Since the 1980s, scientists have been grabbing and tugging tiny particles over microscopic distances with “optical tweezers,” for example to probe the mechanical responses of biomolecules. Now in Physical Review Letters, David Ruffner and David Grier of New York University describe pushing and pulling particles over relatively long distances—tens of microns and, in principle, much longer—using a “tractor beam” that could prove more versatile.
A true tractor beam comes from only one direction. For a particle to be pulled rather than pushed, it must redirect the momentum of enough photons “downstream” to overcome the force of the photons hitting it from upstream. This can happen if the intensity of light changes rapidly along the axis of the beam, for example, where it is tightly focused.
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