From Thales's water to gravitational waves and the Higgs boson
In Greece began the attempt to explain the world through a single underlying principle rather than myth. Then Aristotle built the framework that physics would follow for 2,000 years.
Everything is made of water.
Thales is considered the first to explain natural phenomena not as the will of gods but through a single basic substance. His answer was wrong, but his way of asking, assuming nature has principles we can understand, became the starting point of science.
The first recorded quantitative law of physics.
The Pythagoreans found that strings sound pleasing together when their lengths are in simple whole-number ratios such as 2:1, 3:2 and 4:3. It was the first case of a natural phenomenon expressed as a relation between numbers.
The world is made of indivisible atoms and empty space.
Leucippus and Democritus argued that all matter consists of tiny particles that cannot be divided further (atomos) and the void between them. They had no experimental evidence, but the idea was revived 2,300 years later as modern atomic theory.
The framework of physics for 2,000 years.
Aristotle held that the Earth is made of four elements, earth, water, air and fire, and the heavens of unchanging aether. Heavier things fall faster, objects move only while a force keeps pushing them, and a vacuum cannot exist. Because it fit everyday experience, where friction is everywhere, it lasted a very long time.
Centered on Alexandria, a tradition flourished of deriving physical laws mathematically and measuring the heavens with geometry. The Earth-centered model was perfected into a precise system of prediction.
Heliocentrism 1,800 years before Copernicus.
Aristarchus of Samos was the first to propose that the Sun is at the center of the universe and the Earth goes around it. He also tried to measure the sizes and distances of the Moon and Sun geometrically. His idea was not accepted.
The first mathematical physicist.
From the simple axiom that symmetric objects balance, Archimedes derived the law of the lever mathematically, and he proved the principle of buoyancy the same way. It was the first time physical laws were deduced from axioms rather than gathered from experience.
The size of the Earth from the angle of a shadow.
At noon on the same day, the Sun shone straight down a well in Syene while objects cast shadows in Alexandria. From the distance between the two cities and the angle of the shadow, Eratosthenes calculated the Earth's circumference quite accurately.
The Earth's axis slowly turns.
Comparing old star records with his own observations, Hipparchus found that the stars' positions had all shifted slightly and steadily. This is precession, the turning of the Earth's axis over a cycle of about 26,000 years. He also compiled the first systematic star catalog.
Light takes the shortest path.
Hero of Alexandria explained the law of reflection by the principle that light bouncing off a mirror takes the shortest of all possible paths. It was the seed of Fermat's principle and the principle of least action. He also left the aeolipile, a device spun by steam.
The definitive Earth-centered model, in use for 1,400 years.
Combining epicycles, eccentrics and the equant, Ptolemy predicted the positions of the planets in an Earth-centered universe with considerable accuracy. In his Optics he also measured experimentally how much light bends when it enters water and glass.
Heavier things do not fall much faster.
John Philoponus of Alexandria criticized Aristotle, pointing out that two objects of different weights dropped together land at almost the same time. His explanation of why a thrown object keeps moving became the prototype of the medieval impetus theory.
Greek texts were translated in the Islamic world and tested again by experiment. Medieval European scholars treated accelerated motion mathematically before Galileo.
Light enters the eye from objects.
Ibn al-Haytham, also known as Alhazen, refuted by experiment the old idea that light comes out of the eye, and studied the camera obscura, refraction and reflection systematically. Because he so rigorously framed hypotheses and tested them with controlled experiments, he is sometimes called the father of experimental science.
Straight-line motion from two circular motions.
At the Maragha observatory, Nasir al-Din al-Tusi devised an arrangement in which a point on a small circle rolling inside a larger one moves back and forth in a straight line, aiming to explain planetary motion without Ptolemy's equant. The same device appears in Copernicus's book 300 years later.
Calculating the distance of uniformly accelerated motion.
Scholars at Merton College, Oxford, found that an object accelerating uniformly covers the same distance as one moving steadily at the speed it has at the midpoint in time. The result came 300 years before Galileo's law of falling bodies.
A thrown object flies on with a driving force inside it.
Jean Buridan of Paris explained that a thrown object keeps moving after leaving the hand because of the "impetus" it received when thrown. His idea that, without air resistance, impetus would not diminish anticipated the concept of inertia.
In effect, a velocity-time graph.
