact be realized on a small scale (see paper in Physical Review Letters: “Single Gradientless Light Beam Drags Particles as Tractor Beams” [behind a paywall]). “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 Wang.

Coming up in the description of just how Wang’s tractor beam works is my second reference to Albert Einstein today (in the earlier May 23, 2012 posting: Teaching physics visually), form the news item on Nanowerk,

Based on pioneering work by Albert Einstein and Max Planck more than a hundred years ago, it is known that light carries momentum that pushes objects away. In addition, the intensity that varies across a laser beam can be used to push objects sideways, and for example can be used to move cells in biotechnology applications. Pulling an object towards an observer, however, has so far proven to be elusive. In 2011, researchers theoretically demonstrated a mechanism where light movement can be controlled using two opposing light beams — though technically, this differs from the idea behind a tractor beam.

Wang and co-workers have now studied the properties of 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. What Wang and co-workers have now shown theoretically for Bessel beams is 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. In other words, the behaviour of the particle is the direct opposite of the usual scenario. The size of the tractor beam force depends on parameters such as the electrical and magnetic properties of the particles.

There aren’t too many real life applications for a tractor beam of limited power but the lead scientist, Wang, does suggest it could be helpful in diagnosing malaria at the cellular level.