id
stringlengths 14
20
| title
stringlengths 4
181
| text
stringlengths 1
43k
| source
stringclasses 1
value |
|---|---|---|---|
wiki_39991_chunk_1
|
Eudicella gralli
|
Further reading
Vincent Allard, 1985 - The Beetles of the World, volume 6. Goliathini 2 (Cetoniidae), Sciences Nat, Venette
Vincent Allard, 1985 - Réhabilitation de Eudicella gralli pauperata Kolbe, bona species, (nec trilineata Quedf.) (Cetoniidae), Bulletin de la Société Sciences Nat, 46, p. 11.
Vincent Allard, 1985 - Réflexions sur la classification des groupes gralli et smithi du genre Eudicella White (Cetoniidae), Bulletin de la Société Sciences Nat, 47, p. 27.
|
wikipedia
|
wiki_39991_chunk_2
|
Eudicella gralli
|
External links
Eudicella gralli elgonensis photos at Beetlespace.wz.cz
Eudicella gralli hubini photos at Beetlespace.wz.cz
Natural Worlds Cetoniinae
Beetles of Africa
Beetles described in 1836
|
wikipedia
|
wiki_39992_chunk_0
|
Une jeune Pucelle
|
"Une jeune Pucelle" is a French folk song from 1557, which has a melody that is based loosely on an older French song entitled "Une jeune Fillette".
|
wikipedia
|
wiki_39992_chunk_1
|
Une jeune Pucelle
|
The French words were set to an earlier Italian ballad from the sixteenth century titled "La Monica", which is also known as a dance, in German sources called Deutscher Tanz, and in Italian, French, Flemish, and English sources labeled Alemana, Almande, Almagne, Almande nonette, Balletto alta morona, Balletto celeste Giglio, Aria venetiana, Aria Venetia che cantava Scappino, Balo todesco, The Queen’s Almaine, or Oulde Almaine. It was also used for German texts such as "Ich ging einmal spazieren" and with sacred texts such as "Von Gott will ich nicht lassen" and "Helft mir Gotts Güte preisen".
|
wikipedia
|
wiki_39992_chunk_2
|
Une jeune Pucelle
|
The words of the Huron Carol ("Jesous Ahatonhia"), written probably in 1642 by the Jesuit missionary Jean de Brébeuf for the Hurons at Ste. Marie, were set to an adaptation of this melody.
|
wikipedia
|
wiki_39992_chunk_3
|
Une jeune Pucelle
|
The melody of the closing chorale of Johann Sebastian Bach's Herr, wie du willt, so schicks mit mir, BWV 73, with the incipit "Das ist des Vaters Wille", is based on either "Une jeune Pucelle" or "Une jeune Fillette". Also Marc-Antoine Charpentier used the melody in his Quatrième Kyrie of the Messe de Minuit pour Noël (H9) (Midnight Mass for Christmas). Pierre Dandrieu wrote a set of six variations.
|
wikipedia
|
wiki_39992_chunk_4
|
Une jeune Pucelle
|
Lyrics
Une jeune pucelle de noble cœur,
Priant en sa chambrette son Créateur.
L'ange du Ciel descendant sur la terre
Lui conta le mystère de notre Salvateur.
La pucelle esbahie de ceste voix,
Elle se peint à dire pour ceste fois:
Comment pourra s'accomplir telle affaire?
Car jamais n'eus affaire à nul homme qui soyt.
|
wikipedia
|
wiki_39992_chunk_5
|
Une jeune Pucelle
|
Ne te soucie, Marie, aucunement,
Celui qui Seignerie au firmament,
Son Saint-Esprit te fera apparaître,
Dont tu ne pourras connaître tost cet enfantement.
Sans douleur et sans peine, et sans tourment,
Neuf moys seras enceinte de cet enfant;
Quand ce viendra à le poser sur terre,
Jésus faut qu'on l'appelle, le Roy sur tout triomphant.
|
wikipedia
|
wiki_39992_chunk_6
|
Une jeune Pucelle
|
Lors fut tant consolée de ces beaux dits,
Qu'elle pensait quasi être en Paradis.
Se soubmettant du tout à lui complaire,
disant voicy l'ancelle du Sauveur Jésus-Christ.
Mon âme magnifie, Dieu mon sauveur,
Mon esprit glorifie son Créatuer,
Car il a eu egard à son ancelle;
Que terre universelle lui soit gloire et honneur.
A young maid with a noble heart,
praying to her Creator in her chamber.
The angel, descending from heaven to earth,
told her the mystery of our Saviour.
The maid, astonished at this voice,
was moved to say at this point:
'How can such a thing be accomplished?
For never did I have converse with any man at all.'
|
wikipedia
|
wiki_39992_chunk_7
|
Une jeune Pucelle
|
'Fear not, Mary, at all:
he who is Lord of the firmament will send you his Holy Spirit,
from whom you will soon learn of the Child to be born.
Without sorrow, without pain, without torment,
you will carry this Child for nine months;
when the time comes to give him birth,
you must call him Jesus,
the King triumphing over all things.'
|
wikipedia
|
wiki_39992_chunk_8
|
Une jeune Pucelle
|
Then she was so consoled by these fine words,
that she felt as though she were in Paradise.
She submitted entirely to comply with all he said,
saying 'Here is the handmaid of the Saviour Jesus Christ.
My soul glorifies God my Saviour,
my spirit praises its Creator,
for he has looked upon his handmaiden;
may the whole earth be glory and honour to him.' References Sources Further reading
Glover, Raymond F. (ed.). 1990–94. The Hymnal 1982 Companion, 3 vols. in 4. New York: Church Hymnal Corp. . French folk songs
|
wikipedia
|
wiki_39993_chunk_0
|
Evolutionary multimodal optimization
|
In applied mathematics, multimodal optimization deals with optimization tasks that involve finding all or most of the multiple (at least locally optimal) solutions of a problem, as opposed to a single best solution.
