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Matter is commonly defined as being anything that has mass and that takes up space. However this definition is ambiguous, and leads to some problems, leading some physicists to define matter in terms of certain types of elementary particles.
"Normal matter" matter constitutes about 5% of the mass of the observable universe, the remaining mass being composed of exotic and poorly understood forms of mass, currently known as dark matter and dark energy. There are four phases of macroscopic matter (liquid, gas, solid, and plasma) although some exotic phases exists (like Bose-Einstein condensates) as well.
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Definitions
Common definition
The common definition of matter is anything which both occupies space and has mass. For example, a car would be said to be made of matter, as it occupies space, and has mass. In chemistry, this is often taken to mean what atoms and molecules are made of, meaning anything made of protons, neutrons, and electrons. For example, phosphorus sesquisulfide is a molecule made of four atoms of phosphorus, and three of sulfur (see image on right), and is thus considered to be matter.
However in physics, there is no broad consensus as to an exact definition of matter, partly because the notion of "taking up space" is ambiguous in quantum mechanics, and partly because mass doesn't lead to a "natural classification" of particles. Therefore physicists generally do not use the term matter when precision is needed, preferring instead to speak of the more clearly defined concepts of mass, energy, and particles. In discussions of matter and antimatter, normal matter is also sometimes referred to as koinomatter.[1]
Mass definition
Since space is problematic, a possible definition of matter could be anything that has mass. This leads to some inelegance problems in particle physics, as particles tend to be regrouped into "families" based on properties other than mass. For example, photons (which have no mass) and W bosons (which have mass) are both gauge bosons.
Quarks and leptons definition
A possible definition of matter, which at least some physicists use, is that matter is everything that is composed of elementary fermions, namely quarks and leptons.[2] Leptons (the most famous being the electron), and quarks (of which baryons, such as protons and neutrons, are made) combine to form atoms, which in turn forms molecules. Since atoms and molecules are said to be matter, it is natural to generalize what matter is as being anything that is made of the same things that atoms and molecules are made of. Since electrons are leptons, and protons and neutrons are made of quarks, this leads to the definition of matter as being "quarks and leptons", which are the two elementary types of fermions.
This definition of matter means that mass is not something that is exclusive to matter. For example, some massive particles such as the W and Z bosons are not made of quarks and leptons. This definition of matter leads to "two groups" of particles, matter (quarks and leptons) and force carriers (gauge bosons).
Properties of matter
- See also: Physical property and Materials science
Bulk properties of matter
In bulk, matter can exist in several different forms known as phases, depending on ambient pressure and temperature. A phase is a form of matter that has a relatively uniform chemical composition and physical properties (such as density, specific heat, refractive index, and so forth). These phases include the three familiar ones (solids, liquids, and gases), as well as more exotic states of matter ( such as plasmas, superfluids, supersolids, Bose-Einstein condensates, ...). There are also paramagnetic and ferromagnetic phases of magnetic materials. As conditions change, matter may change from one phase into another. These phenomena are called phase transitions, and are studied in the field of thermodynamics. In small quantities, matter can exhibit properties that are entirely different from those of bulk material and may not be well described by any phase (see nanomaterials for more details).
Phases are sometimes called states of matter, but this term can lead to confusion with thermodynamic states. For example, two gases maintained at different pressures are in different thermodynamic states (different pressures), but in the same phase (both are solids).
Solid
Solids are characterized by a tendency to retain their structural integrity; if left on their own, they will not spread in the same way gas or liquids would. Many solids, like rocks and concrete, have very high hardness and rigidity and will tend to break or shatter when subject to various forms of stress, but others like steel and paper are more flexible and will bend. Solids are often composed of crystals, glasses, or long chain molecules (e.g. rubber and paper).
Liquid
In a liquid, the molecules are frequently touching, but able to move around each other. So unlike a gas, it has cohesion and viscosity. While unlike a solid, it is not highly rigid.
Gas
A gas is a substance composed of small molecules which are spaced far enough apart from each other that they rarely interact and do not impeded each others' motion. Thus a gas has no resistance to changing shape (beyond the inertia of the molecules which have to be knocked aside).
Plasma
A plasma is a gas which is so hot that many of the molecules are electrically charged. Thus it interacts with electric and magnetic fields.
Fundamental properties of matter
Quarks
Quarks are a particles of spin-1⁄2, meaning that they are fermions. They carry an electric charge of −1⁄3 e (down-type quarks) or +2⁄3 e (up-type quarks). For comparison, an electron has a charge of −1 e. They also carry colour charge, which is the equivalent of the electric charge for the strong interaction. Quarks also undergo radioactive decay, meaning that they are subject to the weak interaction. Quarks are massive particles, and therefore are also subject to gravity.
