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Matter

2007 Schools Wikipedia Selection. Related subjects: General Physics

   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.

Definition

   Colloquially and in chemistry, matter is easier to define because it is
   associated with quantitative aspects such as 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, prefering instead to speak of the more clearly defined concepts
   of mass, invariant mass, energy, and particles.

Fermion definition

   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. Fermions are spin-1/2 particles, which are
   thought to have no substructure. They include the leptons (the best
   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-energy 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, would include light ( photons) and the other massless gauge
   bosons, such as gravitons and gluons. Presumably massive gauge bosons
   such as the W and Z bosons which mediate the weak force would also not
   be included in "matter."

Problem with fermion definition: most mass of ordinary objects is not
elementary fermions

   However, the fermionic (or elementary particulate) definition of matter
   is not always satisfying when examined closely. In this scheme,
   elementary massive gauge bosons of the weak force have invariant mass,
   but are not considered matter because they are not fermions.
   Furthermore, a number of other long-lived systems also may have mass
   without being mostly (fermionic) matter, and some of these are more
   familiar than massive gauge bosons. These include ordinary nucleons
   such as protons and neutrons.

   In fact, much of the mass of ordinary matter is not the fermions which
   it contains.
     * Any kind of energy in a closed system will be associated with a
       kind of 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 in weight,
       and therefore in mass.
     * The contribution of energy to mass of systems holds, even if the
       particles which contribute the energy have no rest mass, which is
       mass which would be apparent if they were examined one at a time,
       at rest. Thus, any two photons which are not moving parallel to
       each other, taken as a system, have an invariant mass, even though
       neither of the photons can be examined at rest, or said to have a
       mass. This type of system mass (see mass in special relativity) is
       similar to the kinetic energy of two objects in a system where the
       objects are moving relative to each other, which energy contributes
       a certain amount to the mass of the system. When the reference
       frame is chosen in which the momenta of the particles sums to zero,
       their summed kinetic energy contributes a minimal amount to the
       mass of the system, and this (minimal) contribution cannot be
       removed by choice of reference frame, and is over and above the
       contribution of the sums of the rest energies of the particles
       themselves (should they have any) to the system.
     * More problematically, as noted above, most of the invariant mass
       and weight associated with ordinary objects, is not associated with
       their fermions. A simple example of some fraction of ordinary
       matter not consisting of fermions, is the packing fraction of
       matter associated with the binding of mass in nuclei. This mass is
       composed of nucleonic fields and is not particles like neutrons or
       protons, yet it contributes up to 0.1% of the mass of hydrogen, as
       compared with more tightly bound atoms like iron or nickel.
       The problem becomes more acute when baryons themselves are
       examined, rather than atomic nuclei. Electrons are relatively
       light, but so are the quarks that make up the fermionic
       substructure of protons and neutrons (quarks are less than 1% of
       the weight of these particles). Therefore, more than 99% of the
       mass of baryons, or ordinary atoms, and thus ordinary objects, is
       NOT fermions; rather it is the mass of the systemic kinetic energy
       of the bound quarks, and the mass of the gluons which hold quarks
       together. These gluons have no rest mass by themselves (like
       photons), but even so, they do have energy, and thus contribute
       mass to systems they are bound into.

   For all of these reasons, it appears that there is no easy definition
   of "matter" which includes ordinary kinds of "mass," but would does not
   include the kind of "trapped energy" which massless particles and their
   energies of motion can and do show, when they are considered as
   systems, or when bound into systems. Kinetic energy or light might not
   seem like "matter", but it must be realized scientifically that most of
   the mass of a piece of ponderable matter is actually the pure kinetic
   energy of the quark particles which compose it, as well as the energy
   of the massless "light-like" gluon particles themselves.

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:
    1. The opposite of anti-matter (e.g. electrons, but not positrons)
    2. 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 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.

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.

Phases

   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".

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 = mc^2. 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 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.
   Retrieved from " http://en.wikipedia.org/wiki/Matter"
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