Matter This article is about the concept in the physical sciences. For other uses, see Matter (disambiguation). Matter Matter is usually classified into three classical states, with plasma sometimes added as a fourth state. From top to bottom: quartz (solid), water (liquid), nitrogen dioxide (gas), and a plasma globe (plasma). Matter is a poorly defined term in science (see definitions below). The term has often been used in reference to a substance (often a particle) that has rest mass. Matter is also used loosely as a general term for the substance that makes up all observable physical objects. [1][2] All objects we see with the naked eye are composed of atoms. This atomic matter is in turn made up of interacting subatomic particles—usually a nucleus of protons and neutrons, and a cloud of orbiting electrons.[3][4] Typically, science considers these composite particles matter because they have both rest mass and volume. By contrast, massless particles, such as photons, are not considered matter, because they have neither rest mass nor volume. However, not all particles with rest mass have a classical volume, since fundamental particles such as quarks and leptons (sometimes equated with matter) are considered point particles with no effective size or volume. Nevertheless, quarks and leptons together make up ordinary matter, and their interactions contribute to the effective volume of the composite particles that make up ordinary matter. Matter commonly exists in four states (or phases): solid, liquid and gas, and plasma. However, advances in experimental techniques have revealed other previously theoretical phases, 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. [5] 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).[6] Albert Einstein showed[7] that ultimately all matter is capable of being converted to energy (known as mass-energy equivalence) by the famous formula E = mc2, where E is the energy of a piece of matter of mass m, times c2 the speed of light squared. As the speed of light is 299,792,458 metres per second (186,282 mi/s), a relatively small amount of matter may be converted to a large amount of energy. An example is that positrons and electrons (matter) may transform into photons (non-matter). However, although matter may be created or destroyed in such processes, neither the quantity of mass or energy change during the process. Matter should not be confused with mass, as the two are not quite the same in modern physics.[8] For example, mass is a conserved quantity, which means that its value is unchanging through time, within closed systems. However, matter is not conserved in such systems, although this is not obvious in ordinary conditions on Earth, where matter is approximately conserved. Still, special relativity shows that matter may disappear by conversion into energy, even inside closed systems, and it can also be created from energy, within such systems. However, because mass (like energy) can neither be created nor destroyed, the quantity of mass and the quantity of energy remain the same during a transformation of matter (which represents a certain amount of energy) into non-material (i.e., non-matter) energy. This is also true in the reverse transformation of energy into matter. Different fields of science use the term matter in different, and sometimes incompatible, ways. Some of these ways are based on loose historical meanings, from a time when there was no reason to distinguish mass and matter. As such, there is no single universally-agreed scientific meaning of the word matter. Scientifically, the term mass is well- defined, but matter is not. Sometimes in the field of physics matter is simply equated with particles that exhibit rest mass (i.e., that cannot travel at the speed of light), such as quarks and leptons. However, in both physics and chemistry, matter exhibits both wave-like and particle-like properties, the so-called wave–particle duality.[9][10][11] Definition 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).[12][13] 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.[14] [15] 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 Main article: mass-energy equivalence In the context of relativity, mass is not an additive quantity, in the sense that one can add the rest masses of particles in a system to get the total rest mass of the system.[1] Thus, in relativity usually a more general view is that it is not the sum of rest masses, but the energy– momentum tensor that quantifies the amount of matter. This tensor gives the rest mass for the entire system. Matter therefore is sometimes considered as anything that contributes to the energy– momentum of a system, that is, anything that is not purely gravity.[16][17] This view is commonly held in fields that deal with general relativity such as cosmology. But in this view, light and other types of insubstantial energy may be part of matter. The reason for this is that in this definition, electromagnetic radiation (such as light) as well as the energy of electromagnetic fields contributes to the mass of systems, and therefore appears to add matter to them. For example, light radiation (or thermal radiation) trapped inside a box would contribute to the mass of the box, as would any kind of energy inside the box, including the kinetic energy of particles held by the box. Nevertheless, isolated individual particles of light (photons) and the isolated kinetic energy of massive particles, are normally not considered to be matter. A difference between matter and mass therefore may seem to arise when single particles are examined. In such cases, the mass of single photons is zero. For particles with rest mass, such as leptons and quarks, isolation of the particle in a frame where it is not moving, removes its kinetic energy. A source of definition difficulty in relativity arises from two definitions of mass in common use, one of which is formally equivalent to total energy (and is thus observer-dependent), and the other of which is referred to as rest mass or invariant mass and is independent of the observer. Only the latter type of mass is loosely equated with matter (since it can be weighed). However, energies which contribute to the first type of mass may be weighed also in special circumstances, such as when trapped in a system with no net momentum (as in the box example above). Thus, a photon with no mass may add mass to a system in which it is trapped. Since such mass is measured as part of ordinary matter in complex systems, the matter status of massless particles becomes unclear in such systems. These problems contribute to the lack of a rigorous definition of matter in science, although mass is easier to define as the total stress-energy above (this is also what is weighed on a scale, and what is the source of gravity). Atoms definition A definition of matter based on its physical and chemical structure is: matter is made up of atoms.[18] As an example, deoxyribonucleic acid molecules (DNA) are matter under this definition because they are made of atoms. This definition can extend 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 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 positively charged protons, neutral neutrons, and negatively charged electrons.[19] 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 seen in 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.[20][21] 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 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. [21] (Higher generations particles quickly decay into first-generation particles, and thus are not commonly encountered.[22]) 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.[23] 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.[24] 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.[25] In other words, most of what composes the mass of ordinary matter is due to the binding energy of quarks within protons and neutrons.[26] 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).[22][27] The bottom line is that most of the mass of everyday objects comes from the interaction energy of its elementary components. Smaller building blocks issue 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.[28] 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.[29] Structure In particle physics, fermions are particles that obey Fermi–Dirac statistics. Fermions can be elementary, like the electron—or composite, like the proton and neutron. In the Standard Model, there are two types of elementary fermions: quarks and leptons, which are discussed next. Quarks Main article: Quark Quarks are particles of spin-1⁄2, implying that they are fermions. They carry an electric charge of −1⁄3 e (down-type quarks) or +2⁄3 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[30] name symbol spin electric charge (e) mass (MeV/c2) mass comparable to antiparticle antiparticle symbol up-type quarks up u 1⁄2 +2⁄3 1.5 to 3.3 ~ 5 electrons antiup u charm c 1⁄2 +2⁄3 1160 to 1340 ~ 1 proton anticharm c top t 1⁄2 +2⁄3 169,100 to 173,300 ~ 180 protons or ~ 1 tungsten atom antitop t down-type quarks down d 1⁄2 −1⁄3 3.5 to 6.0 ~ 10 electrons antidown d strange s 1⁄2 −1⁄3 70 to 130 ~ 200 electrons antistrange s bottom b 1⁄2 −1⁄3 4130 to 4370 ~ 5 protons antibottom b Quark structure of a proton: 2 up quarks and 1 down quark. Baryonic matter Main article: Baryon 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 usually refers 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 baryonic matter. About 23% is dark matter, and about 72% is dark energy.[
Posted on: Mon, 11 Nov 2013 09:20:14 +0000
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