“But surely that’s just a metaphor,” said Glenda, a student Kyle sometimes thought he might have pursued had he not already met Heather. “Surely there’s really only one universe, no?”
“Or,” said D’Annunzio, a biker type who always seemed out of place in class, “even if another universe does exist, there’s no way to prove it, so it’s not a falsifiable hypothesis, and therefore not real science.”
Papineau grinned broadly. “You know,” he said, “if this were a nightclub performance, people would accuse me of having planted the two of you in the audience. Let’s look at that question: is there any direct evidence that multiple universes might exist? Roopshand, will you get the lights, please?”
A student in the back stood up and turned off the lights. Papineau moved next to a slide projector sitting on a metal cart; he turned it on. A diagram appeared on the screen.
“This picture shows some experimental apparatus,” said Papineau. “At the top, we have a lightbulb. In the middle there’s a bar representing a horizontal wall as seen from above. You see those two breaks in the bar? Those are two vertical slits that go right through the wall—one on the left and one on the right.” He used a small telescoping pointer to indicate these. “And at the bottom we have a horizontal line representing a sheet of photographic film seen edge-on from above. The wall in the middle is like Queen’s Park, and the two slits are like the two possible paths around the Parliament Buildings—one on the east and one on the west.” He paused while the students digested this. “Now, what happens when we turn on the light-bulb?”
He pushed a key; the carousel clicked around and a new slide came on. The photographic film at the bottom showed a zebra pattern of light and dark lines.
“You all know what that is from high-school physics, right? It’s an interference pattern. Light from the bulb, traveling like a wave, passes through the two slits—which behave now like two separate light sources, each with waves of light emanating from it. Well, when the two sets of waves crash against the photographic plate, some of the waves cancel out, leaving dark areas, and others reinforce each other, making the bright bands.”
Some students nodded.
“But you also know from high-school physics that light doesn’t always behave like a wave—sometimes it behaves like a particle, too. And, of course, we call particles of light ‘photons.’ Now, what happens if we turn down the power going to the lightbulb? What happens when the power is turned down so low that photons are coming out of the lightbulb one at a time? Anyone?”
A redheaded woman held up a hand.
“Yes, Tina?” said Papineau.
“Well, if only one photon is going through, then it should make one little spot of light on the photographic film—assuming it finds its way through one of the slits.”
Papineau smiled. “That’s what you’d expect, yes. But even when photons are released one at a time, you still get the light and dark bands. You still get interference patterns.”
“But how can you get interference if there’s only one particle passing through at a time?” asked Kyle. “I mean, what’s the particle interfering with?”
Papineau raised his index finger. “That is the question! And there are two possible answers. The one that’s simply weird is that in transit between the lightbulb and the film, the single photon breaks up into a series of waves, some of which go through one slit and some through the other, forming the interference pattern.
“But the other answer—the really interesting answer—is that the photon never breaks up, but rather remains a discrete particle, and as such, it has no choice but to go through only one of the two possible slits—in this universe. But just as you, Kyle, could have taken either route around Queen’s Park, so the photon could have taken the path through either slit—and in a parallel universe, it took the other path.”
“But how come we see the interference pattern?” asked D’Annunzio, chewing gum as he spoke. “I mean, if we stood south of the Parliament Buildings, we’d never see two versions of Graves, one coming around the east side and one around the west.”
“Excellent question!” crowed Papineau. “The answer is that the two-slit experiment is a very special example of parallel universes. The original single universe splits into two universes once the photon encounters the slits, but the two universes exist separately only so long as the photon is traveling. Since it makes no difference now or ever which path the photon actually took, the universes collapse back together into a single universe. The only evidence that the two universes ever existed is the interference pattern left behind on the film.”
“But what if it does make a difference which slit was chosen?” asked Roopshand from the back.
“In any experiment you can devise in which the choice of slit actually matters—indeed, in any experiment in which you can detect which slit the photon went through—you don’t get the interference pattern. If it matters at all, the universes never stitch back together into one; they continue on as two separate universes.
It had been a heady class—as all of Papineau’s were. And it had also been a metaphor that Kyle carried with him throughout his life: choices, branching paths.
Back then, back in 1996, even though he and Heather were still students, he knew which choice he wanted. He wanted to live in the universe in which they did have a baby.
And so that November, their first child, Mary Lorraine Graves, was born.
5
Kyle was walking along Willcocks Street, heading from New College back toward Mullin Hall, but he was accosted before he could cross St. George.
“Sir—excuse me. Sir, pardon me! Yes, you. Dale Wong, City-TV. We’d like to ask you a question.”
“A streeter?” said Kyle, the word coming to him from somewhere.
The young man with the camcorder was amused. “Exactly, sir. A streeter. Here’s our question. Today is the tenth anniversary of the receipt of the first radio message from Alpha Centauri.”
“Is it really?”
“Yes, sir. How has it affected you this past decade, knowing that there’s intelligent life elsewhere in the universe?”
Kyle frowned, thinking. “Well, that’s a good question. It’s certainly interesting—my wife actually works on trying to decode the alien radio messages.”
“But how has it changed you—changed your outlook?”
“Well, I suppose it gives me a little perspective on things. You know—all our problems don’t amount to much, compared to the limitless universe.” The words rang false as they came out. Kyle paused—long enough, he knew, that the man wouldn’t be able to use the video clip without editing. “No, no, that’s not it. You want the truth? It hasn’t changed a damn thing. No matter how big the universe gets, we’re always looking inside.”
“Thank you, sir. Thank—Ma’am! Ma’am! A moment of your time, please!”
Kyle continued to walk. He hadn’t really thought about it before, but his current research project clearly had had its genesis back in the spring of 1996, the same day he’d learned that Heather was pregnant.
“So,” Professor Papineau had said, “the interference patterns that result when a single photon passes through two slits might be proof of the existence of multiple universes. But what, you may ask, does this have to do with computing?” He beamed at his seminar students.