It’s a commonplace of time-travel that it necessitates scrupulous care to avoid temporal cross contamination; that visitors from ahead are assiduous, leave nothing behind to risk changing the order of things, no pre-emptive stuff or knowledge; that implacable agencies police this, keep their ancestors’ moments clean of not-yettery with extreme prejudice.

The truth is different. Chrononauts litter no less than any other tourists. The past is a dump, each epoch a tip of its futures’ rubbish. There are no police: only overworked binwomen and binmen endlessly shovelling junk into timefills. They slog uninterrupted: the detritus is all over the place, and unnoticed by us natives. We stub our toes every day on things discarded from times to come.