30
guesses for what the world is made of, the fundamental stuff that the quan-
31
tum theory of reality describes.
32
And that’s almost true, but not quite. Our best theory of the world— at
33
least in the domain of applicability that includes our everyday experience—
34
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
01
space, taking on some particular value at every point. Modern physics says
02
that the particles and the forces that make up atoms all arise out of fields.
03
That viewpoint is called quantum field theory. It’s quantum field theory that
04
gives us confidence that we can’t bend spoons with the power of our minds,
05
and that we know all of the pieces of which you and I are made.
06
And what are the fields made of? There isn’t any such thing. The fields
07
are the stuff that everything else is made of. There could always be a deeper
08
level, but we haven’t found it yet.
09
10
•
11
It’s easy enough to accept that the forces of nature arise from fields filling
12
space. It was our old friend Pierre- Simon Laplace who first showed that
13
Newton’s theory of gravity could be thought of as describing a “gravita-
14
tional potential field” that was pushed around by, and in turn pulled back
15
on, objects moving through the universe. Electromagnetism, the theory put
16
together in the nineteenth century by Scottish physicist James Clerk Max-
17
well and his contemporaries, provides a unified description of electric and
18
magnetic fields.
19
But what about the particles? Particles and fields seem like they’re dia-
20
metrically opposed to each other— particles live at one spot, while fields live
21
everywhere. Surely we’re not going to be told that a particle like an electron
22
comes out of some “electron field” filling space?
23
That is exactly what you are going to be told. And the connection is
24
provided by quantum mechanics.
25
The fundamental feature of quantum mechanics is that what we see
26
when we look at something is different from how we describe the thing
27
when we’re not looking at it. When we measure the energy of an electron
28
orbiting a nucleus, we get a definite answer, and that answer is one of a
29
specific number of allowed outcomes; but when we’re not looking at it, the
30
state of the electron is generally a superposition of all those possible out-
31
comes.
32
Fields are exactly the same way. According to quantum field theory,
33
there are certain basic fields that make up the world, and the wave func-
34
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-
02
ciently precise instruments— what we see are individual particles. For elec-
03
tromagnetism, we call those particles “photons”; for the gravitational
04
field, they’re “gravitons.” We’ve never observed an individual graviton, be-
05
cause gravity interacts so very weakly with other fields, but the basic struc-
06
ture of quantum field theory assures us that they exist. If a field takes on a
07
constant value through space and time, we don’t see anything at all; but
08
when the field starts vibrating, we can observe those vibrations in the form
09
of particles.
10
There are two basic kinds of fields and associated particles: bosons and
11
fermions. Bosons, such as the photon and graviton, can pile on top of each
12
other to create force fields, like electromagnetism and gravity. Fermions
13
take up space: there can only be one of each kind of fermion in one place at
14
one time. Fermions, like electrons, protons, and neutrons, make up the ob-
15
jects of matter like you and me and chairs and planets, and give them all the
16
property of solidity. As fermions, two electrons can’t be in the same place
17
at the same time; otherwise objects made of atoms would just collapse to a
18
microscopic size.
19
•
20
21
The ordinary stuff out of which you and I are made, as well as the Earth
22
and everything you see around you, only really involves three matter
23
particles and three forces. Electrons in atoms are bound to the nucleus by
24
electromagnetism, and the nucleus itself is made of protons and neu-
25
trons held together by the nuclear force, and of course everything feels the
26
force of gravity. Protons and neutrons, in turn, are made out of two kinds
27
of smaller particles: up quarks and down quarks. They are held together by
28
the strong nuclear force, carried by particles called gluons. The “nuclear
29
force” between protons and neutrons is a kind of spillover of the strong
30
nuclear force. There’s also a weak nuclear force, carried by W and Z bosons,
31
which lets other particles interact with a final kind of fermion, the neu-
32
trino. And the four fermions (electron, neutrino, up and down quarks) are
33
just one generation out of a total of three. Finally, in the background
34
lurks the Higgs field, responsible for giving masses to all the particles that
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have them.
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02
electron
Higgs field
(in background)
03
04
electromagnetism
05
(photons)
proton
06
07
up
&
nbsp; up
08
09
down
10
strong nuclear
weak nuclear
11
force (gluons)
force (W/Z)
neutrino
12
13
gravity
14
(gravitons)
15
16
The fields, and associated particles, that make up our everyday world.
17
18
The basic collection of fields and their associated particles is illustrated
19
in the figure, a more sophisticated version of the illustration of a hydrogen
20
atom from chapter 20. The two heavier generations of fermions aren’t in-
21
cluded, as they tend to decay away extremely quickly. The particles we’ve
22
shown here are the only ones that stick around long enough to make up
23
everyday objects; the full set is discussed in the Appendix.
24
25
•
26
Physicists divide our theoretical understanding of these particles and forces
27
into two grand theories: the standard model of particle physics, which in-
28
cludes everything we’ve been talking about except for gravity, and general
29
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
31
of quantum mechanics, and matches onto general relativity when things
32
become classical- looking. Superstring theory is one promising candidate for
33
such a model, but right now we just don’t know how to talk about situations
34
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
02
challenges currently occupying the minds of theoretical physicists around
03
the world.