Nicole Oresme drew time horizontally and speed vertically and proved the mean speed theorem as an area of a figure. He showed, 300 years early, the key idea of integration: distance traveled is the area under the graph.
Copernicus put the Sun at the center, and Tycho Brahe's precise observations broke the belief in unchanging heavens.
The Sun at the center of the universe.
In a book published just before his death, Copernicus presented a system in which the Earth orbits the Sun. He still insisted on perfect circular motion, so he needed epicycles, and his predictions were no more accurate than Ptolemy's. The revolution began with a change of perspective, not of precision.
A new star appears in the unchanging heavens.
Tycho showed that a bright star that suddenly appeared in 1572 lay far beyond the Moon, and in 1577 measured that a comet was also beyond the Moon. His observations, the most precise of the pre-telescope age, later passed into Kepler's hands.
Persistence with observational data and the union of experiment and mathematics opened modern physics. The motions of the heavens and of the Earth each gained new laws.
The Earth is a giant magnet.
Experimenting with a spherical magnet, William Gilbert argued that a compass points north because the Earth itself is a magnet. It was an early example of systematic research grounded in experiment.
What came of refusing to ignore eight minutes of arc.
The gap between Tycho's observations of Mars and a circular-orbit model was a mere eight minutes of arc. Kepler would not ignore it, and by pursuing it he published two laws: planetary orbits are ellipses, and the line from the Sun to a planet sweeps out equal areas in equal times. In 1619 he added a third: the square of the orbital period is proportional to the cube of the orbit's size.
Jupiter's moons, the phases of Venus, the mountains of the Moon.
With a telescope he built himself, Galileo discovered four moons circling Jupiter, showing that not everything revolves around the Earth. Venus waxing and waning like the Moon was evidence the Earth-centered model could not explain.
"And yet it moves."
After defending heliocentrism in his 1632 Dialogue Concerning the Two Chief World Systems, Galileo was forced by the Inquisition the following year to recant, and spent the rest of his life under house arrest. The famous line is thought to be a later legend.
The distance fallen grows with the square of the time.
In this book written under house arrest, Galileo used experiments with inclined planes to reveal the law of falling bodies and showed that a thrown object follows a parabola. He also set out inertia and the principle of relativity: inside a ship moving steadily, you cannot tell whether the ship is moving.
Air has weight.
Torricelli showed that when a glass tube filled with mercury is turned upside down, the mercury column stops at about 76 cm, leaving an empty space above it. The vacuum Aristotle had declared impossible appeared in the laboratory. In 1648 Pascal confirmed air pressure by showing that the column was lower on a mountaintop.
A moving object keeps moving in a straight line.
In his Principles of Philosophy, Descartes stated the law of inertia precisely as straight-line motion and argued that the total quantity of motion in the universe is conserved. His cosmology of a universe filled with vortices of matter, however, turned out to be wrong.
Gases, pendulums and the speed of light were measured by experiment, and Newton united Earth and heavens under a single set of laws. It was physics' first great unification.
At constant temperature, a gas's pressure and volume are inversely proportional.
Experimenting with gases using a vacuum pump built with his assistant Hooke, Boyle established the quantitative relationship between pressure and volume. It was the starting point of gas research and experimental chemistry.
Calculus, universal gravitation and the prism, while the plague closed the university.
In 1665–66, while Cambridge was shut by the plague and he stayed at home, the young Newton invented calculus, conceived universal gravitation and used a prism to show that white light is a mixture of colors.
Measuring time precisely, and finding centripetal acceleration.
Huygens invented the pendulum clock, greatly improving the precision of timekeeping, and in his Horologium Oscillatorium showed that an object in circular motion has centripetal acceleration v²/r. Newton used this result in his calculations of gravity.
Light has a speed.
Rømer noticed that the moment Jupiter's moon Io disappeared into Jupiter's shadow came later or earlier depending on the distance between Earth and Jupiter, and concluded that light travels at a finite speed.
The apple and the Moon obey the same law.
From three laws of motion and the law of universal gravitation, Newton derived Kepler's three laws, the tides and even precession. It united the Earth and the heavens that Aristotle had separated, physics' first great unification. Halley paid for the printing, and Hooke disputed priority for the inverse-square law.
Light is a wave.
