Open Questions:
High-energy Physics
|
See also: Mathematics and Physics
People have a tendency to wonder what things are made of -- at least since
the Greeks, and undoubtedly much earlier.
Artisotle called Thales of Miletus (ca. 585 BCE) the founder of physical
science and wrote that Thales believed everything to be made of water. That
wasn't so far-fetched, considering water can be solid, liquid, or gas
in everyday experience.
Artistotle himself, following his teacher Plato, thought that matter was
based on four fundamental substances: earth, air, and fire, as well as water.
In fact, Aristotle added a fifth essence, "quintessence", that supposedly
resided in the celestial world. (The term has been adopted by some modern
physicists and repurposed.)
Other Greek philosophers took a different approach. Instead of trying to
identify the essences of matter, they were content to view matter in terms
of the ability to divide it into smaller and smaller units until some
ultimate indivisible unit they called the "atom" was reached. Leucippus of
Miletus (ca. 450 BCE) and (especially) his student Democritus are recognized
for promoting this view.
Leucippus and Democritus, of course, weren't that far off, in the view
of modern physics. Except that what we now call atoms are known not to be
indivisible. In the early 20th century it was recognized that atoms are
made of even smaller particles --
electrons, protons, and neutrons. Somewhat later, in the 1960s,
it became apparent that protons and neutrons were themselves composed of
smaller particles -- quarks -- though electrons remained indivisible.
There are now various good reasons to think that electons and quarks
(together with a few other assorted particles) really are
fundamental and indivisible. (Even though, a few decades ago, some
physicists toyed with "bootstrap" theories in which divisibility really
could go on forever, such notions have since been discarded.)
According to the uncertainty principle of quamtum mechanics, the more
precisely the position of a particle is determined, the more uncertainty
there must be about its velocity, and hence it's kinetic energy. This
inverse relationship means that to probe the structure of matter at very
small distance scales, it is necessary to work at very large
energies. This seeming paradox is the reason that the study of things
which are very small is called high-energy physics.
There is now another reason, besides our curiosity
as to what "stuff" is actually made of, for a keen interest in the study
of the smallest possible units of matter. This is the realization, which
gradually dawned on physicists in the last third of the 20th century,
that knowing precisely what matter is on the very smallest scales is
essential to undertanding it on the very largest scale as well -- which
is the subject of cosmology. The connection is that very high energies
are involved in both subjects. In cosmology, this means the highest
energy we can imagine, namely that of the big bang itself.
One might almost say that the cosmological applications of high-energy
physics is what the subject was "really" about from the beginning, though
no one was aware of it. Because the fact is that the energy scale at which
the "true" nature of matter becomes manifest existed
only for the briefest of instants at the time of the big bang.
The bottom line is that it is impossible to separate understanding the origins
of the universe from understanding the behavior of the smallest units of
matter. The two are fundamentally connected.
Around 1970, physical theories of the fundamental nature of matter (and energy)
began to converge on what is now called the "standard model" or particle
physics. This model consists of several things.
First, there are the "elementary particles" that make up matter. These
consist of electrons, neutrinos, and quarks. It is the electrons and quarks
which make up ordinary matter. The neutrinos are uncharged, almost massless
particles which exist in great abundance but hardly interact with normal
matter at all. Electrons and neutrinos, nevertheless, are closely associated,
and each has relatives (metaphorically speaking) which differ only in mass
(and don't occur normally, becaue they are unstable). Likewise, there are
several different types of quarks besides the two that occur in ordinary
matter. And for each of these types of particle there is a corresponding
anti-particle, which differs in electrical charge (except for neutrinos,
which are uncharged).
Along with the "fundamental" particles there are also "fundamental" forces,
which are manifested in interactions between the particles. The precise
number of these forces is a little slippery, since it turns out they
merge into each other at increasingly high energies. The first force is
electromagnetism (which itself was considered two forces, electical and
magnetic, before James Maxwell unified them). The second force is gravity.
The remaining forces are the "weak" force and the "strong" force, which are
observable only in interactions between particles and not a part of
direct everyday experience.
For each of these force types, there are additional particles which are
said to "carry" or "mediate" the force. That is, the force is regarded as
resulting from an exchange of these mediating particles, which are of
a type referred to as "bosons". These particles
are called "photons", "gravitons", "gluons", and W or Z bosons -- corresponding
respectively to the electromagnetic, gravitational, strong, and weak forces.
