Aristotle - Physics

           When the objects of an inquiry, in any department, have principles, conditions, or elements, it is through acquaintance with these that knowledge, that is to say scientific knowledge, is attained. For we do not think that we know a thing until we are acquainted with its primary conditions or first principles, and have carried our analysis as far as its simplest elements. Plainly therefore in the science of Nature, as in other branches of study, our first task will be to try to determine what relates to its principles. 

         The natural way of doing this is to start from the things which are more knowable and obvious to us and proceed towards those which are clearer and more knowable by nature; for the same things are not 'knowable relatively to us' and 'knowable' without qualification. So in the present inquiry we must follow this method and advance from what is more obscure by nature, but clearer to us, towards what is more clear and more knowable by nature.Now what is to us plain and obvious at first is rather confused masses, the elements and principles of which become known to us later by analysis. Thus we must advance from generalities to particulars; for it is a whole that is best known to sense-perception, and a generality is a kind of whole, comprehending many things within it, like parts. Much the same thing happens in the relation of the name to the formula. A name, e.g. 'round', means vaguely a sort of whole: its definition analyses this into its particular senses. Similarly a child begins by calling all men 'father', and all women 'mother', but later on distinguishes each of them.

About the nuclear explosion and radiation

In nuclear explosions, about 90 percent of the energy is released in less than one millionth of a second. Most of this is in the form of the heat and shock waves which produce the damage. It is this immediate and direct explosive power which could devastate the urban centers in a major nuclear war.

Compared with the immediate colossal destruction suffered in target areas, the more subtle, longer term effects of the remaining 10 percent of the energy released by nuclear weapons might seem a matter of secondary concern. But the dimensions of the initial catastrophe should not overshadow the after-effects of a nuclear war. They would be global, affecting nations remote from the fighting for many years after the holocaust, because of the way nuclear explosions behave in the atmosphere and the radioactive products released by nuclear bursts.

When a weapon is detonated at the surface of the earth or at low altitudes, the heat pulse vaporizes the bomb material, target, nearby structures, and underlying soil and rock, all of which become entrained in an expanding, fast-rising fireball. As the fireball rises, it expands and cools, producing the distinctive mushroom cloud, signature of nuclear explosions.

The altitude reached by the cloud depends on the force of the explosion. When yields are in the low-kiloton range, the cloud will remain in the lower atmosphere and its effects will be entirely local. But as yields exceed 30 kilotons, part of the cloud will punch into the stratosphere, which begins about 7 miles up. With yields of 2-5 megatons or more, virtually all of the cloud of radioactive debris and fine dust will climb into the stratosphere. The heavier materials reaching the lower edge of the stratosphere will soon settle out, as did the Castle/Bravo fallout at Rongelap. But the lighter particles will penetrate high into the stratosphere, to altitudes of 12 miles and more, and remain there for months and even years. Stratospheric circulation and diffusion will spread this material around the world.
Both the local and worldwide fallout hazards of nuclear explosions depend on a variety of interacting factors: weapon design, explosive force, altitude and latitude of detonation, time of year, and local weather conditions.

Edison and phonograph

AT the opening of the Electrical Show in New York City in October, 1908, to celebrate the jubilee of the Atlantic Cable and the first quarter century of lighting with the Edison service on Manhattan Island, the exercises were all conducted by means of the Edison phonograph. This included the dedicatory speech of Governor Hughes, of New York; the modest remarks of Mr. Edison, as president; the congratulations of the presidents of several national electric bodies, and a number of vocal and instrumental selections of operatic nature. 

All this was heard clearly by a very large audience, and was repeated on other evenings. The same speeches were used again phonographically at the Electrical Show in Chicago in 1909—and now the records are preserved for reproduction a hundred or a thousand years hence. This tour de force, never attempted before, was merely an exemplification of the value of the phonograph not only in establishing at first hand the facts of history, but in preserving the human voice. What would we not give to listen to the very accents and tones of the Sermon on the Mount, the orations of Demosthenes, the first Pitt's appeal for American liberty, the Farewell of Washington, or the Address at Gettysburg? Until Edison made his wonderful invention in 1877, the human race was entirely without means for preserving or passing on to posterity its own linguistic utterances or any other vocal sound. We have some idea how the ancients looked and felt and wrote; the abundant evidence takes us back to the cave-dwellers. But all the old languages are dead, and the literary form is their embalmment. We do not even know definitely how Shakespeare's and Goldsmith's plays were pronounced on the stage in the theatres of the time; while it is only a guess that perhaps Chaucer would sound much more modern than he scans. 

The analysis of sound, which owes so much to Helmholtz, was one step toward recording; and the various means of illustrating the phenomena of sound to the eye and ear, prior to the phonograph, were all ingenious. One can watch the dancing little flames of Koenig, and see a voice expressed in tongues of fire; but the record can only be photographic. In like manner, the simple phonautograph of Leon Scott, invented about 1858, records on a revolving cylinder of blackened paper the sound vibrations transmitted through a membrane to which a tiny stylus is attached; so that a human mouth uses a pen and inscribes its sign vocal. Yet after all we are just as far away as ever from enabling the young actors at Harvard to give Aristophanes with all the true, subtle intonation and inflection of the Athens of 400 B.C. The instrument is dumb. Ingenuity has been shown also in the invention of "talking-machines," like Faber's, based on the reed organ pipe. These automata can be made by dexterous manipulation to jabber a little, like a doll with its monotonous "ma-ma," or a cuckoo clock; but they lack even the sterile utility of the imitative art of ventriloquism. The real great invention lies in creating devices that shall be able to evoke from tinfoil, wax, or composition at any time to-day or in the future the sound that once was as evanescent as the vibrations it made on the air. 

About magnetism

The branch of science that describes the effects of the interactions between charges due to their motion and spin. These may appear in various forms, including electric currents and permanent magnets. The interactions are described in terms of the magnetic field, although the field hypothesis cannot be tested independently of the electrokinetic effects by which it is defined.

