Physics

A pion is one of three light, spin-zero mesons that play a central role in nuclear forces and the low-energy behavior of the strong interaction.

The Planck constant sets the scale of quantum phenomena, relates photon energy to frequency, and provides the basis for the SI definition of the kilogram.

Plasma is an ionized state of matter whose mobile charged particles exhibit collective electromagnetic behavior.

Polarization describes the orientation and temporal behavior of a wave’s transverse oscillations, particularly the electric field of light.

The positron is the electron’s positively charged antiparticle, central to antimatter physics, particle collisions, and positron emission tomography.

Potential energy is energy associated with a system’s configuration, including the relative positions and interactions of its components.

Pressure is normal force per unit area, governing fluid equilibrium, gas behavior, and mechanical energy transfer.

A proton is a positively charged composite subatomic particle found in every atomic nucleus, whose number determines an element’s identity.

Quantum chromodynamics is the quantum field theory of the strong interaction, describing how quarks and gluons form hadrons and behave at different energy scales.

Quantum decoherence is the suppression of observable quantum interference through interactions that distribute coherence into correlations with an environment.

Quantum electrodynamics is the relativistic quantum field theory describing electromagnetic interactions between charged particles and photons.

Quantum entanglement is a property of composite quantum systems whose joint state cannot be expressed as a mixture of independent subsystem states.

Quantum field theory describes quantum fields and their interactions, providing the framework for elementary particle physics and many quantum many-body systems.

Quantum gravity seeks a quantum description of gravitation and spacetime, extending general relativity into regimes where quantum effects become essential.

The quantum Hall effect is the quantization of transverse electrical conductance in two-dimensional systems, revealing integer and fractional quantum states.

An exactly solvable quantum system with a quadratic potential, equally spaced energy levels, and nonzero ground-state energy.

Quantum mechanics is the fundamental theory describing matter and energy at atomic and subatomic scales, built on quantization, wave-particle duality and probabilistic measurement.

A quantum number labels a quantum state or a measurable property, helping classify states, distinguish energy levels, and express symmetry and conservation laws.

Quantum superposition is the coherent combination of quantum states, whose probability amplitudes can interfere and determine measurement outcomes.

Quantum tunneling is the transmission of a quantum system through an energy barrier that would prevent passage under classical mechanics.

A quark is an elementary particle with fractional electric charge and colour charge, forming the constituents of protons, neutrons, and other hadrons.

Quark–gluon plasma is an extremely hot state of strongly interacting matter in which quarks and gluons are no longer confined within individual hadrons.

A quasiparticle is an emergent excitation of a many-body system that can be described as a particle with effective properties.

Radioactivity is the spontaneous transformation of unstable atomic nuclei, accompanied by the emission of particles or electromagnetic radiation.

A rainbow is a multicoloured arc of light formed when sunlight is refracted, dispersed and reflected inside airborne water droplets, seen opposite the Sun.

A reciprocal lattice represents the spatial periodicity of a crystal in wave-vector space and provides the geometric framework for diffraction and crystal-wave analysis.

Optical reflection is the return of light into its original medium after interaction with a surface or interface.

Refraction is the redirection of a wave caused by changes in its propagation speed across an interface or within a nonuniform medium.

A dimensionless optical quantity describing phase propagation in a medium and governing refraction, dispersion, and related optical effects.

Renormalization relates physical theories across scales and absorbs regulator-dependent quantities into parameters, enabling finite predictions and explanations of universal behavior.

The renormalization group describes how effective physical theories change with scale, explaining running couplings, critical behavior, and universality.

A reversible process is an ideal thermodynamic transformation that can be reversed while restoring both the system and its surroundings exactly to their initial states.

American theoretical physicist whose work transformed quantum electrodynamics, particle physics, and physics education.

A rigid body is an idealized object whose internal distances remain constant, allowing its motion to be described entirely by translation and rotation.

Robert Hooke was an English experimental scientist and architect known for Micrographia, his law of elasticity, and his work for the Royal Society.

The Royal Society is the United Kingdom’s national academy of sciences, founded in 1660 to advance scientific knowledge through research, publication, and international cooperation.

An independent Swedish scientific academy founded in 1739 that promotes research and selects Nobel laureates in physics, chemistry, and economic sciences.

Scattering is the redirection of waves or particles through interactions, used to investigate matter and explain the propagation of radiation.

The Schrödinger equation governs the evolution of quantum states and determines the allowed energies and wave functions of quantum systems.

The second is the SI base unit of time, defined by an exact frequency of the unperturbed caesium-133 atom.

The second law of thermodynamics establishes the direction of spontaneous processes through entropy increase and limits the conversion of heat into work.

Spacetime is the four-dimensional framework combining space and time in which relativity describes events, motion, causality, and gravity.

A theory of space, time, and motion based on equivalent inertial frames and the invariant speed of light in vacuum.

Spectroscopy studies how matter interacts with radiation, using spectra to reveal composition, structure, energy states, and physical conditions.

The speed of light in vacuum is an exact physical constant that defines the metre and sets the local limit for transmitting matter and information.

Spin angular momentum is an intrinsic quantum property governing particles’ rotational behavior, magnetic interactions, and statistical classification.

The spin–statistics theorem connects integer spin with bosonic statistics and half-integer spin with fermionic statistics in relativistic quantum field theory.

Spontaneous symmetry breaking occurs when a system’s equilibrium state lacks a symmetry obeyed by its underlying physical laws.

The Standard Model is the quantum field theory describing known elementary particles and their electromagnetic, weak, and strong interactions.

A state function is a thermodynamic property whose value depends only on a system’s present state, not on the process used to reach it.

Statistical mechanics explains macroscopic physical properties through the probabilistic behavior of microscopic constituents.

The strong interaction binds quarks into hadrons and, through its residual effects, holds protons and neutrons together in atomic nuclei.

A quantity describing how spatial arrangement and correlations determine scattering amplitudes or intensities in crystals, liquids, and other materials.

Superconductivity is a quantum state of matter characterized by zero direct-current electrical resistance and magnetic-field expulsion under suitable conditions.

Superfluidity is a quantum state of matter characterized by nondissipative flow, macroscopic coherence, and distinctive rotational and thermal behavior.

The superposition principle states that a linear combination of inputs or solutions produces the corresponding linear combination of responses in a linear system.

Synchrotron radiation is electromagnetic emission from relativistic charged particles following curved paths, used to study matter and astronomical sources.

A telescope collects radiation from distant objects to produce images or measurements, enabling observations beyond the sensitivity and resolving power of the unaided eye.

Temperature is the physical quantity that determines the direction of heat flow between bodies and reflects the average energy of microscopic motion. Its SI unit is the kelvin.

A framework developed by Albert Einstein that describes space, time, motion, and gravity through special and general relativity.