Finally, there is hypothesized to exist one more type of particle, the
"Higgs boson", which has not yet been definitievely observed, but is absolutely
essential to the theory. The Higgs particle (as it's also called) is the
mechanism by which the non-massless particles acquire their mass. It is
currently the most sought-after "fugitive" in all of physics. Even though
it might seem that the fact it hasn't shown up yet leaves the
standard model in a rather precarious position, the success of the model
in predicting all observations to date makes physicists very sure that the
Higgs particle must exist. There will truly be Hell to pay if it doesn't.
The standard model is described mathematically in terms of what is called
a "gauge field theory". This refers to an elegant piece of mathematics
which describes all the particle types not as discrete entities, but in terms
of "fields" -- mathematical quantities associated with every point of space.
Each particle is viewed as the quantum of its own type of field, just as
the photon is the quantum of the electromagnetic field.
After all, quantum theory says particles can't be localized to a single
point in space, so it is natural to deal with them as something that is
"smeared out" rather than a discrete object.
The gauge field theory also includes symmetry properties which are
essential to the standard model. A symmetry, in the general sense used
here, refers to something that apparently different types of particles
have in common, some sort of "likeness". Particles that are related by
one of these symmetries can "change into" each other by a symmetry
operation that is analogous to a rotation in ordinary space. (This is
in addition to any changes which occur during interactions between
particles, as when a neutron "decays" into a proton and an electron.)
Above all, the standard model brought some order to the explosive discovery
of new "fundamental" particles which were found around 1955-65. In
spite of the still significant diversity of leptons (electons and neutrinos),
quarks, and bosons, especially when counting anti-particles as well, the
situation is much more orderly than before.
Symmetry was a big help. The symmetries which exist, for example, between
different types of quarks are saying that each is really a manifestation of
the "same" thing, rather than representing truly different types of particles.
Further, the symmetries are not just an accounting trick. They are represented
in the theory as mathematical operators, and as such enter into computations,
so that they help determine the calculated results.
And the calculated results are impressive. In every case where a value that
can be computed by the theory can be measured experimentally, there is
complete agreement. There are no experimental contradictions to the theory.
The theory also accounts for a variety of phenomena involving the various
particles and forces, such as:
- The occurrence of particle transitions between members of a family,
such as between electrons and electron neutrinos or u and d quarks.
- The existence and nature of the bosons (W and Z) which transmit
the weak force.
- The fact that quarks never appear either alone (a phenomenon called
quark confinement) or in combinations
which have a net "color" charge. That is, all particles that are composed
of quarks (called hadrons) are "colorless". Hadrons also have integral
electric charges, even though quarks have non-integral charges (such as
1/3 or -2/3 of the charge of an electron).
- The existence and number (8) of the gluons which transmit the strong
force and are capable of changing the "color" charge of quarks.
If it is true all experimental measurements are perfectly
consistent with the theory of the standard model, why do physicists
still feel it is not a satisfactory and complete theory? Simple. There
are still many areas where it just doesn't predict or explain physical
observations. There are many observed phenomena which aren't predicted
by the theory, even though they aren't ruled out either.
Some of these phenomena have to do with particle physics technicalities.
In this category we have:
- The hypothetical "Higgs field" -- which accounts for particle masses
and the breaking of the symmetry between the electromagnetic and weak
forces -- is not well understoood. There is no way to calculate the mass(es)
of its field boson(s) (the Higgs particle(s)), or even how many there are.
- There is no unification among the electroweak, strong, and
gravitational forces that would treat them all as special cases of just
one single force.
- There is no explanation for the existence of exactly three "families"
of quarks and leptons.
- There is no way to calculate important quantities such as relative
masses of the various particles and strengths of the different forces.
- There is no explanation for the striking coincidence that electrons
and protons have the exact same electrical charge (of opposite sign).
But there are a number of other problems which come from outside of
particle physics itself -- from cosmology -- that the standard model
provides little help with, even though a complete theory of matter should.
