Experiments Layer
Experiments
Short, named, citable scenarios that schools disagree about. Unlike Works, an experiment does not stake out its own ontological position — it stages the disagreement and records who takes which side. Two categories: thought experiments (Mary's Room, the Chinese Room, the Ship of Theseus…) and scientific experiments (double-slit, Bell test, Michelson–Morley…), each chosen for the metaphysical fault lines they expose.
152 of 152 visible · 76 thought · 76 scientific
Thought experiments 76
Mary's Room
A complete physical description of colour vision still leaves something out — or does it?
The Chinese Room
A program that passes a Turing test in Chinese — without anyone inside understanding a word.
The Ship of Theseus
If every plank is replaced one by one, is it still the same ship? And if you rebuild the original out of the discarded planks — which one is?
Newcomb's Problem
A near-infallible predictor has already filled the boxes. Take both, or just the opaque one?
Schrödinger's Cat
If quantum superposition is real, a cat in a sealed box can be both alive and dead until someone opens the lid.
Wigner's Friend
A friend performs a measurement inside a sealed lab. To them the outcome is definite; to Wigner outside, the friend is in superposition.
Brain in a Vat
A disembodied brain in a vat, fed simulated experience, has the same inner life as you. How could you tell you are not it?
Twin Earth
On Twin Earth, "water" looks and tastes the same but is XYZ. Do you and your twin mean the same thing by "water"?
Philosophical Zombies
A being physically and behaviourally identical to you, but with no inner experience whatsoever — is it conceivable, and if so, is it possible?
Parfit's Teletransporter
Scan, destroy, rebuild on Mars. Is the person who steps out you, or someone exactly like you?
The Experience Machine
A machine that gives you any experience you want, indistinguishable from reality. Would you plug in for life?
The Trolley Problem
A runaway trolley will kill five; you can divert it onto a track where it will kill one. May you? Must you? And does pushing a man off a bridge to stop it change the answer?
Galileo's Falling Bodies
Aristotle says heavy bodies fall faster. Tie a heavy and a light one together: what speed does the pair fall at? Whatever the answer, Aristotle is wrong.
Einstein's Elevator
Inside a sealed elevator you cannot tell whether you are at rest in a gravitational field or accelerating uniformly in free space.
Maxwell's Demon
A clever gatekeeper sorts fast molecules from slow, creating a temperature difference for free — apparently violating the second law.
Boltzmann Brains
If you wait long enough in a high-entropy universe, random fluctuations produce momentary disembodied brains with all your memories — and they should outnumber real observers astronomically.
Plato's Cave
Prisoners chained in a cave mistake shadows on the wall for reality. The philosopher's task is to turn around — and ultimately to climb out.
Descartes' Evil Demon
Suppose an all-powerful malicious deceiver is manipulating every belief you have. What survives the doubt?
Buridan's Ass
A donkey equidistant from two identical bales of hay, dying of starvation because pure rationality cannot tip the scale.
The Inverted Spectrum
Two people identical in every functional respect could, in principle, have systematically swapped colour qualia. Could we ever know?
Swampman
A lightning strike in a swamp produces a perfect molecule-for-molecule duplicate of Davidson, just as Davidson himself is incinerated nearby. Does Swampman have thoughts?
The Beetle in the Box
Everyone has a box containing what they call a "beetle." No one can look in anyone else's. The thing in the box drops out of the language game as irrelevant.
Gettier Cases
A three-page paper that dismantled two thousand years of definitions of knowledge.
The Sorites Paradox
One grain of sand is not a heap. Adding one grain to a non-heap cannot make a heap. Therefore there are no heaps.
The Sleeping Beauty Problem
Sleeping Beauty is told the protocol, then put to sleep. On waking, what credence should she assign to the fair coin having landed heads — 1/2 or 1/3?
Frankfurt Cases
Black secretly wants Jones to do X. If Jones is about to choose otherwise, Black will intervene — but Jones chooses X on his own. Is Jones responsible?
