23 paradoxes
Paradox of the dayThe grandfather paradoxTravel back in time and stop your grandparents meeting. Do you still exist?Read the storyParadoxes
A paradox is an argument where every step looks right but the ending seems impossible. Some have been solved, some are still argued about, and a few are among the biggest open questions in science. Each one here is told as a story, shown as an animation you can play with, and then explained.
- Philosophy and logicStill argued aboutThe Ship of TheseusReplace every plank of a ship. Is it still the same ship?Story and animation
- Philosophy and logicStill argued aboutThe liar paradox"This sentence is false." Is it true or false?Story and animation
- Philosophy and logicStill argued aboutThe heap (sorites) paradoxTake one grain from a heap of sand. Still a heap. Keep going…Story and animation
- Philosophy and logicSolvedThe barber paradoxA barber shaves everyone who doesn't shave himself. Who shaves the barber?Story and animation
- Philosophy and logicStill argued aboutThe surprise testA teacher promises a surprise test next week. The class proves it can't happen.Story and animation
- TimeSolvedAchilles and the tortoiseThe fastest runner can never catch a tortoise. Or can he?Story and animation
- TimeUnsolvedThe grandfather paradoxTravel back in time and stop your grandparents meeting. Do you still exist?Story and animation
- TimeUnsolvedThe bootstrap paradoxA notebook goes round a time loop forever. Who wrote it?Story and animation
- TimeSolvedThe twin paradoxOne twin flies to a star and back. She comes home younger than her brother.Story and animation
- Space and infinitySolvedOlbers' paradox: why is the night sky dark?If the universe is full of stars, every direction should end on one. So why isn't the night sky blazing?Story and animation
- Space and infinityUnsolvedThe Fermi paradoxThe galaxy is old and huge. So where is everybody?Story and animation
- Space and infinitySolvedHilbert's hotelA hotel with infinitely many rooms is full. A new guest arrives. No problem.Story and animation
- Space and infinitySolvedThe coastline paradoxHow long is India's coastline? It depends on the length of your ruler.Story and animation
- Space and infinitySolvedThe ladder paradoxA 12-metre ladder fits inside a 10-metre garage, if it's moving fast enough. Or does it?Story and animation
- PhysicsStill argued aboutSchrödinger's catA cat in a box is both alive and dead until someone looks. Really?Story and animation
- PhysicsSolvedMaxwell's demonA tiny demon opens a door and makes heat flow backwards. Free energy forever?Story and animation
- PhysicsUnsolvedThe black hole information paradoxThrow a book into a black hole. When the hole evaporates, is the book's information gone forever?Story and animation
- PhysicsStill argued aboutThe double-slit experimentFire particles one at a time through two slits. They make a wave pattern. Watch them and the pattern vanishes.Story and animation
- Maths and chanceSolvedThe Monty Hall problemThree doors, one car. The host opens a goat door. Should you switch?Story and animation
- Maths and chanceSolvedThe birthday paradoxIn a class of just 23, it's more likely than not that two share a birthday.Story and animation
- Maths and chanceSolvedSimpson's paradoxTreatment A wins for every group of patients, yet loses overall.Story and animation
- Maths and chanceSolvedBraess's paradoxBuild a new road to cut traffic, and everyone's journey gets slower.Story and animation
- Maths and chanceSolvedGabriel's hornA horn you can fill with paint, but never paint.Story and animation