Probing the relative speeds of gravitational waves and light acts as animportant test of General Relativity and alternative theories of gravity.Measuring the arrival time of gravitational waves and electromagneticcounterparts can be used to measure the relative speeds, but only if theintrinsic time-lag between emission of the photons and gravitational waves iswell understood. Here we suggest a method that does not make such anassumption, using future strongly lensed GW events and EM counterparts;Biesiada et al forecast that 50-100 strongly lensed GW events will be observedeach year with the Einstein Telescope. A single strongly lensed GW event wouldproduce robust constraints on the ratio of speeds of GWs and light at the$10^{-7}$ level, if a high energy EM counterpart is observed within thefield-of-view of an observing gamma ray burst monitor.
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