the object gives out. during the radiation era, shortly after the big bang, the universe consisted of free moving protons, neutrons and electrons and other particles, including helium ions. all radiation was absorbed by these free electrons, making the universe opaque. when the universe was sufficiently expanded the radiation could no longer interact with the electrons, causing the universe became transparent. this process is called decoupling, and it marked the beginning of the matter era. electrons, now no longer absorbing radiation, instead joined with ions to form neutral atoms. through gravity, these atoms clumped together, eventually forming stars, galaxies and other stellar bodies. these are zero - dimensional ( point - like ) objects which form when a spherical symmetry is broken. monopoles are predicted to be supermassive and carry magnetic charge. the existence of monopoles is an inevitable prediction of grand unified theories ( guts ) ; this is one of the puzzles of the standard cosmology. we have five consistent string theories that can describe both the forces and the matter in our universe. we do not, however, have the tools to explore the theories overall possible values of the parameters in the theories. over the past few years, however, we have been able to explore these theories more thoroughly, and we now believe that these five string theories are all different aspects of the same underlying theory : m - theory. m - theory goes beyond string theory, in that it predicts not ten, but eleven dimensions of spacetime. the theory could have as a fundamental object as a membrane, as opposed to a string, which would look like strings when curled up in the eleventh dimension. it is for this reason that the m in m - theory originally referred to a membrane. nowadays, however, the m doesn ’ t specifically refer to anything, and can stand for mystery, or “ mother of all ”, because m - theory is still largely unknown. vast clouds of interstellar dusk, hydrogen, helium and ionized gas. as the mass of a nebula grows due to the slight gravitational attracts of dust towards each other, the mass compacts enough to form stars. other material within the nebula, such as dust, can clump together to form planets and other planetary objects. originally, any large astronomical object was referred to as a nebula - other galaxies, in particular. a liquid crystal is a phase of matter which exhibits properties somewhere between those exhibited by a liquid and solid crystal. when viewed in high resolution, they can appear to be textured, as the molecules may be free


nebula - other galaxies, in particular. a liquid crystal is a phase of matter which exhibits properties somewhere between those exhibited by a liquid and solid crystal. when viewed in high resolution, they can appear to be textured, as the molecules may be free to flow in a limited manner around, provided that they stay within a crystal like structure. liquid crystals are used extensively in televisions and computer screens. the nematic phase of a liquid crystal is temperature dependent. when in this phase, clamitic ( rod - like ) molecules align themselves individually roughly parallel to each other on their long - side axis, in a similar way to cigarettes in a package. the result of this is that the molecules are free - flowing within this directional order. in this phase, the crystals can show signs of intercommuting and loop production, which are properties expected to be exhibited by cosmic strings. a neutron star is formed from the collapse of a larger star which has undergone supernova. these stars, as the name suggests, are composed mostly of neutrons. neutron stars are extremely hot. they typically have masses between about 1 and 2 solar masses ( 1 solar mass is approximately 2x1030kg, which is about 333, 000 times the mass of the earth ), despite being somewhere on the order of 1015 smaller in radius than the sun, which makes them extremely dense. the more compact a neutron star is, the more likely it is to form a black hole. this occurs when the star ' s density become so great that the gravitational force it exerts on itself is greater than its internal pressure, causing a collapse into a black hole. this was developed in 1983 by stephen hawking and james hartle. describes a situation whereby the universe can spontaneously come into existence from literally nothing. once the universe exists, quantum cosmology can be approximated by general relativity so that time appears. a particle is a nucleon if it is a particle that forms an atomic nucleus. there are two nucleons : protons and neutrons. these are complicated manifolds which, like calabi - yau manifolds, may be the space in which six extra dimensions proposed by certain string theories are found within. the study of the universe up to around 10 - 11 seconds after the big bang. during this time, the electroweak and strong forces were unified in a grand unified phase, which quickly changed to separate out strong and the electroweak forces. further on in time the electroweak interaction separated to become electromagnetism


. during this time, the electroweak and strong forces were unified in a grand unified phase, which quickly changed to separate out strong and the electroweak forces. further on in time the electroweak interaction separated to become electromagnetism and the weak nuclear force. it is possible to reach temperature regimes within this cosmology, allowing us to experimentally test theories. speculation, however, is still required within this time period. a mathematical approach to non - gravitational quantum theory, introduced by richard feynman of caltech. in the path integral approach, the probability that a system in an initial state a will evolve to a final state b is given by adding up a contribution from every possible history of the system that starts in a and ends in b. for this reason a path integral is often referred to as a ' sum over histories '. for large systems, contributions from similar histories cancel each other in the sum and only one history is important. this history is the history that classical physics would predict. for example, a system in the starting position of a ball on a non - symmetrical hill. the probability that the system will end up in the final position of the ball at the bottom of the hill on the side that is steepest is given by the summation of the probabilities of all paths that that ball could take, including going down the other side of the hill. for mathematical reasons, path integrals are formulated in a background with four spatial dimensions rather than three spatial dimensions and one time dimension. there is a procedure known as ' analytic continuation ' which is used to convert results expressed in terms of four spatial dimensions into results expressed in terms of three spatial dimensions and one time dimension. this effectively converts one of the spatial dimensions into the time dimension. this spatial dimension is sometimes referred to as ' imaginary ' time because it involves the use of so - called imaginary numbers. the path integral formulation of quantum gravity has many mathematical problems. it is also not clear how it relates to more modern attempts at constructing a theory of quantum gravity such as string / m - theory. however it can be used to correctly calculate quantities that can be calculated independently in other ways e. g. black hole temperatures and entropies. a phase transition is the change in properties and form of matter due to temperature changes. for example, water changes from solid ice to liquid water to gaseous steam or vapour. as temperature drops and phase transitions occur, the symmetry of the resulting matter is reduced - again, vapour is more symmetric than water


due to temperature changes. for example, water changes from solid ice to liquid water to gaseous steam or vapour. as temperature drops and phase transitions occur, the symmetry of the resulting matter is reduced - again, vapour is more symmetric than water, which is more symmetric than ice. in terms of cosmology, when a phase transition in the early universe occurs, topological defects are formed. some of the symmetries that were broken in the early universe led to the four fundamental forces becoming discrete forces. at higher temperatures, they reunite in a unified state. the photon is an elementary particle. it is a gauge boson, in that it mediates one of the fundamental forces. in the case of the photon, it is the electromagnetic force. as mediators of the electromagnetic force, they allow us to see things through the visible light part of the electromagnetic spectrum, and are therefore often interchanged with " light ". as they have no rest mass, they are able to travel at the fastest possible speed, which is know as the " speed of light " ( 299, 792, 458 metres per second ) in a perfect vacuum. their spin is 1 and no electrical charge. this is simply the planck length squared. given that the planck length is a fundamental unit of length, so too is the planck area a fundamental unit of area. this is the size of energy quanta ( discrete packets of energy ) in quantum mechanics - it is therefore the smallest amount of energy that anything can hold. it is the proportionality constant between the energy of a photon and the frequency of the associated electromagnetic wave, as denoted in the planck - einstein equation which links the two : e = hv, where v is frequency, h is planck ' s constant and e is energy. it ' s value is 6. 6260695729×10−34 j. s this is the earliest period of time, from the beginning of time to 10 - 43 seconds after the beginning of time. during this period, the fundamental forces of nature were all unified due to the unimaginable temperature of the universe, and it is believed that gravity was as strong as the other forces ( it is now by far the weakest of the forces ). a very, very small unit of length. its precise value is 1. 61619997x10 - 35m. it is a base unit within the planck unit system and it is calculated using the speed of light, c, planck ' s constant, h and the gravitational


