“Go ahead,” I said. “I’ll get the drinks.”
Xander moved to a nearby bench while I bought us two gourmet hot chocolates and brought them over. I sat down just as he was hanging up.
“They say he’s doing well,” Xander said. “Cooperating and in high spirits. I couldn’t ask for a better report, all things considered.”
“I’m so glad,” I said, handing him the cup of hot chocolate with whipped cream.
“There’s sort of a beautiful, terrible irony in a man struggling with dementia while explaining to his doctors about his quest for a unified theory.”
“I know.” I gave Xander’s hand a squeeze. “But honestly, he should’ve just asked me. I figured that out ages ago.”
Xander started to smile. “Oh yeah?”
“Sure. It’s pretty simple, really. No long math problems required.”
“Do tell.”
I sipped my hot chocolate. “Magic.”
“That’s it?”
“Yep.”
Xander chuckled. “Well, Einsteindidcall it ‘spooky,’ though not in a complimentary way.”
“See? If Einstein agrees with me, then it must be true. So there you go. Theory of Everything, solved. Your dad is free to quote my work in his paper. I don’t mind.”
Now Xander was laughing, which was exactly my goal in the first place.
“To be honest, there are theories that sound pretty fantastical when it comes to solving this stuff,” he said. “Like the many-worlds theory.”
“Oh, I like the sound of that. Explain.”
“Remember Schrödinger’s cat?”
“I remember you get adorably grouchy about it,” I teased. “And yes. It’s both alive and dead until you look at it.”
“Right. A superposition of both states until observed. According to the MWI, or many-worlds interpretation, the particle—or cat—isn’t only in a superposition, but also represents the coexistence of multiple outcomes across different universes.”
“It does what now?”
“For example, flip a coin. While it’s in the air, it’s in a superposition of both heads and tails. According to the MWI, when it lands, the universe splits into two branches: one where it landed on heads and another where it landed on tails. We experience only one outcome because we exist in a single branch of what is actually a vast multiverse.”
I gave him a doubtful look. “That’s an actual quantum theory?”
“Sure is,” Xander said. “You can then extrapolate the coin flip for literally anything that results in an outcome. Meaning, the universe splits any time we make any decision at all.”
“So there might be different versions of us in different lifetimes, living out the repercussions of every choice we did or didn’t make? Like parallel universes?”
“Theoretically.”
“Theoretically,” I said, “there is a universe in which I received the letters you wrote to me when we were ten. And I wrote you back, and we kept in touch the entire seven years. We shared all of our deepest thoughts and secrets, and maybe we…maybe we even fell in love. So that when you moved to Castle Hill, we didn’t waste a second. You were waiting for me on our rock, and I flew at you, and we kissed even though we hadn’t seen each other in ages, but it didn’t matter because we knew each other and maybe we always have.” I inhaled a breath in the cool air. “Is that possible? And don’t say theoretically.”
“Hypothetically, yes,” Xander said, smiling. “If you take the MWI theory out of the quantum world and apply it to ours, there are infinite possibilities with infinite iterations of us. But that means millions of versions of us who never found each other—universes where we never met. Or where I didn’t write letters. Or I wrote them, and you decided not to reply. Or I never moved to Castle Hill because my dad stayed healthy. Or my mom never left us.”
“And Grant never died,” I murmured, and shook my head. “It’s too much. You could get lost in a maze ofwhat ifsthat all lead to regret. Wanting what can never be.”
Xander nodded. “You could.”
