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Directed Energy Weapons

Directed energy weapons

A directed-energy weapon is a type of energy weapon that directs energy in a particular direction by a means other than a projectile. It transfers energy to a target for a desired effect. Some of these weapons are real or practicable; some are science fiction. The energy is in various forms:-
- Electromagnetic radiation (typically lasers or masers).
- Particles with mass (particle beam weapons).
- In fictional weapons, undefined, or some sort of radiation or energetic particle that does not exist in the real world. See energy weapon for non-directional energy weapons. Some of these weapons are known as death rays or rayguns and are usually portrayed as projecting energy at a person or object in order to kill or destroy. The projected energy can come in many forms, such as a particle beam, laser, or radiation stream. Other forms are described later in this article. So far, weapons described as rayguns are all fictional (or non-functional toys and film props). Some lethal directed-energy weapons are under active research and development, but most examples of such weapons appear in science fiction. Types of directed-energy weapons are:-

Ordinary light

- A flashlight directs light in the visible spectrum but it operates at such low power as to be generally harmless.
- Some searchlights are bright enough to cause permanent or temporary blindness.
- There is said to exist a real non-lethal weapon that disorients a target by shooting disorienting lighting patterns at the eyes.

Lasers

Lasers are very well known in science fiction as a type of raygun. In the real world, lasers are often used for sighting, ranging and targeting for guns; but the laser beam is not the source of the weapon's firepower. There is research on real lasers as dazzling non-lethal weapons. Laser weapons usually generate brief high-energy pulses. A million joules delivered as a laser pulse is roughly the same energy as 200g of high explosive, and has the same basic effect on a target. The primary damage mechanism is mechanical shear, caused by reaction (like a rocket) when the surface of the target is explosively evaporated. Most existing weaponized lasers are gas dynamic lasers. Fuel, or a powerful turbine, pushes the lasing media through a circuit or series of orifices. The high-pressures and heating cause the medium to form a plasma and lase. A big difficulty with these systems is preserving the high-precision mirrors and windows of the laser resonating cavity. Most systems use a low-powered "oscillator" laser to generate a coherent wave, and then amplify it. Some experimental laser amplifiers do not use windows or mirrors, but have open orifices, which cannot be destroyed by high energies.

Problems with lasers

Blooming

Laser beams begin to cause plasma breakdown in air at power densities of around a mega joule per square centimeter. The laser beam is not visible, unless there is air, water or smoke present to scatter its light, or the heating of the air by the laser creates a path, or contrail, made of plasma. A laser beam can be effectively transmitted through the atmosphere a long distance without being absorbed because the air heated by the laser in the path of the beam becomes ionized plasma which undergoes thermodynamic expansion. This creates a plasma vacuum channel through the air that the beam passes through. In addition, some frequencies of laser light are not easily absorbed by air. There are several ways to inhibit blooming:
- The most promising is to distribute the beam over a large mirror that focuses the power on the target, to keep energy density in the air too low for blooming to happen. But this needs a large, very precise, very expensive, fragile mirror, mounted somewhat like a searchlight, requiring bulky machinery to slew the mirror to aim the laser.
- A phased array. For the usual laser wavelengths this method would need billions of micrometre-size antennas, and no way to make these is known. Phased arrays could theoretically also perform phase-conjugate amplification (see below).
- A very short pulse that finishes before blooming interferes.
- A phase-conjugate laser system. Here, a "finder" or "guide" laser illuminates the target. Any mirror-like ("specular") points on the target reflect light that is sensed by the weapon's primary amplifier. The weapon-power amplifier then amplifies inverted waves in a positive feedback loop, destroying the target with shockwaves as the specular regions evaporate. This avoids the blooming problem because the waves from the target passed through the blooming, and therefore show the most conductive optical path; this automatically corrects for the distortions caused by blooming. Experimental systems using this method usually use special chemicals to form a "phase conjugate mirror". In most systems, the mirror overheats dramatically at weaponized powers.
- Trying to induce a shockwave that evacuates the path between the target and the weapon. Without air in the laser's path, blooming will not occur. However, it is difficult to achieve the amount of power needed to blast the air out of the way.

Evaporated target material shading the target

Another problem with weaponized lasers is that the evaporated material from the surface of the target begins to shade the surface. There are several approaches to this problem:
- One is to induce a standing shockwave in the ablation cloud. The shockwave then continues to perform damage.
- Another scheme is to scan the target faster than the shockwave.
- Another theoretical possibility is to induce plasmic optical mixing at the target. In this scheme, the transparency of the target's ablation cloud to one laser is modulated by another laser, perhaps by tuning the laser to the absorption spectra of the ablation cloud, and inducing population inversion in the cloud. The other laser then induces local lasing in the ablation cloud. The beat frequency that results can induce frequencies that penetrate the ablation cloud.

High power consumption

The major drawback of lasers is their high power consumption, limited range especially in inclement weather, and high production cost. Their future use in non-lethal weapon systems seems limited due to the generally perceived cruel nature of victim incapacitation. Despite this, hybrid technologies such as the electrolaser continue to see development, by companies such as [http://www.ionatron.com/ Ionatron] and [http://www.xtremeads.com/ Xtreme Alternative Defense Systems] though both systems suffer from extremely limited range. Some weapons of this type are already in testing for deployment as battlefield anti-missile weapons or riot control devices, and research and development is increasing. One reason that many directed-energy weapons are currently fictional is due to the likely energy requirements. Existing methods of storing, conducting, transforming and directing energy are inadequate to produce a convenient hand-held weapon. Existing energy weapons, such as lasers, need much energy, and also need much cooling equipment because they are relatively inefficient and waste much energy as heat. The speculated weapons might need to fire so much energy to get in reality the effects described in fiction, that their energy storage and conversion systems would need to be almost 100% efficient. This problem is offset in chemical lasers by using energy released in a suitable chemical reaction instead of electrical energy. Chemical oxygen iodine laser and deuterium fluoride laser are two examples of laser types capable of megawatt-range output of a continious beam. The energy is provided in the first case by reaction of hydrogen peroxide with chlorine, in the second case by reaction of atomic fluorine with deuterium.

Electrolaser

An electrolaser lets blooming occur, and then sends a powerful electric shock down the conducting ionized track of plasma so formed, somewhat like lightning. It functions as a giant ultra-high energy long-distance version of the Taser or Stun gun. Alternating current is sent through a series of step-up transformers, increasing the voltage and decreasing the amperage. The final voltage may be between 10⁸ and 10⁹ volts. This current is fed into the laser beam. To complete the electric circuit, there should be either a second laser beam, or a ground return from the target to the last transformer in the step-up series. The target receives a gigavolt electric shock. Anybody hit by this electric arc is electrocuted. Any electric or electronic devices in the target may be seriously damaged, disabled or destroyed. For an electrolaser to work, there must be air or some other gas between the gun and the target. The only defenses against electron particle beam weapons are magnetic fields, electrical insulators, capacitors, electrostatic fields, and Faraday cages. Electrolasers often occur in science fiction and videogames.
- In 1985 the U.S. Navy tested an electrolaser. Its targets were missiles and aircraft. This device was known as the Phoenix project within the Stategic Defense Initiative research program. It was first proved by experiment at long range in 1985.
- Xtreme Alternative Defense Systems in Anderson, Indiana is developing a rifle-sized electrolaser for the U.S. Marines. It will incapacitate men and pre-detonate improvised explosives.

Microwave lasers

Microwave lasers are called masers. Microwave guns powerful enough to injure humans are possible.
- Active Denial System is a microwave laser, to heat the water in the target's skin and thus cause incapacitating pain. Being developed by the Air Force Research Laboratory in New Mexico by researchers working with Raytheon for riot-control duty in Iraq. While intended to cause severe pain while leaving no lasting damage, there has yet to be testing for long-term side effects of exposure to the microwave beam. It can destroy electronics.

