Tractor beams, here we come!

An asteroid is falling towards an alien planet full of peace-loving, flower-toting creatures.  All hope seems lost.  Then suddenly the Enterprise arrives, latches onto the rock, and saves the day.  Their weapon of choice?  The tractor beam, one of the many impossible technologies of the Star Trek universe.  How could mere light affect a massive object in such a way?  Certainly, it seemed, light could never pull something.  Right?

With the proper pattern, these beams of light can actually pull and object.  (Image credit: University of St. Andrews)

Not so fast, suggests new research conducted by a joint team from the University of St. Andrews and the Institute for Scientific Instruments. In a paper published in Nature Photonics, they describe the construction of the world's first tractor beam, capable of pulling molecules towards the device using nothing but light.

This isn't how light usually behaves.  Even though particles of light, called photons, don't weigh anything, they still have a tiny bit of momentum.  This means that when light shines on something, it tends to push it slightly away from the light source.  We don't notice this effect on the Earth, because even the slightest breeze is hugely more powerful than so-called radiation pressure.

On the molecular scale, however, this force can have an effect.  That's what makes this latest experiment so remarkable.  By using certain patterns of light, the team was able to create a negative force, which pulled single molecule against direction of the light.  The pattern of light needed to achieve this is very specific to the material being moved, so scientists hope the beam can be used to sort molecules on a microscopic level.

 Unfortunately, making such a device work on macroscopic objects doesn't seem terribly plausible right now.  The force is so weak and the patterns needed so specific that any interference in the real world would probably swamp the effect.  Even so, space is a much simpler environment than on the Earth, so perhaps the Star Trek dream is possible after all... 

Source: Nature Photonics