I Where Did LIGO Find Merging Black Holes?

[Albertgauss]

Surprisingly, I have not been able to find an answer to this question. How did scientists know where to build LIGO so that it would be able to find merging black holes in the sky? I assume LIGO is a permanent instrument so that it cannot be pointed to various parts of the sky, like an ordinary telescope would. Thus, fixed to the Earth, it would have to rotate with the Earth and hope that in whatever sky it happened to slice to at the right time would have merging black holes contained within the appropriate sky-patch. Was there some other way to know where the black holes merging would be and then the scientists built LIGO on the right steradian of Earth? Without LIGO up and running, there would be no way to know where merging black holes would be unless there is a binary star or space dust producing gamma rays nearby, etc.

 

[Orodruin]

A gravitational wave interferometer such as LIGO is omnidirectional and not something that you point like a regular telescope. There are some directions that are worse in terms of detection, but the different gravitational wave detectors have different orientation, so that makes up for it. Working in unison, they have full sky coverage.

For the latest announced discovery of the neutron star merger - it actually helped that it happened in the blindspot of VIRGO. That you could see it in the LIGO detectors, but not in VIRGO significantly reduced the possible directions.

Edit: It should be mentioned that a single interferometer by itself gives pretty lousy directional sensitivity. However, working in unison, several interferometers can be used for triangulation to find the direction.

 

[pervect]

The directional characteristics of Ligo are more like those of a basic antenna (by which I mean I simple dipole antenna if you are technincally minded) than a telescope. Like an antenna, there are certain directions that will experience moresignal loss (and the signal is very small to start with). . For antenna's, you'll see charts of gain versus direction. I don't know for sure if Ligo's directional sensitivity is exactly like that of a dipole antenna (I'd have to do some calculations and check them, or find a reference to be positive), but it should be a close enough model for conceptual understanding. The real trick is not how Ligo was able to receive the signals, but how they were able to find the direction at all. By having two different recievers, and paying careful attention to exactly when the signals were received and knowing that the signals move at the speed of light, they were able to get a rough estimate of what direction the event happened in. I'd have to look up the details of that calculation, and you didn't ask about it anyway.

 

[Albertgauss]

I think I get it now. It seemed hard to find that information you all gave before, but it is very clear in your answers now. I'm good to go.