The Universal Sense (2 page)

The natural world is based on variation, even among vertebrate animals who tend to share a lot of evolutionary history.
There are plenty of blind animals: cave fish live beyond the reach of light in underground caves and map the world by changes in water flow around their bodies, and the muddy river home of Indus river dolphins blocks most of the light, leaving them to rely on echolocation and the odd habit of swimming on their side to feel what is in the mud. There are plenty of animals (including us) who are unable to detect the songs of electric fields that underlie so much of the behavior of electric fish and the hunting behavior of sharks, or the beauty of the ultraviolet world as seen by bees. Plenty of animals have very limited senses of smell (humans again); animals such as the armadillo have a limited sensitivity to touch; and we can but hope that vultures have a limited sense of taste. But here’s one thing you never find: deaf animals. Why?

Let’s step back a moment. There are a lot of organisms, little one-celled things with no nervous system, that couldn’t be appropriately described as having a sense of hearing. And there are lots of animals, even household pets, whose hearing has deteriorated with age or been blown out by some untoward event. So, speaking more specifically now, there are no normally deaf vertebrates. And that distinguishes hearing from all the other senses we know, including sensitivity to electrical fields and ultraviolet light.

So why do all animals with backbones hear? Or to put the question another way, why is hearing the most universal of all senses? And if it is so crucial a sense, why do we humans so often ignore it at a conscious level, unless we’re trying desperately to block out the noise of the subway or checking out the latest music downloads?

Sound is everywhere. From the night chorus of frogs in the deepest rain forest to the emptiest wind-blown stretch of the
Antarctic, you are surrounded by, embedded in, and molded by sound and vibration. Anywhere there is energy, including the depths of intergalactic space, is a vibratory region. Some are richer than others, but none are totally silent. The range of measured vibration is immense. At the top of the spectrum is the insanely fast 9,192,631,770 cycles per second of an energized cesium-133 atom. Near if not at the bottom are the gravity-wave-induced pulses of the sound of black holes (a B flat 57 octaves below middle C, according to Andrew Fabian of the Institute of Astronomy in Cambridge). But these vibrational extremes are not of biological relevance to us—everything our brains can handle happens in a more constrained time frame, from hundreds of nanoseconds to years, far from either end of the spectrum. Living things are tuned to pick up information of interest and use to them, signals that will let them gather information about the environment, their friends and family, their predators, and their alarm clocks. But even within the limitations imposed by biology, there is a tremendous range of information to be gathered, whether by human hearing, bat sonar, or the head-knocking communication system employed by naked mole rats.

Vision is a relatively fast-acting sense that works slightly faster than our conscious recognition of what we see. Smell and taste are slowpokes, working over the course of seconds or more. Touch, a mechanosensory sense, can work quickly (as in light touch) or slowly (as in pain), but only over a restricted range. By contrast, animals and humans can detect and respond to changes in sound that occur in less than a millionth of a second and to the content of complex sounds over the course of hours. Any detectable vibration represents information, to be used or ignored. And in that simple concept lies the entire realm of sound and mind.
Whether it’s a humpback whale listening to hours-long song cycles during its migration or a bat using a submicrosecond difference in echoes to determine if something is an edible treat or a branch to be avoided, sound helps animals find food, mate, play, and sleep; ignoring it can get them eaten pretty fast. Which is probably why vibration detection, including what we humans with our fair-to-middlin’ ears call hearing, is one of the most basic and universal sensory systems that any earthly organism can have. What is detectable, what is discernable, and what is relevant are the bases for parsing out raw vibration into silence, signal, and noise.

