Testing the speed of gravitational waves over cosmological distances
  with strong gravitational lensing
  Probing the relative speeds of gravitational waves and light acts as an
important test of General Relativity and alternative theories of gravity.
Measuring the arrival time of gravitational waves and electromagnetic
counterparts can be used to measure the relative speeds, but only if the
intrinsic time-lag between emission of the photons and gravitational waves is
well understood. Here we suggest a method that does not make such an
assumption, using future strongly lensed GW events and EM counterparts;
Biesiada et al forecast that 50-100 strongly lensed GW events will be observed
each year with the Einstein Telescope. A single strongly lensed GW event would
produce 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 the
field-of-view of an observing gamma ray burst monitor.