In his Treatise on Light, Huygens treated light as a wave and explained reflection and refraction by the principle that every point on a wavefront becomes a new source. But Newton's particle theory buried it for a hundred years.
Mathematicians on the Continent rewrote Newtonian mechanics with differential equations and variational principles. At the same time, a new phenomenon, electricity, entered the laboratory.
Light is made of particles.
In Opticks, which collected his prism experiments, Newton treated light as a stream of tiny particles. Thanks to his authority, the particle theory dominated for the next hundred years.
Electricity flows, and comes in two kinds.
Stephen Gray discovered conduction, finding that electricity could travel a long way along a thread. A few years later Du Fay showed there are two kinds of electricity that attract or repel each other.
The Earth bulges at the equator.
Cartesians on the Continent saw universal gravitation, bodies attracting each other at a distance with nothing in between, as a kind of mysticism. Newton predicted the Earth would bulge at the equator, and when Maupertuis's expedition to Lapland confirmed it by surveying near the pole, the debate was settled. Voltaire and Émilie du Châtelet introduced Newton to France.
A fast-flowing fluid has lower pressure.
Daniel Bernoulli revealed the relationship between a fluid's speed and its pressure, and first proposed the kinetic theory of gases, explaining gas pressure as the collisions of countless molecules. In the same period a decades-long debate raged over whether the "quantity" of motion was mv or mv². Both turned out to be right: they became momentum and kinetic energy.
Electricity in a bottle.
A device that stored electricity in a water-filled glass jar and released it all at once was built almost simultaneously in Germany and the Netherlands. The first capacitor, it made experiments with powerful charges possible.
A vibrating string as a differential equation.
After proposing a principle that turns problems of dynamics into problems of statics, d'Alembert wrote down and solved a partial differential equation for the motion of a vibrating string. The wave equation reappears again and again, in sound, light and quantum mechanics.
F = ma as a differential equation.
Newton proved things geometrically, but Euler wrote the laws of motion as differential equations, the form we use today. He also created the equations for rotating rigid bodies and moving fluids.
Lightning is electricity.
Benjamin Franklin is said to have shown that lightning is electrical by flying a kite in a storm. He introduced the ideas of positive and negative charge and of conservation of charge, which is neither created nor destroyed, and invented the lightning rod.
Ice doesn't get warmer while it melts.
Joseph Black discovered latent heat: when ice melts or water boils, it absorbs heat without changing temperature. He also established specific heat, the idea that different substances need different amounts of heat to warm up. At the time heat was thought to be a weightless fluid called "caloric."
Electric force also falls off with the square of distance.
Coulomb built a torsion balance that measured tiny forces by the twist of a thin fiber, and showed that the force between two charges follows an inverse-square law, just like gravity.
A mechanics textbook without a single figure.
Lagrange developed all of mechanics from variational principles and boasted in his preface that the book contained no diagrams. His method, which lets you choose any coordinates you like, was powerful for solving complex systems.
Weighing the Earth.
With a torsion balance, Henry Cavendish measured the tiny gravitational pull between lead balls in his laboratory. From his result, the gravitational constant and the mass of the Earth could be calculated.
"I had no need of that hypothesis."
Laplace calculated how the planets perturb one another and showed the solar system to be stable. When Napoleon remarked that the work never mentioned God, Laplace famously replied that he had no need of that hypothesis.
The wave theory of light was revived, electricity and magnetism were linked, and heat and work were shown to be the same thing. The field and energy, core concepts of modern physics, were born.
The first steady electric current.
Volta stacked zinc and copper disks with cloth soaked in brine between them and obtained a continuous current. Beyond the momentary discharge of static electricity, the age of electricity that keeps flowing began.
Light plus light makes darkness.
Thomas Young showed that light passing through two slits forms bright and dark fringes, proving that light is a wave. It was the first powerful counterattack against Newton's particle theory, which had reigned for a hundred years.
A bright spot in the middle of a disk's shadow.
When Fresnel explained diffraction mathematically, Poisson objected that his theory implied a bright spot at the center of a disk's shadow, which seemed absurd. Arago did the experiment and found the spot. The objection became the decisive evidence.
An electric current moves a compass.
During a lecture, Ørsted noticed that a compass needle moved next to a wire carrying current. It was the moment electricity and magnetism were linked. Soon after, Ampère quantified the forces that currents exert on one another.