. its precise value is 1. 61619997x10 - 35m. it is a base unit within the planck unit system and it is calculated using the speed of light, c, planck ' s constant, h and the gravitational constant, g. specifically, it is given by the square root of ħg / c3 where ħ is the reduced planck ' s constant, or planck ' s constant divided by 2π. it is the shortest measureable length in existence. to discuss length on scale shorter than this would be meaningless because it is a physical impossibility to measure below this length. a theory that could describe physical laws at this level would be of great use in the search for a theory of everything. this is the energy that exists in a body due to its position within a system. forces act upon the body to restore it to a lower energy state or configuration, this difference in the energy states is the potential energy. when the force acts upon the body, the energy held within the body is converted into some other form of energy, this occurs because the conservation of energy law states that energy cannot be created or destroyed. an example of potential energy being converted into other energy would be in someone skydiving. the position of the person ( the body in the system ) in the system ( the earth ), i. e. being high up in the air in a plane, gives the person gravitational potential energy. once they leap from the plane, this gravitational potential is turned into kinetic energy as the person falls toward earth. once they have landed, their position, at the surface of the earth, means that they have lower amounts of gravitational potential energy, and they have been restored to a lower energy state. this is the theory that explains the strong nuclear force that is mediated by gluons between different quarks. the charge of this force is known as colour. the force, which occurs due to an exchange of these gluons, does not weaken over distance, as say gravity does, but rather remains constant, on the order of several thousand newtons. this means that at no point does any quark separate from another one, and so quarks can only be observed on a hadron level. this property is called confinement. another property within qcd is asymptotic freedom. this results in a very weak interaction between quarks and gluons during extremely high energy reactions. this is the study of cosmology at temperature regimes where all four fundamental forces were unified. this


qcd is asymptotic freedom. this results in a very weak interaction between quarks and gluons during extremely high energy reactions. this is the study of cosmology at temperature regimes where all four fundamental forces were unified. this unification, it is theorised, occurred from the big bang to some 10 - 43 seconds after the big bang. due to the temperatures involved all quantum cosmology is theoretical and highly speculative. quantum field theory is a framework that allows for the extension of quantum mechanics, which deals with individual particles, to field systems operating relativistically. quantum field theories have been used to describe how three of the four fundamental forces act, being mediated by and exchange of particles called bosons. the photon and the gluon, for example, are exchanged between electrons and quarks in the case of electromagnetism and the strong nuclear force respectively. with quantum field theory, these natural fields pervade an area of space. particles that mediate these fields, the gauge bosons associated with the field ( like the aforementioned photon with electromagnetism ), are quanta of these fields, that is, ripples in the field carrying small amounts of energy, other particles that act within the field, for example the electron within the electromagnetic field, are though of in a similar manner., albeit different ripples and excitations. these fields are of variable range. the colour field within the quantum chromodynamic field theory, for example, acts in a range between quarks within a nucleon. other fields, such as the electromagnetic field, are infinite in scope and range. the search for a theory of quantum gravity is the search for a theory that can explain the effects of the fundamental force of gravity as explained by general relativity at a quantum level, and marry these up with quantum mechanics, which is a series of models which explain the other fundamental forces ; the strong nuclear ; weak nuclear and electromagnetic forces. examples of quantum gravity include string theory, loop quantum gravity and m - theory. this phase transmission occurred approximately one millionth of a second after the big bang. this was when quark - gluon plasma underwent a phase transition, resulting in quarks forming into hadronic matter, i. e. nucleons. quintessence is a theory of dark energy, given to explain the acceleration of the universe ’ s expansion. it is a dynamic equation, resulting in an attractive or repulsive force depending on the amount of kinetic energy


e. nucleons. quintessence is a theory of dark energy, given to explain the acceleration of the universe ’ s expansion. it is a dynamic equation, resulting in an attractive or repulsive force depending on the amount of kinetic energy relative to potential energy in the universe. as a repulsive force, it overcomes gravity ’ s attraction over large scales, resulting in an accelerated expansion. quintessence is hypothesised to have become repulsive approximately 10 billion years ago. this refers to a period of time from just after big bang to approximately 300, 000 years after its beginning. during this time, the universe consisted of free moving protons, neutrons and electrons and other particles. all radiation was absorbed by these free electrons, making the universe opaque. protons and neutrons were combining to form deuterium, a heavy isotope of hydrogen, and then helium, however, the temperature of the universe was so high that these existed as free ions in the plasma that was the universe. it was only when the universe was sufficiently expanded that the electrons no longer absorbed the radiation and instead joined with the ions to form neutral atoms. this forms the beginning of the matter era, in which we still exist. recombination was a time period, approximately 300, 000 years after the big bang, when electrons and protons bound to form atoms of hydrogen. before 300, 000 years had passed, the universe was still too hot for atoms for hydrogen to form. only after the universe had expanded sufficiently did the universe cool down sufficiently, making the formation of hydrogen possible. when the source of a wave moves away from us, we observe a change of frequency of that wave. an example would be an ambulance or fire - truck - we hear a lower pitch in its siren once it has passed us by. this is the doppler effect. it is not, however, limited to sound waves, but any kind of waves, including electromagnetic. this means that as an electromagnetic wave source is moving away from us, the frequency of the wave will decrease. as frequency and wavelength are inversely related, one goes up and the other goes down, the wavelength will increase. this shifts the wavelength closer to the red end of the spectrum ( this, when talking about the visible part of the electromagnetic spectrum, of course the wavelength may not be in the visible part ). this is redshift, and it is something we detect from far away galaxies and other electromagnetic sources. this leads us to the conclusion that the universe is expanding. these


