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Hollow Earth
The Hollow Earth theory, or Hohlweltlehre, holds that Earth is hollow with habitable inner worlds; proposed by Edmond Halley, it was refuted by geophysics and survives in fiction.
Hollow fiber liquid-phase microextraction
Hollow fiber liquid-phase microextraction (HF-LPME) is a sample preparation technique that extracts and concentrates analytes from a liquid sample into microliters of solvent inside a porous hollow fiber membrane.
Hollow Moon
The Hollow Moon, or Spaceship Moon, is a fringe belief that the Moon is hollow or artificial, stemming from a 1970 Soviet article and the Moon's bell-like ringing.
Hollow-core fiber compression
Hollow-core fiber compression is a technique for shortening intense ultrashort laser pulses by spectrally broadening them in a gas-filled hollow-core fiber, first demonstrated in 1996 and now standard for attosecond science.
Holly Krieger
Holly Krieger is an American mathematician, Professor at the University of Cambridge since 2022, known for arithmetic dynamics, work on the Mandelbrot set, and Numberphile videos.
Holmium
Holmium is a silvery chemical element with symbol Ho and atomic number 67, a rare earth metal with the strongest magnetic moment of any element.
Holocene
The Holocene, also known as the Holocene epoch, is the current geological epoch, beginning 11,700 years ago with warming that ended the Younger Dryas and divided since 2018 into the Greenlandian, Northgrippian, and Meghalayan stages.
Holocene calendar
The Holocene calendar, also called the Human Era calendar, adds 10,000 years to AD numbering so that year 1 falls near the start of the Holocene epoch.
Holographic acoustic tweezers
Holographic acoustic tweezers are devices that use holographically shaped sound radiation forces to trap and move multiple particles without contact, first demonstrated in 2018 by Asier Marzo and Bruce Drinkwater.
Holographic dark energy
Holographic dark energy (HDE) is a class of cosmological models in which dark energy's density is set by the holographic principle, proposed by Miao Li in 2004.
Holographic interference microscopy
Holographic interference microscopy is a coherent optical technique applying holographic interferometry to microscopy, quantitatively measuring nearly transparent phase objects like living cells, recovering optical phase with nanometer-scale axial sensitivity.
Holographic interferometry
Holographic interferometry is an optical technique that reveals small changes in an object's shape or refractive index as fringe patterns, used in nondestructive testing and vibration analysis.
Holographic microscopy
Holographic microscopy is a label-free imaging technique that reconstructs the 3D structure of transparent specimens from interference patterns, yielding quantitative phase maps of living cells.
Holographic optical elements
Holographic optical elements (HOEs) are volume holograms used as thin, flat optical components that can replace lenses, prisms, and mirrors, notably in augmented reality displays.
Holographic optical tweezers
Holographic optical tweezers (HOTs) are an optical manipulation technique in which a computer-generated hologram splits a single laser beam into hundreds of traps in 3D, invented by Eric Dufresne and David Grier in 1997.
Holographic principle
The holographic principle, also known as holographic duality, is a conjecture in quantum gravity holding that a spacetime region's physics, including gravity, is fully described by its boundary.
Holographic QCD
Holographic QCD, also called the AdS/QCD correspondence, is a theoretical framework using gauge/gravity duality to study non-perturbative quantum chromodynamics, with applications to neutron stars and the quark-gluon plasma.
Holographic quantum error-correcting code
A holographic quantum error-correcting code is a quantum code encoding bulk degrees of freedom of a hyperbolic space onto its boundary, modeled on the AdS/CFT correspondence.
Holographic reconstruction
Holographic reconstruction is a computational imaging method that numerically recovers an object's amplitude and phase from recorded holograms, applied across optical, electron, and X-ray imaging, where the recovered phase carries quantitative information invisible to ordinary cameras.
Holographic recording
Holographic recording stores a complete light wavefront, amplitude and phase together, by exposing a photosensitive medium to the interference of object and reference waves; it enables 3D imaging, security holograms, and data storage.
Holographic tomography
Holographic tomography is a label-free optical imaging method that reconstructs the 3D refractive index of transparent specimens by combining holographic phase measurements from many illumination angles.
Holography
Holography is a technique for recording and reconstructing the complete light field of an object, capturing both amplitude and phase of light to produce three-dimensional holograms.
Holomorphic function
A holomorphic function, also called a complex analytic function, is a complex-valued function differentiable at every point of an open set, the central object of complex analysis.
Holomorphic functional calculus
The holomorphic functional calculus, also called the Riesz–Dunford functional calculus, assigns to an element of a Banach algebra and a holomorphic function near its spectrum an element f(a) via a contour integral.
Holonomic function
A holonomic function, also called a D-finite function, satisfies a linear differential equation with polynomial coefficients; the class follows Zeilberger's holonomic systems approach and contains most special functions.
Holonomy
Holonomy, or the holonomy group, is a construction in differential geometry measuring how parallel transport around closed loops fails to return geometric data to its starting value.
Holotomography
Holotomography is an X-ray phase-contrast imaging technique, introduced by Cloetens and colleagues in 1999, that reconstructs a sample's 3D refractive index from inline holograms recorded at multiple distances or energies.
Holstein model
The Holstein model is a condensed matter lattice model in which tight-binding electrons couple locally to dispersionless optical phonons, the canonical model for small-polaron formation.
Holy Cross Day
Holy Cross Day, kept on 14 September and also called the Exaltation of the Holy Cross, is a Christian feast honoring the cross of Christ, observed by Anglican, Lutheran, Catholic, and Orthodox churches.
Holy day of obligation
A holy day of obligation, also called a feast of precept, is a day on which the Catholic Church requires the faithful to attend Mass and rest from work.