Many reports have talked about the recent discovery of gravitational waves in terms of sound. Shane L. Larson, one member of the huge team of scientists working on the LIGO project, straightforwardly declared in a blog post, “On 14 September 2015, the two LIGO observatories detected a very loud gravitational wave event.“ In The New York Times, Dennis Overbye began his article by saying that scientists “had heard and recorded the sound of two black holes colliding” and added, a few sentences later, that the signal “seems destined to take its place among the great sound bites of science.” A thorough account written by Nicola Twilley for the Elements section of the New Yorker website reported that, after the two black holes merged, radiated a vast amount of energy, and settled down, “space and time became silent again.”
Those who don’t spend much time reading about science may wonder what this talk of sound really means. Here’s another way of putting it. The Laser Interferometer Gravitational-Wave Observatory (LIGO) consists of two very large detectors, one in Washington State and one in Louisiana. Each detector is basically a pair of yardsticks at right angles to each other; the experiment does nothing more than check whether the length of one of the yardsticks changes in comparison to the other. (I won’t try to explain here how it works, but it updates a classic piece of physics called the Michelson-Morley experiment.) Very little in the world is ever totally still, and so the two detectors right away began picking up minor fluctuations—the sort of thing you can call background noise. Early in the morning on September 14, the frequency and the amplitude of the fluctuations briefly increased, first in Louisiana and shortly thereafter in Washington: over the course of about two-tenths of a second, the detectors caught a vibration that grew from about 35 times a second to 250. Those frequencies correspond to audible pressure fluctuations in air, which is what we mean by the word “sound.” So it’s reasonable to say that we have now heard the universe in a new way, and it’s possible to convert the detector outputs into actual sound—in fact, this has been done—but we should keep in mind that this is a metaphor, an analogy. Vibrations in air can literally be heard, as can vibrations in water and many other solids (put your ear to a train track and you may hear an approaching train); vibrations in spacetime cannot.
Not all pressure fluctuations in air are audible. Atmospheric pressure routinely changes over the course of hours or days; that’s too slow for us to hear. Going up or down in an airplane or an elevator can cause the pressure to change more quickly. Normally we wouldn’t detect that either, but the quirks of our auditory system often momentarily block the pressure change on the inside of our eardrum, and when that block opens up, as happens when we yawn, the pressure change is sudden enough that we can hear it as a pop.
Human hearing can ideally detect air-pressure changes—that is, frequencies—between about 20 and 20,000 cycles per second, more often called Hertz and abbreviated Hz. Anything else that vibrates in that range, or that can be scaled to fall within that range, can be conceptually interpreted as sound, and if you have the right equipment it can be converted into sound. As I write, a candle is burning on a nearby countertop, and its flame seems to be jumping about on the order of three to five times a second. I could build an apparatus to capture the changing position of the flame and convert it into an electrical signal, which I could easily see on an oscilloscope, but if I fed the signal to a loudspeaker I wouldn’t hear anything. I could hear it, though, if I scaled it up—that is, multiplied it—by 100. Three hundred Hz is slightly sharper than the frequency of the musical note D4 (the D just above middle C in A=440 tuning), and 500 is just above B4 (the B above middle C), so my candle would seem to be dancing in the midrange of the piano keyboard. As it happens, the LIGO signal approached the same area; the Times report and others have told readers that it rose to the note of middle C.
Vibrations occur all over the place; nothing above the temperature of absolute zero is truly still. The atoms or molecules that make up many solids can vibrate in a way that contributes to the transmission of heat or electricity through the material. These oscillations are called phonons, a term that comes from a Greek word denoting sound; the Wikipedia entry is short on historical details but reports that the concept dates from the 1930s, which implies that for nearly a century we’ve been able to understand these atomic-scale movements in terms of sound, if not literally to hear them. The Earth itself vibrates, sometimes disastrously and sometimes subtly; so does the moon, as we know from seismographs left there by Apollo missions—moonquakes have more than once been described by scientists as causing the moon to “ring like a bell”; so does the sun.
In the metaphorical sense, then, we’ve been hearing the universe in many ways for some time. We’ve also been seeing it metaphorically. I forgot this myself a few days ago. When I read Shane Larson’s remark that “Within the next few years, we will no longer live in a world where our view of the Cosmos is limited to what light alone can tell us,” my first thought was that astronomy has for decades been looking at the universe via radio waves, X-rays, and gamma rays as well as by means of light. But Larson reminded me, in an exchange of comments, that physicists nowadays talk about all electromagnetic radiation—which includes radio and the other things I just mentioned—in terms of light. If they’re talking about the relatively narrow range of things that we can literally see, they say “visible light.”
These metaphors aren’t surprising. We have only a handful of senses through which to detect the world, and they all have their limits. Much of science consists of efforts to convert all sorts of exotic phenomena into something that our senses can grasp or that our languages can communicate. Mathematics can convey a lot, but it’s not something most people understand, so scientists fall back on the language of the senses, and nonscientists do the same. “I hear you,” we say, or “I see what you mean.” There’s not really much difference between those commonplaces of conversation and the recent talk of “a loud event” or “the sound of black holes colliding.” If only they were able to understand metaphor, even a dog or a bat or a dolphin—animals that are, like us, sensitive to sonic vibrations—could probably see what we mean by that.
(The report of the scientific team can be found here.)