Physics
Einstein's path
Follow the chain from ideas and predictions to observation and evidence.
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1900
Planck's quantum hypothesis
Max Planck
To reproduce the spectrum of glowing matter, Planck treated energy exchange as occurring in discrete packets. He initially regarded this as a mathematical device, not a complete picture of nature.
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1905
Special relativity
Albert Einstein
Einstein reformulated space and time so that the laws of physics—and the measured speed of light—are the same for all inertial observers.
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1905
Light quanta and the photoelectric effect
Albert Einstein
Einstein proposed that light itself sometimes behaves as localized energy quanta, explaining why light below a threshold frequency cannot eject electrons from a material.
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1905
Mass–energy equivalence
Albert Einstein
Einstein showed that mass and energy are equivalent, a result now commonly written as E = mc²: a small decrease in mass can correspond to a large release of energy.
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1908
Minkowski spacetime
Hermann Minkowski
Minkowski showed that special relativity is most naturally described as a four-dimensional geometry combining three dimensions of space with time.
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1915
General relativity
Albert Einstein
Einstein completed general relativity in 1915. It described gravity through a changing geometry of spacetime shaped by matter and energy.
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1916
Schwarzschild solution
Karl Schwarzschild
The first exact solution of Einstein's field equations described spacetime outside a spherical mass and contained a critical radius later understood through black-hole theory.
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1917
Relativistic cosmology and Λ
Albert Einstein
Einstein applied general relativity to the entire Universe and introduced the cosmological constant, Λ, to obtain a static model.
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1910s onward
The block-universe interpretation
Hermann Minkowski · philosophers of time
One influential interpretation treats past, present, and future events as parts of a four-dimensional spacetime whole. It is a philosophical consequence drawn from relativity, not an astronomical observation.
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1919
Eclipse tests of light bending
Arthur Eddington · Frank Dyson · Andrew Crommelin
Expeditions observed star positions near the eclipsed Sun to test the deflection of light predicted by general relativity. The results favored Einstein's value, though early precision was limited.
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1922–1924
Friedmann's expanding solutions
Alexander Friedmann
Friedmann found mathematical solutions of general relativity in which the scale of the Universe changes with time. These were theoretical possibilities, not yet observational discoveries.
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1925–1927
Quantum mechanics takes shape
Werner Heisenberg · Erwin Schrödinger · Max Born · Paul Dirac
Matrix mechanics, wave mechanics, the probabilistic interpretation, and a unifying formalism appeared across several years. Quantum mechanics was a collective development, not a one-day invention.
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1927
The uncertainty principle
Werner Heisenberg
In a quantum state, certain paired quantities, such as position and momentum, cannot both have arbitrarily precise values. This limit is inherent in the state, not merely a problem with imperfect instruments.
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1927
Solvay debates on quantum meaning
Niels Bohr · Albert Einstein · Werner Heisenberg · Erwin Schrödinger
At the fifth Solvay Conference, leading physicists debated measurement, probability, completeness, and the meaning of the new quantum theory.
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1927
Lemaître connects an expanding model with observations
Georges Lemaître
In 1927 Lemaître published an expanding-universe solution to Einstein's equations and estimated a velocity–distance relation from published redshifts and galaxy distances.
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1929
The 1929 distance–velocity relation
Edwin Hubble · Milton Humason · Vesto Slipher
In 1929 Hubble published a correlation between galaxy distance estimates and velocities derived from redshifts. This observational result is distinct from Slipher's earlier spectroscopy and Lemaître's 1927 expansion analysis.
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1998
Accelerating expansion of the Universe
Supernova Cosmology Project · High-Z Supernova Search Team
Two teams using distant Type Ia supernovae found that cosmic expansion has accelerated rather than steadily slowed under gravity.
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2015–2016
First direct detection of gravitational waves
LIGO Scientific Collaboration · Virgo Collaboration
Advanced LIGO recorded GW150914 on 14 September 2015: a characteristic spacetime strain from two merging stellar-mass black holes. The detection was announced after extensive checks in 2016.
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