The magnetic field complements the concept of the electrostatic field used to describe the potential energy between charges due to their relative positions. Special relativity theory relates the two, showing that magnetism is a relativistic modification of the electrostatic forces. The two together form the electromagnetic interactions which are propagated as electromagnetic waves, including light. They control the structure of materials at distances between the long-range gravitational actions and the short-range “strong” and “weak” forces most evident within the atomic nucleus.

The term “magnetism” originates in the material magnetite, an iron ore, which producesweak natural magnets in the formof lodestones, exerting forces on each other and on pieces of iron. Peregrinus showed in 1269 that the behavior can be described in terms of magnetic poles on opposite end surfaces. The analogy between magnetic poles and electric charge greatly influenced the later development of the subject. The Earth provides an example of the subsequent explanation of magnetic behavior in terms of the flow of current and movement of charge, including the quantum state described as spin.

This leaves open the question of whether or not isolated magnetic poles, or monopoles, exist as separate physical entities. The magnetic field can be visualized as a set of lines illustrated by iron filings scattered on a suitable surface. The intensity of the field is indicated by the line spacing, and the direction by arrows pointing along the lines. The sign convention is chosen so that the Earth’smagnetic field is directed from the north magnetic pole toward the south magnetic pole.

Time and time

The dimension of the physical universe that orders the sequence of events at a given place; also, a designated instant in this sequence, such as the time of day, technically known as an epoch, or sometimes as an instant.

Measurement.

Time measurement consists of counting the repetitions of any recurring phenomenon and possibly subdividing the interval between repetitions. Two aspects to be considered in the measurement of time are frequency, or the rate at which the recurring phenomena occur, and epoch, or the designation to be applied to each instant.
A determination of time is equivalent to the establishment of an epoch or the correction that should be applied to the reading of a clock at a specified epoch. A time interval may be measured as the duration between two known epochs or by counting from an arbitrary starting point, as is done with a stopwatch. Time units are the intervals between successive recurrences of phenomena, such as the period of rotation of the Earth or a specified number of periods of radiation derived from an atomic energy-level transition. other units are arbitrary multiples and subdivisions of these intervals, such as the hour being 1/24 of a day, and the minute being 1/60 of an hour.

Bases.

Several phenomena are used as bases with which to determine time. The phenomenon traditionally used has been the rotation of the Earth, where the counting is by days. Days are measured by observing the meridian passages of the Sun or stars and are subdivided with the aid of precision clocks. The day, however, is subject to variations in duration because of the variable rotation rate of the Earth. Thus, when a more uniform time scale is required, other bases for time must be used.

Sidereal time.

The angle measured along the celestial equator between the observer's local meridian and the vernal equinox is the measure of sidereal time. In practice, a conventionally adopted mathematical expression provides this time as a function of civil time. It is reckoned from 0 to 24 hours, each hour being subdivided into 60 sidereal minutes and the minutes into 60 sidereal seconds. Sidereal clocks are used for convenience in many astronomical observatories because a star or other object outside the solar system comes to the same place in the sky at virtually the same sidereal time.

Solar time.

The angle measured along the celestial equator between the observer's local meridian and the Sun is the apparent solar time. The only true indicator of local apparent solar time is a sundial. Mean solar time has been devised to eliminate the irregularities in apparent solar time that arise from the inclination of the Earth's orbit to the plane of the Sun's motion and the varying speed of the Earth in its orbit. In practice it is defined by a conventionally adopted mathematical expression. Intervals of sidereal time can be converted into intervals of mean solar time by dividing by 1.002 737 909 35. Both sidereal and solar time depend on the rotation of the Earth for their time base.

Universal Time (UT).

Historically, the mean solar time determined for the meridian of 0° longitude using astronomical observations was referred to as UT1. Currently UT1 is used only as an angle expressed in time units that depends on the Earth's rotation with respect to the celestial reference system. It is defined by a conventional mathematical expression and continuing astronomical observations. These are made at a number of observatories around the world. The International Earth Rotation and Reference System Service (IERS) receives these data and provides daily values of the difference between UT1 and civil time.
Because the Earth has a nonuniform rate of rotation and a uniform time scale is required for many timing applications, a different definition of a second was adopted in 1967. The international agreement calls for the second to be defined as 9,192,631,770 periods of the radiation derived from an energy-level transition in the cesium atom. This second is referred to as the international or SI (International System) second and is independent of astronomical observations. International Atomic Time (TAI) is maintained by the International Bureau of Weights and Measures (BIPM) from data contributed by time-keeping laboratories around the world.
Coordinated Universal Time (UTC) uses the SI second as its time base. However, the designation of the epoch may be changed at certain times so that UTC does not differ from UT1 by more than 0.9 s. UTC forms the basis for civil time in most countries and may sometimes be referred to unofficially as Greenwich Mean Time. The adjustments to UTC to bring this time scale into closer accord with UT1 consist of the insertion or deletion of integral seconds. These "leap seconds" may be applied preferably at 23 h 59 m 59 s of June 30 or December 31 of each year according to decisions made by the IERS. UTC differs from TAI by an integral number of atomic seconds.

Dynamical time.

Dynamical time is based on the apparent orbital motion of the Sun, Moon, and planets. It is the time inferred in the ephemerides of the positions of these objects, and from its inception in 1952 until 1984 was referred to as Ephemeris Time. Barycentric Dynamical Time (TDB) refers to ephemerides that have been computed by using the barycenter of the solar system as a reference. Terrestrial Dynamical Time (TDT) is the practical realization of dynamical time and is defined as being equal to TAI + 32.184 seconds. In 1991, the International Astronomical Union recommended that TDT be renamed Terrestrial Time (TT), that Geocentric Coordinate Time (TCG) be the time coordinate for the geocenter, and that Barycentric Coordinate Time (TCB) be the time coordinate for the barycenter of the solar system. These times are related by the appropriate relativistic transformations.

Civil and standard times.

