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It is recognized in the Standard Model of particle physics that kinetic energy and force fields contribute most of the mass of ordinary objects. In the natural sciences, the term "matter" is therefore somewhat loosely defined, largely due the fact that use of "matter," as a term opposed to "energy," linguistically forces an artificial distinction between matter and energy which is no longer present in the understanding of modern physics. Ordinary physical objects are mostly composed of fields and kinetic energy by weight, and so exclusion of fields and energy from the term "matter" would require that ordinary physical objects be mostly composed of non-matter.
Colloquially and in chemistry, matter is easy to define because it is directly associated with mass. Matter is what ponderable objects are made of, and consists of identifiable chemical substances. These are made of atoms, which are made of protons, neutrons, and electrons. In this way, matter is contrasted with energy.
In physics, there is no broad consensus as to an exact definition of matter. Physicists generally do not use the word when precision is needed, preferring instead to speak of the more clearly defined concepts of mass, invariant mass, energy, and particles.
A possible definition of matter which at least some physicists use  is that matter is everything that is constituted of truly elementary particles called fermions. These fermions are spin-1/2 particles, which are thought to have no substructure. They include the leptons (an example of which is the familiar electron), and also the quarks, including the up and down quarks of which protons and neutrons are made. Since protons, neutrons and electrons combine to form atoms, the bulk substances which are made of atoms are all "made" of fermionic matter.
In this scheme, matter also includes the various high-mass and short-lived baryons (such as delta particles) which are never seen except in physics experiments, and also the mesons. Things which are not matter include light (photons) and the other massless gauge bosons, such as gravitons and gluons. Massive gauge bosons such as the W and Z bosons which mediate the weak force are not included in this definition (i.e., because they are not fermions, having a spin of 1, they would not be considered matter).
Problems with this definition
The fermion definition of matter is not always satisfying when examined closely. In this scheme, elementary massive gauge bosons of the weak force have mass, but are not considered matter because they are not fermions. Furthermore, only a small fraction of the mass of ordinary nucleons such as protons and neutrons can be accounted for by the mass of their constituent fermions (quarks).
Moreover, any kind of energy in a closed system is associated with an invariant mass which has weight, inertia, and in general acts exactly like all other forms of matter. For example, when an object is heated, according to modern physics, it gains mass. Should heat then be considered matter? Even a system of massless particles can have invariant mass. For instance, any two photons which are not moving parallel to each other constitute a massive system. There may even be massive quasi-stable particles called glueballs which are made entirely of massless, non-fermionic components. See mass in special relativity.
For these reasons, it appears that there is no easy definition of "matter" that correctly takes into account special relativity while still satisfying most people's intuition.
Usage note regarding matter and anti-matter
There is a semantic difficulty with the word matter, since it has two meanings, once of which includes the other. Matter may mean either:
- The opposite of anti-matter (e.g. electrons, but not positrons)
- Both matter as defined in the previous line and anti-matter (e.g. both electrons and positrons)
The same difficulty occurs with the word particle.
Properties of matter
As individual particles
Quarks combine to form hadrons. Because of the principle of color confinement which occurs in the strong interaction, quarks never exist unbound from other quarks. Among the hadrons are the proton and the neutron. Usually these nuclei are surrounded by a cloud of electrons. A nucleus with as many electrons as protons is thus electrically neutral and is called an atom, otherwise it is an ion.
Leptons do not feel the strong force and so can exist unbound from other particles. On Earth, electrons are generally bound in atoms, but it is easy to free them, a fact which is exploited in the cathode ray tube. Muons may briefly form bound states known as muonic atoms. Neutrinos feel neither the strong nor the electromagnetic interactions. They are never bound to other particles.
As bulk matter
Homogeneous matter has a definite composition and properties and any amount of it has the same composition and properties. It may be a mixture, such as brass, or elemental, like pure iron. Heterogeneous matter, such as granite, does not have a definite composition.
In bulk, matter can exist in several different phases, according to pressure and temperature. A phase is a state of a macroscopic physical system that has relatively uniform chemical composition and/or physical properties (i.e., density, crystal structure, index of refraction, and so forth). These phases include the three familiar ones — Solids, liquids, and gases — as well as plasmas, superfluids, supersolids, Bose-Einstein condensates, fermionic condensates, liquid crystals, strange matter and quark-gluon plasmas. There are also the 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 their energetics 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.
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, but the same "state of matter".
In particle physics, 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 that 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 minuscule 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.
In cosmology, most models of the early universe and big bang require the existence of so called dark matter. This matter would have energy and mass, but would not be composed of either elementary fermions (as above) or gauge bosons. As such, it would be composed of particles unknown to present science. Its existence is inferential at this point.
- Povh, Rith, Scholz, Zetche, Particles and Nuclei, 1999, ISBN 3-540-43823-8