electromagnetic spectrum, of course the wavelength may not be in the visible part ). this is redshift, and it is something we detect from far away galaxies and other electromagnetic sources. this leads us to the conclusion that the universe is expanding. these associate a scalar ( either a number, or a physical quantity ) value to every point in a space within the field. examples of scalar fields include pressure distribution, temperature variation, and gravitational fields. this is a point in spacetime where the curvature of spacetime becomes infinite. it is an area of extremely high density into which matter or light is attracted. singularities can be found both at the centre of black holes and on their own. inside a singularity, the laws of physics are distorted to the point that they are no longer applicable. spacetime is the concept of space and time being part of the same continuum. we use the typical three dimensions that are everyday and commonplace - the x, y and z dimensions used in geometry - ascribing a fourth dimension of time. this allows us to map out any event that takes place in the universe by a set of coordinates ; three of space to give us the location, and one of time to give us when the event occurred. this merging of time and space is important and must be accounted for, because relativity tells us that the observed rate of time passing changes with the respect to an objects velocity relative to the observer. gravitational fields can also change the passage of time. on quantum scales, therefore, it is important to account for time within theoretical frameworks, whereas in classical physics this is unnecessary. the structure of spacetime is detailed in einstein ' s theory of special relativity. this theory lays out the structure of spacetime. it draws on the principle of relativity as laid out by galileo, which states that there is no absolute state of rest, and that all motion is relative to other motion. there are two principles that are laid out in the theory ; that the laws of physics are the same for observers whose motion is uniform relative to each other and that the speed of light in a vacuum is the same for all observers, regardless of any relative motion. this means that with different relative velocities, observers will experience different physical laws. effects of these principles can be seen in various manners. one of the most interesting is time dilation. a clock that is sitting stationary in front of you will tick faster than a clock which is moving away from you. this is has been shown to be true for astronauts, who come


in various manners. one of the most interesting is time dilation. a clock that is sitting stationary in front of you will tick faster than a clock which is moving away from you. this is has been shown to be true for astronauts, who come back from space younger than they would have been had they remained on earth. another well known consequence of the theory is the energy - mass equivalence relationship, as defined by the equation e = mc2, probably the most famous equation of all time. this states that energy and mass are interchangeable and are related by a function of the speed of light in a vacuum, c. the speed of light in a vacuum, c, is shown not to be just a speed that photons travel at, it is a key cosmological constant that is related to the nature of space and time. special relativity shows us that any object with rest mass cannot travel at the speed of light. the speed that photons, or indeed any particle with zero rest mass ( as energy and mass are equivalent as shown by the equation e = mc2, a particle that is travelling will have kinetic energy and therefore more mass than a particle at rest ), will travel at in a vacuum. its value is 299, 792, 458 metres per second ( ms - 1 ). as explained in the theory of special relativity, the speed of light is the fastest that any form of energy or information can travel in the universe. an intrinsic quantum property of particles that is defined by a spin number that can be either a whole integer ( 1, 2, 3 etc. ) or a half integer ( 1 / 2, 3 / 2, 5 / 2 etc. ), and can be positive or negative. it is a property that all particles exhibit, the sole known exception being the higgs boson, although other particles with zero spin, such as the inflaton, have been hypothesised. to an extent, it is easy to make an analogy of quantum spin with the classic rotational spin that we encounter in everyday life, for example with a spinning top. particles that are electrically charged, such as electrons or positrons, will generate a magnetic field through their spin, as movement of an electric charge will automatically generate magnetic fields. this analogy, however, only takes us so far. different spin quantum numbers can give us ideas as to the symmetry of these particles. a particle with zero spin looks exactly the same from all sides. a particle with spin will look different if rotated, but will regain its symmetry if it


us so far. different spin quantum numbers can give us ideas as to the symmetry of these particles. a particle with zero spin looks exactly the same from all sides. a particle with spin will look different if rotated, but will regain its symmetry if it is rotated a certain number of time. in this instance, an analogy of a deck of cards is useful. consider any face card, these are symmetrical every time you spin them half way around, or 180 degrees. consider now the ace of spaces. this card, if places with the point of the space facing up as you look at it, will require a full 360 degree rotation until it looks the same again. a particle with spin 1 will act like an ace of spaces, requiring a full rotation, whereas a spin 2 particle will be symmetrical through 180 degree rotations. a half spin particle will require two rotations to be symmetrical. this kind of rotational symmetry does not have an analogue in the macroscopic world. crucially, whether a particle has half or whole integer spin tells us about how it reacts. particles with half integer spin, or fermions, obey a set of statistics known as fermi - dirac statistics. particles with whole integer spin, or bosons, obey a set called bose - einstein statistics. one of the key differences between these two sets of statistics is that those particles which obey fermi - dirac statistics are subject to the pauli exclusion principle. this states that particles may not occupy the same quantum state as each other. crucially, this means that you cannot make fermions of the same quantum state occupy the same space. this is why fermions are the particles which make up the matter of the universe. they include quarks, which combine to make protons and neutrons, and leptons, a set of particles that include electrons. bosons, which do not obey fermi - dirac statistics and are consequently not subject to the pauli exclusion principle, fulfill other roles, some mediate the fundamental forces of nature, these are the gauge bosons, and the higgs boson gives rise to mass in other particles. also known as the λcdm or lambda - cdm model, this is the best and most widely used model to explain the expansion of the universe, origins of the cosmic microwave background, nucleosynthesis of light elements and the formation of galaxies and large - scale structure. this is a set of mathematical tools that allow us to study thermodynamical properties, such as work


, origins of the cosmic microwave background, nucleosynthesis of light elements and the formation of galaxies and large - scale structure. this is a set of mathematical tools that allow us to study thermodynamical properties, such as work, heat and entropy, of a large number of particles, allowing us to look at both atomic level and macroscopic level detail of the system. this allows us to explain thermodynamics in ways that apply to both classical and quantum physics, and allows us to extrapolate macroscopic predictions from microscopic properties. in the standard model of particle physics, particles are considered to be points moving through space, tracing out a line called the world line. to take into account the different interactions observed in nature, one has to provide particles with more degrees of freedom than only their position and velocity. these include mass, electric charge, colour ( which is the “ charge ” associated with the strong interaction ) and spin. this model was designed within a framework known as quantum field theory ( qft ), which allows us to build theories consistent with both quantum mechanics and the special theory of relativity. these theories describe with great success three of the four known interactions in nature : electromagnetism, the strong and weak nuclear forces. unfortunately, gravity, as described by einstein ’ s general relativity, does not fit into this scheme. string theory replaces these different particle types with a single fundamental building block : a “ string ”. these can be closed, like loops, or open, like a hair. as the string moves through time it traces out a tube or a sheet ( depending on whether it is closed or open ). this string is free to vibrate, and different vibrational modes of the string represent the different particle types, as difference modes are seen as different masses or spins. one mode of vibration, or ‘ note ’, makes the string appear as an electron, another as a photon. there is even a mode describing the graviton, the particle carrying the force of gravity. this means we can make sense of the interaction of gravitons in a way we could not in qft. it is this ability of string theory to create a valid model that includes all four fundamental interactions that has dubbed it to be a ‘ theory of everything ’. the problem is that there are five different versions of string theory. this is why we now look to m - theory, which has place for all five theories, as the greatest solution to our ‘ theory of everything ’. as a point of


