We are constantly dealing with waves that are deflected in complex ways: this could be a light beam passing through a glass of milk and being dispersed in all directions, or electromagnetic waves from mobile phone masts being dispersed or absorbed, causing us to complain about poor reception in indoor areas.
Researchers at TU Wien are developing methods for manipulating waves in a targeted manner, so that they can move forward with almost no restriction. In a partnership with research groups from the École Polytechnique Fédérale de Lausanne (EPFL) and the University of Crete, this method has now been implemented in an experiment. Using precisely controlled loudspeakers, it has been possible to send a sound wave through a tube containing various obstacles. In the long term, technologies like this could enable light waves to be manipulated and objects to be made invisible.
Light or sound - it all depends on the wave
Sound waves were chosen in order to test the concept for loss-free wave transmission. "In principle, our technology can be applied to any type of wave", says Prof. Stefan Rotter from the Institute of Theoretical Physics at TU Wien. "From a mathematical perspective, it is irrelevant whether we are dealing with light waves, sound waves or quantum matter waves - acoustics experiments are, however, particularly illustrative in terms of their implementation."
In order to manipulate the wave in precisely the desired manner, energy has to be supplied or removed at specific locations. This is done using special loudspeakers that are mounted along a sound tube with a length of several metres. "The purpose of the loudspeakers is not, however, to simply reproduce the original sound wave on the other side of the tube - that would be too easy", explains Andre Brandstötter, a co-author of the study and doctoral student in Stefan Rotter's group. "The idea is to manipulate the sound wave point by point and to guide it through the tube in such a way that it always has the same strength right in front of the loudspeakers."
The loudspeakers are controlled such as to strengthen or weaken the sound wave locally. "This allows us to counteract the complex dispersal that would otherwise be unavoidable when the wave encounters an obstacle", says Rotter.
The tube maze
The experiment was carried out using an air-filled tube containing irregular obstacles. If you pass a sound wave through the tube, almost no sound reaches the end. However, if the loudspeakers installed in the tube are controlled according to the mathematical rules developed by the team of researchers, the sound wave leaves the tube as if it has not encountered a single obstacle along the way.
The experiment in Lausanne shows that TU Wien's wave manipulation concept works well in practice. The next step is now to build on the potential of these technologies. "The ultimate goal would be to achieve the same results in three-dimensional space with light waves, which would allow us to make objects invisible", says Stefan Rotter. While such a potential 'invisibility cloak' for light will certainly require several more steps to work in practice, the technology could already now be of great interest for a diverse range of message transmission applications.
Prof. Stefan Rotter
Institut für Theoretische Physik
Technische Universität Wien
Wiedner Hauptstraße 8-10, 1040 Wien