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Matter

Matter is a general term for the substance of which all physical objects consist.[1][2] Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume.[3] In practice however there is no single correct scientific meaning of "matter," as different fields use the term in different and sometimes incompatible ways.

For much of the history of the natural sciences people have contemplated the exact nature of matter. The idea that matter was built of discrete building blocks, the so-called particulate theory of matter, was first put forward by the Greek philosophers Leucippus (~490 BC) and Democritus (~470–380 BC).[4] Over time an increasingly fine structure for matter was discovered: objects are made from molecules, molecules consist of atoms, which in turn consist of interacting subatomic particles like protons and electrons.[5][6]

Matter is commonly said to exist in four states (or phases): solid, liquid, gas and plasma. However, advances in experimental techniques have realized other phases, previously only theoretical constructs, such as Bose–Einstein condensates and fermionic condensates. A focus on an elementary-particle view of matter also leads to new phases of matter, such as the quark–gluon plasma.[7]

In physics and chemistry, matter exhibits both wave-like and particle-like properties, the so-called wave–particle duality.[8][9][10]

In the realm of cosmology, extensions of the term matter are invoked to include dark matter and dark energy, concepts introduced to explain some odd phenomena of the observable universe, such as the galactic rotation curve. These exotic forms of "matter" do not refer to matter as "building blocks", but rather to currently poorly understood forms of mass and energy.

 

Summary

The term "matter" is used throughout physics in a bewildering variety of contexts: for example, one refers to "condensed matter physics",[35] "elementary matter",[36] "partonic" matter, "dark" matter, "anti"-matter, "strange" matter, and "nuclear" matter. In discussions of matter and antimatter, normal matter has been referred to by Alfvén as koinomatter.[37] It is fair to say that in physics, there is no broad consensus as to an exact definition of matter, and the term "matter" usually is used in conjunction with some modifier.


Definitions


Common definition

The DNA molecule is an example of matter under the "atoms and molecules" definition.

The common definition of matter is anything that has both mass and volume (occupies space).[38][39] For example, a car would be said to be made of matter, as it occupies space, and has mass.

The observation that matter occupies space goes back to antiquity. However, an explanation for why matter occupies space is recent, and is argued to be a result of the Pauli exclusion principle.[40][41] Two particular examples where the exclusion principle clearly relates matter to the occupation of space are white dwarf stars and neutron stars, discussed further below.


Relativity

In the context of relativity, mass is not an additive quantity.[1] Thus, in relativity usually a more general view is taken that it is not mass, but the energy–momentum tensor that quantifies the amount of matter. Matter therefore is anything that contributes to the energy–momentum of a system, that is, anything that is not purely gravity.[42][43] This view is commonly held in fields that deal withgeneral relativity such as cosmology.


Atoms and molecules definition

A definition of "matter" that is based upon its physical and chemical structure is: matter is made up of atoms andmolecules.[44][not in citation given] As an example, deoxyribonucleic acid molecules (DNA) are matter under this definition because they are made of atoms. This definition can be extended to include charged atoms and molecules, so as to include plasmas (gases of ions) and electrolytes (ionic solutions), which are not obviously included in the atoms and molecules definition. Alternatively, one can adopt the protons, neutrons and electrons definition.


Protons, neutrons and electrons definition

A definition of "matter" more fine-scale than the atoms and molecules definition is: matter is made up of what atoms and moleculesare made of, meaning anything made of protons, neutrons, and electrons.[45][verification needed] This definition goes beyond atoms and molecules, however, to include substances made from these building blocks that are not simply atoms or molecules, for examplewhite dwarf matter — typically, carbon and oxygen nuclei in a sea of degenerate electrons. At a microscopic level, the constituent "particles" of matter such as protons, neutrons and electrons obey the laws of quantum mechanics and exhibit wave–particle duality. At an even deeper level, protons and neutrons are made up of quarks and the force fields (gluons) that bind them together (see Quarks and leptons definition below).


