After spending weeks staying at home during the coronavirus pandemic, people reacted to the killing of George Floyd by spilling into the streets. As they have called attention to police violence, their desires and concerns have found a vehicle in numerous chants and protest songs.From “No Justice, No Peace” and “Say His Name” to full renditions of Lean on Me and We Shall Overcome, musical expression continues to play an indispensable supporting role in political and social action.There is, of course, considerable difference in form and function between highly repetitive collective chanting and rousing melodies derived from gospel songs. Yet I refer to both of them as “music” because they use sonic elements such as rhythm, pitch contour and timbre to convey meaning that goes beyond language.Rather than simply amplifying the words — after all, protest chants and songs aren’t meant to reveal anything we haven’t heard before, and it is unlikely anyone would have to be reminded why they are protesting — this music is important for expressing political messages because it creates a sense of emotional connection and social coherence, even among strangers. It does this through the physical link that develops between participants. In a way, music functions as a social glue that binds the minds and bodies of those who create it.This connection has to do with two features that work in tandem and make music unique among the various forms of human communication. The first is that the meaning of music is deliberately imprecise — in technical terms, music is referentially ambiguous. The same song can be significant in different ways to different listeners, or even to the same person on different occasions. The second feature is that listeners can still connect with each other emotionally by moving together in synchrony with what they hear and with each other.Take Lean on Me, Bill Withers’s 1972 hit, which on June 3 became a rallying cry for thousands of protesters led by Kenny Sway in Washington. The song’s effectiveness comes not just from the lyrics, but from the way the lyrics are supported by the music. The consonant looping harmonies, the warmth of the strings accompanying the voice and the simple swaying melody that dips and rises in a smooth arc all combine in ways that leave lots of room for individual interpretation.For example, the entrance of Withers’s voice on the words “Lean on me” in the first verse — the way his strikingly clear timbre pierces the texture with a syncopated rhythm — could be associated with sincerity and integrity, a direct address to listeners to put their trust in the singer.That is because the glimmering quality of Withers’s voice shares its features with objects that demand attention by virtue of standing out against a more uniform background. Meanwhile, the radically sparse texture in the chorus — “You can call on me, brother” — and the entrance of back-up singers on “We all need somebody to lean on” might signify an invitation for listeners to join in an affirmation of community and kinship. In this instance, there is an analogy between the solo voice ultimately receiving reinforcement from others and the notion of physical space being created for a potential new member of the ensemble. Whatever terms we use to describe the feelings we associate with different sound qualities, the point is that concepts such as kinship and sincerity are soundless in themselves, but those of us who grew up in a given culture can plausibly transfer them on to the song in similar ways. The reasons for this are cognitive and sociocultural. As the music theorist Lawrence Zbikowski has shown, one of the mind’s most basic mechanisms is the capacity for analogical thought. From a young age, we can effortlessly relate things that belong to different domains: here, sounds and feelings, but also sounds and physical objects or human characters. The precise nature of these relationships — for example, whether we consider melodies as dipping and rising in an arc, or whether we hear some sounds as expressing warmth and others coldness — is then conditioned by the social groups to which we belong.Music’s referential ambiguity means that one need not have the vocabulary to describe the exact qualities one hears: a nebulous feeling is enough to make the tune meaningful. What I hear as integrity might feel more like confidence to you, and where I envision physical space for me to join a group of singers you might notice a lonely figure in need of encouragement. Because the sounds don’t specify which of these is correct, our interpretations of the song, as well as its meaning to us as individuals, are going to be different.Despite these differences, you and I can have an extremely powerful shared experience because of music’s ability to co-ordinate our movements with what I call kinetic precision. From the electrical firings in the brain, all the way to the movements of our limbs, we humans have a rare — although, as it turns out, not unique — ability to attune to a regular series of sounds. This process, known as entrainment, explains why different listeners can tap their feet or snap their fingers to the same beat. Because an overwhelming majority of music — especially rhythmically driven music like collective chant — takes advantage of our ability to entrain by making the beat prominent, it can easily co-ordinate the movements of a large group of participants.Using motion-capture technology in my own work, I have found that the effect of entrainment can occur spontaneously and with positive consequences for the emotional wellbeing of listeners. In particular, I asked listeners to dance to music that didn’t always have a beat. Without the beat, the timing of the dancers’ movements was highly variable. But when the beat was present, almost without exception the participants locked on to it and articulated it with their bodies, even though I didn’t explicitly ask them to do so. Afterward, many reported a feeling of pleasure when the music provided structure to which they could entrain.The human ability to entrain is evident in countless videos of protesters chanting together, in anything from rhythmically simple “Take your knees off our necks!” to the far more complex syncopations in “If you stood there and watched”.Like a perfectly tuned system, people with no formal musical training synchronise their voices and entire bodies with each other, creating a unified ensemble.Kinetic precision generates a feeling of social belonging that facilitates cohesion and increases efficiency in communal tasks, something that the historian William McNeill calls “muscular bonding”.The ability to synchronise is so essential to human existence that it strengthens attachment and increases co-operation among group members even when the task itself isn’t particularly pleasurable. Perhaps this is one reason that nearly every culture creates opportunities for synchronised activities, such as playing music, singing or dancing.Although these activities are costly in terms of time and energy, and although they yield no immediately discernible material goals — unlike, say, sex — emotional and physical attachment formed between individuals in sync with each other is enough for music-making to be well worth the effort.In practice, it would be extremely difficult to use music to communicate exact concepts, yet one can easily elicit the same emotions and movements in a group of strangers just by having them listen to the same song. More than that, the beauty of the design lies in, through the powerful social bond created by our synchronised movements, you and I feeling intimately connected even when simultaneously experiencing different emotions. Sitting in my apartment in New York, I may not understand the words chanted by protesters in Berlin, but my body responds to their sounds in a way that makes me feel as if I’m part of a much larger global gathering.It is important that kinetic precision should work together with referential ambiguity. If the meaning of music is too precise, and this precision leads to a conflict between individual interpretations, then even synchronised movement cannot create an emotional connection between listeners.This sometimes happens when popular songs are deployed to support political or social messages with which listeners disagree. One of the more notable examples of this was the 2016 Donald Trump presidential campaign’s use of Bruce Springsteen’s Born in the USA. Regardless of the artist’s personal political stance, the referential ambiguity of music makes it semantically “sticky”, which is to say it easily acquires new and even contradictory meanings because the sounds by themselves are all-too-readily open to different interpretations. And this creates opportunities for potential misuse.One striking aspect of music is that it brings participants together by working around individual goals and interpretations, and directly tapping into a deeper sense of physical connection. As we move and feel together in synchrony, this connection across time and space often feels more sincere, and more elemental than anything created by speech alone. And this not only amplifies the message we try to articulate, but also allows us to share in the common sense of humanity, to love, care and lean on one another in a way that only music can facilitate.• Mariusz Kozak is an associate professor of music at Columbia University and author of Enacting Musical Time: The Bodily Experience of New Music