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Particle Physics
Particle physics
Particle physics is a branch of
physics that studies the
elementary constituents of
matter and
radiations, and the interactions between them. It is also called
high energy physics, because many elementary particles do not occur
independently in Nature, and can only be detected during energetic collisions of
larger particles, as is done in
particle accelerators.
Modern particle physics research is focused on
subatomic particles, which are smaller than
atoms.
These include atomic constituents such as
electrons,
protons, and
neutrons (protons and neutrons are actually composite particles, made up of
quarks), as well as particles produced by
radiative and scattering processes, such as
photons,
neutrinos, and
muons.
Strictly speaking, the term particle is something of
a misnomer. The objects studied by particle physics obey the principles of
quantum mechanics. As such, they exhibit
wave-particle duality, displaying particle-like behavior under certain
experimental conditions and
wave-like
behavior in others. Theoretically, they are described neither as waves nor as
particles, but as
state vectors in an abstract
Hilbert space. For a more detailed explanation, see
quantum field theory. Following the convention of particle physicists, we
will use "elementary particles" to refer to objects such as
electrons and
photons, with the understanding that these "particles" display wave-like
properties as well.
All the particles observed to date have been catalogued in a
quantum field theory called the
Standard Model, which is often regarded as particle physics' best
achievement to date. The model contains 47 species of elementary particles, some
of which can combine to form composite particles, accounting for the hundreds of
other species of particles discovered since the
1960s. The Standard Model has been found to agree with almost all the
experimental tests conducted to date. However, most particle physicists believe
that it is an incomplete description of Nature, and that a more fundamental
theory awaits discovery. In recent years, measurements of
neutrino
mass
have provided the first experimental deviations from the Standard Model.
Particle physics has had a large impact on
philosophy of science. The
reductionist ideas that motivates much of the work in this field has been
criticized by various philosophers and scientists. Part of the debate is
described below.
The idea that
matter is composed on elementary particles dates to at least the
6th century BC. The philosophical doctrine of "atomism" was studied by
ancient Greek
philosophers such as
Leucippus,
Democritus, and
Epicurus. Although
Isaac Newton in the
17th century thought that matter was made up of particles, it was
John Dalton who formally stated in
1802
that everything is made from tiny atoms.
Dmitri Mendeleev's first
periodic table in
1869
helped cement the view, prevalent throughout the
19th century, that matter was made of atoms. Work by
J.J. Thomson established that atoms are composed of light
electrons and massive
protons.
Ernest Rutherford established that the protons are concentrated in a compact
nucleus. The nucleus was initially thought to be composed of protons and
confined electrons (in order to explain the difference between nuclear charge
and mass number), but was later found to be composed of protons and
neutrons.
The
20th century explorations of
nuclear physics and
quantum physics, culminating with proofs of
nuclear fission and
nuclear fusion, gave rise to an active industry of generating one atom from
another, even rendering possible (although not feasible economically) the
transmutation of lead into gold. These theories successfully predicted
nuclear weapons.
Throughout the
1950s and
1960s, a bewildering variety of particles was found in scattering
experiments. This was referred to as the "particle zoo". This term was
deprecated after the formulation of the
Standard Model during the
1970s in which the large number of particles was explained as combinations
of a (relatively) small number of fundamental particles.
The current state of the classification of elementary
particles is called the "Standard
Model". It describes the
strong,
weak, and
electromagnetic
fundamental forces, using mediating
bosons known as "gauge
bosons". The species of gauge bosons are the
photon,
W- and W+ and
Z bosons, and the
gluons. The model also contains 24
fundamental particles, which are the constituents of
matter. Finally, it predicts the existence of a type of boson known as the
Higgs boson, which has yet to be discovered.
In Particle Physics, the major international collaborations
are:
-
CERN,
located on the French-Swiss border near
Geneva. Its main facilities are
LEP, the Large
Electron
Positron collider (now dismantled) and the
LHC, or Large
Hadron Collider (under construction).
-
DESY,
located in
Hamburg, Germany. Its main facility is HERA, which collides
electrons or
positrons and
protons.
-
SLAC,
located near Palo Alto, USA. Its main facility is PEP-II, which collides
electrons and
positrons.
-
Fermilab, located near Chicago, USA. Its main facility is the Tevatron,
which collides
protons and
antiprotons.
-
Brookhaven National Laboratory, located on Long Island, USA. Its main
facility is the
Relativistic Heavy Ion Collider, which collides
heavy ions such as
gold
ions (it is the first heavy ion collider) and
protons.
Many other
particle accelerators exist.
Within physics itself, there are some objections to the
extreme
reductionist approach of attempting to explain everything in terms of
elementary particles and their interaction. These objections are usually raised
by
solid state physicists. While the Standard Model itself is not challenged,
it is held that testing and perfecting the model is not nearly as important as
studying the emerging properties of atoms and molecules, and especially large
statistical ensembles of those. These critics claim that even a complete
knowledge of the underlying elementary particles will not give complete
knowledge of atoms and molecules, knowledge that arguably is more important to
us.
Reductionists typically claim that all progress in the
sciences has involved reductionism to some extent.
Experimental results in particle physics are investigated
using enormous
particle accelerators which typically cost several billion dollars and
require large amounts of government funding. Because of this, particle physics
research involves issues of public policy.
Many have argued that the potential advances do not justify
the money spent, and that in fact particle physics takes money away from more
important research and education efforts. In 1993, the US Congress stopped the
Superconducting Super Collider because of similar concerns, after $2 billion
had already been spent on its construction. Many scientists, both supporters and
opponents of the SSC, believe that the decision to stop construction of the SSC
was due in part to the end of the
Cold War which removed scientific competition with the
Soviet Union as a rationale to spend large amounts of money on the SSC.
Some within the scientific community believe that particle
physics has also been adversely affected by the aging population. The belief is
that the aging population is much more concerned with immediate issues of their
health and their parent's health and that this has driven scientific funding
away from physics toward the biological and health sciences. In addition, many
opponents question the ability of any single country to support the expense of
particle physics results and fault the SSC for not seeking greater international
funding.
Proponents of particle accelerators hold that the
investigation of the most basic theories deserves adequate funding, and that
this funding benefits other fields of science in various ways. They point out
that all accelerators today are international projects and question the claim
that money not spent on accelerators would then necessarily be used for other
scientific or educational purposes.
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