Low-powered lasers

There is an imitation shotgun which fires a low-powered laser beam at a target which is covered with reflective 90° corners designed to send the beam back where it came from to be detected by a detector on the gun. This is only for target practice without using up ammunition; it has the disadvantage (for a shotgun user) that the beam travels instantaneously and straight without teaching the shooter to allow for the effects of wind deflecting the fired shot and the target moving while the shot travels.

THEL

THEL is a real high-energy laser system that can shoot down missiles. It is a deuterium fluoride laser.

Airborne Laser

The U.S. Air Force's Airborne Laser, or Airborne tactical laser, is a plan to mount a CO2 gas laser or COIL chemical laser on a modified Boeing 747 and use it to shoot down missiles.

Ultraviolet laser

HSV Technologies of San Diego is developing a laser weapon to paralyze animals (testing for later use on humans) by an electric charge generated by the laser beam. It is described as an ultraviolet laser and not an electrolaser.

Tactical considerations

Lasers have two advantages:
- They can hit whatever they see, at the speed of light.
- Some lasers run on electricity, which can be cheaply generated, reducing the need for expensive ammunition. Since lasers can defeat artillery and missile attacks, any group fielding an effective laser system will gain decisive advantages in ground, air and space combat. Under radar control, lasers have shot artillery shells in flight, including mortar rounds. This suggests that a primary application of lasers should be as part of a defensive system. Before a projectile can hit a target, it must become visible to the target. The main difficulty with currently practical lasers is the high expense and fragility of their mirrors and mirror-pointing systems. Some believe that mirrors or other countermeasures can reduce the effectiveness of high energy lasers. This has not been demonstrated. Small defects in mirrors absorb energy, and the defects rapidly expand across the surface. Protective mirroring on the outside of a target is liable to damage and getting dirty, much more than a mirror shut away inside a laser's mechanism.

Electric beam in a vacuum

In a vacuum (e.g. in space), an electric discharge can travel a potentially unlimited distance at a velocity slightly slower than the speed of light. This is because there is no significant electric resistance to the flow of electric current in a vacuum. This would make such devices useful to fry the electrical and electronic parts of satellites and spacecraft. However, in a vacuum the electric current cannot ride a laser beam, and some other means must be used to keep the electron beam on track and to prevent it from dispersing: see particle beam.

Particle beam weapons

Particle beam weapons include charged and neutral, endoatmospheric and exoatmospheric. Particle beams as beam weapons are theoretically possible, but practical weapons have not been demonstrated. Certain types of particle beams have the advantage of being self-focusing in the atmosphere. Blooming is not limited to lasers, but is also a problem in particle beam weapons. Energy that would otherwise be focused on the target spreads out; the beam becomes less effective.
- Thermal blooming occurs in both charged and neutral particle beams, and occurs when particles bump into one another under the effects of thermal vibration, or bump into air molecules. It is likely that a particle beam (except electrolasers) fired into air will make merely a short hot flame like a blowtorch.
- Electrical blooming occurs only in charged particle beams, as ions of like charge repel one another.

Plasma weapons

Plasma weapons fire a beam or bolt of plasma, which is excited matter consisting of electrons and also protons or nuclei. Examples are:
- The MARAUDER (Magnetically Accelerated Ring to Achieve Ultra-high Directed Energy and Radiation). See [http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=7369133 this link] for more details; the antiaircraft potential of such a system is mentioned.
- [http://www.forteantimes.com/articles/163_ballsoffire.shtml This article] explains theories about ball lightning, which may be a type of plasma, which if weaponized could produce beam weapons guided in the same sense as an ATGM The plasma rifle is a staple of science fiction. There may have been influence from the real plasma torch used to cut metal.

Pulse rifle

The pulse rifle is a fictional weapon occasionally used in science fiction. In some of its occurrences, (e.g. Aliens) it is a projectile weapon. The rifle in Aliens was said to use caseless ammunition, which is a current-generation technology used in actual weapons such as the Heckler & Koch G11 rifle. Caseless ammunition propels a bullet down a rifled barrel as a result of the combustion of gunpowder, just as regular cased ammunition does. Thus, this sort of "pulse rifle" is not a directed-energy weapon. In some of its incarnations (e.g. the Lost in Space movie) the "pulse rifle" is a directed-energy weapon. whose mode of operation has not been conclusively and consistently explained.

Sonic and ultrasonic beam weapons

See Sonic weaponry.

Rayguns: directed-energy weapons in fiction

See Raygun.

History

Mythology

Before modern technology developed, many mythologies described gods or demons using weapons that make lightning, such as Jupiter's and Zeus's thunderbolt, Thor's hammer Mjolnir, and the Hindu god Indra's spear (the vajrā).

Ancient inventors

According to mythology, the concept of the "burning mirror" or death ray began with Archimedes who created such a mirror with an adjustable focal length to track and set fire to the Roman fleet as it invaded Syracuse. Historians, however, acknowledge that the earliest accounts of the battle did not mention a "burning mirror" and that only Archimedes' ingenuity combined with a way to hurl fire were relevant to the victory. A Byzantine writer hundreds of years later is suggested to have imagined this 2200-year-old death ray, which is attributed to Archimedes. While the MythBusters television program was unsuccessful at creating this "death ray", others have since successfully built other versions. An experiment by students at MIT showed that a mirror-based weapon was at least possible, though the practicality of such a weapon was not established. A Greek called Diocles supposedly invented the parabolic mirror.

Grindell-Matthews

After the astonishing technological advancement during World War I, many such schemes began to appear credible. Harry Grindell-Matthews tried to sell such a ray to the British Air Ministry after that war. He failed to appear to demonstrate his apparatus, however. It was apparently taken to France but has not resurfaced, leading to various conspiracy theory ideas about what might have happened to it, or who might have developed it later. Radar may be a by-product of this research.

Tesla

Nikola Tesla generated the first large scale artificial electric lightning discharges during the 1890s. He invented Tesla coils, transformers, and alternating electric current generators. He was an early pioneer of electronic radio and television technology. He worked on a real Death ray in the early 1900's. He designed it in 1942, and offered the US War Department the secrets of his "teleforce" weapon on January 5 1943, but was assumed to be crazy. Tesla then offered his device to several European countries. Records which recently turned up in Russia showed that his proposed "death ray" was based on a narrow stream of atomic clusters of liquid mercury or tungsten accelerated by high voltage, probably produced by a huge Tesla Coil. If so, this weapon can be classed as a particle beam or as an electromagnetic-powered projectile gun, depending on the size of the projectiles. When he died in 1943, a prototype compact version of the "death ray" called an "Anti-Tank gun" was in a trunk in the basement of his hotel. Immediately after he died, a Russian spy raided the room and the safe containing the schematics of the "death ray". The FBI never found any of the important parts of the schematics nor the trunk with the prototype, as far as we know. Schematics of the projector nozzle have surfaced, though. The U. S. Government classified his electron particle beam weapon design as secret until the 1980s. One of Nicola Tesla's favorite things to do was to watch the natural lightning produced by thunderstorms from his high rise apartment building in New York. See the document Ventura, Timothy, "[http://www.americanantigravity.com/graphics/tesla/Tesla-Death-Ray-Reconstruction.pdf Tesla Death Ray Reconstruction]", 1994. It is in PDF format. It theorizes that Tesla's weapon fired a powerful beam of a special sort of electromagnetic radiation which caused the target to pick up a high electric charge, which then attracted destructive lightning-type electric arcs from the area around. The document is purely hypothetical with no experimentation. Tesla theorized on the use of UV light to ionize a plasma path through the air to a target for his electron particle beam weapon. However, UV lasers were not invented until the 1960s.

Nazis

In the later phases of WWII, Nazi Germany put its hopes on research for technologically revolutionary secret weapons. Through the 1930s the atomic bomb and radiological weapon were proposed, and the related, more selective, surgical idea of a death ray was probably more appealing than wanton and horrific destruction by such means. This belief intensified in and after 1945 after Hiroshima and Nagasaki proved the undesirable physical and political fallout of such weapons of mass destruction. This Nazi research included searching in India in the hope that some of the powerful weapons and flying craft described in the Mahabharata were the real products of a supposed ancient technologization, rather than mythology based on lightning and other destructive natural forces.

