Physicists Have Created A Tractor Beam Using Water

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In recent years, scientists have used lasers to construct crude tractor beams that, one day, might be used as grappling hooks in space. Now, for the first time, physicists have succeeded in creating the same effect using water.

Thanks to Star Trek and Star Wars, the term "tractor beam" immediately conjures up images of spacecraft ensnaring one another with beams of energy. But, at its most basic, a tractor beam is a way to use a forward-propagating force to pull an object backwards. You can do that with photons or—as a new study by Horst Punzmann at the Australian National University reveals—you can do it with water waves.

The physics of waves on the surface of water is somewhat different from light waves in space. Regular plane water waves are rare in nature. Instead, they tend to be diffracted, non-linear and complex. That makes them difficult to study.


However, as the Physics arXiv blog explains, the team of physicists worked with waves generated by an elongated block vibrating up and down on the surface of the water—producing a train of oval waves, which are almost linear in parts:

The crucial factor for floating objects is the way waves influence the flow of fluid along the surface of the water. When the amplitude of these waves is relatively small, Punzmann and co show that the waves generate a surface fluid flow in the same direction, away from the wave generator.

This fluid jet carries any floating particles along with the waves. "Floating particles are pushed in the direction of the wave propagation forming a strong outward jet," say Punzmann and co. At the same time, a compensating return flow occurs towards the edges of the wave-maker.

But as the waves get bigger, they become unstable and their behavior changes dramatically. When this happens, the waves interact and become a much more complex three-dimensional surface.

Punzmann and co say the interaction between the waves in this non-linear regime changes the direction of the jet at the center of the wave maker. "It now pushes floaters inward, towards the wave maker and against the wave propagation," they say. Any floaters caught in this jet, are therefore pulled.


Practical implications for the research include mathematical models that can predict the movement of materials and organisms across the sea. Perhaps not as exciting as laser beams grappling objects in space, but still pretty darn cool.

[Source: Physics arXiv blog]