# Detecting gravity wave sources with LIGO

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## Main Question or Discussion Point

I would have a question to LIGO.

How can LIGO detect the source of a gravity wave? It's a 2-dimensional detector, which has a certain length and a certain width, but no height. How does LIGO know the direction and distance of the source, and if this source is on the one side of LIGO or exactly on its other side?

## Answers and Replies

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There are two different LIGO detectors. The difference in time between the wave measurements of the two detectors can be used to determine the direction it is coming from, sort of, since it would be a point along that angle rotated 3 dimensionally around the base - a ring of possibilities. However, because of the difference in position along the Earth's curve and the orientation of the detectors, the magnitude of the wave helps to define the position of the source along the ring, to determine the direction to the origin. The distance is based on the decrease in the frequency of the gravitational wave. It does fade over time and distance. Estimations have to be made of the original objects to then backtrack to where the collision started, sort of.

FactChecker
Can LIGO only detect sources that are above this detection system? Or can LIGO also detect sources that are exactly on the other side of the Earth?

The waves "pass through" matter so it can detect sources in any direction. Basically you can calculate a single direction and then rotate that vector around the base of the detectors to create a disc-like shape that extends essentially in a plane that passes through the Earth. So the source can be somewhere in the 360 degree radius, not just in front of or behind.

Comeback City
Sorry, i have problems to understand this rotation. What's the base of the detectors? Something like an axis in a 3d-coord-system? A rotation of a vector around an axis doesn't create a plane, but a cone.?.?

Technically not a plane but for simplification purposes I used a plane since on the scale we are talking about the distance between the detectors is so small compared to the distance to the event. Without knowing distance it would not be a cone though, since we are not really bringing the line segments to a single point. So imagine two line segments, one extending from each of the detectors in the roughly indicated direction of the event. Now extend the line segments as lines in the opposite direction from each detector and then rotate each line around its detector on an axis perpendicular to the line creating an infinite circle on each of two planes, with the two circles or planes being parallel to each other. This area between the two circles is where the event occurred. That is the first part of locating the event, a ring around the Earth giving us a slice of the universe to look in. I do not have my drawing software on this computer or this would be easier!.

This may help:

"But gravitational-wave emission is not isotropic: for a compact binary, more power is emitted perpendicular to the binary’s orbital plane than in the plane. Detectors are not uniformly sensitive either. They are best at detecting overhead sources, and are completely insensitive to sources located in the detector plane at a 45-degree angle to the detector arms."

Demystifying Inspiral Range and Detection Rates
by Ilya Mandel

It is from the current issue of LIGO Magazine: http://www.ligo.org/magazine/LIGO-magazine-issue-9.pdf#page=31

Yes, not a formal citation but at least we give credit!