First successful detection of gravitational waves (GW) by LIGO/Virgo opened up a new window on the Universe and gave us unprecedented possiblities of testing gravity. In particular the issue of the speed of gravity, in General Relativity equal to the speed of light, became now accessible for experimental tests. Any observational clue that gravitational waves propagate with speed different from c would mark the breakdown of General Relativity.
In this paper we propose a new method to measure the speed of gravity assuming that strongly lensed gravitational signal accompanied by its electromagnetic (EM) counterpart could be detected. The method is based on measuring the difference between strong lensing time delays registered in GW and EM. The differential setting of our method makes it robust and free from the intrinsic time delays in the source (i.e. different emission times of GW and EM signal).
Strongly lensed GW signals are expected to be registered by the next generation of interferometric detectors like the Einstein Telescope.