You are looking at all articles with the topic "Physics". We found 175 matches.

Hint: To view all topics, click here. Too see the most popular topics, click here instead.

In astrophysics and physical cosmology, Olbers' paradox, named after the German astronomer Heinrich Wilhelm Olbers (1758–1840), also known as the "dark night sky paradox", is the argument that the darkness of the night sky conflicts with the assumption of an infinite and eternal static universe. In the hypothetical case that the universe is static, homogeneous at a large scale, and populated by an infinite number of stars, then any line of sight from Earth must end at the (very bright) surface of a star and hence the night sky should be completely illuminated and very bright. This contradicts the observed darkness and non-uniformity of the night.

The darkness of the night sky is one of the pieces of evidence for a dynamic universe, such as the Big Bang model. That model explains the observed non-uniformity of brightness by invoking spacetime's expansion, which lengthens the light originating from the Big Bang to microwave levels via a process known as redshift; this microwave radiation background has wavelengths much longer than those of visible light, so appears dark to the naked eye. Other explanations for the paradox have been offered, but none have wide acceptance in cosmology.

Clarence Leonard "Kelly" Johnson (February 27, 1910 – December 21, 1990) was an American aeronautical and systems engineer. He is recognized for his contributions to a series of important aircraft designs, most notably the Lockheed U-2 and SR-71 Blackbird. Besides the first production aircraft to exceed Mach 3, he also produced the first fighter capable of Mach 2, the United States' first operational jet fighter, as well as the first fighter to exceed 400 mph, and many other contributions to various aircraft. As a member and first team leader of the Lockheed Skunk Works, Johnson worked for more than four decades and is said to have been an "organizing genius". He played a leading role in the design of over forty aircraft, including several honored with the prestigious Collier Trophy, acquiring a reputation as one of the most talented and prolific aircraft design engineers in the history of aviation. In 2003, as part of its commemoration of the 100th anniversary of the Wright Brothers' flight, Aviation Week & Space Technology ranked Johnson eighth on its list of the top 100 "most important, most interesting, and most influential people" in the first century of aerospace. Hall Hibbard, Johnson's Lockheed boss, referring to Johnson's Swedish ancestry, once remarked to Ben Rich: "That damned Swede can actually see air."

In 2019, the SI base units were redefined in agreement with the International System of Quantities, effective on the 144th anniversary of the Metre Convention, 20 May 2019. In the redefinition, four of the seven SI base units – the kilogram, ampere, kelvin, and mole – were redefined by setting exact numerical values for the Planck constant (h), the elementary electric charge (e), the Boltzmann constant (k_B), and the Avogadro constant (N_A), respectively. The second, metre, and candela were already defined by physical constants and were subject to correction to their definitions. The new definitions aimed to improve the SI without changing the value of any units, ensuring continuity with existing measurements. In November 2018, the 26th General Conference on Weights and Measures (CGPM) unanimously approved these changes, which the International Committee for Weights and Measures (CIPM) had proposed earlier that year after determining that previously agreed conditions for the change had been met.^:23 These conditions were satisfied by a series of experiments that measured the constants to high accuracy relative to the old SI definitions, and were the culmination of decades of research.

The previous major change of the metric system occurred in 1960 when the International System of Units (SI) was formally published. At this time the metre was redefined: the definition was changed from the prototype of the metre to a certain number of wavelengths of a spectral line of a krypton-86 radiation, making it derivable from universal natural phenomena. The kilogram remained defined by a physical prototype, leaving it the only artefact upon which the SI unit definitions depend. At this time the SI, as a coherent system, was constructed around seven base units, powers of which were used to construct all other units. With the 2019 redefinition, the SI is constructed around seven defining constants, allowing all units to be constructed directly from these constants. The designation of base units is retained but is no longer essential to define SI measures.

The metric system was originally conceived as a system of measurement that was derivable from unchanging phenomena, but practical limitations necessitated the use of artefacts – the prototype of the metre and prototype of the kilogram – when the metric system was introduced in France in 1799. Although it was designed for long-term stability, the masses of the prototype kilogram and its secondary copies have shown small variations relative to each other over time; they are not thought to be adequate for the increasing accuracy demanded by science, prompting a search for a suitable replacement. The definitions of some units were defined by measurements that are difficult to precisely realise in a laboratory, such as the kelvin, which was defined in terms of the triple point of water. With the 2019 redefinition, the SI became wholly derivable from natural phenomena with most units being based on fundamental physical constants.

A number of authors have published criticisms of the revised definitions; their criticisms include the premise that the proposal failed to address the impact of breaking the link between the definition of the dalton and the definitions of the kilogram, the mole, and the Avogadro constant.