’. the problem is that there are five different versions of string theory. this is why we now look to m - theory, which has place for all five theories, as the greatest solution to our ‘ theory of everything ’. as a point of note, string theory predicts that spacetime has ten dimensions. although we only have three dimensions of space and one of time, we can assume that six of these dimensions are curled up very tightly, so that we may never be aware of their existence. having these so - called compact dimensions is very beneficial, as we can suggest that the degrees of freedom, such as electric charge of an electron, can simply arise as motion in the extra compact dimensions. there are four fundamental forces in nature. they are electromagnetism, the weak nuclear force, the strong nuclear force and gravitation. the weak nuclear force is associated with radioactivity in unstable nuclei, specifically the decay of a neutron into a proton. when the temperature is hot enough, such as that of the universe shortly after the big bang, electromagnetism and the weak nuclear force will merge to form the electroweak force. the strong nuclear force binds together neutrons and protons inside nuclei. the mathematical theory describing the elementary particles in this theory, quarks and gluons, is known as quantum chromodynamics ( qcd ). theories that unify the strong nuclear force with electroweak theory are known as grand unified theories, of guts. a supercluster is a vast ( the are some of the largest structures in the universe ) grouping of smaller galaxy clusters and groups. they can span between several hundred million light years to over one billion light years. superclusters can contain galaxy bubbles, sheets, voids and filaments, which are smaller structures within the supercluster. nearly all galaxies are found within superclusters, and inbetween superclusters thee are usually large voids. our own supercluster, called the virgo supercluster, contains the local group, the virgo cluster and some 100 other galactic groups and clusters. its diameter is approximately 100 million light years. supergravity is a theory which follows on from supersymmetry. it is theorised that in the same way that photons mediate the electromagnetism, gluons the strong nuclear force and w and z bosons the weak nuclear force, so to does the as - yet undiscovered gravi


theorised that in the same way that photons mediate the electromagnetism, gluons the strong nuclear force and w and z bosons the weak nuclear force, so to does the as - yet undiscovered graviton the gravitational force. in supergravity, the graviton has a heavier superpartner whose spin differs by 1 / 2. so far, as with supersymmetry, there has been no observational evidence for supergravity. this is a very powerful stellar explosion that can quite often outshine galaxies. a star undergoes a supernova either when a very old massive star undergoes sudden gravitational collapse, releasing vast quantities of gravitational energy, or through the reignition of the nuclear fusion reaction in a degenerate star ' s ( such as a white dwarf or neutron star ) core. the explosion releases huge quantities of the star ' s matter, resulting in a supernova remnant. certain types of supernova have luminosities of known quantity, such that they can be used as ' standard candles ', which means that we can detect how far away the object is by comparing its known luminosity with our observed brightness. string theory states that all particles are representations of different vibrations on a fundamental building block ; a string. as a theory, it is able to describe the interactions of the particle that mediates gravitation : the graviton. in this way, and by being able to describe all other particles and interactions thereof, it is able to unite the four fundamental forces in nature, and is therefore a ‘ theory of everything ’. the original string theory only described particles with integer spins, called bosons. these are the particles that mediate the fundamental forces, and include the photon, electron, gluon and graviton. the other class of particle, which have half integer spin, called fermions, were not described. these are particles that constitute matter as we know it, such as quarks and electrons. by introducing supersymmetry to bosonic string theory, we obtain a new theory that describes both the forces and the matter that make up the universe. this is superstring theory. there are three different superstring theories that have no mathematical inconsistencies. in two of them, the fundamental object is a closed string, whilst in the third, the string is open. by mixing the best aspects of bosonic string theory and superstring theory, we can create two other consistent theories of strings, heterotic


of them, the fundamental object is a closed string, whilst in the third, the string is open. by mixing the best aspects of bosonic string theory and superstring theory, we can create two other consistent theories of strings, heterotic string theories supersymmetry is a theory which postulates that for every elementary particle, there is a more massive " superpartner " whose spin is different by 1 / 2. the theory comes about to solve mathematical difficulties related to quantum field theory and the reconciling of general relativity and quantum field theory. these inconsistencies arise because the higgs boson, a gauge boson whose interaction with other particles gives them mass, appears to gain large amounts of mass through interactions with itself. solving these inconsistencies would give physicists a way to marry quantum mechanics and gravity at the smallest scales. these superpartners are a possible candidate for dark matter. no superpartners have yet to be detected and no evidence exists as of yet to support supersymmetry. this is because in order to observe particles of this mass we need to use incredible amounts of energy, which so far we have been unable to generate. it is hoped that the large hadron collider at cern might detect evidence of supersymmetric particles. this is the set of points in space where decoupling occurred, approximately 380, 000 years after the big bang, at the right distance so that we are now seeing these photons reach us as part of the cosmic microwave background relic radiation. this occurs when a system in some state of symmetry moves into a different configuration, resulting in the loss of that symmetry. consider a ball on a hill. the ball is symmetrical. the hills is also symmetrical. if the ball is on top of the hill, the ball and hill in system are symmetrical. if the ball rolls down the hill, the ball and hill are individually still symmetrical, but the system of the ball and the hill is now asymmetrical. this is symmetry breaking. in a cosmological context, this happened as the universe cooled down after the big bang. as this occurred, elementary particles changed state in what is known as a phase transition. as this occurred, symmetry that previously was exhibited by these particles was broken. these symmetries are associated with different fundamental forces. this is why some particles are acted upon by these forces, and others not. these symmetries are restored at higher temperatures, however. these are a type of topological defect that


