Particle physics is a branch of physics that studies the elementary constituents of matter and radiation, and the interactions between them. It is also called high energy physics, because many elementary particles do not occur under normal circumstances in nature, but can be created and detected during energetic collisions of other particles, as is done in particle accelerators.
Modern particle physics research is focused on subatomic particles, which have less structure than atoms. These include atomic constituents such as electrons, protons, and neutrons (protons and neutrons are actually composite particles, made up of quarks), particles produced by radiative and scattering processes, such as photons, neutrinos, and muons, as well as a wide range of exotic particles.
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 the philosophy of science. Some in the field still adhere to reductionism, an older concept which has been criticized by various philosophers and scientists. Part of the debate is described below.
History of particle physics
The idea that matter is composed of 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 profitable) 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 Standard Model of particle physics The current state of the classification of elementary particles is the Standard Model. It describes the strong, weak, and electromagnetic fundamental forces, using mediating 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.
Experimental particle physics 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. Budker Institute of Nuclear Physics (Novosibirsk, Russia) Many other particle accelerators exist.
Theoretical high-energy physics and phenomenology Experimental particle physicists only form a part of the community of particle physicists. The other part contains theorists. In high-energy physics, the word theorist usually refers to a theoretical physicist whose primary goal is to develop theoretical and mathematical tools that may describe physical phenomena in (far) future, while the desire to understand current experiments and experiments in near future is secondary. Nowadays, most high-energy theorists work in the framework of string theory.
On the other hand, the theoreticians whose primary goal is to develop a description of current experiments and experiments in near future are usually referred to as phenomenologists or, which is almost equivalent, model builders. Although they can be inspired by string theory, their main mathematical formalism is effective field theory. Among the possible physical phenomena that they try to study we find supersymmetry, Higgs mechanism, hierarchy problem, Randall-Sundrum models and other models with extra dimensions, and many others.
This separation into theorists and phenomenologists also affects the preprint archive (www.arxiv.org) as is apparent from the names hep-th (theory), hep-ph (phenomenology), hep-ex (experiments), and hep-lat (lattice gauge theory).
Particle physics and reductionism Throughout the development of particle physics, there have been many objections to the extreme reductionist (or greedy reductionist) approach of attempting to explain everything in terms of elementary particles and their interaction. These objections have been raised by people from a wide array of fields, including many modern particle physicists, solid state physicists, chemists, biologists, and metaphysical holists. While the Standard Model itself is not challenged, it is contended that the properties of elementary particles are no more (or less) fundamental than the emergent properties of atoms and molecules, and especially statistically large ensembles of those. Some critics of reductionism claim that even a complete knowledge of the underlying elementary particles will not lend a thorough understanding of more complicated natural processes, while others doubt that a complete knowledge of particle behavior (as part of a larger process) could even be attained, thanks to quantum indeterminacy. Reductionists typically claim that all progress in the sciences has involved reductionism to some extent. Public policy and particle physics Experimental results in particle physics are investigated using enormous particle accelerators which are very expensive (typically several billion US 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 dollars 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 parents' 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.