The Veil of Ignorance
What principles of justice would you choose if you didn't know who in society you would be?
Bostrom's Simulation Argument
If technologically mature civilisations run vast numbers of ancestor-simulations, we are almost certainly in one. The argument: at least one of three claims must be true.
The Violinist
You wake up surgically attached to an unconscious violinist who will die unless you remain plugged in for nine months. Must you stay?
The Drowning Child
A child drowns in a shallow pond near you. Saving her ruins your shoes. You're obviously obliged to act. Why is distant famine relief different?
Block's Chinese Nation
A billion Chinese citizens, each playing the role of one neuron, implement the functional organisation of a human mind. Does the nation feel anything?
The Repugnant Conclusion
For any world with very high quality of life, there is a much larger world of lives barely worth living that is ranked better by total utilitarianism.
The Ring of Gyges
A shepherd finds a ring that makes him invisible at will. Why should he remain just?
Newton's Bucket
A bucket of water hung by a twisted cord: at first the water is flat, then rotates, then climbs the sides. Rotation is detectable absolutely.
Galileo's Ship
In a sealed cabin below deck on a smoothly moving ship, no experiment can tell you whether the ship is moving or at rest.
Hilbert's Hotel
A hotel with countably infinite rooms, all occupied, can still accommodate one more guest — or infinitely many — by shifting everyone.
Pascal's Wager
If God exists, belief yields infinite reward and disbelief infinite loss. Even tiny credence in God's existence makes belief the rational bet.
The Liar Paradox
If "this sentence is false" is true, then it's false; if false, then true. Either way, contradiction.
Tegmark's Mathematical Universe Hypothesis
Every mathematically possible structure exists physically. Our universe is one such structure; so are all the others.
Dennett's 'Where Am I?'
Dennett's brain is removed and kept in a vat in Houston; his body is sent to defuse a warhead in Tulsa. Where, then, is Dennett?
The Doomsday Argument
If you are a randomly-drawn human, you are unlikely to be among the first 5% — so the total number of humans is unlikely to be much larger than your birth rank suggests.
The Lottery Paradox
For each ticket in a million-ticket lottery, you rationally believe it will lose. Conjoin all such beliefs and you believe none will win — contradicting your belief that one will.
The Two Envelopes Paradox
You hold an envelope containing $X. The other envelope contains either $2X or $X/2. Switching has expected value 1.25X. So you should switch — but the same logic applies after switching.
The Surprise Examination Paradox
The class reasons backward from Friday: an exam then would not be a surprise; nor on Thursday; … the exam is impossible. The teacher gives the exam on Wednesday.
Hesperus and Phosphorus
Hesperus (the evening star) and Phosphorus (the morning star) are the same planet. "Hesperus is Hesperus" is trivial; "Hesperus is Phosphorus" is an astronomical discovery. Reference cannot be all there is to meaning.
Quine's Gavagai
A field linguist hears "gavagai!" whenever a rabbit appears. Does the word mean "rabbit," "undetached rabbit-part," "rabbit-stage," or "rabbithood instantiated"? No behavioural evidence can decide.
Hempel's Ravens
"All ravens are black" is logically equivalent to "all non-black things are non-ravens." Observing a white shoe confirms the latter — and therefore the former?
Goodman's Grue
Define "grue" = green if observed before time t, blue otherwise. All emeralds so far are green; equivalently, all are grue. Which predicate is projectible into the future?
Russell's Five-Minute Hypothesis
Suppose the universe sprang into existence five minutes ago, complete with apparent memories, fossils, and historical records. What evidence rules this out?
Fitch's Knowability Paradox
Anti-realists hold that all truths are knowable. Fitch shows this entails that all truths are actually known — provided there are any unknown truths, contradiction.
Eternal Recurrence
A demon tells you: this life — every detail — will recur infinitely, with nothing new. Would you bless or curse the news?
