Physical world and mathematics

Fortnight

A fortnight is a period of two weeks, or fourteen consecutive days, chiefly used in British English and rare in American English, where "two weeks" is normal.

Forward error

Forward error is the difference between the output a numerical computation produces and the exact output, bounded by the condition number times the backward error.

The forward–backward algorithm is a dynamic-programming method for hidden Markov models that computes the posterior probability of each hidden state at each time step, and it forms the computational core of Baum–Welch training.

Fossil Free campaign

The Fossil Free campaign is a climate advocacy campaign led by 350.org that calls on universities, pension funds, and governments to divest from fossil fuel companies.

Fossil fuel industry disinformation

Fossil fuel industry disinformation is the spread of false or misleading climate claims by oil, gas, and coal companies, most extensively documented at ExxonMobil from 1977 onward.

Fotini Katopodes Chow

Fotini "Tina" Katopodes Chow is an American atmospheric scientist and environmental engineer at UC Berkeley known for atmospheric boundary-layer modeling and landfill methane measurement methods.

Foucault pendulum

A Foucault pendulum is a pendulum free to swing in any direction, used to demonstrate Earth's rotation, first shown by Léon Foucault in Paris in 1851.

Foundationalism

Foundationalism is an epistemological view holding that some basic beliefs are justified without depending on other beliefs, and all other justified beliefs ultimately rest on them.

Foundations of mathematics

Foundations of mathematics, also known as foundations of math, studies the logical and set-theoretic frameworks underlying mathematics, from Russell's paradox and Gödel's incompleteness theorems to modern proof assistants.

Foundations of Physics (journal)

Foundations of Physics is a peer-reviewed Springer journal covering the conceptual and fundamental theories of modern physics, founded in 1970 by Henry Margenau and Wolfgang Yourgrau.

Foundations of Science

Foundations of Science is a quarterly peer-reviewed academic journal published by Springer in the Netherlands that covers methodological and philosophical topics about the structure and growth of science.

Four color theorem

The four color theorem, also known as the four color problem, states that any planar map needs at most four colors so that bordering regions differ, proved by computer in 1976.

Four Symbols (四象)

The Four Symbols (四象, also called the Four Guardians) are four animal groupings of seven Lunar Mansions each in Chinese astronomy, tied to directions and seasons.

Four-circle diffractometry

Four-circle diffractometry is a single-crystal diffraction technique in which a crystal is rotated on a goniometer with four circles, three Eulerian angles and the detector angle, to measure each reflection's intensity.

Four-component reaction

A four-component reaction (4CR) is a multicomponent reaction in which four different molecules combine in one step to give a product incorporating atoms from all four.

Four-current

The four-current, Jμ = (cρ, J), is the Lorentz-covariant four-vector combining charge density and current density in relativity, the source term of the covariant Maxwell equations, introduced by Minkowski in 1908.

Four-dimensional space

Four-dimensional space, or tetraspace, is a geometric space in which four independent coordinates are needed to specify the position of a point, generalizing ordinary three-dimensional space.

Four-force

The four-force, also called the Minkowski force, is the Lorentz-covariant generalization of force in relativity, defined as the derivative of a particle's four-momentum with respect to proper time.

Four-momentum

The four-momentum, or energy-momentum 4-vector, is the four-vector in special relativity that combines a particle's energy and momentum, with rest mass as its invariant length.

Four-terminal sensing

Four-terminal sensing, also called Kelvin sensing or four-wire measurement, is an electrical technique that measures resistance accurately by using separate pairs of current-carrying and voltage-sensing leads.

Four-vector

A four-vector, also called a Lorentz vector, is a set of four quantities transforming like spacetime event coordinates under Lorentz transformations, the basic objects of Minkowski space in special relativity.

Four-velocity

The four-velocity is the derivative of a particle's spacetime position with respect to its proper time, written Uμ = dxμ/dτ, making it a Lorentz four-vector.

Fourier analysis

Fourier analysis, also called Fourier theory, is the branch of mathematics that expresses functions as sums of sines and cosines, founded by Joseph Fourier's 1807 work on heat conduction.

Fourier multiplier

A Fourier multiplier is a linear operator acting on a function by multiplying its Fourier transform by a fixed symbol and transforming back, equivalently convolution with a kernel.

Fourier optics

Fourier optics is the treatment of light propagation and imaging with Fourier analysis and linear systems theory, in which a lens performs a Fourier transform of an optical field.

Fourier ptychographic microscopy

Fourier ptychographic microscopy (FPM) is a computational imaging method that combines many low-resolution images captured under varied illumination angles into one high-resolution, wide-field microscope image, introduced at Caltech in 2013.

Fourier ptychography

Fourier ptychography is a computational imaging method that combines many low-resolution images, each captured under a different illumination angle, into a single high-resolution, wide-field image, first reported in 2013.

Fourier series

A Fourier series, also called a Fourier expansion, represents a periodic function as an infinite sum of sines and cosines at multiples of the fundamental frequency.

Fourier shell correlation

Fourier shell correlation (FSC) is a resolution measure in three-dimensional electron microscopy, computed between two independently reconstructed 3D volumes, and the standard global metric in cryo-EM.