Evolutionary multimodal optimization is a branch of evolutionary computation, which is closely related to machine learning. Wong provides a short survey, wherein the chapter of Shir and the book of Preuss cover the topic in more detail.
|
wikipedia
|
wiki_39993_chunk_1
|
Evolutionary multimodal optimization
|
Motivation
Knowledge of multiple solutions to an optimization task is especially helpful in engineering, when due to physical (and/or cost) constraints, the best results may not always be realizable. In such a scenario, if multiple solutions (locally and/or globally optimal) are known, the implementation can be quickly switched to another solution and still obtain the best possible system performance. Multiple solutions could also be analyzed to discover hidden properties (or relationships) of the underlying optimization problem, which makes them important for obtaining domain knowledge.
In addition, the algorithms for multimodal optimization usually not only locate multiple optima in a single run, but also preserve their population diversity, resulting in their global optimization ability on multimodal functions. Moreover, the techniques for multimodal optimization are usually borrowed as diversity maintenance techniques to other problems.
|
wikipedia
|
wiki_39993_chunk_2
|
Evolutionary multimodal optimization
|
Background
|
wikipedia
|
wiki_39993_chunk_3
|
Evolutionary multimodal optimization
|
Classical techniques of optimization would need multiple restart points and multiple runs in the hope that a different solution may be discovered every run, with no guarantee however. Evolutionary algorithms (EAs) due to their population based approach, provide a natural advantage over classical optimization techniques. They maintain a population of possible solutions, which are processed every generation, and if the multiple solutions can be preserved over all these generations, then at termination of the algorithm we will have multiple good solutions, rather than only the best solution. Note that this is against the natural tendency of classical optimization techniques, which will always converge to the best solution, or a sub-optimal solution (in a rugged, “badly behaving” function). Finding and maintenance of multiple solutions is wherein lies the challenge of using EAs for multi-modal optimization. Niching is a generic term referred to as the technique of finding and preserving multiple stable niches, or favorable parts of the solution space possibly around multiple solutions, so as to prevent convergence to a single solution.
|
wikipedia
|
wiki_39993_chunk_4
|
Evolutionary multimodal optimization
|
The field of Evolutionary algorithms encompasses genetic algorithms (GAs), evolution strategy (ES), differential evolution (DE), particle swarm optimization (PSO), and other methods. Attempts have been made to solve multi-modal optimization in all these realms and most, if not all the various methods implement niching in some form or the other. Multimodal optimization using genetic algorithms/evolution strategies
|
wikipedia
|
wiki_39993_chunk_5
|
Evolutionary multimodal optimization
|
De Jong's crowding method, Goldberg's sharing function approach, Petrowski's clearing method, restricted mating, maintaining multiple subpopulations are some of the popular approaches that have been proposed by the community. The first two methods are especially well studied, however, they do not perform explicit separation into solutions belonging to different basins of attraction.
|
wikipedia
|
wiki_39993_chunk_6
|
Evolutionary multimodal optimization
|
The application of multimodal optimization within ES was not explicit for many years, and has been explored only recently.
A niching framework utilizing derandomized ES was introduced by Shir, proposing the CMA-ES as a niching optimizer for the first time. The underpinning of that framework was the selection of a peak individual per subpopulation in each generation, followed by its sampling to produce the consecutive dispersion of search-points. The biological analogy of this machinery is an alpha-male winning all the imposed competitions and dominating thereafter its ecological niche, which then obtains all the sexual resources therein to generate its offspring.
|
wikipedia
|
wiki_39993_chunk_7
|
Evolutionary multimodal optimization
|
Recently, an evolutionary multiobjective optimization (EMO) approach was proposed, in which a suitable second objective is added to the originally single objective multimodal optimization problem, so that the multiple solutions form a weak pareto-optimal front. Hence, the multimodal optimization problem can be solved for its multiple solutions using an EMO algorithm. Improving upon their work, the same authors have made their algorithm self-adaptive, thus eliminating the need for pre-specifying the parameters.
|
wikipedia
|
wiki_39993_chunk_8
|
Evolutionary multimodal optimization
|
An approach that does not use any radius for separating the population into subpopulations (or species) but employs the space topology instead is proposed in. References Bibliography
|
wikipedia
|
wiki_39993_chunk_9
|
Evolutionary multimodal optimization
|
D. Goldberg and J. Richardson. (1987) "Genetic algorithms with sharing for multimodal function optimization". In Proceedings of the Second International Conference on Genetic Algorithms on Genetic algorithms and their application table of contents, pages 41–49. L. Erlbaum Associates Inc. Hillsdale, NJ, USA, 1987.
A. Petrowski. (1996) "A clearing procedure as a niching method for genetic algorithms". In Proceedings of the 1996 IEEE International Conference on Evolutionary Computation, pages 798–803. Citeseer, 1996.
Deb, K., (2001) "Multi-objective Optimization using Evolutionary Algorithms", Wiley (Google Books)
F. Streichert, G. Stein, H. Ulmer, and A. Zell. (2004) "A clustering based niching EA for multimodal search spaces". Lecture Notes in Computer Science, pages 293–304, 2004.
Singh, G., Deb, K., (2006) "Comparison of multi-modal optimization algorithms based on evolutionary algorithms". In Proceedings of the 8th annual conference on Genetic and evolutionary computation, pages 8–12. ACM, 2006.
Ronkkonen, J., (2009). Continuous Multimodal Global Optimization with Differential Evolution Based Methods
Wong, K. C., (2009). An evolutionary algorithm with species-specific explosion for multimodal optimization. GECCO 2009: 923–930
J. Barrera and C. A. C. Coello. "A Review of Particle Swarm Optimization Methods used for Multimodal Optimization", pages 9–37. Springer, Berlin, November 2009.
