When Flitwick described "Harry from the past" and "Harry from the future" as a closed loop, Pomona recalled the description in A Brief History of Time. Suppose someone wanted to travel from Earth to another planet. Even if he travelled at the speed of light, it would take several years, let alone whether he and the rocket carrying him could withstand light speed.
So people proposed a hypothesis—the wormhole. Within a wormhole, one could not only move through space but also travel through time. For example, there was a horse race on Earth and a conference in a neighboring galaxy. After using a wormhole to travel from Earth to the neighboring planet and attend the conference, the time traveler used another wormhole to return to Earth. But this "closed loop" meant that the person who left Earth after the horse race ended arrived in the past, before the conference on the neighboring planet had begun. When he returned to Earth after the conference, he would arrive before he had set out. If he knew the result of the horse race in advance, he could still place a bet before it ended.
Harry and the others had not changed locations, but Harry had not used the Time-Turner to return to the Hospital Wing. Instead, he had followed the flow of time from 7:30 in the evening to midnight. That left exactly half an hour short of the stipulated five hours. Even if it had not been like the timed bombs in films, dismantled only in the final second, it was still thrilling enough.
The biological clock of the Harry who returned to the Hospital Wing was already several hours apart from that of the Harry who had "vanished" with the Time-Turner. A few hours meant little in a human life, but