Any function as a sum of sine waves.
To solve the equation for how heat spreads, Fourier created Fourier series, which represent a function as an infinite sum of sines and cosines. It changed the very concept of a function in mathematics and is the foundation of today's signal processing.
A heat engine's efficiency depends on two temperatures.
Sadi Carnot showed that the maximum efficiency of a heat engine depends not on the kind of engine but only on the temperatures of its hot and cold reservoirs. Though built on the mistaken caloric theory, his conclusion was correct and became the seed of the second law of thermodynamics.
Voltage = current × resistance.
Measuring current while varying the length and thickness of wires, Ohm found the simple relationship among voltage, current and resistance. At first German academics ignored him.
Move a magnet, and a current appears.
Faraday discovered that a changing magnetic field produces an electric current, the principle behind generators and transformers. With almost no mathematical training, he imagined "lines of force" filling space, the concept of the field. In 1845 he found that a magnetic field rotates the polarization of light, the first hint of a link between light and electromagnetism.
Mechanics as optics.
Using an analogy between the paths of light rays and the motion of particles, Hamilton reformulated mechanics. At the time it was seen as merely an elegant restatement, but a hundred years later it became the backbone of quantum mechanics.
A planet found on paper.
From small irregularities in Uranus's orbit, Le Verrier and Adams calculated the position of an unseen planet, and Galle found Neptune right where predicted. It was the high point of Newtonian mechanics.
Heat and work are two faces of the same thing.
In the 1840s Mayer, Joule and Helmholtz each concluded that energy only changes form and never disappears. Joule precisely measured how much work turns into heat with a device in which falling weights stirred water. This is the first law of thermodynamics.
Maxwell united electricity, magnetism and light, and Boltzmann explained heat as the statistics of atoms. At the end of the century, just as classical physics seemed complete, phenomena it could not explain began pouring in.
Heat does not flow by itself from cold to hot.
Clausius and Kelvin each formulated the second law, and in 1865 Clausius expressed it with the concept of "entropy." The same year, Foucault and Fizeau measured that light travels more slowly in water, putting an end to the particle theory of light.
Reading the makeup of stars from bands of light.
Kirchhoff and Bunsen discovered that every element has its own characteristic spectral lines. Just by analyzing the light of the Sun and stars, you could tell which elements they contained. The same year Le Verrier reported an unexplained anomaly in the perihelion of Mercury, a puzzle general relativity would solve 56 years later.
Electricity, magnetism and light are one.
Translating Faraday's field concept into mathematics, Maxwell added a displacement current term, and electromagnetic waves fell out of the equations. Their calculated speed matched the speed of light. It was physics' second great unification. Heaviside later condensed Maxwell's original 20 equations into four using vectors.
S = k log W.
After Maxwell derived the distribution of molecular speeds in a gas, Boltzmann explained entropy as the number of possible microscopic states. The second law was not an absolute law but a matter of overwhelming probability. Yet under attack from Mach, Ostwald and others who denied that atoms were real, Boltzmann felt isolated all his life.
A hidden rule in hydrogen's spectral lines.
Johann Balmer, a Swiss schoolteacher, found that the wavelengths of light emitted by hydrogen all fit one simple formula. No one knew why, and the answer came 28 years later in Bohr's model of the atom.
There was no aether.
If the Earth moves through the aether, the medium thought to carry light, the speed of light should differ by direction. Yet however precisely it was measured, there was no difference. FitzGerald and Lorentz tried to explain this by proposing that moving objects contract.
Maxwell's prediction in the laboratory.
Hertz generated electromagnetic waves with electric sparks and detected them with a loop some distance away, showing that they reflect and refract like light. He said the discovery had no use, but it soon led to wireless communication.
A mysterious light that shows the bones of the hand.
During experiments with a cathode ray tube, Röntgen discovered an unknown radiation that passed through paper and called it "X-rays." His photograph of his wife's hand bones caused a worldwide sensation, and the rays were put to medical use almost immediately. He received the first Nobel Prize in Physics.
Elements that give off radiation on their own.
Becquerel discovered that uranium salts fog photographic plates even without exposure to light. In 1898 Marie and Pierre Curie found the far more radioactive elements polonium and radium. The belief that atoms never change began to waver.
A particle smaller than the atom.
