Atomic Recoil Required by Photon Momentum
By assigning momentum to individual radiation quanta, Einstein concluded that atoms must recoil when they emit or absorb directed packets of light.
Albert Einstein · Year
1917
Albert Einstein records dated 1917, drawn directly from the World History Database source collection.
Cosmological Constant Introduced
Einstein adds the cosmological constant to his gravitational field equations to permit a static universe, creating the mathematical term later associated with dark energy.
Cosmological Term Added to Field Equations
To support a static cosmological solution, Einstein modified the gravitational field equations with an additional cosmological term that could counterbalance large-scale attraction.
Kaiser Wilhelm Institute Directorship
Becomes founding director of the Kaiser Wilhelm Institute for Physics.
Light Quantum Momentum Set by Energy and Light Speed
Einstein's radiation theory assigned each emitted or absorbed light quantum a momentum equal to its energy divided by the speed of light.
Machian Motivation Shapes Einstein Cosmology
Einstein designed his early relativistic cosmology partly around the hope that inertia would be determined by the overall distribution of matter, reflecting his interpretation of Mach's ideas.
Radiation Equilibrium Connects Quantum Events and Thermodynamics
Einstein united discrete atomic transitions with thermodynamic equilibrium, demonstrating that quantum absorption and emission rules could generate the observed black-body spectrum.
Random Emission Directions Linked to Molecular Motion
Einstein showed that randomly directed quantum emission events contribute to molecular momentum fluctuations and must be balanced statistically in thermal equilibrium.
Relativistic Cosmology
Applies general relativity to the universe and introduces the cosmological constant in a static cosmological model.
Repulsive Gravity in Empty Space
Einstein interprets the cosmological constant as a repulsive contribution capable of balancing ordinary gravitational attraction on the largest cosmic scales.
Spontaneous Emission Formalized in Quantum Radiation Theory
Einstein's published quantum theory of radiation formalized spontaneous emission as an intrinsically statistical process occurring without an externally applied radiation field.
Static Closed Universe Model Proposed
Einstein applied general relativity to the universe as a whole and proposed a spatially closed, homogeneous model intended to remain static on cosmic scales.
Stimulated Emission
Formulates stimulated emission in his quantum theory of radiation, a principle later fundamental to lasers.
Stimulated Emission Predicted from Thermal Equilibrium
Einstein's equilibrium analysis required an induced emission process in which incident radiation increases the probability that an excited atom emits matching radiation.