- the magazine - web exclusives gage block length is defined as an interferometric measurement when the gage block is wrung to a flat platen. gage block wring is incorporated into the lengths of the blocks themselves, and wringing film is included in the defined length of the gage block. when gage blocks are assembled in combinations, no additional correction factor for wringing films needs to be added to the length of the combination. wringability itself may be defined as the ability of two surfaces to adhere tightly to each other in the absence of external means - - they are not magnetized or clamped together. the source of the forces holding gage blocks together are thought to come from : - air pressure from the surrounding environment as the air is squeezed out when the blocks are slid together. - surface tension from oil that remains on the gage blocks or water vapor from the air that acts as an adhesive to hold them together. - molecular force caused by the interchange of electrons between the atoms of the separate blocks when two very flat surfaces are brought into such close contact with each other. this force will remain even in a vacuum or if no oil or water is present on the blocks. wringing requires two smooth, flat surfaces with surface finishes of 1 microinch aa or better. for gage blocks, it becomes difficult to wring surfaces once the flatness begins to exceed 5 microinches. remove all nicks and burrs before attempting to wring blocks together because a burr on one block may damage the surface of the other block. blocks may be checked for burrs with a gage block stone before wringing. a gage block stone with serrated grooves is recommended because it gives a better " feel " for nicks and burrs that catch the edges of the serration. badly nicked surfaces will click as a nick passes along the serration. foreign material em - bedded in the grain of the stone may scratch the blocks. gage block stones should be conditioned before use. this may be done by lightly rubbing the surfaces of two stones together in a figure - eight pattern. clean the stones with kerosene before use. will stoning a gage block change its size? it depends largely on the condition of a block. a block in good condition will not be affected by light stoning. stoning removes portions of the block that have been raised above the true gage surface by nicks or scratches that can contribute to more variation or large readings during calibration. stoning will remove this small amount of raised material. repeatability of


narrator : this is science today. one of five nanoscale science research centers funded by the department of energy is located at the lawrence berkeley national laboratory. rick kelly, the environmental health and safety manager of the molecular foundry, explains the intent of the facility is to foster the development of new nanotechnologies that can benefit society. kelly : the next revolution in technology in this country is nanotechnology. and the intent of the department of energy is to excite people and give them opportunities they might not have at their home research institutions to do nanoscale science, so they use our facilities, they use the expertise of our scientists and they use the techniques and materials we ' ve developed over the years to hopefully develop great new technologies that will help society and help medicine and all the fields that are applicable to science. narrator : there are all kinds of applications - from the development of cheap, nanoscale solar cells and nanoparticle - containing sunscreens to medical applications. kelly : we have the ability to use materials such as quantum nanodots to trace things going on in cells - ways to detect cancer ; ways in fact, to treat cancer that we ' ve never had before. narrator : for science today, i ' m larissa branin.


journal of heterocyclic chemistry overview. a heterocyclic compound is defined as any organic compound where their molecules are characterized by rings containing at least one atom other than carbon. these compounds are structurally similar to cyclic organic hydrocarbons, but their properties can vary widely from those of their hydrocarbon counterparts and are largely governed by the identity, location and number of heteroatoms present in the molecule. it is this rich diversity of physical and biological properties that has lead to intense study of heterocyclic compounds. it follows then that heterocyclic chemistry is the study of all aspects of heterocyclic compounds. heterocyclic chemistry has its origin in organic synthesis, natural products chemistry and medicinal chemistry. indeed most any heterocyclic chemist will also consider themselves organic chemists and many will consider themselves to be natural products chemists and medicinal chemists as well. this relationship between disciplines arises because heterocyclic molecules are fundamental building blocks of biological systems. in addition to its importance to biology, heterocyclic chemistry has seen intense study in diverse areas such as dyes, photosensitizers, coordination compounds, polymeric materials and many other fields. policies. the journal of heterocyclic chemistry is interested in publishing research on all aspects of heterocyclic chemistry. a perusal of any issue will show that the majority of manuscripts submitted are on the synthesis and properties of heterocyclic compounds. many of those studies include a short history of the biological and commercial applications of related compounds, and often include biological screening data corresponding to newly reported compounds. this strong connection between the preparation of heterocyclic compounds and their potential benefit to humanity is both welcomed and encouraged. one will also find articles related to physical organic chemistry studies on heterocyclic compounds, mechanistic studies of reactions leading to heterocyclic compounds and studies of the photophysical properties of heterocyclic compounds, just to name a few. heterocyclic chemistry is an ever - expanding subject where scientists regularly discover new and exciting applications for heterocyclic compounds. the journal of heterocyclic chemistry invites authors to submit heterocyclic chemistry research on any aspect of heterocyclic chemistry in the form of articles, notes, reviews, and communications. articles must report the results of comprehensive research and will usually amount to more than four final printed journal pages. shorter papers, usually classified as notes


| the chinese believe that everything that exists in the universe, tangible as well as intangible, can be categorised as water, wood, fire, earth or metal. collectively there are the five elements or wu xing. there are three cycles that define the relationships of these fiveelements - the productive cycle, the destructive cycle and the exhaustive cycle. | with the ever - growing popularity of feng shui books in print all over the world, many people are now very familiar with the concept of the five elements and their cycles. some are familiar only with two cycles - the productive and the destructive - but actually there is a third cycle. this is the exhaustive cycle and it is the most appropriate when devising remedies to counter chi energy afflictions. wu xing or five element therapy is applicable not only in feng shui, but also in improving one ’ s destiny based on the four pillars charts, as well as in the practice of traditional chinese medicine. according to the chinese, everything tangible and everything abstract in the universe can be categorized under five categories, so they are either wood, fire, earth, metal, or water. you will discover as you get deeper into the chinese esoteric sciences that everything from compass directions to moods, feelings, objects, body parts, organs, plants, furniture, animals and so forth are labeled under one of the five elements, so learning how to categorize tangible objects and intangible items into one of the five element categories makes up an important dimension of the study of feng shui. what makes up the elements? for the most part, many of these things are self - explanatory. for instance, plants, flowers and anything made of wood or have the appearance of wood would be labeled under the wood element. similarly, anything that comes from the earth, or is made of earth, such as stones, dirt, ashes, crystal, glass, pebbles and so forth would be categorized as being earth element. it is the same with the other three elements of water, fire and metal. each of these elements is also unique in ways that have a bearing on their attributes and strengths. for example, wood is the only element that has a life of its own. it is growing and it is alive, so there is alive wood and there is dead wood - yang wood and yin word is easy to differentiate. as for water, it is always associated with a flow - so flowing water is yang water, while stagnant water is yin water.


exploring the physical limits to computation researchers at the university of notre dame ’ s center for nano science and technology are laying the groundwork for smaller, faster and more efficient electronic devices by exploring how energy is dissipated in nanoscale electron devices and discovering new ways to represent information and perform computation. a multidisciplinary team of electrical engineers, computer scientists, physicists and chemists is developing electronic devices, microsystems and system architecture that can process larger volumes of data — using thousands of times less energy compared to modern information processing systems. " at present there are theoretical and physical limits that define how much energy is required to process digital information ( bits ) in computing and telecommunication systems, " says alan seabaugh, professor of electrical engineering, and director of the notre dame - based midwest institute for nanoelectronics discovery. " how do we enable more energy - efficient computation? where is energy dissipated, and why? what is the simplest and most cost - effective way to build a computing engine? " these key questions are guiding development of innovative semiconductor switching technologies such as quantum mechanical tunneling, an energy transport process that becomes exponentially more efficient as the energy barriers in devices are reduced to the nanometer scale. innovative methods of electronically controlling the tunneling process are at the core of the low - voltage, low - power tunnel transistors being designed at notre dame. notre dame engineers also are discovering new ways to represent and process information using nanomagnetic logic devices. " these chips use polarization to represent digital information in a magnetic material, " says michael niemier, assistant professor of computer science and engineering. " the computation and signal transmission are achieved through the interaction of magnetic fields at the nanoscale, instead of with charge. " the theory, design, development and testing of these devices is being explored at the center for nano science and technology, utilizing the center ’ s new nanofabrication facility, a state - of - the - art clean room in the stinson - remick hall of engineering. the huge gains in computational power and energy efficiency that appear to be possible at the nanoscale promise lower energy costs, better communications and superior ability to process large volumes of data, and open up a wide array of new applications in healthcare and environmental protection. this new energy - efficient computing technology will lead to processors capable of handling much more complex problems, such as global climate change, weather prediction, nuclear fusion and the behavior of new materials and devices. more energy - efficient, environmentally friendly and cost -


synchrotron radiation occurs when a charge moving at relativistic speeds follows a curved trajectory. in this section, formulas and supporting graphs are used to quantitatively describe characteristics of this radiation for the cases of circular motion ( bending magnets ) and sinusoidal motion ( periodic magnetic structures ). we will first discuss the ideal case, where the effects due to the angular divergence and the finite size of the electron beam — the emittance effects — can be neglected. the angular distribution of radiation emitted by electrons moving through a bending magnet with a circular trajectory in the horizontal plane is given by sb = photon flux ( number of photons per second ) q = observation angle in the horizontal plane y = observation angle in the vertical plane a = fine - structure constant g = electron energy / mec2 ( me = electron mass, c = velocity of light ) w = angular frequency of photon ( e = ow = energy of photon ) i = beam current e = electron charge = 1. 602 ´ 10 – 19 coulomb y = w / wc = e / ec wc = critical frequency, defined as the frequency that divides the emitted power into equal r = radius of instantaneous curvature of the electron trajectory ( in practical units, r [ m ] = 3. 3 e [ gev ] / b [ t ] ) e = electron beam energy b = magnetic field strength ec = owc ( in practical units, ec [ kev ] = 0. 665 e2 [ gev ] b [ t ] ) x = gy x = y ( 1 + x2 ) 3 / 2 / 2 the subscripted k ’ s are modified bessel functions of the second kind. in the horizontal direction ( y = 0 ), eq. ( 1 ) becomes in practical units [ photons · s – 1 · mr – 2 · ( 0. 1 % bandwidth ) – 1 ], the function h2 ( y ) is shown in fig. 2 - 1. fig. 2 - 1. the functions g1 ( y ) and h2 ( y ), where y is the ratio of photon energy to critical photon energy. the distribution integrated over y is given by in practical units [ photons · s – 1 · mr – 1 · ( 0. 1 % bandwidth ) – 1 ], the function g1 ( y ) is also plotted in fig. 2 - 1. radiation from a bending magnet is linearly polarized when observed in the bending plane. out of this plane, the polar


