Every weekday morning I commute to work on BART. When I look around at my fellow passengers, I’m always struck by how few of them seem to be fully present. Some seem incapacitated due to chemical intoxication, but many of them — often, most of them — are staring into little glass rectangles in their hands. Their bodies share this train car with me, but their minds and attention are somewhere else.

I sometimes catch glimpses of what they’re doing: chat bubbles, games with colorful candy explosions, videos of Bollywood dancers, a news website, a cat GIF, Facebook. Sometimes a smile or a frown flashes on their faces, cued by an interaction the rest of us are oblivious to. Their focus is intense; they only come back to the here and now when the train pulls into a station or makes an unexpected stop.

While they’re focused on their glass rectangles, we’ve somehow stopped being in the same place together. The boundaries of the physical environment we share no longer constrain their consciousness. They’re participating in something — a political argument, a shopping expedition, a flirtatious encounter — that’s happening somewhere else.

Where are these people?