A classic technology of science fiction is the tractor beam, which by some means is able to hold and move a remote object without directly touching it. Such functionality has been reproduced optically, but that only works on relatively small objects. Now researchers at the Australian National University have discovered that it is possible to create a tractor beam using water waves quite easily.
Normally one would expect that water waves would push objects away, or leave them where they are. At certain amplitudes and frequencies though, the researchers found that the ping pong ball they were testing with would move against the waves. According to advanced particle tracking tools, the waves were generating flow patterns on the surface of the water that would move the ball around. Different patterns would result in different movements.
Presently, there is no mathematical theory to explain these observations, but we can already envision applications. This could be used to manipulating floating objects and even trap and confine oil spills to certain regions.
Source: Australian National University