One recent morning, I went for a walk and saw a dog. I suppose he had an owner—someone must have held the other end of his leash. But I noticed only the dog: his white fluffy coat with large chestnut-and-black markings like saddlebags on his broad, barrel-shaped body. He grinned at me to share his joy at the miracle of being on a walk—walking! going for a walk!—on this sunny, breezy, every-day-is-wonderful-because-I’m-a-dog-outdoors day. Known to me as just-a-random-Wednesday.
His laughing face (I refused to look carefully at his ears) made my breath catch in my throat. If he weren’t on a leash, and if I squatted and opened my arms wide, would he bound up to me and drop a ball at my feet, the way he used to?
And then I stopped myself: the dog I saw wasn’t Sparky. Sure, he looked a lot like him, and if his prancing gait was any indication, he had a similar personality. But he couldn’t be my dog. I put my broken heart into a box and closed the lid.
Then I ran a thought experiment—a flight of fancy followed to its natural conclusion, an elaborate game of “what if?” Thought experiments can investigate the very nature of the world using nothing other than the human imagination. A thought experiment can propose, challenge, or support a theory; it can clarify an abstraction or illustrate a difficult idea.
Through thought experiments, philosophers and scientists have investigated important questions such as whether the universe has a boundary, how objects fall through space, and how gravity affects the orbits of moons and planets.
One of the most famous thought experiments involves not a dog in a box, but a cat. In 1935, Austrian physicist Erwin Schrödinger proposed an experiment to explore the nature of quantum phenomena—physical interactions of subatomic particles like photons and electrons. In his thought experiment, a cat is placed in a sealed box with a flask of poison, a source of radioactivity, a Geiger counter, and a hammer. If the Geiger counter detects any radioactive decay, it triggers the hammer, which shatters the flask of poison, which kills the cat. But if the radioactive material doesn’t decay, the flask of poison remains intact, and the cat remains alive.
One way of thinking about quantum events, known as the Copenhagen interpretation, says that the cat in the box remains simultaneously both dead and alive until someone observes the outcome. Until that moment of observation, all possible outcomes exist—in the case of a cat, it’s both alive and dead. In the case of a photon, it’s both a particle and a wave.
I first met Sparky when I started dating Mike. Sparky was already two years old by then, so I experienced his puppyhood through Mike’s stories. Even though Sparky was the runt of a very mixed-breed litter, Mike had picked him because Sparky had scampered up to Mike with confidence. In truth, Sparky had picked Mike, right from the start.
Their early time together had its moments. As a young dog, Sparky loved to chew. One morning in early spring, FedEx left a large box of CDs inside the gate to Mike’s yard, and Sparky “opened” it. A snowstorm passed through that afternoon, leaving behind several inches of snow. Mike come home to a yard full of snow-covered lumps. A few CDs appeared when the snow finally melted the next week. For months, Mike found corrugated cardboard bits under bushes. The following summer, he unearthed a stray jewel box that had been partially buried in the flowerbed.
“Darn dog,” Mike said, always with affection, at the end of the story. Because in spite of Sparky’s destructive streak, he was impossible to resist: his cheerful white-furred face, with its one chestnut ear folded at a jaunty angle toward his hazel eyes, made up for a lot.
Though I’d missed Sparky’s puppyhood, I had no trouble falling in love. And within a few months, Mike and I were also comfortably committed to each other. Sparky seemed to like having me around. Two humans doubled his chances that someone would throw something for him to bring back. He loved Fetch, using his upright, waving-flag tail to show his pride in being a good boy, such a good boy.
But scientists can’t control what happens to their experimental results—even the results of a thought experiment.
Schrödinger, for example, intended his cat-in-a-box thought experiment as a criticism of the Copenhagen interpretation. A living-and-dead cat is obviously impossible, according to how we experience the world.
Still, the experiment became popular for reasons other than the absurdity of particles existing in more than one state at a time. For most of us, the setup—a cat, a flask, a radioactive substance, a hammer, a Geiger counter, a box—is much easier to understand than relationships among particles we can’t see.
Eighty years later, not much has changed. The observable world still apparently follows different laws than the quantum one.
And although I wanted, on that afternoon walk, to control my thought experiment, I couldn’t do it. I couldn’t think only of Sparky without including Mike, the person usually at the other end of his leash.
Mike never said, “Love me, love my dog,” but the result was the same: Sparky was always part of our life together. One long weekend soon after we started dating, the three of us took a quick vacation in the mountains. Sparky panted over our shoulders from the back seat, curling up for a nap and then standing again to look out the window in the hope of spotting a deer to bark away.
We stopped for gas, and Mike went into the store to pay while I stayed in the car with Sparky. The moment Mike disappeared from his sight, Sparky began whining.
“It’s okay, boy.” I laid my hand on his head. “I’m still here.”
Feet shifting in his Sit, he glanced at me—who cares, he all but said—and whimpered louder. I scratched behind his ears. He shook off my hand and stood up to check the windows—none open, no way to get out and find Mike. He sagged back into a disappointed, silent Sit. He looked at me.
“Good boy,” I said. He sighed deep in his throat, ears on alert, except for the top of his left one, which folded down in its oh-so-adorable way. He as much as said, Sure, he’s always come back, but what if this time is different? For Sparky, this event could have a different outcome.
When Mike emerged from the convenience store, Sparky yelped with joy. I knew then that however much Sparky loved me—and he did, I was sure—Sparky was Mike’s dog. To him, Mike came first.
