Towards Probing the Origin of Individual Black Hole Mergers

How and where do black hole mergers form in our Universe? After 10 years of GW science with LIGO/Virgo/Kagra, there still seems to be no direct indication of what the underlying astrophysical formation channels are. I will present ideas and ongoing work on how the formation environment can leave observable modulations in the gravitational waveform (GW), from which one can determine the origin of individual GW sources. I further illustrate how a future of strongly lensed GW sources has the potential to provide further insight into the GW source environment that is not possible to measure in any other way. This especially includes the proper motion of the GW source, which can be combined with line-of-sight acceleration measurements, from which one can reconstruct the full orbital trajectory of the GW source.