(BBC)A real-life "tractor beam", which uses light to attract objects, has been developed by scientists. It is hoped it could have medical applications by targeting and attracting individual cells. The team at the University of St Andrews worked with colleagues at the Institute of Scientific Instruments (ISI) in the Czech Republic. In science fiction programmes such as Star Trek, tractor beams are used to move massive objects towards a light source. The tractor beam is very selective in the properties of the particles it acts on, so you could pick up specific particles in a mixture." "Eventually this could be used to separate white blood cells, for example." Usually when microscopic objects are hit by a beam of light, they are forced along the direction of the beam by the light photons. That radiation force was first identified by the German astronomer Johannes Kepler in 1619 when he observed that tails of comets always point away from the Sun. Dr Cizmar's team's technique allows for that force to be reversed which he said some people might find counter-intuitive. "It's surprising," he said. "Only when we looked in detail at the process did we see the reversal. It's quite a narrow field it occurs at." "Unfortunately there is a transfer of energy. On a microscopic scale that is OK, but on a macro scale it would cause huge problems. "It would result in a massive amount of heating of an object, like a space shuttle. So trapping a space ship is out of the question."