Glossari
GLOSSARI
NOTA: Definicions extretes de la base de dades OXFORD REFERENCE ONLINE CORE COLLECTION.
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black-body radiation The thermal radiation that would be emitted by a black body. Black-body radiation has a characteristic spectrum, described by Planck's law, in which the peak of the emission moves to shorter wavelengths as the temperature of the black body rises (see Wien's displacement law). The cosmic background radiation is black-body radiation, and stars often act as black-body radiators in the optical part of the spectrum.
Black-body radiation: The higher the temperature of a black body, the more the peak of the emitted radiation is displaced towards shorter wavelengths. Hot stars have a peak in the ultraviolet, while cool stars peak in the infrared. Solar-type stars have a peak in the visible part of the spectrum.
Citació:
"black-body radiation" A Dictionary of Astronomy. Ed. Ian Ridpath. Oxford University Press, 2003. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 24 January 2005 <http://www.oxfordreference.com/search?siteToSearch=aup&q=Black-body+radiation&searchBtn=Search&isQuickSearch=true>
black hole An object with such a strong gravitational field that its escape velocity exceeds the velocity of light. One way in which black holes are believed to form is when massive stars collapse at the end of their lives. A collapsing object becomes a black hole when its radius has shrunk to a critical size, known as the Schwarzschild radius, and light can no longer escape from it. The surface having this critical radius is referred to as the event horizon, and marks the boundary inside which all information is trapped. Hence events within the black hole cannot be observed from outside. Theory indicates that both space and time become distorted inside the event horizon and that an object collapses to a single point, a singularity , at the centre of a black hole. Black holes may have any mass. Supermassive black holes (105 solar masses) may exist at the centres of active galaxies. At the other extreme, mini black holes of radii 10-10 m and masses similar to that of an asteroid may have been formed in the extreme conditions following the Big Bang.
No black hole has ever been observed directly. However, an accretion disk may form around a black hole when matter falls towards it from a nearby companion star or other source. Energy predominantly at X-ray wavelengths is produced as matter in the accretion disk loses momentum and spirals in; these X-rays can be detected by satellites in orbit. Several black-hole candidates have been located in our Galaxy, most famously Cygnus X-1.
There are several theoretically possible forms of black hole. A non-rotating black hole without electrical charge is known (after K. Schwarzschild ) as a Schwarzschild black hole. A non-rotating black hole with electrical charge is termed a Reissner–Nordström black hole after the German physicist Hans Jacob Reissner (1874–1967) and the Finn Gunnar Nordström (1881–1923). In practice, black holes are likely to be rotating and uncharged, a form known as a Kerr black hole. Black holes are not entirely black; theory suggests that they can emit energy in the form of Hawking radiation.
Citació:
"black hole" A Dictionary of Astronomy. Ed. Ian Ridpath. Oxford University Press, 2003. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 24 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=black+hole&searchBtn=Search&isQuickSearch=true>
Bose- Einstein condensation A phenomenon occurring in a macroscopic system consisting of a large number of bosons at a sufficiently low temperature, in which a significant fraction of the particles occupy a single quantum state of lowest energy (the ground state). Bose—Einstein condensation can only take place for bosons whose total number is conserved in collisions. Because of the Pauli exclusion principle, it is impossible for two or more fermions to occupy the same quantum state, and so there is no analogous condensation phenomenon for such particles. Bose—Einstein condensation is of fundamental importance in explaining the phenomenon of superfluidity . At very low temperatures (around 2 × 10-7 K) a Bose—Einstein condensate can form, in which several thousand atoms become a single entity (a superatom). This effect has been observed with atoms of rubidium and lithium. The effect is named after the Indian physicist Satyendra Nath Bose (1894–1974) and Albert Einstein .
Citació:
"Bose-Einstein condensation" A Dictionary of Physics. Ed. Alan Isaacs. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 http://www.oxfordreference.com/search?siteToSearch=oso&q=Bose-+Einstein+condensation&searchBtn=Search&isQuickSearch=true
Bose, Satyendranath (1894–1974) Indian physicist and mathematician who contributed to the theory of quantum mechanics and statistical mechanics . Bose made the initial advances to describe the statistical properties of certain elementary particles (now called bosons ). These particles all have the property that any number of them can occupy the same quantum state: that is they do not obey Enrico Fermi's exclusion principle . Bose's work was developed by Einstein and the statistics that such particles obey are called Bose-Einstein statistics.