was broken. these symmetries are associated with different fundamental forces. this is why some particles are acted upon by these forces, and others not. these symmetries are restored at higher temperatures, however. these are a type of topological defect that is hypothesised to form when large symmetries are broken. they are unstable and prone to collapse. unlike certain other topological defects, such as magnetic monopoles, these are delocalized and occur over large areas. no evidence has been found of them as yet. this is a gravitational anomaly located in the centaurus supercluster. it is a localized concentration of mass of unknown origin that is equivalent to tens of thousands of galaxies. it mass is so large, that ( as the name suggests ) its gravitational attraction is altering the motion of galaxies and galaxy clusters in a region over hundreds of millions of light years across. in the aftermath of the big bang, the universe was extremely hot and extremely dense. at these energies, the laws of nature that we know were changed. the fundamental forces that we see in nature were unified - it is only as the universe expanded and cooled that gravitation, electromagnetism and the strong and weak nuclear forces all ceased to be as one. electroweak theory describes the unification of the weak nuclear force and electromagnetism. a theory of everything will marry up all the fundamental forces. the issue with this is that whilst quantum chronodynamics and the electroweak theory describe the strong and weak nuclear forces and electromagnetism on a well understood quantum basis, there is no consistent theory for describing gravity on such a basis. m - theory, and the associated string theories behind it are being explored as possible candidates. these are configurations of matter that form during matter phase transitions and symmetry breakings, such as occurred during the very early universe. they are configurations of matter in the old, symmetrical phase that remain stable in the new phase where the symmetry that was previously held is now broken. examples of these defects include monopoles, cosmic strings, domain walls and textures. within quantum field theory, particles may move from higher to lower energy states, such as occurred in the very early universe as the universe was expanding and thus cooling. these lower energy states, or vacuum states, may be different whilst possessing the same amount of energy. this means these states are degenerate. the particle, therefore, has a chance of falling into any of these degenerate vacuum states, unless there is something outside the system described here which


what is a retrofit? main entry : ret · ro · fit pronunciation : \ ’ re - tro - ’ fit, ‘ re - tro - ’ fit \ function : transitive verb 1 : to furnish ( as a computer, airplane, or building ) with new or modified parts or equipment not available or considered necessary at the time of manufacture 2 : to install ( new or modified parts or equipment ) in something previously manufactured or constructed 3 : to adapt to a new purpose or need 4 : to save a lot of money on energy costs! 5 : to update your current lighting system innovation and continuous improvement in the field of lighting have given rise to tremendous energy - saving opportunities. lighting is an area in which there is enormous energy - efficient potential, starting at the design stage by incorporating modern energy - efficient lamps and luminaries. following responsible operational practices also can significantly reduce associated energy costs. lighting is not only a very high priority when considering facility retrofitting, but also is a high - return, low - risk investment. by installing new lighting technologies such as dimmers, photo sensors, occupancy sensors, and timers, facilities can reduce the amount of electricity consumed and energy costs associated with lighting. there are several types of energy efficient lighting and affordable lighting technology : compact fluorescents lights, light - emitting diodes ( leds ), and lighting controls. below are a few examples of energy - saving opportunities with efficient lighting! • installation of energy - efficient fluorescent lamps in place of conventional fluorescent lamps for example converting to t8 or t5 lamps from t12 lamps. • installation of compact fluorescent lamps ( cfls ) in place of incandescent lamps. • installation of high pressure sodium ( hps ) lamps for applications where color rendering is not critical. metal halide lamps should also be considered when correct color is important. • installation of led exit signs to replace incandescents. • installation of high frequency ( hf ) electronic ballasts in place of conventional ballasts. • installation of occupancy sensors, an inexpensive way to ensure that unused lights do not remain on. • installation of microprocessor - based controllers. • installation of photocells, devices that automatically detect the natural light level in a room and adjust the intensity of the artificial light accordingly. • replacing incandescent wall lights and exit sign lighting with cfl or led - lit units will not only save a considerable amount of energy, it also will significantly reduce labor costs associated with changing light bulbs,


i stand at the edge of a dream, my breath a cloud. snow has transformed my familiar landscape and chilled my toes. this careless arm of the east verde where my grandchildren splashed away the summer has frozen over and snow has rendered abstract the shapes of rocks and junipers. water gleams in all its forms about me — still flowing in the stream, gathered in vapor on my breath, crystallized into snow on every hand, frozen into ice underfoot. i wish i knew the proper prayer — the light step of the ritual dance — the intonation of the chant – to offer at such a moment. instead, i kneel at the edge of the stream and study the ice, perhaps the most unlikely of water ’ s forms. here ’ s a nugget to suck on : chill any other liquid and the jittery molecules will slow down — bouncing about less as the temperature drops. eventually, the liquid will settle into a stable crystal lattice — which takes up less space than the liquid did. that ’ s why all other liquids most sensibly condense when they freeze. but not water, thank the lord. water ’ s made of one molecule of hydrogen linked to two molecules of oxygen. these amiable molecular companions actually share electrons to keep everyone happy. moreover, a water molecule has a slight positive electrical charge at one end and a faint negative electrical charge at the other end. this accounts for the nearly miraculous chemistry of water — on which life on the planet depends utterly. for starters, as water cools below 32 degrees f the molecules slip into a strange and counter - intuitive crystalline lattice. once they click into place, they actually take up about 9 percent more space than they did as a warm liquid. now, that didn ’ t work out so well for folks in rim country who left the water on in empty houses during the big freeze, since the expanding ice in the neglected pipes can split open even copper or steel. but water ’ s demented determination to expand when it ought to contract makes life on the planet possible. if water contracted as it froze, then sea ice would form at the surface every winter and sink to the bottom. over time, the oceans would freeze solid — and we could not be here. we could go on and on about the fortunate strangeness of water. for instance, the positive and negative ends of water molecules account for surface tension — so useful to water skiers, stone - skippers and water bugs. but it also explains what ’ s called “ capillary action, ”


may 4, 2012 researchers in spain have found that at least some of the individuals claiming to see the so - called aura of people actually have the neuropsychological phenomenon known as " synesthesia " ( specifically, " emotional synesthesia " ). this might be a scientific explanation of their alleged ability. in synesthetes, the brain regions responsible for the processing of each type of sensory stimuli are intensely interconnected. synesthetes can see or taste a sound, feel a taste, or associate people or letters with a particular color. the study was conducted by the university of granada department of experimental psychology oscar iborra, luis pastor and emilio gomez milan, and has been published in the journal consciousness and cognition. this is the first time that a scientific explanation has been provided for the esoteric phenomenon of the aura, a supposed energy field of luminous radiation surrounding a person as a halo, which is imperceptible to most human beings. in basic neurological terms, synesthesia is thought to be due to cross - wiring in the brain of some people ( synesthetes ) ; in other words, synesthetes present more synaptic connections than " normal " people. " these extra connections cause them to automatically establish associations between brain areas that are not normally interconnected, " professor gomez milan explains. new research suggests that many healers claiming to see the aura of people might have this condition. the case of the " santon de baza " one of the university of granada researchers remarked that " not all ' healers ' are synesthetes, but there is a higher prevalence of this phenomenon among them. the same occurs among painters and artists, for example. " to carry out this study, the researchers interviewed some synesthetes including a ' healer ' from granada, " esteban sanchez casas, " known as " el santon de baza ". many local people attribute " paranormal powers " to el santon, because of his supposed ability to see the aura of people " but, in fact, it is a clear case of synesthesia, " the researchers explained. according to the researchers, el santon has face - color synesthesia ( the brain region responsible for face recognition is associated with the color - processing region ) ; touch - mirror synesthesia ( when the synesthete observes a person who is being touched or is experiencing pain, s / he experiences the same ) ; high empathy ( the ability to feel what other person is feeling ), and schizoty


