The Kelvin scale is an absolute thermodynamic temperature scale using as its null point absolute zero, the temperature at which all thermal motion ceases in the classical description of thermodynamics. The kelvin is the base unit of temperature in the International System of Units; until 2018, the kelvin was defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. In other words, it was defined such that the triple point of water is 273.16 K. On 16 November 2018, a new definition was adopted, in terms of a fixed value of the Boltzmann constant. For legal metrology purposes, the new definition will come into force on 20 May 2019; the Kelvin scale is named after the Belfast-born, Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin, who wrote of the need for an "absolute thermometric scale". Unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to or written as a degree; the kelvin is the primary unit of temperature measurement in the physical sciences, but is used in conjunction with the degree Celsius, which has the same magnitude.
The definition implies that absolute zero is equivalent to −273.15 °C. In 1848, William Thomson, made Lord Kelvin, wrote in his paper, On an Absolute Thermometric Scale, of the need for a scale whereby "infinite cold" was the scale's null point, which used the degree Celsius for its unit increment. Kelvin calculated; this absolute scale is known today as the Kelvin thermodynamic temperature scale. Kelvin's value of "−273" was the negative reciprocal of 0.00366—the accepted expansion coefficient of gas per degree Celsius relative to the ice point, giving a remarkable consistency to the accepted value. In 1954, Resolution 3 of the 10th General Conference on Weights and Measures gave the Kelvin scale its modern definition by designating the triple point of water as its second defining point and assigned its temperature to 273.16 kelvins. In 1967/1968, Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree Kelvin", symbol °K. Furthermore, feeling it useful to more explicitly define the magnitude of the unit increment, the 13th CGPM held in Resolution 4 that "The kelvin, unit of thermodynamic temperature, is equal to the fraction 1/273.16 of the thermodynamic temperature of the triple point of water."In 2005, the Comité International des Poids et Mesures, a committee of the CGPM, affirmed that for the purposes of delineating the temperature of the triple point of water, the definition of the Kelvin thermodynamic temperature scale would refer to water having an isotopic composition specified as Vienna Standard Mean Ocean Water.
In 2018, Resolution A of the 26th CGPM adopted a significant redefinition of SI base units which included redefining the Kelvin in terms of a fixed value for the Boltzmann constant of 1.380649×10−23 J/K. When spelled out or spoken, the unit is pluralised using the same grammatical rules as for other SI units such as the volt or ohm; when reference is made to the "Kelvin scale", the word "kelvin"—which is a noun—functions adjectivally to modify the noun "scale" and is capitalized. As with most other SI unit symbols there is a space between the kelvin symbol. Before the 13th CGPM in 1967–1968, the unit kelvin was called a "degree", the same as with the other temperature scales at the time, it was distinguished from the other scales with either the adjective suffix "Kelvin" or with "absolute" and its symbol was °K. The latter term, the unit's official name from 1948 until 1954, was ambiguous since it could be interpreted as referring to the Rankine scale. Before the 13th CGPM, the plural form was "degrees absolute".
The 13th CGPM changed the unit name to "kelvin". The omission of "degree" indicates that it is not relative to an arbitrary reference point like the Celsius and Fahrenheit scales, but rather an absolute unit of measure which can be manipulated algebraically. In science and engineering, degrees Celsius and kelvins are used in the same article, where absolute temperatures are given in degrees Celsius, but temperature intervals are given in kelvins. E.g. "its measured value was 0.01028 °C with an uncertainty of 60 µK." This practice is permissible because the degree Celsius is a special name for the kelvin for use in expressing relative temperatures, the magnitude of the degree Celsius is equal to that of the kelvin. Notwithstanding that the official endorsement provided by Resolution 3 of the 13th CGPM states "a temperature interval may be expressed in degrees Celsius", the practice of using both °C and K is widespread throughout the scientific world; the use of SI prefixed forms of the degree Celsius to express a temperature interval has not been adopted.