J.J. Thomson showed that cathode rays are streams of negatively charged particles far lighter than atoms. It was the first evidence that the supposedly indivisible atom has an internal structure.
From Kelvin's "two clouds" came two revolutions. The meaning of time, space and gravity changed, and the world of the atom came to be described by probabilities. All of this took just 30 years.
Energy is exchanged only in lumps.
In 1900 Kelvin named two clouds remaining over physics: the failure to detect the aether and the problem of blackbody radiation. The same year, to fit the blackbody spectrum, Planck assumed that energy is exchanged only in units of E = hν. Even he made the assumption reluctantly, regarding it as a mathematical trick.
Light comes in grains, and atoms are real.
Einstein explained the photoelectric effect by proposing that light itself consists of lumps of energy (light quanta). It was for this work that he received the Nobel Prize. The same year he explained the erratic motion of pollen grains in water as the result of molecular collisions, and when Perrin's experiments confirmed it in 1908, the debate over whether atoms are real was over.
The speed of light is the same for everyone.
From the assumption that the speed of light is the same for all observers, Einstein concluded that time and space differ from one observer to another. The aether became unnecessary, and the same year brought E = mc², the equivalence of mass and energy. In 1908 Minkowski recast the theory as the geometry of four-dimensional spacetime.
A tiny, heavy core at the center of the atom.
Firing alpha particles at gold foil, Rutherford saw some bounce almost straight back and realized that most of an atom's mass is concentrated in a tiny nucleus at its center. He said it was as if he had fired a cannon shell at tissue paper and it had come back.
Electrons can only occupy certain orbits.
Bohr explained the Balmer formula with a quantum condition: electrons stay only in particular orbits and emit light equal to the energy difference when they jump between them. He could not explain why only those orbits were allowed.
Gravity is the curvature of spacetime.
Starting from the equivalence principle, that a person in free fall does not feel their own weight, Einstein explained gravity as mass curving spacetime. He had to learn Riemannian geometry with the help of his friend Grossmann, and at the end completed the equations in something like a race with Hilbert. The anomaly in Mercury's perihelion, 43 arcseconds per century, was explained exactly.
Where there is symmetry, there is a conservation law.
Emmy Noether proved that every continuous symmetry of the laws of physics corresponds to a conservation law. Symmetry in time gives conservation of energy; symmetry in space gives conservation of momentum. The result underlies all of modern physics.
Starlight bends near the Sun.
During a total solar eclipse, Eddington's expedition confirmed that stars near the Sun appeared shifted by exactly the amount general relativity predicted. When the news hit the front pages, Einstein became world-famous overnight.
Light acts like particles, electrons like waves.
Compton showed that X-rays change wavelength when they collide with electrons like billiard balls, confirming light quanta. The next year de Broglie proposed the reverse: matter such as electrons also has wave properties.
Quantum mechanics born on the island of Heligoland.
Escaping hay fever on the island of Heligoland, Heisenberg built a theory using only observable quantities, and Born and Jordan recognized it as matrix calculation. The same year Pauli proposed the exclusion principle, that no two electrons can be in the same state, and Uhlenbeck and Goudsmit proposed electron spin.
The square of the wave function is a probability.
Schrödinger wrote a wave equation for matter waves. At first it looked nothing like matrix mechanics, but the two were soon proved mathematically equivalent. Born interpreted the square of the wave function as the probability of finding the particle. It was the moment probability entered the fundamental laws of nature.
"God does not play dice."
Heisenberg published the uncertainty principle: position and momentum cannot both be known precisely at once. Together with Bohr's complementarity, it formed the Copenhagen interpretation. The same year, at the Solvay Conference, the famous debate between Einstein and Bohr began.
Relativity meets quantum mechanics, and antimatter appears.
Dirac wrote a quantum equation for the electron that satisfied relativity, and it produced solutions with negative energy. Dirac interpreted them as antiparticles, identical to electrons but with opposite charge, and in 1932 Anderson discovered the positron in cosmic rays.
The galaxies are moving away.
Hubble showed by observation that the farther away a galaxy is, the faster it is receding. Lemaître had already predicted an expanding universe from general relativity in 1927. The picture of an eternal, unchanging universe came to an end.
The particles and forces inside the atomic nucleus were revealed one by one, and the discovery of fission led to the atomic bomb. After the war, the theory of light and electrons overcame the problem of infinities and was completed.