. 1 % bandwidth ) – 1 ], the function g1 ( y ) is also plotted in fig. 2 - 1. radiation from a bending magnet is linearly polarized when observed in the bending plane. out of this plane, the polarization is elliptical and can be decomposed into its horizontal and vertical components. the first and second terms in the last bracket of eq. ( 1 ) correspond, respectively, to the intensity of the horizontally and vertically polarized radiation. figure 2 - 2 gives the normalized intensities of these two components, as functions of emission angle, for different energies. the square root of the ratio of these intensities is the ratio of the major and minor axes of the polarization ellipse. the sense of the electric field rotation reverses as the vertical observation angle changes from positive to negative. synchrotron radiation occurs in a narrow cone of nominal angular width ~ 1 / g. to provide a more specific measure of this angular width, in terms of electron and photon energies, it is convenient to introduce the effective rms half - angle sy as follows : fig. 2 - 2. normalized intensities of horizontal and vertical polarization components, as functions of the vertical observation angle y, for different photon energies. ( adapted from ref. 1. ) fig. 2 - 3. the function c ( y ). the limiting slopes, for e / ec < < 1 and e / ec > > 1, are indicated. where sy is given by the function c ( y ) is plotted in fig. 2 - 3. in terms of sy, eq. ( 2 ) may now be rewritten as in a wiggler or an undulator, electrons travel through a periodic magnetic structure. we consider the case where the magnetic field b varies sinusoidally and is in the vertical direction : b ( z ) = b0 cos ( 2pz / lu ), ( 8 ) where z is the distance along the wiggler axis, b0 the peak magnetic field, and lu the magnetic period. electron motion is also sinusoidal and lies in the horizontal plane. an important parameter characterizing the electron motion is the deflection parameter k given by in terms of k, the maximum angular deflection of the orbit is d = k / g. for, radiation from the various periods can exhibit strong interference phenomena, because the angular excursions of the electrons are within the nominal 1 / g radiation cone ; in this case, the structure is referred to


of the orbit is d = k / g. for, radiation from the various periods can exhibit strong interference phenomena, because the angular excursions of the electrons are within the nominal 1 / g radiation cone ; in this case, the structure is referred to as an undulator. in the case k > > 1, interference effects are less important, and the structure is referred to as a wiggler. in a wiggler, k is large ( typically ) and radiation from different parts of the electron trajectory adds incoherently. the flux distribution is then given by 2n ( where n is the number of magnet periods ) times the appropriate formula for bending magnets, either eq. ( 1 ) or eq. ( 2 ). however, r or b must be taken at the point of the electron ’ s trajectory tangent to the direction of observation. thus, for a horizontal angle q, ecmax = 0. 665 e2 [ gev ] b0 [ t ]. when y = 0, the radiation is linearly polarized in the horizontal plane, as in the case of the bending magnet. as y increases, the direction of the polarization changes, but because the elliptical polarization from one half - period of the motion combines with the elliptical polarization ( of opposite sense of rotation ) from the next, the polarization remains linear. in an undulator, k is moderate ( ) and radiation from different periods interferes coherently, thus producing sharp peaks at harmonics of the fundamental ( n = 1 ). the wavelength of the fundamental on axis ( q = y = 0 ) is given by the corresponding energy, in practical units, is the relative bandwidth at the nth harmonic is on axis the peak intensity of the nth harmonic is given by here, the j ’ s are bessel functions. the function fn ( k ) is plotted in fig. 2 - 4. in practical units [ photons · s – 1 · mr – 2 · ( 0. 1 % bandwidth ) – 1 ], eq. ( 13 ) becomes the angular distribution of the nth harmonic is concentrated in a narrow cone whose half - width is given by fig. 2 - 4. the function fn ( k ) for different values of n, where k is the deflection parameter. here l is the length of the undulator ( l = nlu ). additional rings of radiation of the same frequency also appear at angular distances the angular structure of undulator radiation is illustrated in fig. 2 - 5


the deflection parameter. here l is the length of the undulator ( l = nlu ). additional rings of radiation of the same frequency also appear at angular distances the angular structure of undulator radiation is illustrated in fig. 2 - 5 for the limiting case of zero beam emittance. we are usually interested in the central cone. an approximate formula for the flux integrated over the central cone is or, in units of photons · s – 1 · ( 0. 1 % bandwidth ) – 1, the function qn ( k ) = ( 1 + k2 / 2 ) fn / n is plotted in fig. 2 - 6. equation ( 13 ) can also be written as fig. 2 - 5. the angular distribution of fundamental ( n = 1 ) undulator radiation for the limiting case of zero beam emittance. the x and y axes correspond to the observation angles q and y ( in radians ), respectively, and the z axis is the intensity in photons · s – 1 · a – 1 · ( 0. 1 mr ) – 2 · ( 1 % bandwidth ) – 1. the undulator parameters for this theoretical calculation were n = 14, k = 1. 87, lu = 3. 5 cm, and e = 1. 3 gev. ( figure courtesy of r. tatchyn, stanford university. ) fig. 2 - 6. the function qn ( k ) for different values of n. away from the axis, there is also a change in wavelength : the factor ( 1 + k2 / 2 ) in eq. ( 11 ) must be replaced by [ 1 + k2 / 2 + g2 ( q2 + y2 ) ]. because of this wavelength shift with emission angle, the angle - integrated spectrum consists of peaks at ln superposed on a continuum. the peak - to - continuum ratio is large for k < < 1, but the continuum increases with k, as one shifts from undulator to wiggler conditions. the total power radiated by an undulator or wiggler is where z0 = 377 ohms, or, in practical units, the angular distribution of the radiated power is or, in units of w · mr – 2, the behavior of the angular function fk ( gq, gy ), which is normalized as fk ( 0, 0 ) = 1, is shown in fig. 2 - 7. the function g ( k ), shown in fig. 2 - 8, quickly approaches unity


fk ( gq, gy ), which is normalized as fk ( 0, 0 ) = 1, is shown in fig. 2 - 7. the function g ( k ), shown in fig. 2 - 8, quickly approaches unity as k increases from zero. electrons in storage rings are distributed in a finite area of transverse phase space — position ´ angle. we introduce the rms beam sizes sx ( horizontal ) and sy ( vertical ), and beam divergences ( horizontal ) and ( vertical ). the quantities and are known as the horizontal and vertical emittances, respectively. in general, owing to the finite emittances of real electron beams, the intensity of the radiation observed in the forward direction is less than that given by eqs. ( 2a ) and ( 13a ). finite emittances can be taken into account approximately by replacing these equations by for bends and undulators, respectively. for bending magnets, the electron beam divergence effect is usually negligible in the horizontal plane. fig. 2 - 7. the angular function fk, for different values of the deflection parameter k, ( a ) as a function of the vertical observation angle y when the horizontal observation angle q = 0 and ( b ) as a function of q when y = 0. fig. 2 - 8. the function g ( k ). for experiments that require a small angular divergence and a small irradiated area, the relevant figure of merit is the beam brightness b, which is the photon flux per unit phase space volume, often given in units of photons · s – 1 · mr – 2 · mm – 2 · ( 0. 1 % bandwidth ) – 1. for an undulator, an approximate formula for the peak brightness is where, for example, and where the single - electron radiation from an axially extended source of finite wavelength is described by brightness is shown in fig. 2 - 9 for several sources of synchrotron radiation, as well as some conventional x - ray sources. that portion of the flux that is transversely coherent is given by a substantial fraction of undulator flux is thus transversely coherent for a low - emittance beam satisfying exey ( l / 4p ) 2. longitudinal coherence is described in terms of a coherence length for an undulator, the various harmonics have a natural spectral purity of dl / l = 1 / nn [ see eq. ( 12 ) ] ; thus, the co


