> What are the practical applications of gravitational wave research?

Currently, not much. We’re Galileo grinding lenses at home.

Theoretically? Everything from a whole new mode of active sensing, e.g. deep into the Earth and/or Sun, all the way to communications. Being able to directly measure gravity also lets us do new classes of experiments (not yet, but if we can measure more sensitively). We don’t know what we don’t know when it comes to gravity, so that’s another pot of potential.

Less fantastically, these are immensely precise instruments. The manufacturing methods and know how will almost certainly translate to other domains.

It's another type of telescope. In history we first observed waves in the visible spectrum, then perhaps infrared, radio, microwave, ultraviolet, X-ray, etc.

* https://en.wikipedia.org/wiki/Telescope#By_spectrum

This will allow measurements of another type of wave.

* https://en.wikipedia.org/wiki/Gravitational-wave_astronomy

The several already existing gravitational wave detectors have already detected signals believed to come from black hole-black hole, black hole-neutron star and neutron star-neutron star mergers.

Using the time delta between detecting the same events at different detectors around the globe, we have inferred the directions the signals are coming from.

The intensity curves of the events tell us details about the masses and other characteristics of the colliding object. Also over time the distribution of their masses and other characteristics will give us information about the populations and distribution of these objects, which can help validate our theories of star, neutron star, black hole and galaxy formation.

No one expects the science to be of practical value, but the development of the technology and engineering practices needed to perform the science may have many applications elsewhere.

I don't know of any field that necessitates better noise reduction than gravitational wave detection. Seems to me it has the potential to spin off tons of researchers/techniques able to handle noisy data.

If and when ET goes whipping by earth, these things might detect it. Setting all the science and engineering breakthroughs aside, every scientist wants a warp drive detector network.

Get them working well enough and we might be able to see the cosmic gravity wave background. That might let us see how/if the big bang actually happened, pre-inflation.

Better measuring macro effects of gravity might help us build better model of physics which can enable better quantum theory which can provide us with better technologies.