Wong, K. C., (2010). Effect of Spatial Locality on an Evolutionary Algorithm for Multimodal Optimization. EvoApplications (1) 2010: 481–490
Deb, K., Saha, A. (2010) Finding Multiple Solutions for Multimodal Optimization Problems Using a Multi-Objective Evolutionary Approach. GECCO 2010: 447–454
Wong, K. C., (2010). Protein structure prediction on a lattice model via multimodal optimization techniques. GECCO 2010: 155–162
Saha, A., Deb, K. (2010), A Bi-criterion Approach to Multimodal Optimization: Self-adaptive Approach. SEAL 2010: 95–104
Shir, O.M., Emmerich, M., Bäck, T. (2010), Adaptive Niche Radii and Niche Shapes Approaches for Niching with the CMA-ES. Evolutionary Computation Vol. 18, No. 1, pp. 97-126.
C. Stoean, M. Preuss, R. Stoean, D. Dumitrescu (2010) Multimodal Optimization by means of a Topological Species Conservation Algorithm. In IEEE Transactions on Evolutionary Computation, Vol. 14, Issue 6, pages 842–864, 2010.
S. Das, S. Maity, B-Y Qu, P. N. Suganthan, "Real-parameter evolutionary multimodal optimization — A survey of the state-of-the-art", Vol. 1, No. 2, pp. 71–88, Swarm and Evolutionary Computation, June 2011.
|
wikipedia
|
wiki_39993_chunk_10
|
Evolutionary multimodal optimization
|
External links
Multi-modal optimization using Particle Swarm Optimization (PSO)
Niching in Evolution Strategies (ES)
Multimodal optimization page at Chair 11, Computer Science, TU Dortmund University
IEEE CIS Task Force on Multi-modal Optimization Cybernetics
Evolutionary algorithms
Machine learning algorithms
Articles containing video clips
|
wikipedia
|
wiki_39994_chunk_0
|
Degrees of freedom (physics and chemistry)
|
In physics and chemistry, a degree of freedom is an independent physical parameter in the formal description of the state of a physical system. The set of all states of a system is known as the system's phase space, and the degrees of freedom of the system are the dimensions of the phase space.
|
wikipedia
|
wiki_39994_chunk_1
|
Degrees of freedom (physics and chemistry)
|
The location of a particle in three-dimensional space requires three position coordinates. Similarly, the direction and speed at which a particle moves can be described in terms of three velocity components, each in reference to the three dimensions of space. If the time evolution of the system is deterministic, where the state at one instant uniquely determines its past and future position and velocity as a function of time, such a system has six degrees of freedom. If the motion of the particle is constrained to a lower number of dimensions – for example, the particle must move along a wire or on a fixed surface – then the system has fewer than six degrees of freedom. On the other hand, a system with an extended object that can rotate or vibrate can have more than six degrees of freedom.
|
wikipedia
|
wiki_39994_chunk_2
|
Degrees of freedom (physics and chemistry)
|
In classical mechanics, the state of a point particle at any given time is often described with position and velocity coordinates in the Lagrangian formalism, or with position and momentum coordinates in the Hamiltonian formalism. In statistical mechanics, a degree of freedom is a single scalar number describing the microstate of a system. The specification of all microstates of a system is a point in the system's phase space. In the 3D ideal chain model in chemistry, two angles are necessary to describe the orientation of each monomer.
|
wikipedia
|
wiki_39994_chunk_3
|
Degrees of freedom (physics and chemistry)
|
It is often useful to specify quadratic degrees of freedom. These are degrees of freedom that contribute in a quadratic function to the energy of the system. Depending on what one is counting, there are several different ways that degrees of freedom can be defined,
each with a different value. Thermodynamic degrees of freedom for gases
|
wikipedia
|
wiki_39994_chunk_4
|
Degrees of freedom (physics and chemistry)
|
By the equipartition theorem, internal energy per mole of gas equals cv T,
where T is temperature in kelvins and the specific heat at constant volume is cv = (f)(R/2).
R = 8.314 J/(K mol) is the universal gas constant, and "f" is the number of thermodynamic (quadratic) degrees of freedom,
counting the number of ways in which energy can occur.
|
wikipedia
|
wiki_39994_chunk_5
|
Degrees of freedom (physics and chemistry)
|
Any atom or molecule has three degrees of freedom associated with translational motion (kinetic energy) of the center of mass with respect to the x, y, and z axes. These are the only degrees of freedom for noble gases (helium, neon, argon, etc.), which do not form molecules.
|
wikipedia
|
wiki_39994_chunk_6
|
Degrees of freedom (physics and chemistry)
|
A molecule (two or more joined atoms) can have rotational kinetic energy.
A linear molecule, where all atoms lie along a single axis,
such as any diatomic molecule and some other molecules like carbon dioxide (CO2),
has two rotational degrees of freedom, because it can rotate about either of two axes perpendicular to the molecular axis.
A nonlinear molecule, where the atoms do not lie along a single axis, like water (H2O), has three rotational degrees of freedom, because it can rotate around any of three perpendicular axes.
In special cases, such as adsorbed large molecules, the rotational degrees of freedom can be limited to only one.
|
wikipedia
|
wiki_39994_chunk_7
|
Degrees of freedom (physics and chemistry)
|
A molecule can also vibrate. A diatomic molecule has one molecular vibration mode, where the two atoms oscillate back and forth with the chemical bond between them acting as a spring. A molecule with atoms has more complicated modes of molecular vibration, with vibrational modes for a linear molecule and modes for a nonlinear molecule.
As specific examples, the linear CO2 molecule has 4 modes of oscillation, and the nonlinear water molecule has 3 modes of oscillation
Each vibrational mode has two degrees of freedom for energy. One degree of freedom involves the kinetic energy of the moving atoms,
and one degree of freedom involves the potential energy of the spring-like chemical bond(s).