coherence is described in terms of a coherence length for an undulator, the various harmonics have a natural spectral purity of dl / l = 1 / nn [ see eq. ( 12 ) ] ; thus, the coherence length is given by which corresponds to the relativistically contracted length of the undulator. thus, undulator radiation from low - emittance electron beams [ exey ( l / 4p ) 2 ] is transversely coherent and is longitudinally coherent within a distance described by eq. ( 27 ). in the case of finite beam emittance or finite angular acceptance, the longitudinal coherence is reduced because of the change in wavelength with emission angle. in this sense, undulator radiation is partially coherent. transverse and longitudinal coherence can be enhanced when necessary by the use of spatial and spectral filtering ( i. e., by use of apertures and monochromators, respectively ). the references listed below provide more detail on the characteristics of synchrotron radiation. fig. 2 - 9. spectral brightness for several synchrotron radiation sources and conventional x - ray sources. the data for conventional x - ray tubes should be taken as rough estimates only, since brightness depends strongly on such parameters as operating voltage and take - off angle. the indicated two - order - of - magnitude ranges show the approximate variation that can be expected among stationary - anode tubes ( lower end of range ), rotating - anode tubes ( middle ), and rotating - anode tubes with microfocusing ( upper end of range ). 1. g. k. green, “ spectra and optics of synchrotron radiation, ” in proposal for national synchrotron light source, brookhaven national laboratory, upton, new york, bnl - 50595 ( 1977 ). 2. h. winick, “ properties of synchrotron radiation, ” in h. winick and s. doniach, eds., synchrotron radiation research ( plenum, new york, 1979 ), p. 11. 3. s. krinsky, “ undulators as sources of synchrotron radiation, ” ieee trans. nucl. sci. ns - 30, 3078 ( 1983 ). 4. d. f. alferov, yu. bashmakov, and e. g. bessonov, “ undulator radiation, ” sov. phys. tech. phys


a dynamic theory of history ( 1904 ) a dynamic theory, like most theories, begins by begging the question : it defines progress as the development and economy of forces. further, it defines force as anything that does, or helps to do work. man is a force ; so is the sun ; so is a mathematical point, though without dimensions or known existence. man commonly begs the question again taking for granted that he captures the forces. a dynamic theory, assigning attractive force to opposing bodies in proportion to the law of mass, takes for granted that the forces of nature capture man. the sum of force attracts : the feeble atom or molecule called man is attracted ; he suffers education or growth ; he is the sum of the forces that attract him ; his body and his thought are alike their product ; the movement of the forces controls the progress of his mind, since he can know nothing but the motions which impinge on his senses, whose sum makes education. for convenience as an image, the theory may liken man to a spider in its web, watching for chance prey. forces of nature dance like flies before the net, and the spider pounces on them when it can ; but it makes many fatal mistakes, though its theory of force is sound. the spider - mind acquires a faculty of memory, and, with it, a singular skill of analysis and synthesis, taking apart and putting together in different relations the meshes of its trap. man had in the beginning no power of analysis or synthesis approaching that of the spider, or even of the honey - bee ; he had acute sensibility to the higher forces. fire taught him secrets that no other animal could learn ; running water probably taught him even more, especially in his first lessons of mechanics ; the animals helped to educate him, trusting themselves into his hands a dynamic theory of history 475 merely for the sake of their food, and carrying his burdens or supplying his clothing ; the grasses and grains were academies of study. with little or no effort on his part, all these forces formed his thought, induced his action, and even shaped his figure. long before history began, his education was complete, for the record could not have been started until he had been taught to record. the universe that had formed him took shape in his mind as a reflection of his own unity, containing all forces except himself. either separately, or in groups, or as a whole, these forces never ceased to act on him, enlarging his mind as they enlarged the surface foliage of a vegetable


mind as a reflection of his own unity, containing all forces except himself. either separately, or in groups, or as a whole, these forces never ceased to act on him, enlarging his mind as they enlarged the surface foliage of a vegetable, and the mind needed only to respond, as the forests did, to these attractions. susceptibility to the highest forces is the highest genius ; selection between them is the highest science ; their mass is the highest educator. man always made, and still makes, grotesque blunders in selecting and measuring forces, taken at random from the heap, but he never made a mistake in the value he set on the whole, which he symbolized as unity and worshipped as god. to this day, his attitude towards it has never changed, though science can no longer give to force a name. man ' s function as a force of nature was to assimilate other forces as he assimilated food. he called it the love of power. he felt his own feebleness, and he sought for an ass or a camel, a bow or a sling, to widen his range of power, as he sough fetish or a planet in the world beyond. he cared little to know its immediate use, but he could afford to throw nothing away which he could conceive to have possible value in this or any other existence. he waited for the object to teach him its use, or want of use, and the process was slow. he may have gone on for hundreds of thousands of years, waiting for nature to tell him her secrets ; and, to his rivals among the monkeys, nature has taught no more than at their start ; but certain lines of force were capable of acting on individual apes, and mechanically selecting types of race or sources of variation. the individual that responded or reacted to lines of 476 the education of henry adams new force then was possibly the same individual that reacts on it now, and his conception of the unity seems never to have changed in spite of the increasing diversity of forces ; but the theory of variation is an affair of other science than history, and matters nothing to dynamics. the individual or the race would be educated on the same lines of illusion, which, according to arthur balfour, had not essentially varied down to the year 1900. to the highest attractive energy, man gave the name of divine, and for its control he invented the science called religion, a word which meant, and still means, cultivation of occult force whether in detail or mass. unable


down to the year 1900. to the highest attractive energy, man gave the name of divine, and for its control he invented the science called religion, a word which meant, and still means, cultivation of occult force whether in detail or mass. unable to define force as a unity, man symbolized it and pursued it, both in himself, and in the infinite, as phiiosophy and theology ; the mind is itself the subtlest of all known forces, and its self - introspection necessarily created a science wh. ch had the singular value of lifting his education, at the start, to the finest, subtlest, and broadest training both in analysis and synthesis, so that, if language is a test, he must have reached his highest powers early in his history ; while the mere motive remained as simple an appetite for power as the tribal greed which led him to trap an elephant. hunger, whether for food or for the infinite, sets in motion multiplicity and infinity of thought, and the sure hope of gaining a share of infinite power in eternal life would lift most minds to effort. he had reached this completeness five thousand years ago, and added nothing to his stock of known forces for a very long time. the mass of nature exercised on him so feeble an attraction that one can scarcely account for his apparent motion. only a historian of very exceptional knowledge would venture to say at what date between 3000 b. c. and 1900., the momentum of europe was greatest ; but such progress as the world made consisted in economies of energy rather than in its development ; it was proved in mathematics, measured by names like archimedes, aristarchus, ptolemy, and euclid ; or in civil law, measured by a number of names which adams had begun life by failing to learn ; or in coinage, a dynamic theory of history 477 which was most beautiful near its beginning, and most barbarous at its close ; or it was shown in roads, or the size of ships, or harbors ; or by the use of metals, instruments, and writing ; all of them economies of force, sometimes more forceful than the forces they helped ; but the roads were still travelled by the horse, the ass, the camel, or the slave ; the ships were still propelled by sails or oars ; the lever, the spring, and the screw bounded the region of applied mechanics. even the metals were old. much the same thing could be said of religious or supernatural forces. down to the year 300 of


of stoppage is illusive. the theory seems to exact excess, rather than deficiency, of action and reaction to account for the dissolution of the roman empire, which should, as a problem of mechanics, have been torn to pieces by acceleration. if the student means to try the experiment of framing a dynamic law, he must assign values to the forces of attraction that caused the trouble ; and in this case he has them in plain evidence. with the relentless logic that stamped roman thought, the empire, which had established unity on earth, could not help establishing unity in heaven. it was induced by its dynamic necessities to economize the gods. the church has never ceased to protest against the charge that christianity ruined the empire, and, with its usual force, has pointed out that its reforms alone saved the state. any dynamic theory gladly admits it. all it asks is to find and follow the force that attracts. the church points out this force in the cross, and history needs only to follow it. the empire loudly asserted its mo - tive. good taste forbids saying that constantine the great speculated as audaciously as a modern stock - broker on values of which he knew at the utmost only the volume ; or that he merged all un - certain forces into a single trust, which he enormously overcapitalized, and forced on the market ; but this is the substance of what constantine himself said in his edict of milan in the year 313, which admitted christianity into the trust of state religions. regarded as an act of congress, it runs : " we have resolved to grant to christians as well as all others the liberty to practice the religion they prefer, in order that whatever exists of divinity or celestial power may help and favor us and all who are under our a dynamic theory of history 479 government. " the empire pursued power - - not merely spiritual but physical - - in the sense in which constantine issued his army order the year before, at the battle of the milvian bridge : in hoc signo vinces! using the cross as a train of artillery, which, to his mind, it was. society accepted it in the same character. eighty years afterwards, theodosius marched against his rival eugene with the cross for physical champion ; and eugene raised the image of hercules to fight for the pagans ; while society on both sides looked on, as though it were a boxing - match, to decide a final test of force between the divine powers. the church was powerless to raise the ideal. what


theory of history 487 man of science stood at first as bewildered and helpless as, in the fourth century, a priest of isis before the cross of christ. this, then, or something like this, would be a dynamic formula of history. any schoolboy knows enough to object at once that it is the oldest and most universal of all theories. church and state, theology and philosophy, have always preached it, differing only in the allotment of energy between nature and man. whether the attractive energy has been called god or nature, the mechanism has been always the same, and history is not obliged to decide whether the ultimate tends to a purpose or not, or whether ultimate energy is one or many. every one admits that the will is a free force, habitually decided by motives. no one denies that motives exist adequate to decide the will ; even though it may not always be conscious of them. science has proved that forces, sensible and occult, physical and metaphysical, simple and complex, surround, traverse, vibrate, rotate, repel, attract, without stop ; that man ' s senses are conscious of few, and only in a partial degree ; but that, from the beginning of organic existence, his consciousness has been induced, expanded, trained in the lines of his sensitiveness ; and that the rise of his faculties from a lower power to a higher, or from a narrower to a wider field, may be due to the function of assimilating and storing outside force or forces. there is nothing unscientific in the idea that, beyond the lines of force felt by the senses, the universe may be - as it has always been - - either a supersensuous chaos or a divine unity, which irresistibly attracts, and is either life or death to penetrate. thus far, religion, philosophy, and science seem to go hand in hand. the schools begin their vital battle only there. in the earlier stages of progress, the forces to be assimilated were simple and easy to absorb, but, as the mind of man enlarged its range, it enlarged the field of com - plexity, and must continue to do so, even into chaos, until the reservoirs of sensuous or supersensuous energies are exhausted, or cease to affect him, or until he succumbs to their excess.


