You don't need to be a beach bum to understand waves: they move objects along with them, pushing boats and swimmers to the shore. There's even a name for it, the Stokes drift model. But for the first time, physicists have figured out how to do the opposite, using waves to bring a floating object backward to the source. It's a real-life tractor beam.
First off, I know what you're thinking, and no, this is not the same thing as a rip current or undertow. Those are actually two different phenomena, but neither is capable of bringing an object all the way back to the source of the wave. Got it? Cool.
So anyway, those tractor beams. At the Australian National University in Canberra, Horst Punzmann and fellow researchers used a wide vibrating block to create surface waves in a water tank. Small waves behave as expected: the surface fluid moves away from the wave generator, in the way that's familiar to surfers and sailors.
But as the amplitude of the waves gets bigger, they become unstable, and a strange phenomenon occurs: the two-dimensional flow away from the vibrating block turns into complex 3D interaction. At just the right amplitude, this non-linear interaction creates a jet that pushes floating objects inward, against the waves and toward the vibrating block. There's your tractor beam right there.
The team demonstrated the effect with a ping-pong ball. On the left, normal waves push the ball away. On the right, Tractor Beam Waves pull it in.
As Physics arXiv points out, that behavior contradicts our standard thinking about how water behaves in waves. Stokes drift had us thinking that drift effect always follows the direction of waves—now we see that's not always true.
That's important for understanding fluid dynamics, which affects things as wide-ranging as ocean ecosystems, global transport, and vehicle aerodynamics. But more importantly, it means that tractor beams aren't just a science fiction fantasy any more. [Physics arXiv via Medium]