In 2005 the CIPM embarked on a programme to redefine the kelvin using a more experimentally rigorous methodology. In particular, the committee proposed redefining the kelvin such that Boltzmann's constant takes the exact value 1.3806505×10−23 J/K. The committee had hoped tha
Nobel Prize in Physics
The Nobel Prize in Physics is a yearly award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions for humankind in the field of physics. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the first Nobel Prize in Physics was awarded to physicist Wilhelm Röntgen in recognition of the extraordinary services he rendered by the discovery of the remarkable rays. This award is administered by the Nobel Foundation and regarded as the most prestigious award that a scientist can receive in physics, it is presented in Stockholm at an annual ceremony on 10 December, the anniversary of Nobel's death. Through 2018, a total of 209 individuals have been awarded the prize. Only three women have won the Nobel Prize in Physics: Marie Curie in 1903, Maria Goeppert Mayer in 1963, Donna Strickland in 2018. Alfred Nobel, in his last will and testament, stated that his wealth be used to create a series of prizes for those who confer the "greatest benefit on mankind" in the fields of physics, peace, physiology or medicine, literature.
Though Nobel wrote several wills during his lifetime, the last one was written a year before he died and was signed at the Swedish-Norwegian Club in Paris on 27 November 1895. Nobel bequeathed 94% of his total assets, 31 million Swedish kronor, to establish and endow the five Nobel Prizes. Due to the level of skepticism surrounding the will, it was not until April 26, 1897 that it was approved by the Storting; the executors of his will were Ragnar Sohlman and Rudolf Lilljequist, who formed the Nobel Foundation to take care of Nobel's fortune and organise the prizes. The members of the Norwegian Nobel Committee who were to award the Peace Prize were appointed shortly after the will was approved; the prize-awarding organisations followed: the Karolinska Institutet on June 7, the Swedish Academy on June 9, the Royal Swedish Academy of Sciences on June 11. The Nobel Foundation reached an agreement on guidelines for how the Nobel Prize should be awarded. In 1900, the Nobel Foundation's newly created statutes were promulgated by King Oscar II.
According to Nobel's will, The Royal Swedish Academy of sciences were to award the Prize in Physics. A maximum of three Nobel laureates and two different works may be selected for the Nobel Prize in Physics. Compared with other Nobel Prizes, the nomination and selection process for the prize in Physics is long and rigorous; this is a key reason why it has grown in importance over the years to become the most important prize in Physics. The Nobel laureates are selected by the Nobel Committee for Physics, a Nobel Committee that consists of five members elected by The Royal Swedish Academy of Sciences. In the first stage that begins in September, around 3,000 people – selected university professors, Nobel Laureates in Physics and Chemistry, etc. – are sent confidential forms to nominate candidates. The completed nomination forms arrive at the Nobel Committee no than 31 January of the following year; these nominees are scrutinized and discussed by experts who narrow it to fifteen names. The committee submits a report with recommendations on the final candidates into the Academy, where, in the Physics Class, it is further discussed.
The Academy makes the final selection of the Laureates in Physics through a majority vote. The names of the nominees are never publicly announced, neither are they told that they have been considered for the prize. Nomination records are sealed for fifty years. While posthumous nominations are not permitted, awards can be made if the individual died in the months between the decision of the prize committee and the ceremony in December. Prior to 1974, posthumous awards were permitted; the rules for the Nobel Prize in Physics require that the significance of achievements being recognized has been "tested by time". In practice, it means that the lag between the discovery and the award is on the order of 20 years and can be much longer. For example, half of the 1983 Nobel Prize in Physics was awarded to Subrahmanyan Chandrasekhar for his work on stellar structure and evolution, done during the 1930s; as a downside of this approach, not all scientists live long enough for their work to be recognized.
Some important scientific discoveries are never considered for a prize, as the discoverers die by the time the impact of their work is appreciated. A Physics Nobel Prize laureate earns a gold medal, a diploma bearing a citation, a sum of money; the Nobel Prize medals, minted by Myntverket in Sweden and the Mint of Norway since 1902, are registered trademarks of the Nobel Foundation. Each medal has an image of Alfred Nobel in left profile on the obverse; the Nobel Prize medals for Physics, Physiology or Medicine, Literature have identical obverses, showing the image of Alfred Nobel and the years of his birth and death. Nobel's portrait appears on the obverse of the Nobel Peace Prize medal and the Medal for the Prize in Economics, but with a different design; the image on the reverse of a medal varies according to the institution awarding the prize. The reverse sides of the Nobel Prize medals for Chemistry and Physics share the same design of Nature, as a Goddess, whose veil is held up by the Genius of Science.
