Physical world and mathematics

Emodin

Emodin is a naturally occurring anthraquinone compound found in rhubarb, Japanese knotweed, and aloe, long used as a purgative and now studied for its pharmacological effects and liver toxicity.

Oct 10, 2026

Emory Leon Chaffee (1885–1975) was an American physicist and electronics pioneer at Harvard whose 1911 dissertation established the Chaffee gap, a method of producing continuous electrical oscillations for radiotelephony.

EMPAD pixel array detector

The EMPAD, short for electron microscope pixel array detector, is a direct electron detector developed at Cornell University for scanning transmission electron microscopy and sold commercially from late 2017.

Empirical adequacy

Empirical adequacy is the property of a theory whose claims about observable things and events are true, a notion central to van Fraassen's constructive empiricism.

Empirical Bayes estimation

Empirical Bayes estimation is a statistical approach that estimates the prior distribution from observed data rather than fixing it in advance, introduced by Herbert Robbins in 1955.

Empirical Bayes method

The empirical Bayes method is a statistical technique that estimates a prior distribution's parameters from the data, then computes posterior distributions as if the prior were known.

Empirical Bayes methods

Empirical Bayes methods are statistical techniques that estimate the prior distribution of a model from the data itself, then apply Bayes' rule with that estimated prior; Herbert Robbins named the approach in 1956.

Empirical distribution function

The empirical distribution function (EDF), also called the empirical cumulative distribution function, is a statistical estimator that assigns each of n observations probability 1/n and underlies goodness-of-fit tests.

Empirical formula

An empirical formula is a chemical formula showing the simplest whole-number ratio of atoms in a compound; butene (C4H8) and ethylene (C2H4) both reduce to CH2.

Empirical interpolation method

The empirical interpolation method is a greedy numerical technique that approximates parametrized functions at automatically selected points, introduced in 2004 to restore the offline-online decomposition in reduced-basis solvers of parametrized partial differential equations.

Empirical likelihood

Empirical likelihood is a nonparametric statistical method that assigns probability weights to observed data points and maximizes the likelihood under constraints, yielding tests and confidence regions without parametric assumptions.

Empirical measure

An empirical measure is a random probability measure constructed from observed data by placing equal mass at each sampled point; in statistics it proxies the unknown underlying distribution.

Empirical mode decomposition

Empirical mode decomposition (EMD) is an adaptive signal-processing method that decomposes nonlinear, nonstationary time series into intrinsic mode functions; with the Hilbert transform it forms the Hilbert–Huang transform, developed at NASA in 1998.

Empirical orthogonal functions

Empirical orthogonal function (EOF) analysis decomposes a geophysical field, such as sea level pressure, into orthogonal spatial patterns paired with uncorrelated time series; it is also known as principal component analysis.

Empirical process

An empirical process is the centered, √n-scaled version of an empirical distribution function, G_n(x) = √n(F_n(x) − F(x)), underlying goodness-of-fit tests like Kolmogorov–Smirnov; Donsker's theorem shows it converges to a Brownian bridge.

Empirical research

Empirical research, also known as an empirical study, is research that draws conclusions from data gathered through direct observation, experiment, or survey, rather than from pure reasoning or theory.

Empiricism

Empiricism, also known as empiricist epistemology, is the philosophical position that sense experience is the only source of knowledge, opposed to rationalism and associated with Locke, Berkeley, and Hume.

Empty set

The empty set, or empty subset, is the unique set containing no elements, denoted ∅; it has cardinality zero and is a subset of every set.

Emulsion

An emulsion is a fluid colloidal system in which droplets of one liquid are dispersed in another immiscible liquid, most commonly oil in water or water in oil.

Emulsion polymerization

Emulsion polymerization is a free-radical process that polymerizes water-insoluble monomers in water with surfactant and a water-soluble initiator, yielding a latex of about 100 nm polymer particles.

En number

The En number is a normalized error statistic used in proficiency testing to compare a laboratory's measured value with a reference value, defined in ISO 13528.

Enal

An enal, a contraction of alkene aldehyde, is an α,β-unsaturated aldehyde in which an aldehyde group is conjugated with an alkene; simple examples are acrolein and crotonaldehyde, and the ketone analog is the enone.

Enalaprilat

Enalaprilat is an intravenous angiotensin-converting enzyme (ACE) inhibitor used to treat hypertension when oral therapy is not practical; it is the active metabolite of enalapril.

Enamine

An enamine is an alkene bearing an amine on a double-bond carbon, formed from an aldehyde or ketone and a secondary amine, and key to asymmetric organocatalysis.

Enamine REAL

Enamine REAL (REadily AccessibLe) is a make-on-demand compound library from Enamine Ltd, founded in Kyiv in 1991, with compounds synthesized to order in a few weeks and over 80 percent success.

Enantioconvergent catalysis

Enantioconvergent catalysis is a form of asymmetric catalysis in which a chiral catalyst converts both enantiomers of a racemic starting material into one enantioenriched product.

Enantioconvergent cross-coupling

Enantioconvergent cross-coupling is a reaction in which both enantiomers of a racemic alkyl electrophile yield one product enantiomer under a chiral catalyst, pioneered by Gregory Fu's 2005 nickel-catalyzed Negishi couplings.

Enantioconvergent metallaphotoredox cross-coupling

Enantioconvergent metallaphotoredox cross-coupling is an asymmetric bond-forming method where a photoredox catalyst generates carbon radicals and a chiral metal catalyst, usually nickel, captures them into one enantioenriched product.

Enantiomer

An enantiomer, also called an optical isomer, is one of a pair of molecules that are mirror images and cannot be superimposed, differing only in how they rotate light.