Justin Valdez was shot on a San Francisco train, and no one saw it coming:

A man standing on a crowded Muni train pulls out a .45-caliber pistol.

He raises the gun, pointing it across the aisle, before tucking it back against his side. He draws it out several more times, once using the hand holding the gun to wipe his nose. Dozens of passengers stand and sit just feet away – but none reacts.

Their eyes, focused on smartphones and tablets, don’t lift until the gunman fires a bullet into the back of a San Francisco State student getting off the train.

Investigators say this scene was captured by a Muni camera on Sept. 23, the night Nikhom Thephakaysone, 30, allegedly killed 20-year-old Justin Valdez in an apparently random encounter.

There were no witnesses to a murder with dozens in close proximity, despite the gunman repeatedly raising and pointing the .45-caliber gun, withdrawing it, raising it again, wiping his nose with a weapon of normally quite noticeable size.

These weren’t concealed movements – the gun is very clear,” said District Attorney George Gascón. “These people are in very close proximity with him, and nobody sees this. They’re just so engrossed, texting and reading and whatnot. They’re completely oblivious of their surroundings.

It is a surreal moment in our technological life, when devices that we, to be honest, primarily use for trivialities have come to exert such a powerful suck on our attention that not a single person could spare the brainpower for a subconscious alert to the mortal danger by their elbow, or in front of their face.

“When you used to go into a public place, you assumed everyone was in that place with you,” said Jack Nasar, an Ohio State University professor in city and regional planning who specializes in environmental psychology. “What happens to public places when everybody is talking on a cell phone? Everyone is somewhere else.

“Someone can take a gun, hold it up, and nobody will notice it.”

Justin Valdez’s fellow passengers must be asking themselves just what was so absorbing that they couldn’t look up. Were they on the brink of a new level of Candy Crush? Were they dashing off the perfect subtweet, or catching up on their gossip? Reaching for a distraction to occupy the commute is certainly nothing new, and a novel can be quite absorbing. But one can’t shake the sense that Jack Nasar has a point, that there’s something unprecedented about a crowd of people all being attached, each in their own atomized way, to Elsewhere.

Will Oremus over at Slate acknowledges this, but says “On the other hand, it’s hard to see an easy solution on the horizon, unless Google Glass can build in some handgun-recognition software.” The San Francisco shooting seems to demand more of us, however, than a shrug. Our relationship to the world around us is an ethical issue, and we choose what technologies to use, when, and where.

It may be time for us to ask ourselves: if someone waved a gun in your face, would you notice?

If not, what are you going to do about it?