Therefore, the number of vibrational degrees of freedom for energy is for a linear molecule and modes for a nonlinear molecule.
|
wikipedia
|
wiki_39994_chunk_8
|
Degrees of freedom (physics and chemistry)
|
Both the rotational and vibrational modes are quantized, requiring a minimum temperature to be activated. The "rotational temperature" to activate the rotational degrees of freedom is less than 100 K for many gases. For N2 and O2, it is less than 3 K.
The "vibrational temperature" necessary for substantial vibration is between 103 K and 104 K, 3521 K for N2 and 2156 K for O2. Typical atmospheric temperatures are not high enough to activate vibration in N2 and O2, which comprise most of the atmosphere. (See the next figure.) However, the much less abundant greenhouse gases keep the troposphere warm by absorbing infrared from the Earth's surface, which excites their vibrational modes.
Much of this energy is reradiated back to the surface in the infrared through the "greenhouse effect."
|
wikipedia
|
wiki_39994_chunk_9
|
Degrees of freedom (physics and chemistry)
|
Because room temperature (≈298 K) is over the typical rotational temperature but lower than the typical vibrational temperature, only the translational and rotational degrees of freedom contribute, in equal amounts, to the heat capacity ratio. This is why ≈ for monatomic gases and ≈ for diatomic gases at room temperature.
|
wikipedia
|
wiki_39994_chunk_10
|
Degrees of freedom (physics and chemistry)
|
Because air is dominated by diatomic gases nitrogen and oxygen, its molar internal energy is close to cv T = (5/2)RT, determined by the 5 degrees of freedom exhibited by diatomic gases.
See the graph at right. For 140 K < T < 380 K, cv differs from (5/2) Rd by less than 1%.
Only at temperatures well above temperatures in the troposphere and stratosphere do some molecules have enough energy to activate the vibrational modes of N2 and O2. The specific heat at constant volume, cv, increases slowly toward (7/2) R as temperature increases above T = 400 K, where cv is 1.3% above (5/2) Rd = 717.5 J/(K kg).
|
wikipedia
|
wiki_39994_chunk_11
|
Degrees of freedom (physics and chemistry)
|
Counting the minimum number of co-ordinates to specify a position
|
wikipedia
|
wiki_39994_chunk_12
|
Degrees of freedom (physics and chemistry)
|
One can also count degrees of freedom using the minimum number of coordinates required to specify a position. This is done as follows:
For a single particle we need 2 coordinates in a 2-D plane to specify its position and 3 coordinates in 3-D space. Thus its degree of freedom in a 3-D space is 3.
For a body consisting of 2 particles (ex. a diatomic molecule) in a 3-D space with constant distance between them (let's say d) we can show (below) its degrees of freedom to be 5.
Let's say one particle in this body has coordinate and the other has coordinate with unknown. Application of the formula for distance between two coordinates
|
wikipedia
|
wiki_39994_chunk_13
|
Degrees of freedom (physics and chemistry)
|
results in one equation with one unknown, in which we can solve for .
One of , , , , , or can be unknown. Contrary to the classical equipartition theorem, at room temperature, the vibrational motion of molecules typically makes negligible contributions to the heat capacity. This is because these degrees of freedom are frozen because the spacing between the energy eigenvalues exceeds the energy corresponding to ambient temperatures (). Independent degrees of freedom
The set of degrees of freedom of a system is independent if the energy associated with the set can be written in the following form: where is a function of the sole variable .
|
wikipedia
|
wiki_39994_chunk_14
|
Degrees of freedom (physics and chemistry)
|
example: if and are two degrees of freedom, and is the associated energy:
If , then the two degrees of freedom are independent.
If , then the two degrees of freedom are not independent. The term involving the product of and is a coupling term that describes an interaction between the two degrees of freedom. For from 1 to , the value of the th degree of freedom is distributed according to the Boltzmann distribution. Its probability density function is the following:
, In this section, and throughout the article the brackets denote the mean of the quantity they enclose.
|
wikipedia
|
wiki_39994_chunk_15
|
Degrees of freedom (physics and chemistry)
|
The internal energy of the system is the sum of the average energies associated with each of the degrees of freedom: Quadratic degrees of freedom
A degree of freedom is quadratic if the energy terms associated with this degree of freedom can be written as
, where is a linear combination of other quadratic degrees of freedom.
|
wikipedia
|
wiki_39994_chunk_16
|
Degrees of freedom (physics and chemistry)
|
example: if and are two degrees of freedom, and is the associated energy:
If , then the two degrees of freedom are not independent and non-quadratic.
If , then the two degrees of freedom are independent and non-quadratic.
If , then the two degrees of freedom are not independent but are quadratic.
If , then the two degrees of freedom are independent and quadratic. For example, in Newtonian mechanics, the dynamics of a system of quadratic degrees of freedom are controlled by a set of homogeneous linear differential equations with constant coefficients.
|
wikipedia
|
wiki_39994_chunk_17
|
Degrees of freedom (physics and chemistry)
|
Quadratic and independent degree of freedom
are quadratic and independent degrees of freedom if the energy associated with a microstate of the system they represent can be written as: Equipartition theorem In the classical limit of statistical mechanics, at thermodynamic equilibrium, the internal energy of a system of quadratic and independent degrees of freedom is: Here, the mean energy associated with a degree of freedom is: Since the degrees of freedom are independent, the internal energy of the system is equal to the sum of the mean energy associated with each degree of freedom, which demonstrates the result.
