At a concert hall near Woodstock, N.Y., in August 1952, the pianist David Tudor played John Cage’s three-movement composition 4'33″. Doing so did not require enormous jumps with the right hand. Most people could play the piece with equal skill. Tudor set a stopwatch for 33 seconds and sat in front of the piano without touching the keys. He opened and shut the lid before sitting for another two minutes and 40 seconds and then did so again for a final interval of one minute and 20 seconds. Then he bowed and left the stage.

As Cage put it, 4'33″ was a “silent piece.” The composer wanted to push the audience members to listen to the other sounds that surrounded them. “There is no such thing as an empty space or an empty time,” Cage later said. “There is always something to see, something to hear. In fact, try as we may to make a silence, we cannot.”

The way we traditionally think of listening is that we hear a noise, a song, our friend’s voice, a car honking. But those sounds are inevitably punctuated with silent pauses that mark an absence of acoustic waves. Silence is integral to our everyday experience: the awkward pause in a conversation, the second after a thunderclap, the moment after a piece of music ends before the applause begins. The term “deafening silence” is even a common figure of speech.

Disentangling the way we perceive silence is like a Zen koan for neuroscientists—they must literally confront the sound of one hand clapping. The challenge reduces to just a few questions: Does the brain actually “hear” silence as an input processed by its auditory system in the same way it does a car horn? Or does the organ instead infer these empty spaces by inserting place markers between sounds that are then perceived as the silent bits?

A study published on July 10 in Proceedings of the National Academy of Sciences USA figured out a way to answer those questions. A group of interdisciplinary researchers at Johns Hopkins University set up an experiment that shows that, indeed, our brain actively perceives silence in the same way it hears sound.

The conundrum the group took on relates more generally to the question of how sensory perception works—and what happens in its absence—which has long bothered philosophers and psychologists. Vision is the light that reaches our eyes; touch is about what comes into contact with our body; and hearing has to do with sound. All that seems obvious—but maybe not.

“Silence, whatever it is, is not a sound,” says Chaz Firestone, an assistant professor of psychological and brain sciences at Johns Hopkins and co-author of the paper. “It’s the absence of sound. And yet it often feels like we can hear it. If silence isn’t really a sound, and yet it turns out that we can hear it, then hearing is more than just sound.”

To probe whether we actually “hear” silence, the researchers adapted a series of well-established auditory illusions used by experimental psychologists to show that the mind reacts to silence in similar ways as it does to sound. A total of 1,000 people participated in seven silence experiments using three silence illusions.

In an illusion called “one is more,” two shorter separate tones are played, followed by a single extended tone. People react by saying that the single noise is longer than the two distinct ones together, even though its overall duration is the same. This stems from a perceptual process called “event segmentation,” in which the mind processes sounds by dividing continuous input into discrete “events.” This can lead to perceptual illusions such as a single beep appearing to be longer than two separate ones.

In the silence-adapted version of this cognitive exercise, people were immersed in ambient noise such as the sounds of a busy restaurant or a train station. The soundtrack turned off for two silent interludes, each followed by a very brief resumption of the noisy background, and then there was a continuous interval of silence.

The same “one is more” illusion was produced, just as strongly as in the variation that compares the lengths of sounds. Study participants said that the sole silent pause was longer than the two punctuated ones, including the moment of noise resumption. “This suggests that our mind constructs similar auditory representations that might underpin our experience of silence,” says Rui Zhe Goh, a Ph.D. candidate at Johns Hopkins and first author of the paper. Goh is the first Johns Hopkins doctoral student to pursue a joint Ph.D. in psychology and philosophy.

      Credit: Johns Hopkins University

Another part of the study involved the “oddball illusion”: People were immersed in a soundscape in which two different sounds—such as a high-pitched organ and a low engine rumbl—were played at the same time. Four “silences” intervened in which the organ stopped and participants just heard the engine. Finally, a fifth “oddball silence” occurred in which the engine cut out and the organ continued to play. Listeners erroneously thought the oddball organ-only interval was longer. For each illusion, participants’ brain responded to the silent versions as the organ would respond to the sound illusions—the different noise produced the illusion that it was longer. That suggests that silence isn’t just the absence of sound but something that we actively perceive as we do with noises. You can try out the various silent illusions yourself.

The study used the tools of cognitive science to address old philosophical questions, a collaboration of methods and ideas that Firestone hopes to see continue into the future.

Studying silences can be an entry point into studying other kinds of absences, such as shadows or holes, says Nico Orlandi, a philosopher of mind and of cognitive science at the University of California, Santa Cruz, who wasn’t involved with the research.

“One of the advantages of thinking about holes, shadows and silence is: they’re perceptual, we have control over them,” says Roy Sorensen, a philosopher at the University of Texas at Austin and author of the books Seeing Dark Things: The Philosophy of Shadows and Nothing: A Philosophical History. Silence can help us understand absences in a new way, and get a grasp on how they work. “They’re like the fruit flies of metaphysics,” says Sorensen, who also wasn’t involved in the study.

The kinds of silences this work tests are called relative or contrastive, and they have some overlap with the study of holes. Philosophers have asked similar questions about the latter: Can we actually see a hole? Or is it just what’s around a hole that’s seen? “Holes require a host,” Firestone says. “Silences also require a host. Doughnut holes require a doughnut. The kinds of silences that we study require a soundtrack to go silent first.”

Knowing that we might actively perceive silence helps us better understand the moments when we’re confronted with it. For philosopher and study co-author Ian Phillips, he and his colleagues’ findings led him to a new appreciation for silence such as its use in music. He notes that the critic Alex Ross wrote that Anton Webern’s funeral march “is among the loudest musical phenomena in history, but even louder is the ensuing silence, which smacks the ears like thunder.”

“Prior to doing the work we have done, it would have been easy to worry that these were rather overblown metaphorical descriptions,” Phillips says.

Silences aren’t just a window to the nature of physical reality but deepen our understanding of our basic cognition. “The ability to relate to absence is a defining feature of all psychological creatures,” Orlandi says. “Humans, for example, are distinctive in their ability to relate to things that are not immediately present to them.”