The Big Picture
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guesses for what the world is made of, the fundamental stuff that the quan-
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tum theory of reality describes.
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And that’s almost true, but not quite. Our best theory of the world— at
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least in the domain of applicability that includes our everyday experience—
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takes unification one step further, to say that both particles and forces arise
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a specific location in space, a field is something that stretches all throughout
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space, taking on some particular value at every point. Modern physics says
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that the particles and the forces that make up atoms all arise out of fields.
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That viewpoint is called quantum field theory. It’s quantum field theory that
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gives us confidence that we can’t bend spoons with the power of our minds,
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and that we know all of the pieces of which you and I are made.
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And what are the fields made of? There isn’t any such thing. The fields
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are the stuff that everything else is made of. There could always be a deeper
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level, but we haven’t found it yet.
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It’s easy enough to accept that the forces of nature arise from fields filling
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space. It was our old friend Pierre- Simon Laplace who first showed that
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Newton’s theory of gravity could be thought of as describing a “gravita-
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tional potential field” that was pushed around by, and in turn pulled back
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on, objects moving through the universe. Electromagnetism, the theory put
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together in the nineteenth century by Scottish physicist James Clerk Max-
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well and his contemporaries, provides a unified description of electric and
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magnetic fields.
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But what about the particles? Particles and fields seem like they’re dia-
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metrically opposed to each other— particles live at one spot, while fields live
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everywhere. Surely we’re not going to be told that a particle like an electron
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comes out of some “electron field” filling space?
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That is exactly what you are going to be told. And the connection is
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provided by quantum mechanics.
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The fundamental feature of quantum mechanics is that what we see
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when we look at something is different from how we describe the thing
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when we’re not looking at it. When we measure the energy of an electron
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orbiting a nucleus, we get a definite answer, and that answer is one of a
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specific number of allowed outcomes; but when we’re not looking at it, the
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state of the electron is generally a superposition of all those possible out-
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comes.
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Fields are exactly the same way. According to quantum field theory,
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there are certain basic fields that make up the world, and the wave func-
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tion of the universe is a superposition of all the possible values those fields
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can take on. If we observe quantum fields— very carefully, with suffi-
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ciently precise instruments— what we see are individual particles. For elec-
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tromagnetism, we call those particles “photons”; for the gravitational
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field, they’re “gravitons.” We’ve never observed an individual graviton, be-
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cause gravity interacts so very weakly with other fields, but the basic struc-
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ture of quantum field theory assures us that they exist. If a field takes on a
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constant value through space and time, we don’t see anything at all; but
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when the field starts vibrating, we can observe those vibrations in the form
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of particles.
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There are two basic kinds of fields and associated particles: bosons and
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fermions. Bosons, such as the photon and graviton, can pile on top of each
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other to create force fields, like electromagnetism and gravity. Fermions
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take up space: there can only be one of each kind of fermion in one place at
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one time. Fermions, like electrons, protons, and neutrons, make up the ob-
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jects of matter like you and me and chairs and planets, and give them all the
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property of solidity. As fermions, two electrons can’t be in the same place
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at the same time; otherwise objects made of atoms would just collapse to a
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microscopic size.
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The ordinary stuff out of which you and I are made, as well as the Earth
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and everything you see around you, only really involves three matter
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particles and three forces. Electrons in atoms are bound to the nucleus by
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electromagnetism, and the nucleus itself is made of protons and neu-
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trons held together by the nuclear force, and of course everything feels the
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force of gravity. Protons and neutrons, in turn, are made out of two kinds
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of smaller particles: up quarks and down quarks. They are held together by
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the strong nuclear force, carried by particles called gluons. The “nuclear
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force” between protons and neutrons is a kind of spillover of the strong
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nuclear force. There’s also a weak nuclear force, carried by W and Z bosons,
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which lets other particles interact with a final kind of fermion, the neu-
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trino. And the four fermions (electron, neutrino, up and down quarks) are
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just one generation out of a total of three. Finally, in the background
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lurks the Higgs field, responsible for giving masses to all the particles that
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have them.
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electron
Higgs field
(in background)
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electromagnetism
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(photons)
proton
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up
&
nbsp; up
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down
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strong nuclear
weak nuclear
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force (gluons)
force (W/Z)
neutrino
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gravity
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(gravitons)
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The fields, and associated particles, that make up our everyday world.
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The basic collection of fields and their associated particles is illustrated
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in the figure, a more sophisticated version of the illustration of a hydrogen
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atom from chapter 20. The two heavier generations of fermions aren’t in-
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cluded, as they tend to decay away extremely quickly. The particles we’ve
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shown here are the only ones that stick around long enough to make up
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everyday objects; the full set is discussed in the Appendix.
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Physicists divide our theoretical understanding of these particles and forces
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into two grand theories: the standard model of particle physics, which in-
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cludes everything we’ve been talking about except for gravity, and general
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relativity, Einstein’s theory of gravity as the curvature of spacetime. We lack 30
a full “quantum theory of gravity”— a model that is based on the principles
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of quantum mechanics, and matches onto general relativity when things
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become classical- looking. Superstring theory is one promising candidate for
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such a model, but right now we just don’t know how to talk about situations
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where gravity is very strong, like near the Big Bang or inside a black hole, in
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quantum- mechanical terms. Figuring out how to do so is one of the greatest
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challenges currently occupying the minds of theoretical physicists around
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the world.