in coding theory, one typically maps a string to a code such that with some small amount of error in the code one can still recover the original string. what if the amount of error is too much to give a unique decoding? in the 1950s peter elias suggested the idea of list decoding, coming up with a short list of possibilities one of which is correct. madhu sudan showed that list decoding can be achieved in scenarios where one cannot do unique decoding. in this paper sudan gives a polynomial - time list - decoding algorithm that can deal with errors in the code beyond what regular codes can handle. later guruswami and sudan give a polynomial - time algorithm that handles what is believed to be the best possible amount of error. list - decodable codes have had applications to many areas including pseudo - random generators, extractors, hard - core predicates, probabilistically - checkable proofs and a neat result by sivakumar on the implications of sat being membership - comparable. we ' ve seen many other important papers in coding theory from computer scientists over the last decade. besides the work on list decoding i should also mention spielman ' s breakthrough result showing linear time encodable and decodable codes building on the initial work of using expander graphs for codes developed by sipser and spielman.


knowledge discovery from graphs ( invited talk ) jensen, david ( 2001 ) knowledge discovery from graphs ( invited talk ). in : graph drawing 8th international symposium, gd 2000, september 20 – 23, 2000, colonial williamsburg, va, usa, p. 170 ( official url : http : / / dx. doi. org / 10. 1007 / 3 - 540 - 44541 - 2 _ 16 ). full text not available from this repository. knowledge discovery is the process of discovering useful and previously unknown knowledge by analyzing large databases. knowledge discovery is also sometimes called " data mining " or " applied machine learning. " a new generation of knowledge discovery tools are beginning to address data that can be expressed as large graphs. example applications include fraud detection in telecommunication networks and classifying web pages based on hyperlink structure. these new technologies for knowledge discovery are becoming increasingly relevant to graph drawing. specifically, graph drawing can aid the process of knowledge discovery by providing visualizations that reveal useful patterns in the data. conversely, knowledge discovery can provide guidance for graph drawing by identifying recurring substructures or by classifying nodes into distinct types. attempts to exploit the synergy between the two fields raises interesting new research questions. how should knowledge about a domain affect the drawing of graphs about that domain? what types of knowledge are most easily discovered using visualization, as opposed to automated statistical algorithms? these questions were posed in the context of several examples of knowledge discovery applied to large graphical data sets. repository staff only : item control page


new scan ' sees ' tumors deep inside body imaging technique could help surgeons remove all cancerous tissue wednesday, april 2 ( healthday news ) - - stanford university researchers have developed a new imaging system that illuminates tumors deep inside the body and lets doctors view details 1, 000 times smaller than previously possible. raman spectroscopy uses tiny nanoparticles injected into the body to serve as beacons for lasers, according to a description the method published in this week ' s online issue of the proceedings of the national academy of sciences. when a laser beam outside the body hits them, the specialized particles emit signals that can be converted into a visible indicator of their location in the body. these strong, long - lived signals can simultaneously transmit information about multiple molecular targets. " usually we can measure one or two things at a time, " senior author dr. sanjiv sam gambhir, a professor of radiology at stanford ' s school of medicine, said in a prepared statement. " with this, we can now likely see 10, 20, 30 things at once. " the new system utilizes the raman effect, which occurs when light is shined on an object. the light causes roughly one in 10 million photons to bounce off the object ' s molecules with an increase or decrease in energy, called raman scattering. this forms a unique measurable pattern, called a spectral fingerprint, for each type of molecule. the stanford research team tested the system on mice, injecting them with various engineered raman nanoparticles and then viewing the anesthetized mice under a special microscope where they were exposed to laser light. the nanoparticles, for example, would be " tagged " with different pieces of proteins that sought out different tumor molecules. in these experiments, the team spotted targets 1, 000 times smaller than what is viewable with the most precise fluorescence imaging available. since the raman effect lasts indefinitely, as long as the particles stay in the body they can work as signals. because of these findings, the technique could be useful during tumor surgery on humans by aiding in the removal of even the most microscopic bits of cancerous tissue, the researchers said. gambhir ' s lab is further studying these raman nanoparticles, including optimizing their size and dosage and evaluating possible toxicity. a clinical trial using gold nanoparticles in humans in conjunction with a colonoscopy to indicate early - stage colorectal cancer is being planned. the u. s


two types of forces in structural engineering : tension the force felt in your arms while you hang from a bar. structural element subjected to tension is elongated. the force felt in your arms while you stand on your structural element subjected to compression is shortened. is defined as force per unit area that the force acts upon. are either tensile or compressive. structural materials are chosen by their ability to resist tensile or compressive forces, depending upon the application. most materials are better at resisting one or the other. for instance, concrete is strong in compression and relatively weak in tension. steel is equally strong in both tension example of a tensile structural element would be a example of a compressive structural element would be a column. is defined as the change in length of a stressed structural element divided by the original length of the unstressed element. tensile strength is determined in the laboratory by pulling on a specimen until it breaks. while the test is conducted, both the stress and strain are recorded. the maximum stress that the specimen can withstand is called the ultimate strength of that particular material. from a design stand - point, we are mainly interested in the stress where the material stops behaves elastically when it returns to its original shape when an applied load is no longer applied. this point is found by plotting stress versus strain during the test and determining the stress at which the plot becomes non - linear. this stress is called the yield stress, of the stress - strain curve in the elastic region is defined as the elastic modulus, e. structures should be designed so that any applied load would not cause the stress in the structure to be greater structural elements that are subjected to bending forces. when bending occurs, the beam is subjected to tension and a sponge beam. say we draw a grid on the side of the beam, so that the sponge is divided into two rows of rectangles of equal length, lo, and height, h / 2. force is applied to the beam, the rectangles deform. of the upper row of rectangles are shortened, and the bottoms of the lower row of rectangles are elongated. thus, we see that the top of the beam is in compression and the bottom of the beam is in tension. that the middle of the beam is in neither tension or compression. this is called the neutral axis. the bending stress at the neutral axis is zero. to designing a beam is to locate the point of maximum stress. for a simply - supported beam under a uniform load, the maximum stress occurs


operation between people. thomas defined social disorganisation as " the inability of a neighbourhood to solve its problems together " which suggested a level of social pathology and personal disorganisation, so the term, " differential social organisation " was preferred by many, and may have been the source of sutherland ' s ( 1947 ) differential association theory. the researchers have provided a clear analysis that the city is a place where life is superficial, where people are anonymous, where relationships are transitory and friendship and family bonds are weak. they have observed the weakening of primary social relationships and relate this to a process of social disorganisation ( comparison with the concept of anomie and the strain theories is instructive ). vasishth and sloane ( 2000 ) argue that while it is tempting to draw analogies between organisms in nature and the human condition, the problem lies in reductionism, i. e. that the science of biology is oversimplified into rules that are then applied mechanically to explain the growth and dynamics of human communities. the most fundamental difficulties are definitional. if a community is a group of individuals who inhabit the same place, is the community merely the sum of individuals and their activities, or is it something more that an aggregation of individuals? this is critical in planning research into group interactions. will research be effective if it focuses on the individuals comprising a group, or is the community itself a proper subject of research independently of the individuals who comprise it? if the former, then data on individuals will explain the community, but if the community either directly or indirectly affects the behaviour of its members, then research must consider the patterns and processes of community as distinct from patterns and processes in populations of individuals. but this requires a definition and distinction between " pattern " and " process ". the structures, forms, and patterns are relatively easy to observe and measure, but they are nothing more than evidence of underlying processes and functions which are the real constitutive forces in nature and society. the chicago school wanted to develop tools by which to research and then change society by directing urban planning and social intervention agencies. it recognised that urban expansion was not haphazard but quite strongly controlled by community - level forces such as land values, zoning ordinances, landscape features, circulation corridors, and historical contingency. this was characterised as ecological because the external factors were neither chance nor intended, but rather arose from the natural forces in the environment which limit the adaptive spatial and temporal relationships between individuals. the school sought to derive patterns from a


