The sounds of spacetime: Overview

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 nanometers) than the sun (hundreds of thousands of kilometers in size), lightbulbs (several centimeters) and typical objects in our vicinity. This separation of lengthscales between light and the objects that produce or reflect ligth is key to our brains’ ability to form images.

Gravitational waves (GWs) are typically in a rather different regime. By virtue of the gravitational dynamics that generate them, GWs 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 or 15 kilometers in size) separated by a few hundred kilometers. The waves this binary generates 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 idea.

A more fruitful analogy can be formed based on sound. The sounds our ears are sensitive to have wavelengths of meters, or tens of meters. This is far too long to form images of the sources that generate them. That’s fine; you would never imagine using the sound of a person’s voice to build an accurate 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.

(It’s worth noting that some animal, such as bats, do in fact use sound for imaging, at least in a crude way. Also, humans have developed ways to make images based on sound. Key to these “audio images” is using sound waves with very short wavelengths. Medical ultrasound wavelengths are typically millimeters or smaller, and are used to image structures inside bodies that are centimeters or tens of centimeters in size.)

It must be emphasized that the sonic connection is an analogy: sound waves are pressure waves moving in a medium (air or water); GWs are ripples of spacetime curvature which exerts an oscillating tidal force on widely separated masses. Our ear cannot hear a GW, but you can regard detectors like LIGO as a transducer than converts an astrophysical GW into a signal you can hear.

Continuing with this analogy, one can regard the two distict polarizations of GWs as carrying “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 is a lot of value in regarding a GW signal as sound-like, and the detectors which measure them as ear-like.