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But we don’t live inside a black hole, and the Big Bang was quite a few
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years ago. We live in a world where gravity is relatively weak. And as long as
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the force is weak, quantum field theory has no trouble whatsoever describ-
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ing how gravity works. That’s why we’re confident in the existence of gravi-
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tons; they are an inescapable consequence of the basic features of general
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relativity and quantum field theory, even if we lack a complete theory of
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quantum gravity.
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The domain of applicability of our present understanding of quantum
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gravity includes everything we experience in our everyday lives. There is,
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therefore, no reason to keep the standard model and general relativity sepa-
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rate from each other. As far as the physics of the stuff you see in front of you
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right now is concerned, it is all very well described by one big quantum field
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theory. Nobel Laureate Frank Wilczek has dubbed it the Core Theory. It’s
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the quantum field theory of the quarks, electrons, neutrinos, all the families
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of fermions, electromagnetism, gravity, the nuclear forces, and the Higgs.
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In the Appendix we lay it out in a bit more detail. The Core Theory is not
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the most elegant concoction that has ever been dreamed up in the mind of
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a physicist, but it’s been spectacularly successful at accounting for every
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experiment ever performed in a laboratory here on Earth. (At least as of
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mid- 2015— we should always be ready for the next surprise.)
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In the previous chapter we concluded that “what the world is” is a quan-
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tum wave function. A wave function is a superposition of configurations of
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stuff. The next question is “What is the stuff that the wave function is a
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function of?” The answer, as far as the regime of our everyday life is con-
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cerned, is “the fermion and boson fields of the Core Theory.”
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We don’t need nearly all of the Core Theory to describe almost all of our
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everyday lives. The heavier fermions decay away very quickly. The Higgs
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field lurks in the background, but to make an actual Higgs boson— the par-
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ticle that you see when the Higgs field starts vibrating— requires a $10-
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billion particle accelerator like the Large Hadron Collider in Geneva, and
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even then the particle decays in about a zeptosecond. Neutrinos are all
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around us, but the weak nuclear force is so weak that they are very hard to
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detect. The sun is emitting neutrinos like mad, so that about a hundred
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trillion of them pass through your body every second, but I suspect you’ve
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never noticed.
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Almost all of human experience is accounted for by a very small number
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of ingredients. The various atomic nuclei that we find in the elements of the
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periodic table; the electrons that swirl around them; and two long- range
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forces through which they all interact, gravity and electromagnetism. If you
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want to describe what goes on in rocks and puddles, pineapples and
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armadillos— that’s all you need. And gravity, let’s face it, is pretty simple.
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Everything pulls on everything else. All of the real structure and complex-
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ity we see in the world come from electrons (and the fact that they can’t lie
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on top of each other) interacting with nuclei and with other electrons.
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There are exceptions, of course. The weak nuclear force plays an impor-
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tant role in nuclear fusion, which powers the sun, so we wouldn’t want to
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do without that. Muons, which are the heavier cousins of electrons, can be
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produced when cosmic rays hit the Earth’s atmosphere, and may be in-
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volved in the rate at which DNA mutates, and therefore in the evolution of
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life. These and other phenomena are important to keep track of— and the
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Core Theory does a fantastic job accounting for them. But the vast majority
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of life is gravity and electromagnetism pushing around electrons and nuclei.
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br />
We can be confident that the Core Theory, accounting for the sub-
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stances and processes we experience in our everyday life, is correct. A thou-
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sand years from now we will have learned a lot more about the fundamental
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nature of physics, but we will still use the Core Theory to talk about this
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particular layer of reality. From the perspective of poetic naturalism, there
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is one story of reality we can tell with confidence, in a well- defined domain
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of applicability. We can’t be metaphysically certain of this; it’s not some-
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thing we can prove mathematically, since science never proves things. But
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in any good Bayesian accounting, it seems overwhelmingly likely to be true.
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The laws of physics underlying everyday life are completely known.
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The Stuff of Which We Are Made
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Quantum field theory is an immensely powerful framework. If
Godzilla and the Hulk had a baby, and that baby was a frame-
work describing a certain kind of physical theory, that baby would
be quantum field theory.
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“Powerful” doesn’t mean “capable of smashing cities to rubble.” (Al-
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though quantum field theory is that, since it’s the only way we have of de-
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scribing one kind of particle transforming into another one, which is a
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crucial part of nuclear reactions and therefore nuclear weapons.) When
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we’re talking about scientific theories, powerful actually means restrictive—
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a powerful theory is one in which there are many things that simply cannot
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happen. The power we’re talking about here is the ability to start with very
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few assumptions and draw conclusions that are reliable and wide- ranging
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in their scope. Quantum field theory doesn’t knock down buildings lying
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in its path; it knocks down our speculations about what kinds of things can
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happen in physical reality.
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The claim we’re making is pretty audacious:
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Claim: The laws of physics underlying everyday life are com-
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pletely known.