|
wikipedia
|
wiki_39994_chunk_18
|
Degrees of freedom (physics and chemistry)
|
Generalizations
The description of a system's state as a point in its phase space, although mathematically convenient, is thought to be fundamentally inaccurate. In quantum mechanics, the motion degrees of freedom are superseded with the concept of wave function, and operators which correspond to other degrees of freedom have discrete spectra. For example, intrinsic angular momentum operator (which corresponds to the rotational freedom) for an electron or photon has only two eigenvalues. This discreteness becomes apparent when action has an order of magnitude of the Planck constant, and individual degrees of freedom can be distinguished. References Physical quantities
Dimension
|
wikipedia
|
wiki_39995_chunk_0
|
Field (physics)
|
In physics, a field is a physical quantity, represented by a number or another tensor, that has a value for each point in space and time. For example, on a weather map, the surface temperature is described by assigning a number to each point on the map; the temperature can be considered at a certain point in time or over some interval of time, to study the dynamics of temperature change. A surface wind map, assigning an arrow to each point on a map that describes the wind speed and direction at that point, is an example of a vector field, i.e. a 1-dimensional (rank-1) tensor field. Field theories, mathematical descriptions of how field values change in space and time, are ubiquitous in physics. For instance, the electric field is another rank-1 tensor field, while electrodynamics can be formulated in terms of two interacting vector fields at each point in spacetime, or as a single-rank 2-tensor field.
|
wikipedia
|
wiki_39995_chunk_1
|
Field (physics)
|
In the modern framework of the quantum theory of fields, even without referring to a test particle, a field occupies space, contains energy, and its presence precludes a classical "true vacuum". This has led physicists to consider electromagnetic fields to be a physical entity, making the field concept a supporting paradigm of the edifice of modern physics. "The fact that the electromagnetic field can possess momentum and energy makes it very real ... a particle makes a field, and a field acts on another particle, and the field has such familiar properties as energy content and momentum, just as particles can have." In practice, the strength of most fields diminishes with distance, eventually becoming undetectable. For instance the strength of many relevant classical fields, such as the gravitational field in Newton's theory of gravity or the electrostatic field in classical electromagnetism, is inversely proportional to the square of the distance from the source (i.e., they follow Gauss's law).
|
wikipedia
|
wiki_39995_chunk_2
|
Field (physics)
|
A field can be classified as a scalar field, a vector field, a spinor field or a tensor field according to whether the represented physical quantity is a scalar, a vector, a spinor, or a tensor, respectively. A field has a consistent tensorial character wherever it is defined: i.e. a field cannot be a scalar field somewhere and a vector field somewhere else. For example, the Newtonian gravitational field is a vector field: specifying its value at a point in spacetime requires three numbers, the components of the gravitational field vector at that point. Moreover, within each category (scalar, vector, tensor), a field can be either a classical field or a quantum field, depending on whether it is characterized by numbers or quantum operators respectively. In this theory an equivalent representation of field is a field particle, for instance a boson.
|
wikipedia
|
wiki_39995_chunk_3
|
Field (physics)
|
History
To Isaac Newton, his law of universal gravitation simply expressed the gravitational force that acted between any pair of massive objects. When looking at the motion of many bodies all interacting with each other, such as the planets in the Solar System, dealing with the force between each pair of bodies separately rapidly becomes computationally inconvenient. In the eighteenth century, a new quantity was devised to simplify the bookkeeping of all these gravitational forces. This quantity, the gravitational field, gave at each point in space the total gravitational acceleration which would be felt by a small object at that point. This did not change the physics in any way: it did not matter if all the gravitational forces on an object were calculated individually and then added together, or if all the contributions were first added together as a gravitational field and then applied to an object.
|
wikipedia
|
wiki_39995_chunk_4
|
Field (physics)
|
The development of the independent concept of a field truly began in the nineteenth century with the development of the theory of electromagnetism. In the early stages, André-Marie Ampère and Charles-Augustin de Coulomb could manage with Newton-style laws that expressed the forces between pairs of electric charges or electric currents. However, it became much more natural to take the field approach and express these laws in terms of electric and magnetic fields; in 1849 Michael Faraday became the first to coin the term "field".
|
wikipedia
|
wiki_39995_chunk_5
|
Field (physics)
|
The independent nature of the field became more apparent with James Clerk Maxwell's discovery that waves in these fields propagated at a finite speed. Consequently, the forces on charges and currents no longer just depended on the positions and velocities of other charges and currents at the same time, but also on their positions and velocities in the past.
|
wikipedia
|
wiki_39995_chunk_6
|
Field (physics)
|
Maxwell, at first, did not adopt the modern concept of a field as a fundamental quantity that could independently exist. Instead, he supposed that the electromagnetic field expressed the deformation of some underlying medium—the luminiferous aether—much like the tension in a rubber membrane. If that were the case, the observed velocity of the electromagnetic waves should depend upon the velocity of the observer with respect to the aether. Despite much effort, no experimental evidence of such an effect was ever found; the situation was resolved by the introduction of the special theory of relativity by Albert Einstein in 1905. This theory changed the way the viewpoints of moving observers were related to each other. They became related to each other in such a way that velocity of electromagnetic waves in Maxwell's theory would be the same for all observers. By doing away with the need for a background medium, this development opened the way for physicists to start thinking about fields as truly independent entities.
|
wikipedia
|
wiki_39995_chunk_7
|
Field (physics)
|
In the late 1920s, the new rules of quantum mechanics were first applied to the electromagnetic field. In 1927, Paul Dirac used quantum fields to successfully explain how the decay of an atom to a lower quantum state led to the spontaneous emission of a photon, the quantum of the electromagnetic field. This was soon followed by the realization (following the work of Pascual Jordan, Eugene Wigner, Werner Heisenberg, and Wolfgang Pauli) that all particles, including electrons and protons, could be understood as the quanta of some quantum field, elevating fields to the status of the most fundamental objects in nature. That said, John Wheeler and Richard Feynman seriously considered Newton's pre-field concept of action at a distance (although they set it aside because of the ongoing utility of the field concept for research in general relativity and quantum electrodynamics).
|
wikipedia
|
wiki_39995_chunk_8
|
Field (physics)
|
Classical fields There are several examples of classical fields. Classical field theories remain useful wherever quantum properties do not arise, and can be active areas of research. Elasticity of materials, fluid dynamics and Maxwell's equations are cases in point. Some of the simplest physical fields are vector force fields. Historically, the first time that fields were taken seriously was with Faraday's lines of force when describing the electric field. The gravitational field was then similarly described. Newtonian gravitation A classical field theory describing gravity is Newtonian gravitation, which describes the gravitational force as a mutual interaction between two masses.