All research into hearing seems to home in on two basic facts: (1) if a channel of information is available, it will get used by living things, and (2) sound is everywhere there is life (and other places). Anywhere there is matter and energy, there is vibration, and any vibration can transfer energy and information to a receiver who is listening. And the wide range of vibration perceivable by living things, from the single thud of a footstep that shuts up a frog chorus to the incredibly high-frequency sounds that form a dolphin’s natural ultrasound, requires a sensory system thousands of times faster than its slower cousins, vision, smell, and taste. It is this faster-than-thought auditory speed, with a wide range of tones and timbres that visual color cannot hope to match and greater flexibility than the chemical sensitivities of taste and smell, that lets sound underlie and drive a fantastic range of subconscious elements in the living organism. Combined with wildly divergent ways of listening by different species and the increasingly complex ways of using information by living things, the presence of sound drives the evolution, development, and day-to-day function of the mind. How it does this is what the rest of the book is about.

Chapter 1
In the Beginning Was the Boom

In the summer of 2009, my wife and I were invited to go to NASA’s Ames Research Center to try out some sound recording work on the Vertical Gun. The Vertical Gun is a 0.30 caliber light gas gun that fires custom-made projectiles (ranging from ice to steel) into targets at incredibly high speeds, up to 15 km/sec (about 33,000 miles per hour, or around fifteen times the muzzle velocity of an M16 rifle). It is used to simulate meteorite and asteroid impacts. The gun’s force comes from a powerful explosion, about half a pound of gunpowder detonated in 50 liters of hydrogen gas.

The bright red barrel is three stories high, pointing down at the ground through a sealed central chamber—imagine an elevator shaft that you really, really don’t want to stand in at the wrong time. The gun’s angle is changed by an old Nike missile launch elevator and fired through one of four ports at different angles into the central chamber. The main chamber, painted a cheery sky blue with walls thick as battleship armor, can be brought down to vacuum levels near that of outer space to simulate impacts on airless bodies such as the moon or filled
with custom gas mixes to replicate an atmosphere like the Earth’s. The test chamber is about 2.5 meters around with a central “target” that can be filled with sand, water, ice, or any other substance that might do something interesting when hit by a projectile at tens of thousands of Gs, and has a variety of ports to allow images of impacts to be captured by stereo video and thermal cameras with frame rates of up to 1 million frames per second. The walls of the room in which the chamber is located are lined with targets and projectiles from previous experiments—Lucite, Lexan, glass, steel—all showing the cratering and shattering effects of high-speed impacts. The big red button that fires the gun is safely in another room.

We were invited by Peter Schultz, a friend and colleague from the Brown University Planetary Geology department who is one of the world’s experts on crater formation. He can wax eloquent on rates of crater formation to help determine the age of planetary surfaces, geological formation and remodeling of planetary structures based on their impact history, and how such impacts have shaped our Earth through both deep and recent time scales. But mostly he is one of the world’s experts on blowing holes in things. Most people with expertise in this area talk about entrance and exit wounds in forensics, calculations of ballistic impact effects for launches and weapons design, or demolition issues that arise when blowing up buildings. Pete thinks bigger. He blew a hole in a comet to see what was inside it. He blew a huge hole in the moon to look for traces of water. Pete enjoys himself immensely when he gets to watch the ejecta fly, whether it’s tons of rocks, ice, and volatiles from the comet Tempel 1 or a pile of toothpicks used to model the trees that were blown flat during the Tunguska event in Siberia in 1908.

I was invited along to help Pete with a new way of recording
an impact. Using broad-spectrum recording techniques, we thought we could capture information about the impact and the material ejected from the target zone to create a sound-based 3-D model of how material flew away from an impact site. This could be important and useful for several reasons. First of all, the flow of ejecta after the impact is based not only on the physical characteristics of the projectile (our model asteroid) and target (a dish full of sand) but also on the way in which energy is transferred from one to the other. Think of a rock hitting water—those rippling circles that come from the center are waves of vibration caused by the transfer of energy from the rock to the water. The same thing happens when a rock hits something solid (like an asteroid smashing into the Earth), only it’s harder to see.