wall is sitting still, the machines can detect the rise and fall of the person ' s chest with each breath. the radars transmit millions of very short pulses. what they see through a wall is related to the timing of the return pulses. radarvision generates 10 million 300 - to 500 - picosecond - long pulses every second - - each one at well below 100 microwatts. its receiver knows to within a few picoseconds when any one of the pulses will return and will switch on only for a brief sampling window, after which it shuts off again. this feature greatly improves the signal - to - noise ratio of the return signal and reduces the radar ' s power consumption. either device can run for a couple of hours on a single battery charge. each also has the added benefit of making it difficult for the bad guys to know they are being monitored, because signal detection devices can ' t distinguish the devices ' low - power transmissions from background noise. on return, the pulses are picked up by a linear array of antennas. the time of arrival for each return pulse is measured at each antenna, providing an accurate determination of where the moving object is with respect to the machine ' s field of view. the radar systems look for changes in the range and angle at which successive pulses strike an object on the other side of the wall. if, say, pulse 1 comes back revealing that there is an object at range x and angle y, a difference in range or angle for pulse 2 is registered as movement. an onscreen representation of that is shown to the user. whenever there is no difference between the latest pulse return and the one preceding it, which is the case for pulses that bounce off inanimate objects, the system disregards those objects and omits them from the display. what the user sees is a plain view of what lies on the other side of the wall, but seen onscreen from above [ see illustration, " looking over " ]. an optional mode shows the space on the other side of the wall the way it would appear from the side. this option allows an experienced operator to distinguish between tall and short objects, such as an adult and a small child or pet. to get around spectrum interference rules and to make the radar even more immune to detection, the pulses, which are spread across frequencies ranging from 1 to 5 gigahertz, are pseudorandomly dithered in time. dithering requires a time code that determines the position of the pulse within a time window. this


that mother nature herself came up with. that ' s where foldit plays a role. the game is designed so that players can manipulate virtual molecular structures that look like multicolored, curled - up tinkertoy sets. the virtual molecules follow the same chemical rules that are obeyed by real molecules. when someone playing the game comes up with a more elegant structure that reflects a lower energy state for the molecule, his or her score goes up. if the structure requires more energy to maintain, or if it doesn ' t reflect real - life chemistry, then the score is lower. more than 236, 000 players have registered for the game since its debut in 2008. the monkey - virus puzzle was one of several unsolved molecular mysteries that a colleague of khatib ' s at the university, frank dimaio, recently tried to solve using a method that took advantage of a protein - folding computer program called rosetta. " this was one of the cases where his method wasn ' t able to solve it, " khatib said. fortunately, the challenge fit the current capabilities of the foldit game, so khatib and his colleagues put the puzzle out there for foldit ' s teams to work on. " this was really kind of a last - ditch effort, " he recalled. " can the foldit players really solve it? " they could. " they actually did it in less than 10 days, " khatib said. university of washington a screen shot shows how the foldit program posed the monkey - virus molecular puzzle. one floppy loop of the molecule, visible on the left side of this image, was particularly tricky to figure out. but players belonging to the foldit contenders group worked as a tag team to come up with an incredibly elegant, low - energy model for the monkey - virus enzyme. " standard autobuilding and structure refinement methods showed within hours that the solution was almost certainly correct, " the researchers reported in the paper published today. " using the foldit solution, the final refined structure was completed a few days later. " khatib said the seattle team ' s collaborators in poland were in such a celebratory mood that they insisted on organizing a simultaneous champagne toast, shared over a skype video teleconference. " although much attention has recently been given to the potential of crowdsourcing and game playing, this is the first instance that we are aware of in which online gamers solved a longstanding scientific problem, " khatib and his colleagues wrote. the


how does it come about that alongside of the idea of ponderable matter, which is derived by abstraction from everyday life, the physicists set the idea of the existence of another kind of matter, the ether? the explanation is probably to be sought in those phenomena which have given rise to the theory of action at a distance, and in the properties of light which have led to the undulatory theory. let us devote a little while to the consideration of these two subjects. outside of physics we know nothing of action at a distance. when we try to connect cause and effect in the experiences which natural objects afford us, it seems at first as if there were no other mutual actions than those of immediate contact, e. g. the communication of motion by impact, push and pull, heating or inducing combustion by means of a flame, etc. it is true that even in everyday experience weight, which is in a sense action at a distance, plays a very important part. but since in daily experience the weight of bodies meets us as something constant, something not linked to any cause which is variable in time or place, we do not in everyday life speculate as to the cause of gravity, and therefore do not become conscious of its character as action at a distance. it was newton ' s theory of gravitation that first assigned a cause for gravity by interpreting it as action at a distance, proceeding from masses. newton ' s theory is probably the greatest stride ever made in the effort towards the causal nexus of natural phenomena. and yet this theory evoked a lively sense of discomfort among newton ' s contemporaries, because it seemed to be in conflict with the principle springing from the rest of experience, that there can be reciprocal action only through contact, and not through immediate action at a distance. it is only with reluctance that man ' s desire for knowledge endures a dualism of this kind. how was unity to be preserved in his comprehension of the forces of nature? either by trying to look upon contact forces as being themselves distant forces which admittedly are observable only at a very small distance and this was the road which newton ' s followers, who were entirely under the spell of his doctrine, mostly preferred to take ; or by assuming that the newtonian action at a distance is only apparently immediate action at a distance, but in truth is conveyed by a medium permeating space, whether by movements or by elastic deformation of this medium. thus the endeavour toward a unified view of the nature of forces leads to the hypothesis of an ether.


immediate action at a distance, but in truth is conveyed by a medium permeating space, whether by movements or by elastic deformation of this medium. thus the endeavour toward a unified view of the nature of forces leads to the hypothesis of an ether. this hypothesis, to be sure, did not at first bring with it any advance in the theory of gravitation or in physics generally, so that it became customary to treat newton ' s law of force as an axiom not further reducible. but the ether hypothesis was bound always to play some part in physical science, even if at first only a latent part. when in the first half of the nineteenth century the far - reaching similarity was revealed which subsists between the properties of light and those of elastic waves in ponderable bodies, the ether hypothesis found fresh support. it appeared beyond question that light must be interpreted as a vibratory process in an elastic, inert medium filling up universal space. it also seemed to be a necessary consequence of the fact that light is capable of polarisation that this medium, the ether, must be of the nature of a solid body, because transverse waves are not possible in a fluid, but only in a solid. thus the physicists were bound to arrive at the theory of the " quasi - rigid " luminiferous ether, the parts of which can carry out no movements relatively to one another except the small movements of deformation which correspond to light - waves. this theory - also called the theory of the stationary luminiferous ether - moreover found a strong support in an experiment which is also of fundamental importance in the special theory of relativity, the experiment of fizeau, from which one was obliged to infer that the luminiferous ether does not take part in the movements of bodies. the phenomenon of aberration also favoured the theory of the quasi - rigid ether. the development of the theory of electricity along the path opened up by maxwell and lorentz gave the development of our ideas concerning the ether quite a peculiar and unexpected turn. for maxwell himself the ether indeed still had properties which were purely mechanical, although of a much more complicated kind than the mechanical properties of tangible solid bodies. but neither maxwell nor his followers succeeded in elaborating a mechanical model for the ether which might furnish a satisfactory mechanical interpretation of maxwell ' s laws of the electro - magnetic field. the laws were clear and simple, the mechanical interpretations clumsy and contradictory. almost imperceptibly the theoretical physicists adapted themselves to a situation which


which might furnish a satisfactory mechanical interpretation of maxwell ' s laws of the electro - magnetic field. the laws were clear and simple, the mechanical interpretations clumsy and contradictory. almost imperceptibly the theoretical physicists adapted themselves to a situation which, from the standpoint of their mechanical programme, was very depressing. they were particularly influenced by the electro - dynamical investigations of heinrich hertz. for whereas they previously had required of a conclusive theory that it should content itself with the fundamental concepts which belong exclusively to mechanics ( e. g. densities, velocities, deformations, stresses ) they gradually accustomed themselves to admitting electric and magnetic force as fundamental concepts side by side with those of mechanics, without requiring a mechanical interpretation for them. thus the purely mechanical view of nature was gradually abandoned. but this change led to a fundamental dualism which in the long - run was insupportable. a way of escape was now sought in the reverse direction, by reducing the principles of mechanics to those of electricity, and this especially as confidence in the strict validity of the equations of newton ' s mechanics was shaken by the experiments with b - rays and rapid cathode rays. this dualism still confronts us in unextenuated form in the theory of hertz, where matter appears not only as the bearer of velocities, kinetic energy, and mechanical pressures, but also as the bearer of electromagnetic fields. since such fields also occur in vacuo - i. e. in free ether - the ether also appears as bearer of electromagnetic fields. the ether appears indistinguishable in its functions from ordinary matter. within matter it takes part in the motion of matter and in empty space it has everywhere a velocity ; so that the ether has a definitely assigned velocity throughout the whole of space. there is no fundamental difference between hertz ' s ether and ponderable matter ( which in part subsists in the ether ). the hertz theory suffered not only from the defect of ascribing to matter and ether, on the one hand mechanical states, and on the other hand electrical states, which do not stand in any conceivable relation to each other ; it was also at variance with the result of fizeau ' s important experiment on the velocity of the propagation of light in moving fluids, and with other established experimental results. such was the state of things when h a lorentz entered upon the scene. he brought theory into harmony with experience by means of a wonderful simplification


