Ferengi smugglers shake in their boots as the first step towards tractor beams is announced. In the past, lasers have been used only to impart forward momentum to their targets. They hit objects with photons, and those objects move forward with the beam of light (or burst into flame). Now scientists in Hong Kong have figured out how to use a special kind of laser to pull objects toward the laser's source.
Image by Thomas Harriman
The special laser that the scientists need for a tractor beam is called a Bessel beam. Bessel beams have a few extremely unusual properties. While most lasers do give off a focused burst of light, that light naturally diffuses as it goes. Pointing a pen laser at an object a foot away will result in a small point of light. Pointing it at a screen from across the room will get a larger, more blurry point. Point it at something a mile away, and if it reaches that something, the point produced will be very large. Bessel beams don't do that. Point a Bessel beam at a nearby object and it will appear as a focused point surrounded by little rings. Point it at an object across a room and the central point will still be focused. Point it at an object a mile away and the central point will still be focused.
Even more impressively, a Bessel beam is able to reconstruct itself. Place an object in front of an ordinary laser and it will cut off that laser. Place it in front of a Bessel beam, and the beam will appear again on the other side.
It's these qualities that allow Bessel beams to become tractor beams. Most laser beams will spread out and push the entire object foward. A Bessel beam allows the amount of energy to an object to be controlled and focused at a precise spot. It's a stilleto, not a bat. By keeping the amount of energy pushing on the object low and under precise control, scientists allow any fluctuations in energy around the object to make a big difference.
With the amount of ‘pushing' energy kept controlled and minimal, the Bessel beam's other remarkable quality comes into play. It reconstructs itself on the other side of the object, surrounding the object in a ‘sea' of light. That light travels in waves. When the high crests of two waves meet, they build to an even higher wave. When a crest meets a trough, a low point, it annihilates. Objects that come into contact with light usually absorb and re-emit energy. If the light that the far side of the object emits comes together with the existing light of the Bessel beam in two crests, then it builds a large concentration of energy on the far side. If the light emitted by the near side annihilates with the Bessel beam, there's no energy on the near side. The object will experience a push of energy from the far side, almost like the beam was coming from the other direction, and it will be pulled towards the origin of the light.