Gravitational lenses as long-baseline gravitational-wave detectors

Gravitational waves produce a time delay between the different images of a gravitational lens. The measurements of the time delay in the gravitational lens 0957±561 put new limits on the amplitude of low-frequency gravitational waves h<2×${10}^{\mathrm{-}5}$[ν/(3×${10}^{\mathrm{-}18}$ Hz)] in the frequency range 3×${10}^{\mathrm{-}18}$ Hz<ν. Future measurements of the time delay in other gravitational lenses may permit this limit to be improved.