An invisible particle that carries off missing energy.
When energy seemed not to be conserved in beta decay, Pauli proposed that a new, uncharged particle that barely interacts with anything was carrying the energy away. It was detected only 26 years later, in 1956.
The second particle in the nucleus.
Chadwick discovered the neutron, a nuclear particle with no charge. The same year von Neumann placed quantum mechanics on a rigorous mathematical footing in Hilbert space.
Far more mass than we can see.
Zwicky found that the galaxies in the Coma Cluster move so fast that, with only the visible mass, the cluster should fly apart. He proposed that invisible "dark matter" was present.
A new force behind beta decay.
Fermi built a theory of how a neutron turns into a proton while emitting an electron and a neutrino. After gravity and electromagnetism, a new fundamental force, the weak interaction, entered physics.
A particle that carries the nuclear force, and is quantum mechanics incomplete?
Yukawa predicted that the force binding protons and neutrons in the nucleus is carried by a particle of intermediate mass. The same year Einstein, with Podolsky and Rosen, wrote the EPR paper arguing that quantum mechanics is incomplete. The debate led to Bell's inequality 30 years later.
The uranium nucleus splits in two.
Hahn and Strassmann found much lighter barium produced in uranium bombarded with neutrons, and Meitner and Frisch explained that the nucleus had split in two, nuclear fission, releasing enormous energy.
A chain reaction under a Chicago stadium.
In a reactor built of graphite and uranium under the stands of the University of Chicago's stadium, Fermi's team produced the first self-sustaining nuclear chain reaction.
Physics changes the outcome of a war.
At Los Alamos, led by Oppenheimer, and other sites, the US developed the atomic bomb, conducted the first nuclear test in July and dropped bombs on Japan in August. Physicists found themselves at the center of debates over responsibility for and control of nuclear weapons.
A tiny discrepancy, and quantum mechanics as technology.
Lamb precisely measured that two energy levels of the hydrogen atom differ very slightly from Dirac's prediction. This small discrepancy spurred the completion of quantum electrodynamics. The same year, at Bell Labs, the transistor was invented on the basis of the quantum theory of solids.
Taming the infinities.
Calculating how electrons and light interact gave infinity for every answer. Feynman, Schwinger and Tomonaga each devised a way to handle the infinities through renormalization, and Dyson showed the three methods were the same. QED agrees with experiment to more than ten decimal places, making it the most precisely tested theory there is.
Accelerators produced hundreds of new particles, and symmetry and gauge theory brought order to the chaos. The cosmic microwave background confirmed the Big Bang, and by the mid-1970s the Standard Model was complete.
The gauge theory that would become the language of the Standard Model.
Chen-Ning Yang and Robert Mills extended the symmetry of electromagnetism to a more complex symmetry in a gauge theory. It seemed useless at the time, but 20 years later it became the language of the Standard Model, which describes all the fundamental forces.
A ghost particle caught after 26 years.
Cowan and Reines succeeded in detecting neutrinos streaming from a nuclear reactor, 26 years after Pauli predicted them.
Nature can break mirror symmetry.
In 1956 Tsung-Dao Lee and Chen-Ning Yang proposed that mirror symmetry (parity) might be broken in the weak interaction, and the next year Chien-Shiung Wu confirmed it with an experiment on cobalt-60. The same year Bardeen, Cooper and Schrieffer put forward the BCS theory explaining superconductivity.
Light in one direction and one color.
Theodore Maiman built the first working laser from a ruby crystal. Based on the stimulated emission Einstein predicted in 1917, it was jokingly called "a solution looking for a problem," but today it is used everywhere, in communications, medicine and manufacturing.
Order in the particle zoo, the origin of mass, and a test for entanglement.
Gell-Mann and Zweig classified hundreds of particles using group theory and proposed smaller constituents called quarks. The same year Higgs, Englert, Brout and others proposed a mechanism by which particles acquire mass, and Bell produced an inequality that could test quantum entanglement experimentally.
Antenna noise turns out to be the afterglow of the Big Bang.
Penzias and Wilson tracked down an antenna noise they could not get rid of and found it was radiation of about 3 K coming evenly from the entire sky. It was the afterglow of the Big Bang, and it settled the Big Bang theory.
Electromagnetism and the weak force are one force.