Quarks and leptons definition

Under the "quarks and leptons" definition, the elementary and composite particles made of the quarks (in purple) and leptons (in green) would be "matter"; while the gauge bosons (in red) would not be "matter". However, interaction energy inherent to composite particles (for example, gluons involved in neutrons and protons) contribute to the mass of ordinary matter.

As may be seen from the above discussion, many early definitions of what can be called ordinary matter were based upon its structure or "building blocks". On the scale of elementary particles, a definition that follows this tradition can be stated as: ordinary matter is everything that is composed of elementary fermions, namely quarks and leptons.[46][not in citation given][47][not in citation given] The connection between these formulations follows.

Leptons (the most famous being the electron), and quarks (of which baryons, such as protons and neutrons, are made) combine to formatoms, which in turn form molecules. Because atoms and molecules are said to be matter, it is natural to phrase the definition as: ordinary matter is anything that is made of the same things that atoms and molecules are made of. (However, notice that one also can make from these building blocks matter that is not atoms or molecules.) Then, because electrons are leptons, and protons and neutrons are made of quarks, this definition in turn leads to the definition of matter as being "quarks and leptons", which are the two types of elementary fermions. Carithers and Grannis state: Ordinary matter is composed entirely of first-generation particles, namely the [up] and [down] quarks, plus the electron and its neutrino.[48] (Higher generations particles quickly decay into first-generation particles, and thus are not commonly encountered.[49])

This definition of ordinary matter is more subtle than it first appears. All the particles that make up ordinary matter (leptons and quarks) are elementary fermions, while all the force carriers are elementary bosons.[50] The W and Z bosons that mediate the weak force are not made of quarks or leptons, and so are not ordinary matter, even if they have mass.[51] In other words, mass is not something that is exclusive to ordinary matter.

The quark–lepton definition of ordinary matter, however, identifies not only the elementary building blocks of matter, but also includes composites made from the constituents (atoms and molecules, for example). Such composites contain an interaction energy that holds the constituents together, and may constitute the bulk of the mass of the composite. As an example, to a great extent, the mass of an atom is simply the sum of the masses of its constituent protons, neutrons and electrons. However, digging deeper, the protons and neutrons are made up of quarks bound together by gluon fields (see dynamics of quantum chromodynamics) and these gluons fields contribute significantly to the mass of hadrons.[52] In other words, most of what composes the "mass" of ordinary matter is due to thebinding energy of quarks within protons and neutrons.[53] For example, the sum of the mass of the three quarks in a nucleon is approximately 12.5 MeV/c2, which is low compared to the mass of a nucleon (approximately 938 MeV/c2).[49][54] The bottom line is that most of the mass of everyday objects comes from the interaction energy of its elementary components.


Smaller building blocks?

The Standard Model groups matter particles into three generations, where each generation consists of two quarks and two leptons. The first generation is the up and down quarks, the electron and theelectron neutrino; the second includes the charm and strange quarks, the muon and the muon neutrino; the third generation consists of the top and bottom quarks and the tau and tau neutrino.[55] The most natural explanation for this would be that quarks and leptons of higher generations are excited states of the first generations. If this turns out to be the case, it would imply that quarks and leptons are composite particles, rather than elementary particles.[56]


Structure

In particle physics, fermions are particles which obey Fermi–Dirac statistics. Fermions can be elementary, like the electron, or composite, like the proton and the neutron. In the Standard Model there are two types of elementary fermions: quarks and leptons, which are discussed next.


Quarks

Quarks are a particles of spin-12, implying that they are fermions. They carry an electric charge of −13 e (down-type quarks) or +23 e (up-type quarks). For comparison, an electron has a charge of −1 e. They also carry colour charge, which is the equivalent of the electric charge for the strong interaction. Quarks also undergo radioactive decay, meaning that they are subject to the weak interaction. Quarks are massive particles, and therefore are also subject to gravity.