Citació:
"Bose, Satyendranath" World Encyclopedia. Philip's, 2004. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 http://www.oxfordreference.com/search?siteToSearch=oso&q=Bose%2C+Satyendranath&searchBtn=Search&isQuickSearch=true
boson Elementary particle that has an integer spin . Named after the Indian physicist Satyendranath Bose , bosons are those particles not covered by the exclusion principle which says no two electrons in an atom can have the same energy and spin. This means that the number of bosons occupying the same quantum state is not restricted. Bosons are force-transmitting particles, such as photons and gluons (the particles that hold quarks together). See also fermion
Citació:
"boson" World Encyclopedia. Philip's, 2004. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 http://www.oxfordreference.com/search?siteToSearch=oso&q=boson&searchBtn=Search&isQuickSearch=true
Brownian motion A continuous-time version of the random walk , named after Robert Brown (1773–1858), a Scottish botanist. In 1827, Brown noticed the erratic movement of pollen grains under water. The first explanation of this motion (in termsof the bombardment of the pollen by the surrounding water molecules) was given by Einstein in 1905. Norbert Wiener provided a concise mathematical description in 1918 and the motion is also called a Wiener process.
Starting from the origin at time 0, the path of a particle is made up of independent increments (in d dimensions) which are such that its distance from the origin at time t is an observation from a normal distribution with mean 0 and variance proportional to t.
An alternative description is provided by assuming that it is the velocity rather than the position which is changing through time as a consequence of collisions and friction. This model is called the Ornstein–Uhlenbeck process.
Citació:
"Brownian motion" A Dictionary of Statistics. Graham Upton and Ian Cook. Oxford University Press, 2002. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=Brownian+motion&searchBtn=Search&isQuickSearch=true>
Brownian movement Random, zigzag movement of particles suspended in a fluid (liquid or gas). It is caused by the unequal bombardment of larger particles, from different sides, by the smaller molecules of the fluid. The movement is named after the Scottish botanist Robert Brown (1773–1858), who in 1827 observed the movement of plant spores floating in water.
Citació:
"Brownian movement" World Encyclopedia. Philip's, 2004. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=Brownian+movement&searchBtn=Search&isQuickSearch=true>
cosmology The study of the structure and evolution of the Universe. Observational cosmology is concerned with the physical properties of the Universe, such as its chemical composition, density, and rate of expansion, as well as the distribution of galaxies and clusters of galaxies. Physical cosmology tries to understand these properties by applying known laws of physics and astrophysics. Theoretical cosmology involves making models that give a mathematical description of the observed properties of the Universe based on this physical understanding. Cosmology also has philosophical, or even theological, aspects in that it seeks to understand why the Universe has its observed properties. Active areas of research in cosmology include: the use of large-scale galaxy surveys to map the distribution of matter on cosmological scales, according to the redshift–distance relation; the investigation of fluctuations in the temperature of the cosmic background radiation and their implications for theories of galaxy formation; the construction of the cosmological distance scale using observations of stars in distant galaxies; and the search for dark matter and the identification of its nature.
Theoretical cosmology is based on the general theory of relativity, Einstein's theory of gravitation. Of all the forces of nature, gravity has the strongest effect on large scales and dominates the overall behaviour of the Universe. General relativity allows cosmologists to create mathematical models that describe the relationship between space-time and the material contents of the Universe. All cosmological theories incorporate the cosmological principle, which states that the Universe looks much the same from all places. The standard cosmological theory is called the Big Bang theory, and is based on a particular solution of the equations of general relativity, called the Friedmann universe. The Big Bang theory is supported by a great deal of observational evidence and is accepted by most cosmologists.
