Tandem

“It’s, um … kind of a joke,” Thomas said, with a soft snort of self-effacing laughter.

“Oh really? What’s so funny about it?”

“Not funny,” he said. “It’s just that Juliana’s KES code name is ‘Sparrow.’ Sparrows and starlings are related, I guess.”

“They’re from the same biological order,” Dr. Moss explained. “Passeriformes.”

“Fascinating.”

“It was his idea,” Thomas said, pointing at Dr. Moss. “As you can probably imagine.”

“Well, children, I haven’t got all day,” Dr. Moss said. “And I assume it wasn’t an interest in ornithology that brought you here, so why don’t you tell me what did, eh?”

Thomas drew in a deep breath. “We have a situation.”

“Oh? What sort of situation?”

“Go ahead,” Thomas said, putting a hand on the small of my back and pushing me forward. “Tell Mossie what you told me.”

I still wasn’t quite sure about Dr. Moss, but I didn’t see that I had a better option, so I explained everything as best I could. This time, it came out easier, and more intelligibly. As he listened to me describe my “situation,” excitement bloomed across Dr. Moss’s face.

“Well,” he said when I was finished, with soft but potent enthusiasm. “That is interesting.”

Just interesting? “Have you heard of that happening to other people?”

“Other people?” He tapped his chin with his fingers. “Not as such.” The wheels of Dr. Moss’s mind were turning swiftly and energetically, fueled by a new mystery, and his eyes held that same glint of excitement Granddad used to get when he was on the verge of a particularly gratifying discovery in his own work. “What do you know about parallel universes, Ms. … ?”

“Lawson,” Thomas and I provided in unison.

“Right.” His hands flapped in the air, an impatient gesture meant to spur me to answer.

“Not a lot,” I admitted. “My grandfather is a physicist, but it’s not something I was personally very interested in—up till now, anyway.”

“Hmmm.” My lack of knowledge disappointed Dr. Moss, but he soldiered on with an air of great burden. “Then I guess I’ll have to start with the basics, won’t I?”

“That would probably be best,” I said.

Dr. Moss nodded, rushing to a keyboard and typing a series of quick commands. There was a large screen on the opposite end of the laboratory that took up almost the entire wall. When Dr. Moss was finished, the screen, previously blank, held one inscrutable image.

I know that symbol, I thought with a sudden, grave certainty. I struggled to remember where I had last seen it, but I couldn’t. Was it something Granddad had once shown me? Then I realized—it was the Libertas insignia, except that instead of stars it was made up of dots.

“What’s that?” I asked, advancing toward the screen.

“No touching the equipment, if you please,” Dr. Moss scolded. I drew my hand away.

“That is a tetractys,” Dr. Moss told me. “A mathematical symbol dating back to about 500 BC. But the shape doesn’t matter—it’s what the tetractys represents that’s important.”

“And what’s that?” I asked, squinting so that I could read the type.

“Universes.” I could hear a grin in his voice when he said the word. If nothing else, Dr. Moss certainly had a flare for the dramatic.

“As we are all well aware, there is a large number—perhaps infinite number—of universes in existence,” Dr. Moss continued. “But what you may not know is that there are many different kinds of universes. What you see on the screen is a method I’ve developed for categorizing universes in their many forms.” Dr. Moss took one look at my baffled expression and said, “Allow me to explain further.”

“Please do,” I urged.

“For the purposes of this demonstration, let us consider Earth to be our home universe,” Dr. Moss said with a nod in my direction. “At the top of the pyramid, you have Earth, and all of the universes that are separated from it by what I’ve taken to calling zero degrees. These are universes that are virtually indistinguishable from Earth itself. They are nearly identical, apart from minuscule changes that have no global consequences. For instance, Thomas is wearing a blue tie today. In a zero-degree universe, he’s wearing a black tie. A small difference with no measurable impact.

“The second row is representative of all universes that are separated from Earth by one degree. These are distinguishable from Earth, but are not so different that the world appears substantially changed. For instance, if you were comparing Earth to another world with one degree of separation, at any given time the President of the United States could be a different person in each world, but there would still be a President of the United States. Do you understand?”

“I guess.”

“The third row is where it gets interesting,” Dr. Moss said. “It represents universes that are substantially differentiated from one another. At some point in time, history took a very different turn, resulting in an altered worldscape. Earth and Aurora are universes separated by two degrees.”

“Because of the Last Common Event,” I said, remembering what Thomas had told me—in Aurora, George Washington had died during the Revolutionary War, and the war was lost. As a result, history had forged a new route.

“Precisely!” Dr. Moss was growing more and more excited by the second.

“So what’s the fourth row?” I asked. Granddad would’ve thought Dr. Moss was a lunatic, but he would’ve loved his theories, and his penchant for organization.

“Universes separated by three degrees are highly differentiated from each other,” Dr. Moss said. “In these universes, something happened that was so major that it completely changed the world scape. Of course, at the moment these third-degree universes are entirely theoretical. We’re incapable of proving that they actually exist. And there might be other universes still, separated by four, five, six degrees, with entirely different laws of physics, perhaps! You can see how difficult it is to imagine what such universes might look like.”

“What does any of this have to do with analogs?”

“Everything!” Dr. Moss cried. “People are products of their environments. The more different two universes are, the more different the analogs in those universes will be from each other, not in appearance but in circumstance. In a universe where the differences are subtle—universes of zero, or even one degree—your analog is much more likely to be like you. In these universes, analogs share names and genealogical backgrounds and identities, and if you were to compare the lives of analogs in these universes, you would find that they are being lived almost entirely in parallel with your own, allowing for relatively few slight variations. Of course, there are exceptions. There are always exceptions.”

“Can I ask you something?” Dr. Moss gave me a curious look. “How come Juliana and I have different parents?”

“I’m not sure I understand your meaning,” Dr. Moss said. “You’re different people.”

“I know that, but, if we’re analogs, shouldn’t our parents be analogs, too? And our grandparents? Isn’t that how biology works?” This was something I had been wondering for a while, but no one seemed capable of explaining it to me. If anyone knew the answer, though, Dr. Moss would.

“Not necessarily,” Dr. Moss said. “Because of the LCEs, those linchpin moments that create divergent histories, analogs in second-degree universes and higher do not, for the most part, share the same genealogical backgrounds or identities. Although, again, there are outliers; analogs in second-degree universes can, in unique cases, live their lives along very similar paths, but it’s rare. And your analog in a third-degree universe is likely to be even more different from yourself than Juliana is.”

“I don’t see how that’s possible,” I said. I was a normal high school junior; Juliana was a princess. How much more different could two lives be?

“Believe me, Ms. Lawson—anything is possible.”

“I still don’t understand,” I said. “What about DNA? If we look the same, shouldn’t we come from the same