Star Wars

In the 1980s, Ronald Reagan revived the idea as a matter for public funding with his Strategic Defense Initiative program, which was immediately nicknamed "Star Wars", due to its objective to put weapons in space. Lasers could destroy ICBMs in flight. The program had limited success but there were numerous attempts to find practical death ray technologies. It is not clear whether this was part of a general plan to facilitate the collapse of the Soviet Union by misdirecting the Soviets into investing in research that had no practical outputs (this was a common Cold War strategy on both sides). Enthusiasm for these ideas, and the arms race they implied, waned in the 1990s. By this point, science fiction was more interested in the very real potential of personal-scale biological warfare, chemical warfare, robots, artificial intelligence and nanotechnology to kill selected individuals - without necessarily having to come directly into their sights to do so. The Project for the New American Century, for instance, noted that genetically-selective plagues might become a politically useful tool.

THEL

Research proceeded, however, and by 2003, this led to the Tactical High Energy Laser project, a joint research project of Israel and the US, has demonstrated a weaponized laser which can shoot down aircraft and missiles.

Energy devices which may be confused with directed-energy weapons

The term electroshock gun includes two sorts of weapons, but neither of these is a directed-energy weapon, despite its name:-
- Electric shock prod: it administers an electric shock on contact. It is not strictly a gun, as it does not cause any effect at a distance.
- Guns which fire an electrified projectile. The real thermic lance is not a gun. Occasionally science fiction authors misuse the name "thermic lance" to mean a raygun.

External links

- [http://www.wired.com/news/technology/0,1282,68152,00.html?tw=wn_7techhead Wired News (AP) article on weapons deployment in Iraq, Active Denial System and Stunstrike, July 10, 2005]
- [http://www.wired.com/news/technology/0,1282,64437,00.html?tw=newsletter_topstories_html Wired News article "Weapons Freeze, Microwave Enemies"] (and copied in at least 661 other web pages including [http://www.foxnews.com/story/0,2933,127763,00.html this link])
- [http://web.mit.edu/2.009/www/lectures/10_ArchimedesResult.html Archimedes Death Ray: Idea Feasibility Testing] Category:Science fiction weapons Category:Energy weapons Category:Star Wars weapons Category:Less-lethal weapons Category:Fictional weapons Category:Physics in fiction Category:Electromagnetic radiation ja:殺人光線

Energy weapon

Many types of real and fictional weapon which emit energy, rather than a physical projectile, fire it in one direction, and so can be classed as guns: see Directed-energy weapon for a full description of them. Among omni-directional energy weapons are:-
- Explosives, e.g. grenades, bombs.
- Reportedly, electromagnetic bombs which deliver a wide-area electromagnetic pulse are in production by various militaries.

Patents

- - Electric whaling apparatus - Albert Sonnenburg and Philipp Rechten
- - Electric weapon - Thomas D. Ryan

Projectile

A projectile is any object sent through space by the application of a force. In a general sense, even a football or baseball may be considered a projectile, but in practice most projectiles are designed as weapons.

Motive force

Arrows, darts, spears, and similar weapons are fired using pure mechanical force applied by another solid object; conversely, other weapons use the compression or expansion of gases as their motive force. Blowguns and pneumatic rifles use compressed gases, while most other guns and firearms utilize expanding gases liberated by sudden chemical reactions. Light gas guns use a combination of these mechanisms. Railguns provide a constant acceleration along the entire length of the device, greatly increasing the muzzle velocity. Some projectiles provide propulsion during (part of) the flight by means of a rocket engine or jet engine. In military terminology, a rocket is unguided, while a missile is guided. Note the two meanings of "rocket": an ICBM is a missile with rocket engines.

Blunt or sharp

Although blowguns use small darts, most types of guns and firearms hurl bullets, pellets, or shot made of a metal, usually lead, that are designed to deform and fragment inside a target, causing significant damage. Items like arrows, hand darts, and spears are generally tipped with sharp metallic or lithic artifacts called projectile points that allow them to more easily penetrate a target, although some types of arrows used for hunting are designed to stun or kill through shock rather than to penetrate. Projectiles designed to be non-lethal, for example for use against riots, include rubber bullets and flexible baton rounds.

Kinetic projectiles

Some projectiles do not contain an explosive charge (as opposed to projectiles with explosive charge, such as shells). They are termed kinetic projectile, kinetic energy weapon or kinetic penetrator. The classic kinetic energy weapon is the bullet. Among projectiles which do not contain explosives are railguns, mass drivers, and kinetic energy penetrators, in addition to smaller weapons such as bullets. All of these weapons work by attaining a high muzzle velocity (hypervelocity), and collide with their objective, releasing kinetic energy. Some kinetic weapons for targeting objects in spaceflight are anti-satellite weapons and anti-ballistic missiles. Since they need to attain a high velocity anyway, they can destroy their target with their released kinetic energy alone; high explosive is not strictly necessary. Compare the energy of TNT, 4.6 MJ/kg, to the energy of a kinetic kill vehicle with a closing speed of 10 km/s, which is 50 MJ/kg. This saves costly weight and there is no detonation to be done at the right time, but on the other hand it requires a more accurate hit. With regard to anti-missile weapons, the Arrow missile and MIM-104 Patriot have explosives, but of the KEI, LEAP, and THAAD being developed, none has (see Missile Defense Agency). See also Hypervelocity terminal ballistics, Exoatmospheric Kill Vehicle (EKV). For an idea for kinetic projectiles from Earth orbit, see kinetic bombardment. For a fictional kinetic weapon, see Relativistic kill vehicle.

Miscellaneous

Ballistics analyses the projectile trajectory, the forces acting upon the projectile, and the impact that a projectile has on a target. A guided missile is not called a projectile. An explosion, whether or not by a weapon, causes debris to act as projectiles. An explosive weapon may be designed to produce shrapnel. The term projectile also refers to weapons or any other objects thrown, shot or otherwise directed to enemies in video games or computer games.

Science Fiction

Science fiction

Electromagnetic spectrum

s
SX = Soft X-Rays
EUV = Extreme ultraviolet
NUV = Near ultraviolet
Visible light
NIR = Near infrared
MIR = Moderate infrared
FIR = Far infrared

Radio waves:
EHF = Extremely high frequency (Microwaves)
SHF = Super high frequency (Microwaves)
UHF = Ultrahigh frequency
VHF = Very high frequency
HF = High frequency
MF = Medium frequency
LF = Low frequency
VLF = Very low frequency
VF = Voice frequency
ELF = Extremely low frequency]] The electromagnetic spectrum is the range of all possible electromagnetic radiation. Also, the "electromagnetic spectrum" (usually just spectrum) of an object is the range of electromagnetic radiation that it emits, reflects, or transmits. The electromagnetic spectrum, shown in the table, extends from frequencies used in the electric power grid (at the long-wavelength end) to gamma radiation (at the short-wavelength end), covering wavelengths from thousands of kilometres down to fractions of the size of an atom, though in principle the spectrum is actually infinite. Electromagnetic energy at a particular wavelength λ (in vacuum) has an associated frequency ν and photon energy E. Thus, the electromagnetic spectrum may be expressed equally well in terms of any of these three quantities. They are related according to the equations: :

\lambda = \frac  \,\!

and :

E=h\nu \,\!

where:
- c is the speed of light, 299792458 m/s

(c \approx 3 \cdot 10^8 \ \mbox/\mbox = 300,000 \ \mbox/\mbox)

.
- h is Planck's constant,

(h \approx 6.626069 \cdot 10^ \ \mbox \cdot \mbox \approx 4.13567 \ \mathrm \mbox/\mbox)

Spectra of objects

Nearly all objects in the universe emit, reflect and/or transmit some light. The distribution of this light along the electromagnetic spectrum (called the spectrum of the object) is determined by the object's composition. Several types of spectra can be distinguished depending upon the nature of the radiation coming from an object:
- If the spectrum is composed primarily of thermal radiation emitted by the object itself, an emission spectrum occurs.
- Some bodies emit light more or less according to the blackbody spectrum.
- If the spectrum is composed of background light, parts of which the object transmits and parts of which it absorbs, an absorption spectrum occurs. Electromagnetic spectroscopy is the branch of physics that deals with the characterization of matter by its spectra.

