How can they "aim" a LIGO?
Anonymous
11 Mar 2005 17:16:11 UTC
Topic 12866
(moderation:
Sorry, haven't much time, but to give you a rough idea:
The method we are using is called "matched filering". In principle we generate a signal with the expected properties and search for it in the data. The gravity waves sent out by the pulsars should be pure sinosoids of a given frequency. Arriving at the detectors, however, they got modulated in amplitude as the sensitivity of the detectors varies with their orientation relative to the source, and in frequency because of doppler effects caused by the motion of the earth (again relative to the source). These two modulations have to be taken into account, on one hand making the calculations much more complicated, but on the other allowing us to look in a certain direction of the sky. Gravity waves are not affected by matter, so it doesn't matter from which side of the earth they arrive.
BM
How can they "aim" a LIGO?
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I see you're getting the idea. Thwo things to add:
- The strength of the gravity wave remains (more or less) the same, however it depends on the angle in which it hits the detector how strong the detected signal is. The detector actually measueres the difference of the lengths of its arms, so the signal is seen strongest when the wave comes in from the direction one arm points in and weakest (ideally zero) when it comes in right in the middle between them, deforming both arms the same way.
- Movement of the earth relative to the source doesn't only mean its own rotation, but it also movement around the sun in the solar system, and this solar system also moves relative to the center of our galaxy. These movements, btw., are described in the "earth" and "sun" files you downloaded.
BM
BM
Actually the current WUs
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Actually the current WUs consist of three steps or passes: Two almost identical ones, in which they perform an all-sky search on two different data sets (currently two different time stretches from the same detector), and a third coincidence step, in which the results of the first two passes are compared. So with the current WUs, the crosshair should make two passes around the sphere.
BM
BM