Innovative design draws on the pressure light exerts as a driving force
The force is with us, even if it is very small indeed. Scientists have long been aware that radiation of any kind exerts pressure, and light is no exception. In the 1920s, the Soviet rocket-designer Friedrich Zander suggested that a spaceship could be propelled solely by sunbeams and over time could reach very high velocities.
Sure enough, in May 2010 the Japan Aerospace Exploration Agency launched the world’s first spacecraft designed to use solar propulsion as its primary driving force. IKAROS – or the ‘Interplanetary Kite-craft Accelerated by Radiation Of the Sun’ – unfurled a polyimide sail, and set out for Venus, which it reached, as expected, in December. The sail is 14 metres square but only 7.5 microns thick, and its highly reflective surface doubles the thrust the sun gives it, to all of 0.112 grams-force.
Now a team of researchers at the Rochester Institute of Technology in New York has demonstrated another trick light can do. Take a transparent rod, semicircular in cross-section and thinner than a human hair, and direct a beam at it. As the stream of photons flows through it, the pressure they exert first rotates it until it finds equilibrium and then pushes it, not in the direction of the stream but at a particular angle to it. It’s not unlike the way the flow of air round an aerofoil creates ‘lift’.
Tiny as this effect is, such a ‘lightfoil’ could have applications not only in the near-vacuum of space but even on Earth. “The advantage of these rods”, explains Ortwin Hess, Professor in Physics and Leverhulme Chair in Metamaterials at Imperial College London, “is that you can use a lot of them together.” And on the nano scale, he adds, even very small effects can have a gigantic impact.
So what is it good for? The pressure light exerts is minuscule, and so there are only two contexts in which it can be harnessed: in space, where there is a near-total vacuum and therefore almost no resistance to the force it applies; and in nanotechnology, where the things that need moving have a very small mass.
It’s possible that the energy of light could be harnessed to drive micromachines, or transport very small loads through a liquid. “It’s
still a big challenge,” says Hess, “but then the challenges that have already been mastered are quite amazing!”
– Huw Spanner
Image credit: Japan Aerospace Exploration Agency