Science

Half-life is the time required for a decaying quantity to decrease to half its initial value, especially in radioactive decay and chemical kinetics.

Hamiltonian mechanics describes classical dynamics through generalized coordinates, conjugate momenta, and a Hamiltonian function on phase space.

The Hamiltonian operator represents a quantum system’s energy and generates its time evolution, linking measurable energy spectra with dynamical behavior.

Heat is energy transferred between systems because of a temperature difference, rather than a property stored within a body.

Heat capacity measures the heat required to raise a system’s temperature under specified conditions, reflecting its size, composition, and microscopic energy-storage mechanisms.

A heat pump uses an external energy input to transfer heat from a lower-temperature source to a higher-temperature destination, providing heating, cooling, or both.

Hebbian theory explains how coordinated neuronal activity can strengthen connections, providing a foundation for models of learning, memory, and neural computation.

Helmholtz free energy is a thermodynamic potential that determines equilibrium at fixed temperature and volume and bounds the work obtainable in an isothermal process.

Hemoglobin is an iron-containing protein that transports oxygen in blood, helps carry carbon dioxide, and contributes to acid–base balance.

Henry Cavendish was a British experimental scientist known for characterizing hydrogen, investigating water’s composition, and determining Earth’s mean density.

The Higgs boson is the elementary particle associated with the Higgs field, whose interactions explain how many fundamental particles acquire mass.

The Higgs field is a quantum field whose nonzero vacuum value generates the masses of several elementary particles in the Standard Model.

The Higgs mechanism generates masses for gauge bosons through a scalar field’s nonzero vacuum value while preserving gauge invariance.

High-temperature superconductivity is superconductivity at unusually high transition temperatures, found notably in copper oxides, iron-based compounds, and compressed hydrides.

A histone is a DNA-associated protein that helps organize chromatin and regulate access to genetic information.

Homeostasis is the active regulation of internal conditions within ranges compatible with the functioning of a living system.

A homologous chromosome is one of a corresponding pair that carries the same genes at matching locations, although their alleles may differ.

Horizontal gene transfer is the movement of genetic material between organisms outside ordinary parent-to-offspring inheritance, shaping genome evolution and microbial adaptation.

Host–guest chemistry studies how molecular hosts selectively bind guest species through complementary structures and noncovalent interactions.

The Hubble constant is the present-day fractional expansion rate of the universe, relating cosmic recession velocities to distances and setting a fundamental cosmological scale.

A hydrocarbon is a compound composed exclusively of carbon and hydrogen, encompassing major fuels and important raw materials for chemical manufacturing.

Hydrogen is the lightest chemical element, the most abundant element in the universe, and an important industrial feedstock and energy carrier.

The hydrophobic effect is the solvent-mediated tendency of nonpolar molecules or surfaces to associate in water, influencing molecular assembly and biological structure.

A hypothesis is a proposed explanation or claim evaluated through reasoning, observation, or empirical testing.

A theoretical gas whose pressure, volume, and absolute temperature obey the ideal gas law, providing a reference model for dilute gases.

Inbreeding is reproduction between genetically related individuals, increasing homozygosity and potentially reducing offspring survival and reproductive success.

An inertial reference frame is a frame in which freely moving bodies maintain constant velocity and physical laws take their standard unaccelerated form.

Infrared spectroscopy analyzes how matter interacts with infrared radiation to identify substances, investigate molecular vibrations, and measure composition.

An independent research institution in Princeton, New Jersey, founded in 1930 to support fundamental inquiry across the sciences and humanities.

Interference is the reinforcement or cancellation produced when overlapping waves or quantum probability amplitudes combine.

Interleukin-2 is a cytokine that regulates lymphocyte growth, immune responses, and the maintenance of regulatory T cells.

Internal energy is the energy associated with a system’s microscopic constituents and interactions, excluding its overall motion and external potential energy.

The International Bureau of Weights and Measures coordinates international measurement standards, maintains reference time scales, and supports worldwide measurement comparability.

The International System of Units is the internationally agreed measurement system, defined through seven constants and organized into base units, derived units, and decimal prefixes.

An intron is a transcribed segment of a gene that is removed from precursor RNA during splicing.

An ion is an atom or bonded group of atoms with a net electric charge, involved in chemical bonding, electrical conduction, and biological processes.

Ionization is the formation of electrically charged atoms or molecules through electron removal, electron attachment, or chemical reactions.

Ionization energy is the minimum energy required to remove an electron from an isolated atom, molecule, or ion in a specified state.

The Ising model describes interacting two-state variables and provides a fundamental framework for studying magnetism, phase transitions, and collective behavior.

Isomerism is the relationship between chemical species with the same molecular formula but different atomic connectivity or spatial arrangement.

Isotopes are forms of the same chemical element that have equal numbers of protons but different numbers of neutrons.

British physicist who discovered the electron, developed an early atomic model, and helped establish mass spectrometry.

English experimental physicist whose measurements established the quantitative relationship between mechanical work and heat and helped found thermodynamics.

Johannes Kepler was a German astronomer and mathematician who formulated three laws of planetary motion and contributed to optics, geometry, and astronomical calculation.

English chemist, physicist, and meteorologist who developed a quantitative atomic theory and the law of partial pressures.

The Josephson effect is phase-dependent supercurrent flow between weakly coupled superconductors, linking electrical voltage to oscillation frequency and enabling quantum sensors, voltage standards, and superconducting qubits.

The joule (J) is the SI unit of energy, work, and heat, equal to one newton metre or one watt second.

Joule heating is the conversion of electrical energy into thermal energy when an electric current flows through a resistive material.

The kelvin is the SI base unit of thermodynamic temperature, defined by fixing the numerical value of the Boltzmann constant.

The kilogram is the SI base unit of mass, defined by fixing the numerical value of the Planck constant.

Kinetic energy is the energy associated with motion, described by different expressions in classical mechanics, relativity, and quantum mechanics.

A Lagrangian is a function that encodes a physical system’s dynamics through stationary action, or combines an optimization objective with constraints.

The standard cosmological model describing an expanding universe with a cosmological constant, cold dark matter, ordinary matter, and radiation.

The Large Hadron Collider is CERN’s underground particle accelerator, used to investigate fundamental particles and interactions through high-energy proton and heavy-ion collisions.

Lattice QCD is a discretized formulation of quantum chromodynamics used to calculate strong-interaction phenomena nonperturbatively through numerical simulations.

A lepton is an elementary spin-½ particle that does not participate in the strong interaction; the known leptons comprise three charged particles and three neutrino flavors.

Leukemia is a group of cancers of blood-forming cells, classified by cellular lineage, disease progression, and genetic characteristics.

A Lie group is a group with a compatible smooth manifold structure, providing a mathematical framework for continuous symmetry and its infinitesimal generators.

A ligand is an atom, ion, or molecule that binds to a central atom or molecular target, forming a coordination entity or biological complex.

Light is electromagnetic radiation, particularly the visible portion, exhibiting wave-like propagation and quantized exchanges of energy with matter.