on the velocity of the propagation of light in moving fluids, and with other established experimental results. such was the state of things when h a lorentz entered upon the scene. he brought theory into harmony with experience by means of a wonderful simplification of theoretical principles. he achieved this, the most important advance in the theory of electricity since maxwell, by taking from ether its mechanical, and from matter its electromagnetic qualities. as in empty space, so too in the interior of material bodies, the ether, and not matter viewed atomistically, was exclusively the seat of electromagnetic fields. according to lorentz the elementary particles of matter alone are capable of carrying out movements ; their electromagnetic activity is entirely confined to the carrying of electric charges. thus lorentz succeeded in reducing all electromagnetic happenings to maxwell ' s equations for free space. as to the mechanical nature of the lorentzian ether, it may be said of it, in a somewhat playful spirit, that immobility is the only mechanical property of which it has not been deprived by h a lorentz. it may be added that the whole change in the conception of the ether which the special theory of relativity brought about, consisted in taking away from the ether its last mechanical quality, namely, its immobility. how this is to be understood will forthwith be expounded. the space - time theory and the kinematics of the special theory of relativity were modelled on the maxwell - lorentz theory of the electromagnetic field. this theory therefore satisfies the conditions of the special theory of relativity, but when viewed from the latter it acquires a novel aspect. for if k be a system of coordinates relatively to which the lorentzian ether is at rest, the maxwell - lorentz equations are valid primarily with reference to k. but by the special theory of relativity the same equations without any change of meaning also hold in relation to any new system of co - ordinates k ' which is moving in uniform translation relatively to k. now comes the anxious question : - why must i in the theory distinguish the k system above all k ' systems, which are physically equivalent to it in all respects, by assuming that the ether is at rest relatively to the k system? for the theoretician such an asymmetry in the theoretical structure, with no corresponding asymmetry in the system of experience, is intolerable. if we assume the ether to be at rest relatively to k, but in motion relatively to k ', the physical equivalence of k and k ' seems


theoretical structure, with no corresponding asymmetry in the system of experience, is intolerable. if we assume the ether to be at rest relatively to k, but in motion relatively to k ', the physical equivalence of k and k ' seems to me from the logical standpoint, not indeed downright incorrect, but nevertheless unacceptable. the next position which it was possible to take up in face of this state of things appeared to be the following. the ether does not exist at all. the electromagnetic fields are not states of a medium, and are not bound down to any bearer, but they are independent realities which are not reducible to anything else, exactly like the atoms of ponderable matter. this conception suggests itself the more readily as, according to lorentz ' s theory, electromagnetic radiation, like ponderable matter, brings impulse and energy with it, and as, according to the special theory of relativity, both matter and radiation are but special forms of distributed energy, ponderable mass losing its isolation and appearing as a special form of energy. more careful reflection teaches us however, that the special theory of relativity does not compel us to deny ether. we may assume the existence of an ether ; only we must give up ascribing a definite state of motion to it, i. e. we must by abstraction take from it the last mechanical characteristic which lorentz had still left it. we shall see later that this point of view, the conceivability of which i shall at once endeavour to make more intelligible by a somewhat halting comparison, is justified by the results of the general theory of relativity. think of waves on the surface of water. here we can describe two entirely different things. either we may observe how the undulatory surface forming the boundary between water and air alters in the course of time ; or else - with the help of small floats, for instance - we can observe how the position of the separate particles of water alters in the course of time. if the existence of such floats for tracking the motion of the particles of a fluid were a fundamental impossibility in physics - if, in fact nothing else whatever were observable than the shape of the space occupied by the water as it varies in time, we should have no ground for the assumption that water consists of movable particles. but all the same we could characterise it as a medium. we have something like this in the electromagnetic field. for we may picture the field to ourselves as consisting of lines of force. if


for the assumption that water consists of movable particles. but all the same we could characterise it as a medium. we have something like this in the electromagnetic field. for we may picture the field to ourselves as consisting of lines of force. if we wish to interpret these lines of force to ourselves as something material in the ordinary sense, we are tempted to interpret the dynamic processes as motions of these lines of force, such that each separate line of force is tracked through the course of time. it is well known, however, that this way of regarding the electromagnetic field leads to contradictions. generalising we must say this : - there may be supposed to be extended physical objects to which the idea of motion cannot be applied. they may not be thought of as consisting of particles which allow themselves to be separately tracked through time. in minkowski ' s idiom this is expressed as follows : - not every extended conformation in the four - dimensional world can be regarded as composed of world - threads. the special theory of relativity forbids us to assume the ether to consist of particles observable through time, but the hypothesis of ether in itself is not in conflict with the special theory of relativity. only we must be on our guard against ascribing a state of motion to the ether. certainly, from the standpoint of the special theory of relativity, the ether hypothesis appears at first to be an empty hypothesis. in the equations of the electromagnetic field there occur, in addition to the densities of the electric charge, only the intensities of the field. the career of electromagnetic processes in vacuo appears to be completely determined by these equations, uninfluenced by other physical quantities. the electromagnetic fields appear as ultimate, irreducible realities, and at first it seems superfluous to postulate a homogeneous, isotropic ether - medium, and to envisage electromagnetic fields as states of this medium. but on the other hand there is a weighty argument to be adduced in favour of the ether hypothesis. to deny the ether is ultimately to assume that empty space has no physical qualities whatever. the fundamental facts of mechanics do not harmonize with this view. for the mechanical behaviour of a corporeal system hovering freely in empty space depends not only on relative positions ( distances ) and relative velocities, but also on its state of rotation, which physically may be taken as a characteristic not appertaining to the system in itself. in order to be able to look upon the rotation of


relative positions ( distances ) and relative velocities, but also on its state of rotation, which physically may be taken as a characteristic not appertaining to the system in itself. in order to be able to look upon the rotation of the system, at least formally, as something real, newton objectivises space. since he classes his absolute space together with real things, for him rotation relative to an absolute space is also something real. newton might no less well have called his absolute space " ether " ; what is essential is merely that besides observable objects, another thing, which is not perceptible, must be looked upon as real, to enable acceleration or rotation to be looked upon as something real. it is true that mach tried to avoid having to accept as real something which is not observable by endeavouring to substitute in mechanics a mean acceleration with reference to the totality of the masses in the universe in place of an acceleration with reference to absolute space. but inertial resistance opposed to relative acceleration of distant masses presupposes action at a distance ; and as the modern physicist does not believe that he may accept this action at a distance, he comes back once more, if he follows mach, to the ether, which has to serve as medium for the effects of inertia. but this conception of the ether to which we are led by mach ' s way of thinking differs essentially from the ether as conceived by newton, by fresnel, and by lorentz. mach ' s ether not only conditions the behaviour of inert masses, but is also conditioned in its state by them. mach ' s idea finds its full development in the ether of the general theory of relativity. according to this theory the metrical qualities of the continuum of space - time differ in the environment of different points of space - time, and are partly conditioned by the matter existing outside of the territory under consideration. this space - time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that " empty space " in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions ( the gravitation potentials gmn ), has, i think, finally disposed of the view that space is physically empty. but therewith the conception of the ether has again acquired an intelligible content although this content differs widely from that of the ether of the mechanical undulatory theory of light. the ether of the general theory


the view that space is physically empty. but therewith the conception of the ether has again acquired an intelligible content although this content differs widely from that of the ether of the mechanical undulatory theory of light. the ether of the general theory of relativity is a medium which is itself devoid of all mechanical and kinematical qualities, but helps to determine mechanical ( and electromagnetic ) events. what is fundamentally new in the ether of the general theory of relativity as opposed to the ether of lorentz consists in this, that the state of the former is at every place determined by connections with the matter and the state of the ether in neighbouring places, which are amenable to law in the form of differential equations ; whereas the state of the lorentzian ether in the absence of electromagnetic fields is conditioned by nothing outside itself, and is everywhere the same. the ether of the general theory of relativity is transmuted conceptually into the ether of lorentz if we substitute constants for the functions of space which describe the former, disregarding the causes which condition its state. thus we may also say, i think, that the ether of the general theory of relativity is the outcome of the lorentzian ether, through relativation. as to the part which the new ether is to play in the physics of the future we are not yet clear. we know that it determines the metrical relations in the space - time continuum, e. g. the configurative possibilities of solid bodies as well as the gravitational fields ; but we do not know whether it has an essential share in the structure of the electrical elementary particles constituting matter. nor do we know whether it is only in the proximity of ponderable masses that its structure differs essentially from that of the lorentzian ether ; whether the geometry of spaces of cosmic extent is approximately euclidean. but we can assert by reason of the relativistic equations of gravitation that there must be a departure from euclidean relations, with spaces of cosmic order of magnitude, if there exists a positive mean density, no matter how small, of the matter in the universe. in this case the universe must of necessity be spatially unbounded and of finite magnitude, its magnitude being determined by the value of that mean density. if we consider the gravitational field and the electromagnetic field from the standpoint of the ether hypothesis, we find a remarkable difference between the two. there can be no space nor any part of space without gravitational potentials ; for these confer upon space its metrical qualities,


gravitational field and the electromagnetic field from the standpoint of the ether hypothesis, we find a remarkable difference between the two. there can be no space nor any part of space without gravitational potentials ; for these confer upon space its metrical qualities, without which it cannot be imagined at all. the existence of the gravitational field is inseparably bound up with the existence of space. on the other hand a part of space may very well be imagined without an electromagnetic field ; thus in contrast with the gravitational field, the electromagnetic field seems to be only secondarily linked to the ether, the formal nature of the electromagnetic field being as yet in no way determined by that of gravitational ether. from the present state of theory it looks as if the electromagnetic field, as opposed to the gravitational field, rests upon an entirely new formal motif, as though nature might just as well have endowed the gravitational ether with fields of quite another type, for example, with fields of a scalar potential, instead of fields of the electromagnetic type. since according to our present conceptions the elementary particles of matter are also, in their essence, nothing else than condensations of the electromagnetic field, our present view of the universe presents two realities which are completely separated from each other conceptually, although connected causally, namely, gravitational ether and electromagnetic field, or - as they might also be called - space and matter. of course it would be a great advance if we could succeed in comprehending the gravitational field and the electromagnetic field together as one unified conformation. then for the first time the epoch of theoretical physics founded by faraday and maxwell would reach a satisfactory conclusion. the contrast between ether and matter would fade away, and, through the general theory of relativity, the whole of physics would become a complete system of thought, like geometry, kinematics, and the theory of gravitation. an exceedingly ingenious attempt in this direction has been made by the mathematician h weyl ; but i do not believe that his theory will hold its ground in relation to reality. further, in contemplating the immediate future of theoretical physics we ought not unconditionally to reject the possibility that the facts comprised in the quantum theory may set bounds to the field theory beyond which it cannot pass. recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities ; in this sense, therefore, there exists an ether. according to the general theory of relativity space without ether is unthinkable ; for in such space there