Citació:
"cosmology" A Dictionary of Astronomy. Ed. Ian Ridpath. Oxford University Press, 2003. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 24 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=cosmology&searchBtn=Search&isQuickSearch=true>
Einstein, Albert (1879–1955) US physicist, b. Germany , who devised the famous theories of relativity . Einstein published many important theoretical papers: his explanation of Brownian movement confirmed the reality of atoms, his application of quantum theory to photoelectricity won him the 1921 Nobel Prize in physics. In 1905, he devised the special theory of relativity, which completely revolutionized physics and led, through its equivalence of mass and energy (E = mc2), to the invention of the atomic bomb. In 1916, Einstein produced the general theory of relativity. He also made other fundamental contributions to quantum theory.
Citació:
"Einstein, Albert" World Encyclopedia. Philip's, 2004. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=Einstein%2C+Albert++&searchBtn=Search&isQuickSearch=true>
Einstein–Podolsky–Rosen thought experiment Thought experiment introduced in 1935 by these three authors in their Physical Review paper ‘Can Quantum-Mechanical Description of Physical Reality be Considered Complete?' Suppose two quantum systems 1 and 2 that briefly interact, and are then separated and in no kind of physical contact. Then a measurement on system 1 for property P will yield some definite value, p1 for 1. Corresponding to this can be calculated a value of P, p2, for system 2. Similarly if we measured a property Q yielding a value q1 for 1, we can calculate the value q2 for 2. So definite magnitudes can be assigned to properties of 2 from measurements that do not affect 2. But if we now consider conjugate pairs (see Heisenberg uncertainty principle ), such as the position and momentum of a particle, this makes the existence of states of 2 depend upon which processes of measurement we choose to carry out on 1, although there is no signal betwen them. Attributing an antecedent determinate nature to 2 that can be revealed by a measurement on 1 contradicts the Heisenberg uncertainty principle, if that is taken to apply to reality itself, rather than to mere indeterminacies of measurement. The contentious conclusion of the paper was that the quantum mechanical description of physical reality is not complete. See also Bell's theorem .
Citació:
"Einstein–Podolsky–Rosen thought experiment" The Oxford Dictionary of Philosophy. Simon Blackburn. Oxford University Press, 1996. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=Einstein%E2%80%93Podolsky%E2%80%93Rosen+thought+experiment&searchBtn=Search&isQuickSearch=true>
Einstein–de Sitter universe A type of universe in which the mean density of matter is precisely matched to the critical density. Such a model will not actually collapse, but will expand for ever with a continually decreasing expansion rate. This model lies on the dividing line between a closed Friedmann universe (which collapses) and an open Friedmann universe (which does not). This model has the mathematical virtue of simplicity, in that it is spatially flat (see curvature of space-time ). It is named after A. Einstein and W. de Sitter.
Citació:
"Einstein–de Sitter universe" A Dictionary of Astronomy. Ed. Ian Ridpath. Oxford University Press, 2003. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=Einstein%E2%80%93de+Sitter+universe&searchBtn=Search&isQuickSearch=true>
Einstein–Smoluchowski equation A relation between the diffusion coefficient D and the distance 
that a particle can jump when diffusing in a time 
. The Einstein–Smoluchowski equation, which is D = 
2/2 
, gives a connection between the microscopic details of particle diffusion and the macroscopic quantities associated with the diffusion, such as the viscosity. The equation is derived by assuming that the particles undergo a random walk. The quantities in the equation can be related to quantities in the kinetic theory of gases, with 
/ 
taken to be the mean speed of the particles and 
their mean free path. The Einstein–Smoluchowski equation was derived by Albert Einstein and the Polish physicist Marian Ritter von Smolan-Smoluchowski.
Citació:
"Einstein–Smoluchowski equation" A Dictionary of Chemistry. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=Einstein%E2%80%93Smoluchowski+equation+&searchBtn=Search&isQuickSearch=true>
Einstein coefficients Coefficients used in the quantum theory of radiation , related to the probability of a transition occurring between the ground state and an excited state (or vice versa) in the processes of induced emission and spontaneous emission . For an atom exposed to electromagnetic radiation , the rate of absorption Ra is given by Ra = B 
, where 
is the density of electromagnetic radiation and B is the Einstein B coefficient associated with absorption. The rate of induced emission is also given by B 
, with the coefficient B of induced emission being equal to the coefficient of absorption. The rate of spontaneous emission is given by A, where A is the Einstein A coefficient of spontaneous emission. The A and B coefficients are related by A = 8 
hv3B/c3, where h is the Planck constant , v is the frequency of electromagnetic radiation, and c is the speed of light. The coefficients were put forward by Albert Einstein in 1916–17 in his analysis of the quantum theory of radiation.