Because rotational time scales are local angular measures, at any instant they vary from place to place on the Earth. When the mean solar time is 12 noon at Greenwich, the mean solar time for all places west of Greenwich is earlier than noon and for all places east of Greenwich later than noon, the difference being 1 hour for each 15° of longitude. Thus, at the same instant at short distances east of the 180th meridian the mean solar time is 12:01 A.M., and at a short distance west of the same meridian it is 11:59 P.M. of the same day. Thus persons traveling westward around the Earth must advance their time 1 day, and those traveling eastward must retard their time 1 day in order to be in agreement with their neighbors when they return home. The International Date Line is the name given to a line where the change of date is made. It follows approximately the 180th meridian but avoids inhabited land. To avoid the inconvenience of the continuous change of mean solar time with longitude, zone time or civil time is generally used. The Earth is divided into 24 time zones, each approximately 15° wide and centered on standard longitudes of 0°,15°,30°, and so on (see illustration). Within each of these zones the time kept is related to the mean solar time of the standard meridian.
Zone time is reckoned from 0 to 24 hours for most official purposes, the time in hours and minutes being expressed by a four-figure group followed by the zone designation. For example, "1009 zone plus five" refers to the zone 75° west of Greenwich, where zone time must be increased by 5 hours to obtain UTC. The various zones are sometimes designated by letters, especially the Greenwich zone which is Z, "1509 Z" meaning 1509 UTC. The zone centered on the 180th meridian is divided into two parts, the one east ofthe date line being designated plus 12 and the other minus 12. The time July 2,2400 is identical with July 3,0000.
In civil life the designations A.M. and PPM. are often used, usually with punctuation between hours and minutes. Thus 1009 may be written as 10:09 A.M. and 1509 as 3:09 P.M. The designations for noon and midnight, however, are often confused, and it is better to write 12:00 noon and July 2-3, 12:00 midnight, in order to avoid ambiguity. In some occupations where time is of special importance, there is a rule against using 12:00 at all, 11:59 or 12:01 being substituted. The time 1 minute after midnight is 12:01 A.M. and 1 minute after noon is 12:01 PPM.
The illustration shows the designations ofthe various time zones, the longitudes ofthe standard meridians, and the letter designations and the times in the various zones when it is noon at Greenwich. In the United States the boundaries of the time zones are fixed by the Department of Transportation. Frequently the actual boundaries depart considerably from the meridians exactly midway between the standard meridians. Ships at sea and transoceanic planes usually use UTC for navigation and communication, but for regulating daily activities onboard they use any convenient approximation to zone time, avoiding frequent changes during daylight hours.
Many countries, including the United States, advance their time 1 hour, particularly during the summer months, into "daylight saving time." For example, 6 A.M. is redesignated as 7 A.M. Such a practice effectively transfers an hour of little-used early morning light to the evening.
Time scales are coordinated internationally by the BIPM. Most countries maintain local time standards to provide accurate time within their borders by radio, telephone, and TV services. These national time scales are often intercompared by using the Global Positioning System (GPS) or time signals transferred by artificial Earth satellites.

About X-Ray

X-rays, or Roentgen rays, are electromagnetic waves; they are the same as visible light, except that they have shorter wavelengths (higher photon energies). Thus x-rays, visible light, ultraviolet, infrared, microwaves, and radio waves are all electromagnetic radiation in different wavelength (energy) spectral regions. X-rays are generated when fast-moving electrons slow down and stop in matter, when an innershell vacancy in an atom is filled by another electron, and when electrons moving at relativistic speeds (speeds near the speed of light) change their direction of motion in space.

Roentgen's findings.

X-rays were discovered by W.C. Roentgen in 1895. This discovery came about by accident. Roentgen was studying gas discharges in his laboratory when he noticed that unknown radiation from a gas discharge could induce fluorescence in certain materials. In his first communication, Roentgen described the properties ofthese rays as follows: They were invisible; moved in straight lines; were unaffected by electric or magnetic fields, and hence not electrically charged; passed through matter opaque to ordinary light (since they penetrated through the black cardboard around his cathode-ray tube); were differentially absorbed by matter of different densities or of different atomic weights; affected photographic plates; produced fluorescence in certain chemicals, such as in the barium platinocyanide screen with which the initial discovery was made and in the wall of his glass tube opposite the cathode; produced ionization in gases; and were evidently produced at the anode by the beam of rays (identified by J. J. Thomson in 1897 as electrons) issuing from the cathode in his vacuum tube.
Along with all these definitive characteristics of the rays, however, other crucial experiments designed to establish similarity or differences from visible light were clearly called for. The fundamental optical properties of visible light were well established in 1895: reflection from mirrors; refraction in prisms (change in direction in passing from air into glass, for example), by means of which a beam of white light could be spread out into a rainbow or spectrum of colors; diffraction by narrow slits or ruled gratings, also a method of producing spectra; and polarization, or constraint of the electric field of the light wave to a single direction. In spite of the best efforts
of Roentgen, no evidence of any of these four optical phenomena could be found. Hence the designation "x"—unknown—was assigned by Roentgen. Many theories were proposed to account for the apparently unique quality of x-rays, which seemed to be so closely similar and yet so greatly different from visible light.

Later discoveries.