Quark properties[57]
namesymbolspinelectric charge
(e)
mass
(MeV/c2)
mass comparable toantiparticleantiparticle
symbol
up-type quarks
upu12+231.5 to 3.3~ 5 electronsantiupu
charmc12+231160 to 1340~ 1 protonanticharmc
topt12+23169,100 to 173,300~ 180 protons or
~ 1 tungsten atom
antitopt
down-type quarks
downd12133.5 to 6.0~ 10 electronsantidownd
stranges121370 to 130~ 200 electronsantistranges
bottomb12134130 to 4370~ 5 protonsantibottomb
Quark structure of a proton: 2 up quarks and 1 down quark.


Baryonic matter

Baryons are strongly interacting fermions, and so are subject to Fermi-Dirac statistics. Amongst the baryons are the protons and neutrons, which occur in atomic nuclei, but many other unstable baryons exist as well. The term baryon is usually used to refer to triquarks — particles made of three quarks. "Exotic" baryons made of four quarks and one antiquark are known as the pentaquarks, but their existence is not generally accepted.

Baryonic matter is the part of the universe that is made of baryons (including all 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. Microwave light seen by Wilkinson Microwave Anisotropy Probe (WMAP), suggests that only about 4.6% of that part of the universe within range of the best telescopes (that is, matter that may be visible because light could reach us from it), is made of baryionic matter. About 23% is dark matter, and about 72% is dark energy.[58]

A comparison between the white dwarf IK Pegasi B (center), its A-class companion IK Pegasi A (left) and the Sun (right). This white dwarf has a surface temperature of 35,500 K.


Degenerate matter

In physics, degenerate matter refers to the ground state of a gas of fermions at a temperature near absolute zero.[59] The Pauli exclusion principlerequires that only two fermions can occupy a quantum state, one spin-up and the other spin-down. Hence, at zero temperature, the fermions fill up sufficient levels to accommodate all the available fermions, and for the case of many fermions the maximum kinetic energy called the Fermi energy and the pressure of the gas becomes very large and dependent upon the number of fermions rather than the temperature, unlike normal states of matter.

Degenerate matter is thought to occur during the evolution of heavy stars.[60] The demonstration by Subrahmanyan Chandrasekhar that white dwarf stars have a maximum allowed mass because of the exclusion principle caused a revolution in the theory of star evolution.[61]

Degenerate matter includes the part of the universe that is made up of neutron stars and white dwarfs.


Strange matter

Strange matter is a particular form of quark matter, usually thought of as a 'liquid' of up, down, and strange quarks. It is to be contrasted with nuclear matter, which is a liquid of neutrons and protons(which themselves are built out of up and down quarks), and with non-strange quark matter, which is a quark liquid containing only up and down quarks. At high enough density, strange matter is expected to be color superconducting. Strange matter is hypothesized to occur in the core of neutron stars, or, more speculatively, as isolated droplets that may vary in size from femtometers(strangelets) to kilometers (quark stars).


Two meanings of the term "strange matter"

In particle physics and astrophysics, the term is used in two ways, one broader and the other more specific.

  1. The broader meaning is just quark matter that contains three flavors of quarks: up, down, and strange. In this definition, there is a critical pressure and an associated critical density, and when nuclear matter (made of protons and neutrons) is compressed beyond this density, the protons and neutrons dissociate into quarks, yielding quark matter (probably strange matter).
  2. The narrower meaning is quark matter that is more stable than nuclear matter. The idea that this could happen is the "strange matter hypothesis" of Bodmer [62] and Witten.[63] In this definition, the critical pressure is zero: the true ground state of matter is always quark matter. The nuclei that we see in the matter around us, which are droplets of nuclear matter, are actually metastable, and given enough time (or the right external stimulus) would decay into droplets of strange matter, i.e. strangelets.


Leptons

Leptons are a particles of spin-12, meaning that they are fermions. They carry an electric charge of −1 e (charged leptons) or 0 e (neutrinos). Unlike quarks, leptons do not carry colour charge, meaning that they do not experience the strong interaction. Leptons also undergo radioactive decay, meaning that they are subject to the weak interaction. Leptons are massive particles, therefore are subject to gravity.

