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In science, 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. It is normally defined as anything that has mass and takes up space.
Anything which occupies space and has mass is known as matter. In physics, there is no broad consensus as to an exact definition of matter. Physicists generally do not use the saying when precision is needed, preferring instead to speak of the more clearly defined concepts of mass, energy, and particles.
A possible definition of matter which at least some physicists use is that matter is everything that is composed of elementary fermions. These are the leptons, including the electron, and the quarks, including the up and down quarks of which protons and neutrons are made. Since protons, neutrons and electrons combine to form atoms and molecules, thus they comprise the bulk substances which make up all ordinary matter. Matter also includes the various other baryons, but excludes the "true mesons". The key relevant property of fermions is that they have half-integral spin (ie, 1/2, 3/2, 5/2,...,etc.) and thus, by the spin-statistics theorem of quantum field theory, obey the Pauli Exclusion Principle, which forbids two fermions from occupying the same quantum state. This seems to correspond closely to the more primitive notion that matter is "impenetrable", and takes up space.
On this view, things which are not matter include light (photons), gravitons, mesons (except for the muon, a lepton which was misnamed a meson before the distinction became clear) and the other gauge bosons. These all have half-even spin (0,1,2,...), do not respect the Exclusion Principle, and so do not occupy space in the same sense. These may all be regarded as field quanta, and may be exchanged freely by fermions without the fermions changing their own statistics, or thus their essential identity. However, these bosons do always have energy and, (according to the mass-energy equivalence of special relativity) therefore mass, so that under this definition some particles have mass without being matter: W and Z bosons have rest mass, but are not elementary fermions. Also, any two photons which are not moving parallel to each other, taken as a system, have an invariant mass. Glueballs have mass due to their binding energy, but contain no particle with rest mass, nor any elementary fermions.
Properties of matter
Quarks combine to form hadrons. Because of the principle of colour 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.
Homogeneous matter has a uniform composition and properties. It may be a mixture, such as brass, a chemical compound like water, 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 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".
Chemical matter is the part of the universe which is made of chemical 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 chemical matter. About 22% is dark matter, and about 74% is dark energy.
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.
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 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.