If gravitational wave pass through specific points (which are know as blind spots) current GW detectors aren't able to detect the passed waves so over the time will we be able to completely overcome this problem or will it be there always upto certain extent?


The blind spots are caused by the way the detectors work. They are sensitive to a gravitational wave (GW) changing the relative path length along interferometer arms at right angles to each other.

Gravitational waves come in two polarisations (plus and cross). These polarisations cause alternate perpendicular expansions and contractions in space, but are rotated by 45 degrees with respect to each other (plot from Kalmus 2009).

Plus and cross polarisation

A GW source would normally be a mixture of both. The sensitivity to each polarisation depends on the orientation of the interferometer arms with respect to the source direction. For example, a source that is directly "overhead" will only be detected in the polarisation state that lines up with the arms and not in the other, because it would make the arm lengths change by the same amount. e.g. Imagine your detector is lined up with the x and y axis indicators in the picture above, then only the plus ($+$) polarised waves would be detected.

However, if the waves come from a source in the plane of the interferometer, then neither polarisation causes a relative difference in the arm lengths if the source lies along the bisector of the two arms or along an equivalent line at right angles to this. Thus there are 4 blind spots on the sky. Note that if a source is perfectly polarised (e.g. an edge-on merging binary system), then there will be addditional blind spots.

There are several solutions: make your interferometer 3 dimensional (i.e. add a third arm at right angles to the other two); make a triangular interferometer (this is what is planned for space interferometers); build another interferometer with a different plane/orientation to the first.

As an example, the plots on the left in the picture below show the sensitivity of LIGO (Washington and Louisiana) to unpolarized GWs at the top. These two interferometers are nearly lined up and nearly in the same plane, so you see the 4 blind spots clearly. The middle plot on the left shows how things improve when you add VIRGO (in Italy) - there are essentially no blind spots in the combined instrument, and then in a planned 6-detector network at the bottom. (Plot from Andersson et al. 2013).

Sensitivity plots

  • $\begingroup$ This is an excellent answer and links to some really interesting papers! $\endgroup$
    – uhoh
    Feb 9 '20 at 5:22

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