Factoring Humanity Read Online Free PDF

then. As today, it was red, even though his hair was black. But in those days he’d kept it scruffy, unkempt—never trimming it, never shaving his neck beneath. He cringed now to think about it. “Down the west,” he said, shrugging to convey that it was a purely arbitrary selection.
    “A fine choice,” said Papineau. “But it’s not the only choice. And in the many-worlds interpretation of quantum mechanics, we believe that any time a choice can be made one way, the alternative choice is also made—but in a parallel universe. If Kyle did indeed come down the west side in this universe, there would also exist a parallel universe in which he came down the east side.”
    “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