Citació:
"Einstein coefficients" A Dictionary of Physics. Ed. Alan Isaacs. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=Einstein+coefficients+&searchBtn=Search&isQuickSearch=true>
Einstein equation
1. The mass–energy relationship announced by Albert Einstein in 1905 in the form E = mc2, where E is a quantity of energy, m its mass, and c is the speed of light. It presents the concept that energy possesses mass.
2. The relationship Emax = hf - W, where Emax is the maximum kinetic energy of electrons emitted in the photoemissive effect, h is the Planck constant, f the frequency of the incident radiation, and W the work function of the emitter. This is also written Emax = hf - 
e, where e is the electronic charge and 
a potential difference, also called the work function. (Sometimes W and 
are distinguished as work function energy and work function potential.) The equation can also be applied to photoemission from gases, when it has the form: E = hf - I, where I is the ionization potential of the gas.
Citació:
"Einstein equation" A Dictionary of Physics. Ed. Alan Isaacs. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 24 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=Einstein+equation+&searchBtn=Search&isQuickSearch=true>
Einstein theory of specific heat A theory of the specific heat capacity of solids put forward by Albert Einstein in 1906, in which it was assumed that the specific heat capacity is a consequence of the vibrations of the atoms of the lattice of the solid. Einstein assumed that each atom has the same frequency . The theory leads to the correct conclusion that the specific heat of solids tends to zero as the temperature goes to absolute zero, but does not give a correct quantitative description of the low-temperature behaviour of the specific heat capacity. In the Debye theory of specific heat , and in other analyses of this problem, Einstein's simplifying approximation was improved on by taking account of the fact that the frequencies of lattice vibrations can have a range of values.
Citació:
"Einstein theory of specific heat" A Dictionary of Chemistry. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=Einstein+theory+of+specific+heat++&searchBtn=Search&isQuickSearch=true>
general theory of relativity A theory announced by A. Einstein in 1915 that describes how space and time are affected by the gravitational fields of matter. The theory predicts that gravitational fields change the geometry of space and time, causing it to become curved. This curvature is apparent in a number of ways. First, light is bent in a gravitational field, a prediction that was confirmed by photographic measurements of the positions of stars near the limb of the Sun made during a total solar eclipse in 1919. The same effect manifests itself in a delay in radio signals from distant space probes as the signals pass the limb of the Sun. The curvature of space near the Sun also causes the perihelion point of Mercury's orbit to move forward, by 43" per century more than predicted by I. Newton's theory of gravity (see advance of perihelion) . In the orbits of pulsars in binary systems, the advance of periastron can amount to several degrees per year.
Another effect predicted by general relativity is the redshift of light caused by gravity. This has been demonstrated in the redshift of lines in the spectra of the Sun and, more noticeably, white dwarfs. Other predictions of the general theory include the gravitational lens effect; gravitational waves; singularities; and the invariance of the universal gravitational constant, G. General relativity was developed from the principle of equivalence between gravitational and inertial forces.
Citació:
"general theory of relativity" A Dictionary of Astronomy. Ed. Ian Ridpath. Oxford University Press, 2003. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 24 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=general+theory+of+relativity++&searchBtn=Search&isQuickSearch=true>
gravitation Force of attraction that is exercised by every particle of matter as a result of its mass. Gravitation is the weakest of the four fundamental forces, but it is apparent because of the great mass of the Earth. The Moon has only 1/6 of the Earth's gravitational force. Gravitation was first described (1687) by Sir Isaac Newton, whose law of gravitation stated that gravitational force is directly proportional to the masses of the interacting bodies and inversely proportional to the square of the distance between them. Thus, the gravitational force will decrease by 1/4 if the distance between two objects is doubled. Albert Einstein developed a more complete treatment of gravitation, showing in his general theory of relativity that gravitation is a manifestation of space-time.
