42,772 articles
Pomeranchuk instability
The Pomeranchuk instability is an instability of a Fermi liquid in which the Fermi surface spontaneously deforms or the liquid phase-separates, identified by Isaak Pomeranchuk in 1958.
Pomeron
The Pomeron is the Regge-theory exchange, carrying vacuum quantum numbers, that accounts for diffractive scattering of hadrons at high energies and the slow rise of total cross sections.
Ponceau 4R
Ponceau 4R, also known as Cochineal Red A, is a synthetic azo dye used as a red food colorant, authorized in the EU but not in the United States.
Ponceau S
Ponceau S is a red anionic azo dye used as a rapid, reversible stain for total protein on western blot membranes, and as a histological tissue stain.
Ponderomotive force
The ponderomotive force is the time-averaged, nonlinear force that an inhomogeneous oscillating electromagnetic field exerts on charged particles, pushing them from high toward low intensity.
Ponderomotive squeezing
Ponderomotive squeezing, also called optomechanical squeezing, produces squeezed light when radiation pressure drives a mirror or membrane whose motion imprints phase shifts, first proposed by Braginsky and Manukin in 1967 and demonstrated in 2013.
Pons asinorum
Pons asinorum, or "bridge of asses," is the traditional name of the isosceles triangle theorem, Proposition 5 of Book I of Euclid's Elements: in an isosceles triangle the base angles are equal.
Pontecorvo–Maki–Nakagawa–Sakata matrix
The Pontecorvo–Maki–Nakagawa–Sakata matrix, or PMNS matrix, is the unitary matrix connecting neutrino flavor states to their mass eigenstates, making neutrino oscillation possible. It plays the role for leptons that the CKM matrix plays for quarks.
Pontryagin's maximum principle
Pontryagin's maximum principle is a set of necessary conditions for optimality in optimal control theory, announced in 1956 by Lev Pontryagin and students at the Steklov Mathematical Institute.
Pood
The pood (Russian: пуд, pud) is a historical unit of mass in Russia, Belarus, and Ukraine, equal to 40 funts and about 16.38 kilograms, abolished in 1924.
Pooled analysis
Pooled analysis combines the individual participant data of multiple studies into a single dataset, estimating an effect with greater precision; the two-stage approach, which preserves study membership, is the most common method.
Pooled variance
Pooled variance, also called combined variance, is a single estimate of a common population variance that weights group sample variances by degrees of freedom, used in the equal-variance two-sample t-test and analysis of variance.
POPC
POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, PC(16:0/18:1)) is a phosphatidylcholine lipid with a palmitoyl and an oleoyl chain that forms fluid bilayers, making it a standard model membrane lipid and simulation benchmark.
Popper's experiment
Popper's experiment is a quantum thought experiment proposed by philosopher Karl Popper in 1982 to test the Copenhagen interpretation, first realized with entangled photons by Kim and Shih in 1999.
Poppers
Poppers is the street name for volatile alkyl nitrite liquids, chiefly amyl and isopropyl nitrite, inhaled for a brief rush of warmth, dizziness, and muscle relaxation from vasodilation.
Population dynamics
Population dynamics is the study of how the size, structure, and distribution of populations change over time, a central branch of ecology built on mathematical models.
Population I
Population I, or Pop I, is the class of relatively young, metal-rich stars that inhabit the disks of spiral galaxies, including the Sun and most Milky Way disk stars.
Population II
Population II, or Pop II, stars are the old, metal-poor stars of a galaxy's bulge, halo, and globular clusters, formed early with 11 to 13 billion-year typical ages.
Population III
Population III (Pop III) stars are the hypothetical first stars, formed from metal-free primordial hydrogen and helium about 100 million years after the Big Bang.
Population III remnant seed
A Population III remnant seed is a stellar-mass black hole left by the death of one of the first stars, proposed as a seed of supermassive black holes.
Population inversion
Population inversion is a non-equilibrium condition in which an upper energy level holds more population per state than a lower one, enabling laser light amplification.
Population protocol
A population protocol is a model of distributed computation, introduced in 2004, in which anonymous identical finite-state agents collectively compute a function through random pairwise interactions.
Population synthesis
Population synthesis is a computational astrophysics method that evolves simulated stellar and binary populations to compact remnants, linking gravitational-wave sources detected by LIGO and Virgo to binary evolution physics.
Porcelain
Porcelain, also known as fine china, is a hard, white, translucent ceramic made mainly from kaolin clay and fired above 1250 °C, first made in Tang dynasty China and reinvented in Europe by 1710.
Pore-scale imaging
Pore-scale imaging is the microscopy and tomography techniques that resolve individual pores in rocks, chiefly X-ray CT, so pore geometry can be tied to fluid flow and transport.
Pore-scale simulation
Pore-scale simulation models fluid flow, solute transport, and chemical reaction directly inside the pore geometry of a porous medium, computing permeability and capillary pressure from 3D images.
Poroelastic model
A poroelastic model is a continuum-mechanics description coupling fluid flow in a deformable porous solid to elastic stress and strain, based on Biot's 1941 consolidation theory.
Pororoca
Pororoca: the tidal bore of the Amazon River, waves up to 4 m traveling 800 km upstream during spring tides, February-March; famous for river surfing, the 37-minute record ride, and preventing an Amazon delta.
Porosimetry
Porosimetry is a family of measurement techniques that determine the pore size distribution, porosity, and related characteristics of porous materials, most commonly by mercury intrusion or gas adsorption.
Porosity
Porosity is the fraction of a solid material's bulk volume made up of voids or pores rather than solid matter, a key property of rocks, ceramics, and catalysts.