Matter encyclopedia topics

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.

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. 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).

This definition is also problematic inasmuch as most of the mass which is present in ordinary matter is not the intrinsic mass of the fermions which make it up. The up and down quarks which make up ordinary matter have only about 2% of the mass of the baryons which they compose. This means that about 98% of the mass of ordinary matter is due to the kinetic energy of confined quarks and their binding energy, which is due to gluons which have no rest mass themselves: the kinetic energy of particles on a confined system contributes to the invariant mass to the system (see mass in special relativity).

Properties of matter

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.

Liquid

Gas

Plasma

Fundamental properties of matter

Quarks

Quarks are a particles of spin-, meaning that they are fermions. They carry an electric charge of − e (down-type quarks) or + 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.

Quark properties
Name	Electric charge (e)	Mass (MeV/c²)	Mass comparable to	Antiparticle
Up-type quarks
Up	+	1.5 to 3.3	~ 5 electrons	Antiup
Charm	+	1160 to 1340	~ 1 proton	Anticharm
Top	+	169,100 to 173,300	~ 180 protons or ~ 1 tungsten atom	Antitop
Down-type quarks
Down	−	3.5 to 6.0	~ 10 electrons	Antidown
Strange	−	70 to 130	~ 200 electrons	Antistrange
Bottom	−	4130 to 4370	~ 5 protons	Antibottom

Leptons

Leptons are a particles of spin-, 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.

Lepton properties
Name	Electric charge (e)	Mass (MeV/c²)	Mass comparable to	Antiparticle
Electron-like leptons
Electron	−1	0.5110	1 electron	Antielectron (positron)
Muon	−1	105.7	~ 200 electrons	Antimuon
Tauon	−1	1,777	~ 2 protons	Antitauon
Neutrinos
Electron neutrino	0	< 0.000460	Less than a thousandth of an electron	Electron antineutrino
Muon neutrino	0	< 0.19	Less than half of an electron	Muon antineutrino
Tauon neutrino (or tau neutrino)	0	< 18.2	Less than ~ 40 electrons	Tauon antineutrino (or tau antineutrino)

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.

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...