Classification systems

While the classification scheme is generally accurate, in reality there is often some overlap between neighboring types of electromagnetic energy. For example, SLF radio waves at 60 Hz may be received and studied by astronomers, or may be ducted along wires as electric power. Also, some low-energy gamma rays actually have a longer wavelength than some high-energy X-rays. This is possible because "gamma ray" is the name given to the photons generated from nuclear decay or other nuclear and subnuclear processes, whereas X-rays on the other hand are generated by electronic transitions involving highly energetic inner electrons. Therefore the distinction between gamma ray and X-ray is related to the radiation source rather than the radiation wavelength. Generally, nuclear transitions are much more energetic than electronic transitions, so usually, gamma-rays are more energetic than X-rays. However, there are a few low-energy nuclear transitions (e.g. the 14.4 keV nuclear transition of Fe-57) that produce gamma rays that are less energetic than some of the higher energy X-rays. Use of the radio frequency spectrum is regulated by governments. This is called frequency allocation.

Electric energy

Electrical energy covers the low-frequency, long-wavelength end of the spectrum. The radiation is usually ducted along 2-wire and 3-wire transmission lines and sent to various devices besides antennas. At zero frequency the energy is emitted by batteries and DC power supplies, while at 50 Hz and 60 Hz it is produced by rotary magnetic generators and ducted through the international power grids. At frequencies between 20 Hz to 30 kHz the EM energy is translated to and from acoustic energy and is distributed over telephone lines, as well as being used to operate loudspeakers for public address or in music systems. Note that other than its frequency, there is no functional difference between the VHF energy guided along a television coaxial cable, versus the 60 Hz travelling along the cord leading to a light bulb. When connected to the appropriate antenna, both will radiate into space.

Radio frequency

telephone lines Radio waves generally are utilized by antennas of appropriate size, with wavelengths ranging from hundreds of meters to about one millimeter. They are used for transmission of data, via modulation. Television, mobile phones, wireless networking and amateur radio all use radio waves.

Microwaves

The super high frequency (SHF) and extremely high frequency (EHF) of Microwaves come next. Microwaves are waves which are typically short enough to employ tubular metal waveguides of reasonable diameter. Microwave energy is produced with klystron and magnetron tubes, and with solid state diodes such as Gunn and IMPATT devices. Microwaves are absorbed by molecules that have a dipole moment in liquids. In a microwave oven, this effect is used to heat food. Low-intensity microwave radiation is used in Wi-Fi. It should be noted that an average microwave oven in active condition is, in close range, powerful enough to cause interference with poorly shielded electromagnetic fields such as those found in mobile medical devices and cheap consumer electronics.

Terahertz radiation

This is a region of the light spectrum between far infrared and microwaves. Until recently, the range was rarely studied and few sources existed for microwave energy at the high end of the band (sub-millimeter waves or so-called terahertz waves), but applications are now appearing. The proposed WiMAX standard for wireless networking, a long-range enhancement of Wi-Fi, lies within this region. Scientists are also looking to apply Terahertz technology in the armed forces, where high frequency waves will be sent at enemy troops to incapacitate them.

Infrared radiation

The infrared part of the electromagnetic spectrum covers the range from roughly 300 GHz (1 mm) to 400 THz (750 nm). It can be divided into three parts:
- Far-infrared, from 300 GHz (1 mm) to 30 THz (10 μm). The lower part of this range may also be called microwaves. This radiation is typically absorbed by so-called rotational modes in gas-phase molecules, by molecular motions in liquids, and by phonons in solids. The water in the Earth's atmosphere absorbs so strongly in this range that it renders the atmosphere effectively opaque. However, there are certain wavelength ranges ("windows") within the opaque range which allow partial transmission, and can be used for astronomy. The wavelength range from approximately 200 μm up to a few mm is often referred to as "sub-millimeter" in astronomy, reserving far infrared for wavelengths below 200 μm.
- Mid-infrared, from 30 to 120 THz (10 to 2.5 μm). Hot objects (black-body radiators) can radiate strongly in this range. It is absorbed by molecular vibrations, that is, when the different atoms in a molecule vibrate around their equilibrium positions. This range is sometimes called the fingerprint region since the mid-infrared absorption spectrum of a compound is very specific for that compound.
- Near-infrared, from 120 to 400 THz (2,500 to 750 nm). Physical processes that are relevant for this range are similar to those for visible light.

Visible radiation (light)

Color	Wavelength interval	Frequency interval
violet	~ 380 to 430 nm	~ 790 to 700 THz
blue	~ 430 to 500 nm	~ 700 to 600 THz
cyan	~ 500 to 520 nm	~ 600 to 580 THz
green	~ 520 to 565 nm	~ 580 to 530 THz
yellow	~ 565 to 590 nm	~ 530 to 510 THz
orange	~ 590 to 625 nm	~ 510 to 480 THz
red	~ 625 to 740 nm	~ 480 to 405 THz
Continuous spectrum Image:Spectrum441pxWithnm.png The spectrum of visible light Designed for monitors with gamma 1.5.

After infrared comes visible light. This is the range in which the sun and stars similar to it emit most of their radiation. It is probably not a coincidence that the human eye is sensitive to the wavelengths that the sun emits most strongly. Visible light (and near-infrared light) is typically absorbed and emitted by electrons in molecules and atoms that move from one energy level to another. The light we see with our eyes is really a very small portion of the electromagnetic spectrum. A rainbow shows the optical (visible) part of the electromagnetic spectrum; infrared (if you could see it) would be located just beyond the red side of the rainbow with ultraviolet appearing just beyond the violet end.

Ultraviolet light

Next comes ultraviolet. This is radiation whose wavelength is shorter than the violet end of the visible spectrum. Being very energetic, UV can break chemical bonds, make molecules unusually reactive or ionize them, in general changing their mutual behavior. Sunburn, for example, is caused by the disruptive effects of UV radiation on skin cells, which can even cause skin cancer, if the radiation damages the complex DNA molecules in the cells (UV radiation is a proven mutagen). The Sun emits a large amount of UV radiation, which could quickly turn Earth into a barren desert, but most of it is absorbed by the atmosphere's ozone layer before reaching the surface.

X-rays

After UV come X-rays. Hard X-rays are of shorter wavelengths than soft X-rays. X-rays are used for seeing through some things and not others, as well as for high-energy physics and astronomy. Neutron stars and accretion disks around black holes emit X-rays, which enable us to study them.

Gamma rays

After hard X-rays come gamma rays. These are the most energetic photons, having no lower limit to their wavelength. They are useful to astronomers in the study of high-energy objects or regions and find a use with physicists thanks to their penetrative ability and their production from radioisotopes. The wavelength of gamma rays can be measured with high accuracy by means of Compton scattering. Note that there are no defined boundaries between the types of electromagnetic radiation. Some wavelengths have a mixture of the properties of two regions of the spectrum. For example, red light resembles infra-red radiation in that it can resonate some chemical bonds.

