Why does time only go forward?

Time goes one way because every repair throws information away, and nothing can put it back. When two neighboring readings disagree, both move to their midpoint. A seam holding 6 and 0 ends at 3 and 3, and so do 5 and 1, 4 and 2, and four other starting pairs. Seven arrangements go in, one comes out, and about 2.8 bits about the past are gone. The loss is arithmetic, so it needs no special low-entropy beginning for the universe and holds on the first repair as on the ten billionth. The arrow is a ratio. A step producing ten bits of entropy runs forward about a thousand times as often as backward, so a bead in water watched for a hundredth of a second can be caught running backwards, and a cup of coffee never is.