Is it possible to overcome the problem of blind spot(s) of current gravitational wave detectors?

Question

If gravitational waves (GWs) pass through specific points (which are known as blind spots), current GW detectors aren't able to detect the passing waves. In the future, will we be able to completely overcome this problem or will it always be there to a certain extent?

Answer 1

The blind spots are caused by the way the current detectors ("L-shaped" interferometers) work. They are sensitive to a gravitational wave (GW) changing the difference in path length along the 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).

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 with respect to the plane defined by the "L-shaped" interferometer arms will only be detected in the polarisation state that lines up with the arms and not in the other, because it wouldn't make the arm lengths change at all. 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 because it changes the lengths of the arms in anti-phase (i.e. whilst the x-arm gets longer, the y-arm gets shorter and vice-versa). The cross polarisation would leave the x- and y-arms unchanged.

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 a line defined by the bisector of the two arms or along an equivalent line at right angles to this. The reason is either that the polarisation of the GW changes the length of both arms by the same amount, or not at all.

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); build another interferometer with a different plane/orientation to the first (which is essentially what has been done by adding VIRGO, and now KAGRA, to the two LIGO interferometers). Hybrid "3D" interferometers have also been proposed, using a pyramidal shape for the arms (e.g. Liu & Gong 2020).

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 (and the most sensitive "hot spot" directions, which are directly above and below the detectors). The middle plot on the left shows how things improve when you add VIRGO (in Italy), which has a different interferometer plane - 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).

A further way to tackle the blind spots is to construct multiple interferometers within the same triangular design. This is what is planned for the proposed ground-based Einstein Telescope).

The Figure below is taken from the Einstein Telescope Design Report (2020) and shows the response of the propsed Einstein Telescope configuration (right) compared with that of the VIRGO interferometer (an "L-shaped" interferometer). In this design, there are multiple interferometers using each adjacent pair of the arms, which eliminates the blind spots in the summed response. Instead the detector response is equal in all azimuthal directions that are at a similar angle to the detector plane and only reduced by a factor of two in the detector plane.

Another possibility is offered by space interferometers like LISA. This also has a triangular shape, and consists of multiple interferometers, which eliminate the blind spots in the interferometer plane. However, a further key advantage of any single| space interferometer observing low-frequency sources (inaccessible to ground-based detectors) is that most sources are expected to be persistent or long-lived signals (as opposed to chirps; e.g. short period white dwarf binaries, thousands of years pre-merger, should dominate LISA detections, Lamberts et al. 2019). This means they can be observed continuously whilst the detector orientation changes with time (during its orbit) and means that any anisotropies in the time-averaged detector response will become smoothed out.