1. deeply moved. 5. an elaborate song for solo voice. 9. type genus of the amiidae. 13. ( judaism ) sacred chest where the ancient hebrews kept the two tablets containing the ten commandments. 16. a river that rises in northeastern turkey ( near the source of the euphrates ) and flows generally eastward through armenia to the caspian sea. 17. the central meaning or theme of a speech or literary work. 18. city in northern austria on the danube. 19. common black - and - gray eurasian bird noted for thievery. 20. ( irish ) mother of the tuatha de danann. 21. great coolness and composure under strain. 23. capital and largest city of cuba. 25. pertaining to or containing any of a group of organic compounds of nitrogen derived from ammonia. 27. a violent hot sand - laden wind on the deserts of arabia and north africa. 29. an island of central hawaii. 30. a summary that repeats the substance of a longer discussion. 31. before noon. 32. deciduous shrub widely cultivated for its white or pink or red flowers. 34. a state in new england. 36. the longer of the two telegraphic signals used in morse code. 39. the territory of athens in ancient greece. 45. any of various strong liquors distilled from the fermented sap of toddy palms or from fermented molasses. 47. being one more than six. 48. a river that rises in western new mexico and flows westward through southern arizona to become a tributary of the colorado river. 49. qualities that attract by seeming to promise some kind of reward. 50. permeate or impregnate. 53. ( archaic ) of persons. 57. true firs. 59. an area in which something acts or operates or has power or control. 60. small genus of eurasian aquatic perennial herbs. 61. capital and largest city of italy. 62. a master ' s degree in fine arts. 64. a highly unstable radioactive element ( the heaviest of the halogen series ). 65. the branch of engineering science that studies the uses of electricity and the equipment for power generation and distribution and the control of machines and communication. 67. type genus of the annonaceae. 69. a native or inhabitant of iran. 71. a silvery ductile metallic element found primarily in bauxite. 72. ( old testament ) cain and abel were the first children of adam and eve born after the fall of man


mechanical and aerospace engineering and computing and information science at cornell university, “ this result brings us closer to the idea of programmable matter — where computer programs and materials merge to form a new kind of matter whose shape and function can be programmed — not unlike biology. “ many people are excited today to learn about 3 - d printing and its ability to fabricate any shape ; gershenfeld ’ s group is already thinking about the next episode, where we don ’ t just control the shape of objects, but also their behavior. ” the milli - motein is part of a family of such devices being explored at size scales ranging from protein - based “ nanoassemblers ” to a version where the chain is as big as a person, gershenfeld says. ultimately, a reconfigurable robot should be “ small, cheap, durable and strong, ” knaian says, adding that right now, “ it ’ s not possible to get all of those. ” still, he points out, “ biology is the existence proof that it is possible. ” the mit researchers ’ work could lead to robotic systems that can be dynamically reconfigured to do many different jobs rather than repeating a fixed function, and that can be produced much more cheaply than conventional robotics. the development of the milli - motein was supported by the u. s. defense advanced research projects agency ’ s maximum mobility and manipulation and programmable matter projects. - ara n. knaian et al., the milli - motein : a self - folding chain of programmable matter with a one centimeter module pitch, 2012, http : / / cba. mit. edu / docs / papers / 12. 10. iros. pdf - ara nerses knaian, electropermanent magnetic connectors and actuators : devices and their application in programmable matter, 2012, http : / / cba. mit. edu / docs / theses / 10. 06. knaian. pdf


joined : 16 mar 2004 | posted : mon nov 27, 2006 11 : 42 am post subject : high - definition tv from your cell phone | new mems technology could lead to dime - sized, high - resolution projectors. a silicon mirror suspended by carbon fibers can vibrate very quickly, scanning a laser across a surface fast enough to draw high - resolution images. a cell phone that can project a high - definition television image could soon be possible, say researchers at cornell university who have developed a new microelectromechanical system ( mems ) for rapidly scanning wide areas with a laser. a projector based on the device would be about the size of dime and could cast a meter - wide image on a surface only half a meter away. the key is a small mirror, about half a millimeter across, suspended by carbon fibers - - rolled - up sheets of crystalline carbon commonly used to reinforce materials. the fibers amplify the vibrations of a piezoelectric motor, moving the mirror. this movement deflects a laser at different angles, causing it to sweep back and forth across a surface. while the current device only moves the laser side to side, the researches say it can be easily mounted on a stage that tilts up and down to allow the device to sequentially draw each line of an image, using complex electronics that turn the laser on and off as it is directed across the screen to create the light and dark pixels. a full - color display would mix light from red, green, and blue lasers. mems - based displays already exist in commercial products. texas instruments, based in dallas, tx, for example, has developed a chip that uses millions of tiny mirrors, each of which turns pixels on and off by either turning toward or away from a light source ( see " may the micro force be with you " ). this chip is now used in a variety of televisions and movie projectors. another company, microvision, in redmond, wa, uses a single mirror mems device more like the one being developed at cornell, but without the carbon fibers. the company is developing a full - color display. the cornell researchers say what sets their device apart is the high scanning speed of the mirror, combined with its ability to scan over a wide angle. the wide angle of the system is made possible, says michael thompson, a materials science and engineering professor and one of the researchers on the project, because the carbon fibers can bend sharply without breaking, giving the mirror a wide - range


each of the three terminals. here are three more potentiometers, more specialized than the set just shown : the large " helipot " unit is a laboratory potentiometer designed for quick and easy connection to a circuit. the unit in the lower - left corner of the photograph is the same type of potentiometer, just without a case or 10 - turn counting dial. both of these potentiometers are precision units, using multi - turn helical - track resistance strips and wiper mechanisms for making small adjustments. the unit on the lower - right is a panel - mount potentiometer, designed for rough service in industrial applications. let ' s take another look at our example series circuit, this time numbering the points in the circuit for voltage reference : if we were to connect a voltmeter between points 2 and 1, red test lead to point 2 and black test lead to point 1, the meter would register + 45 volts. typically the " + " sign is not shown, but rather implied, for positive readings in digital meter displays. however, for this lesson the polarity of the voltage reading is very important and so i will show positive numbers explicitly : when a voltage is specified with a double subscript ( the characters " 2 - 1 " in the notation " e2 - 1 " ), it means the voltage at the first point ( 2 ) as measured in reference to the second point ( 1 ). a voltage specified as " ecd " would mean the voltage as indicated by a digital meter with the red test lead on point " c " and the black test lead on point " d " : the voltage at " c " in reference to " d ". if we were to take that same voltmeter and measure the voltage drop across each resistor, stepping around the circuit in a clockwise direction with the red test lead of our meter on the point ahead and the black test lead on the point behind, we would obtain the following readings : we should already be familiar with the general principle for series circuits stating that individual voltage drops add up to the total applied voltage, but measuring voltage drops in this manner and paying attention to the polarity ( mathematical sign ) of the readings reveals another facet of this principle : that the voltages measured as such all add up to zero : this principle is known as kirchhoff ' s voltage law ( discovered in 1847 by gustav r. kirchhoff, a german physicist ), and it can be stated as such : " the algebraic sum of all


search our database of handpicked sites looking for a great physics site? we ' ve tracked down the very best and checked them for accuracy. just fill out the fields below and we ' ll do the rest. you searched for we found 16 results on physics. org and 241 results in our database of sites 241 are websites, 0 are videos, and 0 are experiments ) search results on physics. org search results from our links database play golf with a twist using protons in magnetic fields. start with simple fields, then move all the way up to golfing in earth ’ s magnetic field. magnetic fields are produced by electric currents, which can be macroscopic currents in wires, or microscopic currents associated with electrons in atomic orbits. the magnetic field b is defined in... shows magnetic field patterns for wire, single turn coil and solenoid. index from eric weisstein ' s world of physics leading to information on lots of topics related to magnetic fields. an introduction to magnetic fields with various diagrams and links through to related sections. a simple demonstration enabling us to visualize magnetic fields in 3d. an interactive practical demonstration of a magnetic force on a current carrying wire and the change cause when the magnetic field is introduced. efield allows users to simulate the motion of charged particles moving under the influence of electric fields. a static magnetic field in the z direction is also allowed. the secret lives of invisible magnetic fields are revealed as chaotic ever - changing geometries in this sci - art film. an article about the huge magnetic field that surrounds the earth and stretches into space. showing 1 - 10 of 241


