Browse All 842 Physics Terms A–Z

Mass Foundations of Physics kg A fundamental intrinsic property of matter that quantifies both the amount of substance in an object and its inertia (resistance to acceleration). m = F / a

Mass Defect Nuclear & Particle Physics u (Atomic Mass Units) or MeV/c² The difference between the total mass of an atom's individual free nucleons and the actual smaller mass of the bound atomic nucleus. Δm = Z m_p + (A - Z) m_n - M_nucleus

Mass-Energy Equivalence Astrophysics, Cosmology & Relativity Joules or MeV / GeV The fundamental relativistic principle stating that mass and energy are directly proportional and interchangeable physical quantities. E = m c² = γ m₀ c²

Matrix Mechanics Mathematical & Computational Physics Matrix Operator Algebra An algebraic formulation of quantum mechanics created by Werner Heisenberg in 1925 expressing physical observables as infinite-dimensional matrices. p̂ q̂ - q̂ p̂ = -i ℏ I, Ĥ = H(q̂, p̂)

Matter Foundations of Physics kg, m³ Anything that possesses mass, occupies volume, and is composed of fundamental particles such as quarks and leptons. E = mc²

Maximum Height Classical Mechanics m The maximum vertical elevation reached by a projectile above its launch level, where vertical velocity momentarily becomes zero. H = (v0² sin² θ) / (2 g)

Maxwell Relations Thermodynamics & Statistical Physics Various A set of thermodynamic equations derived from equality of second partial derivatives of thermodynamic potentials (U, H, F, G). (∂T/∂V)_S = -(∂P/∂S)_V, (∂T/∂P)_S = (∂V/∂S)_P, etc.

Maxwell's Equations Electromagnetism & Electronics Various SI Units The set of four fundamental partial differential equations that unify electricity, magnetism, and optics into classical electrodynamics. ∇·E = ρ/ε₀, ∇·B = 0, ∇×E = -∂B/∂t, ∇×B = μ₀J + μ₀ε₀∂E/∂t

Maxwell-Boltzmann Distribution Thermodynamics & Statistical Physics s/m The probability distribution describing particle speeds in an ideal gas in thermal equilibrium. f(v) = 4π (m / (2π kB T))^(3/2) v² exp(-m v² / (2 kB T))

Mean Free Path Thermodynamics & Statistical Physics m The average distance traveled by a moving gas particle between successive collisions with other particles. λ = 1 / (√2 π d² n_V)

Mean Life Nuclear & Particle Physics Seconds or Years The average expected lifetime of a radioactive nucleus before undergoing radioactive decay. τ = 1 / λ = t_½ / ln(2) ≈ 1.443 t_½

Mechanical Advantage Classical Mechanics Dimensionless The factor by which a simple machine multiplies input effort force to overcome load resistance. MA = F_output / F_input

Mechanical Energy Classical Mechanics J The sum of kinetic energy and potential energy in a physical system. E_mech = KE + PE

Medical Lasers Biophysics & Medical Physics W or J/cm² (Fluence) Laser devices engineered for surgical cutting, tissue coagulation, and photomedical treatments exploiting specific light-tissue absorption interactions. Selective Photothermolysis: Laser Pulse Duration ≤ Thermal Relaxation Time τ_r

Medical Linear Accelerator (LINAC) Biophysics & Medical Physics MV (Megavoltage Photon Beams 6–18 MV) A high-energy medical accelerator generating megavoltage X-ray photon or electron beams for external beam radiation cancer therapy. RF Cavity Electron Acceleration to 6–18 MeV → Bremsstrahlung X-Rays

Medical Ultrasound Biophysics & Medical Physics MHz (Frequency 2–15 MHz) & MRayl (Acoustic Impedance) A non-invasive diagnostic imaging modality using high-frequency acoustic waves (2–15 MHz) to image soft tissue reflection interfaces. R = ((Z₂ - Z₁) / (Z₂ + Z₁))², Z = ρ c (Acoustic Impedance)

Meissner Effect Condensed Matter & Materials Physics T (Magnetic Flux Density B) The active expulsion of magnetic flux from the interior of a superconductor as it cools below its critical temperature T_c. B_internal = 0, M = -H (Perfect Diamagnetism)

Membrane Potential Biophysics & Medical Physics mV (Millivolts) The electrical potential difference across a biological cell membrane produced by asymmetric ion concentration gradients. V_m = V_in - V_out (Resting Potential ≈ -70 mV)

Mesons Nuclear & Particle Physics Composite Boson Class Hadronic subatomic particles composed of one valence quark and one valence antiquark bound by gluons. Quark Content: q q̄, Spin = 0, 1, Baryon Number = 0

Metamaterials Condensed Matter & Materials Physics Dimensionless Index n < 0 Artificially engineered composite structures designed with subwavelength resonant features that display electromagnetic properties not found in natural materials. n_refractive = - √(ε_r μ_r) (Negative Refractive Index)

Michelson Interferometer Optics m An optical instrument that splits a beam of light into two perpendicular paths and recombines them to create high-precision interference fringes. 2 d = m λ

Micrometer Screw Gauge Foundations of Physics mm An ultra-precise measuring tool that uses a calibrated screw thread mechanism to measure microscopic thicknesses or diameters. Reading = Main Scale + (Circular Scale Division × Least Count)

Microstate Thermodynamics & Statistical Physics N/A A specific detailed microscopic configuration of a thermodynamic system, specifying exact positions and momenta of all constituent particles. S = kB ln Ω

Milankovitch Cycles Geophysics & Environmental Physics Geologic Climate Cycles (Years) Cyclical variations in Earth's orbital eccentricity, axial tilt (obliquity), and precession that alter the seasonal and latitudinal distribution of solar insolation. Eccentricity (100ky), Obliquity (41ky), Precession (23ky)

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