|
wikipedia
|
wiki_39995_chunk_9
|
Field (physics)
|
Any body with mass M is associated with a gravitational field g which describes its influence on other bodies with mass. The gravitational field of M at a point r in space corresponds to the ratio between force F that M exerts on a small or negligible test mass m located at r and the test mass itself:
Stipulating that m is much smaller than M ensures that the presence of m has a negligible influence on the behavior of M. According to Newton's law of universal gravitation, F(r) is given by
|
wikipedia
|
wiki_39995_chunk_10
|
Field (physics)
|
where is a unit vector lying along the line joining M and m and pointing from M to m. Therefore, the gravitational field of M is The experimental observation that inertial mass and gravitational mass are equal to an unprecedented level of accuracy leads to the identity that gravitational field strength is identical to the acceleration experienced by a particle. This is the starting point of the equivalence principle, which leads to general relativity. Because the gravitational force F is conservative, the gravitational field g can be rewritten in terms of the gradient of a scalar function, the gravitational potential Φ(r): Electromagnetism
|
wikipedia
|
wiki_39995_chunk_11
|
Field (physics)
|
Michael Faraday first realized the importance of a field as a physical quantity, during his investigations into magnetism. He realized that electric and magnetic fields are not only fields of force which dictate the motion of particles, but also have an independent physical reality because they carry energy. These ideas eventually led to the creation, by James Clerk Maxwell, of the first unified field theory in physics with the introduction of equations for the electromagnetic field. The modern version of these equations is called Maxwell's equations. Electrostatics
|
wikipedia
|
wiki_39995_chunk_12
|
Field (physics)
|
A charged test particle with charge q experiences a force F based solely on its charge. We can similarly describe the electric field E so that . Using this and Coulomb's law tells us that the electric field due to a single charged particle is The electric field is conservative, and hence can be described by a scalar potential, V(r): Magnetostatics
|
wikipedia
|
wiki_39995_chunk_13
|
Field (physics)
|
A steady current I flowing along a path ℓ will create a field B, that exerts a force on nearby moving charged particles that is quantitatively different from the electric field force described above. The force exerted by I on a nearby charge q with velocity v is
where B(r) is the magnetic field, which is determined from I by the Biot–Savart law: The magnetic field is not conservative in general, and hence cannot usually be written in terms of a scalar potential. However, it can be written in terms of a vector potential, A(r): Electrodynamics
|
wikipedia
|
wiki_39995_chunk_14
|
Field (physics)
|
In general, in the presence of both a charge density ρ(r, t) and current density J(r, t), there will be both an electric and a magnetic field, and both will vary in time. They are determined by Maxwell's equations, a set of differential equations which directly relate E and B to ρ and J. Alternatively, one can describe the system in terms of its scalar and vector potentials V and A. A set of integral equations known as retarded potentials allow one to calculate V and A from ρ and J, and from there the electric and magnetic fields are determined via the relations
|
wikipedia
|
wiki_39995_chunk_15
|
Field (physics)
|
At the end of the 19th century, the electromagnetic field was understood as a collection of two vector fields in space. Nowadays, one recognizes this as a single antisymmetric 2nd-rank tensor field in spacetime. Gravitation in general relativity Einstein's theory of gravity, called general relativity, is another example of a field theory. Here the principal field is the metric tensor, a symmetric 2nd-rank tensor field in spacetime. This replaces Newton's law of universal gravitation.
|
wikipedia
|
wiki_39995_chunk_16
|
Field (physics)
|
Waves as fields
Waves can be constructed as physical fields, due to their finite propagation speed and causal nature when a simplified physical model of an isolated closed system is set . They are also subject to the inverse-square law. For electromagnetic waves, there are optical fields, and terms such as near- and far-field limits for diffraction. In practice though, the field theories of optics are superseded by the electromagnetic field theory of Maxwell. Quantum fields
|
wikipedia
|
wiki_39995_chunk_17
|
Field (physics)
|
It is now believed that quantum mechanics should underlie all physical phenomena, so that a classical field theory should, at least in principle, permit a recasting in quantum mechanical terms; success yields the corresponding quantum field theory. For example, quantizing classical electrodynamics gives quantum electrodynamics. Quantum electrodynamics is arguably the most successful scientific theory; experimental data confirm its predictions to a higher precision (to more significant digits) than any other theory. The two other fundamental quantum field theories are quantum chromodynamics and the electroweak theory.
|
wikipedia
|
wiki_39995_chunk_18
|
Field (physics)
|
In quantum chromodynamics, the color field lines are coupled at short distances by gluons, which are polarized by the field and line up with it. This effect increases within a short distance (around 1 fm from the vicinity of the quarks) making the color force increase within a short distance, confining the quarks within hadrons. As the field lines are pulled together tightly by gluons, they do not "bow" outwards as much as an electric field between electric charges.
|
wikipedia
|
wiki_39995_chunk_19
|
Field (physics)
|
These three quantum field theories can all be derived as special cases of the so-called standard model of particle physics. General relativity, the Einsteinian field theory of gravity, has yet to be successfully quantized. However an extension, thermal field theory, deals with quantum field theory at finite temperatures, something seldom considered in quantum field theory. In BRST theory one deals with odd fields, e.g. Faddeev–Popov ghosts. There are different descriptions of odd classical fields both on graded manifolds and supermanifolds.
|
wikipedia
|
wiki_39995_chunk_20
|
Field (physics)
|
As above with classical fields, it is possible to approach their quantum counterparts from a purely mathematical view using similar techniques as before. The equations governing the quantum fields are in fact PDEs (specifically, relativistic wave equations (RWEs)). Thus one can speak of Yang–Mills, Dirac, Klein–Gordon and Schrödinger fields as being solutions to their respective equations. A possible problem is that these RWEs can deal with complicated mathematical objects with exotic algebraic properties (e.g. spinors are not tensors, so may need calculus for spinor fields), but these in theory can still be subjected to analytical methods given appropriate mathematical generalization.
