Photon–photon scattering and other effects of nonlinear optics in vacuum is an active area of experimental research, with current or planned technology beginning to approach the Schwinger limit. It has already been observed through inelastic channels in SLAC Experiment 144. However, the direct effects in elastic scattering have not been observed. As of 2012, the best constraint on the elastic photon–photon scattering cross section belonged to PVLAS, which reported an upper limit far above the level predicted by the Standard Model.