The time difference of the arrival of the gravitational wave in the two Ligo detectors had a certain value. This difference would have been zero if the source of the wave had an equal distance to both detectors. In that case, the source of the wave had to lie on a plane perpendicular to the line connecting the detectors. It would have been maximal if the source lied on the extended line connecting the two detectors. But because the speed of gravity is unknown, only in the case of equal arrival time the source could have been said to lie on the circular plane. In all other cases, such a plane can't be identified.

So, therefore, my question: Can the direction from which the wave came be restricted to a part of the heavens, as is the case of equal arrival time on both detectors? If the speed of gravity is equal to c then in the case of equal arrival time there has to be seen (in the e.m. spectrum) an event that will arrive at the same time as the gravitational wave. Of course, you're always walking behind the facts, because you don't know when a gravitational wave arrives (if the speed of gravity is less than c this could, of course, be done, but the speed is unknown), let alone one that arrives at the same time in both detectors. If by accident a huge event sending out e.m. radiation which reaches the earth is detected, and the gravitational waves emerging from it are strong enough to reach the earth then the location of the source can be identified as well the conclusion that the speed of gravity is indeed c (the chances that it's a coincidence is virtually zero).

  • $\begingroup$ The source was constrained to lie within one of two large "bananas," as indicated in Lawrence B. Crowell's answer below. I'm not sure whether they had to assume that the speed of gravity was c, though. $\endgroup$
    – tparker
    Mar 9, 2017 at 22:28
  • $\begingroup$ They did assume that the speed of gravity is c. It's the best guess we have for the speed of any massless field, or any classical force that goes as 1/$r^2$, i.e. long range forces. Too many things become ins consistent if that's not true. Einsteins GR has the speed of grav waves c. We assumed that GR was right in some of the other conclusions we made of their sources like the masses of the black holes, and indeed even their existence. $\endgroup$
    – Bob Bee
    Mar 10, 2017 at 2:12

1 Answer 1


I have the following picture. The dense area on the left is the area around SgrA* or the center of the Milky Way. The contour lines indicate intensity and thus increased probability with the inner red of locating the source.

enter image description here


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