|
wikipedia
|
wiki_39995_chunk_21
|
Field (physics)
|
Field theory
Field theory usually refers to a construction of the dynamics of a field, i.e. a specification of how a field changes with time or with respect to other independent physical variables on which the field depends. Usually this is done by writing a Lagrangian or a Hamiltonian of the field, and treating it as a classical or quantum mechanical system with an infinite number of degrees of freedom. The resulting field theories are referred to as classical or quantum field theories.
|
wikipedia
|
wiki_39995_chunk_22
|
Field (physics)
|
The dynamics of a classical field are usually specified by the Lagrangian density in terms of the field components; the dynamics can be obtained by using the action principle.
|
wikipedia
|
wiki_39995_chunk_23
|
Field (physics)
|
It is possible to construct simple fields without any prior knowledge of physics using only mathematics from several variable calculus, potential theory and partial differential equations (PDEs). For example, scalar PDEs might consider quantities such as amplitude, density and pressure fields for the wave equation and fluid dynamics; temperature/concentration fields for the heat/diffusion equations. Outside of physics proper (e.g., radiometry and computer graphics), there are even light fields. All these previous examples are scalar fields. Similarly for vectors, there are vector PDEs for displacement, velocity and vorticity fields in (applied mathematical) fluid dynamics, but vector calculus may now be needed in addition, being calculus for vector fields (as are these three quantities, and those for vector PDEs in general). More generally problems in continuum mechanics may involve for example, directional elasticity (from which comes the term tensor, derived from the Latin word for stretch), complex fluid flows or anisotropic diffusion, which are framed as matrix-tensor PDEs, and then require matrices or tensor fields, hence matrix or tensor calculus. The scalars (and hence the vectors, matrices and tensors) can be real or complex as both are fields in the abstract-algebraic/ring-theoretic sense.
|
wikipedia
|
wiki_39995_chunk_24
|
Field (physics)
|
In a general setting, classical fields are described by sections of fiber bundles and their dynamics is formulated in the terms of jet manifolds (covariant classical field theory). In modern physics, the most often studied fields are those that model the four fundamental forces which one day may lead to the Unified Field Theory. Symmetries of fields
A convenient way of classifying a field (classical or quantum) is by the symmetries it possesses. Physical symmetries are usually of two types: Spacetime symmetries
|
wikipedia
|
wiki_39995_chunk_25
|
Field (physics)
|
Fields are often classified by their behaviour under transformations of spacetime. The terms used in this classification are:
scalar fields (such as temperature) whose values are given by a single variable at each point of space. This value does not change under transformations of space.
vector fields (such as the magnitude and direction of the force at each point in a magnetic field) which are specified by attaching a vector to each point of space. The components of this vector transform between themselves contravariantly under rotations in space. Similarly, a dual (or co-) vector field attaches a dual vector to each point of space, and the components of each dual vector transform covariantly.
tensor fields, (such as the stress tensor of a crystal) specified by a tensor at each point of space. Under rotations in space, the components of the tensor transform in a more general way which depends on the number of covariant indices and contravariant indices.
spinor fields (such as the Dirac spinor) arise in quantum field theory to describe particles with spin which transform like vectors except for the one of their components; in other words, when one rotates a vector field 360 degrees around a specific axis, the vector field turns to itself; however, spinors would turn to their negatives in the same case.
|
wikipedia
|
wiki_39995_chunk_26
|
Field (physics)
|
Internal symmetries Fields may have internal symmetries in addition to spacetime symmetries. In many situations, one needs fields which are a list of spacetime scalars: (φ1, φ2, ... φN). For example, in weather prediction these may be temperature, pressure, humidity, etc. In particle physics, the color symmetry of the interaction of quarks is an example of an internal symmetry, that of the strong interaction. Other examples are isospin, weak isospin, strangeness and any other flavour symmetry.
|
wikipedia
|
wiki_39995_chunk_27
|
Field (physics)
|
If there is a symmetry of the problem, not involving spacetime, under which these components transform into each other, then this set of symmetries is called an internal symmetry. One may also make a classification of the charges of the fields under internal symmetries. Statistical field theory Statistical field theory attempts to extend the field-theoretic paradigm toward many-body systems and statistical mechanics. As above, it can be approached by the usual infinite number of degrees of freedom argument.
|
wikipedia
|
wiki_39995_chunk_28
|
Field (physics)
|
Much like statistical mechanics has some overlap between quantum and classical mechanics, statistical field theory has links to both quantum and classical field theories, especially the former with which it shares many methods. One important example is mean field theory.
|
wikipedia
|
wiki_39995_chunk_29
|
Field (physics)
|
Continuous random fields
Classical fields as above, such as the electromagnetic field, are usually infinitely differentiable functions, but they are in any case almost always twice differentiable. In contrast, generalized functions are not continuous. When dealing carefully with classical fields at finite temperature, the mathematical methods of continuous random fields are used, because thermally fluctuating classical fields are nowhere differentiable. Random fields are indexed sets of random variables; a continuous random field is a random field that has a set of functions as its index set. In particular, it is often mathematically convenient to take a continuous random field to have a Schwartz space of functions as its index set, in which case the continuous random field is a tempered distribution.
