Researchers at the Fudan University of China believe they have worked out the theory behind creating a tractor beam from a laser.
Such a thing is difficult to imagine outside of science fiction, and of course Star Wars and Star Trek, as it involves getting a pulling force generated from a “forward propagating beam”.
The paper detailing the theory has been posted at the Cornell University Library entitled Backward Pulling Force from a Forward Propagating Beam. The description is as follows:
A photon carries a momentum of, so one may anticipate light to “push” on any object standing in its path via the scattering force. In the absence of intensity gradient, using a light beam to pull a particle backwards is counter intuitive. Here, we show that it is possible to realize a backward scattering force which pulls a particle all the way towards the source without an equilibrium point. The underlining physics is the maximization of forward scattering via interference of the radiation multipoles. We show explicitly that the necessary condition to realize a negative (pulling) optical force is the simultaneous excitation of multipoles in the particle and if the projection of the total photon momentum along the propagation direction is small (as in some propagation invariant beams), attractive optical force is possible. This possibility adds “pulling” as an additional degree of freedom to optical micromanipulation.
In other words, under the right conditions they believe a tractor beam can be created from a laser to pull very small objects: a particle, at the very least.
While such technology is a very long way off dragging ships through space, if proven to work in a real experiment it could come in very handy for those working with objects at the nano scale at least.