Particle physics, also called high energy physics, is that branch of physics which studies the elementary constituents of matter and the interactions between them. In modern Quantum Mechanics, particles and field quanta are interchangeable - for instance electrons are just the quanta of an electron field, and the electromagnetic field is carried by particles called photons. The name particle is somewhat of a misnomer. They are actually subject to Wave-Particle duality: objects that appear to be particles in some circumstances can be interpreted as waves in others.
This article describes current research in the particle physics field itself. For a discussion of the implications of claims made in this branch of science on physics in general, and a much more general discussion of cultural acceptance of such models as "real", see foundation ontology and cosmology. Particle physics has had major impact on philosophy of science, notably with respect to the difficulty of its empirical validation , and continues to be a source of some controversy - although less than in the 20th century when, before the Standard Model was established, a bewildering variety of particles had been described; this was sometimes called the "particle zoo". That term is now generally deprecated.
The current "Standard Model" of particle physics
The current state of the classification of elementary particles is called the Standard Model. It holds that all everyday matter is composed of 4 different particles, and that there are two similar families of 4 particles of higher mass and short lifespan. The fundamental forces are thought to be transmitted by 4-5 other particles. It is unknown whether there are any more classes of particles.
Fundamental particles are subdivided into bosons and fermions, the former with integer spin (for example, 0, 1, 2) and the latter with half-integer spin (for example, 1/2, 1 1/2). Fermions are subject to the Pauli exclusion principle which states that no two particles can share the same quantum numbers--that is, two identical fermions can't be in the same position, momentum, energy, and angular momentum state at the same time, whereas bosons can. At extremely low energies, fermions pair up to form pseudo-bosons which can then share the same quantum numbers. This state of matter has been achieved in Bose-Einstein condensates, superfluids and superconductors.
The field quanta of the fundamental forces are all bosons:
- photons are responsible for generating the electromagnetic interaction
- The W+ and W- bosons together with the Z0 boson are responsible for generating the weak nuclear force (weak interaction).
- The 8 colorful gluons are responsible for the strong nuclear force (strong interaction). Six of these gluons come in color pairs of colors and anti-colors (for example, a gluon can carry red and anti-green) while the other two are a complicated mix of colors and anti-colors.
- The graviton is believed to be responsible for the gravitational interaction, No gravitons have thus far been discovered. They remain theoretical objects until some evidence of their existence is found.
- Various high mass bosons are predicted by unification models, notably the Higg bosons or the higgson.
The basic constituents of matter are fermions, including the well-known proton, neutron, and electron. Of these, though, only the electron is really elementary, the other two being aggregates of smaller particles held together by the strong interaction. What appear to be elementary fermions come in four basic varieties, each of which come in three generations with different masses, for a total of twelve different "flavors":
| Symbol | Electromagnetic charge | Weak charge* | Strong charge (color) | Mass | |
| Electron | e- | -1 | -1/2 | 0 | 0.511 MeV |
| Muon | μ- | -1 | -1/2 | 0 | 105.6 MeV |
| Tau | τ- | -1 | -1/2 | 0 | 1.784 GeV |
| Up quark | u | +2/3 | +1/2 | R/G/B | ~5 MeV |
| Charm quark | c | +2/3 | +1/2 | R/G/B | ~1.5 GeV |
| Top quark | t | +2/3 | +1/2 | R/G/B | > 30 GeV |
| Down quark | d | -1/3 | -1/2 | R/G/B | ~10 MeV |
| Strange quark | s | -1/3 | -1/2 | R/G/B | ~100 MeV |
| Bottom quark | b | -1/3 | -1/2 | R/G/B | ~4.7 GeV |
| Electron neutrino | Ve | 0 | +1/2 | 0 | < 50 eV |
| Muon neutrino | Vμ | 0 | +1/2 | 0 | < 0.5 MeV |
| Tau neutrino | Vτ | 0 | +1/2 | 0 | < 70 MeV |
* - particles on the table only have a weak charge when they have left-handed spin, and their antiparticles, right-handed.
These particles can be arranged in three "generations", the first one consisting of the electron, the up and down quarks, and the electron neutrino. All ordinary matter is made from first generation particles; the higher generation particles decay quickly into the first generation ones and can only be generated for a short time in high-energy experiments.
Colorless particles (leptons) occur free; this is because their interactions (weak and electromagnetic) fall off rapidly with distance. On the other hand (and for somewhat complicated reasons) the strong force, between quarks, gets stronger with distance, and so colored particles (quarks) are always found in colorless combinations called hadrons. These are either fermionic baryons composed of three quarks (for example, protons and neutrons) or bosonic mesons composed of a quark-antiquark pair (for example, pions). The total mass of such aggregates exceeds that of the components thanks to the binding energy and in fact each comes in a series of energy states.
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 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 Lab], located on Long Island, USA. Its main facility is the Relativistic Heavy Ion Collider, which collides heavy ions such as Au (it is the first heavy ion collider) and protons.
Many other particle accelerators exist.
Origins of particle physics
Atomism - the first theory of matter that assumed any model of "particle physics goes back 2000 years to the Greeks; and Isaac Newton thought that matter was made up of particles in the 17th century. However, it was John Dalton who formally stated in 1802 that everything is made from tiny atoms." - schoolscience, UK
Dmitri Mendeleev's first periodic table in 1869 helped cement the view, prevalent throughout the 19th century, that matter was made of atoms composed of protons, neutrons, and electrons - and nothing else. The atomic nucleus of protons and neutrons was assumed to be an unbreakable unit, with electrons as their only mobile separable particle.
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.
Objections against particle physics as reductionism
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.
Public policy and particle physics
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 these 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.
External links:
- http://library.advanced.org/10380/: An interactive walkthrough of the atom]
- http://ParticleAdventure.org: A site with more detail, and news from particle physics]
- GISAI glossary, Yudkowsky
- "Philosophy Redivivus? Science, Ethics, and Faith"