Citació:
"gravitation" World Encyclopedia. Philip's, 2004. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=gravitation+&searchBtn=Search&isQuickSearch=true>
gravitational redshift The redshift of light or other electromagnetic radiation caused by a strong gravitational field; also known as the Einstein shift. It arises because radiation loses energy as it passes out of the gravitational field of the emitting body. As a consequence, the frequency of the radiation decreases and its wavelength is shifted to the red end of the spectrum. The redshift at wavelength 
is given by Gm 
/c2 r, where m is the mass of the body, r is the distance of the emitting region from the centre of mass, c is the speed of light, and G is the universal gravitational constant. A gravitational redshift has been observed in the light from some white dwarfs, and would result in the rapid fading out of a black hole in the process of formation as seen from outside.
Citació:
"gravitational redshift" A Dictionary of Astronomy. Ed. Ian Ridpath. Oxford University Press, 2003. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=gravitational+redshift&searchBtn=Search&isQuickSearch=true>
induced emission (stimulated emission) The emission of a photon by an atom in the presence of electromagnetic radiation. The atom can become excited by the absorption of a photon of the right energy and, having become excited, the atom can emit a photon. The rate of absorption is equal to the rate of induced emission, both rates being proportional to the density of photons of the electromagnetic radiation. The relation between induced emission and spontaneous emission is given by the Einstein coefficients. The process of induced emission is essential for the operation of lasers and masers. See also quantum theory of radiation.
Citació:
"induced emission" A Dictionary of Physics. Ed. Alan Isaacs. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 24 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=induced+emission&searchBtn=Search&isQuickSearch=true>
mass Measure of the quantity of matter in an object. The standard unit of mass is the kilogram (one kg = 1000 grams). Scientists recognize two types of mass. The gravitational mass of a body is determined by its mutual attraction to another reference body, such as the Earth, as expressed in Newton's law of gravitation. Spring balances and platform balances proved a measure of gravitational mass. The inertial mass of a body is determined by its resistance to a change in state of motion, as expressed in the second law of motion. Inertia balances provide a measure of inertial mass. According to Einstein's principle of equivalence, upon which his general theory of relativity is based, the inertial mass and the gravitational mass of a given body are equivalent. See also weight
Citació:
"mass" World Encyclopedia. Philip's, 2004. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=mass&searchBtn=Search&isQuickSearch=true>
mass action The law of mass action states that the rate at which a chemical reaction takes place at a given temperature is proportional to the product of the active masses of the reactants. The active mass of a reactant is taken to be its molar concentration. For example, for a reaction xA + yB ? products
the rate is given by R = k[A]x[B]y where k is the rate constant. The principle was introduced by C. M. Guldberg and P. Waage in 1863. It is strictly correct only for ideal gases. In real cases activities can be used. See also equilibrium constant.
Citació:
"mass action" A Dictionary of Chemistry. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=mass+action&searchBtn=Search&isQuickSearch=true>
Nernst–Einstein equation An equation relating the limiting molar conductivity [Lambda]m0 (see Kohlrausch's law) to the ionic diffusion coefficients, devised by Nernst and Albert Einstein. The Nernst–Einstein equation is [Lambda]m0 = (F2/RT)(v+z+2D+ + v–z–2D–), where F is the Faraday constant, R is the gas constant, T is the thermodynamic temperature, v+ and v– are the number of cations and anions per formula unit of electrolyte, z+ and z– are the valences of the ions, and D+ and D– are the diffusion coefficients of the ions. An application of the Nernst–Einstein equation is to calculate the ionic diffusion coefficients from experimental determinations of conductivity.
Citació:
"Nernst–Einstein equation" A Dictionary of Chemistry. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=Nernst%E2%80%93Einstein+equation+&searchBtn=Search&isQuickSearch=true>
nuclear bomb The first and only nuclear bomb to have been used in warfare was the atomic bomb, which was developed in the Manhattan Project, and exploded over Hiroshima and Nagasaki on 6 and 9 August 1945 respectively. It worked through fission of highly enriched uranium, though since then plutonium has also been used. It was the first weapon of mass destruction. Through their explosive power and heat they killed 150,000 people, while even more suffered from radiation subsequently. Immediately, the atomic bomb became a symbol, if not a fact, of superpower status, as the Soviet Union developed its bomb by 1949, the UK by 1952, France by 1960, and China by 1962. Since then, India and Pakistan, Israel, and Nort Korea have also achieved nuclear capability.