These are phenomena for which we have observational evidence, often in
great abundance, from a large diversity of astronomical studies. In
this category we have:
- There is abundant evidence that over 90% of the mass in the universe
consists of "dark" matter which does not compose luminous objects like
stars. Some of this dark matter may exist in the form of neutrinos, but
much more must consist of particles which are not even included in the
standard model.
- In addition to the dark matter, there must also apparently exist
"dark energy", which has been identified with the "cosmological constant"
in Einstein's equation of general relativity. This is needed to explain the
recently ovserved accelerating rate of expansion of the universe.
The constant should theoretically
be calculable as "vacuum energy", but computations based on existing
theories produce a result that is 120 orders of magnitude too large for
the observed effect -- a factor of 10120.
- There seems to be, at most, an insignificant amount of antimatter in the
universe. Almost everything we can detect appears to be familiar matter.
The theoretical conditions for this asymmetry to exist are known, but
can't be accounted for by the standard model.
- The big bang theory of the origin of the universe needs to be supplemented
with the addition of a phase of extremely rapid "inflation", in order to
accord with the observed flatness and homogeneity of the universe. Such
inflation is presumed to have been driven by some sort of "phase transition"
which occurred in the first instant after the big bang. This type of
transition could have occurred when the strong force differentiated from
the weak force -- but we don't understand exactly how this
might have occurred.
And then there's the biggest question in all of physics. Right now there
are two broad theories which describe physical behavior, usually in very
different areas, but sometimes overlapping -- quantum theory and general
relativity. Yet they have proven very incompatible. When will we have
a consistent quantum theory of gravity, and what will it look like?
Site indexes
-
Open Directory Project: Particle Physics
- Categorized and annotated links. A version of this
list is at
Google, with entries sorted in "page rank" order.
-
The World-Wide Web Virtual Library: High-Energy Physics
- Links to high-energy physics information resources (mostly
laboratories and university departments).
-
Yahoo High Energy and Particle Physics Links
- Annotated list of links.
-
Fermilab: More About Particle Physics: Resources
- Good list of links on particle physics, astrophysics, and
general physics. There are also links to
other high energy physics research laboratories.
-
Particle Physics Education Sites
- An index of overview & tutorial pages, maintained by PDG at the
Particle Adventure site.
-
SLAC Library: Online Particle Physics Information
- An index oriented to technical/professional sites.
-
Galaxy: High-Energy and Particle Physics
- Categorized site directory. Entries usually include
descriptive annotations.
-
Physics Internet Resources
- Short annotated list, from the
American Physical Society.
Sites with general resources
-
Particle Physics News and Resources
- Known more simply as Interactions.org. "A communications
resource for the world's particle physics laboratories."
In addition to recent news, the site contains special
sections for
cosmic physics and
string theory,
an image bank,
and
links to other reources.
-
Yahoo News Full Coverage: High Energy and Particle Physics
- Links to recent news stories from various sources. Also includes
links to sites dealing with high energy physics.
-
Particle Data Group
- "The PDG is an international collaboration
that reviews Particle Physics and related areas of Astrophysics, and
compiles/analyzes data on particle properties." Publishes the main
journal in the field, Review of Particle Physics.
-
Best of Physics Web: Particle and Nuclear Physics
- Directory of best feature articles, news stories, and
external links on particle and nuclear physics at the
Physics Web site.
-
Level 5: Particle Physics
- Relatively short collection of survey articles and
external links to some of the best
particle physics sites. (From the
Level 5 project.)
-
Bibliography of Particle Physics Educational Materials
- Introductory and intermediate resources.
-
The Large Hadron Collider Home Page
- Information, mostly technical, about the Large Hadron Collider
under construction at CERN. There are some
press releases.
-
The ATLAS Experiment
- Public site with general information on ATLAS
("A Toroidal LHC ApparatuS"). Site has an overview of the
relevant physics, a description of the experiment, and information
on the LHC.
-
CERN
- "CERN is the European Organization for Nuclear Research,
the world's largest particle physics centre."
This site contains public information about CERN.
It's main research tool will be the
Large Hadron Collider.
Use the
site map to find your way around.
-
CERN Courier
- Monthly journal of high energy physics news. Produced by
CERN. Although the journal is print-based, the content (including
news stories and articles) is archived online.
-
Stanford Linear Accelerator Center
- This is the principal home page of SLAC. For a general
overview the
Virtual Visitor
Center is a good place to start. There are many other
subsidiary pages.
