The "sound" of spacetime: Overview


The character of gravitational waves generated by a source is quite different from that of electromagnetic waves a source might generate. In many circumstances, electromagnetic waves — be they light, x-rays, radio waves — have a wavelength much smaller than the size of their emitter. For example, visible light has a far smaller wavelength (a few hundreds of nanometers) than the sun (hundreds of thousands of kilometers in size), lightbulbs (several centimeters) and even typical objects in our vicinity. This separation of lengthscales between the light and the objects that produce or reflect it is key to our brains' ability to form images.

Gravitational waves are typically in a rather different regime. By virtue of the gravitational dynamics that generate them, these waves always have a wavelength quite a bit larger than their source. For example, a compact binary that is radiating in LIGO's sensitive band may consist of neutron stars (each about 10 kilometers in size) separated by a few hundred kilometers. The waves that they generate will have wavelengths of thousands or tens of thousands of kilometers. We cannot even in principle use this radiation to form an image of its source; thinking about what GWs can help us "see" is just not a well-posed analogy.

A more fruitful analogy can be formed based on sound. The sounds that our ears are sensitive to have wavelengths that can be many meters or tens of meters, far too long 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. (There are some species, such as bats, that use sound for imaging, at least in a crude way; and, humans have developed techniques for making internal images based on sound. Key to these "audio images" is making the sound waves have very short wavelengths; ultrasound wavelengths are typically millimeters or smaller.)

The information content of GWs is analogous to that of sound. GWs 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 speak the language in which that story is told. Also, GW detectors probe the entire sky much as human ears can hear sounds from essentially all directions.

There are differences, of course: Rather than pressure waves moving in an atmosphere, the waves are ripples of spacetime curvature. And, GWs don't act upon membranes in our ear, but rather as oscillating tidal forces upon widely separated masses. As such, it should of course be understood that the analogy to sound waves is really just an analogy. But, it has proven to be a very useful one for communicating the information that GWs carry, and clarifying how GWs can be used for astronomy.

The links in the sidebar will take you to pages that present catalogs of sounds corresponding to particular sources. Colleagues: If you have sounds that you would be willing to share on this page, please drop us a line!