  "If one could ever prove the existence of gravitational waves, the processes
responsible for their generation would probably be much more curious and
interesting than even the waves themselves." (Gustav Mie, 1868 - 1957) The
discovery of gravitational waves has opened new windows on astrophysics,
cosmology and physics beyond the Standard Model (BSM). Measurements by the
LIGO, Virgo and KAGRA Collaborations of stellar-mass binaries and neutron star
mergers have shown that gravitational waves travel at close to the velocity of
light, and also constrain BSM possibilities such as a graviton mass and Lorentz
violation in gravitational wave propagation. Follow-up measurements of neutron
star mergers have provided evidence for the production of heavy elements,
possibly including some essential for human life. The gravitational waves in
the nanoHz range observed by Pulsar Timing Arrays (PTAs) may have been emitted
by supermassive black hole binaries, but might also have originated from BSM
cosmological scenarios such as cosmic strings, or phase transitions in the
early Universe. The answer to the question in the title may be provided by
gravitational-wave detectors at higher frequencies, such as LISA and atom
interferometers.
