Matter
Matter is commonly defined as the substance of which physical objects are composed. It constitutes the
observable Universe. According to the
theory of relativity there is no distinction between matter and
energy, because matter can be converted to energy (see
annihilation), and vice versa (see
matter creation).
Philosophically, 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 everything that is constituted of
elementary fermions. Matter occupies
space and has
mass. It is composed predominantly of
atoms, which consist of
protons,
neutrons, and
electrons. All
gauge bosons (of which the
photon is one), which mediate the
four fundamental forces, are not considered matter, even though they certainly have
energy and also some mass.
Matter thus consists 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 circumstances 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, as elementary particles such as quarks and leptons are not known to participate in
chemical reactions.
In the
physical sciences, a
phase is a
set of states of a macroscopic physical system that have 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".
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 particleâ€"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.
*
Materialism*
Particle physics (provides a historical background)
*
Laycock, H., Theories of matter (essay; PDF)