-
SLAC Virtual Visitor Center
- Overview of the facilities, experiments, history, and other
information about the Stanford Linear Accelerator Center.
-
BaBar
- BaBar is the name of an experiement using a special particle
detector for study of the phsics of B mesons at
SLAC.
It is the principal high-energy physics program there for the
coming decade.
-
SLAC Library: Databases and Documents
- Databases maintained by the SLAC library on vaious high-energy
physics topics.
-
SLAC Beam Line
- Quarterly journal of SLAC and particle physics news. Individual
issues are in PDF format.
-
Fermi National Accelerator Laboratory
- The laboratory was once a leading center of particle physics
research, now a casualty of U. S. failure to support particle
physics. Neverthless, the site still has useful features, such as
Inquiring Minds and
FermiNews.
There is a related site for the
Fermilab Education Office,
which contains educational outreach material and information.
-
Fermi National Accelerator Laboratory: Inquiring Minds
- This is a collection of resources on particle physics at
Fermilab. It includes various
tutorials,
links
to external sites, and other useful resources.
-
FermiNews
- Biweekly newsletter dealing with research and community
information related to the Fermi National Accelerator Laboratory.
Past issues are archived. Has some good permanent features, such as
Particle Physics for Regular People-Recommended Readings.
-
High Energy Physics Information Center (HEPIC)
- Operated by Fermilab. General resources for the high energy
physics community.
-
Particle Physics Picture of the Week
- People, places, and things associated with particle physics.
-
Next Linear Collider Home Page
- The NLC is a new facility being planned to answer fundamental
questions about the nature of matter. A successor to the Stanford
Linear Accelerator (SLAC), it will be 20 miles long, 10 times the
length of SLAC. This site contains information about the project
as well as background information.
-
European Particle Physics Outreach Group
- "A network of people, representing the CERN member states,
who are involved with the popularization of Particle Physics."
The site consists mainly of external links, in categories such
as "learning about particle physics". However, it is a little
hard to use since it is organized at the top level by country.
-
Research Topics in Theoretical Particle Physics
- Capsule summaries of research areas pursued by the
DESY Theory Group, such as electroweak
physics, supersymmetry, applications to cosmology.
Surveys, overviews, tutorials
-
Particle physics
- Article from
Wikipedia.
See also
Elementary particle,
Fundamental force.
-
The Particle Adventure
- Maintained by PDG.
"Introduces the theory of fundamental particles and forces,
called the Standard Model. It explores the experimental evidence
and the reasons physicists want to go beyond this theory. In
addition, it provides information on particle decay and
a brief history section." Features of the site include
Particle Physics News and external links to
particle physics education sites. An
alternative version of this site is maintained at CERN.
-
Particle Adventure Glossary
- From the Particle Adventure site.
-
ATLAS Glossary of high-energy physics terms
- Useful reference.
-
To the LHC and beyond
- September 2004 article from
Physics World, by
Peter Rodgers. Relates history, status, and future of the
CERN laboratory where the Large Hadron Collider is located.
The research program for the LHC is briefly described.
-
Why Do We Need a Linear Collider
- Slide presentation given by Martinus Veltman at the
2001: A Spacetime Odyssey conference.
Discusses some open questions that could be studied by the
proposed "New Linear Collider".
-
Particle physics: the next generation
- December 1999 article from
Physics World, by
John Ellis. "Although the basic building blocks of matter and their
interactions have been placed on a firm theoretical footing, many
fundamental questions remain unanswered and await the experiments of the
future."
-
Elementary Particle Physics PHY-653
- Material from lectures by
Steve Lloyd.
Main topics includes hadrons and quarks, electroweak interactions,
particle astrophysics, and developments beyond the standard model.
Many lectures are available in PDF format. Also contains a section
on Feynman diagrams. There are also a few external links of
general interest and more
specialized.
-
Introduction to Particle Physics
- It's very elementary, but nicely done. From York University (UK).
-
An Introduction to Particle Physics
- Overview, including material on the top quark and
connections with the Big Bang theory, by Phil Bradley.
-
Introduction to Particle Physics
- Outline of a
college course at Tel Aviv University.
-
HEPAP'S Subpanel on Vision for the Future of High-Energy
Physics
- More succinctly known as the Drell Report. A detailed report
on the state of high-energy physics in 1994.
