On Christmas Eve 195 years ago, one of the most beloved holiday songs -- Stille Nacht or Silent Night -- was performed publicly for the first time, in a village church in Austria. The song is one of many examples of our idealization of quiet during the holidays. The poem In It Came Upon a Midnight Clear, the angels arrive to find a "world in solemn stillness." To what degree can this desire be fulfilled? Is is possible to have a silent night?
Probably not. To understand why, think of the riddle "If a tree falls in a forest and no one is there to hear it, does it make a sound?" The implication is that perception may be a prerequisite for something to occur. Silence presents the opposite conundrum: It only exists if you aren't there to perceive it. The sounds of your inner workings will always be present. Even the inner ear, in the absence of sound, will itself sometimes emit noises called spontaneous otoacoustic emissions.
Still, there are places where you will hear little beyond your own heartbeat. In his book Zero Decibels: The Quest for Absolute Silence, George Michelsen Foy found near-silence at SNOLAB in Ontario, a laboratory specializing in neutrino and dark matter physics. The facility is buried two kilometres below ground to help scientists block out cosmic rays that hit the Earth's surface at a rate of 50 million per day per square meter. Being at that depth makes it a pretty quiet place.
But SNOLAB is not quite the quietest place on Earth. That distinction belongs to the anechoic chamber at Orfield Laboratories, an acoustics and lighting testing company in Minneapolis. Foy describes it as a sort of "Russian nesting doll," with the inner chamber enclosed within two larger structures. The walls are lined with sound absorbers. The floors and walls are lined with mesh that also dampens any sound. The room isn't technically silent: The noise level hovers around minus-9.4 decibels. (Zero decibels represents a volume just below which an average person can perceive sounds, a scale that makes negative decibel measurements possible.) As far as humans are concerned, though, the anechoic chamber contains no ambient sounds. But you'll still hear yourself.
"At first there is absolutely no ambient sound," Foy writes of his experience at Orfield, "Then, after a moment, I started to hear my heartbeat and my own breathing. When I frowned, I could hear the skin scraping over my scalp."
Silence on Earth may be impossible, but what about in space? It's a widespread misconception that the vacuum of space is incapable of propagating sound. In fact, an ionized gas permeates the entire universe, and that gas is capable of carrying sound. German physicist Heinrich Barkhausen was among the first scientists to detect the sounds that traveled through this space plasma.
Assigned to eavesdrop on British telephone lines during World War I, Barkhausen attempted to pick up transmissions using a large antenna. Rather than hearing conversations, he heard a peculiar whistling sound. Barkhausen likened the sound to grenades being tossed across a battlefield. He never figured out exactly what caused these "whistlers," but a series of scientific discoveries demonstrated that the sounds had to do with lightning travelling through the plasmasphere -- or ionized gas cloud -- that floats in space far above the Earth's surface.
We now know that whistlers aren't the only sounds traveling through space. The "dawn chorus" is a set of spontaneously generated disturbances in the Van Allen radiation belt that hovers between 960 and 61,000 kilometres above Earth. The dawn chorus is so named because it sounds so much like morning birdsong.
Moreover, the Voyager spacecraft have picked up space sounds well beyond Earth, such as some linked to Jupiter's weather. In 2012, Voyager 1 picked up the sound of a plasma storm that had traveled from beyond the solar system, proving sound exists even in interstellar space.
To be sure, most of these sounds are imperceptible to humans without the help of sophisticated scientific equipment, so space is very close to silent, from a human perspective. But the larger question is whether silence is actually desirable. Canadian astronaut Julie Payette told Foy that, in space anyway, silence is more disturbing than noise. The life-support systems inside a spacesuit are pretty noisy, and that's a good thing, she said. Noise means the machines are working. Silence is death. The experience most people have in the anechoic chamber reinforces this: Foy was the first person who managed to stay in the chamber for 45 minutes. Every previous guest found the silence maddening.
If the pitter-patter of children's feet -- or the roar of a motorcycle engine -- interrupts your long winter's nap this evening, keep this lesson in mind: Noise is irritating, and even harmful in big doses. Studies have shown that stress hormones and blood pressure tend to rise when a person is exposed to the ambient noise levels of some cities. But silence presents its own challenges. Wish for a quiet night, not a silent night.