How To Turn A Laser Into A Tractor Beam - Technology Review: Jun Chen from Fudan University in China and a few pals demonstrate the counterintuitive result that photons can pull things too. In other words, they've worked out how to generate a backward pulling force from a forward propagating beam.
Chen and buddies say this is possible when the system meets two conditions. First, it works only for beams in which the momentum in the direction of propagation is small, as is the case for beams that merely glance off an object. Second, the photons must simultaneously excite several multipoles within the particle, which scatter the beam.
If the scattering angle is just right, the total momentum in the direction of propagation can be negative, meaning the particle is pulled back towards the source and the light becomes a tractor beam.