External links

- [http://www.ntia.doc.gov/osmhome/allochrt.html U.S. Frequency Allocation Chart] - Covering the range 3 kHz to 300 GHz (from Department of Commerce)
- [http://strategis.ic.gc.ca/epic/internet/insmt-gst.nsf/vwapj/spectallocation.pdf/%24FILE/spectallocation.pdf Canadian Table of Frequency Allocations] (from Industry Canada)
- [http://www.ofcom.org.uk/static/archive/ra/topics/spectrum-strat/future/strat02/strategy02app_b.pdf UK frequency allocation table] (from Ofcom, which inherited the Radiocommunications Agency's duties, pdf format)
- [http://www.scienceofspectroscopy.info The Science of Spectroscopy] - supported by NASA, includes OpenSpectrum, a Wiki-based learning tool for spectroscopy that anyone can edit
- [http://www.e-builds.com/EM%20spectrum/ An EM Spectrum Overview in Flash] by e-builds ja:電磁スペクトル

Maser

A maser is a device that produces coherent electromagnetic waves through amplification due to stimulated emission. Historically the term came from the acronym "microwave amplification by stimulated emission of radiation", although modern masers emit over a broad portion of the electromagnetic spectrum. This has lead some to replace "microwave" with "molecular" in the acronym, as suggested by Townes [http://nobelprize.org/physics/laureates/1964/townes-lecture.pdf]. When optical coherent oscillators were first developed, they were called optical masers, but it has become more common to refer to these as lasers. See the section on terminology below for more on this.

History

Theoretically, the principle of the maser was described by Nikolay Basov and Alexander Prokhorov from Lebedev Institute of Physics at an All-Union Conference on Radio-Spectroscopy held by USSR Academy of Sciences in May 1952. They subsequently published their results in October 1954. Independently, Charles H. Townes, J. P. Gordon, and H. J. Zeiger built the first maser at Columbia University in 1953. The device used stimulated emission in a stream of energised ammonia molecules to produce amplification of microwaves at a frequency of 24 gigahertz. Townes later worked with Arthur L. Schawlow to describe the principle of the optical maser, or laser, which Theodore H. Maiman first demonstrated in 1960. For their research in this field Townes, Basov and Prokhorov were awarded the Nobel Prize in Physics in 1964.

Technology

The maser is based on the principle of stimulated emission proposed by Albert Einstein in 1917. When atoms have been put into an excited energy state, they can amplify radiation at the proper frequency. By putting such an amplifying medium in a resonant cavity, feedback is created that can produce coherent radiation. Stimulated microwave and radio wave emission is observed in astronomy, and this is usually called "masing", even in the absence of the resonant feedback that would be required for a true maser. Technically this form of stimulated emission is called superradiant emission, and it is closely associated with lasing and masing. Such emission is observed from water (H₂O), hydroxyl ion (HO⁻), methanol (CH₃OH), formaldehyde (CH₂O), and silicon monoxide (SiO).

Some common types of masers

Atomic beam masers
- Ammonia maser
- Hydrogen maser Gas masers
- Rubidium maser Solid State masers
- Ruby maser The dual noble gas maser [http://cfa-www.harvard.edu/Walsworth/Activities/DNGM/old-DNGM.html] is an example of a masing medium which is nonpolar.

Uses

Masers serve as high precision frequency references. These "atomic frequency standards" are one form of atomic clock. They are also used as electronic amplifiers in radio telescopes. Maser-like stimulated emission also occurs in nature in interstellar space. Water molecules in star-forming regions can undergo a population inversion and emit radiation at 22 GHz, creating the brightest spectral line in the radio universe. Some water masers also emit radiation from a vibrational mode at 96 GHz.

Hydrogen maser

Today, the most important type of maser is the hydrogen maser which is currently used as an atomic frequency standard. Together with other types of atomic clocks, they constitute the "Temps Atomic International" or TAI. This is the international time scale, which is coordinated by the Bureau International des Poids et Mesures, or BIPM. It was Norman Ramsey and his colleagues who first realized this device. Today's masers are identical to the original design. The maser oscillation relies on stimulated emission between two hyperfine levels of atomic hydrogen. Here is a brief description of how it works:
- First, a beam of atomic hydrogen is produced. This is done by submitting the gas at low pressure to a RF discharge.
- The next step is "state selection"—in order to get some stimulated emission, it is necessary to create a population inversion of the atoms. This is done in a way that is very similar to the famous Stern-Gerlach experiment. After passing through an aperture and a magnetic field, many of the atoms in the beam are left in the upper energy level of the lasing transition. From this state, the atoms can decay to the lower state and emit some microwave radiation.
- A high quality factor microwave cavity confines the microwaves and reinjects them repeatedly into the atom beam. The stimulated emission amplifies the microwaves on each pass through the beam. This combination of amplification and feedback is what defines all oscillators. The resonant frequency of the microwave cavity is exactly tuned to the hyperfine structure of hydrogen: 1.420405751 GHz.
- A small fraction of the signal in the microwave cavity is coupled into a coaxial cable and then sent to a coherent receiver.
- The microwave signal coming out of the maser is very weak (a few pW) and the frequency is extremely stable but can not be changed. The coherent receiver is used to amplify the signal and change the frequency. This is done using a series of phase-locked loops and a high performance quartz oscillator.

Terminology

The meaning of the term maser has changed slightly since its introduction. Initially the acronym was universally given as "microwave amplification by stimulated emission of radiation," which described devices which emitted in the microwave region of the electromagnetic spectrum. The principle of stimulated emission has since been extended to more devices and frequencies, and so the original acronym is sometimes modified, as suggested by Charles H. Townes [http://nobelprize.org/physics/laureates/1964/townes-lecture.pdf], to "molecular amplification by stimulated emission of radiation." Molecular is used here in the sense of kinetic theory, where the base element of a kinetic system is a molecule, even if it happens to be monatomic. This should not be confused with the usage of the term in the molecular sciences, where it refers to a bound state comprising two or more atoms. Initially, visible light oscillators based on stimulated emission were called optical masers, but this terminology has become uncommon. It is more conventional now to refer to devices that emit in the X-ray through infrared portions of the spectrum as lasers, and devices that emit in the microwave region and below as masers. There is some debate over whether maser or laser is the correct generic term for all devices that produce coherent electromagnetic waves through stimulated emission. Distinct names were originally proposed for devices that emit in each portion of the spectrum, including grasers (gamma ray lasers), xasers (x-ray lasers), uvasers (ultraviolet lasers), lasers (visible lasers), irasers (infrared lasers), masers (microwave masers), and rasers (rf masers). Most of these terms never caught on, however, and all have now become obsolete except for maser and laser.

Masers in science fiction

Masers are the most recognizable weapon in the Godzilla series and Toho's other monster movies. Maser tanks are often deployed against monsters. The maser tank fires a bolt of electricity, presumably created by amplified microwaves. As in most science fiction, the science behind the maser tank is dubious and not supported by real world physics. The maser tank is also present as a unit in Outpost 2. It is one of the weaker units. In the film Batman Begins (2005), the villain Ra's Al Ghul tries to destroy Gotham City using a device he refers to as a "focused microwave emitter," which seems to be some sort of maser. Masers are used predominantly as weaponry, both from spaceships and by ground troops in Peter F. Hamilton's 'Night's Dawn' universe. Arvin Sloane refers to a maser in the show Alias (season 3, episode 7) during a private meeting with the CIA.

External links

- [http://arxiv.org/find/grp_physics/1/ti:+maser/0/1/0/all/0/1?per_page=100; arXiv.org search for "maser"]
- [http://cfa-www.harvard.edu/Walsworth/Activities/DNGM/old-DNGM.html Noble gas Maser]
- [http://cfa-www.harvard.edu/hmc/ The Red Shift (Hydrogen Maser Clock) Experiment]

References

- J.R. Singer, Masers, John Whiley and Sons Inc., 1959.
- J. Vanier, C. Audoin, The Quantum Physics of Atomic Frequency Standards, Adam Hilger, Bristol, 1989. Category:Lasers Category:Optical devices Category:Microwave technology ja:メーザー

Particles

In physics, a particle is an object, or body, with only a few degrees-of-freedom, including position, and perhaps orientation in space. The homogeneity and isotropy of space, being symmetries with respect to translation and rotation, may provide for particle properties such as momentum and angular momentum. These are common to classical mechanics and quantum mechanics. Certain internal degrees-of-freedom provide fixed physical properties, such as (electric) charge, or variable ones, such as spin angular momentum, most of which can only be understood within quantum mechanics.

