§ I — The Physics
Theories
From light quanta to curved spacetime: what each idea says, the equation at its heart, how it was tested, and where it stands today.
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1905
The Photoelectric Effect and Light Quanta
In March 1905 Einstein proposed that light itself comes in discrete packets of energy. It was the idea he called revolutionary, and the one named in his Nobel Prize.
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1905
Brownian Motion and the Reality of Atoms
Einstein showed that the restless jiggling of tiny particles in a liquid is caused by molecular collisions, and that measuring it lets you count atoms.
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1905
Special Relativity
Two simple postulates — the laws of physics are the same for everyone moving steadily, and so is the speed of light — force time and space to depend on the observer.
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1905
Mass–Energy Equivalence
The most famous equation in science began as a three-page afterthought: the mass of a body is a measure of the energy it contains.
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1907 – 1915
General Relativity
Gravity is not a force pulling across space. It is the shape of space and time themselves, curved by matter and energy.
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1915 – Today
Testing General Relativity
From a solar eclipse in 1919 to the shadow of a black hole in 2019, a century of experiments has tried to break the theory. None has.
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1916 – 1937
Gravitational Waves
Einstein predicted ripples in spacetime in 1916, doubted them in 1936, and thought they would never be detected. A century later they were.
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1917 – 1932
Cosmology and the Cosmological Constant
In 1917 Einstein applied his theory to the entire universe, added a term to hold it still, abandoned the term, and was proved half right eighty years later.
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1907
The Quantum Theory of Solids
In 1907 Einstein used the quantum idea to explain why solids take less and less heat to warm as they get colder, the first time quantum theory was applied to ordinary matter.
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1916 – 1917
Stimulated Emission
Working out how atoms and light exchange energy, Einstein found a third process nobody had noticed. Forty years later it became the laser.
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1924 – 1925
Bose–Einstein Statistics and the Condensate
A letter from an unknown physicist in Dhaka led Einstein to predict a new state of matter, in which atoms cooled almost to absolute zero merge into a single quantum wave.
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1927 – 1935
The EPR Paradox and Entanglement
Einstein's sharpest objection to quantum mechanics turned out to describe one of its most real and useful features.
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1912 – 1936
Gravitational Lensing
A massive object bends the light passing it and can act as a lens. Einstein worked it out, published it reluctantly, and thought nobody would ever see it.
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1935
The Einstein–Rosen Bridge
The original wormhole was not a shortcut through space. It was an attempt to build elementary particles out of pure geometry.
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1920s – 1955
The Search for a Unified Field Theory
Einstein spent his last thirty years looking for one theory to contain gravity and electromagnetism. He did not find it, and was still working on it in the hospital in his final days.
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1909 – 1930
Refrigerators, Magnets, and Blue Skies
The lesser-known Einstein: an experiment with a spinning iron bar, a refrigerator with no moving parts, and an explanation of why the sky is blue.