The Bilking Argument
If a future cause C produces a past effect E, then once you observe E happening, you can resolve not to do C — bilking the alleged causal link.
What Is It Like to Be a Bat?
Bats navigate by echolocation. Even given a complete physical description, we cannot know what it is *like* to be one.
Block's Blockhead
A machine with a pre-recorded response for every possible conversation passes the Turing test. Does it think?
Wittgenstein's Lion
"If a lion could speak, we could not understand him." Language is bound up with a form of life.
Reid's Brave Officer
A general remembers being a brave officer; the officer remembers being a flogged schoolboy; the general remembers nothing of the schoolboy. Locke's memory criterion fails.
Locke's Prince and the Cobbler
The consciousness of a prince enters the body of a cobbler. The prince persists — wherever the consciousness goes.
Williams' Self and the Future
Tell the same body-swap story two ways — once psychologically, once first-personally — and intuition flips.
Strawson's Reactive Attitudes
Resentment, gratitude, indignation — these reactive attitudes are constitutive of moral life. No general truth of determinism could rationally compel us to abandon them.
Russell's Paradox
Consider the set R of all sets that do not contain themselves. Does R contain itself? Either answer is a contradiction.
Cantor's Diagonal Argument
No list of real numbers can be complete: construct a new real differing from the n-th listed real in the n-th digit.
The Cogito
However thoroughly I doubt, the doubting itself requires a doubter. The existence of the thinking self survives any skeptical scenario.
Nozick's Tale of the Slave
A slave's situation is liberalised step by step until he is a citizen voting for his own laws. Is he still a slave? If not, when did he stop being one?
Singer's Expanding Circle
Moral concern has expanded historically — from family, to tribe, to nation, to humanity, to sentient animals. The trajectory is the test of moral progress.
Kripke's "Plus" vs "Quus"
How do you know you have always meant plus, rather than "quus" (= plus except returning 5 for arguments above some bound)? No fact about your past use settles it.
The Survival Lottery
Two patients will die without transplants. Random lottery selects a healthy person to harvest. Greater good is served — yet the intuition recoils.
Meno's Slave Boy
A slave with no geometric education is led, by Socratic questioning alone, to demonstrate the diagonal-square theorem. He must have known it all along.
BonJour's Clairvoyant
Norman reliably forms true beliefs about the President's location via clairvoyance, with no idea his clairvoyance is reliable. Does he know?
Williamson's Anti-Luminosity Argument
Feeling cold "luminously" means: whenever you feel cold, you know you feel cold. Williamson argues no condition is luminous in this sense.
The Frame Problem
A robot moves a battery into another room. How does it know the colour of its body, the position of the door, the time of day, and a million other things haven't changed?
Pascal's Mugging
A stranger demands your wallet. Refuse, he claims, and he'll inflict 3↑↑↑3 days of agony. Expected utility says hand it over — but obviously don't.
Curry's Paradox
Let C = "If C is true, then arithmetic is inconsistent." Trivial reasoning derives the conclusion. The paradox is harder than the Liar.
Berry's Paradox
Consider "the smallest positive integer not nameable in fewer than nineteen syllables." That phrase has eighteen.
Searle's Wisdom Tooth
A wisdom tooth has chemical-causal properties that are not "physical" in a reductive sense, but no one thinks teeth are non-physical. Mind is like that — biologically real but irreducible.
Anscombe's Intention
A man pumps water that he knows is poisoned. He poisons the household; he kills the inhabitants. One action or many — and intentional under which description?
Davidson's Triangulation
Determinate thought-content requires a triangle of self, other, and shared world. Solipsistic content is impossible.
Scientific experiments 76
The Double-Slit Experiment
A single electron interferes with itself — until you watch which slit it goes through.
Bell Test Experiments
No theory that is both local and admits definite pre-existing values can match the observed correlations.
The Michelson–Morley Experiment
A precision interferometer fails to detect Earth's motion through the aether — because there is no aether.
