The Physics of Very Small Things

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Atoms, electrons, and light obey rules that differ from the rules governing cricket balls and planets. Physicists worked those rules out between 1900 and 1930 and named them quantum mechanics. The century since has extended the rules to chemistry, electronics, nuclear power, the particles inside the proton, and the first instant of the universe. Every experiment so far agrees with the theory. What the theory says about reality remains in dispute.

Three failures of nineteenth-century physics

By 1890 physicists had Newton’s mechanics for matter and Maxwell’s equations for light, and many expected only refinements to follow. Three measurements broke that confidence.

The glow of hot objects. A stove element glows red, then orange, then white as the element heats. Classical theory predicted that any hot object should pour unlimited energy into ultraviolet light, a result later named the ultraviolet catastrophe. In 1900 Max Planck found a formula that matched the measured glow. The formula worked only if matter emitted light in packets, each carrying energy equal to the light’s frequency multiplied by a tiny constant. Planck’s constant equals 6.6 × 10⁻³⁴ joule-seconds, and its smallness explains why nobody had noticed the packets before.

Light knocking electrons out of metal. Shine ultraviolet light on zinc and electrons fly off. Shine red light on the zinc, however bright, and the electrons stay put. A brighter wave should shake electrons harder, so wave theory had no answer. In 1905 Einstein proposed that light travels as particles, later named photons, and that each photon’s energy depends on its colour. One ultraviolet photon carries enough energy to free one electron. A red photon falls short, and a billion red photons fall short a billion times. Arthur Compton confirmed the particle picture in 1923 by bouncing X-ray photons off electrons like billiard balls.

The atom that should collapse. Ernest Rutherford, born near Nelson, discovered in 1911 that an atom’s mass sits in a tiny central nucleus with electrons outside. Classical physics says an orbiting electron radiates light, loses energy, and spirals into the nucleus within a hundred-billionth of a second. Atoms last billions of years. In 1913 Niels Bohr patched the model by decree: electrons occupy only certain orbits, each with a fixed energy, and emit light only when jumping between orbits. Bohr’s rule reproduced the colours of glowing hydrogen to four significant figures. Nobody knew why the rule worked.

Matter is a wave

In 1924 a French doctoral student, Louis de Broglie, reversed Einstein’s move. If light waves behave as particles, particles should behave as waves, with a wavelength equal to Planck’s constant divided by momentum. In 1927 Clinton Davisson and Lester Germer fired electrons at a nickel crystal and watched the electrons bounce off in the ripple pattern that X-ray waves produce. George Thomson found the same result independently. His father, J. J. Thomson, had won a Nobel Prize for proving the electron is a particle; George won his for proving the electron is a wave.

De Broglie’s idea explained Bohr’s orbits. An electron wave wrapped around a nucleus must join up with itself, like a standing wave on a guitar string. Only whole numbers of wavelengths fit, so only certain orbits exist.

Electrons sent one at a time through two slits land as single dots, and the dots accumulate into the striped pattern that overlapping ripples make. Physicists have repeated the experiment with atoms and with molecules, and in 2019 a Vienna team recorded stripes from molecules of 2,000 atoms each. Every object has a wavelength. A cricket ball bowled at 140 km/h has a wavelength one ten-billion-billionth the width of a proton, far too short for any slit to reveal.

The rules of the game

Two complete theories arrived within months of each other. Werner Heisenberg, recovering from hay fever on the island of Heligoland in 1925, built a theory from tables of numbers describing jumps between energy levels. Erwin Schrödinger wrote a wave equation in 1926. Paul Dirac proved the two theories identical in content. They share these rules.

The state. A list of numbers called amplitudes describes a quantum system completely, one amplitude for each possible outcome of a measurement. For an electron’s position the list becomes a continuous wave, the wave function.

Amplitudes have direction. An amplitude behaves like an arrow on a clock face, with a length and an angle. When an event can happen two ways, the arrows for each way add tip to tail. Arrows pointing the same way reinforce; arrows pointing opposite ways cancel. The dark stripes in the two-slit pattern mark places where the arrows for “left slit” and “right slit” cancel.

The Born rule. Max Born supplied the link to experiment in 1926: the probability of an outcome equals the squared length of the outcome’s arrow. Ordinary probabilities only ever add. Amplitudes can cancel, and cancellation is the source of every quantum oddity.