An even more powerful nuclear weapon was developed in the hydrogen bomb, based on destruction through nuclear fusion, and was acquired by the USA in 1952, the USSR in 1953, the UK (1957), China (1967), and France (1968). Finally, an even more potent weapon was created in the enhanced hydrogen bomb, developed by the USA in 1977, which used a beryl coating to vastly expand its radioactive power.
The nuclear bomb in its various forms shaped politics throughout the Cold War era (1948–91). Despite fundamental tensions between the great power blocs (mainly NATO and the Warsaw Pact) , as the USA and the USSR amassed stockpiles of nuclear weapons capable of destroying the earth several times over, an all-out war became too costly to contemplate. Less tangibly, it shaped the political and social culture of postwar generations, articulated by organizations like CND and Pugwash, as well as individuals like Einstein, Russell, and Sakharov, as nuclear weapons became the epitome of the dangers of scientific research in particular, and ‘modernity' in general.
Citació:
"nuclear bomb" A Dictionary of Contemporary World History. Jan Palmowski. Oxford University Press, 2003. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=nuclear+bomb+++&searchBtn=Search&isQuickSearch=true>
photoelectric effect The liberation of electrons from a substance exposed to electromagnetic radiation. The number of such electrons (photoelectrons) emitted depends on the intensity of the radiation. The kinetic energy of the electrons emitted depends on the frequency of the radiation. The effect is a quantum process in which the radiation is regarded as a stream of photons, each having an energy hf, where h is the Planck constant and f is the frequency of the radiation. A photon can only eject an electron if the photon energy exceeds the work function, 
, of the solid, i.e. if hf0 = 
an electron will be ejected; f0 is the minimum frequency (or threshold frequency) at which ejection will occur. For many solids the photoelectric effect occurs at ultraviolet frequencies or above, but for some materials (having low work functions) it occurs with light. The maximum kinetic energy, Em, of the photoelectron is given by Einstein's equation: Em = hf – 
.
Apart from the liberation of electrons from atoms, other phenomena are also referred to as photoelectric effects. These are the photoconductive effect and the photovoltaic effect. In the photoconductive effect, an increase in the electrical conductivity of a semiconductor is caused by radiation as a result of the excitation of additional free charge carriers by the incident photons. Photoconductive cells, using such photosensitive materials as cadmium sulphide, are widely used as radiation detectors and light switches (e.g. to switch on street lighting).
In the photovoltaic effect, an e.m.f. is produced between two layers of different materials as a result of irradiation. The effect is made use of in photovoltaic cells, most of which consist of p–n semiconductor junctions. When photons are absorbed near a p–n junction new free charge carriers are produced (as in photoconductivity); however, in the photovoltaic effect the electric field in the junction region causes the new charge carriers to move, creating a flow of current in an external circuit without the need for a battery.
Citació:
"photoelectric effect" A Dictionary of Chemistry. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=photoelectric+effect&searchBtn=Search&isQuickSearch=true>
quantum mechanics Quantum theory, introduced by Max Planck (1858–1947) in 1900, was the first serious scientific departure from Newtonian mechanics. It involved supposing that certain physical quantities can only assume discrete values. In the following two decades it was applied successfully to different physical problems by Einstein and the Danish physicist Neils Bohr (1885–1962). It was superseded by quantum mechanics in the years following 1924, when the French physicist Louis de Broglie (1892–1987) introduced the idea that a particle may also be regarded as a wave. The Schrödinger wave equation relates the energy of a system to a wave function: the square of the amplitude of the wave is proportional to the probability of a particle being found in a specified position. The wave function expresses the lack of possibility of defining both the position and momentum of a particle (see Heisenberg uncertainty principle) . The allowed wave functions, or ‘eigenfunctions', have ‘eigen-values' that describe stationary states of the system.