These medals and the ones for Physiology/Medicine and Literature were designed by Erik Lindberg in 1902. Nobel laureates receive a diploma directly from the hands of the
A nitrogen-vacancy center is one of numerous point defects in diamond. Its most explored and useful property is photoluminescence, which can be detected from an individual N-V center those in the negative charge state. Electron spins at N-V centers, localized at atomic scales, can be manipulated at room temperature by applying a magnetic field, electric field, microwave radiation or light, or a combination, resulting in sharp resonances in the intensity and wavelength of the photoluminescence; these resonances can be explained in terms of electron spin related phenomena such as quantum entanglement, spin-orbit interaction and Rabi oscillations, analysed using advanced quantum optics theory. An individual N-V center can be viewed as a basic unit of a quantum computer, it has potential applications in novel, more efficient fields of electronics and computational science including quantum cryptography and masers; the nitrogen-vacancy center is a point defect in the diamond lattice. It consists of a nearest-neighbor pair of a nitrogen atom, which substitutes for a carbon atom, a lattice vacancy.
Two charge states of this defect, neutral N-V0 and negative N-V−, are known from spectroscopic studies using optical absorption, photoluminescence, electron paramagnetic resonance and optically detected magnetic resonance, which can be viewed as a hybrid of PL and EPR. A nitrogen atom has five valence electrons. Three of them covalently bond to the carbon atoms and two remain non-bonded and are called a lone pair; the vacancy has three unpaired electrons. Two of them make one remains unpaired; the overall symmetry, however, is axial. The N-V0 is paramagnetic. However, despite extensive efforts, electron paramagnetic resonance signals from N-V0 avoided detection for decades until 2008. Optical excitation is required to bring the N-V0 defect into the EPR-detectable excited state; the N-V0 centers can be converted into N-V− by changing the Fermi level position. This can be achieved by applying external voltage to a p-n junction made from doped diamond, e.g. in a Schottky diode. In the negative charge state N-V−, an extra electron is located at the vacancy site forming a spin S=1 pair with one of the vacancy electrons.
As in N-V0, the vacancy electrons are "exchanging roles" preserving the overall trigonal symmetry. This N-V− state is what is and somewhat incorrectly, called "the nitrogen-vacancy center"; the neutral state has not yet been explored for spin manipulations. The N-V centers are randomly oriented within a diamond crystal. Ion implantation techniques can enable their artificial creation in predetermined positions. Nitrogen-vacancy centers are produced from single substitutional nitrogen centers by irradiation followed by annealing at temperatures above 700 °C. A wide range of high-energy particles are suitable for such irradiation, including electrons, neutrons and gamma photons. Irradiation produces lattice vacancies; those vacancies are immobile at room temperature, annealing is required to move them. Single substitutional nitrogen produces strain in the diamond lattice. During chemical vapor deposition of diamond, a small fraction of single substitutional nitrogen impurity traps vacancies generated as a result of the plasma synthesis.
Such nitrogen-vacancy centers are preferentially aligned to the growth direction. Diamond is notorious for having a large lattice strain. Strain splits and shifts optical transitions from individual centers resulting in broad lines in the ensembles of centers. Special care is taken to produce sharp N-V lines required for most experiments: high-quality, pure natural or better synthetic diamonds are selected. Many of them have sufficient concentrations of grown-in N-V centers and are suitable for applications. If not, they are annealed. Selection of a certain irradiation dose allows tuning the concentration of produced N-V centers such that individual N-V centers are separated by micrometre-large distances. Individual N-V centers can be studied with standard optical microscopes or, near-field scanning optical microscopes having sub-micrometre resolution. N-V− centers emit bright red light which can be conveniently excited by visible light sources, such as argon or krypton lasers, frequency doubled Nd:YAG lasers, dye lasers, or He-Ne lasers.