Other scientists studying x-rays found the essential experimental conditions to prove that x-rays can be polarized (C. Barkla, 1905, by scattering from carbon); diffracted by crystals (M. von Laue, W Friedrich, and P. Knipping, 1912); refracted in prisms and in crystals; reflected by mirrors; and diffracted by ruled gratings (A. Compton, 1921-1922). Instead of being refracted in passing from a less dense medium (air) to a more dense medium (a glass prism or a crystal) in the same direction as light (the index of refraction for visible light is always greater than 1), x-rays are deviated in the opposite direction by a very small amount: the index of refraction is less than 1 by an amount as small as 10-6. Total reflection from mirrors is observed only when the beam impinges at a very small angle (grazing angle), a necessary condition understandably missed by Roentgen. Similarly, the beam must be incident at a very small angle on a ruled diffraction grating if diffraction (a spectrum, or spatial dispersion by wavelength) is to be observed.
From 1895 to 1912 there seemed to be no analyzer capable of dispersing an x-ray beam into a spectrum. The spectacular Laue diffraction pattern of a zinc sulfide crystal in 1912 proved the electromagnetic wave nature of x-rays and the ordered structure of crystals, with atoms lying on families of planes to constitute three-dimensional diffraction gratings, all governed by the simple Bragg law nk = 2d sin 6 (which must be corrected for refraction in extremely accurate work). Here n is an integer indicating the order of the spectrum, k the wavelength, d the crystal lattice spacing of one set of planes, and 6 the angle between the incident ray and this set of planes.
The wavelength range of x-rays in the electromagnetic spectrum, as excited in x-ray tubes by the bombardment of a target by electrons under a high accelerating potential, overlaps the ultraviolet range on the order of 100 nanometers on the long-wavelength side, and the shortest-wavelength limit moves downward as voltages increase. An accelerating potential of 109 V, now readily generated, produces a k of 10-6 nm. An average wavelength used in research is 0.1 nm, or about 1/6000 the wavelength of yellow light.

Quantum mechanics.

X-rays (and visible light) can be considered as an electromagnetic wave. X-rays can also be considered as discontinuous bundles of energy, or quanta, in accordance with the laws first enunciated by M. Planck and extended by A. Einstein early in the twentieth century. In diffraction, refraction, polarization, and interference phenomena, x-rays, together with all other electromagnetic radiation, appear to act as waves and k has a real significance. There is duality, meaning that light and x-rays have both wave and particle properties, although these are generally not observed at the same time in a given experiment. Beams of electrons and neutrons also have wave properties, and are diffracted in appropriate media. In other phenomena—such as the appearance of sharp spectral lines, a definite short-wavelength limit k0 of the continuous "white" spectrum [defined by k0 = hc/eV, where h is Planck's constant, c the velocity of electromagnetic radiation (including light and x-rays), e the charge of electron, and V the accelerating voltage], the shift in wavelength of x-rays scattered by electrons in atoms (Compton effect), and the photoelectric effect— the energy is propagated and transferred in quanta (called photons) defined by values of hv, where the frequency v is c/x.

Applications.

X-rays are a valuable probe of matter, as they interact selectively with electrons; electrons in matter account for most of the significant properties of matter (excepting nuclear properties). Medical uses abound, as x-rays can penetrate matter, and are selectively absorbed by atoms containing many electrons—elements with a high atomic number. Thus dental x-rays show fillings as dark, teeth and bone in grey, and are only lightly absorbed in the cheek. X-rays in high intensities are also used for therapy, to treat certain cancers. X-rays are also used in many industrial processes, as in checking the integrity of welds. Tomographic techniques allow three-dimensional images to be obtained, which are invaluable in medical diagnosis.
X-rays are used in research for the study of both the electronic structure of matter and its spatial structure. X-ray microscopes and microprobes have been developed which allow imaging with high spatial resolution while obtaining contrast with different elemental discrimination, distinguishing chemical bonds, and, by use of circularly polarized x-rays, the orientation of magnetization. X-rays are used in protein crystallography to determine the spatial structure of proteins, viruses, and other objects that can be made into crystals but that otherwise cannot be seen.

Quantum Teleportation

A way to transfer the state of a quantum system over large distances by employing entanglement. Entanglement is a nonclassical connection between objects that Albert Einstein called "spooky."

To be able to travel from one place to another instantly and over arbitrary distances, or at least to move objects in this way, is an ancient dream. The concept of teleportation is frequently utilized in the literature of science fiction to overcome limitations imposed on space travel by the laws of physics.

In the standard science fiction approach, the sender, Alice, scans the object to be teleported in order to read out all the information needed to describe it. She then sends that information to the receiver, Bob, who uses this information to reconstitute the object, not necessarily from the same material as that of the original. However, according to quantum mechanics, it is impossible to succeed in this way. If only one individual object is at hand, it is impossible to determine its quantum state by measurement. The quantum state represents all that can be known about the object, that is, all possible (in general, probabilistic) predictions that can be made about future observations of the object.

In fact, it is quantum mechanics that comes to the rescue and makes quantum teleportation possible using a very deep feature of the theory, quantum entanglement. It is important to realize that there are significant differences between teleportation as portrayed in science fiction and quantum teleportation as realized in the laboratory. In the experiments, what is teleported is not the substance an object is made of but the information it represents.
Quantum entanglement. Entangled quantum states as used in teleportation were introduced into the discussion of the foundations of quantum mechanics by Einstein, Boris Podolsky, and Nathan Rosen in 1935. In the same year, Erwin Schrodinger introduced the notion of entanglement, which he called the essence of quantum mechanics.

In order to discuss entanglement, one specific case will be considered, and the possible experimental results will be examined . There are many possible sources that can create many different sorts of entangled states. The source under consideration will be assumed to be the one used in the first tele-portation experiments, which produced photons in a singlet polarization state. This means that neither photon enjoys a state of well-defined polarization; each one of the photons on its own is maximally unpolarized. Yet, when one of the two photons is subject to a polarization measurement, it assumes one specific polarization. That specific experimental outcome is completely random. As a consequence of the two photons being in the entangled singlet state, the other photon is instantly projected into a state orthogonal to that of the first photon. The fact that the measurement result on the second photon can be perfectly predicted on the basis of the measurement result of the first photon, even as neither one carries a well-defined quantum state, is known as the Einstein-Podolsky-Rosen paradox.

In 1964 John Bell showed that these perfect correlations cannot be understood on the basis of properties that the entangled photons carry individually before the measurement. The resulting conflict between the philosophical position of local realism and the predictions of quantum mechanics, which have been confirmed beyond reasonable doubt in experiment, is known as Bell's theorem.
From an information-theoretic point of view, the interesting feature of entanglement is that neither of the two photons carries any information on its own. All information is stored in joint properties.

