This is an archive of entries that have appeared or will appear on Portal:Physics's Selected Article section in 2007. Articles for previous weeks should not be edited.
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January
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| Schematic of a neutron |
In physics, the neutron is a baryon that consists of two down quarks and one up quark. Its strong force radiation is the primary force that holds atomic nuclei. It also has a mass of 1.008 664 915 (78) u (1.6749 × 10−27 kg (939.573 MeV/c²), but no electric charge. Neutrons additionally radiate beta decay and have a spin of ½. Neutrons are present in the majority of nuclei, however in a weighted majority they are not. The number of neutrons in a nucleus determines the isotope of the atom. For example, protium, or 1H, is an isotope of Hydrogen with no neutrons, while deuterium, or 2H, contains one. Neutrons were discovered in 1932 by Physicist James Chadwick two years after they were unexplainably detected by Physicist Walther Bothe.
February
editErwin Schrödinger (German pronunciation: [ˈɛrviːn ˈʃrøːdɪŋɐ]; August 12, 1887 – January 4, 1961) was an Austrian - Irish physicist who achieved fame for his contributions to quantum mechanics. In 1935, after correspondence with Albert Einstein, he proposed the Schrödinger's cat thought experiment. In 1933, Schrödinger was awarded the Nobel Prize in Physics, along with physicist Paul Dirac for "the discovery of new productive forms of atomic theory" in regard to the Schrödinger equation which he proposed in 1926. One of Schrödinger's lesser-known areas of scientific contribution was his study of color, color perception, and the field of colorimetry
March
editA planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. The name originates from a similarity in appearance to giant planets when viewed through a small optical telescope, and is unrelated to planets of the solar system. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years.
In recent years, Hubble Space Telescope images have revealed many planetary nebulae to have extremely complex and varied morphologies. About a fifth are roughly spherical, but the majority are not spherically symmetric. Their formation depends on the mass of its star; lower mass stars, such as the sun usually form planetary nebulae, while higher mass stars, such as Zeta Ophiuchi will not. The average nebula is approximately 1 light year across.
April
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Microwaves are electromagnetic waves with wavelengths between one meter and one millimeter, or with frequencies between 300 megahertz and 300 gigahertz.
Apparatuses and techniques may be described qualitatively as "microwave" when the wavelengths of signals are roughly the same as the dimensions of the equipment, so that lumped-element circuit theory is inaccurate. As a consequence, devices that utilize microwaves, such as the microwave communications tower pictured, tend to move away from the discrete resistors, capacitors, and inductors used with lower frequency radio waves. Open-wire and coaxial transmission lines give way to waveguides, and lumped-element tuned circuits are replaced by cavity resonators or resonant lines. Similarly to visible light, the phenomena of reflection, polarization, scattering, diffraction, and atmospheric absorption are observed in microwaves.
Microwaves were first theorized in 1864 by James Clerk Maxwell, and were later detected in the 1940s by Sir John Randall and Dr Harry Boot. Since then, microwaves have been used in communication, remote sensing of both earth based and astronomical bodies, food preparation, and non lethal weaponry.
May
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The Speed of Light, or c, is a physical constant equal to exactly 299,792,458 meters per second, or approximately 983,571,056 feet per second, and is the speed of all frequencies of light in a vacuum. Since 1983, the speed of light is used to define the SI unit if one meter. Light can travel at average lower speeds when passing through translucent materials. Denser materials than air, slow the observed speed of light down more than lighter ones do. For instance, light traveling through a diamond travels at approximately 124,000,000 meters per second.
Experimental evidence has shown that the speed of light is independent of the motion of the source. It has also been confirmed experimentally that the two-way speed of light (for example from a source, to a mirror, and back again) is constant. It is not, however, possible to measure the one-way speed of light (for example from a source to a distant detector) without some convention as to how clocks at the source and receiver should be synchronized. Einstein postulated that the speed of light should be taken as constant in all cases, one-way and two-way.
Astronomical distances are sometimes measured in light years (the distance that light would travel in one Earth year, roughly 9.46×1012 kilometres or about 5.88×1012 miles). Because light travels at a large but finite speed, it takes time for light to cover large distances. Thus, the light we observe from distant objects in the universe was emitted from them long ago (pictured is the Crab Nebula as it looked ~6.5 X 103 years ago).
June
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Spacecraft propulsion is any method used to change the velocity of spacecraft and artificial satellites. Most spacecraft today are propelled by expelling a gas from the rear of the vehicle at very high speed through a rocket engine nozzle. Most current spacecraft use chemical rockets (bipropellant or solid-fuel) for launch (some, such SpaceShipOne, used jet engines in their first stage). Most satellites have simple reliable chemical thrusters (often monopropellant rockets) or resistojet rockets for orbital station-keeping and some use momentum wheels for attitude control. Soviet bloc satellites have used electric propulsion for decades, and newer Western geo-orbiting spacecraft are starting to use them for north-south stationkeeping. Interplanetary vehicles mostly use chemical rockets as well, although a few have experimentally used ion thrusters. Spacecraft propulsion systems are often first statically tested on the Earth's surface, within the atmosphere but many systems require a vacuum chamber be accurately tested.
July
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Uranium is a silver-gray metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. It has 92 protons and electrons, 6 of them valence electrons. Uranium has the highest atomic weight of the naturally occurring elements. It occurs naturally in low concentrations (a few parts per million) in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite. There are many current applications of uranium, including the fissile explosive material used in nuclear weapons, fuel for nuclear power plants, high density bullets, and, prior to the discovery of the harmful effects of its radiation, glass and ceramic dyes, and in luminescent paints...
August
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Mars /ˈmɑːrz/ is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance. Mars has two tiny natural moons, Phobos and Deimos, discovered in 1877 by Asaph Hall, which orbit very close to the planet and are thought to be captured asteroids.
Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts and polar ice caps of Earth. It is the site of Olympus Mons, the highest known mountain in the Solar System, and of Valles Marineris, the largest canyon. Mars has been the focus of dozens of missions, the most recent being the Mars Scout Program's Phoenix probe...
September
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A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. As white dwarfs have mass comparable to the Sun's and their volume is comparable to the Earth's, they are very dense. Their faint luminosity comes from the emission of stored heat. They comprise roughly 6% of all known stars in the solar neighborhood The unusual faintness of white dwarfs was first recognized in 1910 by Henry Norris Russell, Edward Charles Pickering and Williamina Fleming, and the name white dwarf was coined by Willem Luyten in 1922. White dwarves continue to lose energy and fade, becoming Black dwarfs...