Excitation can be achieved at energies below that of zero phonon emission. Laser illumination, however converts some N-V− into N-V0 centers. Emission is quick. At room temperature, no sharp peaks are observed because of the thermal broadening. However, cooling the N-V− centers with liquid nitrogen or liquid helium narrows the lines down to a width of a few megahertz. An important property of the luminescence from individual N-V− centers is its high temporal stability. Whereas many single-molecular emitters bleach after emission of 106–108 photons, no bleaching is observed for the N-V centers at room temperature; because of these properties, the ideal technique to address the N-V centers is confocal microscopy, both at room temperature and at low temperature. In particular, low temperature operation is required to address only the zero-phonon line
The noble gases make up a group of chemical elements with similar properties. The six noble gases that occur are helium, argon, krypton and the radioactive radon. Oganesson is variously predicted to be a noble gas as well or to break the trend due to relativistic effects. For the first six periods of the periodic table, the noble gases are the members of group 18. Noble gases are highly unreactive except when under particular extreme conditions; the inertness of noble gases makes them suitable in applications where reactions are not wanted. For example, argon is used in incandescent lamps to prevent the hot tungsten filament from oxidizing; the properties of the noble gases can be well explained by modern theories of atomic structure: their outer shell of valence electrons is considered to be "full", giving them little tendency to participate in chemical reactions, it has been possible to prepare only a few hundred noble gas compounds. The melting and boiling points for a given noble gas are close together, differing by less than 10 °C.
Neon, argon and xenon are obtained from air in an air separation unit using the methods of liquefaction of gases and fractional distillation. Helium is sourced from natural gas fields that have high concentrations of helium in the natural gas, using cryogenic gas separation techniques, radon is isolated from the radioactive decay of dissolved radium, thorium, or uranium compounds. Noble gases have several important applications in industries such as lighting and space exploration. A helium-oxygen breathing gas is used by deep-sea divers at depths of seawater over 55 m. After the risks caused by the flammability of hydrogen became apparent, it was replaced with helium in blimps and balloons. Noble gas is translated from the German noun Edelgas, first used in 1898 by Hugo Erdmann to indicate their low level of reactivity; the name makes an analogy to the term "noble metals", which have low reactivity. The noble gases have been referred to as inert gases, but this label is deprecated as many noble gas compounds are now known.
Rare gases is another term, used, but this is inaccurate because argon forms a considerable part of the Earth's atmosphere due to decay of radioactive potassium-40. Pierre Janssen and Joseph Norman Lockyer discovered a new element on August 18, 1868 while looking at the chromosphere of the Sun, named it helium after the Greek word for the Sun, ἥλιος. No chemical analysis was possible at the time, but helium was found to be a noble gas. Before them, in 1784, the English chemist and physicist Henry Cavendish had discovered that air contains a small proportion of a substance less reactive than nitrogen. A century in 1895, Lord Rayleigh discovered that samples of nitrogen from the air were of a different density than nitrogen resulting from chemical reactions. Along with Scottish scientist William Ramsay at University College, Lord Rayleigh theorized that the nitrogen extracted from air was mixed with another gas, leading to an experiment that isolated a new element, from the Greek word ἀργός. With this discovery, they realized.
During his search for argon, Ramsay managed to isolate helium for the first time while heating cleveite, a mineral. In 1902, having accepted the evidence for the elements helium and argon, Dmitri Mendeleev included these noble gases as group 0 in his arrangement of the elements, which would become the periodic table. Ramsay continued his search for these gases using the method of fractional distillation to separate liquid air into several components. In 1898, he discovered the elements krypton and xenon, named them after the Greek words κρυπτός, νέος, ξένος, respectively. Radon was first identified in 1898 by Friedrich Ernst Dorn, was named radium emanation, but was not considered a noble gas until 1904 when its characteristics were found to be similar to those of other noble gases. Rayleigh and Ramsay received the 1904 Nobel Prizes in Physics and in Chemistry for their discovery of the noble gases; the discovery of the noble gases aided in the development of a general understanding of atomic structure.
In 1895, French chemist Henri Moissan attempted to form a reaction between fluorine, the most electronegative element, argon, one of the noble gases, but failed. Scientists were unable to prepare compounds of argon until the end of the 20th century, but these attempts helped to develop new theories of atomic structure. Learning from these experiments, Danish physicist Niels Bohr proposed in 1913 that the electrons in atoms are arranged in shells surrounding the nucleus, that for all noble gases except helium the outermost shell always contains eight electrons. In 1916, Gilbert N. Lewis formulated the octet rule, which conc
Gordon Gould was an American physicist, but not universally, credited with the invention of the laser. Gould is best known for his thirty-year fight with the United States Patent and Trademark Office to obtain patents for the laser and related technologies, he fought with laser manufacturers in court battles to enforce the patents he subsequently did obtain. Born in New York City, Gould was the oldest of three sons, his father was the founding editor of Scholastic Magazine Publications in New York City. He grew up in Scarsdale, a small suburb of New York, attended Scarsdale High School, he earned a Bachelor of Science degree in physics at Union College, where he became a member of the Sigma Chi Fraternity, a master's degree at Yale University, specializing in optics and spectroscopy. Between March 1944 and January 1945 he worked on the Manhattan Project but was dismissed due to his activities as a member of the Communist Political Association. In 1949 Gould went to Columbia University to work on a doctorate in optical and microwave spectroscopy.