Superposition. Any weighted blend of allowed states is also an allowed state. An electron can be in a blend of “here” and “there”; an atom can be in a blend of “decayed” and “intact”.

Two kinds of change. Left alone, the wave function changes smoothly and predictably according to Schrödinger’s equation. When measured, the system yields one outcome at random, with odds set by the Born rule, and the wave function resets to match that outcome. The theory never says which physical processes count as measurements. That gap, the measurement problem, remains open.

Quantised values. A confined wave, like a guitar string, vibrates only in whole-number patterns, so a confined electron’s energy takes only certain values. A free electron’s energy can take any value. Quantum mechanics is named after the confined case.

Richard Feynman found a third formulation in 1948. A particle travelling from A to B takes every possible path at once, each path contributing an arrow. For large objects the arrows of all paths cancel except those near the single path Newton predicts.

Uncertainty and the energy of confinement

A wave with one precise wavelength extends forever, like a pure sustained tone. A wave concentrated at one spot, like a hand clap, mixes many wavelengths. Wavelength sets momentum, so an electron with a sharp position has a broad spread of momentum, and the reverse. Heisenberg put the trade-off in numbers in 1927: the spread in position multiplied by the spread in momentum always exceeds Planck’s constant divided by 4π. Better instruments make no difference, because the limit belongs to the wave.

The trade-off solves the collapsing atom. Squeeze an electron closer to the nucleus and the electron’s position sharpens, the momentum spread grows, and the energy of motion rises. The electron settles where the pull of the nucleus balances that rising energy, about a twentieth of a nanometre out. Uncertainty sets the size of every atom, and therefore the density of every solid.

The same principle forbids perfect stillness. A particle at rest at a known spot would have sharp position and sharp momentum together, so every confined particle keeps a minimum jiggle called zero-point energy. Zero-point motion keeps helium liquid all the way down to absolute zero at ordinary pressure: the light atoms jiggle too hard to lock into a crystal.

Spin, and why matter is solid

In 1922 Otto Stern and Walther Gerlach sent a beam of silver atoms through an uneven magnetic field. Tiny classical magnets pointing in random directions would smear the beam into a band. The beam split into two clean spots. Electrons carry a built-in magnetism called spin, and spin measured along any direction gives one of two answers: up or down.

Spin resembles rotation and differs from rotation in one startling respect. Turn an electron through a full circle and the electron’s wave function flips sign. Two full turns restore the original.

Particles divide into two families according to spin.

Fermions have half-unit spin: electrons, protons, neutrons, quarks. Wolfgang Pauli found their law in 1925: two identical fermions never occupy the same state.

Bosons have whole-unit spin: photons, helium-4 atoms, the Higgs particle. Identical bosons crowd into a shared state, and the more that share, the more join.

Pauli’s exclusion principle builds the periodic table. Electrons in an atom fill the lowest energy level two at a time, one spin up and one spin down, then the next level, and so on outward. The outermost electrons determine how an element bonds, which explains why the table’s columns share chemical behaviour. Exclusion also makes matter solid. Your hand stops at the tabletop because the electrons in your hand and in the table refuse to share states. In a white dwarf, a dead star the size of Earth, the same refusal holds up a Sun’s worth of mass against gravity. Subrahmanyan Chandrasekhar calculated in 1930 that above 1.4 solar masses exclusion fails and the star collapses further, into a neutron star or a black hole.

Bosons give the opposite show.

Lasers. Einstein showed in 1917 that a passing photon prompts an excited atom to emit an identical photon. Theodore Maiman built the first laser on that principle in 1960. A laser beam consists of trillions of photons in one shared state.

Bose–Einstein condensates. Cool a gas of boson atoms to within a millionth of a degree of absolute zero and the atoms merge into one wave function, a single quantum object visible to a camera. Eric Cornell, Carl Wieman, and Wolfgang Ketterle made the first condensates in 1995.

Superfluids. Below 2.17 kelvin, liquid helium-4 flows without friction, climbs the walls of its container, and leaks through pores too fine for any ordinary liquid.

Tunnelling

A ball rolling toward a hill with too little energy to reach the top rolls back. A wave meeting a barrier fades gradually inside the barrier, and if the barrier is thin a remnant of the wave emerges on the far side. The particle therefore has some probability of appearing beyond a barrier that classical physics calls impassable. The probability falls exponentially with the barrier’s thickness.