Concept of quantum teleportation. It was first realized by Charles H. Bennett and his colleagues that entanglement can be utilized to make teleportation possible. Alice, who is in possession of the original teleportee photon in a quantum state not known to her, and Bob initially share an ancillary pair of entangled photons, say in the singlet state described above. Alice then subjects her teleportee photon and her member of the ancillary pair to a Bell-state measurement. A Bell-state measurement is designed in such a way that it projects the two photons into an entangled state even if they were previously unentangled. This is a very tricky procedure both conceptually and experimentally. Conceptually it means that the measurement must be performed in such a way that it is not possible, even in principle, to determine from the measurement result which photon was the teleportee and which was Alice's ancillary. They both have to lose their individuality. The result of the measurement must reveal only how the two photons relate to each other, and ignore individual properties. A Bell measurement has four possible results if the objects considered are denned in a two-dimensional Hilbert space just as is done to describe the photon's polarization. One of the four states is the singlet state discussed above. The other three states also define specific relations between the two photons, though different ones than those for the singlet state.

By the Bell-state measurement, Alice now knows how the unknown state of the teleportee photon relates to her ancillary one. She also knows in which entangled state the two ancillaries were produced, that is, how these two relate to each other. Thus she finally knows precisely how the teleportee relates to Bob's photon. More formally speaking, as a result of Alice's measurement Bob's photon is projected into a state which is uniquely related to the original state; the specific relationship is expressed by which of the four Bell states Alice obtained. Alice therefore informs Bob of her measurement result via a classical communication channel, and he, by applying a simple unitary transformation on his photon, changes it into the original state.
In one of the four cases, Alice obtains the information that her two photons have been projected into the singlet state, the same state in which the ancil-laries were produced. Then, she knows that Bob's photon is instantly projected into the original state; the transformation that Bob has to apply is an identity transformation, that is, one that makes no change to his photon. That Bob's photon then instantly becomes an exact replica of the original seems to violate relativity.

Yet, while Alice knows instantly that Bob's photon, no matter how far away, is already an exact replica, she has to inform Bob of the Bell mea-sûrement result such that he knows that his photon is already in the correct state. That classical information can arrive only at the speed of light. This requirement is also true for the other possible Bellstate measurement results. Bob has to know them in order to apply the correct transformation to his photon.
The result of the Bell measurement is not related at all to any properties that the original photon carries. Thus, that measurement does not reveal any information about its state. Therefore, the operation that Bob has to apply is also completely independent of any properties of the original photon. The reason that quantum measurement succeeds is that entanglement makes it possible to completely transfer the information that an object carries without measuring this information.
Experimental realization. An experiment therefore faces a number of challenges. They include (1) how to produce the entangled photon pairs and (2) how to perform a Bell measurement for independent photons. In the experimental realization by D. Bouwmeester and his colleagues in 1997, the entangled photons were produced in the process of spontaneous parametric downconversion.

This is a second-order nonlinear process where a suitable crystal, in the experiment beta barium borate (BBO), is pumped with a beam of ultraviolet radiation. A photon from that beam has a very small probability to decay spontaneously into two photons, which then are polarization-entangled in just the way necessary for the experiment. The more tricky part is the Bellstate measurement because, in essence, it requires that the two photons are registered such that all information about which was the teleportee photon and which the ancillary is irrevocably erased. This is a nontrivial requirement since the two photons are coming from different directions, they might arrive at different times, and so forth.
In the experiment, the Bell-state measurement was performed using a semireflecting mirror, which acted as a 50/50 beam splitter.

Two photons were incident on the beam splitter, one from its front side and one from its back, and each one had the same probability of 50% to be either reflected or transmitted. If each of the two detectors in the two outgoing beams, again one in the front and one in the back, registered a photon simultaneously, then no information existed as to which incoming photon was registered in which detector, and the two were projected into the entangled singlet state. Narrow-bandwidth filters in front of the detectors further served to erase any time information which could also serve to identify the photons.
In this experiment, only one of the four possible Bell states could be identified, the singlet state. This certainly reduced the efficiency of the procedure, though in those cases in which the two detectors at the Bell-state analyzer registered, teleportation worked with a fidelity escaping all possible classical explanation.
In another experiment, also called entanglement swapping, it was even possible to teleport a photon that was still entangled to another one. That experiment started with two entangled pairs. A joint Bellstate measurement on one photon from each pair projected the other two photons onto an entangled state. In that way, two photons that neither came from the same source nor ever interacted with one another became entangled.

What all these experiments reveal is that the quantum state is really just a representation of the information that has been acquired. In the case of entanglement, it is only information on how objects relate to each other without any information on their individual properties. And in the case of teleportation, Alice's observation changes the quantum state that Bob observes. In other words, what can be said about the situation changes due to an observation by Alice. This gives very strong support to the Copenhagen interpretation of quantum mechanics. The first experiments were done with polarization-entangled photon pairs. Since then a number of experiments teleporting other properties such as continuous variables carried by the electromagnetic field of light, instead of the discrete ones discussed above, have been performed.
Prospects. While the teleportation distance in the first experiments was of the order of 1 m (3 ft), experiments in 2004 extended the distance to the order of 600 m (2000 ft), and there are plans to perform such experiments over much larger distances and even from a satellite down to laboratories on the ground. Other important experimental steps include the teleportation of quantum states of atoms (2004) and the teleportation of the quantum state of a photon onto that of an atomic cloud (2006).

Today quantum teleportation and entanglement swapping—the teleportation of an entangled state— are considered to be key building blocks of future quantum computer networks. At present there is intense research in the development of both quantum communication networks and quantum computers. Future quantum computers would use individual quantum states, for example those of atoms, to represent information in so-called quantum bits. They are expected to allow some algorithms to be performed with significantly higher speed than any existing computers. Quantum tele-portation would allow the transfer of the quantum output of one quantum computer to the quantum input of another quantum computer.