it security is, generally, defined as a defensive approach to protect a company and its assets from unauthorized access by an intruder. it security efforts include network security appliances, honeypots, robust authentication, limiting authorization to least necessary privileges, as well as other perimeter security defenses. however, these approaches do not provide definitive protection of the company ' s most valuable asset, its data, because a single intrusion could result in sensitive data being compromised. additionally, in today ' s workplace culture the disgruntled employee may be as much of a threat as any external threat. data encryption is a direct response to internal and external security threats that may also meet compliance regulations. encryption provides strong security for data " at - rest " ; in our case, the data stored in the database, but to be effective should be implemented as a part of a broader security plan. there are many issues involved with the implementation of encryption, details that require decisions and actions to ensure the success of the implementation and the security of the data. this document will discuss the issues associated with database encryption implemented using sql server ' s native transparent database encryption ( tde ) mechanism. encryption has been integral to human history beginning with the babylonian use of intaglio other historical examples include the caesar cipher, scytale transposition cipher, enigma, and even jimkryptos sculpture. throughout history our society has enjoyed the ability to protect information using cryptographic methods including steganography, microdots, invisible ink, digital watermarks, and encryption which may be defined as the conversion of data so as to keep its meaning private. as the amount of sensitive data collected by commercial entities continues to grow the regulatory requirements for protecting the sensitive data will become more robust ; meeting the regulatory requirements will necessarily require the continued use of data encryption methods. encryption requires the application of an algorithm to transform the target data into a form that is unusable to anyone that does not have access to the encryption process used. in practical terms encryption applies a cryptographic algorithm with a " key " to the target data producing the encrypted form of the data which cannot be accessed without the key used to encrypt the data. the two primary forms of key encryption are symmetric and asymmetric which are distinguished by the number of keys used in the encryption / decryption process. symmetric encryption uses a single key while asymmetric encryption uses a pair of keys generally referred to as public and private keys. while asymmetric encryption appears ideal for implementation because only the public key need ever be shared


encryption / decryption process. symmetric encryption uses a single key while asymmetric encryption uses a pair of keys generally referred to as public and private keys. while asymmetric encryption appears ideal for implementation because only the public key need ever be shared there are disadvantages with regard to performance. a sampling of asymmetric algorithms includes rsa, dsa, elgamal, ecdsa, and xtr. figure 1 demonstrates the asymmetric encryption process. figure 1 asymmetric key encryption / decryption process symmetric algorithms require a single key for both encryption and decryption which allows for high - performance ; however, with this approach the strength of the encryption is dependent on the security of the key. common symmetric algorithms include aes / rijndael, blowfish, des, triple des, serpent, and idea to name only a few. figure 2 demonstrates the symmetric encryption process. figure 2 symmetric key encryption process both symmetric and asymmetric encryption approaches are vulnerable to brute force attacks and cryptanalysis. brute force is an attack during which every possible permutation of the key value is attempted. cryptanalysis, on the other hand, applies computational techniques to circumvent the encryption. in general, the use of sufficiently long keys will mitigate these attacks. in summary, a symmetric key algorithm is fast but less secure than an asymmetric algorithm. another approach is a hybrid wherein a symmetric key is used to encrypt the data while an asymmetric key is used to encrypt the symmetric key. it may be important to know in order to maintain perspective that there is only one encryption algorithm that is impossible to crack, one - time pad ( otp ), any other algorithm may be broken given sufficient time and / or computer resources. security concerns, in general, and encryption, specifically, are new concepts for most it professionals ; therefore, a glossary of security / encryption terms is included as an appendix for reference. overview of transparent database encryption the primary benefit of transparent database encryption ( tde ) is the ability to encrypt data without affecting any application that uses the data while providing security for the entire database. tde is implemented at the database - level, unlike cell - level encryption tde does not require modification to applications or database column data types ; furthermore, database - level encryption allows for higher performance than cell - level encryption. however, tde may allow more data leakage because encrypted data is decrypted when read into the buffer


to applications or database column data types ; furthermore, database - level encryption allows for higher performance than cell - level encryption. however, tde may allow more data leakage because encrypted data is decrypted when read into the buffer pool ; therefore, the data is not protected if the operating system writes data from memory to disk during paging operations, or during hibernation, or memory dumps, nor is the data protected while in memory. database encryption is achieved by leveraging the data protection api ( dpapi ) in windows® which protects the service master key ( smk ) which protects the database master key ( dmk ) which is used to protect the certificate or asymmetric keys which are used to protect the database encryption key ( dek ). these dependencies create a security chain from the operating system to the data eliminating user interaction thus strengthening security. the relationships and dependencies between keys is represented in figure 3 below : figure 3 sql server encryption key hierarchy with tde and ekm ( source : bol - http : / / msdn. microsoft. com / en - us / library / cc278098. aspx ) the hierarchy of keys in tde is protected from the dpapi to the dek allowing the server to manage encryption and decryption automatically. the dmk and the certificate are stored in the master database while the dek is stored in the user database. this hierarchy and the key management chain provide tde the capability to transparently encrypt and decrypt the database. the process for encrypting a database is conceptually simple : - create a master key - obtain an authentication certificate - create dek - enable tde on the database however, significant complexity will be introduced if the database encryption strategy is undertaken without proper planning that addresses important implementation issues. those issues are discussed in the following section. the level of security necessary to protect the database should be documented during the planning phase. individually and in combination the following encryption mechanisms are available to secure the database : - encrypting file system ( efs ) - transparent database encryption ( tde ) discussion of the benefits and performance implications of each mechanism and their combinations is beyond the scope of this paper. data encryption must address two equally important issues : encryption technology and cryptographic key ( key ) management. encryption technology provides for variable granularity of data protection, performance, and integration with existing applications, as well as ease of implementation and management. however,


