There’s no escaping it: though the tractor beam is a staple of sci-fi space-faring scenarios, it’s also extremely counter-intuitive. How does one pull something in via an outward propagating beam? Now a few Chinese researchers think they’ve found the answer via a theoretical method that should generate a backward pulling force from a forward traveling stream of photons.
The Fudan University team won’t be capturing rebel tranports with their beam any time soon--it only works (theoretically) at the nano-level--but it does achieve an interesting turnabout of physical force. We know photons exert an outward momentum; this is what allows solar sails to harness sunlight to generate small amounts of thrust. But carefully tuned to meet two conditions, a system can be created to turn “push” into “pull.”