Part of the difficulty with the notions involved is that a system may be in an indeterminate state at a time, characterized only by the probability of some result for an observation, but then ‘become' determinate (‘the collapse of the wave packet') when an observation is made (see also Einstein–Podolsky–Rosen thought experiment, Schrödinger's cat) . It is as if there is nothing but a potential for observation or a probability wave before observation is made, but when an observation is made the wave becomes a particle. The wave–particle duality seems to block any way of conceiving of physical reality in quantum terms. In the famous two-slit experiment, an electron is fired at a screen with two slits (like a tennis ball thrown at a wall with two doors in it). If one puts detectors at each slit, every electron passing the screen is observed to go through exactly one slit. But when the detectors are taken away, the electron acts like a wave process going through both slits, and interfering with itself. A particle such as an electron is usually thought of as always having an exact position, but its wave nature ensures that the amplitude of its waves is not absolutely zero anywhere (there is therefore a finite probability of it ‘tunnelling' from one position to emerge at another).
Citació:
"quantum mechanics" The Oxford Dictionary of Philosophy. Simon Blackburn. Oxford University Press, 1996. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=quantum+mechanics+&searchBtn=Search&isQuickSearch=true>
quantum statistics A statistical description of a system of particles that obeys the rules of quantum mechanics rather than classical mechanics. In quantum statistics, energy states are considered to be quantized. If the particles are treated as indistinguishable, Bose–Einstein statistics apply if any number of particles can occupy a given quantum state. Such particles are called bosons. All known bosons have an angular momentum nh, where n is zero or an integer and h is the Planck constant. For identical bosons the wave function is always symmetric. If only one particle may occupy each quantum state, Fermi–Dirac statistics apply and the particles are called fermions. All known fermions have a total angular momentum (n + 1/2)h/2 
and any wave function that involves identical fermions is always antisymmetric.
Citació:
"quantum statistics" A Dictionary of Chemistry. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=quantum+statistics++&searchBtn=Search&isQuickSearch=true>
quantum theory The foundation of 20th-century physics, together with the theory of relativity. It concerns itself with the relationship between matter and energy at the elementary or subatomic level, and with the behaviour of elementary particles. The theory is the basis of quantum mechanics. See also quantum numbers
"quantum theory" World Encyclopedia. Philip's, 2004. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=quantum+theory++&searchBtn=Search&isQuickSearch=true>
relativity theory The assumptions on which Einstein's special theory of relativity (1905) depends are (i) all inertial frameworks are equivalent for the description of all physical phenomena, and (ii) the speed of light in empty space is constant for every observer, regardless of the motion of the observer or the light source. (Although the second assumption may seem plausible in the light of the Michelson–Morley experiment of 1887, which failed to find any difference in the speed of light when measured in the direction of the earth's rotation or when measured perpendicular to it, it seems likely that Einstein was not influenced by the experiment, and may not even have known the result.) As a consequence of the second postulate, no matter how fast she travels, an observer can never overtake a ray of light, and see it as stationary beside her. However near her speed approaches to that of light, light still retreats at its classical speed. The consequences are that space, time, and mass become relative to the observer. Measurements made of quantities in an inertial system moving relative to one's own reveal slower clocks, contracted lengths, and heavier masses, with the effect increasing as the relative speed of the systems approaches the speed of light. Events deemed simultaneous as measured within one such system will not be simultaneous as measured from the other: time and space thus lose their separate identity, and become parts of a single space-time. The special theory also has the famous consequence (E = mc2) of the equivalence of energy and mass.
Einstein's general theory of relativity (1916) treats of non-inertial systems, i.e. those accelerating relative to each other. The leading idea is that the laws of motion in an accelerating frame are equivalent to those in a gravitational field. The theory treats gravity not as a Newtonian force acting in an unknown way across distance, but as a metrical property of a space-time continuum that is curved in the vicinity of matter. Gravity can be thought of as a field described by the metric tensor (see geometry) at every point. The classic analogy is with a rock sitting on a bed. If a ball-bearing is thrown across the bed, it is deflected towards the rock not by a mysterious force, but by the deformation of the space, i.e. the depression in the sheet around the rock. Interestingly, the general theory lends some credit to a version of the Newtonian absolute theory of space, in the sense that space itself is regarded as a thing with metrical properties of its own (see also clock paradox) . The search for a unified field theory is the attempt to show that just as gravity is explicable as a consequence of the nature of space-time, so are the other three fundamental physical forces: the strong and the weak nuclear forces, and the electromagnetic force (see also physics, philosophy of) . The theory of relativity is the most radical challenge to the ‘common-sense' view of space and time as fundamentally distinct from each other, with time as an absolute linear flow in which events are fixed in objective relationships.
