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squirunny.”
    “I prefer ‘quantum particle,’ but I guess that’s the basic idea. Everything in the universe is built of these quantum particles.”
    “That’s pretty weird.”
    “Oh, that’s just the beginning of the weird stuff . . .”

* Sir Isaac Newton, of the falling apple story, set forth three laws of motion that govern the behavior of moving objects. The first law is the principle of inertia, that objects at rest tend to remain at rest, and objects in motion tend to remain in motion unless acted on by an external force. The second law quantifies the first, and is usually written as the equation
F = ma,
force equals mass times acceleration. The third law says that for every action there is an equal and opposite reaction—a force of equal strength in the opposite direction. These three laws describe the motion of macroscopic objects at everyday speeds, and form the core of classical physics.

CHAPTER 2
Where’s My Bone? The Heisenberg Uncertainty Principle
    I’m grading papers on the couch when Emmy comes into the room, looking concerned. “What’s the matter?” I ask.
    “I can’t find my bone,” she says. “Do you know where my bone is?”
    “I have no idea where your bone is,” I say, “but I can tell you exactly how fast it’s moving.”
    There’s silence in response, and when I look up, she’s staring at me blankly.
    “It’s a physics joke,” I explain, because that always makes things funnier. “You know, Heisenberg’s uncertainty principle? The uncertainty in the position of an object multiplied by the uncertainty in the momentum is greater than Planck’s constant over four pi? Which means that when one uncertainty is small, the other must be very large.”
    Now she’s glaring at me, almost growling. “Stop doing that!” she says.
    “What? It’s not all that funny, but it wasn’t that bad.”
    “It’s your fault that I can’t find my bone.”
    “How is it my fault?”
    “You went and measured how fast it’s moving, and the position got all uncertain. And now I can’t find my bone.”

    “That’s not what happened,” I say. “The uncertainty principle doesn’t work like that.”
    “Yes it does. You just said. You know how fast my bone is moving, and now I can’t find it.”
    “First of all, that was a joke. I didn’t really measure the velocity of your bone. Second, that’s a slightly mistaken view of the uncertainty principle. It’s not just that measurement changes the state of the system, it’s that what we
can
measure is limited by the fact that position and momentum are undefined until we measure them.”
    She looks puzzled. “I don’t see the difference.”
    “Well, in the picture where you attribute everything to the effects of measurement, you implicitly assume that whatever you’re measuring has some definite and well-defined properties, and the uncertainty in those values arises only from perturbations that occur through the act of measuring them. That’s not what happens, though—in quantum theory, there are no definite values for those quantities. They’re not uncertain because of limits on your measurement, they’re uncertain because they are not defined, and they can’t
be
defined, due to the quantum nature of reality.”
    “Oh.” She looks thoughtful for a moment, then resumes glaring. “I think you lost my bone, and you’re just trying to weasel out of this by being all confusing.”
    “No, that’s really how the theory works. It’s a moot point, though, since even if I had perturbed the position of your bone by measuring its velocity, there’s no way that would’ve prevented you from finding it.”
    “Yeah? Why not?”
    “Well, because the uncertainty involved would be tiny. I mean, your bone has a mass of a couple hundred grams, and if I measured its velocity to within one millimeter per second, that would give an uncertainty in position of only about 10 -31 meters. That’s a trillionth of the size of a