The transfer of impact energy can be analyzed in part by the multiple pressure waves that emerge from the impact, both as a result of the initial impact itself and from the way the ejecta move through the surrounding medium (for instance, air or water). Sound is a great way to model wave phenomena such as impacts and pressure flow. So after Peter set up the sand target and the techs loaded the gun, I placed a series of seismic microphones on the platform around the target, positioned a couple of pressure zone microphones (PZMs) along the chamber walls, and mounted an ultrasonic microphone near the port that the projectile would come through. The spacing between the mikes let us know how fast the sound was traveling, which in turn would let us start building up our 3-D sound model. My wife started up the digital recorders, the gun barrel was raised to the appropriate angle, the chamber was depressurized until it was nearly a vacuum, and we were all sent into the data center, a room full of video monitors that would let us watch the impact
remotely, through the high-speed cameras. (It would also keep us safe from any possible incidents involving meteoritic-speed projectiles, gunpowder, and flammable gases, though no such episodes have ever occurred in the gun’s forty-year history.) In the darkened room, the firing alarm went off and all we heard was a muffled thump. But the video screen shortly showed a slow-motion view of the pile of sand lighting up like a thousand suns followed by a hollow cone of sand and glass (from fused sand particles) flying outward, some particles moving even faster than the 5 km/sec of the original projectile and turning the chamber into a maelstrom of glass and dust.

We went back into the room, where Peter opened the chamber and gave one of his huge happy grins at the new crater while I fussed around, checking if any of his “babies” had destroyed my microphones. I tend to be rough on equipment, but my level of abuse usually is limited to dropping a microphone in a pile of bat guano or putting the recorder’s batteries in backward, not subjecting them to a miniature Martian sandstorm. Luckily, everything was still working, so the target was cleaned, the sand replaced, and another projectile loaded, but this time the chamber was left full of a normal Earth atmosphere to simulate a strike here rather than on some airless body in space. Once again we checked the recorders, then retreated to the data room and waited for the alarm. This time it was different. Again, through the safety doors and the battleship-thick armor of the chamber, all we heard was the thump of the ignition, but the presence of atmospheric gases changed things. Again there was the cone of ejecta, but there seemed to be a
lot
more motion of sand particles. And it wasn’t until I actually listened to the sound recordings that I could tell the difference.

The sounds from the first experiment in the vacuum were
very discreet, almost dull. The seismic microphones (called geophones and used to record earthquakes) picked up the initial impact and some faint pattering sounds as some of the sand hit the walls of the chamber and fell back to the target plate. The PZM microphones were silent—no surprise there, since most microphones used for what we think of as normal recordings detect changes in air pressure, and the near vacuum of the chamber gave them little to work with. The ultrasonic microphone was similarly quiet.

But the impact experiment with the atmosphere was radically different. The ultrasonic microphone picked up a submillisecond
whiiisssssh
of the projectile’s flight right before impact. But rather than a thump and some gentle pattering in the seismic microphone and silence from the in-air microphone, we heard an explosion of sound at the impact, followed by almost a full minute of a sandstorm that sounded like it could have buried the Sahara. The presence of an atmosphere had completely changed the ejection and acoustics dynamics of our simulated asteroid impact, changing it from a dull thump to a raging event of continuous noise. And I began to see how these contained simulations can help us understand events that happened long before the invention of recording equipment or even ears to listen with, around the time the Earth was born.

About four and half billion years ago, the Earth coalesced from a pile of dust and rubble into a large rocky sphere surrounded by a cloud of gases. It was a noisy place, with a constant bombardment by chunks of rock, the occasional comet, and asteroids trying desperately to figure out a place to orbit without running into cosmic traffic, along with the eruption of volcanoes that redeposited liquid rock and hurled boulders everywhere. A few hundred million years after this, another planet,
about the size of Mars, smacked the new Earth a glancing blow, ripping off chunks of the crust and sending the debris into orbit, where it slowly recoalesced into the moon.

It must have made a hell of a bang.