LIGO Detects Gravitational Waves for Third Time

There are loads of possible ways to increase sensitivity, but none of them are easy or cheap given that the low hanging fruit was all picked off in previous generation detectors.

Increasing arm length is the "easiest" but definitely the most expensive option. Try finding a 40km L-shaped area that's seismically stable and free from significant anthropogenic activity. There may only be a handful of places in North America. However, 4km is already on the cusp of being long enough that gravity misaligns the two mirrors are each end of each arm due to the curvature of the Earth. Going to 40km would prompt the need for static corrections to mirror alignment, which will increase the amount of seismic noise that couples into the longitudinal direction in which gravitational waves are sensed. There are other problems such as the need to either refocus light at points along the arms (very susceptible to alignment and thermal noise) or use much, much bigger mirrors. The Advanced LIGO mirrors are already ~40kg, ~30 x 15cm cylinders of the purest fused silica known to man circa ~2012. There is talk of increasing the mirrors to 200kg and ~50 x 25 cm, and no facility is currently capable of producing pure enough fused silica at that size.

An "easier" option is to increase the laser power. This gives diminishing returns, and leads to an increase in high frequency sensitivity at the expense of low frequency sensitivity (due to photon pressure pushing the mirrors around noisily). However, the challenges are to make stable lasers that are also powerful - very tricky - and to mitigate the effect that laser absorption has on the mirrors within the interferometer - as you increase laser power, things heat up. Hot mirrors can lens the light, misaligning it and creating extra loss (i.e. reducing sensitivity). It's trickly to mitigate. Another effect of higher laser power is the introduction of parametric instabilities, where the mechanical body modes of the mirrors are amplified by the high laser power, leading to huge spikes of noise at narrow frequencies which are difficult to damp out.

Another is to use a different interferometer topology: instead of an L-shaped Michelson interferometer, suggestions have been made for Sagnac interferometers which possess an interesting property called quantum non-demolition, which can potentially reduce the limiting noise source in Advanced LIGO which directly increases sensitivity. Research into this is at a very early stage and will not be seen in detector facilities for decades, if ever.

So, the short answer is: there are lots of potential methods to increase sensitivity, but all of them are challenging and require significant R&D and money.

No filtering is applied to data containing such anthropogenic/geological noise after it is measured to remove such effects. In fact, such data is usually junked and not used for analysis. The sites have thousands of witness channels which listen for things like trucks, ground motion, magnetic storms, etc. that could possibly influence the mirrors in the way a gravitational wave would. If the same signal appears in both the gravitational wave channel and some auxiliary sensor, it's thrown away.

Instead, the mirrors are highly isolated from the ground (suspended from pendulums, motion damped by actuators, that sort of thing) so that such effects do not have significant impact on the motion of the mirrors.

In any case, seismic noise can't be fully isolated and creates a sensitivity wall below around 10Hz. To get sensitivity much below 10Hz, you have to go to space (look up LISA).

Also, I read somewhere that they had 6 additional signals they haven't reported yet (can't find it at the moment).

I guess we will see once the count gets into the dozens. If it happens without any kind of outside way to verify these signals are inspiraling black hole events it will definitely be interesting to see how the physics community deals with it.

Hi Nonbel, I work within the LIGO Scientific Collaboration, and as another posted commented, manual intervention (in the case of GW170104 by me) was only necessary for the online analysis. The purpose of online analysis is for fast coordination with EM partners so that potentially interesting opportunities are not missed. In the case of binary black holes, the expectation is that there will be no electromagnetic counterpart, as the region is expected to be cleared of matter well before we observed the black holes merging. If one to were to be found however, that would be exciting.

By design, detection statements and significance estimates come solely from the offline analysis which is conducted separately (i.e. not triggered by) the online analysis. No human intervention is required here, as the issue with the online status information was known about at the time and was not an issue with the data itself. Even if there were no candidate events at the time, it would be been included in the offline analysis of the period containing the event.

In regards to GW150914 and iDQ, you should know that iDQ has never been approved as a veto for CBC (compact binaries such as neutron stars and black holes) searches. Again, no intervention was required to "remove" the veto as it was never used in the offline analysis nor would be in the first place. It's only use that I am aware of is as a veto against Burst triggers in online analysis. These searches look for generic signals, but may also detect some of the louder CBC sources, such as GW150914. In case you were wondering, there weren't dedicated online CBC searches at the time of GW150914, but there were offline analysis, and those produced the results reported in the original detection paper.

We do account for the expansion of the universe in fact. We estimated that this source was at about z ~ 0.2 (see https://en.wikipedia.org/wiki/Redshift). Roughly speaking this means there'll be only ~20% effect as the scale of the universe (a) has increased by (1+z) over the time it has traveled.