His doctoral supervisor was Nobel laureate Polykarp Kusch, who guided Gould to develop expertise in the then-new technique of optical pumping. In 1956, Gould proposed using optical pumping to excite a maser, discussed this idea with the maser's inventor Charles Townes, a professor at Columbia and won the 1964 Nobel prize for his work on the maser and the laser. Townes gave Gould advice on how to obtain a patent on his innovation, agreed to act as a witness. By 1957, many scientists including Townes were looking for a way to achieve maser-like amplification of visible light. In November of that year, Gould realized that one could make an appropriate optical resonator by using two mirrors in the form of a Fabry–Pérot interferometer. Unlike considered designs, this approach would produce a narrow, intense beam. Since the sides of the cavity did not need to be reflective, the gain medium could be optically pumped to achieve the necessary population inversion. Gould considered pumping of the medium by atomic-level collisions, anticipated many of the potential uses of such a device.
Gould recorded his analysis and suggested applications in a laboratory notebook under the heading "Some rough calculations on the feasibility of a LASER: Light Amplification by Stimulated Emission of Radiation"—the first recorded use of this acronym. Gould's notebook was the first written prescription for making a viable laser and, realizing what he had in hand, he took it to a neighborhood store to have his work notarized. Arthur Schawlow and Charles Townes independently discovered the importance of the Fabry–Pérot cavity—about three months later—and called the resulting proposed device an "optical maser". Gould's name for the device was first introduced to the public in a conference presentation in 1959, was adopted despite resistance from Schawlow and his colleagues. Eager to achieve a patent on his invention, believing incorrectly that he needed to build a working laser to do this, Gould left Columbia without completing his doctoral degree and joined a private research company, TRG, he convinced his new employer to support his research, they obtained funding for the project from the Advanced Research Projects Agency with support from Charles Townes.
For Gould, the government declared the project classified, which meant that a security clearance was required to work on it. Because of his former participation in communist activities, Gould was unable to obtain a clearance, he continued to work at TRG, but was unable to contribute directly to the project to realize his ideas. Due to technical difficulties and Gould's inability to participate, TRG was beaten in the race to build the first working laser by Theodore Maiman at Hughes Research Laboratories. During this time, Gould and TRG began applying for patents on the technologies; the first pair of applications, filed together in April 1959, covered lasers based on Fabry–Pérot optical resonators, as well as optical pumping, pumping by collisions in a gas discharge, optical amplifiers, Q-switching, optical heterodyne detection, the use of Brewster's angle windows for polarization control, applications including manufacturing, triggering chemical reactions, measuring distance and lidar. Schawlow and Townes had applied for a patent on the laser, in July 1958.
Their patent was granted on March 22, 1960. Gould and TRG launched a legal challenge based on his 1957 notebook as evidence that Gould had invented the laser prior to Schawlow and Townes's patent application. While this challenge was being fought in the Patent Office and the courts, further applications were filed on specific laser technologies by Bell Labs, Hughes Research Laboratories and others. Gould lost the battle for the U. S. patent on the laser itself on the grounds that his notebook did not explicitly say that the sidewalls of the laser medium were to be transparent though he planned to optically pump the gain medium through them, considered loss of light through the sidewalls by diffraction. Questions were raised about whether Gould's notebook provided sufficient information to allow a laser to be constructed, given that Gould's team at TRG was unable to do so. Gould was able to obtain patents on the laser in several other countries, he continued fighting for U. S. patents on specific laser technologies for many years afterward.