encryption must address two equally important issues : encryption technology and cryptographic key ( key ) management. encryption technology provides for variable granularity of data protection, performance, and integration with existing applications, as well as ease of implementation and management. however, the success of the selected encryption strategy may depend most on key management policies and processes. key management issues include : key access, key storage, and cryptographic algorithm. key management is one of many important issues that must be considered when planning the encryption project. the important issues to consider during the planning phase of the encryption project are listed below : - encryption algorithm : des, triple des, triple _ des _ 3key, rc2, rc4, 128 - bit rc4, desx, 128 - bit aes, 192 - bit aes, and 256 - bit aes - key management : key storage, hardware security module ( hsm ), key scheduling, key availability / mobility / security - performance impact. encryption / decryption - microsoft claims 3 - 5 % ; however, independent tests indicate 6 - 12 %.. - tempdb encryption - encryption of any one db will encrypt tempdb. - transaction log is encrypted. - log shipping implementation changes - encrypted database log shipping requires the recipient database to possess the key in order to apply the logs. - backup and recovery plan changes - encrypted databases cannot be recovered to a different instance without the key. - disaster recovery plan changes - encrypted databases cannot be recovered to a different instance without the key. - increased disk space requirements - no sql server native backup compression. third party tools may be available ; however, in general, encrypted data cannot be significantly compressed. - tde operates during i / o ; therefore, any data written to disk outside of the buffer pool is not protected - no support for filestream data - type the diagram in figure 4 represents a nominal encryption project planning process with each major area of consideration represented. the end result of the planning process is to produce a document detailing the decisions made that address the issues related to encrypting the database. figure 4 encryption planning process a comprehensive it security policy provides a layered defense against threats to the system. however, even the most thorough perimeter network and physical defenses do not obviate the vulnerability of plaintext data stored in databases. data encryption provides a means to protect sensitive data from unauthorized access as a part of a coordinated it security policy that includes network security, robust authentication


most thorough perimeter network and physical defenses do not obviate the vulnerability of plaintext data stored in databases. data encryption provides a means to protect sensitive data from unauthorized access as a part of a coordinated it security policy that includes network security, robust authentication and authorization, as well as other physical security considerations. sql server and windows® provide several mechanisms for the protection of data either at the file, database, or data levels. transparent database encryption ( tde ) is a new technology available in sql server 2008 enterprise edition which provides a simplified the data encryption option. tde is a database - level encryption mechanism that reduces the implementation complexity by negating the need to modify the data and / or the client applications. however, the benefits of performance and simplicity are balanced by tde ' s potential for data leakage ; therefore, for the most sensitive data tde alone may not suffice as a data security strategy. any data protection strategy must weigh the costs and benefits of implementation to arrive at a usable solution that meets the security requirements defined by the business. tde ' s protection of sensitive data in low to moderate threat environments may be sufficient for some business requirements while highly sensitive data or data in high threat environments will require the combination of tde with other encryption mechanisms such as cell - level encryption, efs, or bitlocker.


with implications similar to cyclical views of time and history. ( left ) an event is visible through time, like a pebble thrown into a bond creates outward waves / / / ( right ) an event in the present can only be perceived in a certain region of space - time. knowing that our time in the sun is limited, sometimes we try to capture time and light with images. albrecht durer ’ s etching, “ melancholia i ” associates light with order and darkness with chaos. the composition places the products of the imagination – geometry, mathematics, tools, and architecture – within the timeframe of an hourglass running out. in this picture, the imagination succeeds in creating a mental zone that overrides both astrophysics and religion – it holds together past, present and future with rays of perpetual sunlight – messengers of time etched in metal. / / / next week : skin, shell and skeleton part i nuclear bomb test, bikini atoll, 1946. the small black figures just outside the cloud are decommissioned world war ii battleships from the us navy. / / in addition to an urban investigation into the power structures of moscow, i ' ve been looking at ideas about the sublime and its relation to... - - yesterday china launched tiangong - 1 ( heavenly palace - 1 ), its first step towards a manned orbital space station. i remembered reading that the last space shuttle mission, sts - 135, finished earlier this year, signalling an end to america ’ s utopian dream of colonizing space. as i read more... i ' ve just started on the cooper union march ii course, and am excited to be here in new york to say the least! i ' ll be sharing my project work for studio, sculpture, and elsewhere over the upcoming year. for this first post i thought i ' d share something i wrote for a course appropriately titled...


with somewhat bizarre behaviors like the tunneling effect, which are governed by the laws of quantum mechanics and relativity. the orderly, deterministic world of classical physics gives way to a world of wave functions, probability distributions, uncertainty principles, and wave - particle dualities. instead of a deterministic world, we now have a world based on probabilities. you cannot predict all the future states of an object or a particle based on its present state. you can map out its behavior, but only as probability distributions of all the possible states it could be at. moreover, the heisenberg uncertainty principle tells you that it is impossible to know the exact state of a particle. you cannot simultaneously determine its exact position and velocity with any great degree of accuracy no matter how good your measurement tools are. the world is intrinsically unpredictable. in addition, there is no such thing as absolute reality. in classical mechanics something either has the properties of a particle, e. g., a planet, a baseball ; or of a wave, e. g, light, sound. in quantum mechanics all objects exhibit both kinds of properties. the concept of wave - particle duality explains that reality depends on what question you are asking and what experiment you perform to answer the question. the very act of observing an object will change the object being observed. any instruments used to measure its properties will invariable alter the properties being measured. this transition, from a world view based on scientific determinism to one based on probability distributions, uncertainty principles and subjective reality is not intuitive and difficult to get used to. even albert einstein had trouble accepting it, and famously said “ god does not play dice with the universe. ” stephen hawking, one of world ’ s top theoretical physicists, concluded in this brilliant lecture : “... it seems einstein was doubly wrong when he said, god does not play dice. not only does god definitely play dice, but he sometimes confuses us by throwing them where they can ’ t be seen... the universe does not behave according to our pre - conceived ideas. it continues to surprise us. ” but, the worlds of the very small, as well as the very large, are not the only ones that exhibit counter - intuitive, seemingly magical behaviors. so is the world of highly complex systems, especially those systems whose components and interrelationships are themselves quite complex, as is the case with systems biology and evolution. such is also the case with organizational and sociotechnical systems whose main components


. so is the world of highly complex systems, especially those systems whose components and interrelationships are themselves quite complex, as is the case with systems biology and evolution. such is also the case with organizational and sociotechnical systems whose main components are people. even though these chaotic systems are in principle deterministic, their dynamic, non - linear nature renders them increasingly unpredictable and accounts for their emergent behavior. new terms, like long tails, freakonomics and black swan theory, – every bit as fanciful as quarks, charm and strangeness, – have begun to enter our lexicon. artificial intelligence ( ai ) is an example of a discipline that has transitioned from its original classical, deterministic approach to an approach more suitable to a highly complex, inherently unpredictable topic like intelligence. ai was one of the hottest areas in computer sciences, in the 1960s and 1970s. many of the ai leaders in those days were convinced that you could build a machine as intelligent as a human being based on logical deductions and the kind of step - by - step reasoning that humans use when solving puzzles or proving theorems. they obtained considerable government funding in the us, uk and japan to implement their vision. but eventually it became clear that all these various projects had grossly underestimated the difficulties of developing any kind of ai system based on logic programming and deductive reasoning. the field went through a so - called ai winter in the 1980s. but things started to change in the 1990s when ai switched paradigms and embraced data mining and information analytics, the precursors of today ’ s big data. instead of trying to program computers to act intelligently, ai embraced a statistical, brute force approach based on analyzing vast amounts of information using powerful computers and sophisticated algorithms. we discovered that such a statistical, information - based approach produced something akin to intelligence or knowledge. moreover, unlike the earlier programming - based projects, the statistical approaches scaled very nicely. the more information you had, the more powerful the supercomputers, the more sophisticated the algorithms, the better the results. deep blue ibm ' s chess playing supercomputer, demonstrated the power of such a statistical approach by beating then reigning chess champion gary kasparov in a celebrated match in may of 1997. since that time, analyzing or searching large amounts of information has become increasingly important and commonplace in a wide variety of disciplines. today, most of us use search engines as the primary mechanism for finding information in the world wide web. researchers have been developing sophisticated question -


