It is intuitive to think of light as interacting with matter and not with itself, but this ignores the predictions of physics, which hold that photon-photon scattering is possible. Researchers at Umeå University in Umeå and at Chalmers University of Technology in Göteborg, both in Sweden, have suggested a means to observe this scattering.In 1864, James Clerk Maxwell published his famous four equations of electromagnetism, which led to the wave equation, the crucial leap describing light as electromagnetic radiation. In short order, however, it was clear that the picture was incomplete. The newly developed quantum mechanics was brought to bear on the problem of the wave-particle duality of light, yielding quantum electrodynamics (QED), which found its first articulation by Dirac, Heisenberg and Pauli in the 1920s and its footing following World War II in the work of Tomonaga, Schwinger, Feynman and others.One consequence of the theory is a nonzero vacuum energy that produces observable effects, including photon-photon scattering. "According to QED, there is no such thing as a completely empty vacuum, since virtual electron-positron pairs are always spontaneously created and destroyed," said Gert Brodin, an associate professor at Umeå University. "These virtual particles interact with the photons, leading to effective nonlinearities in Maxwell's equations."The researchers propose trapping photons in a high-Q cavity of specific dimensions to resonantly couple inserted parallel propagating TEM01 and TEM10 waves of different frequencies, using waveguide filtering to eliminate the pump signals. "This can be fulfilled for any range of frequencies of the pump waves, from the radio-wave regime to the optical or shorter wavelengths," Brodin said, but using millimeter-wave photons should cut down on the expense of the apparatus.According to their calculations, based on a superconducting niobi-um cavity and pump waves of field strength 30 MV/m, the exchange of pairs of virtual electrons and positrons -- producing photon-photon scattering -- should yield a real and detectable excited mode. The required geometry of the cavity for the experiment, however, means that the shape of existing niobium cavities must be modified.Before designing a genuine vacuum scattering experiment, Brodin said, it might be fruitful to examine simpler setups in which the vacuum would be replaced by a material with moderately strong cubic nonlinearity.Experiments to comeOther teams have detected photon-photon scattering; notably, researchers from Stanford University, the University of Tennessee, Princeton University and the University of Rochester. But Brodin noted that this would be a significant departure from that experiment.In that case, the researchers monitored a collision event between terawatt laser pulses and a 46.6-GeV electron beam that produced backscattered high-energy photons that could interact with those from the laser and that produced real positrons. "Their underlying microscopic mechanism involved [the] creation of real particles (in contrast to ours), and, consequently, our proposal would test different aspects of QED," he said.He said that the group has discussed the possibility of constructing the experimental apparatus with researchers at Rutherford Appleton Laboratory in Didcot, UK, but that it also welcomes the interest of other experimental groups.