-
Is There a Theory of Everything?
- Elementary overview by Michio Kaku.
-
The ATLAS Experiment Homepage
- Simple overview of the basic concepts of high-energy physics:
particles, forces, fields. Produced by the team working on an
experiment to be performed at the CERN Large Hadron Collider.
Much of the same information is at
U. S. Atlas Education Pages hosted at CERN.
-
UCT PHY400/1W Particle Physics
- Notes and information related to a course at the University
of Cape Town, by D. G. Aschman.
-
High Energy Physics: A Challenge to Uncover the Secrets of the
Creation and Evolution of Our Universe
- Good overview produced by the
JLC project.
-
Big Bang Science ...exploring the origins of matter
- Overview produced by the UK Particle Physics and Astronomy
Research Council.
-
Particle Physics
- Materials, including lecture notes and PDF files from a course,
by Niels Walet.
-
Small Stuff
- Brief overview in outline form of some topics in high-energy
physics. Part of a physics course by Jess Brewer.
-
SLAC Virtual Visitor Center: Theory
- Hypertext document that gives an overview of high-energy
physics.
-
The Particle Detector BriefBook
- Evertything you ever wanted to know about particle detectors.
-
CERN prepares for the LHC and beyond
- May 2000 article from
Physics World, by
Peter Rodgers about CERN's plans for the next generation of
particle accelerators.
- Particle Accelerators Test Cosmological Theory
David N. Schramm; Gary Steigman
Scientific American, June 1988, pp. 66-72
- Three distinct "generations" of quarks and leptons are known
to exist. Comparisons of the observed abundance of a few light
atomic isotopes with calculations of big bang nucleosynthesis
indicate that there can be only three generations. Observations
of the decay rates of Z0 bosons in accelerator
experiments may soon provide a completely independent confirmation
of this.
- Lisa Randall – Warped Passages: Unraveling the Mysteries
of the Universe's Hidden Dimensions
HarperCollins Publishers, 2005
- This may be the book to read (as of its publication
date) to learn about the whole expanse of modern elementary
particle physics. It begins at the beginning, with relativity,
quantum mechanics, and the standard model. From there is leaps
into the more speculative stuff – supersymmetry, grand
unified theories, string theory, and M-theory. A major theme
throughout is dimensionality and the way that the most
sophisticated theories of modern physics seem to require more
than 3 or 4 dimensions.
[Book review]
- Lawrence M. Krauss – Hiding in the Mirror: The
Mysterious Allure of Extra Dimensions, from Plato to String Theory
and Beyond
Viking, 2005
- This book, like Lisa Randall's, is themed around the idea
of higher dimensions of spacetime. But Krauss writes more
briefly, having already covered some of the prerequisite
ideas in other books of his. He also adopts a more detached,
philosophical, and (somewhat) skeptical attitude towards
some of the trendier concepts like superstrings, branes, and
"theories of everything".
[Book review]
- Martinus Veltman -- Facts and Mysteries in Elementary Particle
Physics
World Scientific, 2003
- Veltman won a Nobel Prize for his work on the mathematical
consistency of the standard model, and this book demonstrates
his command of the subject. It is largely a history of the
experiments and theories of the middle decades of the 20th
century which underlie the standard model. Capsule biographies
are provided of many of the important participants in this
development. Along the way, the major concepts are explained
clearly, but without the more challenging details. The author's
predisposition is to stick with physics that has been experimentally
confirmed, so more speculative ideas such as supersymmetry and
string theory are carefully avoided.
- R. Michael Barnett; Henry Mühry; Helen R. Quinn -- The
Charm of Strange Quarks: Mysteries and Revolutions of Particle
Physics
Springer-Verlag, 2000
- This is an introductory book for general readers, but with
a good deal of meat in it. It mostly dispenses with historical
filler on who did what when, in order to focus on essential
concepts: the standard model, quarks, leptons, fundamental forces,
and the workings of particle accelerators. There's also a chapter
on the relation of particle physics to cosmology. However, a lot
of the "good stuff" is ensconced in one 50-page appendix (along
with other appendices, such as a table of the Greek alphabet).
The bibliography is nice too.