Cosmic rays - Extraterrestrial radiation

Cosmic rays Electrons and the nuclei of atoms—largely hydrogen—that impinge upon Earth from all directions of space with nearly the speed of light. These nuclei with relativistic speeds are often referred to as primary cosmic rays, to distinguish them from the cascade of secondary particles generated by their impact against air nuclei at the top of the terrestrial atmosphere. The secondary particles shower down through the atmosphere and are found all the way to the ground and below.

The primary cosmic rays provide the only direct sample of matter from outside the solar system. Measurement of their composition can aid in understanding which aspects of the matter making up the solar system are typical of the Milky Way Galaxy as a whole and which may be so atypical as to yield specific clues to the origin of the solar system. Cosmic rays are electrically charged; hence they are deflected by the magnetic fields which are thought to exist throughout the Galaxy, and may be used as probes to determine the nature of these fields far from Earth. Outside the solar system the energy contained in the cosmic rays is comparable to that of the magnetic field, so the cosmic rays probably play a major role in determining the structure of the field.

Collisions between the cosmic rays and the nuclei of the atoms in the tenuous gas which permeates the Galaxy change the cosmic-ray composition in a measurable way and produce gamma rays which can be detected at Earth, giving information on the distribution of this gas.
Cosmic-ray detection. All cosmic-ray detectors are sensitive only to moving electrical charges. Neutral cosmic rays (neutrons, gamma rays, and neutrinos) are studied by observing the charged particles produced in the collision of the neutral primary with some type of target. At low energies the ionization of the matter through which they pass is the principal means of detection. A single measurement of the ionization produced by a particle is usually not sufficient both to identify the particle and to determine its energy. However, since the ionization itself represents a significant energy loss to a low-energy particle, it is possible to design systems of detectors which trace the rate at which the particle slows down and thus to obtain unique identification and energy measurement.

At energies above about 500 MeV per nucleon, almost all cosmic rays will suffer a catastrophic nuclear interaction before they slow appreciably. An ionization measurement is commonly combined with measurements of physical effects which vary in a different way with mass, charge, and energy. Cerenkov detectors and the deflection of the particles in the field of large superconducting magnets or the magnetic field of the Earth itself provide the best means of studying energies up to a few hundred GeV per nucleon. Detectors employing the phenomenon of x-ray transition radiation promise to be useful for measuring composition at energies up to a few thousand GeV per nucleon.
Above about 1012 eV, direct detection of individual particles is no longer possible since they are so rare. Such particles are studied by observing the large showers of secondaries they produce in Earth's atmosphere. These showers are detected either by counting the particles which survive to strike ground-level detectors or by looking at the flashes of light the showers produce in the atmosphere with special telescopes and photomultiplier tubes.

Atmospheric cosmic rays. The primary cosmic-ray particles coming into the top of the terrestrial atmosphere make inelastic collisions with nuclei in the atmosphere. When a high-energy nucleus collides with the nucleus of an air atom, a number of things usually occur. Rapid deceleration of the incoming nucleus leads to production of pions with positive, negative, or neutral charge. A few protons and neutrons (in about equal proportions) may be knocked out with energies up to a few GeV. They are called knock-on protons and neutrons.
All these protons, neutrons, and pions generated by collision of the primary cosmic-ray nuclei with the nuclei of air atoms are the first stage in the development of the secondary cosmic-ray particles observed inside the atmosphere. Since several secondary particles are produced by each collision, the total number of energetic particles of cosmic-ray origin will increase with depth, even while the primary density is decreasing.

The uncharged n0 mesons decay into two gamma rays with a life of about 8 x 10-17 s. The two gamma rays each produce a positron-electron pair. Upon passing sufficiently close to the nucleus of an air atom deeper in the atmosphere, the electrons and positrons convert their energy into bremsstrahlung. The bremsstrahlung in turn create new positron-electron pairs, and so on. This cascade process continues until the energy of the initial n0 has been dispersed into a shower of positrons, electrons, and photons with insufficient individual energies (< 1 MeV) to continue the pair production. The electrons and photons of such showers are referred to as the soft component of the atmospheric (secondary) cosmic rays.

The n ± mesons produced by the primary collisions have a life of about 2.6 x 10-8 s before they decay into muons. Most low-energy n ± decay into muons before they have time to undergo nuclear interactions. Except at very high energy (above 500 GeV), muons interact relatively weakly with nuclei, and are too massive (207 electron masses) to produce bremsstrahlung. They lose energy mainly by the comparatively feeble process of ionizing an occasional air atom as they progress downward through the atmosphere. Because of this ability to penetrate matter, they are called the hard component.

The high-energy nucleons—the knock-on protons and neutrons—produced by the primary-particle collisions and a few pion collisions proceed down into the atmosphere. They produce nuclear interactions of the same kind as the primary nuclei, though of course with diminished energies. This cascade process constitutes the nucleonic component of the secondary cosmic rays.
Solar modulation. The cosmic-ray intensity is lower during the years of high solar activity and sunspot number, which follow an 11-year cycle. This effect has been extensively studied with ground-based and spacecraft instruments.

The primary cause of solar modulation is the solar wind, a highly ionized gas (plasma) which boils off the solar corona and propagates radially from the Sun at a velocity of about 250 mi s (400 km/s). The wind is mostly hydrogen, with typical density of 80 protons per cubic inch (5 protons per cubic centimeter). This density is too low for collisions with cosmic rays to be important. Rather, the high conductivity of the medium traps part of the solar magnetic field and carries it outward.

In addition to the bulk sweeping action, another effect of great importance occurs in the solar wind, adiabatic deceleration. Because the wind is blowing out, only those particles which chance to move upstream fast enough are able to reach Earth. However, because of the expansion of the wind, particles interacting with it lose energy. Thus, particles observed at Earth with energy of 10 MeV per nucleon actually started out with several hundred MeV per nucleon in nearby interstellar space, and those with initial energy of only 100-200 MeV per nucleon probably never reach Earth at all.

Atomic bomb - The world is not ready ...

