Physical world and mathematics

Negative differential resistance

Negative differential resistance is the phenomenon in which current through a device falls as applied voltage rises, enabling oscillation, switching, and amplification in tunnel diodes.

Negative energy

Negative energy in classical mechanics is the negative gravitational potential energy, U = -GMm/r, taken when zero is placed at infinite separation; it signals that masses are gravitationally bound.

Negative gas adsorption

Negative gas adsorption is a counterintuitive phenomenon where a porous solid spontaneously releases gas as pressure rises, first observed in 2015 in the metal-organic framework DUT-49 with methane.

Negative mass

Negative mass is hypothetical matter whose mass is negative, distinguished into inertial, passive gravitational, and active gravitational cases; negative rest mass has never been observed.

Negative number

A negative number, also called a directed number, is a real number less than zero; Brahmagupta gave early rules in 628, though Western acceptance came only in the 19th century.

Negative refraction

Negative refraction is the bending of a wave at an interface to the opposite side of the normal, predicted by Veselago in the 1960s and first observed in metamaterials in 2001.

Negative refraction of surface acoustic waves

Negative refraction of surface acoustic waves is the bending of a surface-guided elastic wave to the same side of the normal as the incident wave, demonstrated in phononic crystals.

A negative result, also called a null result, is a scientific study outcome that fails to confirm the hypothesis tested. It is published far less often than positive findings, causing publication bias.

Negative room pressure

Negative room pressure is a technique keeping a room below the pressure of adjacent spaces, so air flows inward and airborne contaminants cannot escape, used in hospital isolation rooms and laboratories.

Sep 29, 2026

Negative staining

Negative staining is an electron microscopy sample preparation method in which a specimen appears light against a dark heavy-metal stain, used to screen samples before cryo-EM.

Negative stiffness

Negative stiffness is a mechanical property in which an element deforms to assist the applied force, making such elements unstable unless embedded in positive stiffness material; metamaterials exploiting it use snap-through curved beams.

Negative stiffness metamaterial

A negative stiffness metamaterial is an architected material containing unstable elements, typically post-buckled beams or columns, whose force decreases as displacement increases, so they assist deformation and boost damping beyond ordinary composite bounds.

Negative temperature

Negative absolute temperature is a thermodynamic temperature below zero on the Kelvin scale, possible when a system's energy states have an upper limit; such systems are hotter than any positive temperature.

Negative thermal expansion

Negative thermal expansion is the property of a material whose volume contracts rather than expands when heated. Discovered in solids by Scheel in 1907, it is used to counteract expansion in precision devices.

Negative triangularity

Negative triangularity is a reversed-D plasma shaping regime for tokamak fusion devices, first explored on the TCV tokamak in Switzerland in the 1990s, offering high confinement without damaging ELMs.

Negative-index metamaterial

A negative-index metamaterial, also called a left-handed or double-negative material, is an engineered composite with a negative refractive index, first demonstrated at microwave frequencies in 2001 after Veselago's 1967 prediction.

Negativity (quantum entanglement)

Negativity is an entanglement measure for bipartite quantum states, introduced by G. Vidal and R. F. Werner in 2002 as a computable quantification of the Peres positive-partial-transpose criterion.

Negishi coupling

Negishi coupling, or the Negishi reaction, is a palladium- or nickel-catalyzed cross-coupling of organozinc reagents with organic halides, first reported by Ei-ichi Negishi in 1977.

Nehari manifold

The Nehari manifold is a constraint set in the calculus of variations, named after Zeev Nehari, containing every nontrivial critical point of a functional, so minimizing on it yields ground state solutions.

NEID

NEID, named for NN-EXPLORE Exoplanet Investigations with Doppler spectroscopy, is an extreme-precision spectrograph on the WIYN 3.5-meter Telescope at Kitt Peak, Arizona, detecting exoplanets since 2019.

Neighborhood system

A neighborhood system assigns to each point of a topological space the collection of all its neighborhoods; these systems form filters and determine the topology completely.

Neighboring group participation

Neighboring group participation, or anchimeric assistance, is the interaction of a reaction center with a lone pair or bond electrons within the same molecule, accelerating substitution.

Neighbourhood (mathematics)

A neighborhood of a point in a topological space is a set containing the point in its interior, formalizing local closeness without any notion of distance.