Lepton properties
namesymbolspinelectric charge
(e)
mass
(MeV/c2)
mass comparable toantiparticleantiparticle
symbol
charged leptons[64]
electrone
12−10.51101 electronantielectrone+
muonμ
12−1105.7~ 200 electronsantimuonμ+
tauτ
12−11,777~ 2 protonsantitauτ+
neutrinos[65]
electron neutrinoν
e
120< 0.00046011000 electronelectron antineutrinoν
e
muon neutrinoν
μ
120< 0.1912 electronmuon antineutrinoν
μ
tau neutrinoν
τ
120< 18.2< 40 electronstau antineutrinoν
τ


Phases

Phase diagram for a typical substance at a fixed volume. Vertical axis is Pressure, horizontal axis is Temperature. The green line marks thefreezing point (above the green line is solid, below it is liquid) and the blue line the boiling point (above it is liquid and below it is gas). So, for example, at higher T, a higher P is necessary t

Matter

Matter is a general term for the substance of which all physical objects are made.[1][2] Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume.[3] In practice however there is no single correct scientific meaning of "matter," as different fields use the term in different and sometimes incompatible ways.

For much of the history of the natural sciences people have contemplated the exact nature of matter. The idea that matter was built of discrete building blocks, the so-called particulate theory of matter, was first put forward by the Greek philosophers Leucippus (~490 BC) and Democritus (~470–380 BC).[4] Over time an increasingly fine structure for matter was discovered: objects are made from molecules, molecules consist of atoms, which in turn consist of interacting subatomic particles like protons and electrons.[5][6]

Matter is commonly said to exist in four states (or phases): solid, liquid, gas and plasma. However, advances in experimental techniques have realized other phases, previously only theoretical constructs, such asBose–Einstein condensates and fermionic condensates. A focus on an elementary-particle view of matter also leads to new phases of matter, such as the quark–gluon plasma.[7]

In physics and chemistry, matter exhibits both wave-like and particle-like properties, the so-called wave–particle duality.[8][9][10]

In the realm of cosmology, extensions of the term matter are invoked to include dark matter and dark energy, concepts introduced to explain some odd phenomena of the observable universe, such as the galactic rotation curve. These exotic forms of "matter" do not refer to matter as "building blocks", but rather to currently poorly understood forms of mass and energy

 

Summary

The term "matter" is used throughout physics in a bewildering variety of contexts: for example, one refers to "condensed matter physics",[35] "elementary matter",[36] "partonic" matter, "dark" matter, "anti"-matter, "strange" matter, and "nuclear" matter. In discussions of matter and antimatter, normal matter has been referred to by Alfvén as koinomatter.[37] It is fair to say that in physics, there is no broad consensus as to an exact definition of matter, and the term "matter" usually is used in conjunction with some modifier.


Definitions


Common definition

The DNA molecule is an example of matter under the "atoms and molecules" definition.

The common definition of matter is anything that has both mass and volume (occupies space).[38][39] For example, a car would be said to be made of matter, as it occupies space, and has mass.

The observation that matter occupies space goes back to antiquity. However, an explanation for why matter occupies space is recent, and is argued to be a result of the Pauli exclusion principle.[40][41] Two particular examples where the exclusion principle clearly relates matter to the occupation of space are white dwarf stars and neutron stars, discussed further below.


Relativity

In the context of relativity, mass is not a conserved quantity.[1] Thus, in relativity usually a more general view is taken that it is not mass, but theenergy–momentum tensor that quantifies the amount of matter. Matter therefore is anything that contributes to the energy–momentum of a system, that is, anything that is not pure gravity.[42][43] This view is commonly held in fields that deal with general relativity such as cosmology.


Atoms and molecules definition

A definition of "matter" that is based upon its physical and chemical structure is: matter is made up of atoms and molecules.[44] As an example,deoxyribonucleic acid molecules (DNA) are matter under this definition because they are made of atoms. This definition can be extended to include charged atoms and molecules, so as to include plasmas (gases of ions) and electrolytes (ionic solutions), which are not obviously included in the atoms and molecules definition. Alternatively, one can adopt the protons, neutrons and electrons definition.


