When MIT grad student Steve Mann began wearing a computer and a head-mounted camera every moment of the day in 1981, he wasn't thinking of Black Lives Matter, racism, or police violence. But Mann, now a professor at the University of Toronto, may have given us the key concept for understanding the role of ubiquitous cameras in documenting police violence against people of color.

The recent shootings of Alton Sterling in Baton Rouge, Louisiana, and Philando Castile in Falcon Heights, Minnesota, were documented on multiple cameras. Sterling's death outside a convenience store, where he worked selling CDs, was captured on the store's surveillance camera and by two bystanders on their cell-phone cameras. Castile's death may have been captured by a squad car camera when he was stopped for a broken taillight. But we know about his death because Diamond Reynolds, Castile's fiancée, broadcast his final moments, her handcuffing, and her four-year-old daughter's attempts to calm her on Facebook Live, ultimately reaching millions of viewers.

Thirty-five years ago, Mann began thinking about a future that seems increasingly real—a world in which cameras are ubiquitous and can store and share what they see. Mann believed that his Eyetap system would be helpful in enhancing human capability and memory, allowing complete recall of past events. But he also predicted that it would have powerful social effects as millions of people with connected cameras could collectively hold authorities responsible for their misdeeds. Mann called this phenomenon "sousveillance," watching from below, an inversion of "surveillance," watching from above.

In addition to inexpensive, networked surveillance cameras, the ubiquity of cell-phone cameras combined with video streaming services like Periscope, YouTube, and Facebook Live has set the stage for citizens to hold the police responsible for excessive use of force. But a funny thing happened on the way to sousveillance: video doesn't always lead to accountability.

Consider Eric Garner, choked to death by a police officer in Staten Island after being arrested on suspicion of selling loose cigarettes. Garner's death was documented by his friend Ramsey Orta, and the video was widely disseminated. Despite the video evidence, a grand jury declined to indict Garner's killer, leading to widespread outrage and protest. (In an ironic twist, the only person indicted in connection with Garner's death was Orta, who came under police scrutiny and was arrested on an "unrelated" weapons possession charge. Orta is now in prison in New York. Sousveillance is not without its costs.)

I asked law professors and defense attorneys why they thought the video of Garner's death hadn't led to the arrest of the police who killed him. Some referenced the challenge of getting grand juries to indict police in general, others to the fact that Staten Island—home to a large number of NYPD officers—was especially unlikely to indict an officer. But one former attorney made the argument that video can make it harder to gain an indictment or a conviction: "When there's a video, you know the narrative the jury's going to see. Your job as a lawyer is to explain why what they saw proves your client was innocent." Lawyers for Officer Daniel Pantaleo were able to persuade the Staten Island grand jury that what it saw was appropriate police behavior, and Pantaleo was not charged with Garner's death.

It's worth remembering that the most famous video of police violence, the Rodney King video, recorded in 1991, led to the acquittal of the officers who beat King. While the video showed the assault on King, it also showed him charging at officers after being Tased. The lawyers for the LAPD explained that the video demonstrated that King had been violent and resistant, justifying the use of extreme force.

Video evidence can lead to justice, especially in egregious cases. We would not have known the circumstances of Walter Scott's death after a traffic stop in North Charleston, South Carolina, without video filmed by Dominican immigrant Feidín Santana. On seeing the video, in which Officer Michael Slager shoots Scott in the back as he was fleeing and then appears to plant a weapon on his body, the North Charleston police chief arrested his officer and supported his indictment. Slager faces trial this October, and Santana's video and testimony will likely be key evidence in his trial.

If video doesn't lead to the indictment of officers who shoot civilians, are we wrong to expect justice from sousveillance? The police who shot Castile and Sterling knew they were likely to be captured on camera—from their police cars, surveillance cameras, and cameras held by bystanders—but still used deadly force in situations that don't appear to have merited it. Is Mann's hope for sousveillance simply wrong?

Not quite. While these videos rarely lead to grand jury indictments, they have become powerful fuel for social movements demanding racial justice and fairer policing. In the wake of Sterling and Castile's deaths, protests brought thousands into the streets in major U.S. cities and led to the temporary closure of interstate highways.

Powerful as these videos are for mobilizing activists, they may be more powerful in bringing new participants into the racial justice movement. As law professor Paul Butler remarked, "A lot of white people are truly shocked by what these videos depict; I know very few African-Americans who are surprised." Watching how horrifically Diamond Reynolds is treated by officers who've just shot her fiancé in front of her four-year-old child may help white people understand why so many black and brown people are terrified of encounters with the police, and why movements like Black Lives Matter are so important.

We may be at an inflection point for the power of sousveillance. When George Holliday filmed the beating of Rodney King, he offered the footage to the LAPD, who reportedly ignored his offer. Local television station KTLA broadcast the footage two days later—had they refused, it's likely that King's name would be entirely unknown. Now our cameras are connected to small networked computers, and we can stream video directly to our social networks. Given the massive reach of a platform like Facebook, Diamond Reynolds's video reached over 3.2 million people within a day of her broadcast. Assuming Facebook continues to permit the broadcast of disturbing and violent videos, we may no longer need a coöperative television station to turn police violence into a national discussion.

The ubiquity of cameras and the power of live-streaming to social networks won't automatically hold powerful institutions accountable. Thus far, it hasn't prevented police from using deadly force in situations that could have been handled in other ways. But sousveillance offers fuel for the activists who are trying to turn powerful images into justice and systemic change. And most critically, ubiquitous cameras and viral video means we are now all aware of America's epidemic of civilian death at the hands of the police. With over 500 civilians killed by American police this year, perhaps sousveillance can help reform a police system that is broken in a deadly way.