- Frank Close -- Lucifer's Legacy: The Meaning of Asymmetry
Oxford University Press, 2000
- Yet another introduction to high energy physics for the general
audience, with an emphasis on the concept of symmetry. The spontaneous
breaking of exact symmetry receives special attention. As the book
explains, the breaking of symmetry at low temperatures ultimately
results in the fact that particles of matter have mass.
The world as we know it follows from that.
- Richard Morris -- The Universe, the Eleventh Dimension, and
Everything: What We Know and How We Know It
Four Walls Eight Windows, 1999
- This is a lightweight, but relatively recent, account of
high energy physics and how it relates to the origins of the
universe itself. A substantial part of the discussion concerns
philosophical questions about physics and science in general.
The book is a good choice for a first introduction to these
topics.
- Gordon Kane -- The Particle Garden
Addison Wesley, 1995
- Short, elementary, non-mathematical, and relatively recent
overview of particle physics. Good explanations of topics like
the meaning of "understanding", Higgs bosons, supersymmetry,
unification. Good glossary. Kane is one of the leading theorists
of supersymmetry, and his book is one of the best and meatiest
available to explain the difficult issues that the standard model
cannot handle.
- Leon Lederman, Dick Teresi -- The God Particle: If the Universe
Is the Answer, What is the Question?
Bantam Doubleday Dell Publishing Group, 1993
- Lederman is the former director of
Fermilab. The title refers to
the long-sought Higgs particle -- and (in spite of the title),
this is not one of those books of metaphysical speculation. It's
about the business of doing high energy physics -- the history,
the accelerators, and the underlying science. Eventually it
even discusses the Higgs. And the author has a sense of humor.
- David Lindley -- The End of Physics: The Myth of a Unified
Theory
Basic Books, 1993
- Will a "theory of everything" be nothing more than a new kind
of mythology? As the title indicates, this is a somewhat skeptical
view that reflects on the philosophical questions involved.
- Steven Weinberg -- Dreams of a Final Theory: The Scientist's
Search for the Ultimate Laws of Nature
Pantheon Books, 1992
- General survey of the idea of a "theory of everything".
The book is more about the philosophy of such a theory than
specific details such as superstrings.
- T. D. Lee -- Symmetries, Asymmetries, and the World of
Particles
University of Washington Press, 1988
- A very short essay on the role of symmetry in particle
physics.
- Harvey R. Brown, Rom Harré -- Philosophical Foundations
of Quantum Field Theory
Oxford University Press, 1988
- Essays from a philosophical perspective on topics such as
virtual particles, renormalization, and gauge theory.
- Michio Kaku, Jennifer Trainer -- Beyond Einstein: The Cosmic Quest for
the Theory of the Universe
Bantam Books, 1987
- One of the early popular expositions of superstring theory.
Considers the problems of "grand unified theories" and develops
the notions of symmetry and sypersymmetry.
- Robert K. Adair -- The Great Design: Particles, Fields, and
Creation
Oxford University Press, 1987
- Thorough introduction with some mathematics to fundamental
concepts used in particle physics, such as special and general
relativity, symmetry and conservation laws, gauge invariance,
and unification.
- Richard P. Feynman, Steven Weinberg -- Elementary Particles and
the Laws of Physics
Cambridge University Press, 1987
- Two introductory lectures. Feynman's lecture is on "The reason
for antiparticles." Weinberg's is on progress towards a theory
of everything.
- A. Zee -- Fearful Symmetry: The Search for Beauty in Modern
Physics
Macmillan Publishing Company, 1986
- Symmetry is a fundamental concept in physics. It is not only
about beauty -- in fact it is a basic organizing principle of
theory. Symmetry, for instance, is closely associated with
conservation laws.
- Richard P. Feynman -- QED: The Strange Theory of Light and
Matter
Princeton University Press, 1985
- Quantum electrodynamics is Feynman's own theory of the
electromagnetic force through the exchange of photons -- the
theory for which Feynman diagrams were invented.
- Heinz Pagels -- The Cosmic Code: Quantum Physics as the
Language of Nature
Simon and Schuster, 1982
- The exposition covers two related but distinct areas:
quantum theory and particle physics. Although the material is
somewhat dated, the author's expertise and writing skill provide
many lucid insights. The coverage of "quantum reality" is
especially good.
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