Protons, neutrons and electrons definition

A definition of "matter" more fine-scale than the atoms and molecules definition is: matter is made up of what atoms and molecules are made of, meaning anything made of protons, neutrons, and electrons.[45] This definition goes beyond atoms and molecules, however, to include substances made from these building blocks that are not simply atoms or molecules, for example white dwarf matter — typically, carbon and oxygen nuclei in a sea of degenerate electrons. At a microscopic level, the constituent "particles" of matter such as protons, neutrons and electrons obey the laws of quantum mechanics and exhibit wave–particle duality. At an even deeper level, protons and neutrons are made up of quarks and the force fields (gluons) that bind them together (see Quarks and leptons definition below).


Quarks and leptons definition

Under the "quarks and leptons" definition, the elementary and composite particles made of the quarks (in purple) and leptons (in green) would be "matter"; while the gauge bosons (in red) would not be "matter". However, interaction energy inherent to composite particles (for example, gluons involved in neutrons and protons) contribute to the mass of ordinary matter.

As may be seen from the above discussion, many early definitions of what can be called ordinary matter were based upon its structure or "building blocks". On the scale of elementary particles, a definition that follows this tradition can be stated as: ordinary matter is everything that is composed of elementary fermions, namely quarks and leptons.[46][47] The connection between these formulations follows.

Leptons (the most famous being the electron), and quarks (of which baryons, such as protons and neutrons, are made) combine to form atoms, which in turn form molecules. Because atoms and molecules are said to be matter, it is natural to phrase the definition as: ordinary matter is anything that is made of the same things that atoms and molecules are made of. (However, notice that one also can make from these building blocks matter that is notatoms or molecules.) Then, because electrons are leptons, and protons and neutrons are made of quarks, this definition in turn leads to the definition of matter as being "quarks and leptons", which are the two types of elementary fermions. Carithers and Grannis state: Ordinary matter is composed entirely of first-generation particles, namely the [up] and [down] quarks, plus the electron and its neutrino.[48] (Higher generations particles quickly decay into first-generation particles, and thus are not commonly encountered.[49])

This definition of ordinary matter is more subtle than it first appears. All the particles that make up ordinary matter (leptons and quarks) are elementary fermions, while all the force carriers are elementary bosons.[50]. The W and Z bosons that mediate the weak force are not made of quarks or leptons, and so are not ordinary matter, even if they have mass.[51] In other words, mass is not something that is exclusive to ordinary matter.

The quark–lepton definition of ordinary matter, however, identifies not only the elementary building blocks of matter, but also includes composites made from the constituents (atoms and molecules, for example). Such composites contain an interaction energy that holds the constituents together, and may constitute the bulk of the mass of the composite. As an example, to a great extent, the mass of an atom is simply the sum of the masses of its constituent protons, neutrons and electrons. However, digging deeper, the protons and neutrons are made up of quarks bound together by gluon fields (see dynamics of quantum chromodynamics) and these gluons fields contribute significantly to the mass of hadrons.[52] In other words, most of what composes the "mass" of ordinary matter is due to the binding energy of quarks within protons and neutrons.[53] For example, the sum of the mass of the three quarks in a nucleon is approximately 12.5 MeV/c2, which is low compared to the mass of a nucleon (approximately 938 MeV/c2).[49][54] The bottom line is that most of the mass of everyday objects comes from the interaction energy of its elementary components.


Smaller building blocks?

The Standard Model groups matter particles into three generations, where each generation consists of two quarks and two leptons. The first generation is the up and down quarks, the electron and the electron neutrino; the second includes the charm and strange quarks, the muon and the muon neutrino; the third generation consists of the top and bottom quarks and the tau and tau neutrino.[55] The most natural explanation for this would be that quarks and leptons of higher generations are excited states of the first generations. If this turns out to be the case, it would imply that quarks and leptons are composite particles, rather than elementary particles.[56]


Structure

In particle physics, fermions are particles which obey Fermi–Dirac statistics. Fermions can be elementary, like the electron, or composite, like the proton and the neutron. In the Standard Model there are two types of elementary fermions: quarks and leptons, which are discussed next. 