what is data communications? the distance over which data moves within a computer may vary from a few thousandths of an inch, as is the case within a single ic chip, to as much as several feet along the backplane of the main circuit board. over such small distances, digital data may be transmitted as direct, two - level electrical signals over simple copper conductors. except for the fastest computers, circuit designers are not very concerned about the shape of the conductor or the analog characteristics of signal transmission. frequently, however, data must be sent beyond the local circuitry that constitutes a computer. in many cases, the distances involved may be enormous. unfortunately, as the distance between the source of a message and its destination increases, accurate transmission becomes increasingly difficult. this results from the electrical distortion of signals traveling through long conductors, and from noise added to the signal as it propagates through a transmission medium. although some precautions must be taken for data exchange within a computer, the biggest problems occur when data is transferred to devices outside the computer ' s circuitry. in this case, distortion and noise can become so severe that information is lost. data communications concerns the transmission of digital messages to devices external to the message source. " external " devices are generally thought of as being independently powered circuitry that exists beyond the chassis of a computer or other digital message source. as a rule, the maximum permissible transmission rate of a message is directly proportional to signal power, and inversely proportional to channel noise. it is the aim of any communications system to provide the highest possible transmission rate at the lowest possible power and with the least possible explain communications channels? a communications channel is a pathway over which information can be conveyed. it may be defined by a physical wire that connects communicating devices, or by a radio, laser, or other radiated energy source that has no obvious physical presence. information sent through a communications channel has a source from which the information originates, and a destination to which the information is delivered. although information originates from a single source, there may be more than one destination, depending upon how many receive stations are linked to the channel and how much energy the transmitted in a digital communications channel, the information is represented by individual data bits, which may be encapsulated into multibit message units. a byte, which consists of eight bits, is an example of a message unit that may be conveyed through a digital communications channel. a collection of bytes may itself be grouped into a frame or other higher - level message unit. such multiple levels of encapsul


, which consists of eight bits, is an example of a message unit that may be conveyed through a digital communications channel. a collection of bytes may itself be grouped into a frame or other higher - level message unit. such multiple levels of encapsulation facilitate the handling of messages in a complex data communications network. explain asynchronous vs. synchronous transmission? serialized data is not generally sent at a uniform rate through a channel. instead, there is usually a burst of regularly spaced binary data bits followed by a pause, after which the data flow resumes. packets of binary data are sent in this manner, possibly with variable - length pauses between packets, until the message has been fully transmitted. in order for the receiving end to know the proper moment to read individual binary bits from the channel, it must know exactly when a packet begins and how much time elapses between bits. when this timing information is known, the receiver is said to be synchronized with the transmitter, and accurate data transfer becomes possible. failure to remain synchronized throughout a transmission will cause data to be corrupted or lost. two basic techniques are employed to ensure correct synchronization. in synchronous systems, separate channels are used to transmit data and timing information. the timing channel transmits clock pulses to the receiver. upon receipt of a clock pulse, the receiver reads the data channel and latches the bit value found on the channel at that moment. the data channel is not read again until the next clock pulse arrives. because the transmitter originates both the data and the timing pulses, the receiver will read the data channel only when told to do so by the transmitter ( via the clock pulse ), and synchronization is guaranteed. explain parity and checksums? noise and momentary electrical disturbances may cause data to be changed as it passes through a communications channel. if the receiver fails to detect this, the received message will be incorrect, resulting in possibly serious consequences. as a first line of defense against data errors, they must be detected. if an error can be flagged, it might be possible to request that the faulty packet be resent, or to at least prevent the flawed data from being taken as correct. if sufficient redundant information is sent, one - or two - bit errors may be corrected by hardware within the receiver before the corrupted data ever reaches its destination. a parity bit is added to a data packet for the purpose of error detection. in the even - parity convention, the value of the parity bit is chosen


be corrected by hardware within the receiver before the corrupted data ever reaches its destination. a parity bit is added to a data packet for the purpose of error detection. in the even - parity convention, the value of the parity bit is chosen so that the total number of ' 1 ' digits in the combined data plus parity packet is an even number. upon receipt of the packet, the parity needed for the data is recomputed by local hardware and compared to the parity bit received with the data. if any bit has changed state, the parity will not match, and an error will have been detected. in fact, if an odd number of bits ( not just one ) have been altered, the parity will not match. if an even number of bits have been reversed, the parity will match even though an error has occurred. however, a statistical analysis of data communication errors has shown that a single - bit error is much more probable than a multibit error in the presence of random noise. what is data compression? if a typical message were statistically analyzed, it would be found that certain characters are used much more frequently than others. by analyzing a message before it is transmitted, short binary codes may be assigned to frequently used characters and longer codes to rarely used characters. in doing so, it is possible to reduce the total number of characters sent without altering the information in the message. appropriate decoding at the receiver will restore the message to its original form. this procedure, known as data compression, may result in a 50 percent or greater savings in the amount of data transmitted. even though time is necessary to analyze the message before it is transmitted, the savings may be great enough so that the total time for compression, transmission, and decompression will still be lower than it would be when sending an uncompressed message. some kinds of data will compress much more than others. data that represents images, for example, will usually compress significantly, perhaps by as much as 80 percent over its original size. data representing a computer program, on the other hand, may be reduced only by 15 or 20 percent. a compression method called huffman coding is frequently used in data communications, and particularly in fax transmission. clearly, most of the image data for a typical business letter represents white paper, and only about 5 percent of the surface represents black ink. it is possible to send a single code that, for example, represents a consecutive string of 1000 white pixels rather than a separate code for each


statistical conclusion. all statistical conclusions involve constructing two mutually exclusive hypotheses, termed the null ( labeled h0 ) and alternative ( labeled h1 ) hypothesis. together, the hypotheses describe all possible outcomes with respect to the inference. the central decision involves determining which hypothesis to accept and which to reject. for instance, in the typical case, the null hypothesis might be : h0 : program effect = 0 while the alternative might be h1 : program effect < > 0 the null hypothesis is so termed because it usually refers to the " no difference " or " no effect " case. usually in social research we expect that our treatments and programs will make a difference. so, typically, our theory is described in the alternative hypothesis. figure 1 below is a complex figure that you should take some time studying. first, look at the header row ( the shaded area ). this row depicts reality - - whether there really is a program effect, difference, or gain. of course, the problem is that you never know for sure what is really happening ( unless youre god ). nevertheless, because we have set up mutually exclusive hypotheses, one must be right and one must be wrong. therefore, consider this the view from gods position, knowing which hypothesis is correct. the first column of the 2x2 table shows the case where our program does not have an effect ; the second column shows where it does have an effect or make a difference. the left header column describes the world we mortals live in. regardless of whats true, we have to make decisions about which of our hypotheses is correct. this header column describes the two decisions we can reach - - that our program had no effect ( the first row of the 2x2 table ) or that it did have an effect ( the second row ). now, lets examine the cells of the 2x2 table. each cell shows the greek symbol for that cell. notice that the columns sum to 1 ( i. e., a + ( 1 - a ) = 1 and b + ( 1 - b ) = 1 ). why can we sum down the columns, but not across the rows? because if one column is true, the other is irrelevant - - if the program has a real effect ( the right column ) it cant at the same time not have one. therefore, the odds or probabilities have to sum to 1 for each column because the two rows in each column describe the only possible decisions ( accept or reject the


the tendency to notice instances of a particular phenomenon once one starts to look for it, and to therefore believe erroneously that the phenomenon occurs frequently. the word issues bothers a fair number of people, including reader david devore, who recently sent me a link to a language complaint in the times of london — along with the warning, " watch out for escaping steam. " and indeed, the times letter writer was at the boiling point. " in the media, in the pub, at the bus stop, " fulminated g. b., " no one ever refers to their ' problems ' ; they only have ' issues. ' " mr. b. is a victim of the frequency illusion, to use the term coined by linguist arnold zwicky. he ' s listening for issues, so he hears the word often, and imagines that it ' s everywhere. — jan freeman, " the issue with issues, " the boston globe, june 28, 2009 crystal does an excellent job exposing these illusions in txtng, even if he doesn ' t designate them as such. and people seem to be listening. on his blog, crystal notes that british media coverage has fairly addressed the book ' s six main points. the first three map precisely to the zwickyan trifecta of illusions : text messages aren ' t full of abbreviations — typically less than ten percent of the words use them. [ frequency illusion ] these abbreviations aren ' t a new language — they ' ve been around for decades. [ recency illusion ] they aren ' t just used by kids — adults of all ages and institutions are the leading texters these days. [ adolescent illusion ] another selective attention effect... is the frequency illusion : once you ' ve noticed a phenomenon, you think it happens a whole lot, even " all the time ". your estimates of frequency are likely to be skewed by your noticing nearly every occurrence that comes past you. people who are reflective about language — professional linguists, people who set themselves up as authorities on language, and ordinary people who are simply interested in language — are especially prone to the frequency illusion. — arnold zwicky, " just between dr. language and i, " language log, august 7, 2005