I think the crucial detail you are missing from the article is this:<p><i>&quot;According to the equations physicists have settled on, gravitational waves would compress space in one direction and stretch it in another as they traveled outward.&quot;</i><p>LIGO is two sets of 2 L-shaped antennas spread far apart on the globe, so that we can compare the compression of space in orthogonal directions and measure the very short delay between the gravitational wave hitting the first detector followed by the second. In this case, that difference was 7 milliseconds, which is also consistent with the speed of gravitational waves (also the speed of light)