Physicists have known for over a century that photons have momentum which they can impart to anything they hit. That means light can be a form of propulsion. Indeed, rocket scientists have tested laser propulsion in the hope of one day using it to accelerate spacecraft to the far reaches of the solar system and beyond.

But a couple of years ago, researchers in China showed that as well as pushing objects, laser beams can also pull them. The trick is to ensure that instead of hitting the object head-on, the light merely glances off it. At the same time, it must interact with the object to generate a backward pulling force. The result is a tractor beam capable of pulling objects towards it.

Today, Horst Punzmann at The Australian National University in Canberra and a few pals take this idea a stage further. Instead of light waves, these guys have been playing with water waves. Using similar ideas, they’ve worked out how to generate waves that push floating objects away and pull them back again. In other words, they can generate a backward pulling force from forward propagating 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, waves tend to be diffracted, non-linear and complex. That makes them difficult to study.

Punzmann and co begin by considering a mathematical model of waves generated by an elongated block vibrating up and down on the surface of the water. This wave-maker produces 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 behaviour 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 centre 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.

To test this idea, Punzmann and co have recreated exactly this situation in a wave tank with an elongated wave maker. They place a ping-pong ball on the water and then measure its movement as well as the shape of the water surface and the fluid flow on the surface.

Sure enough, when the amplitude of the waves is small, the ping-pong ball moves in the same direction as the waves. But as the waves become larger, the ping-pong ball reverses direction and moves back towards the wave maker. In effect, it is pulled by a tractor beam.

That’s fascinating, not least because it contradicts standard thinking about wave generated flows. Fluid dynamicists generally use an idea known as Stokes drift to describe this behaviour. The thinking is that the drift velocity is determined by the average velocity of a specific parcel of fluid on the surface of the water. And this always follows the direction of wave propagation.

The new work shows that this is not always the case and that it is relatively straightforward to set up a flow in the opposite direction to wave propagation.

That could have important implications for mathematical models that predict the movement of materials and organisms across the sea and in fluid flows in general.

Ref: arxiv.org/abs/1407.0745 : Tractor beam on the water surface