Optical tweezers (originally called single-beam gradient force trap) are scientific instruments that use a highly focused laser beam to provide an attractive or repulsive force (typically on the order of piconewtons), depending on the relative refractive index between particle and surrounding medium; these forces can be used to physically hold and move microscopic objects, in a manner similar to tweezers. They are able to trap and manipulate small particles, whose size is typically in microns, including dielectric and absorbing particles. Optical tweezers have been particularly successful in studying a variety of biological systems in recent years.

The flyby anomaly is a discrepancy between current scientific models and the actual increase in speed (i.e. increase in kinetic energy) observed during a planetary flyby (usually of Earth) by a spacecraft. In multiple cases, spacecraft have been observed to gain greater speed than scientists had predicted, but thus far no convincing explanation has been found. This anomaly has been observed as shifts in the S-band and X-band Doppler and ranging telemetry. The largest discrepancy noticed during a flyby has been 13 mm/s.

LK-99 is a proposed ambient pressure and room-temperature superconductor with a gray‒black appearance.^: 8 LK-99 has a hexagonal structure slightly modified from lead‒apatite and is claimed to function as a superconductor below 400 K (127 °C; 260 °F).^: 1 The material was investigated by a team of Sukbae Lee et al. from the Korea Institute of Science and Technology (KIST).^: 1 As of 26 July 2023 the discovery of LK-99 has not been peer reviewed or independently replicated.

The chemical composition of LK-99 is approximately Pb₉Cu(PO₄)₆O such that—compared to pure lead-apatite (Pb₁₀(PO₄)₆O)^: 5—approximately one quarter of Pb(2) ions are replaced by Cu(II) ions.^: 9 This partial replacement of Pb²⁺ ions (measuring 133 picometre) with Cu²⁺ ions (measuring 87 picometre) is said to cause a 0.48% reduction in volume, creating internal stress inside the material.^: 8

The internal stress is claimed to cause a heterojunction quantum well between the Pb(1) and oxygen within the phosphate ([PO₄]³⁻) generating a superconducting quantum well (SQW).^: 10 Lee et al claim to show LK-99 exhibits a response to a magnetic field (Meissner effect) when chemical vapor deposition is used to apply LK-99 to a non-magnetic copper sample.^: 4 Pure lead-apatite is an insulator, but Lee et al claim copper-doped lead-apatite forming LK-99 is a superconductor, or at higher temperatures, a metal.^: 5

Stochastic resonance (SR) is a phenomenon where a signal that is normally too weak to be detected by a sensor, can be boosted by adding white noise to the signal, which contains a wide spectrum of frequencies. The frequencies in the white noise corresponding to the original signal's frequencies will resonate with each other, amplifying the original signal while not amplifying the rest of the white noise (thereby increasing the signal-to-noise ratio which makes the original signal more prominent). Further, the added white noise can be enough to be detectable by the sensor, which can then filter it out to effectively detect the original, previously undetectable signal.

This phenomenon of boosting undetectable signals by resonating with added white noise extends to many other systems, whether electromagnetic, physical or biological, and is an area of research.

Norton's dome is a thought experiment that exhibits a non-deterministic system within the bounds of Newtonian mechanics. It was devised by John D. Norton in 2003. It is a special limiting case of a more general class of examples from 1997 due to Sanjay Bhat and Dennis Bernstein. The Norton's dome problem can be regarded as a problem in physics, mathematics, or philosophy.

A traveling-wave reactor (TWR) is a proposed type of nuclear fission reactor that can convert fertile material into usable fuel through nuclear transmutation, in tandem with the burnup of fissile material. TWRs differ from other kinds of fast-neutron and breeder reactors in their ability to use fuel efficiently without uranium enrichment or reprocessing, instead directly using depleted uranium, natural uranium, thorium, spent fuel removed from light water reactors, or some combination of these materials. The concept is still in the development stage and no TWRs have ever been built.

The name refers to the fact that fission remains confined to a boundary zone in the reactor core that slowly advances over time. TWRs could theoretically run self-sustained for decades without refueling or removing spent fuel.

Oliver Heaviside FRS (; 18 May 1850 – 3 February 1925) was an English self-taught electrical engineer, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, invented mathematical techniques for the solution of differential equations (equivalent to Laplace transforms), reformulated Maxwell's field equations in terms of electric and magnetic forces and energy flux, and independently co-formulated vector analysis. Although at odds with the scientific establishment for most of his life, Heaviside changed the face of telecommunications, mathematics, and science.