The Delayed-Choice Quantum Eraser
Choosing whether to read which-path information *after* the photon has been detected still determines whether interference appears.
The Stern–Gerlach Experiment
Silver atoms passed through an inhomogeneous magnetic field split into two discrete beams — not a continuous distribution. Angular momentum is quantised.
Eddington's Eclipse Expedition
During a total solar eclipse, the positions of stars near the Sun shift by 1.75 arc seconds — exactly as Einstein predicted, and twice what Newton allowed.
Hafele–Keating
Four cesium clocks flown eastward and westward around the world agree with relativity to within nanoseconds.
The Pound–Rebka Experiment
A gamma ray climbing 22.5 metres in Earth's gravitational field is redshifted by one part in 10¹⁵ — exactly as Einstein predicted.
Foucault's Pendulum
A 28-kg pendulum in the Panthéon traces out a slowly rotating plane — direct visual evidence that the Earth rotates beneath it. But beneath it relative to what?
The Cavendish Experiment
A torsion balance detects the gravitational pull between lead spheres in a basement laboratory — confirming Newton at tabletop scale and yielding the first measurement of *G*.
The Wu Experiment
Cobalt-60 nuclei aligned in a magnetic field emit beta particles preferentially in one direction. Nature can tell its left from its right.
LIGO Gravitational-Wave Detection
Two 4-km laser interferometers detect a strain of 10⁻²¹ as two black holes a billion light-years away merge.
The Cosmic Microwave Background
A persistent 4-GHz hiss in a Bell Labs horn antenna turns out to be 14-billion-year-old afterglow of the hot early universe.
Libet's Free Will Experiments
A readiness-potential builds in the motor cortex hundreds of milliseconds *before* the subject reports deciding to act.
The Millikan Oil-Drop Experiment
Charged oil droplets suspended in an electric field carry charges that come only in integer multiples of *e*.
The Rutherford Gold-Foil Experiment
Alpha particles fired at gold foil mostly pass through, but a few bounce nearly straight back — "as if you fired a 15-inch shell at tissue paper and it came back at you."
Newton's Prism Experiment
A glass prism splits white sunlight into a spectrum — and a second prism, properly placed, recombines it. White light is not pure; it is a mixture of all colours.
Joule's Mechanical Equivalent of Heat
A paddle wheel driven by falling weights warms water in a jar by a precise amount. Mechanical work converts to heat at a fixed ratio.
Faraday's Electromagnetic Induction
Move a magnet near a coil of wire — current flows. The electromagnetic field is born.
Hertz's Electromagnetic Waves
A spark gap on one side of a Karlsruhe laboratory induces sparks in a tuned loop on the other. Maxwell's waves are real, and they travel at the speed of light.
Brownian Motion / Perrin's Confirmation
Pollen grains dance erratically in water because invisible molecules are kicking them. Atoms cease to be a theoretical convenience and become a settled fact.
The Photoelectric Effect
Light below a threshold frequency ejects no electrons no matter how bright; above it, ejection is instantaneous. Light is delivered in quanta.
Pasteur's Swan-Neck Flask
Boiled broth in an S-curved flask never spoils. Spontaneous generation is refuted; microbes come from microbes.
Mendel's Pea Plants
Cross tall and short pea plants; the next generation is all tall. Cross those; the generation after is tall and short in a 3:1 ratio. Inheritance is particulate.
Hubble's Redshift Law
Distant galaxies recede with velocities proportional to their distance. The universe is not static; it is expanding.
The Higgs Boson Discovery
Two LHC detectors independently observe a new boson at ~125 GeV — the predicted Higgs particle that completes the Standard Model.
Neutrino Oscillations
Neutrinos born as one flavour arrive at Earth as another. Neutrinos must have mass — a long-standing solar puzzle resolved.
Quantum Teleportation
Alice can transfer the quantum state of a particle to Bob — destroying it in the process — using only a shared entangled pair and two classical bits.
Compton Scattering
X-rays scattered by electrons emerge at shifted wavelengths — exactly as if they were particles bouncing off particles.
