Even though gravitational wave astronomy is a new field, it would nonetheless make sense to try to use gravitational waves as we use light waves. After all, the most natural way we humans have to learn about something is to look at it. So why isn’t gravitational wave astronomy just another form of optical astronomy?

Simply put, the means by which light —electromagnetic radiation— are generated is very different from the way that gravitational radiation is generated. In almost all circumstances, we find that light waves have a wavelength that is much smaller than its emitter, or than the objects that they reflect from. For example, the wavelength of light from the sun is a few hundred nanometers — far smaller than the size of the sun (several hundred thousand kilometers) or even the various objects that we look at (fractions of a millimeter or larger). Because of this "separation of lengthscales", light can be used to form images of these objects. Gravitational waves, on the other hand, typically have a wavelength that is close to or much larger than the size of its emitter. We cannot even in principle use this radiation to form an image of its source. Thinking about what gravitational waves can help us to "see" is just not a well-posed idea.

A more fruitful analogy for thinking about the information that gravitational waves convey is instead based on sound. The sounds that our ears are sensitive to have wavelengths that can be many meters or even tens of meters long — far too long to be used to form images of the sources that generate them, such as a person talking. That's fine — you would never imagine using the sound of a person's voice to build a mental picture of what they looked like! (Think how many radio DJs don't look how you imagined them.) Instead, you have learned to speak languages. Rather than using sound waves to build an image of the emitter, you use your knowledge of the language that is being spoken to understand the information that the emitter is transmitting.

The information content of gravitational waves is wonderfully analogous to the information content of sound. There are differences, of course: Rather than pressure waves moving in an atmosphere, the waves are ripples of spacetime curvature; rather than acting upon membranes in our ear, the waves act via oscillating tidal forces upon widely separated masses. But, in some ways they are remarkably similar. Gravitational waves have two distinct polarizations, so they carry "stereophonic" information about their source. Each polarization carries the imprints of the source's dynamics and evolution, telling a story about what that source is doing. We use the tools of general relativity to try to learn to speak the language in which that story is told. Also, gravitational wave detectors probe the entire sky — not just a small well-chosen piece of it — much as human ears can hear sounds from essentially all directions.

It should of course be understood that the analogy to sound waves is simply an analogy. Gravity doesn't really generate sound, and sound doesn't generate gravity. But, presenting gravitational wave sources as sounds is a fantastic way to communicate the information that they represent, and helps to clarify how gravitational waves can be used for astronomy.