 

MATTER

In science, matter is commonly defined as the substance of which physical objects are composed, not counting the contribution of various energies 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.

Matter (energy) can be in several different states, the most common being high energy physics, solids, liquids and gases.

Definition

Anything which both occupies space and has mass is known as matter. In physics, there is no broad consensus as to an exact definition of matter, partly because the notion of "taking up space" must be ill-defined for quantum reasons. 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.[1] 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.

Most of the mass of protons and neutrons comes from the kinetic energy of the quarks, and from the mass of the gluons (a type of boson) which bind the quarks, not from the masses of the quarks themselves. Thus, the definition of "matter" as being fermions suffers primarily from the problem that most (more than 99%) of the mass of "ordinary matter" is not composed of fermions (leptons and quarks) but rather from bosons and fermion energy of motion.

In an extreme example, Glueballs have mass, but contain no particle with rest mass, nor any elementary fermions.

 

Properties of matter

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.[1]

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.

 

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".

 

Chemical 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.[2]

 

Antimatter

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.

 

Dark matter

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

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.

MATTER

‘Matter’ is the name that scientists give to anything that has mass and occupies space (volume). You and I are made of matter; so is this book and so is the air you are breathing.

Chairs we sit, food we eat, all stars, from simple tools to complicated computers that we use and all the invisible particles of gases forming the atmosphere are some examples of matter. Even all living things; from small organisms to big animals, are all examples of matter.

Scientist also use the word ‘substances’. This means a particular type of matter, which you can put a name to. Salt is a substance, and so is water. Light is not a substance, because it has no mass and volume and it is not matter.

Example: Which of the following are substances?

a) water b) sugar c) electricity d) alcohol

e) sound f) oxygen g) angel h) Vinegear

The states of matter

There are three states of matter: solid, liquid and gas. Solids have a fixed shape—think of an ice cube.

Liquids have no fixed shape, but they take up the shape of their container and their volume is fixed—think of a litre of water. Gases have no fixed shape or volume. They spread out (diffuse) to fill all the available

space—think of steam coming out of a kettle.

Gases are usually invisible, which makes it difficult to think of them as matter at all. But we know gases are a form of matter because they have mass. You can weigh gases—though their density is low, so they don't weigh much. A balloonful of air weighs about 10 g. A bedroomful of air weighs about 75 kg—as much as a person!

Most substances can exist in all three states, depending on the temperature. Water is a solid (ice) below 0°C, a gas (steam) above 100°C and a liquid between these temp eratures.

When we say 'water is a liquid', we mean that the substance scientists know as H
2O is a liquid at normal temperatures. 'Normal' temperature is around 25°C in Moldova. But if you live in the Arctic it might be more sensible to say 'water is a solid', because it certainly is one most of the time.

You can decide the normal state of a substance if you know its melting point and boiling point. For example,

the element bromine has a melting point of -7°C and a boiling point of 59°C. So at the 'normal' temperature of 20°C, bromine will be melted but not boiled. In other words it will be a liquid.

Solid state: Matter existing in solid state has a definite shape and volume. The atoms and molecules composing a solid are very close to each other. There is very little empty space between particles. Particles of solid states can't move or move very very slowly.

Liquid state: Matter in liquid state doesn't have a definite shape since the molecules of liquids flow over each other. They take the shapes of container in which they are placed. Distances between particles in liquid state are bigger than in solid state. And also the liquid particles move faster than solid particles.

Gaseous state: In gas state, atoms and molecules are quite far apart and move randomly. Gas molecules can form homogeneous mixture. A gas has no definite shape and volume but flows and expands to fill any container in which it is placed and takes its shape and volume. The fastest particles are gas particles.







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