“When we see the frozen hands of a clock with a dead battery, and we sit there and watch it, we tend to have a sinking feeling. Something feels wrong. We like to see time flow, as it is only natural that it seek its natural progression forward. On the other hand, when a clock is completely hidden we tend not to question its flow and instead experience an unsettling sense of uncertainty as to what time it might be. Seeing a clock’s secondhand tick-tick forward is a reassuring sign that all is well. (…) A frozen computer is like a frozen clock, and thus ways to psychologically deal with this torturous experience of waiting emerged in the form of progress bars.”
The Laws of Simplicity by John Maeda