Glossary
Plain definitions for the physics terms used on this site. Each entry defines the term and notes where Einstein touched it.
Aether
The hypothetical medium once thought to carry light waves. The Michelson–Morley experiment (1887) failed to detect it; special relativity made it unnecessary.
Annus mirabilis
Latin for “miracle year” — 1905, when Einstein published the four papers on the photoelectric effect, Brownian motion, special relativity, and mass–energy equivalence.
Black-body radiation
The spectrum of light emitted by a hot object. Max Planck’s 1900 explanation introduced energy quanta; Einstein’s 1905 light-quantum paper extended the quantum idea to light itself.
Bose–Einstein condensation
The state predicted by Einstein (1924–1925), extending Bose’s statistics: atoms cooled near absolute zero collapse into a single quantum state and behave as one wave. Achieved in the lab in 1995.
Brownian motion
The random jitter of particles suspended in a fluid, explained by Einstein (1905) as collisions with molecules — the paper that made atoms experimentally inescapable.
Cosmological constant (Λ)
The term Einstein added to his field equations in 1917 to permit a static universe. Abandoned by him after the expansion evidence; revived in modern cosmology as a candidate explanation for the accelerating expansion (dark energy).
Copenhagen interpretation
The standard interpretation of quantum mechanics associated with Niels Bohr: quantum systems have no definite properties until measured. Einstein argued from the 1920s onward that it must be incomplete (see EPR paradox).
Equivalence principle
Einstein’s 1907 insight (“the happiest thought of my life”): gravity and acceleration are locally indistinguishable — an observer in a closed box cannot tell whether the box is accelerating in space or sitting in a gravitational field. The foundation of general relativity.
EPR paradox
The 1935 Einstein–Podolsky–Rosen thought experiment arguing that quantum mechanics, as then understood, was incomplete. It introduced what is now called entanglement.
Entanglement
The quantum phenomenon in which two particles share a single state so that measuring one instantly constrains the other, regardless of distance. Einstein called it “spooky action at a distance” (German: “spukhafte Fernwirkung”, in a 1947 letter to Max Born). Bell-test experiments (1960s–2020s) confirm the predictions while the interpretation remains debated.
Field equations
Einstein’s ten equations of general relativity (1915), relating the curvature of spacetime to the distribution of matter and energy.
Frame of reference
A coordinate system from which motion is measured. Special relativity’s postulates concern inertial frames — frames moving at constant velocity with no acceleration.
General relativity
Einstein’s 1915 theory of gravity: mass and energy curve spacetime, and curved spacetime dictates how matter moves. Tested by Mercury’s orbit, the 1919 eclipse, gravitational redshift, gravitational waves (detected 2015), and GPS corrections.
Geodesic
The straightest possible path through curved spacetime — the path a freely falling object follows in general relativity. Planets orbit because they follow geodesics in the sun’s curved spacetime.
Gravitational redshift
The loss of energy (shift toward red) of light climbing out of a gravitational field. Predicted by general relativity; measured by Pound and Rebka in 1959.
Gravitational waves
Ripples in spacetime predicted by general relativity (1916), produced by accelerating masses — detected directly by LIGO in 2015 from merging black holes.
Inertial frame
See frame of reference.
Lorentz transformation
The equations (due to Hendrik Lorentz) relating space and time coordinates between inertial frames moving relative to each other. Einstein derived them in 1905 from his two postulates, giving them physical meaning.
Mass–energy equivalence
The 1905 result that mass is a form of energy (E=mc² for a body at rest). See Key Ideas.
Michelson–Morley experiment
The 1887 experiment that failed to detect Earth’s motion through the aether. Einstein later said he was aware of it but that it played little direct role in his 1905 reasoning.
Photoelectric effect
The ejection of electrons from metal by light, explained by Einstein (1905) with light quanta — the work cited by his Nobel Prize.
Photon
The quantum of light. Einstein proposed light quanta in 1905; the word “photon” was coined by Gilbert Lewis in 1926.
Postulates (of special relativity)
Einstein’s two 1905 assumptions: (1) the laws of physics are identical in all inertial frames; (2) the speed of light in vacuum is the same for all observers.
Quantum
A discrete packet of energy. Planck introduced the idea for oscillators (1900); Einstein extended it to light itself (1905) and to the specific heats of solids (1907).
Singularity
A point where general relativity’s equations break down (e.g., at the center of a black hole or the Big Bang). Einstein was uneasy about singularities and hoped a future theory would remove them.
Spacetime
The four-dimensional union of space and time. Special relativity treats them as interwoven; general relativity treats spacetime as curved by mass and energy.
Special relativity
Einstein’s 1905 theory of space and time for unaccelerated motion: time dilation, length contraction, and the relativity of simultaneity, built on the constancy of light speed.
Stimulated emission
The process Einstein described in 1917: an incoming photon stimulates an excited atom to emit a second, in-phase photon. The principle of the laser.
Thought experiment
German Gedankenexperiment. Einstein’s favorite tool: chasing a light beam, the moving train, the falling elevator — imagined scenarios used to derive physical consequences from principles.
Time dilation
The slowing of clocks in motion (special relativity) or in strong gravity (general relativity) relative to other observers. Measured with atomic clocks, muons, and GPS.
Unified field theory
Einstein’s decades-long, unsuccessful Princeton project: a single theory uniting gravity and electromagnetism (later extended toward the nuclear forces). He never completed it.
Wave–particle duality
The fact that light (and matter) shows both wave behavior (interference) and particle behavior (discrete detection). Einstein’s light-quantum paper (1905) forced the issue; the duality remains a feature, not a contradiction, of quantum theory.
Source notes
- Definitions are standard physics, consistent with the CPAE papers and the Collected Papers editorial apparatus; Einstein-specific dates and papers are cited in key-ideas.md.
- The “happiest thought” phrasing for the equivalence principle comes from Einstein’s 1922 Kyoto lecture notes.
- “Spooky action at a distance” (spukhafte Fernwirkung) is from Einstein’s letter to Max Born, 3 March 1947.