I understood Sparky’s devotion: Mike came first in my life, and I came first in Mike’s. That’s how relationships worked. You have each other’s best interests at heart when you make decisions. Or so I thought.
It comforts me to know that scientists debate the nature of the particles, visible and invisible, that make up our world. I like knowing that they don’t have all the answers, either. The world is a complex place, no matter how many ways we try to simplify it.
Sometimes, a group of subatomic particles interacts in a way that makes it impossible to determine the quantum state of the individual particles. Instead, you can describe the quantum state only of all of the particles together—the system as a whole. Schrödinger gave this form of interaction the German word Vershränkung, which he translated as entanglement.
Sparky, Mike, and me. Our life together felt so … inevitable. Mike and I both loved music. We talked together, listened to each other. Events followed, one upon another, and led to the expected outcome: we got married and had a puppy, Gizmo. Gizmo was pleasant enough to Mike and me but remained 100% Sparky’s dog through the years.
The four of us played outdoors together, sledding and hiking. We made supper and piled on the couch to watch TV. We made a family, loving and happy together.
The earth turned on its axis and completed orbits around the sun. My aging parents lived 800 miles away—800 miles that felt like 800 light-years—and I made frequent trips to visit them. There, my father cared for my mother, whose Alzheimer’s rapidly changed her personality and her abilities. I worried about them and supported them as best I could, but my nuclear family was with Mike.
Any time a suitcase appeared, Sparky whimpered and moped. He rallied quickly when I was the only one packing, but when Mike went on a business trip, Sparky spent much of the time resting his chin on the bedroom windowsill, waiting. Sometimes I’d sit next to him and lean against him. He’d lean back against me for a bit. Then, with a sigh, he’d bring me his ball for Fetch, for which he mustered about 85% enthusiasm.
I didn’t take it personally. I missed Mike, too.
Einstein is said to have called entanglement “Spooky action at a distance.” I amuse myself by thinking of it as “Sparky action at a distance.”
In German, Vershränkung can also refer to the act of folding together. For example, as in the clasping of hands, perhaps across a table at a special anniversary celebration, or while two people walk through a sunny park together.
Also as in the crossing of your arms over your chest, perhaps while listening to a spouse’s slurred explanation after a late arrival home, or while sitting in a marriage counselor’s office, apportioning blame.
During those trips around the sun, Mike’s father died suddenly. My mother’s death was no less devastating for being expected. Both losses created new worlds for us to adjust to—but neither death caused the problems between us.
Instead, at some point, a single atom decayed and a hammer shattered a flask of poison that killed our marriage. Or maybe something else happened. Even in hindsight, I can’t pinpoint one moment, one decision or choice that led inevitably to catastrophe.
And I can’t de-tangle the elements of the system. I can’t separate Sparky and Gizmo from Mike, no matter how many thought experiments I do.
Perhaps the individual elements of a thought experiment don’t really matter. Perhaps the outcome is all that’s important.
One necessary, extremely helpful element of Schrödinger’s thought experiment was a time limit. He specified that the experiment was to last only an hour—long enough that some radioactive decay might occur, but not so long that decay would occur. Plus, without a time limit, the cat would eventually die, one way or another. As do we all.
How long do you give someone—grieve someone—husband or dog? After years of trying to fix our family, I accepted just how many light-years had come between Mike and me. In the version of the universe in which that marriage existed, the box was open, and our marriage had died.
One late-August morning, with the dogs keeping me company, I assembled boxes and began. For weeks, I sorted linens, knickknacks, and photos, making decisions that led to only one possible outcome. Gizmo crept under the bed and poked his nose out from under the dust ruffle, but Sparky sat near me. Occasionally, I’d toss his ball for him as I worked. By this time, Sparky was an old dog, almost 14, and although he was still devoted to Fetch, he could complete only a few trips before he needed a rest. Often, instead of chasing the ball, he’d simply sit and watch its arcing flight before turning back to look at me.
Eventually, my belongings—and one CD case with Sparky’s teeth marks in it—sat packed in a row of cardboard boxes. As I put the last lid on the last box, Sparky rested his chin on it for a moment before turning to lie down close beside me. He had whimpered when we raised our voices; he had listened, one ear alert and one drooping, during our ever-lengthening silences. He knew I would not be coming back.
They’ll be fine without me, I told myself. Gizmo would still have Sparky, and Sparky would have Mike. I scratched behind Sparky’s ears and dropped a kiss on the top of his dear, furry head.
Schrödinger’s thought experiment wasn’t the last theory scientists developed to explain quantum phenomena.
A newer one, the Many-Worlds theory, says that the act of observing the cat in the box doesn’t irrevocably determine one outcome from among all the possibilities. Instead, the universe creates a copy of itself for each possible outcome.
So according to the Many-Worlds theory, a universe exists in which the flask in the box is intact, and one in which it isn’t—a universe, therefore, in which the cat lives, and one in which it dies.
A decade out of that marriage, I sometimes ask myself, “How could I have left him?”—but only about Sparky or Gizmo, never about Mike. After the divorce was final, Mike and I dropped contact.
Still, I can do arithmetic. As much as I would love to believe that the dog I saw on my recent walk was Sparky, I also know it couldn’t have been. Whatever quantum theorists may posit about the possibilities of cats and boxes and decisions and outcomes, they also knew enough to place a time limit on their experiments.
But time doesn’t limit everything, and the Many-Worlds theory helps mend my heart. I now think of Sparky in a parallel universe somewhere, where dogs live as long as our love for them does. There, Sparky’s joyfully fetching a ball for someone he loves—maybe even for me.
(Names in this essay, though changed, remain entangled.)