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An assertion like that invites a great deal of skepticism. It’s bombas-
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tic, self- congratulatory, and it doesn’t seem that hard to think of plausible
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ways in which our understanding could be dramatically incomplete.
01
It sounds an awful lot like all the many times throughout history when
02
some great thinker or another boasted that the quest for perfect knowledge
03
was nearly complete. Every one of which turned out to be hilariously pre-
04
mature.
05
But we’re not claiming that all the laws of physics are known, only
06
a restricted set that suffices to describe what happens at the level underly-
07
ing everyday life. Even that sounds pretty presumptuous. Surely there must
08
be all sorts of ways to add new particles or forces to the Core Theory that
09
could be important to everyday- level physics, or for that matter new kinds
10
of phenomena that fall outside the scope of quantum field theory en-
11
tirely. Right?
12
Not so. The situation now really is different from the way it has ever been
13
at previous moments in the history of science. Not only do we have a success-
14
ful theory, but we also know how far that theory can be extended before it
15
ceases to be reliable. That’s just how powerful quantum field theory is.
16
17
•
18
The logic behind our audacious claim is simple:
19
20
1. Everything we know says that quantum field theory is the
21
correct framework for describing the physics underlying ev-
22
eryday life.
23
2. The rules of quantum field theory imply that there can’t be
24
any new particles, forces, or interactions that could be rele-
25
vant to our everyday lives. We’ve found them all.
26
27
Could quantum field theory not apply in the appropriate regime? Of
28
course. As good Bayesians, we know better than to set our credences all the
29
way to zero even for the most extreme options. In particular, quantum field
30
theory could fail to completely describe human behavior, since physics could
31
fail to describe human behavior. There could be a miraculous intervention,
32
or some inherently non physical phenomenon that affects the behavior of
33
physical matter. No amount of scientific progress will ever rule that out
34
entirely. What we can do is show that physics by itself is fully up to the task
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of accounting for what we see.
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01
Einstein’s special relativity (as opposed to general relativity) is the theory 02
that melds space and time together and posits the speed of light as an abso-
03
lute limit on the universe. Let’s say you want to invent a theory that simul-
04
taneously embraces these three ideas:
05
06
1. Quantum mechanics
07
2. Special relativity
08
3. Sufficiently separated regions of space behave independently
09
from one another
10
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Nobel laureate Steven Weinberg has argued that every theory that
12
fits these requirements will look like a quantum field theory at (relatively)
13
long distances and low energies—say, anything bigger than a proton. No
14
matter what happens at the ultimate, most fundamental and comprehen-
15
sive level of nature, in the regime that humans can probe, the world will be
16
well described by quantum field theory.
17
If we are interested in describing the everyday low- energy world around
18
us, therefore, and we want to stick purely to physics, we should work in the
19
framework of quantum field theory.
20
•
21
22
Let’s accept the idea that quantum field theory works in the everyday re-
23
gime, and ask why there couldn’t be undiscovered particles that are relevant
24
to the everyday world.
25
First, we need to establish that there can’t be real, tangible particles
26
buzzing around and bumping into us, somehow affecting the behavior of
27
the particles we know about. Then we’ll have to assure ourselves that there
28
aren’t any virtual particles or new interactions that could likewise affect the 29
particles we see. In quantum field theory, virtual particles are ones that
30
quickly flick in and out of existence as quantum fluctuations, affecting
31
what regular particles do without ever being observed themselves. We’ll
32
look at this second issue in the next chapter, and for the moment focus on
33
the possibility of real particles.
34
The reason why we know there are no new fields or particles that play an
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important role in the physics underlying our everyday lives is a crucial
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property of quantum field theory known as crossing symmetry. This amaz-
01
ing feature helps us be sure that certain kinds of particles do not exist; oth-
02
erwise we would have found them already. Crossing symmetry basically
03
says that if one field can interact with another one (for example, by scatter-
04
ing off of it), then the second field can create particles of the first one under 05
the right conditions. It can be thought of as the quantum- field- theory ana-
06
logue of the principle that every action implies a reaction.
07
Consider a new particle X that you might suspect leads to subtle but
08
important physical effects in the everyday world, whether it’s the ability to
09
bend spoons with your mind or consciousness itself. That means that the X
10
particle must interact with ordinary particles like quarks and electrons, ei-
11
ther directly or indirectly. If it didn’t, there would be no way for it to have
12
any effect on the world we directly see.
13
Interactions between particles in quantum field theory can be visualized
14
by the lovely mechanism of Feynman diagrams. Think of an X particle
15
bouncing off of an electron by the exchange of some other new particle, Y.
16
From left to right in the diagram, an X and an electron came in, exchanged
17
a Y particle, then went off on their own ways.
18
19
X
X
20
21
22
Y
23
24
25
electron
electron
26
time
27
28
The diagram isn’t just a picture of what can happen; it’s associated with
29
a number, which tells us how strong the interaction is— in this case, how
30
likely an X is to scatter off an electron. Crossing symmetry says that for
31
every such process, there is another process of the same strength, obtained
32
by rotating the diagram by ninety degrees, and switching any lines that
33
changed directions from particle to antiparticles. One result of crossing
34
symmetry is shown in the next figure.
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01
electron
X
02
03
04
Y
05
06
07
08
positron
time
anti- X
09
A diagram representing the annihilation of an electron and a positron (anti-
10
particle of an electron) into a Y particle, which then decays into an X and an 11
anti- X. This diagram is related to the previous one by crossing symmetry.