Mass

:For other senses of this word, see mass (disambiguation). Mass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. Unlike weight, the mass of something stays the same regardless of location. It is a central concept of classical mechanics and related subjects. Strictly speaking, there are three different quantities called mass:
- Inertial mass is a measure of an object's inertia: its resistance to changing its state of motion when a force is applied. An object with small inertial mass changes its motion more readily, and an object with large inertial mass does so less readily.
- Passive gravitational mass is a measure of the strength of an object's interaction with the gravitational field. Within the same gravitational field, an object with a smaller passive gravitational mass experiences a smaller force than an object with a larger passive gravitational mass. (This force is called the weight of the object. In informal usage, the word "weight" is often used synonymously with "mass", because the strength of the gravitational field is roughly constant everywhere on the surface of the Earth. In physics, the two terms are distinct: an object will have a larger weight if it is placed in a stronger gravitational field, but its passive gravitational mass remains unchanged.)
- Active gravitational mass is a measure of the strength of the gravitational field due to a particular object. For example, the gravitational field that one experiences on the Moon is weaker than that of the Earth because the Moon has less active gravitational mass.

Introduction

Although inertial mass, passive gravitational mass and active gravitational mass are conceptually distinct, no experiment has ever unambiguously demonstrated any difference between them. One of the consequences of the equivalence of inertial mass and passive gravitational mass is the fact, famously demonstrated by Galileo Galilei, that objects with different masses fall at the same rate, assuming factors like air resistance are negligible. The theory of general relativity, the most accurate theory of gravitation known to physicists to date, rests on the assumption that inertial and passive gravitational mass are completely equivalent. This is known as the weak equivalence principle. Classically, active and passive gravitational mass were equivalent as a consequence of Newton's third law, but a new axiom is required in the context of relativity's reformulation of gravity and mechanics. Thus, standard general relativity also assumes the equivalence of inertial mass and active gravitational mass; this equivalence is sometimes called the strong equivalence principle. If one were to treat inertial mass m_i, passive gravitational mass m_p, and active gravitational mass m_a distinctly, Newton's law of universal gravitation would give as force on the second mass due to the first mass :

m_a_2=\frac.

Newton's third law, of reciprocal actions, shows that active and passive mass are proportional. As a result they can be defined to be equal.

Units of mass

In the SI system of units, mass is measured in kilograms (kg). Many other units of mass are also employed, such as: grams (g), metric tons, pounds, ounces, long and short tons, quintals, slugs, atomic mass units, Planck masses, solar masses, and eV/c². The eV/c² unit is based on the electron volt (eV), which is normally used as a unit of energy. However, because of the relativistic connection between (rest) mass and energy, E = mc² (see below), it is possible to use any unit of energy as a unit of mass instead. Thus, in particle physics where mass and energy are often interchanged, it is common to use not only eV/c² but even simply eV as a unit of mass (roughly 1.783 × 10^-36 kg). Because the gravitational acceleration is approximately constant on the surface of the Earth, a unit like the pound is often used to measure either mass or force (e.g. weight), although the pound is officially defined as a unit of mass. For more information on the different units of mass, see Orders of magnitude (mass).

Inertial mass

Inertial mass is the mass of an object measured by its resistance to acceleration. To understand what the inertial mass of a body is, one begins with classical mechanics and Newton's Laws of Motion. Later on, we will see how our classical definition of mass must be altered if we take into consideration the theory of special relativity, which is more accurate than classical mechanics. However, the implications of special relativity will not change the meaning of "mass" in any essential way. According to Newton's second law, we say that a body has a mass m if, at any instant of time, it obeys the equation of motion :

F = \frac (mv)

where F is the force acting on the body and v is its velocity. For the moment, we will put aside the question of what "force acting on the body" actually means. Now, suppose that the mass of the body in question is a constant. This assumption, known as the conservation of mass, rests on the ideas that (i) mass is a measure of the amount of matter contained in a body, and (ii) matter can never be created or destroyed, only split up or recombined. These are very reasonable assumptions for everyday objects, though, as we will see, the situation gets more complicated when we take special relativity into account. Another point to note is that, even in classical mechanics, it is sometimes useful to treat the mass of an object as changing with time. For example, the mass of a rocket decreases as the rocket fires. However, this is an approximation, based on ignoring pieces of matter which enter or leave the system. In the case of the rocket, these pieces correspond to the ejected propellent; if we were to measure the total mass of the rocket and its propellent, we would find that it is conserved. When the mass of a body is constant, Newton's second law becomes :

F = m \frac = m a

where a denotes the acceleration of the body. This equation illustrates how mass relates to the inertia of a body. Consider two objects with different masses. If we apply an identical force to each, the object with a bigger mass will experience a smaller acceleration, and the object with a smaller mass will experience a bigger acceleration. We might say that the larger mass exerts a greater "resistance" to changing its state of motion in response to the force. However, this notion of applying "identical" forces to different objects brings us back to the fact that we have not really defined what a force is. We can sidestep this difficulty with the help of Newton's third law, which states that if one object exerts a force on a second object, it will experience an equal and opposite force. To be precise, suppose we have two objects A and B, with constant inertial masses m_A and m_B. We isolate the two objects from all other physical influences, so that the only forces present are the force exerted on A by B, which we denote F_AB, and the force exerted on B by A, which we denote F_BA. As we have seen, Newton's second law states that :

F_ = m_A a_A \,

and

F_ = m_B a_B \,

where a_A and a_B are the accelerations of A and B respectively. Suppose that these accelerations are non-zero, so that the forces between the two objects are non-zero. This occurs, for example, if the two objects are in the process of colliding with one another. Newton's third law then states that :

F_ = - F_ \,

Substituting this into the previous equations, we obtain :

m_A = - \frac \, m_B

Note that our requirement that a_A be non-zero ensures that the fraction is well-defined. This is, in principle, how we would measure the inertial mass of an object. We choose a "reference" object and define its mass m_B as (say) 1 kilogram. Then we can measure the mass of every other object in the universe by colliding it with the reference object and measuring the accelerations.

Gravitational mass

Gravitational mass is the mass of an object measured using the effect of a gravitational field on the object. The concept of gravitational mass rests on Newton's law of gravitation. Let us suppose we have two objects A and B, separated by a distance |r_AB|. The law of gravitation states that if A and B have gravitational masses M_A and M_B respectively, then each object exerts a gravitational force on the other, of magnitude :

|F| =

where G is the universal gravitational constant. The above statement may be reformulated in the following way: if g is the acceleration of a reference mass at a given location in a gravitational field, then the gravitational force on an object with gravitational mass M is :

F = Mg \,

This is the basis by which masses are determined by weighing. In simple bathroom scales, for example, the force F is proportionate to the displacement of the spring beneath the weighing pan (see Hooke's law), and the scales are calibrated to take g into account, allowing the mass M to be read off.

Equivalence of inertial and gravitational masses

The equivalence of inertial and gravitational masses is sometimes referred to as the Galilean equivalence principle or weak equivalence principle. The most important consequence of this equivalence principle applies to freely falling objects. Suppose we have an object with inertial and gravitational masses m and M respectively. If the only force acting on the object comes from a gravitational field g, combining Newton's second law and the gravitational law yields the acceleration :

K = \frac g

This says that the ratio of gravitational to inertial mass of any object is equal to some constant K if and only if all objects fall at the same rate in a given gravitational field. This phenomenon is referred to as the universality of free-fall. (In addition, the constant K can be taken to be 1 by defining our units appropriately.) The first experiments demonstrating the universality of free-fall were conducted by Galileo. It is commonly stated that Galileo obtained his results by dropping objects from the Leaning Tower of Pisa, but this is unlikely to be true; actually, he performed his experiments with balls rolling down inclined planes. Increasingly precise experiments have been performed, such as those performed by Roland Eötvös, using the torsion balance pendulum, in 1889. To date, no deviation from universality, and thus from Galilean equivalence, has ever been found. More precise experimental efforts are still being carried out. It should be noted that the universality of free fall only applies to systems in which gravity is the only force acting. All other forces, especially friction and air resistance, must be absent or at least negligible. For example, if a hammer and a feather are dropped from the same height, we all know that the feather will take much longer to reach the ground. This happens because the feather is not really in free fall: the force of air resistance on it is about as strong as the force of gravity. On the other hand, if the experiment is performed in a vacuum, where there is no air resistance, the hammer and the feather should fall at the same rate and reach the ground together. This demonstration was, in fact, carried out in 1971 during the Apollo 15 Moon walk, by Commander David Scott. A stronger version of the equivalence principle, known as the Einstein equivalence principle or the strong equivalence principle, lies at the heart of the general theory of relativity. Einstein's equivalence principle states that it is impossible to distinguish between a uniform acceleration and a uniform gravitational field. Thus, the theory postulates that inertial and gravitational masses are fundamentally the same thing. All of the predictions of general relativity, such as the curvature of spacetime, are ultimately derived from this principle.