Radioactive decay. George Gamow explained alpha decay in 1928: a cluster of two protons and two neutrons rattles inside a heavy nucleus and tunnels out through the wall of nuclear attraction. Small differences in the wall produce half-lives from microseconds to billions of years. For any single nucleus the moment of decay is random. For a large collection the half-life is exact, which lets carbon-14, half-life 5,730 years, date bones and charcoal.

Starlight. The Sun’s core sits at 15 million degrees. Classical physics requires about 10 billion degrees to force two protons together against their electrical repulsion. The protons tunnel. The odds for any one collision are so poor that a typical proton waits billions of years to fuse, which is why the Sun burns steadily for ten billion years.

Microscopes. Gerd Binnig and Heinrich Rohrer built the scanning tunnelling microscope in 1981. A needle hovers a nanometre above a surface, electrons tunnel across the gap, and the current changes tenfold when the gap changes by the width of one atom. In 1989 IBM researchers used such a needle to spell the company’s name with 35 xenon atoms.

Memory chips. Flash memory stores each bit by tunnelling electrons through an insulating layer onto an isolated island of silicon, where the electrons stay trapped for years.

Enzymes. Several enzymes speed up reactions by letting hydrogen nuclei tunnel between molecules.

Tunnelling also limits chip makers: shrink a transistor’s insulating layer below about a nanometre and electrons leak straight through.

Chemistry is applied quantum mechanics

Solve Schrödinger’s equation for one electron around a nucleus and the solutions are three-dimensional standing waves called orbitals: spheres, dumbbells, cloverleafs. An orbital maps where the electron is likely to be found.

A chemical bond forms when orbitals from two atoms overlap and merge. In a hydrogen molecule the two electrons spread across both nuclei. A wider box means a longer wavelength and lower energy of motion, so the merged state has less energy than two separate atoms, and the difference is the bond’s strength. Linus Pauling built the modern theory of bonding on that foundation in the 1930s.

Colour follows from energy levels. A carrot is orange because the electrons in carotene absorb blue photons, whose energy matches the gap between two of the molecule’s levels, and reflect the rest. Astronomers read the same fingerprints in starlight to learn what stars are made of.

Dirac wrote in 1929 that the laws underlying all of chemistry were known, and that the only difficulty was equations too hard to solve. The difficulty persists. The list of amplitudes doubles with each added particle, so the wave function of 50 interacting electrons requires more numbers than any computer can store. Chemists rely on clever approximations, and the exponential wall is the original motive for building quantum computers.

Living cells exploit quantum effects in at least one confirmed way, enzyme tunnelling. Migratory birds appear to sense Earth’s magnetic field through pairs of entangled electrons in a retinal protein called cryptochrome; laboratory evidence supports the mechanism, and proof inside a living bird is still missing. Early claims that photosynthesis depends on long-lived quantum waves have faded under later measurements.

Solids: bands, chips, and superconductors

Bring 10²³ atoms together into a crystal and each atomic energy level spreads into a band of closely spaced levels. Electrons fill the bands from the bottom up, and the filling pattern decides the material’s electrical character.

Metals have a partly filled band. Electrons shift into empty neighbouring levels at the slightest push, so current flows.

Insulators have a full band and a wide energy gap above. The electrons have nowhere to go.

Semiconductors have a narrow gap. Heat, light, or a sprinkling of foreign atoms lifts a controlled number of electrons across.

John Bardeen, Walter Brattain, and William Shockley used that control to build the first transistor at Bell Labs in 1947: a switch in which a small voltage turns a semiconductor’s conduction on and off. A modern phone processor holds more than fifteen billion transistors. A light-emitting diode drops electrons across the gap to make photons of a colour set by the gap’s width, and a solar cell runs the same process backwards.

In 1911 Heike Kamerlingh Onnes cooled mercury to 4 kelvin and watched the metal’s electrical resistance vanish completely. Bardeen, Leon Cooper, and Robert Schrieffer explained superconductivity in 1957. Vibrations of the crystal bind electrons into pairs, the pairs behave as bosons, and the pairs condense into one shared state that flows without friction. Superconducting magnets run every hospital MRI scanner and steer the beams at CERN. In 1986 Georg Bednorz and Alex Müller found copper-oxide ceramics that superconduct at far higher temperatures, today up to about 135 kelvin at ordinary pressure. Forty years later physicists still lack an agreed explanation.

