THE VOCABULARY
Instruments, concepts, and phenomena — the shared vocabulary of the site.
Weak equivalence principle
The statement that gravitational mass equals inertial mass for any test particle, regardless of composition — equivalently, that all bodies in vacuum fall with identical acceleration. Galileo's Pisa-tower observation, Eötvös's torsion-balance precision result, and the MICROSCOPE 2017 satellite null result confirm it to ≲ 10⁻¹⁵.
work
The energy transferred to a body by a force acting over a distance: W = F · d · cos θ.
Working substance
The matter — gas, vapour, or fluid — that an engine repeatedly expands and compresses to convert heat into work.
World-line
The locus of events in spacetime traced out by a particle as it moves; a 1D curve in the 4D manifold. Massive particles have timelike world-lines; photons have null world-lines; the proper time elapsed on the particle's clock is the arc length of its world-line in the Minkowski metric.
Yang-Mills equations
The non-abelian generalisation of Maxwell's equations: D_μ F^{aμν} = J^{aν}, where the covariant derivative D_μ = ∂_μ + i g [A_μ, ·] couples the gauge field to itself through the structure constants of the gauge group. The field equations of every non-abelian gauge theory in physics.
Zeroth law of thermodynamics
If A is in thermal equilibrium with B, and B with C, then A is in equilibrium with C — the transitivity that makes temperature meaningful.