BOOK REVIEW
Tiny Particles, Big Questions
Kate Scholberg
Understanding the Universe: From Quarks to the Cosmos. Don
Lincoln. xxiv + 567 pp. World Scientific Publishing, 2004. Cloth,
$88; paper, $28.
In his new book, Don Lincoln takes us on a rollicking tour of the
universe: The reader finds out what we particle physicists
understand about it, how we arrived at that understanding and where
we think we're going next with our research (we may, of course, end
up somewhere altogether different). Although some of the territory
will be familiar to many readers, Lincoln enlivens the landscape
with fresh details, irreverent (yet never unkind) remarks on the
cast of characters, and explanations that are homey, humorous and
often completely original.
Understanding the Universe: From Quarks to the
Cosmosreveals that the story of particle physics has been
marked by a series of tensions and resolutions, of proliferations
followed by unifications. Lincoln underscores this point with a
quotation from William James: "Any one will renovate his
science who will steadily look after the irregular phenomena. And
when the science is renewed, its new formulas often have more of the
voice of the exceptions in them than of what were supposed to be the rules."
In the early 20th century, physics seemed to be tidily wrapped up,
until some quiet but persistent voices demanded attention. Among the
first discrepancies were these discoveries: cathode rays (William
Crookes), x rays (Wilhelm Roentgen) and radioactivity (Marie and
Pierre Curie). By mid-century, the particle "zoo" had
proliferated. The picture came into focus only gradually, but by the
end of the century physicists were able to tease order from the
apparent chaos.
The hunt for this underlying order led to increasingly sophisticated
experimental techniques, culminating in today's town-sized particle
accelerators and building-sized detectors. These colossal and
complex machines, attended by village-sized collaborations of
experimentalists, are designed to observe colliding particles
traveling at tremendous velocities in order to reveal the essential
elements of their composition and interactions.
Lincoln does a first-class job of explaining just how these
fundamental properties emerge from the experimental results. Our
current best description of the basic constituents of the universe
and their interactions is quite simple: Fundamental matter is made
up of only two main types of particles, quarks and leptons, which
come in three successively heavier generations. These
constituents interact with one another in only four ways, determined
by the four forces: gravity, electromagnetism, the strong (or
nuclear) force and the weak force. For the latter three, we now know
what the associated force transfer particles are: the
photon (electromagnetism), the gluon (the strong force) and the
W and Z bosons (the weak force).
Lincoln, an experimental physicist at Fermilab, fondly describes his
experiences as a member of the D0 (pronounced D-zero) experiment,
one of two large international research collaborations involved in
the search for the heaviest of the quark flavors, the top quark. It
was finally discovered simultaneously by both sets of researchers in
1995. In a chapter on accelerators and detectors, Lincoln explains
in some detail the workings of Fermilab's Tevatron, which is four
miles in circumference.
Our current picture, the Standard Model of particle physics, may be
the most concise and elegant yet, but it's still unsatisfying in
many ways. The known particles and forces have been enumerated and
described, but no underlying explanation for the observed patterns
has been found. Why do quarks and leptons come in three generations
of increasing mass? Nobody knows. Why are the masses of the lightest
particles, the neutrinos, minuscule compared with those of the
quarks? Again, nobody knows. Another yawning gap in our
understanding concerns the nature of the forces: We believe that at
high energy, electromagnetic, weak and strong forces have the same
strength: they unify. But how exactly does the unification
happen? Even murkier are questions of whether and how gravity, the
weakest force, merges with the other three.
Ask a particle experimentalist to list the field's most compelling
questions and the list will certainly include the existence and
nature of the Higgs boson (referred to by Leon Lederman, with tongue
in cheek, as the "God particle"), which is intimately
connected with electroweak unification. Why the Higgs matters is
notoriously difficult to explain to the uninitiated: The arguments
requiring its existence are quite abstract, and the Higgs seems even
to some physicists to be born of "just a math trick."
Fortunately, Lincoln is up to the task of making the Higgs concept
accessible. There's a chance that the Higgs will be detected in the
near term at the Tevatron, but more likely we'll have to wait for
the newest, shiniest accelerator on the world scene to be turned on:
the Large Hadron Collider (LHC) at CERN on the Swiss-French border.
The LHC will collide protons and antiprotons. We expect results
around the end of the decade. If the telltale signatures of the
Higgs fail to appear, we may have an exception raising its voice to
herald a new round of glorious confusion.
Lincoln explores in detail many of the questions accessible to
physicists in the relatively near term. For example, the reader
learns about slippery, faintly interacting neutrinos, which for many
years were suspected to have exactly zero mass. However,
measurements made recently in giant underground experiments reveal
neutrinos to have tiny masses. A new generation of experiments is
determining the properties of neutrinos and attempting to understand
how they fit into the big picture.
One of the most looming questions of all: Where is all the
antimatter? Anti-matter has properties very similar to those of
ordinary matter, but matter and antimatter annihilate each other to
liberate energy. One would naively think that the universe had been
created with equal quantities of matter and antimatter, but the
latter exists in our universe only in very tiny quantities. Why the
universe we live in is vastly asymmetric in this regard is
completely baffling. Tiny differences between the properties of
matter and antimatter may give us clues to an explanation for this asymmetry.
Exotic phenomena that may be explored soon include the postulates of
supersymmetry, which imply that every known particle has a massive
partner with different spin properties. If those postulates hold,
theoretical discomforts are eased and force unification is smoothly
achieved. Another idea that may be tested in the short term in
collider experiments is the possibility that spatial dimensions
beyond our familiar three may exist, allowing gravity to partially
"leak away," which would account for its extreme
feebleness compared with the other forces. Tests of concepts that
are even more exotic, such as the idea that the universe consists of
tiny wriggling strings, are as yet out of reach but may become
possible in the distant future.
Lincoln ends the book with a discussion of cosmology, the study of
the history and evolution of the universe. In the last decade,
cosmology has become a precision experimental science, thanks to
recent tour de force measurements of the cosmic microwave
background, galaxy distributions and distant supernovae. The biggest
questions on astrophysical scales now overlap those on microscopic
scales, and recently the distinction between cosmology and particle
physics has become increasingly blurred. We cannot understand the
evolution of the universe without understanding the microscopic
processes that took place at the earliest times. Astronomers have
long puzzled over the nature of the dark matter that makes up a
significant fraction of the universe. The hypothesis that this
substance is supersymmetric matter will be tested at the LHC. Even
more mysterious is the dark energy responsible for the
observed accelerating expansion of the universe. That too may have a
deep connection with particle physics. Future experiments, both on
the scale of the quark and on the scale of the cosmos, will bring
new understanding.
In his epilogue Lincoln addresses explicitly the question of why
particle physicists ask why. That why
is what really animates this book, just as it animates those engaged in
research. One can make compelling arguments for the long-term benefits
to society of gaining fundamental knowledge, and certainly few particle
physicists would deny feeling pleased about applications for their work.
But the real
reason we do research is simply this: It's tremendously fun to figure
the universe out.—Kate Scholberg, Physics, Duke University