Other collective quantum states keep turning up. In 1980 Klaus von Klitzing found that a thin semiconductor layer in a strong magnetic field has an electrical resistance locked to exact fractions of 25,812.807 ohms, a number built only from Planck’s constant and the electron’s charge. National laboratories use the effect to define the ohm, and since 2019 the kilogram itself has been defined by fixing the value of Planck’s constant. Graphene, a sheet of carbon one atom thick isolated in 2004, carries electrons that behave as if they had no mass. Topological insulators block current through their interior and conduct along their surfaces.

Quantum fields

Schrödinger’s equation ignores Einstein’s relativity and fails for fast particles. In 1928 Dirac found an equation for the electron that respects both theories. Dirac’s equation produced spin automatically, and the equation had a second set of solutions describing a particle with the electron’s mass and the opposite charge. Carl Anderson found that particle, the positron, in cosmic-ray tracks in 1932. Every particle has an antimatter twin, and a particle meeting its twin converts into photons. PET scanners map the paired photons from positrons annihilating inside a patient’s body.

Relativity allows energy to become new particles, so a theory with a fixed number of particles cannot be complete. Quantum field theory, the repair, makes fields the basic ingredients of the world. One electron field fills all of space, and every electron is a travelling ripple in that field, which explains why all electrons are exactly alike. One electromagnetic field fills space likewise, and photons are the electromagnetic field’s ripples.

A quantum field keeps zero-point motion even in empty space. The vacuum seethes with fleeting disturbances, and the disturbances have measurable effects. Hendrik Casimir predicted in 1948 that two uncharged metal plates a few hundred nanometres apart would be pushed together, because the gap excludes some of the vacuum’s longer waves. Steve Lamoreaux measured the force in 1997. The same vacuum jitter nudges the energy levels of hydrogen by less than one part in a million, a shift Willis Lamb measured in 1947.

Early calculations of such effects returned infinity. In the late 1940s Sin-Itiro Tomonaga, Julian Schwinger, Feynman, and Freeman Dyson learned to absorb the infinities into the measured mass and charge of the electron, a procedure called renormalisation. The result, quantum electrodynamics, is the most accurate theory in science. Theory and experiment agree on the strength of the electron’s magnetism to about one part in a trillion, equivalent to measuring the distance from Auckland to London to within a quarter of a hair’s width.

The Standard Model

Through the 1960s and 1970s physicists extended the field idea to the forces inside the nucleus. The resulting theory, the Standard Model, lists 17 fields.

Six quarks: up, down, strange, charm, bottom, top. Two ups and a down make a proton; two downs and an up make a neutron.

Six leptons: the electron, the electron’s heavier copies the muon and the tau, and three neutrinos.

Four force carriers: the photon for electromagnetism, the gluon for the strong force, and the W and Z particles for the weak force.

The Higgs field: the field whose drag on other particles gives them mass. CERN’s Large Hadron Collider found the Higgs particle in July 2012, forty-eight years after the prediction.

The strong force binds quarks with a strength that grows as quarks separate, like a rubber band. David Gross, Frank Wilczek, and David Politzer discovered the rule in 1973. Pull two quarks apart and the energy stored in the band creates new quarks at the broken ends, so nobody has ever isolated a single quark. The three quarks in a proton supply about one per cent of the proton’s mass. The energy of the gluon field supplies the remainder, via E = mc². Most of the mass of your body is the energy of confined fields.

The weak force changes one kind of quark into another. In beta decay a neutron becomes a proton, an electron, and a neutrino, and the first step of fusion in the Sun depends on the same process. Sheldon Glashow, Abdus Salam, and Steven Weinberg showed in the 1960s that the weak force and electromagnetism are two faces of one electroweak force, and CERN found the predicted W and Z particles in 1983.

About 100 trillion neutrinos from the Sun pass through your body every second, and almost all pass through the whole Earth untouched. Detectors in Japan and Canada showed between 1998 and 2001 that neutrinos change type in flight, which requires neutrinos to have mass. The original Standard Model gave neutrinos none, and neutrino mass remains the theory’s one confirmed laboratory failure. For two decades the muon’s magnetism looked like a second failure; Fermilab’s final measurement in 2025 and improved supercomputer calculations of the prediction now agree within their uncertainties.

