"We'll try not to," Ed said.
"Okay. First off, the universe you're in—the universe we're in right at this moment—is only one of an infinite number of possible universes whose existence is allowed for within quantum physics. Every time we spot an electron in a particular position, for example, our universe is functionally defined by that electron's position, while in the alternate universe, that electron's position is entirely different. You following me?"
"Not at all," said Ed.
"You nonscientists. Well, just trust me on it, then. The point is: multiple universes. The multiverse. What the skip drive does is open a door to another one of those universes."
"How does it do that?" I asked.
"You don't have the math for me to explain it to you," Alan said.
"So it's magic," I said.
"From your point of view, yes," Alan said. "But it's well allowed in physics."
"I don't get it," Ed said. "We've been through multiple universes then, yet every universe we've been in has been exactly like ours. Every 'alternate universe' I ever read about in science fiction has major differences. That's how you know you're in an alternate universe."
"There's actually an interesting answer to that question," Alan said. "Let us take as a given that moving an object from one universe to another is a fundamentally unlikely event."
"I can accept that," I said.
"In terms of physics, this is allowable, since at its most basic level, this is a quantum physics universe and pretty much anything can happen, even if as a practical matter it doesn't. However, all other things being equal, each universe prefers to keep unlikely events to a bare minimum, especially above the subatomic level."
"How does a universe 'prefer' anything?" Ed asked.
"You don't have the math," Alan said.
"Of course not," Ed said, rolling his eyes.
"But the universe does prefer some things over others. It prefers to move toward a state of entropy, for example. It prefers to have the speed of light as a constant. You can modify or mess with these things to some extent, but they take work. Same thing here. In this case, moving an object from one universe to another is so unlikely that typically the universe to which you move the object is otherwise exactly like the one you left—a conservation of unlikeliness, you might say."
"But how do you explain us moving from one place to another?" I asked. "How do we get from one point in space in one universe, to an entirely different point in space in another?"
"Well, think about it," Alan said. "Moving an entire ship into another universe is the incredibly unlikely part. From the universe's point of view, where in that new universe it appears is really very trivial. That's why I said that the word 'drive' is a misnomer. We don't really go anywhere. We simply arrive."
"And what happens in the universe that you just left?" asked Ed.
"Another version of the Modesto from another universe pops right in, with alternate versions of us in it," Alan said. "Probably. There's an infinitesimally small chance against it, but as a general rule, that's what happens."
"So do we ever get to go back?" I asked.
"Back where?" Alan said.
"Back to the universes where we started from," I said.
"No," Alan said. "Well, again, it's theoretically possible you could, but it's extremely unlikely. Universes are continually being created from branching possibilities, and the universes we go to are generally created almost instantly before we skip into them—it's one of the reasons why we can skip to them, because they are so very close to our own in composition. The longer in time you're separated from a particular universe, the more time it has to become divergent, and the less likely you are to go back to it. Even going back to a universe you left a second before is phenomenally unlikely. Going back to the one we left over a year ago, when we first skipped to Phoenix from Earth, is really out of the question."
"I'm depressed," Ed said. "I liked my universe."
"Well, get this, Ed," Alan said. "You don't even come from the same original universe as John and I, since you didn't make that first skip when we did. What's more, even the people who did make that same first skip with us aren't in the same universe as us now, since they've since skipped into different universes because they're on different ships—any versions of our old friends that we meet up with will be alternate versions. Of course, they will look and act the same, because except for the occasional electron placement here and there, they are the same. But our originating universes are completely different."
"So you and I are all that's left of our universe," I said.
"It's a pretty good bet that universe continues to exist," Alan said. "But we are almost certainly the only two people from it in this universe."