04
But we don’t live inside a black hole, and the Big Bang was quite a few
05
years ago. We live in a world where gravity is relatively weak. And as long as
06
the force is weak, quantum field theory has no trouble whatsoever describ-
07
ing how gravity works. That’s why we’re confident in the existence of gravi-
08
tons; they are an inescapable consequence of the basic features of general
09
relativity and quantum field theory, even if we lack a complete theory of
10
quantum gravity.
11
The domain of applicability of our present understanding of quantum
12
gravity includes everything we experience in our everyday lives. There is,
13
therefore, no reason to keep the standard model and general relativity sepa-
14
rate from each other. As far as the physics of the stuff you see in front of you
15
right now is concerned, it is all very well described by one big quantum field
16
theory. Nobel Laureate Frank Wilczek has dubbed it the Core Theory. It’s
17
the quantum field theory of the quarks, electrons, neutrinos, all the families
18
of fermions, electromagnetism, gravity, the nuclear forces, and the Higgs.
19
In the Appendix we lay it out in a bit more detail. The Core Theory is not
20
the most elegant concoction that has ever been dreamed up in the mind of
21
a physicist, but it’s been spectacularly successful at accounting for every
22
experiment ever performed in a laboratory here on Earth. (At least as of
23
mid- 2015— we should always be ready for the next surprise.)
24
In the previous chapter we concluded that “what the world is” is a quan-
25
tum wave function. A wave function is a superposition of configurations of
26
stuff. The next question is “What is the stuff that the wave function is a
27
function of?” The answer, as far as the regime of our everyday life is con-
28
cerned, is “the fermion and boson fields of the Core Theory.”
29
•
30
31
We don’t need nearly all of the Core Theory to describe almost all of our
32
everyday lives. The heavier fermions decay away very quickly. The Higgs
33
field lurks in the background, but to make an actual Higgs boson— the par-
34
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
36N
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
01
detect. The sun is emitting neutrinos like mad, so that about a hundred
02
trillion of them pass through your body every second, but I suspect you’ve
03
never noticed.
04
Almost all of human experience is accounted for by a very small number
05
of ingredients. The various atomic nuclei that we find in the elements of the
06
periodic table; the electrons that swirl around them; and two long- range
07
forces through which they all interact, gravity and electromagnetism. If you
08
want to describe what goes on in rocks and puddles, pineapples and
09
armadillos— that’s all you need. And gravity, let’s face it, is pretty simple.
10
Everything pulls on everything else. All of the real structure and complex-
11
ity we see in the world come from electrons (and the fact that they can’t lie
12
on top of each other) interacting with nuclei and with other electrons.
13
There are exceptions, of course. The weak nuclear force plays an impor-
14
tant role in nuclear fusion, which powers the sun, so we wouldn’t want to
15
do without that. Muons, which are the heavier cousins of electrons, can be
16
produced when cosmic rays hit the Earth’s atmosphere, and may be in-
17
volved in the rate at which DNA mutates, and therefore in the evolution of
18
life. These and other phenomena are important to keep track of— and the
19
Core Theory does a fantastic job accounting for them. But the vast majority
20
of life is gravity and electromagnetism pushing around electrons and nuclei.
21<
br />
We can be confident that the Core Theory, accounting for the sub-
22
stances and processes we experience in our everyday life, is correct. A thou-
23
sand years from now we will have learned a lot more about the fundamental
24
nature of physics, but we will still use the Core Theory to talk about this
25
particular layer of reality. From the perspective of poetic naturalism, there
26
is one story of reality we can tell with confidence, in a well- defined domain
27
of applicability. We can’t be metaphysically certain of this; it’s not some-
28
thing we can prove mathematically, since science never proves things. But
29
in any good Bayesian accounting, it seems overwhelmingly likely to be true.
30
The laws of physics underlying everyday life are completely known.
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02
03
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04
05
The Stuff of Which We Are Made
06
07
08
09
10
11
12
13
14
15
16
17
18
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.
19
“Powerful” doesn’t mean “capable of smashing cities to rubble.” (Al-
20
though quantum field theory is that, since it’s the only way we have of de-
21
scribing one kind of particle transforming into another one, which is a
22
crucial part of nuclear reactions and therefore nuclear weapons.) When
23
we’re talking about scientific theories, powerful actually means restrictive—
24
a powerful theory is one in which there are many things that simply cannot
25
happen. The power we’re talking about here is the ability to start with very
26
few assumptions and draw conclusions that are reliable and wide- ranging
27
in their scope. Quantum field theory doesn’t knock down buildings lying
28
in its path; it knocks down our speculations about what kinds of things can
29
happen in physical reality.
30
The claim we’re making is pretty audacious:
31
32
Claim: The laws of physics underlying everyday life are com-
33
pletely known.
The Big Picture Page 31