The Standard Model omits gravity. The model has no particle for dark matter, which astronomers infer from the motion of galaxies and which outweighs ordinary matter five to one. The model also fails to explain why the Big Bang left more matter than antimatter.

Entanglement

Two particles that interact can end up sharing one state that assigns definite properties to the pair and none to either particle alone. Prepare two photons so that their polarisations always match, and send the photons to opposite ends of a laboratory, or a continent. Each photon measured on its own gives a random result. Compare the records and the results match every time.

Einstein, Boris Podolsky, and Nathan Rosen argued in 1935 that such particles must carry hidden instructions, like a pair of gloves posted in separate boxes, and that quantum mechanics was incomplete for leaving the instructions out. In 1964 the Belfast-born physicist John Bell proved that the two pictures make different predictions. When the experimenters measure along tilted directions, hidden instructions can produce correlations only up to a fixed ceiling, and quantum mechanics predicts correlations above the ceiling.

John Clauser and Stuart Freedman ran the first test in 1972, Alain Aspect closed a major loophole in 1982, and teams in Delft, Vienna, and Boulder closed the remaining loopholes in 2015. Every test broke Bell’s ceiling. No theory in which particles carry pre-set answers and influences travel at or below light speed can reproduce the results. Clauser, Aspect, and Anton Zeilinger shared the 2022 Nobel Prize.

Three consequences follow.

No faster-than-light messages. Each experimenter sees pure randomness locally. The correlation shows up only when the two records are brought together by ordinary means.

No cloning. William Wootters, Wojciech Zurek, and Dennis Dieks proved in 1982 that no machine can copy an unknown quantum state. An eavesdropper who intercepts a quantum signal always leaves traces, which is the basis of quantum cryptography.

Teleportation. A sender who shares an entangled pair with a receiver can transfer a particle’s exact state by sending two ordinary bits of data. The process destroys the original state, so the no-cloning rule survives. Laboratories first teleported photon states in 1997. In 2017 the Chinese satellite Micius delivered entangled photons to ground stations 1,200 kilometres apart.

The measurement problem

Schrödinger devised his cat in 1935 to show the absurdity of taking the theory literally. Seal a cat in a box with a radioactive atom rigged to release poison on decay. Schrödinger’s equation says the atom becomes a blend of decayed and intact, then the poison vial a blend of broken and whole, then the cat a blend of dead and alive. Nobody has seen a blended cat.

Decoherence, worked out from the 1970s onward by Dieter Zeh and Wojciech Zurek, explains part of the puzzle. Air molecules and photons strike a large object constantly, and each collision entangles the object with its surroundings. The delicate arrow relationships between “alive” and “dead” leak into trillions of scattered particles in far less than a trillionth of a second. After that the two branches never again interfere, and each behaves as an ordinary classical world. Decoherence explains why large superpositions go unseen. Decoherence leaves one question untouched: why an observer finds one branch, and what becomes of the other.

Physicists have proposed several answers.

Copenhagen. Bohr and Heisenberg held that the wave function is a tool for calculating the odds of measurement results, and that asking what happens between measurements has no meaning.

Many worlds. Hugh Everett proposed in 1957 that the wave function never collapses. Every outcome happens, each in its own branch of reality, and each copy of the observer sees one.

Pilot wave. David Bohm showed in 1952 that particles can have definite positions at all times if a real wave guides them. The wave must respond instantly to distant events, as Bell’s theorem demands.

Objective collapse. Giancarlo Ghirardi, Alberto Rimini, and Tullio Weber, and separately Roger Penrose, proposed that wave functions collapse spontaneously, rarely for one particle and rapidly for large clusters. Collapse theories alone predict deviations from standard quantum mechanics, and experiments with ever-larger objects in superposition have already excluded some versions.

Information-based views. QBism and relational quantum mechanics treat the wave function as one observer’s knowledge, or as a relation between systems, and never as a thing in itself.

Every interpretation except objective collapse predicts identical results for every experiment, so evidence cannot yet choose among them.

Engineering with single quanta

The first quantum technologies exploit the collective behaviour of many particles: transistors, lasers, LEDs, MRI, and the atomic clocks aboard GPS satellites, where a timing error of one nanosecond shifts a position fix by 30 centimetres. A second wave of technology controls individual quantum states.

