Overwrite Traces

After reality has quietly been covered over by a new version, it leaves behind a few things that don't line up — the cloud layer in a photograph flatter than it was on site, the characters on a sign turning for an instant into another kind, an alley running half a body's width off from where it ran yesterday.

Most people see it and forget it, then talk themselves round afterwards to "I misread it," "I was tired." It is always small enough to be exactly ignorable: the edge of a cloud smoothed by a few degrees, one character with its radical swapped out, a brick wall with an extra metre-wide patch of black in it. Set against the volume of information on a whole street, a deviation that size doesn't carry enough weight to make anyone stop walking.

There aren't many ways to force it into the open. Original files are one — what goes onto the memory card in RAW is that data and nothing else; run SHA-256 on the card, on the machine and on the backup drive, and the three hash strings match to the character. The file hasn't moved at the byte level, and still the "wrong" the eye picks up gets fainter by the day. The time-sync photo is another: on the way out and on the way back, shoot the street corner, the traffic light, your own shoes, and use them afterwards to nail the timeline down. Two surveillance cameras of different makes and different angles, with their clock offsets subtracted out, landing on the same second rules out dropped frames and compression artefacts. The stupidest method is printing: the version that laser toner presses into the paper stops on that day, while the digital file goes on moving.

The same trace, and the accounts fork. The weather authorities' copy says thin upper atmosphere. A witness at the roadside says, "Just one line, straight, no colour, I can't say, either." Someone deletes his own post the next day; asked why he deleted it, he answers, "Got bored." A man who runs a plumbing-and-wiring shop in Yonghe said first that his head was black, and only after watching a stretch of phone video went back and added, "Yeah. That's it." Then he frowned and asked himself why he hadn't been able to bring it back a moment ago.

Write it down, measure it, cross-check it — for the few people who haven't been altered yet, that is the only thing there is to do.