Physical world and mathematics

Equilibrium level

The equilibrium level, also called the level of neutral buoyancy or limit of convection, is the height where a rising air parcel again equals its environment's temperature, approximating the height of thunderstorm tops.

Equilibrium molecular dynamics

Equilibrium molecular dynamics (EMD) is a simulation method in which a molecular system is run at fixed thermodynamic conditions until it reaches equilibrium, and properties are extracted from the trajectory.

Equilibrium reconstruction

Equilibrium reconstruction is a computational plasma physics method that infers a tokamak plasma's MHD equilibrium state, including its flux surfaces and current profiles, from diagnostic measurements.

Equilibrium temperature

The equilibrium temperature of a planet is the temperature at which absorbed stellar power equals emitted thermal power, set by stellar luminosity, orbital distance, and albedo.

Equinox

An equinox, also called an equinoctial point, is either of the two moments each year when the Sun crosses the plane of Earth's equator, in March and September.

Equinox Gold

Equinox Gold Corp. is a Canadian gold mining company headquartered in Vancouver, founded by Ross Beaty, with mines in Canada, the United States, Mexico, and Nicaragua.

Equipartition theorem

The equipartition theorem, or equipartition of energy, states that each quadratic degree of freedom in a system at thermal equilibrium carries an average energy of ½kBT.

Equipotential surface

An equipotential surface is a surface over which an electric or gravitational potential field has one constant value, requiring no work to move a charge along it.

Equivalence class

An equivalence class is the subset collecting all elements related to a given element by an equivalence relation; the classes form a partition of the set.

Equivalence of adiabatic quantum computation and the circuit model

The equivalence of adiabatic quantum computation and the circuit model is a 2004 theorem, proved by Aharonov and colleagues, that the two models are polynomially equivalent.

Equivalence of categories

Equivalence of categories is the standard category-theoretic notion of sameness between two categories: a pair of functors going in opposite directions, inverse up to natural isomorphism.

Equivalence point

The equivalence point, also called the stoichiometric endpoint, is the stage of a titration at which the titrant has completely reacted with the analyte according to the reaction's stoichiometry.

Equivalence principle

The equivalence principle, or principle of equivalence, states that all bodies fall with the same acceleration and that locally gravity is indistinguishable from acceleration, forming the foundation of general relativity.

Equivalence ratio

The equivalence ratio (φ) is a dimensionless measure of a fuel–oxidizer mixture, the actual fuel-to-oxidizer ratio divided by the stoichiometric one. A value of 1 is stoichiometric, below 1 lean, above 1 rich.

Equivalence relation

An equivalence relation is a binary relation that is reflexive, symmetric, and transitive, so it behaves like equality; its equivalence classes form a partition of the set.

Equivalence test

An equivalence test is a statistical hypothesis test concluding two treatments or measurements differ by no more than a prespecified equivalence margin, typically using two one-sided tests.

Equivalent (chemistry)

The equivalent, also called the molar or gram-equivalent, is a chemistry unit of reactive capacity, the amount of a substance supplying or consuming one mole of a reaction unit.

Equivalent continuous sound level

The equivalent continuous sound level (Leq) is the constant sound level carrying the same acoustic energy as a fluctuating sound over a stated time interval.

Equivalent dose

Equivalent dose, also known as radiation dose equivalent, is a radiological protection quantity in sieverts, given by tissue absorbed dose multiplied by a radiation weighting factor reflecting biological effectiveness.

Equivalent potential temperature

Equivalent potential temperature, or theta-e, is the temperature an air parcel would reach if all its water vapor condensed and it were brought adiabatically to 1000 mb.

Equivalent source method

The equivalent source method, also called equivalent sources, is a geophysical technique that fits fictitious sources to gravity or magnetic data for interpolation and continuation.

Equivalent weight

Equivalent weight, also called equivalent mass, is the mass of a substance that supplies or reacts with one equivalent, equal to formula weight divided by n.

Equivalents of the axiom of choice

The equivalents of the axiom of choice are statements provably equivalent to it over ZF, including Tychonoff's theorem, Zorn's lemma, and every vector space having a basis.

Equuleus

Equuleus, the Little Horse, is a small northern constellation between Pegasus and Delphinus, cataloged by Ptolemy and the second-smallest of the 88 modern constellations, best viewed in September and October.

ER = EPR

ER = EPR is a conjecture in quantum gravity proposed by Juan Maldacena and Leonard Susskind in 2013, holding that entangled particles or black holes are connected by non-traversable wormholes.

Er:YAG laser

The Er:YAG laser is a solid-state medical laser that emits at 2940 nm, matching a strong water absorption peak, making it effective for ablating skin, tooth, and bone.

ER=EPR conjecture

The ER=EPR conjecture, proposed by Juan Maldacena and Leonard Susskind in 2013, holds that quantum entanglement between systems, especially black holes, corresponds to wormhole connections.

ERA-40

ERA-40 is a global atmospheric reanalysis dataset from the European Centre for Medium-Range Weather Forecasts, reprocessing archived observations from September 1957 to August 2002 with a fixed forecasting model, and succeeded by ERA-Interim and ERA5.