In 1967, Gould left TRG and joined the Polytechnic Institute of Brooklyn
A spacecraft is a vehicle or machine designed to fly in outer space. Spacecraft are used for a variety of purposes, including communications, earth observation, navigation, space colonization, planetary exploration, transportation of humans and cargo. All spacecraft except single-stage-to-orbit vehicles cannot get into space on their own, require a launch vehicle. On a sub-orbital spaceflight, a space vehicle enters space and returns to the surface, without having gained sufficient energy or velocity to make a full orbit of the Earth. For orbital spaceflights, spacecraft enter closed orbits around the Earth or around other celestial bodies. Spacecraft used for human spaceflight carry people on board as crew or passengers from start or on orbit only, whereas those used for robotic space missions operate either autonomously or telerobotically. Robotic spacecraft used to support scientific research are space probes. Robotic spacecraft that remain in orbit around a planetary body are artificial satellites.
To date, only a handful of interstellar probes, such as Pioneer 10 and 11, Voyager 1 and 2, New Horizons, are on trajectories that leave the Solar System. Orbital spacecraft may be recoverable or not. Most are not. Recoverable spacecraft may be subdivided by method of reentry to Earth into non-winged space capsules and winged spaceplanes. Humanity has achieved space flight but only a few nations have the technology for orbital launches: Russia, the United States, the member states of the European Space Agency, China, Taiwan (National Chung-Shan Institute of Science and Technology, Taiwan National Space Organization, Israel and North Korea. A German V-2 became the first spacecraft when it reached an altitude of 189 km in June 1944 in Peenemünde, Germany. Sputnik 1 was the first artificial satellite, it was launched into an elliptical low Earth orbit by the Soviet Union on 4 October 1957. The launch ushered in new political, military and scientific developments. Apart from its value as a technological first, Sputnik 1 helped to identify the upper atmospheric layer's density, through measuring the satellite's orbital changes.
It provided data on radio-signal distribution in the ionosphere. Pressurized nitrogen in the satellite's false body provided the first opportunity for meteoroid detection. Sputnik 1 was launched during the International Geophysical Year from Site No.1/5, at the 5th Tyuratam range, in Kazakh SSR. The satellite travelled at 29,000 kilometers per hour, taking 96.2 minutes to complete an orbit, emitted radio signals at 20.005 and 40.002 MHz While Sputnik 1 was the first spacecraft to orbit the Earth, other man-made objects had reached an altitude of 100 km, the height required by the international organization Fédération Aéronautique Internationale to count as a spaceflight. This altitude is called the Kármán line. In particular, in the 1940s there were several test launches of the V-2 rocket, some of which reached altitudes well over 100 km; as of 2016, only three nations have flown crewed spacecraft: USSR/Russia, USA, China. The first crewed spacecraft was Vostok 1, which carried Soviet cosmonaut Yuri Gagarin into space in 1961, completed a full Earth orbit.
There were five other crewed missions. The second crewed spacecraft was named Freedom 7, it performed a sub-orbital spaceflight in 1961 carrying American astronaut Alan Shepard to an altitude of just over 187 kilometers. There were five other crewed missions using Mercury spacecraft. Other Soviet crewed spacecraft include the Voskhod, flown uncrewed as Zond/L1, L3, TKS, the Salyut and Mir crewed space stations. Other American crewed spacecraft include the Gemini spacecraft, Apollo spacecraft, the Skylab space station, the Space Shuttle with undetached European Spacelab and private US Spacehab space stations-modules. China developed, but did not fly Shuguang, is using Shenzhou. Except for the Space Shuttle, all of the recoverable crewed orbital spacecraft were space capsules. Crewed space capsules The International Space Station, crewed since November 2000, is a joint venture between Russia, the United States and several other countries; some reusable vehicles have been designed only for crewed spaceflight, these are called spaceplanes.
The first example of such was the North American X-15 spaceplane, which conducted two crewed flights which reached an altitude of over 100 km in the 1960s. The first reusable spacecraft, the X-15, was air-launched on a suborbital trajectory on July 19, 1963; the first reusable orbital spacecraft, a winged non-capsule, the Space Shuttle, was launched by the USA on the 20th anniversary of Yuri Gagarin's flight, on April 12, 1981. During the Shuttle era, six orbiters were built, all of which have flown in the atmosphere and five of which have flown in space. Enterprise was used only for approach and landing tests, launching from the back of a Boeing 747 SCA and gliding to deadstick landings at Edwards AFB, California; the first Space Shuttle to fly into space was Columbia, followed by Challenger, Discovery and Endeavour. Endeavour was built to replace Challenger when it was lost in January 1986. Columbia broke up during reentry in February 2003; the first automatic reusable spacecraft was the Buran-class shuttle, launched by the USSR on November 15, 1988, although it made only one flight and this was uncrewed.
