Matter

In philosophy, matter constitutes the formless substratum of all things, which exists only potentially and from which reality is produced. In the sense of content, matter is also used in contrast to form.

In physics, matter is commonly defined as the substance of which physical objects are composed, not counting the contribution of various energy or force-fields, which are not usually considered to be matter per se (though they may contribute to the mass of objects). Matter constitutes much of the observable Universe, although again, light is not ordinarily considered matter. Unfortunately, for scientific purposes, "matter" is somewhat loosely defined.

Overview
In physics, a loose definition of matter which corresponds reasonably well to what is colloquially called "matter," is that matter is everything that is constituted of particles called elementary fermions. Matter occupies space and has mass (therefore, by this definition, not all mass is matter, but all matter does have mass). Matter, after it is incorporated into particles within a bulk substance, appears predominantly as atoms, which consist of protons, neutrons, and electrons.

In contrast to fermions, the gauge bosons (of which the photon is one), which mediate the four fundamental forces, are usually not considered matter, even though they have energy and some of them (W and Z bosons) also have mass.

Because matter consists of fermions, matter must be composed of quarks and leptons. There are six types of quarks (strange, charm, top, bottom, up, and down) which combine to form hadrons, primarily baryons and mesons, through the strong interaction and are actually thought to always be confined. Among the baryons are the proton and the neutron, which further combine to form the nuclei of all elements of the periodic table. Usually these nuclei are surrounded by a cloud of electrons. A nucleus with as many electrons as protons, which is thus electrically neutral, is called an atom, otherwise it is an ion.

In bulk, matter can exist in several different phases, according to particle density and energy density or alternatively pressure and temperature. These phases include gases, plasmas, liquids, fluids, superfluids, solids, and Bose-Einstein condensates. 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.

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 chemistry, matter is often restricted to chemical substances. Only the electrons are relevant to chemical reactions and chemical properties of the material, and the nuclei determine the mass of the atoms. Other fermions are irrelevant to chemistry.

Phases
In the physical sciences, a phase is a state of a macroscopic physical system that has relatively uniform chemical composition and physical properties (i.e. density, crystal structure, index of refraction, and so forth). The most familiar examples of phases are solids, liquids, and gases. Less familiar phases include: plasmas and quark-gluon plasmas; Bose-Einstein condensates and fermionic condensates; strange matter; liquid crystals; superfluids and supersolids; and the paramagnetic and ferromagnetic phases of magnetic materials.

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".

Antimatter
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. This gives rise to 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 miniscule 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, 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.

External link

 * Laycock, H., Theories of matter (essay; PDF)

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