|
wikipedia
|
wiki_39995_chunk_30
|
Field (physics)
|
We can think about a continuous random field, in a (very) rough way, as an ordinary function that is almost everywhere, but such that when we take a weighted average of all the infinities over any finite region, we get a finite result. The infinities are not well-defined; but the finite values can be associated with the functions used as the weight functions to get the finite values, and that can be well-defined. We can define a continuous random field well enough as a linear map from a space of functions into the real numbers. See also
|
wikipedia
|
wiki_39995_chunk_31
|
Field (physics)
|
Conformal field theory
Covariant Hamiltonian field theory
Field strength
History of the philosophy of field theory
Lagrangian and Eulerian specification of a field
Scalar field theory
Velocity field Notes References Further reading
Landau, Lev D. and Lifshitz, Evgeny M. (1971). Classical Theory of Fields (3rd ed.). London: Pergamon. . Vol. 2 of the Course of Theoretical Physics. External links Particle and Polymer Field Theories Mathematical physics
Theoretical physics
Physical quantities
|
wikipedia
|
wiki_39996_chunk_0
|
Plenty of Power
|
Plenty of Power is the tenth studio album by Canadian heavy metal band Anvil, released in 2001. Track listing Personnel
Anvil
Steve "Lips" Kudlow – vocals, lead guitar
Ivan Hurd – lead guitar
Glenn Five – bass
Robb Reiner – drums Production
Pierre Rémillard – engineer, mixing
Andy Khrem – mastering
Torsten Hartmann – executive producer References Anvil (band) albums
2001 albums
Massacre Records albums
|
wikipedia
|
wiki_39997_chunk_0
|
Africofusus ocelliferus
|
Africofusus ocelliferus, common name the long-siphoned whelk, is a species of sea snail, a marine gastropod mollusk in the family Fasciolariidae, the spindle snails, the tulip snails and their allies.
|
wikipedia
|
wiki_39997_chunk_1
|
Africofusus ocelliferus
|
Spelling
The specific name was originally spelled "ocelliferus"; although this is not a correct latinization it is not liable to a justified emendation (cf. ICZN art. 32.5.1. "Incorrect transliteration or latinization ... are not to be considered inadvertent errors"). The spelling ocellifer is therefore an unjustified emendation, which may be traced to Kaicher, 1976 (card no 1857) Description Distribution
This marine species occurs off South Africa. References
|
wikipedia
|
wiki_39997_chunk_2
|
Africofusus ocelliferus
|
Lamarck J.B. (1816). Liste des objets représentés dans les planches de cette livraison. In: Tableau encyclopédique et méthodique des trois règnes de la Nature. Mollusques et Polypes divers. Agasse, Paris. 16 pp.
|
wikipedia
|
wiki_39997_chunk_3
|
Africofusus ocelliferus
|
External links
Branch, G.M. et al. (2002). Two Oceans. 5th impression. David Philip, Cate Town & Johannesburg.
Lamarck [J.B.P.A. de M. de]. (1816). Tableau encyclopédique et méthodique des trois règnes de la nature, Mollusques et polypes divers. Part 23 [Livraison 84, 14 December 1816], Tome 3, pp. 1–16, pls. 391-431, 431 bis, 431 bis*, 432-488, Paris: Vve Agasse
Smith E.A. (1899). Descriptions of new species of South African marine shells. Journal of Conchology. 9: 247-252, pl. 5
|
wikipedia
|
wiki_39997_chunk_4
|
Africofusus ocelliferus
|
ocelliferus
Gastropods described in 1816
|
wikipedia
|
wiki_39998_chunk_0
|
Renal stem cell
|
Renal stem cells are self-renewing, multipotent stem cells which are able to give rise to all the cell types of the kidney. It is involved in the homeostasis and repair of the kidney, and holds therapeutic potential for treatment of kidney failure.
|
wikipedia
|
wiki_39998_chunk_1
|
Renal stem cell
|
Structure
Strong evidence suggests that renal stem cells are located in the renal papilla. Using stain-retaining assay (with bromodeoxyuridine, or BrdU), a low-cycling cell population was found in the papillary region, which was able to divide rapidly to repair the damaged caused by transcient renal ischemia. These cells are able to incorporate into other renal tissues, and was able to repeatedly form spheres in 3D cultures, and clonal analysis also exhibited its multipotency.
|
wikipedia
|
wiki_39998_chunk_2
|
Renal stem cell
|
Other reports have suggested the renal tubule and renal capsule to be the site of stem cells. The renal capsule contain stain-retaining cells which exhibited markers for mesenchymal stem cells; after their removal, recovery was significantly slower post-ischemic injury. These evidence suggests a stem cell population exists within the renal capsule.
|
wikipedia
|
wiki_39998_chunk_3
|
Renal stem cell
|
Development
Using in vivo lineage tracing techniques, Lgr5+ cells were found to contribute to the nephron, specifically to the ascending limb of the loop of Henle and the distal convoluted tubule. Thus, Lgr5+ cells can potentially be a marker for renal stem and/or progenitor cells.
|
wikipedia
|
wiki_39998_chunk_4
|
Renal stem cell
|
Clinical significance
There is much debate regarding the cells involved in repair after injury; while some suggests that stem cells are the sole driving force of repair, others suggests that cells dedifferentiate after damage to act like stem cells. Alternately, it was also reported that differentiated tubular epithelial cells are the driving mechanism for regeneration after injury, using proliferative expansion as the mechanism.
|
wikipedia
|
wiki_39998_chunk_5
|
Renal stem cell
|
Multipotent mouse kidney progenitor cells (MKPC) were obtained from Myh9 targeted mutant mice. Injection of MKPC into mice post-ischemic injury saw the MKPC regenerating different cell lineages and was able to regenerate renal function and enhanced survival. Renal induced pluripotent stem cells
It has been reported that endogenous kidney tubular renal epithelial cells can be dedifferentiated into induced pluripotent stem cells by the treatment of only two factors - Oct4 and Sox2. See also
List of human cell types derived from the germ layers References Stem cells
|
wikipedia
|
wiki_39999_chunk_0
|
Leucozonia ocellata
|
Leucozonia ocellata is a species of sea snail, a marine gastropod mollusk in the family Fasciolariidae, the spindle snails, the tulip snails and their allies. Description Distribution References Fasciolariidae
Gastropods described in 1791
Taxa named by Johann Friedrich Gmelin
|
wikipedia
|
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.