I’ve already told them what happened at the deposition. My twin brother, Nigel, had worked on Project CYCLONE for two years by the time they recruited me. They needed a control, you see.

I don’t know the mathematics. Nigel tried to explain it to me once, but every time he got to the part about non-Euclidean geometries and pseudo-relativistic velocities, my eyes glazed over.

They’d named the project after an old roller coaster in America and built their test tracks in an abandoned amusement park. Their set-up had two symmetrical tracks. They rose side by side to a height of 100 metres and then dropped like a corkscrew or a helter-skelter.

They let me watch as they tested the track with two dogs. The dog on the right went up, looped a dozen times, and rolled to a stop a few metres from where it started. It bounded out of the carrier and ran to its trainer, looking for a treat.

The left-hand dog … how do I explain? You know how shadows shorten and lengthen depending on the Sun’s angle? Well, the roller coaster stretched and twisted like a shadow. At times the cart was long and thin enough to coil from the top of the coaster to the bottom.

The coaster rolled to a stop. The dog seemed no worse for wear, except that it would turn to its partner every so often, seemingly at random, with a look of surprise.

“Don’t worry,” Nigel said. “You’re just the control. For you, it’s just an ordinary roller-coaster ride. When we get to the bottom, I’ll be three seconds younger than you. The instruments will pick up the difference and we will advance our understanding of physics by centuries.”

I said OK, and they strapped me in. I noticed he had a book under his arm.

“You brought reading material?” I asked, over the clank of the safety cage falling into place.

“I might be a while,” he said with a grin.

I waved as we accelerated and climbed. As we made that final turn, I thought to myself, what if they got it wrong? What if I’m on the experimental track?

As we entered the first turn, I veered left, and my stomach veered right. Something inside me twisted and stretched behind me. With each turn, I saw Nigel’s car twist further out of shape. Midway through turn three, something flashed. By turn four, I saw nothing at all.

Thirty seconds later, as my car slowed on the final stretch, the emergency response team leapt into action, but they had nothing to do. Nigel’s track was empty.

They helped me out of my car and scanned me. I walked away from the crash feeling numb, until I heard his voice behind me.

“Hey!”

I turned back to the track, and it was empty but … not. Its shadow remained. I knew, without understanding how, that he would offer to take me to a restaurant, to celebrate. I went with him. I heard his footsteps beside mine for three blocks.

I knew he wouldn’t open the door when I got there. I only ordered for one. I knew ghosts didn’t eat. I wasn’t crazy.

I listened to him explain to me how our test would change physics forever, how we would be on the cover of magazines, and how we deserved the Nobel prize as much as the physicists and engineers did. I kept my wireless earpiece in so that I could nod and answer back without raising suspicion.

Later that evening, I asked him what he would say to our parents. I wrote it down, finding that his handwriting came easily to me. The next morning, I pretended to find the letter and gave it to them. I explained that he was away doing secret research and could not speak to anyone. That wasn’t a lie, was it?

The government officials notified our parents of the accident a few days later.

I cried during his memorial, not because I missed him but because I knew I was standing him up. I was supposed to be having lunch with him. As the priest started to speak, Nigel was across town, introducing me to an old college friend named Leslie. Leslie and I would get married and have three kids one day.

I decided not to contact Leslie on my own. Too awkward.

I felt Nigel’s pull less often after that. I catch snippets of our conversations at our booth in the diner or in the rooms of our parents’ house, but the context slipped away. Our paths diverged.

I pass Leslie on the street a few times a year in our small town and feel a flash of recognition. I hear my phone ring on my birthdays, but I don’t know how to answer it anymore. Not the calls from Nigel, anyway.

It boils down to sibling rivalry. I’m jealous, and why shouldn’t I be? He got there first. He lived his life, all of it, in that instant. He’s out there, stretched impossibly thin across the decades. I’m the wayward shadow that couldn’t keep up.

I leave an empty seat for him on holidays, but he never shows. He has a family too, I’ll bet. If he does visit, he’ll be waiting for me in a house I’ll never see, meant for a family I’ll never have.

The inquiry blamed miscalibration of the machines and unexpected mass in my brother’s car. They never answered the important question, though. Should I have been a dutiful shadow and followed his trail of what-ifs? Sometimes I wonder if I made the right choice.

The story behind the story

S. R. Algernon reveals the inspiration behind Entanglement.

As with my previous story, Private i, I used a physics concept as a jumping-off point for a narrative. This story isn’t about quantum entanglement; if anything, Nigel’s experience more closely resembled time and space distortions near the speed of light, Ted Chiang’s Heptapods, or Kurt Vonnegut’s Tralfamadorians.

That being said, I did want the story to echo the idea of quantum interference, that a particle can interfere with a version of itself that took a path it did not take. I wanted to play with the idea of ghosts as ripples in probability left by our alternate selves. Any life improbably cut short leaves behind ‘what-ifs’. Even in real life, it is easy for people to get lost in the echoes that other people leave behind.

I will probably revisit this idea of ghostly echoes of our ‘more probable’ selves. I hope to have time to explore the idea more, as I am sure there are many good stories in that vein.