The Hershey–Chase Experiment
Bacteriophage protein tagged with sulphur-35, DNA tagged with phosphorus-32. Only the phosphorus enters infected bacteria. DNA is the genetic material.
The Meselson–Stahl Experiment
DNA labelled with heavy nitrogen, then grown for one generation in light nitrogen. The result settles whether replication is semi-conservative, conservative, or dispersive.
Milgram's Obedience Experiments
A majority of ordinary volunteers, instructed by a white-coated experimenter, administer what they believe are lethal electric shocks to a stranger.
Asch's Conformity Experiments
Subjects asked which of three lines matches a reference line answer correctly alone — and incorrectly when confederates unanimously give the wrong answer.
Galileo's Inclined Plane
Balls rolled down a smooth inclined plane traverse distances proportional to the square of the elapsed time. The law of falling bodies is empirically established.
Lavoisier's Conservation of Mass
Combustion in a sealed vessel changes nothing about total mass. Phlogiston is dead; chemistry is quantitative.
Coulomb's Torsion Balance
A delicate torsion balance reveals that electrostatic force follows the same inverse-square law as gravity.
CP Violation in Kaon Decay
The long-lived neutral kaon occasionally decays by a CP-forbidden channel — a tiny but consequential asymmetry between matter and antimatter.
Bose–Einstein Condensation
A dilute gas cooled to nanokelvin temperatures undergoes a phase transition into a single quantum state — thousands of atoms occupying one wavefunction.
The Lamb Shift
Two hydrogen energy levels Dirac's theory predicts to be degenerate are separated by 1057 MHz — the signature of quantum-vacuum effects.
The Avery–MacLeod–McCarty Experiment
A purified extract from virulent pneumococci converts harmless strains into virulent ones. Destroy the DNA in the extract; the conversion stops.
Eratosthenes' Measurement of Earth
At noon on the summer solstice, the sun is directly overhead at Syene but casts a 7° shadow at Alexandria. From this, the Earth's circumference: 40,000 km.
Galileo's Moons of Jupiter
Four bright points near Jupiter shift position from night to night — they orbit Jupiter. The geocentric assumption that all bodies orbit Earth is empirically wrong.
Tycho's Supernova
A new bright "star" appears in Cassiopeia in November 1572. Parallax measurements place it beyond the Moon — the unchangeable Aristotelian heavens have changed.
Hess's Cosmic-Ray Balloon Flights
Ionising radiation, expected to decrease with altitude (away from terrestrial sources), instead *increases* by a factor of four at 5,300 metres. The radiation comes from above.
The Aharonov–Bohm Effect
Electrons whose path lies in a region of zero magnetic field, but encloses one, show interference shifts depending on the field — the vector potential is doing the work.
Tonomura's Single-Electron Interference
Electrons fired one at a time at a biprism build, dot by dot, into an interference pattern over 28 minutes of exposure. Each electron interferes with itself.
The Discovery of Pulsars
Bell Burnell's array picks up a regular radio pulse every 1.337 seconds. After ruling out interference and "little green men," the explanation: rotating neutron stars.
The November Revolution
Two groups independently discover the same narrow resonance at 3.1 GeV. The charm quark is real, and the quark model becomes physics.
The Discovery of W and Z Bosons
The Super Proton Synchrotron at CERN produces the W and Z bosons at exactly the masses predicted by electroweak unification — 80 and 91 GeV.
The Top Quark Discovery
After 18 years of searching, two Fermilab experiments independently observe the top quark at 173 GeV — far heavier than anyone expected.
Trapped Anti-Hydrogen at CERN ALPHA
CERN traps 38 antihydrogen atoms for 172 milliseconds, opening direct precision tests of CPT symmetry and gravitational behaviour.
Lunar Laser Ranging
A laser pulse from Earth bounces off retroreflectors on the lunar surface and returns 2.5 seconds later. Distance to the Moon: known to within a millimetre.