Building on earlier work by Glashow, Weinberg and Salam each unified electromagnetism and the weak interaction into a single gauge theory. It was another great unification after Maxwell's.
The outer stars of galaxies move too fast.
Vera Rubin and Kent Ford observed that stars in the outer parts of galaxies orbit far faster than expected. Their observations through the 1970s turned Zwicky's dark matter into an unavoidable problem.
A strong force that weakens up close.
Gross, Wilczek and Politzer discovered asymptotic freedom: the strong force between quarks actually weakens at short distances. Quantum chromodynamics (QCD), the theory of the strong force, took hold, and by the mid-1970s the Standard Model, explaining the strong, weak and electromagnetic forces, was complete.
Black holes glow too.
Hawking showed that applying quantum mechanics at the edge of a black hole means it has a temperature, gives off radiation and very slowly evaporates. It was the first result to bring general relativity, quantum mechanics and thermodynamics together in one place.
The particles predicted by the Standard Model were found one by one, and quantum entanglement was confirmed by experiment. Yet at the end of the century, signals from beyond the Standard Model appeared.
Exact quantization in the macroscopic world.
Von Klitzing discovered that the electrical resistance of two-dimensional electrons in a strong magnetic field forms precise steps set only by fundamental constants. The phenomenon is explained by topology and became the starting point of research into topological materials.
Quantum entanglement is real.
Using pairs of entangled photons, Aspect's team showed precisely that Bell's inequality is violated. The debate began to settle in favor of the reality of quantum entanglement, which Einstein had called "spooky action at a distance."
The particles that carry the weak force.
Proton–antiproton collisions at CERN revealed the W and Z bosons predicted by electroweak theory. A central prediction of the Standard Model was confirmed.
Vibrating strings instead of particles.
Green and Schwarz showed that the contradictions (anomalies) that threatened to ruin string theory could cancel out. Interest in string theory as a candidate for explaining every force, gravity included, exploded. It has not been confirmed by experiment to this day.
The last quark, and atoms in chorus.
Fermilab discovered the top quark, the sixth and heaviest quark. The same year Cornell and Wieman cooled atoms close to absolute zero and created the first Bose–Einstein condensate, predicted by Einstein and Bose in the 1920s.
Two signals from beyond the Standard Model.
Japan's Super-Kamiokande confirmed that neutrinos change type as they travel, which means neutrinos have mass, contrary to the Standard Model's assumption of zero. The same year observations of distant supernovae revealed that the expansion of the universe is speeding up. The cause, dark energy, makes up about 70% of the universe.
Giant experiments confirmed predictions a century old. But unifying gravity with quantum theory, the nature of dark matter and dark energy, and why there is more matter than antimatter remain open questions.
27 km around, the largest experiment ever built.
CERN's Large Hadron Collider (LHC) began operating. Deep beneath the Swiss–French border, it accelerates protons close to the speed of light and smashes them together.
The Standard Model's last piece, found after 48 years.
The ATLAS and CMS experiments at the LHC announced the discovery of the Higgs boson, 48 years after it was predicted in 1964. Higgs and Englert received the Nobel Prize the following year.
The tremor of a black hole collision 1.3 billion years ago.
LIGO directly detected, for the first time, the gravitational waves from two black holes merging (announced in 2016). It came a century after Einstein's prediction and opened a new astronomy that "listens" to the universe.
Topology explains states of matter.
Thouless, Haldane and Kosterlitz received the Nobel Prize for their work on topological phase transitions and topological phases of matter. It was a case of mathematical topology becoming a core tool of physics.
Seeing the shadow of an event horizon.
The Event Horizon Telescope (EHT), linking radio telescopes around the world into one, captured the first image of the shadow of the black hole at the center of the galaxy M87.
Quantum entanglement becomes the foundation of quantum information science.
Aspect, Clauser and Zeilinger received the Nobel Prize for showing violations of Bell's inequality with entangled photons and pioneering quantum information science. The EPR debate of 1935 was settled with a Nobel Prize 87 years later.
Quantum effects in a circuit you can hold in your hand.
Clarke, Devoret and Martinis received the Nobel Prize for demonstrating macroscopic quantum tunneling and energy quantization in superconducting electrical circuits in the 1980s. Their work became the foundation of today's superconducting-qubit quantum computers.