Atomic bomb - A device for suddenly producing an explosive neutron chain reaction in a fissile material such as uranium-235 (235U) or plutonium-239 (239Pu). In a wider sense, any explosive device that derives its energy from nuclear reactions, including not only the foregoing fission weapon but also a fusion weapon, which gets its energy largely from fusion reactions of heavy hydrogen isotopes, and a fission-fusion weapon, which derives its energy from both fission and fusion.

Because an atomic bomb derives its energy from nuclear reactions, it is properly called a nuclear explosive or nuclear weapon.
of the two principal fissile materials, the cheaper but less potent 235Uispresent in natural uranium usually in the proportion of 1 part to 139 parts of 238U and is separated from it by various enrichment processes. Weapons-grade plutonium is manufactured from 238Uina special military production reactor that has enough excess neutrons for the reaction below.

U + n - 239U (23-min half-life) 239Np(2.3-day half-life) 239Pu

Weapon cores are made of very high fractions of fissile materials: highly enriched 93% uranium-235 or weapon-grade 94% plutonium-239.

A fission bomb before ignition consists of a mass of fissile material and surrounding tamper—beryllium oxide or other reflector of neutrons intended ultimately to improve the neutron multiplication factor k—arranged in a geometry so favorable to neutron leakage that k is less than 1. These materials are suddenly compressed into a geometry where k substantially exceeds 1. This is done with chemical explosives that either implode a spherical subcritical mass of fission material or else drive two subcritical sections together in a gun-barrel type of arrangement. At the same time, enough neutrons are artificially introduced to start an explosively divergent (expanding) chain reaction. Fission-explosive devices intended for military application are highly sophisticated combinations of pure materials, precise design, and reliable electronics.

The explosive energy (yield) of a nuclear weapon is usually expressed in kilotons or megatons. A kiloton is the amount of energy liberated in the explosion of 1000 tons of TNT (1012 calories or 4.18 x 1012 J), and a megaton is a thousand times as large. The fission bombs that destroyed Hiroshima (gun-barrel type) and Nagasaki (implosion type) had estimated explosive yields of 13 and 22 kilotons, respectively. Fractional kiloton yields can be obtained (tactical nuclear weapons). Fission weapons have been tested up to approximately 500 kilotons, overlapping the yield of multistage fusion explosives (strategic nuclear weapons).

The nuclear explosive energy is communicated by mechanical shock and radiative transport to the surrounding water, earth, or air, ionizing it out to a radius which, in the case of explosions in air, is known as the fireball radius (150 yd or 140 m in about 1 s after a 20-kiloton nuclear explosion). Energy goes out from such a fireball into the surrounding relatively transparent air, in not very different orders of magnitude in the form of a shock wave and in the form of heat radiation that may continue for a number of seconds.

All about Geomagnetism

Geomagnetism The magnetism of the Earth; also, the branch of science that deals with the Earth's magnetism. Formerly called terrestrial magnetism, geomagnetism involves any topic pertaining to the magnetic field observed near the Earth's surface, within the Earth, and extending upward to the magne-tospheric boundary.

Modern usage of the term is generally confined to historically recorded observations to distinguish it from the sciences of archeomagnetism and paleomagnetism, which deal with the ancient magnetic field frozen respectively in arche-ological artifacts and geologic structures.The primary component of the magnetic field observed at the Earth's surface is caused by electric currents flowing in its liquid core, and is called the main field. Vectorially added to this component are the crustal field of magnetized rocks, transient variations imposed from external sources, and the field from electric currents induced in the Earth from these variations.The geomagnetic field is specified at any point by its vector F. Its direction is that of a magnetized needle perfectly balanced before it is magnetized, and freely pivoted about that point, when in equilibrium. The north pole of such a needle is the one that at most places on the Earth takes the more northerly position.

Over most of the Northern Hemisphere, that pole dips downward . The elements used to describe the vector F are H, the component of the vector projected onto a horizontal plane; its north and east components X and Y. respectively; Z the vertical component; F the magnitude of the vector F; the angles I, the dip of the field vector below the horizontal; and D the magnetic declination or deviation of the compass from geographic north. By convention, Z and I are positive downward, and D is positive eastward (or may be indicated as east or west of north). These elements can be related to each other by trigonometric equations.A magnetic pole is a location where the field is vertically aligned, H = 0. Due to the presence of sometimes strong (for example, >1000 nanoteslas) magnetic anomalies at the Earth's surface, there are a number of locations where the field is locally vertical.

However, those field components that extend to sufficient altitude to control charged particles can be accurately located by using the computations from a spherical harmonic expansion using degrees up to only about n = 10. Indeed, a pole can be defined by using only the main dipole (n = 1), or many terms.The n = 1 poles are sometimes referred to as the geomagnetic poles, and those computed using higher terms as dip poles. The term geomagnetic could also refer to the eccentric geomagnetic pole, which can be computed from n = 1 and n = 2 harmonics so as to be the best representation of a dipole offset from the center of the Earth. The latter has been used as a simplified field model at distances of 3 or 4 earth radii. Due to the more rapid fall-off of the higher terms with distance from the Earth, the two principal poles approach those of the n = 1 term with increasing altitude, until the distortions due to external effects begin to predominate.

The distribution of the dip angle I over the Earth's surface can be indicated on a globe or map by contours called isoclines, along which I is constant. The isocline for which I = 0 (where a balanced magnetized needle rests horizontal) is called the dip equator. The dip equator is geophysically important because there is a region in the ionospheric E layer in which small electric fields can produce a large electric current called the equatorial electrojet.
A magnetized compass needle can be weighted so as to rest and move in a horizontal plane at the latitudes for which it is designed, thus measuring the declination D. The lines on the Earth's surface along which D is constant are called isogonic lines or isogones. The compass points true geographic north on the agonic lines where D = 0. At nonpolar latitudes. D is a useful tool for marine and aircraft navigational reference. Indeed, isogones appear on navigation charts, electronic navigational aids are referenced to D, and airport runways are marked with D/10.A runway painted with the number 11 indicates that its direction has a compass heading of 110s. The compass needle becomes less reliable in polar regions because the horizontal component H becomes smaller as the magnetic poles are approached.