Citació:
"relativity theory" The Oxford Dictionary of Philosophy. Simon Blackburn. Oxford University Press, 1996. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 24 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=relativity+theory++&searchBtn=Search&isQuickSearch=true>
space-time In relativity theory, a central concept that unifies the three space dimensions (length, breadth, and height) with time to form a four-dimensional frame of reference. Durations and rates of processes depend on the relative state of motion of the observer and the system observed. In 1907, Hermann Minkowski clarified relativity theory by describing space-time in terms of a four-dimensional geometry. Three co-ordinates of space and a time co-ordinate specify an event in space-time. A line drawn in this space represents a particle's path both in space and time. Albert Einstein incorporated this viewpoint into his theory of relativity: in the General Theory, gravity is a distortion of space-time by matter.
Citació:
"space-time" World Encyclopedia. Philip's, 2004. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=space-time++&searchBtn=Search&isQuickSearch=true>
special theory of relativity A theory proposed by A. Einstein in 1905, based on the proposition that the speed of light in a vacuum is constant throughout the Universe, and is independent of the motion of the observer and the emitting body. A consequence of this proposition is that three things happen as an object's velocity approaches the speed of light: its mass goes up, its length shortens in the direction of motion, and time slows down. Hence, according to special relativity, no object can ever reach the speed of light because its mass would then become infinite, its length would become zero, and time would stand still. In addition, Einstein concluded that the mass of a body is a measure of its energy content, according to the famous equation E = mc2, where c is the speed of light. This equation describes the conversion of mass into energy in nuclear reactions within stars. See also relativity.
Citació:
"special theory of relativity" A Dictionary of Astronomy. Ed. Ian Ridpath. Oxford University Press, 2003. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=special+theory+of+relativity+&searchBtn=Search&isQuickSearch=true>
Stark–Einstein law The law stating that in a photochemical process (such as a photochemical reaction) one photon is absorbed by each molecule causing the main photochemical process. In some circumstances, one molecule, having absorbed a photon, initiates a process involving several molecules. The Stark–Einstein law is named after Johannes Stark and Albert Einstein.
Citació:
"Stark–Einstein law" A Dictionary of Chemistry. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 20 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=Stark%E2%80%93Einstein+law+&searchBtn=Search&isQuickSearch=true>
unified-field theory A comprehensive theory that would relate the electromagnetic, gravitational, strong, and weak interactions(see fundamental interactions) in one set of equations. In its original context the expression referred only to the unification of general relativity and classical electromagnetic theory. No such theory has yet been found but some progress has been made in the unification of the electromagnetic and weak interactions (see electroweak theory) .
Einstein attempted to derive quantum mechanics from unified-field theory, but it is now thought that any unified-field theory has to start with quantum mechanics. Attempts to construct unified-field theories, such as supergravity and Kaluza-Klein theory, have run into great difficulties. At the present time it is not clear whether the framework of relativistic quantum field theory is adequate to give a unified theory for all the known fundamental interactions and elementary particles, or whether one has to go to extended objects, such as superstrings or supermembranes. Unified-field theories and other fundamental theories, such as superstring theory and supermembrane theory, are of great importance in understanding cosmology, particularly the early universe. In turn cosmology puts constraints on unified-field theories. See also grand unified theory.
Citació:
"unified-field theory" A Dictionary of Physics. Ed. Alan Isaacs. Oxford University Press, 2000. Oxford Reference Online. Oxford University Press. Universitat Politècnica de Catalunya. 24 January 2005 <http://www.oxfordreference.com/search?siteToSearch=oso&q=unified-field+theory&searchBtn=Search&isQuickSearch=true>
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