The practical size limit on Ligo and other Earth-bound detectors means they are best suited for the end stages of binaries or supernova explosions. White dwarf binaries or paired black holes at the centres of galaxies would produce gravitational waves measured in millions of kilometres, impossibly large for detectors built on Earth. Building a significantly larger antenna on Earth simply won't work, 4 kilometres is long enough that the arms of Ligo stick out of the ground due to Earth's curvature, and going underground carries its own expenses and risks (though that's the route the Clio detector takes). If you want to go bigger, you have to go into space.
Spoiled for signals
At this point, gravitational-wave researchers begin to swear and put their fingers in their ears. The reason is that a long-planned project called Lisa (Laser Interferometer Space Antenna), a joint effort between Nasa and Esa, has experienced many delays that were exacerbated when Nasa backed out entirely. The updated version, known as the Next Gravitational-wave Observatory (NGO), is a simplified design, but its launch date is currently unknown.
NGO is based on the same principles as Ligo. The role of the arms is played by three spacecraft flying in a fixed V-formation, separated by 2 million kilometres (1.25 million miles), over 150 times the diameter of the Earth. Mirrors inside the spacecraft move when gravitational waves pass. Isolation from noise found on Earth, as well as the much larger arm length, make NGO far more sensitive – and able to detect stable binaries.
If NGO is completed and flies before I'm aged and grey, it will be our best chance to prove Einstein right, and possibly even find objects we haven't predicted. Historically, every new way of seeing in astronomy has revealed something new to be seen, so it wouldn't be surprising if gravitational-wave astronomy also has a few surprises in store.
Meanwhile, Earth-bound detectors are not guaranteed to ever observe gravitational waves, though I personally do not side with the cynics on that issue. Even if the vastly increased sensitivity from Advanced Ligo, the updated Virgo, and other projects worldwide are insufficient to do the trick, one lesson of history is that we should not underestimate human ingenuity to create new methods. If we collectively support space-based gravitational-wave observatories – which should be international for practical reasons, if nothing else – researchers will be spoiled for signals. Then, the question of “Will we ever detect gravitational waves?” may best be rephrased as “When will we detect gravitational waves?”