Witness is protest.

You need only to video or livestream an event to inject it into mass consciousness, as if to say: This. This exists. And very often that act carries this implicit yet passionate message: And we have to change it.

On Wednesday, Diamond Reynolds used Facebook Live to livestream from Falcon Heights, Minn. Her video showed her boyfriend, Philando Castile, dying of wounds after being shot by a police officer. With purpose and self-possession, she addresses her virtual audience: "We got pulled over for a busted taillight in the back and the police just. . . . He killed my boyfriend. . . . He had a firearm and he was reaching for his wallet, and the officer just shot him in his arm." When police take her cellphone, she says: "They threw my phone, Facebook!"

Her video went viral, with immediate impact. The next day, in Philadelphia and cities throughout the country, people took to the streets to protest the death of Castile and a string of other police-involved shootings.

On CNN, Reynolds made clear why she did what she did, telling New Day host Chris Cuomo: "I want my justice. I want that police officer's name to go public and I want people to know who did this to us. . . . We didn't do this to ourselves. And I just want the world to know that."

Robert Thompson, professor of television and popular culture at Syracuse University, said, "The most important role that we've seen in social media . . . is simply the distribution of the videos themselves." From the 1991 Rodney King beating, through the 2009 Iranian elections, on into the #BlackLivesMatter movement, video distribution has shaped public opinion.

Arranged by Lauren Footman, president of the NAACP Pennsylvania Youth and College Division, the Philadelphia march was quickly set up, and it was peaceful. Ernest Owens of Philadelphia, who was there, said most of the participants had learned of it via social media. He was among dozens recording and publishing in real time. "Everyone was using" social media, Owens said. "Snapchat. Twitter. The minute I posted it on my Twitter, I got a ton of retweets." He said, "Video has been very productive in letting people see" events as they unfold.

Walter Bilderback of University City posted his march experience in videos and photos on Instagram and Facebook, "to let friends in other cities know it was happening, to let people see how peaceful the demonstration was. . . . There were tons of people using cellphones in similar manner: Some marchers seemed to be videotaping the entire 15-block march; bystanders were photographing and videoing. . . . I can only hope this sort of 'witness as protest' has an effect."

Thursday night's demonstration in Dallas began as an anti-police-violence march. It is likely that many there saw it as an extension of #BlackLivesMatter. The police protected demonstrators, even, in a sad irony, live-tweeting and livestreaming the event:

But then shots rang out, and five police officers were fatally shot in the streets. Now the object of protest turned from police violence to violence itself. And the crowds, dozens upon dozens, held up cellphones, to record, make real, witness, protest. The Dallas catastrophe became one of the most documented police killings in U.S. history.

Yessica Hernandez of Dallas was there. "I was originally just posting the videos to show . . . solidarity with what happened," she said. "I just wanted to show that it was a peaceful protest, that no one was getting harmed." Hernandez said livestreaming is important precisely because it is unedited: "It was important that people [knew] what actually happened before the media got their hands on it and put their spin on it."

Christopher Harper, professor of journalism at Temple University, warned about the dark side of all the posting and tweeting.

"Social media allowed people to coordinate protests, to speak their mind, to make change. Social media can do that very well, but it can also turn on a dime and start creating really negative actions" - hate speech, threats, harmful distortions of fact.

The more video, the more documentation, Harper said, the better. But while "social media can serve as a good check and balance on the media," he stressed that there are no filters, no arbiters.

In a tweet, Erin Simpson of Washington summed up the lacerating swing of the week's events: "I don't want black men shot at traffic stops. I don't want cops shot by snipers. I don't want kids shot at school. I don't want any of this."

jt@phillynews.com

215-854-4406@jtimpane

Staff writer Sofiya Ballin contributed to this article.