# Why do we say gravitational waves are analogous to sound?

In every popsci discussion of gravitational waves, the waves are said to be like "sound", and that gravitational waves allow us to "hear" the universe. Despite this, I have no idea how gravitational waves are any more like sound than like light. Some possible explanations are:

• Gravitational waves are produced by the vibrations of objects, just like sound waves. But light waves are also produced by vibrating (charged) objects.
• Gravitational waves propagate in a medium (spacetime), just like sound propagates in some material medium. But so does light; it propagates in the quantum electromagnetic field. This is as much a "medium" as spacetime is.
• Gravitational waves are described by rank 2 tensors, like sound waves in a solid. But no popular explanation ever brings this up; tensors are not popsci level.
• Gravitational waves are classical, like sound waves. But there's quantum sound (phonons) and a perfectly good classical description of light.

I must be missing something basic here. What makes gravitational waves more analogous to sound waves?

• Specifically in the case of the gravitational waves discovered in 2015, the frequency ranged from 35-250Hz, well within the range of human hearing, so we could actually hear a direct transposition of the data. More of a cute coincidence than deep physical insight though. – The Geoff May 29 '16 at 15:51
• You're right, of course. I think the comparison to sound is largely because (1) the signals we hope to detect are at approximately audio frequencies; and (2) the detectors are fairly omnidirectional; and (3) we only receive a single "monophonic" signal from a given detector. We don't get a picture made of pixels. All of these properties are qualitatively a bit more like sound than our day-to-day experience of light. – nibot Jun 1 '16 at 16:13

I was using the "sound" analogy for gravitational waves exclusively in my popular talks – although I also mentioned that at the fundamental level, the gravitational waves are more similar to electromagnetic waves because they propagate by the speed of light and they are both waves on a fundamental field.

However, the "sound" analogy sounds vastly superior to me for these reasons:

• the frequency of the gravitational waves in LIGO is actually of order of 100 Hz, so the signal may be converted to a perfectly audible sound signal (it's the chirping if the black hole signal is long, but the discovered LIGO black holes were large enough and the signal was short which is why it was more similar to a heartbeat)

• creatures living at a planet close enough to the source of the gravitational waves (the two merging gravitational black holes) would actually hear (or did hear, if there were some observers) the signal literally in the form of the sound because the gravitational waves made the pressure and planetary radius oscillate with the same dependence of time. Gravitational waves cause the stretching and shrinking of all masses, just like the sound, so they are a form of sound that also propagates in the vacuum.

• for light, our gadgets (telescopes plus eyes or detectors) may easily identify the direction from which the light is coming, and we may always shield specific or most regions of the (celestial) sphere and only look into one direction. On the other hand, our ears hear the sound from the whole space and it's not easy to "shield" against the sources of sound that are coming from a particular direction. We always hear all five children screaming around us, there's no easy way to "focus". The same property of the sound holds for the gravitational waves, too. The LIGO detectors also hear the sound coming from all directions simultaneously. Helpfully, the number of LIGO detectors was two – just like the number of human ears – and the information from the two sources may be used to deduce the direction from which the gravitational waves are coming in a similar way in which ears+brains are doing it, but one can't ever "shield the rest of the sky", something that is easy to do with light.

• The gravitational waves are qualitatively different from the electromagnetic waves, and because the electromagnetic waves are generally linked to "eyes" (even though they include waves of many frequencies that eyes are not sensitive to), it's natural to pick different organs as the symbols of the gravitational waves, and the ears are clearly the most suitable ones.

The experiment that purported to detect gravity waves was by Weber, who attempted to measure them by looking at the disturbances generated in massive aluminium cylinders, apparently designed to vibrate with a frequency of $$1660 Hz$$ when disturbed by the gravity waves. What is of note though, is the sensors had to be capable of detecting a change in the cylinders' lengths by about $$10^{−16}$$ $$m$$ so any Musica universalis wouldn't be very loud.

Also, have a look here, in particular "Sound also has significance in the history of gravitational-wave detection. In the 1970s, physicist Robert Forward built a rudimentary laser interferometer, a forerunner to LIGO. He reported that the instrument’s output came in the form of “a wideband analog signal in the audio region” that was “recorded directly onto magnetic tape through one channel of a high quality stereo tape recorder” and “analyzed by ear.” Had the interferometer been sensitive enough to detect gravitational waves, Forward could have made the historic discovery by hearing it. Even today engineers at LIGO convert their signal into sound, not for detection but as a means of troubleshooting."

Assume you have a Fabry-Perot or Michelson-Morely interfereometer and you are measuring the phase difference of one path relative to another. You then decide to yell at the device. You will then measure an undulating change in the phase difference in the two paths. This is analogous to how a gravitational wave acts, and is a model of the LIGO. Acoustical and gravitational waves cause masses separated by some distance to exhibit an oscillating variation in that distance.

There are departures of course. Acoustical wave are longitudinal, while gravitational wave are transverse. Acoustical waves are usually linear, being of the form $p_{xx} - v^{-2}p_{tt} = 0$ for $v$ the velocity of the wave and $p$ pressure. Gravitational waves are linear only in the weakest approximation .

They are similar in that they involve the oscillatory motion of test masses in response to them. They are different though in some formal ways, in particular longitudiinal vs transverse waves.