The intensity of the field can also be represented by maps, and the lines of equal intensity are called isodynamic lines. The dipole dominates the patterns of magnetic intensity on Earth in that the intensity is about double at the two poles compared to the value near the Equator. However, it can also be seen that the next terms of the spherical harmonic expansion also have a significant effect in that there is a second maximum in Siberia, and an area near Brazil that is weaker than any other. This so-called Brazilian anomaly allows charged particles trapped in the magnetic field to reach a low altitude and be lost by collisions with atmospheric gases. The highest intensity of this smooth field is about 70 microteslas near the south magnetic pole in Antarctica, and the weakest is about 23 microT near the coast of Brazil.

The term magnetic anomaly has become clearer than it was previously because it is recognized that the geomagnetic field has a continuous spectrum but with two distinct contributors. Originally, the term meant a field pattern that was very local in extent; the modern definition is that portion of the field whose origin is the Earth's crust. The sizes of the strong and easily observable features are generally up to only a few tens of kilometers. Their intensity ranges typically from a few hundred nanoteslas up to several thousand, and they are highly variable depending on the geology of the region.

The electromagnetic spectrum

The electromagnetic spectrum is the full range of electromagnetic radiation and includes radio waves, heat and light rays, X rays, and gamma rays.

Electromagnetic radiation is a form of energy that travels at the velocity of light. 186.000 mi/sec (300.000km/sec). As it travels, its energy switches back and forth between electric and magnetic fields. As one field increases in strength, the other decreases. The rate at which this exchange happens is called the frequency of the radiation. Different types of electromagnetic radiation have different frequencies. Radio waves have lower frequencies than light rays, for example, and blue light has a higher frequency than red. The frequency of electromagnetic radiation, measured in hertz (Hz), is the number 61 times in one second that the electrical field reaches its maximum value.

Scientists say that electromagnetic radiation travels in waves. This is because the strengths oi the electric and magnetic fields vary continually as they travel through space. The wavelength is the distance the wave travels in the time it takes the electric field to fall from its maximum value to its minimum value and then rise back to its maximum value. Because of this, the wavelength is the speed of light divided by the frequency. The signal from a radio station whose broadcast frequency is 1,200 kilohertz, or 1,200,000 Hz, has a wavelength of around 820 feet (250m), for example.

RADIO AND MICROWAVE

Radio stations broadcast using frequencies in a range from 150,000 Hz to around 20.000,000 Hz. Each station uses a particular frequency, so receivers tunc to a given station by only accepting waves at the correct
frequency for that station. Land-based television transmitters send signals between about 70 MHz and 800 MHz. (One megahertz is one million hertz.)
Satellite TV works at even higher frequencies. These electromagnetic waves arc captured by dish-shaped antennae that point toward the satellite.
Radars bounce radio waves off planes, ships, and clouds to show their positions, which can be many miles away. They use wavelengths of about 1 inch (2.5 cm). Doppler radar measures the speed of moving objects from the minute change in the frequency of the reflected waves.
Microwave ovens use wavelengths of a few millimeters, which correspond to frequencies of billions of hertz. The radiation heats food by causing water molecules to vibrate.

INFRARED LIGHT AND BEYOND

Infrared radiation has frequencies just lower than those of visible light. Its wavelength ranges from 1 millimeter to 750 nanometers. A nanometer, or 1 nm,
is one billionth of a meter, or 1/25,000,000 of an
inch. Hot objects give off infrared radiation, which is felt as heat. Visible light is the tiny part of the electromagnetic spectrum that human eyes can sense.The spectrum of colors stretches from red light at 770 nm to violet light at 400 nm.

The energy of electromagnetic radiation increases as the wavelength becomes shorter. Invisible ultraviolet rays cause sunburn and have shorter wavelengths (100-400 nm) than visible light.
X rays have even shorter wavelengths, usually less than the diameter of an atom (0.1 nm) .They penetrate flesh and bone can be used to produce images of cracks deep inside pieces of metal.
Reference : The Kingfisher Science Encyclopedia De Charles Taylor

Relativity Theory explained by Einstein

The principle of relativity in order to attain the greatest possible clearness , let us return to our example of the railway carriage supposed to be traveling uniformly.We call its motion a uniform translation ( "uniform" because it is of constant velocity and direction, "translation" because although the carriage changes its position relative to the embankment yet it does not rotate in so doing ).

Let us imagine a raven flying through the air in such a manner that its motion , as observed from the embankment , is uniform and in a straight line.If we were to observe the flying raven from the moving railway carriage.We should find that the motion of the raven would be one of different velocity and direction, but that it would still be uniform and in a straight line.
Expressed in an abstract manner we may say : If a mass m is moving uniformly in a straight line with respect to a co-ordinate system K, then it will also be moving uniformly and in a straight line relative to a second co-ordinate system K1 provided that the latter is executing a uniform translatory motion with respect to K. In accordance with the discussion contained in the preceding section , it follows that :If K is a Galileian co-ordinate system , the every other co-ordinate system K' is a Galileian one , when , in relation to K , it is in a condition of uniform motion of translation.Relative to K1 the mechanical laws of Galilei-Newton hold good exactly as they do with respect to K.

We advance a step farther in our generalization when we express the tenet thus: If , relative to K , K1 is a uniformly moving co-ordinate system devoid of rotation , then natural phenomena run their course with respect to K1 according to exactly the same general laws as with respect to K.This statement is called the principle of relativity ( in restricted sense).
As long as one was convinced that all natural phenomena were capable of representation with the help of classical mechanics , there was no need to doubt the validity of this principle of relativity.But in view of the more recent development of electrodynamics and optics it became more and more evident that classical mechanics affords an insufficient foundation for the physical description of all natural phenomena.

Reference : Relativity: the special and the general theory : a popular exposition
De Albert Einstein