| Name | Symbol | Spin | Electric charge (e) |
Mass (MeV/c2) |
Mass comparable to | Antiparticle | Antiparticle symbol |
|---|---|---|---|---|---|---|---|
| Up-type quarks | |||||||
| Up | u | 1⁄2 | +2⁄3 | 1.5 to 3.3 | ~ 5 electrons | Antiup | u |
| Charm | c | 1⁄2 | +2⁄3 | 1160 to 1340 | ~ 1 proton | Anticharm | c |
| Top | t | 1⁄2 | +2⁄3 | 169,100 to 173,300 | ~ 180 protons or ~ 1 tungsten atom |
Antitop | t |
| Down-type quarks | |||||||
| Down | d | 1⁄2 | −1⁄3 | 3.5 to 6.0 | ~ 10 electrons | Antidown | d |
| Strange | s | 1⁄2 | −1⁄3 | 70 to 130 | ~ 200 electrons | Antistrange | s |
| Bottom | b | 1⁄2 | −1⁄3 | 4130 to 4370 | ~ 5 protons | Antibottom | b |
Leptons
Leptons are a particles of spin-1⁄2, meaning that they are fermions. They carry an electric charge of −1 e (electron-like leptons) or 0 e (neutrinos). Unlike quarks, leptons do not carry colour charge, meaning that they do not experience the strong interaction. Leptons also undergo radioactive decay, meaning that they are subject to the weak interaction. Leptons are massive particles, therefore are subject to gravity.
| Name | Symbol | Spin | Electric charge (e) |
Mass (MeV/c2) |
Mass comparable to | Antiparticle | Antiparticle symbol |
|---|---|---|---|---|---|---|---|
| Electron-like leptons[4] | |||||||
| Electron | e− | 1⁄2 | −1 | 0.5110 | 1 electron | Antielectron (positron) |
e+ |
| Muon | μ− | 1⁄2 | −1 | 105.7 | ~ 200 electrons | Antimuon | μ+ |
| Tauon | τ− | 1⁄2 | −1 | 1,777 | ~ 2 protons | Antitauon | τ+ |
| Neutrinos[5] | |||||||
| Electron neutrino | νe | 1⁄2 | 0 | < 0.000460 | Less than a thousandth of an electron | Electron antineutrino | νe |
| Muon neutrino | νμ | 1⁄2 | 0 | < 0.19 | Less than half of an electron | Muon antineutrino | νμ |
| Tauon neutrino (or tau neutrino) |
ντ | 1⁄2 | 0 | < 18.2 | Less than ~ 40 electrons | Tauon antineutrino (or tau antineutrino) |
ντ |
Baryonic matter
Baryonic matter is the part of the universe which is made of baryons (including all atoms). This part of the universe does not include dark energy, dark matter, black holes or various forms of degenerate matter, such as compose white dwarf stars and neutron stars. Recent data from the Wilkinson Microwave Anisotropy Probe (WMAP), suggests that only about 4% of the total mass of the part of the universe which is within range of the best theoretical telescopes (i.e., which may be visible, because light has reached us from it), is made of baryionic matter. About 22% is dark matter, and about 74% is dark energy.[6]
Antimatter
In particle physics and quantum chemistry, antimatter is matter that is composed of the antiparticles of those that constitute normal matter. If a particle and its antiparticle come into contact with each other, the two annihilate; that is, they may both be converted into other particles with equal energy in accordance with Einstein's equation E = mc2. These new particles may be high-energy photons (gamma rays) or other particle–antiparticle pairs. The resulting particles are endowed with an amount of kinetic energy equal to the difference between the rest mass of the products of the annihilation and the rest mass of the original particle-antiparticle pair, which is often quite large.
Antimatter is not found naturally on Earth, except very briefly and in vanishingly small quantities (as the result of radioactive decay or cosmic rays). This is because antimatter which came to exist on Earth outside the confines of a suitable physics laboratory would almost instantly meet the ordinary matter that Earth is made of, and be annihilated. Antiparticles and some stable antimatter (such as antihydrogen) can be made in tiny amounts, but not in enough quantity to do more than test a few of its theoretical properties.
There is considerable speculation both in science and science fiction as to why the observable universe is apparently almost entirely matter, whether other places are almost entirely antimatter instead, and what might be possible if antimatter could be harnessed, but at this time the apparent asymmetry of matter and antimatter in the visible universe is one of the great unsolved problems in physics. Possible processes by which it came about are explored in more detail under baryogenesis.
Dark matter
In cosmology, effects at the largest scales seem to indicate the presence of incredible amounts of dark matter which is not associated with electromagnetic radiation. Observational evidence of the early universe and the big bang theory require that this matter have energy and mass, but is not composed of either elementary fermions (as above) OR gauge bosons. As such, it is composed of particles as yet unobserved in the laboratory (perhaps supersymmetric particles).
Exotic matter
Exotic matter is a hypothetical concept of particle physics. It covers any material which violates one or more classical conditions or is not made of known baryonic particles...
References
- ^ Sukys, Paul (1999). Lifting the Scientific Veil: Science Appreciation for the Nonscientist. Rowman & Littlefield, 87. ISBN 0-847-69600-6.
- ^ Povh, Rith, Scholz, Zetche, Reigthinger Particles and Nuclei, 1999, ISBN 3540438238
- ^ C. Amsler et al. (Particle Data Group), PL B667, 1 (2008) (URL: http://pdg.lbl.gov/2008/tables/rpp2008-sum-quarks.pdf)
- ^ C. Amsler et al. (Particle Data Group), PL B667, 1 (2008) (URL: http://pdg.lbl.gov/2008/tables/rpp2008-sum-leptons.pdf)
- ^ C. Amsler et al. (Particle Data Group), PL B667, 1 (2008) (URL: http://pdg.lbl.gov/2008/listings/s066.pdf)
- ^ "Five Year Results on the Oldest Light in the Universe". NASA (2008). Retrieved on May 2, 2008.
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