radiates 632. 8 nm. without helium, the neon atoms would be excited mostly to lower excited states responsible for non - laser lines. a neon laser with no helium can be constructed but it is much more difficult without this means of energy coupling. therefore, a hene laser that has lost enough of its helium ( e. g., due to diffusion through the seals or glass ) will most likely not lase at all since the pumping efficiency will be too low. the energy or pump source of the laser is provided by a high voltage electrical discharge passed through the gas between electrodes ( anode and cathode ) within the tube. a dc current of 3 to 20 ma is typically required for cw operation. the optical cavity of the laser usually consists of two concave mirrors or one plane and one concave mirror, one having very high ( typically 99. 9 % ) reflectance and the output coupler mirror allowing approximately 1 % transmission. commercial hene lasers are relatively small devices, among gas lasers, having cavity lengths usually ranging from 15 cm to 50 cm ( but sometimes up to about 1 meter to achieve the highest powers ), and optical output power levels ranging from 0. 5 to 50 mw. the red hene laser wavelength of 633 nm has an actual vacuum wavelength of 632. 991 nm, or about 632. 816 nm in air. the wavelength of the lasing modes lie within about 0. 001 nm above or below this value, and the wavelengths of those modes shift within this range due to thermal expansion and contraction of the cavity. frequency - stabilized versions enable the wavelength of a single mode to be specified to within 1 part in 108 by the technique of comparing the powers of two longitudinal modes in opposite polarizations. absolute stabilization of the laser ' s frequency ( or wavelength ) as fine as 2. 5 parts in 1011 can be obtained through use of an iodine absorption cell. the mechanism producing population inversion and light amplification in a hene laser plasma originates with inelastic collision of energetic electrons with ground state helium atoms in the gas mixture. as shown in the accompanying energy level diagram, these collisions excite helium atoms from the ground state to higher energy excited states, among them the 23s1 and 21s0 long - lived metastable states. because of a fortuitous near coincidence between the energy levels of the two he metastable states, and the 3s2 and 2s2 ( paschen notation ) levels of neon, collisions between these helium


- lived metastable states. because of a fortuitous near coincidence between the energy levels of the two he metastable states, and the 3s2 and 2s2 ( paschen notation ) levels of neon, collisions between these helium metastable atoms and ground state neon atoms results in a selective and efficient transfer of excitation energy from the helium to neon. this excitation energy transfer process is given by the reaction equations : - he * ( 23s1 ) + ne1s0 → he ( 1s0 ) + ne * 2s2 + δe - he * ( 21s ) + ne1s0 + δe → he ( 1s0 ) + ne * 3s2 where ( * ) represents an excited state, and δe is the small energy difference between the energy states of the two atoms, of the order of 0. 05 ev or 387 cm−1, which is supplied by kinetic energy. excitation energy transfer increases the population of the neon 2s2 and 3s2 levels manyfold. when the population of these two upper levels exceeds that of the corresponding lower level neon state, 2p4 to which they are optically connected, population inversion is present. the medium becomes capable of amplifying light in a narrow band at 1. 15 μm ( corresponding to the 2s2 to 2p4 transition ) and in a narrow band at 632. 8 nm ( corresponding to the 3s2 to 2p4 transition at 632. 8 nm ). the 2p4 level is efficiently emptied by fast radiative decay to the 1s state, eventually reaching the ground state. the remaining step in utilizing optical amplification to create an optical oscillator is to place highly reflecting mirrors at each end of the amplifying medium so that a wave in a particular spatial mode will reflect back upon itself, gaining more power in each pass than is lost due to transmission through the mirrors and diffraction. when these conditions are met for one or more longitudinal modes then radiation in those modes will rapidly build up until gain saturation occurs, resulting in a stable continuous laser beam output through the front ( typically 99 % reflecting ) mirror. the gain bandwidth of the hene laser is dominated by doppler broadening rather than pressure broadening due to the low gas pressure, and is thus quite narrow : only about 1. 5 ghz full width for the 633 nm transition. with cavities having typical lengths of 15 cm to


line - of - sight propagation refers to electro - magnetic radiation or acoustic wave propagation. electromagnetic transmission includes light emissions traveling in a straight line. the rays or waves may be diffracted, refracted, reflected, or absorbed by atmosphere and obstructions with material and generally cannot travel over the horizon or behind obstacles. at low frequencies ( below approximately 2 mhz or so ) radio signals travel as ground waves, which follow the earth ' s curvature due to diffraction with the layers of atmosphere. this enables am radio signals in low - noise environments to be received well after the transmitting antenna has dropped below the horizon. additionally, frequencies between approximately 1 and 30 mhz can be reflected by the f1 / f2 layer, thus giving radio transmissions in this range a potentially global reach ( see shortwave radio ), again along multiple deflected straight lines. the effects of multiple diffraction or reflection lead to macroscopically " quasi - curved paths ". however, at higher frequencies and in lower levels of the atmosphere, neither of these effects are significant. thus any obstruction between the transmitting antenna and the receiving antenna will block the signal, just like the light that the eye may sense. therefore, since the ability to visually see a transmitting antenna ( disregarding the limitations of the eye ' s resolution ) roughly corresponds to the ability to receive a radio signal from it, the propagation characteristic of high - frequency radio is called " line - of - sight ". the farthest possible point of propagation is referred to as the " radio horizon ". in practice, the propagation characteristics of these radio waves vary substantially depending on the exact frequency and the strength of the transmitted signal ( a function of both the transmitter and the antenna characteristics ). broadcast fm radio, at comparatively low frequencies of around 100 mhz, are less affected by the presence of buildings and forests. radio horizon the radio horizon is the locus of points at which direct rays from an antenna are tangential to the surface of the earth. if the earth were a perfect sphere and there were no atmosphere, the radio horizon would be a circle. the radio horizon of the transmitting and receiving antennas can be added together to increase the effective communication range. antenna heights above 1, 000, 000 feet ( 189 miles ; 305 kilometres ) will cover the entire hemisphere and not increase the radio horizon. radio wave propagation is affected by atmospheric conditions, ionospheric absorption, and the presence of obstructions, for example mountains or trees. simple formulas that include the effect of the atmosphere give the range as : the simple formulas