Relativistic relation among mass, energy and momentum

Special relativity is a necessary extension of classical physics. In particular, special relativity succeeds where classical mechanics fails badly in describing objects moving at speeds close to the speed of light. In relativistic mechanics, the mass (m) of a free particle is related to its energy (E) and momentum (p) by the equation :

\frac = m^2 c^2 + p^2

. where c is the speed of light. This is sometimes referred to as the mass-energy-momentum relation. The first thing to notice about this equation is that it can cope with massless objects (m = 0), for which it reduces to :

E = pc \,

In classical mechanics, massless objects are an ill-defined concept, since applying any force to one would produce, via Newton's second law, an infinite acceleration - a nonsensical result. In relativistic mechanics, they are objects that are always traveling at the speed of light; an example being light itself, in the form of photons. The above equation says that the energy carried by a massless object is directly proportional to its momentum. Let us now consider objects with non-zero mass. For these, the quantity m has a simple physical meaning: it is the inertial mass of the object as measured in its rest frame, the frame of reference in which its velocity is zero. (Note: massless objects do not possess a rest frame; they are moving at the speed of light in any frame of reference.) The way we would measure m is exactly the same as in classical mechanics, which we described above: bouncing it off a reference object and measuring the accelerations. As long as the velocity of each object remains much smaller than the speed of light during this procedure, relativistic corrections to classical mechanics will be utterly negligible. In the rest frame, the velocity is zero, and thus so is the momentum p. The mass-energy-momentum relation thus reduces to :

E = mc^2 \,

which states that the energy of an object as measured in its rest frame - its "rest energy" - is equal to its mass times the square of the speed of light. Some books follow this up by stating that "mass and energy are equivalent", but this is somewhat misleading. The mass of an object, as we have defined it, is a quantity intrinsic to the object, and independent of our current frame of reference. The energy E, on the other hand, varies with the frame of reference; if the frame is moving at a high velocity relative to the object, E will be very large, simply because the object has a lot of kinetic energy in that frame. Thus, E = mc² is not a "good" relativistic statement; it is true only in the rest frame of the object. Some authors define a quantity known as the relativistic mass, which is basically the quantity E/c². This makes the "equivalence" of "mass" and energy true by definition, though neither quantity is frame-independent! "Relativistic mass" was used in many early writings on relativity, and it is still used in books for laymen as well as introductory physics classes. However, the concept is downplayed or discouraged by many physicists nowadays, for reasons explained in the article on relativistic mass. Following the modern usage, whenever we refer to "mass" in this article we always mean the rest mass, unless otherwise identified. Having defined the mass of an object, let us look at how it behaves when not at rest. We can arrange the mass-energy-momentum relation in the following way: :

E = mc^2 \sqrt

When the momentum p is much smaller than mc, we can Taylor expand the square root, with the result :

E = mc^2 +  + \cdots

The leading term, which is the largest, is of course the rest energy. The object always has this minimum amount of energy, regardless of its momentum. The second term is the classical expression for the kinetic energy of the particle, and the higher-order terms are basically relativistic corrections for the kinetic energy. Under normal circumstances, the rest energy of an object is inaccessible, in the sense that it cannot be used to do mechanical work. When the object hits something, it can do work by transferring its momentum, and thus its kinetic energy, to whatever it hit. However, the rest energy depends only on the mass of the object, which does not change during collisions, so it cannot be transferred along with the kinetic energy. On the other hand, it is possible to access the rest energy using processes that split or combine particles. The reason is that mass, as we have defined it, is not conserved during such processes. The simplest example is the process of electron-positron annihilation, in which an electron and a positron annihilate each other to produce a pair of photons: the electron and positron both have non-zero mass, but the photons are massless. Other examples include nuclear fusion and nuclear fission. Metabolism, fire and other exothermic chemical processes also convert mass to energy, however the mass change from these is negligible. Energy, unlike mass, is always conserved in special relativity, so, roughly speaking, what is happening in these reactions is that the rest energy of the reactants is being transformed into the kinetic energy of the reaction products. The fact that rest energy can be liberated in this way is one of the most important predictions of special relativity.

References

- R.V. Eötvös et al, Ann. Phys. (Leipzig) 68 11 (1922)

External links

- [http://math.ucr.edu/home/baez/physics Usenet Physics FAQ]
- [http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html Does mass change with velocity?]
- [http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html Does light have mass?]
- [http://www.teleles.nl/pdf/total_artikel.pdf Mass & energy]
- [http://www.geocities.com/physic1525/inertiaenergy.html The law of the inertia of the energy and the speed of the gravity. See chapter 3 The energy has mass ]
- [http://www.geocities.com/physics_world/stp/title.htm Dialog: Use and abuse of the concept of mass (from Spacetime physics by Edwin F. Taylor and John A. Wheeler)]
- [http://arxiv.org/PS_cache/physics/pdf/0111/0111134.pdf Photons, Clocks, Gravity and the Concept of Mass by L.B.Okun]
- [http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_15_feather_drop.html The Apollo 15 Hammer-Feather Drop]
- [http://calc.skyrocket.de/en/ Online Unit Converter - Conversion of many different units] Category:Physical chemistry Category:Classical mechanics
- ko:질량 ms:Jisim ja:質量 simple:Mass th:มวล

Energy weapon

Patents

- - Electric whaling apparatus - Albert Sonnenburg and Philipp Rechten
- - Electric weapon - Thomas D. Ryan

Particle beam

A particle beam is an accelerated stream of atoms or subatomic particles (often moving at very near the speed of light) directed by magnets and focused by lenses. Subatomic particles as electrons, positrons, and protons can be accelerated to high velocities and energies, usually expressed in terms of center-of-mass energy, by machines that impart energy to the particles in small stages or nudges, ultimately achieving in this way very high energy particle beams, measured in terms of billions and even trillions of electron volts. Thus, in terms of their scale, particles can be made to perform as powerful missiles for bombarding other particles in a target substance or for colliding with each other as they assume intersecting orbits. A charged particle is drawn forward by a magnetic field with a charge the opposite of the particle (like charges repel one another, opposites attract); as the particle passes through a series of magnetic fields in sequence, each accelerates it until the charged particle is moving at a high speed. A natural analogy to particle beams is lightning, where electrons flow from negatively charged clouds to positively charged clouds or the earth.