Rømer's Measurement of the Speed of Light
Eclipses of Jupiter's moon Io arrive systematically late when Earth is far from Jupiter, early when close. The light takes time to cross the difference.
Torricelli's Barometer
Invert a sealed tube of mercury in a basin of mercury. The mercury falls until it stands at about 76 cm — supported by the weight of the atmosphere.
Ørsted's Compass Deflection
A current-carrying wire deflects a nearby compass needle. Electricity and magnetism, long thought independent, are unified.
Röntgen's X-Rays
A barium platinocyanide screen glows on the far side of a covered cathode-ray tube. Invisible rays pass through cardboard, wood, and (with the famous photo of his wife's hand) flesh.
Discovery of Radioactivity
Uranium salts blacken a photographic plate through opaque wrapping — without any external excitation. Atoms are not stable; they decay.
Seafloor Spreading
Magnetic stripes on the ocean floor, symmetric about mid-ocean ridges, record the Earth's field reversals as new crust forms and spreads.
The Cesium Atomic Clock
Cesium-133's hyperfine transition at 9,192,631,770 Hz provides a clock more stable than the Earth's rotation. By 1967, the SI second is redefined accordingly.
JWST's Surprisingly Mature Early Galaxies
The James Webb Space Telescope finds galaxies at high redshift more massive and structured than ΛCDM cosmology comfortably predicts.
Boyle's J-Tube
Mercury poured into a J-shaped sealed tube compresses trapped air. Pressure and volume vary inversely.
The Faraday Cage
Faraday sits inside a 12-foot wire cage and applies hundreds of thousands of volts to its exterior. He feels nothing; instruments inside read zero field.
Volta's Pile
Stack alternating zinc and copper discs separated by salt-soaked cloth; current flows through a wire from top to bottom. The first continuous source of electric current.
The Hubble Deep Fields
Stare for 10 days at an apparently empty patch of sky 1/13 the diameter of the Moon. The image contains 3,000 galaxies. The universe is full.
Galvani's Twitching Frogs
Dead frog legs twitch when touched by two different metals. Is the twitch driven by animal electricity, or by the metals?
Olbers' Paradox
In an infinite, eternal, static universe uniformly filled with stars, every line of sight ends on a star. The sky should be ablaze.
Anderson's Discovery of the Positron
A cosmic-ray track in a cloud chamber curves the wrong way: a particle with the electron's mass but opposite charge. Dirac's antimatter prediction confirmed.
The Discovery of the Muon
Cosmic-ray cloud chambers reveal particles with the electron's charge but ~200 times its mass. "Who ordered that?" — I. I. Rabi.
Cherenkov Radiation
Charged particles passing through a transparent medium faster than light travels in that medium emit a characteristic blue glow.
Rossi-Hall Cosmic-Ray Muon Time Dilation
Muons created in the upper atmosphere decay in 2.2 microseconds. They shouldn't reach sea level — but they do, in numbers that confirm special relativity.
The Quantum Hall Effect
Hall resistance of a 2D electron gas at low temperature in strong magnetic field is quantised at h/(ne²) to extraordinary precision.
High-Tc Superconductivity
A copper-oxide ceramic superconducts at 35 K — and within a year, 92 K, above liquid nitrogen. Decades-old theoretical limits are shattered.
The First Image of a Black Hole
A virtual Earth-sized telescope images the supermassive black hole at the heart of M87 — a bright accretion ring surrounding a dark central shadow.
WMAP and Planck CMB Anisotropy Maps
Tiny temperature variations in the CMB (one part in 100,000) encode the geometry, content, and age of the universe to percent precision.
The Casimir Effect
Two uncharged parallel conducting plates in vacuum attract each other. The vacuum is doing the pulling.
Voyager 1 Crossing the Heliopause
On 25 August 2012, Voyager 1 — launched in 1977 — crossed the heliopause into interstellar space and reported the plasma density of the local interstellar medium.