This spaceplane was designed for a crew and resembled the U
A resonator is a device or system that exhibits resonance or resonant behavior, that is, it oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. The oscillations in a resonator can be either mechanical. Resonators are used to either generate waves of specific frequencies or to select specific frequencies from a signal. Musical instruments use acoustic resonators. Another example is quartz crystals used in electronic devices such as radio transmitters and quartz watches to produce oscillations of precise frequency. A cavity resonator is one. In electronics and radio, microwave cavities consisting of hollow metal boxes are used in microwave transmitters and test equipment to control frequency, in place of the tuned circuits which are used at lower frequencies. Acoustic cavity resonators, in which sound is produced by air vibrating in a cavity with one opening, are known as Helmholtz resonators. A physical system can have as many resonant frequencies.
Systems with one degree of freedom, such as a mass on a spring, balance wheels, LC tuned circuits have one resonant frequency. Systems with two degrees of freedom, such as coupled pendulums and resonant transformers can have two resonant frequencies. A crystal lattice composed of N atoms bound together can have N resonant frequencies; as the number of coupled harmonic oscillators grows, the time it takes to transfer energy from one to the next becomes significant. The vibrations in them begin to travel through the coupled harmonic oscillators in waves, from one oscillator to the next; the term resonator is most used for a homogeneous object in which vibrations travel as waves, at an constant velocity, bouncing back and forth between the sides of the resonator. The material of the resonator, through which the waves flow, can be viewed as being made of millions of coupled moving parts. Therefore, they can have millions of resonant frequencies, although only a few may be used in practical resonators.
The oppositely moving waves interfere with each other, at its resonant frequencies reinforce each other to create a pattern of standing waves in the resonator. If the distance between the sides is d, the length of a round trip is 2 d. To cause resonance, the phase of a sinusoidal wave after a round trip must be equal to the initial phase so the waves self-reinforce; the condition for resonance in a resonator is that the round trip distance, 2 d, is equal to an integer number of wavelengths λ of the wave: 2 d = N λ, N ∈ If the velocity of a wave is c, the frequency is f = c / λ so the resonant frequencies are: f = N c 2 d N ∈ So the resonant frequencies of resonators, called normal modes, are spaced multiples of a lowest frequency called the fundamental frequency. The above analysis assumes the medium inside the resonator is homogeneous, so the waves travel at a constant speed, that the shape of the resonator is rectilinear. If the resonator is inhomogeneous or has a nonrectilinear shape, like a circular drumhead or a cylindrical microwave cavity, the resonant frequencies may not occur at spaced multiples of the fundamental frequency.
They are called overtones instead of harmonics. There may be several such series of resonant frequencies in a single resonator, corresponding to different modes of vibration. An electrical circuit composed of discrete components can act as a resonator when both an inductor and capacitor are included. Oscillations are limited by the inclusion of resistance, either via a specific resistor component, or due to resistance of the inductor windings; such resonant circuits are called RLC circuits after the circuit symbols for the components. A distributed-parameter resonator has capacitance and resistance that cannot be isolated into separate lumped capacitors, inductors, or resistors. An example of this, much used in filtering, is the helical resonator. A single layer coil, used as a secondary or tertiary winding in a Tesla coil or magnifying transmitter is a distributed resonator. A cavity resonator is a hollow closed conductor such as a metal box or a cavity within a metal block, containing electromagnetic waves reflecting back and forth between the cavity's walls.
When a source of radio waves at one of the cavity's resonant frequencies is applied, the oppositely-moving waves form standing waves, the cavity stores electromagnetic energy. Since the cavity's lowest resonant frequency, the fundamental frequency, is that at which the width of the cavity is equal to a half-wavelength, cavity resonators are only used at microwave frequencies and above, where wavelengths are short enough that the cavity is conveniently small in size. Due to the low resistance of their conductive walls, cavity resonators have very