“But suppose,” he continued, “time isn’t like that at all. Suppose time really looks like this.”

“Handy that you happened to have a picture of twigs with you, wasn’t it?” said Brian.

“Yes,” said Professor Stefan. “I have to explain this often. So, imagine that, every time you made a decision, like whether to come here to work with us—”

“So a bad decision?”

“Yes. No. Maybe. Anyway, suppose that every time you made a decision, the universe branched off, and another universe came into being. So there’s this universe, the universe in which you work here, and there’s another universe, in which—”

“In which I work somewhere there are no ghosts,” said Brian. “Sorry, no not-ghosts.”

“Precisely. Now, if you think about all the decisions and actions that you take in a single day, suddenly time begins to seem a lot more complicated, doesn’t it, with lots and lots of lines running alongside each other. And perhaps they’re not straight lines either. Perhaps they tangle and cross over at points, just like those twigs. And sometimes, if the circumstances are right, we get a glimpse of one of those other universes, those alternative realities.”

“And you believe that’s what’s happening here?”

“It’s a possibility,” Professor Stefan said. He decided not to mention that some of these universes might not contain just other, equally slow, versions of Brian, but potentially destructive beings. He was making some progress with Brian, and didn’t want to spoil it all by introducing nameless horrors from the beyond.

“But how has this happened?” said Brian.

Professor Stefan shifted awkwardly on his seat.

“What may have occurred—and I stress ‘may,’ because we don’t want people blaming us for things that we might not have done, and especially not for things that we might actually have done—is that, in the course of the Collider experiments, the skins separating some of the universes within the Multiverse might have been worn a little thin, thus enabling us to peer through them into other realms.”

“Weren’t we talking about twigs a moment ago?”

“We were, but forget the twigs. We’re back on skins.”

“So why can’t the people in these other universes see us when we see them?”

Brian really was asking the most awkward questions, thought Professor Stefan. He began mentally weighing his empty patience bucket and practicing his swing.

“Think of them as those windows in police stations that look like plain old mirrors on one side but, if you’re sitting on the other side, allow you to watch suspects being questioned.” Professor Stefan had just thought of this explanation, and was quite pleased with it, even if it meant moving from twigs to skins to police stations. “That would explain why we can see them, but they can’t see us.”

“Oh,” said Brian.

It made a kind of sense, in a not very sensible way.

“So we’re not going to talk about ghosts anymore, okay?” said Professor Stefan.