Particle Beams as Weapons

Though particle beams are perhaps most famously employed as weapon systems in Science Fiction, the U.S. Advanced Research Projects Agency (now called DARPA) started work on particle beam weapons as early as 1958 [http://www.airpower.maxwell.af.mil/airchronicles/aureview/1984/jul-aug/roberds.html], two years before the first scientific demonstration of Lasers. The general idea of particle-beam weaponry is to hit a target object with a stream of accelerated particles moving at near the speed of light and therefore carrying tremendous kinetic energy; the particles transfer their kinetic energy to the atoms in the molecules of the target upon striking, much as a cue ball transfers its energy to the racked balls in billiards, thus exciting the target's atoms and superheating the target object in such a short time that it explodes. Currently, the materials for such weapons are "high-risk" and may not be developed for some time. [http://www.airpower.maxwell.af.mil/airchronicles/aureview/1984/jul-aug/roberds.html]. Particle cannons are not likely to be used in a near future conflict as the power needed to project such a highly powered beam surpasses the production capabilities of any standard battlefield powerplant. This beam weapon is undoubtedly also very hard not to notice; particles travelling near the speed of light are not usually seen on a battlefield. It may be possible to use particle beams as part of the Strategic Defense Initiative (dubbed "Star Wars"), but the problem of a viable power source still stands, even more so in space. It may be possible in the future with possible fusion generators, but this technology is not expected to be perfected and be in mass use for several decades. If perfected this could be a new horizon in conventional weaponry, being easily as dangerous as any kinetic weapon, and effectively making any substance it comes into contact with an explosive. Particle weapons are often overlooked when compared to lasers, even though they have some advantages over their more well known counterpart. Because the particle beam is in fact millions of tiny projectiles, it transmits its kinetic energy to the atomic structure of the target making it difficult to protect a target from it. This obviously makes the weapon very destructive, and highly lethal to anyone it hits. However, it is easier to deflect a particle beam if one posseses powerful magnetic or electromagnetic fields, but at the speeds the beam would travel, one cannot be sure how effective these countermeasures would be. Also, the atmosphere would reduce the beam strength very quickly, because the air molecules would slow down and scatter the particles. Only actual testing will prove how effective particle beams would really be for military applications.

Radiation

Radiation can refer to one of the following:
- Alpha radiation
- Beta radiation
- Gamma radiation
- Delta radiation
- Epsilon radiation
- Neutron radiation
- Cherenkov radiation, radiation by a particle moving through an insulating medium faster than the speed of light in that medium.
- Electromagnetic radiation, a stream of photons of a variety of different energies.
- Ionizing radiation, a stream of particles with sufficient energy to cause ionization.
- Gravitational radiation, a predicted consequence of general relativity.
- Non-ionizing radiation, electromagnetic radiation that does not carry enough energy to ionize living material.
- Particle radiation, any kind of radiation in which the individual elements behave like particles.
- Synchrotron radiation, the emission of radiation by a charged particle undergoing acceleration.
- Thermal radiation, the process by which a hot object emits electromagnetic radiation.
- Radiant energy, radiation emitted by a source into the surrounding environment.
- Adaptive radiation, in evolutionary biology, a process by which one species becomes many in order to adapt to specific ecological niches. In fiction, radiation can also refer to:
- Theta radiation and Omicron radiation, which are found in Star Trek

Fictional

, were the goddesses of charm, beauty, nature, human creativity and fertility in Greek mythology.]] Fiction is storytelling of imagined events and stands in contrast to non-fiction, which makes factual claims about reality. A large part of the appeal of fiction is its ability to evoke the entire spectrum of human emotions: to distract our minds, to give us hope in times of despair, to make us laugh, or to let us experience empathy without attachment. Fictional works—novels, stories, fairy tales, fables, films, comics, interactive fiction—may be partly based on factual occurrences but always contain some imaginary content. The term is also often used synonymously with fictional prose. In this sense, fiction refers only to novels or short stories and is often divided into two categories, popular fiction (e.g., science fiction or mystery fiction) and literary fiction (e.g., Victor Hugo or William Faulkner). Fiction is largely perceived as a form of art and/or entertainment, although not all fiction is necessarily artistic. Fiction may be created for the purpose of educating, such as fictional examples used in school textbooks. Fiction is also frequently instrumentalized by propaganda and advertising. Fiction may be propagated by parents to their children out of tradition (e.g. Santa Claus) or in order to instill certain beliefs and values. Fables with an explicit moral goal are not necessarily targeted at children, however. Fiction may over time blend with factual accounts and develop into mythology. Many atheists perceive religion as no different from any fictional tale, whereas members of religious groups typically explain their beliefs with faith and claim they are fundamentally different from fictional tales (although they may call other religious views fictional). The sociological school of constructivism argues that every view of reality is fundamentally a construction of the self and that a safe distinction between fact and fiction is impossible, whereas the philosophy of naturalism holds that reality can be approximated and truth can be demonstrated through usefulness, allowing the distinction from fiction. Fiction has often been the target of censorship or boycotts, escalating into book burnings or bans. Extremist regimes like the Taliban have been even more prohibitive, restricting all reading to religious texts. There is an ongoing debate regarding sexual content in fiction and whether or not juveniles can be safely exposed to it; opponents of fiction with sexual content typically label it pornography. The Internet has had a massive impact on the distribution of fiction, calling into question the feasibility of copyright as a means to ensure royalties are payed to copyright holders. Also digital libraries such as Project_Gutenberg have come into being which make public domain texts more readily available. The combination of inexpensive home computers, the Internet and the creativity of its users has also led to new forms of fiction, such as interactive computer games or computer-generated comics. Countless forums for fan fiction can be found online, where loyal followers of specific fictional realms create and distribute derivative stories. Through open writing systems like wikis, collaboratively written fiction is also becoming possible (see the [http://meta.wikipedia.org/wiki/Wikifiction Wikifiction] initiative). Fiction is a fundamental part of human culture, and the ability to create fiction and other artistic works is frequently cited as one of the defining characteristics of humanity.

Elements of fiction

- antagonists
- conflicts
- climax
- characters
- plots
- protagonists
- resolution
- structures
- subplots
- themes
- fictional character
- suspension of disbelief

External links

- [http://book.awardannals.com/genre/fiction/ Most Honored Fiction] at the Book Award Annals
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- ja:フィクション

Film prop

Prop has multiple meanings:
- For carriable items used in performances, see theatrical properties.
- For bladed propulsion devices, see propeller.
- For "prop forward", see rugby union positions.
- For various referenda named using "prop" or "proposition", see referendum.
- To give props to somebody means to give "proper respect" to him/her; for example, "I've got to give props to Dave for the way he handled that situation."

Weapons research

The research of weapons and weapon capabilities. A good example of weapon research is the Manhattan Project. Much of the ongoing research is conducted at, or through the auspices of, institutions of higher learning such as the University of California, which operates (as it has for sixty years) the Los Alamos National Laboratory (LANL). Category: Military

Flashlight

:"Flashlight" is the NATO designation for the Yakovlev Yak-25 Soviet military jet. A flashlight or torch is a hand-held portable electric spotlight. It is known as a flashlight mainly in the United States and Canada and as a torch in most Commonwealth countries. A typical flashlight consists of a small electric lightbulb with associated parabolic reflector, powered by electric batteries, and with an electric power switch. The components are mounted in a housing that contains the necessary electric circuit and provides ease of handling, a means of access to the batteries for replacement, and a clear covering over the lightbulb for its protection. Although a relatively simple device, its invention did not occur until the late 19th century because it depended upon the earlier invention of the electric battery and electric light. The batteries in the first ones were of such short useful life that the common method of operating them was to flash them just long enough to discern the environs, and only as needed; hence the term "flash-light". electric light Recently, flashlights which use light-emitting diodes (LEDs) instead of conventional lightbulbs have become available. LEDs are far more efficient, and use less power than normal lightbulbs. Such torches have longer battery lifetimes. LEDs also survive sharp blows that often break conventional lightbulbs. Another innovation in flashlight design is the headlamp, a flashlight worn on the head for hands-free operation. Powerful headlamps mounted on helmets have been used in mining for decades, but general-purpose ones with fabric straps are now also available. Most flashlights are cylindrical in design, with the lamp assembly attached to one end. However, early designs came in a variety of shapes. Many resembled lanterns of the day, consisting largely of a box with a handle and the lamp attached to the front. Some others were made to have a similar appearance to candles. It is possible that future developments of battery and LED technology will bring interesting new designs. For instance, one very small light that exists now in 2004 consists of a few LEDs with a switch, designed to be an endcap for a 9-volt battery.