Quantum computers. A quantum bit, or qubit, holds a blend of 0 and 1, and a register of n entangled qubits carries 2ⁿ amplitudes at once. A quantum algorithm choreographs the amplitudes so that wrong answers cancel and right answers reinforce. Peter Shor showed in 1994 that such a machine could factor large numbers fast enough to break the RSA encryption protecting most internet traffic. Feynman proposed the other major use in 1981: simulating molecules and materials that defeat ordinary computers. The obstacle is decoherence. Qubits lose their state within microseconds to seconds, so one reliable “logical” qubit must be woven from many physical qubits that check each other for errors. In 2024 Google’s 105-qubit Willow chip showed for the first time that adding physical qubits to a logical qubit reduces the error rate. A 2025 estimate put RSA within reach of a machine with just under a million physical qubits, when the largest machines held about a thousand. Governments are already replacing RSA: the US standards agency NIST published the first quantum-resistant encryption standards in August 2024.

Quantum cryptography. Two parties exchange single photons to build a secret key. Measuring a photon disturbs the photon’s state, so any eavesdropper raises the error rate and gives the intrusion away. Such links already run over optical fibre and via satellite.

Quantum sensors. The best optical atomic clocks would gain or lose one second in 30 billion years, and they detect the slowing of time that Einstein’s gravity predicts across a height difference of one millimetre. Clouds of falling cold atoms measure gravity finely enough to find buried tunnels. Single atomic defects in diamond map the magnetic fields of living cells. The LIGO gravitational-wave detectors use “squeezed” light, with reduced uncertainty in one property bought by increased uncertainty in another, to hear fainter collisions of black holes.

The quantum universe

Quantum rules shape the largest structures in the universe.

Exclusion pressure holds up white dwarfs and neutron stars. Tunnelling lights the stars, and the energy levels of nuclei decide which elements stars forge. Carbon exists in abundance because the carbon nucleus has an energy level at exactly the right height for three helium nuclei to fuse. Fred Hoyle predicted that level in 1953 from the mere fact that carbon-based astronomers exist.

According to the theory of cosmic inflation, the universe expanded enormously in its first fraction of a second, and the expansion stretched quantum jitters in a primordial field to astronomical size. The jitters became regions of slightly higher and lower density, visible today as temperature variations of one part in 100,000 in the cosmic microwave background. The denser regions grew into galaxies. The statistical pattern of the variations matches the quantum prediction.

Stephen Hawking showed in 1974 that quantum fields near a black hole’s horizon make the black hole glow faintly and slowly evaporate. A black hole with the Sun’s mass would glow at 60 billionths of a degree above absolute zero, far too cold to detect. Evaporation raises a paradox: quantum mechanics says information about everything that fell in must survive, and a vanished black hole seems to leave nowhere for the information to be. Work since 2019 suggests the information escapes encoded in subtle correlations within the radiation.

What remains unknown

Quantum gravity. General relativity treats space and time as a smooth stage; quantum mechanics says everything jitters. Combining the two by the methods that worked for the other forces yields infinities that renormalisation cannot absorb. String theory and loop quantum gravity are the leading attempts. The effects should appear at the Planck length, 10⁻³⁵ metres, a scale 10²⁰ times smaller than a proton and far beyond any experiment. Proposed tabletop experiments aim to test whether gravity can entangle two small masses, which would show that gravity itself is quantum.

Vacuum energy. Add up the zero-point energy of all quantum fields and the total exceeds the measured energy of empty space, the dark energy accelerating cosmic expansion, by up to 120 orders of magnitude. No other prediction in physics misses by so much.

The quantum-classical boundary. Interference has been shown for molecules of 2,000 atoms. Experiments with glass nanospheres of billions of atoms will show whether superposition has an upper size limit.

Missing pieces of the particle table. Dark matter, neutrino masses, and the surplus of matter over antimatter all call for physics beyond the Standard Model.

Strongly interacting electrons. High-temperature superconductors and related materials defeat current methods of calculation, and may be the first problems a quantum computer solves.

Feynman told a lecture audience in 1964, “I think I can safely say that nobody understands quantum mechanics.” The remark still holds. The theory matches experiment to twelve significant figures, runs the hardware of the global economy, and explains why the Sun shines and the table is solid. What kind of world the theory describes is a question physicists have yet to answer.

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