12
13
In field theory, every particle has an antiparticle with the opposite elec-
14
tric charge. The antiparticle of an electron is a particle called the positron,
15
which is positively charged. Crossing symmetry says that the first process,
16
scattering of an X off an electron, implies the existence of a related process 17
in which an electron and positron annihilate to create one of our X parti-
18
cles as well as its antiparticle.
19
Here is the payoff. We have smashed electrons and positrons together,
20
often and with great care. From 1989 to 2000, a particle accelerator called
21
the Large Electron- Positron Collider (predecessor of today’s Large Hadron
22
Collider) operated underground outside Geneva. Within its experiments,
23
electrons and positrons collided at enormous energies, and physicists kept
24
extremely careful track of everything that came out. They were hoping with
25
all their hearts to find new particles; discovering new particles, especially
26
unexpected ones, is what keeps particle physics exciting. But they didn’t see
27
any. Just the known particles of the Core Theory, produced in great numbers.
28
•
29
30
The same has been done for protons smashing into antiprotons, and various
31
other combinations. The verdict is unambiguous: we’ve found all of the
32
particles that our best current technology enables us to find. Crossing sym-
33
metry assures us that, if there were any particles lurking around us that
34
interact with ordinary matter strongly enough to make a difference to the
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behavior of everyday stuff, those particles should have easily been produced
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in experiments. But there’s nothing there.
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There are probably more particles yet to be found. They just won’t be
01
relevant to our everyday world. The fact that we haven’t yet found such
02
particles tells us a great deal about what properties they must have; that’s
03
the power of quantum field theory. Any particle that we haven’t yet de-
04
tected must have one of the following features:
05
06
1. It could be so very weakly interacting with ordinary matter
07
that it is almost never produced; or—
08
2. It could be extremely massive, so that it takes collisions at
09
energies even higher than what our best accelerators can
10
achieve in order to make it; or—
11
3. It could be extremely short- lived, so that it gets made but
12
then almost immediately decays away into other particles.
13
14
If any particle we haven’t yet found lasted long enough and interacted
15
with ordinary matter with sufficient strength that it could possibly affect
16
the physics of everyday goings-on, we would have produced it in experi-
17
ments by now.
18
One as- yet- undiscovered particle we believe exists is dark matter.
19
Astronomers, studying the motions of stars and galaxies as well as the large-
20
scale structure of the universe, have become convinced that most matter is
21
“dark”— some kind of new particle that is not part of the Core Theory. The
22
dark- matter particle must be quite long- lived, or it would have decayed away
23
long ago. But it cannot interact strongly with ordinary matter, or it would
24
have already been found in one of the many dark- matter detection experi-
25
ments that physicists are currently running. Whatever the dark matter is,
26
it certainly plays no role in determining the weather here on Earth, or any-
27
thing having to do with biology, consciousness, or human life.
28
29
•
30
There is an apparent loophole in this analysis. There is a particle that we think
31
exists but have never directly detected: the graviton. It is light and stable
32
enough to be produced, but gravity is such a weak force that any gravitons we
33
might make in a particle accelerator will be swamped by the huge number of
34
other particles produced. And yet, gravity does affect our everyday lives.
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The basic reason why gravity matters to us is that it is a long- range force
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01
that accumulates— the more stuff you have causing the gravity, the stronger
02
its influence is. (That’s not necessarily true for electromagnetism, for ex-
03
ample, since positive and negative charges can cancel out; gravity always just
04
adds up.) So while we have no hope of making or detecting an individual
05
graviton by smashing two particles together, the combined gravitational
06
effect of the whole Earth creates a noticeable amount of gravitational force.
07
Is it possible that some other force takes advantage of this loophole— it
08
would be weak if we look at just a few particles, but could accumulate if we
09
had a lot of matter working together? Absolutely— and physicists have been
10
looking for such a “fifth force” for many years now. They haven’t found one.
11
The search for new forces is greatly abetted by the fact that ordinary
12
objects are made only of three kinds of particles: protons, neutrons, and
13
electrons. Another feature of quantum field theory is that you can’t turn
14
the forces from individual particles on and off; the associated fields are al-
15
ways there. You can create macroscopic forces by arranging positive and
16
negative charges in the right way, as in an electromagnet, but particle by
17
particle the fields are always present. So we just have to look for forces
18
between those three kinds of particles. Physicists have done precisely that:
19
constructing impeccably precise experiments that bring objects of different
20
compositions close together and then apart again, searching for any hint of
21
an influence outside the known forces of nature.
22
The results, as of 2015, are shown schematically in the figure. Any pos-
23
sible force between two given kinds of particles is parameterized by two
24
numbers: how strong it is, and the distance over which it reaches. (Gravity
25
and electromagnetism are “ long- range” forces, stretching essentially infi-
26
nitely far; the strong and weak nuclear forces have very short ranges, smaller
27
than individual atoms.) It’s easiest to measure forces that are strong, and
28
that reach over long distances. Those are the possible forces that we’ve al-
29
ready ruled out.
30
The result is that, if a new force stretches for more than a tenth of a
31
centimeter— which it would have to, if you wanted to use it to bend spoons
32
or reach from Saturn to the time and place of your birth— it would have to
The Big Picture Page 32