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Hertz
The hertz (Hz) is the SI unit of frequency, equal to one cycle per second, named for the German physicist Heinrich Hertz and adopted into the SI in 1960.
Hertz's experiments on electromagnetic waves
Heinrich Hertz's experiments of 1887–1888 at Karlsruhe first generated and detected electromagnetic waves with a spark-gap oscillator, confirming Maxwell's theory that light itself is an electromagnetic wave.
Hertzsprung gap
The Hertzsprung gap is a sparsely populated region of the Hertzsprung-Russell diagram between the main sequence and the giant branch, where intermediate-mass stars cross so quickly during the short subgiant phase that few are caught there.
Hertzsprung–Russell diagram
The Hertzsprung–Russell diagram, or HR diagram, plots stellar luminosity against surface temperature, named for Ejnar Hertzsprung and Henry Norris Russell, who used it to study stellar evolution.
Herwig (software)
Herwig is a general-purpose Monte Carlo event generator for particle physics that simulates collider collisions; with Pythia and Sherpa it produces simulated events at the LHC.
Herwig Kogelnik
Herwig Kogelnik, born 1932 in Graz, Austria, is a physicist and electrical engineer known for coupled wave theory, stable optical resonators, and the distributed feedback laser at Bell Laboratories.
Hesperetin
Hesperetin is a flavanone of the flavonoid class that occurs naturally in citrus fruits and is best known as the aglycone of hesperidin, the predominant flavonoid in lemons and oranges.
Hesperian (Mars)
The Hesperian is the middle of Mars's three geologic epochs, spanning roughly 3.7 to 3.0 billion years ago and marked by outflow channels carved by catastrophic floods.
Hesperidin
Hesperidin, also known as cirantin, is a flavanone glycoside and the predominant polyphenol in citrus fruits, first isolated from orange peel by Lebreton in 1828.
Hess's law
Hess's law, also called the law of constant heat summation, states that the total enthalpy change of a reaction is the sum of the enthalpy changes of its steps.
Hessdalen lights
The Hessdalen lights, also called the Hessdalen phenomenon, are recurring unexplained balls of light of varying shapes and colors in the Hessdalen valley of central Norway, reported since the 1940s.
Hessian matrix
The Hessian matrix, or Hesse matrix, is the square matrix of a function's second-order partial derivatives, used to classify critical points as minima, maxima, or saddle points.
Heston model
The Heston model is a stochastic volatility model in mathematical finance in which an asset's variance is itself random and mean-reverting, introduced by Steven Heston in 1993.
Hetero-Diels–Alder reaction
The hetero-Diels–Alder reaction is a [4+2] cycloaddition extending the all-carbon Diels–Alder reaction, in which a diene reacts with a heteroatom-containing dienophile to build six-membered heterocycles used in pharmaceutical synthesis.
Heteroaryl ethers
Heteroaryl ethers are ethers in which one group attached to oxygen is a heteroaryl group, a structural motif found in drugs such as lansoprazole and flecainide.
Heterocyclic compound
A heterocyclic compound is a cyclic compound whose ring contains atoms of at least two different elements, most commonly carbon with nitrogen, oxygen, or sulfur.
Heterodyne detection
Heterodyne detection is a measurement technique that mixes a weak signal with a local oscillator at a slightly different frequency, recovering signals too weak or fast to measure directly.
Heterodyne spectroscopy
Heterodyne spectroscopy is a coherent detection technique that mixes radiation with a slightly offset local oscillator, shifting it to a lower frequency where amplitude, frequency, and phase can be measured.
Heteroepitaxy
Heteroepitaxy is the growth of a single-crystalline film on a substrate of a different material, with the film's orientation dictated by the substrate; examples include GaN LEDs on sapphire and GaAs films on silicon.
Heterogenite
Heterogenite is a cobalt oxyhydroxide mineral, CoO(OH), the most abundant oxidized cobalt mineral and principal cobalt ore in the Katanga Copperbelt of the Democratic Republic of Congo.
Heterojunction
A heterojunction is the interface between two different semiconductor materials, where band offsets control carrier flow; Herbert Kroemer's heterostructure work earned the 2000 Nobel Prize in Physics.
Heteronuclear multiple bond correlation
Heteronuclear multiple bond correlation (HMBC) is a two-dimensional NMR experiment that correlates protons with heteronuclei, typically carbon-13, over two to four bonds, a standard tool for small-molecule structure elucidation.
Heteronuclear multiple quantum coherence
Heteronuclear multiple quantum coherence (HMQC) is a two-dimensional NMR experiment, introduced by Luciano Mueller in 1979, that correlates each proton with the heteronucleus it is directly bonded to.
Heteronuclear single quantum coherence spectroscopy
Heteronuclear single quantum coherence (HSQC) spectroscopy is a two-dimensional NMR experiment correlating each proton signal with the carbon-13 or nitrogen-15 directly bonded to it, reported by Bodenhausen and Ruben in 1980.
Heterotic string theory
Heterotic string theory is a class of closed superstring theories in which left- and right-moving modes differ, combining the 26-dimensional bosonic string with the 10-dimensional superstring. It was constructed at Princeton in 1984.
Heun function
The Heun function solves Heun's equation, the most general second-order differential equation with four regular singular points, proposed by Karl Heun in 1889 as a generalization of the hypergeometric equation.
Heuristic
A heuristic, also called a heuristic technique, is a problem-solving or decision-making method that finds adequate answers quickly without guaranteeing an optimal solution, used where exact methods are too slow or unavailable.
Heusler alloy
A Heusler alloy is a magnetic intermetallic compound with an ordered face-centered cubic structure, named after Fritz Heusler, who discovered in 1903 that Cu₂MnAl is ferromagnetic.
Hewitt–Savage zero–one law
The Hewitt–Savage zero–one law is a probability theorem stating that any event invariant under finite permutations of i.i.d. random variables has probability 0 or 1.
Hexabromobiphenyl
Hexabromobiphenyl is a polybrominated biphenyl in which six hydrogen atoms of biphenyl are replaced by bromine atoms; it was used as a flame retardant and is now banned internationally as a persistent organic pollutant.