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Logic
Barbershop paradox: The supposition that, 'if one of two simultaneous assumptions leads to a contradiction, the other assumption is also disproved' leads to paradoxical consequences. Not to be confused with the Barber paradox.
What the Tortoise Said to Achilles: "Whatever Logic is good enough to tell me is worth writing down…" Also known as Carroll's paradox and is not to be confused with the "Achilles and the tortoise" paradox by Zeno of Elea.
Catch-22: A situation in which someone is in need of something that can only be had by not being in need of it. A soldier who wants to be declared insane to avoid combat is deemed not insane for that very reason and will therefore not be declared insane.
Drinker paradox: In any pub there is a customer of whom it is true to say: if that customer is drinking, everybody in the pub is drinking.
Paradox of entailment: Inconsistent premises always make an argument valid.
Lottery paradox: If there is one winning ticket in a large lottery, it is reasonable to believe of any particular lottery ticket that it is not the winning ticket, but it is not reasonable to believe that no lottery ticket will win.
Raven paradox: (or Hempel's Ravens): Observing a green apple increases the likelihood of all ravens being black.
Ross' paradox: Disjunction introduction poses a problem for imperative inference by seemingly permitting arbitrary imperatives to be inferred.
Unexpected hanging paradox: The day of the hanging will be a surprise, so it cannot happen at all, so it will be a surprise. The surprise examination and Bottle Imp paradox use similar logic.
Self-reference
These paradoxes have in common a contradiction arising from either self-reference or circular reference, in which several statements refer to each other in a way that following some of the references leads back to the starting point.
Barber paradox: A male barber shaves all and only those men who do not shave themselves. Does he shave himself? (Russell's popularization of his set theoretic paradox.)
Bhartrhari's paradox: The thesis that there are some things which are unnameable conflicts with the notion that something is named by calling it unnameable.
Berry paradox: The phrase "the first number not nameable in under ten words" appears to name it in nine words.
Crocodile dilemma: If a crocodile steals a child and promises its return if the father can correctly guess exactly what the crocodile will do, how should the crocodile respond in the case that the father guesses that the child will not be returned?
Paradox of the Court: A law student agrees to pay his teacher after (and only after) winning his first case. The teacher then sues the student (who has not yet won a case) for payment.
Curry's paradox: "If this sentence is true, then Santa Claus exists."
Epimenides paradox: A Cretan says: "All Cretans are liars". This paradox works in mainly the same way as the liar paradox.
Grelling–Nelson paradox: Is the word "heterological", meaning "not applicable to itself", a heterological word? (A close relative of Russell's paradox.)
Hilbert-Bernays paradox: If there was a name for a natural number that is identical to a name of the successor of that number, there would be a natural number equal to its successor.
Kleene–Rosser paradox: By formulating an equivalent to Richard's paradox, untyped lambda calculus is shown to be inconsistent.
Knower paradox: "This sentence is not known."
Liar paradox: "This sentence is false." This is the canonical self-referential paradox. Also "Is the answer to this question 'no'?", and "I'm lying."
Card paradox: "The next statement is true. The previous statement is false." A variant of the liar paradox in which neither of the sentences employs (direct) self-reference, instead this is a case of circular reference.
No-no paradox: Two sentences that each say the other is not true.
Pinocchio paradox: What would happen if Pinocchio said "My nose grows now"?[1]
Quine's paradox: "'Yields a falsehood when appended to its own quotation' yields a falsehood when appended to its own quotation." Shows that a sentence can be paradoxical even if it is not self-referring and does not use demonstratives or indexicals.
Yablo's paradox: An ordered infinite sequence of sentences, each of which says that all following sentences are false. While constructed to avoid self-reference, there is no consensus whether it relies on self-reference or not.
Opposite Day: "It is opposite day today." Therefore, it is not opposite day, but if you say it is a normal day it would be considered a normal day, which contradicts the fact that it has previously been stated that it is an opposite day.
Richard's paradox: We appear to be able to use simple English to define a decimal expansion in a way that is self-contradictory.
Russell's paradox: Does the set of all those sets that do not contain themselves contain itself?
Socratic paradox: "All I know is that I know nothing."
Vagueness
Ship of Theseus: It seems like you can replace any component of a ship, and it is still the same ship. So you can replace them all, one at a time, and it is still the same ship. However, you can then take all the original pieces, and assemble them into a ship. That, too, is the same ship you began with.
See also List of Ship of Theseus examples
Sorites paradox (also known as the paradox of the heap): If you remove a single grain of sand from a heap, you still have a heap. Keep removing single grains, and the heap will disappear. Can a single grain of sand make the difference between heap and non-heap?
Mathematics
See also: Category:Mathematics paradoxes and Paradoxes of set theory
All horses are the same color: A fallacious argument by induction that appears to prove that all horses are the same color.
Ant on a rubber rope: An ant crawling on a rubber rope can reach the end even when the rope stretches much faster than the ant can crawl.
Cramer's paradox: The number of points of intersection of two higher-order curves can be greater than the number of arbitrary points needed to define one such curve.
Elevator paradox: Elevators can seem to be mostly going in one direction, as if they were being manufactured in the middle of the building and being disassembled on the roof and basement.
Interesting number paradox: The first number that can be considered "dull" rather than "interesting" becomes interesting because of that fact.
Potato paradox: If you let potatoes consisting of 99% water dry so that they are 98% water, they lose 50% of their weight.
Russell's paradox: Does the set of all those sets that do not contain themselves contain itself?
Statistics
See also: Category:Statistical paradoxes
Abelson's paradox: Effect size may not be indicative of practical meaning.
Accuracy paradox: Predictive models with a given level of accuracy may have greater predictive power than models with higher accuracy.
Berkson's paradox: A complicating factor arising in statistical tests of proportions.
Freedman's paradox: Describes a problem in model selection where predictor variables with no explanatory power can appear artificially important.
Friendship paradox: For almost everyone, their friends have more friends than they do.
Inspection paradox: Why one will wait longer for a bus than one should.
Lindley's paradox: Tiny errors in the null hypothesis are magnified when large data sets are analyzed, leading to false but highly statistically significant results.
Low birth weight paradox: Low birth weight and mothers who smoke contribute to a higher mortality rate. Babies of smokers have lower average birth weight, but low birth weight babies born to smokers have a lower mortality rate than other low birth weight babies. This is a special case of Simpson's paradox.
Simpson's paradox, or the Yule–Simpson effect: A trend that appears in different groups of data disappears when these groups are combined, and the reverse trend appears for the aggregate data.
Will Rogers phenomenon: The mathematical concept of an average, whether defined as the mean or median, leads to apparently paradoxical results—for example, it is possible that moving an entry from an encyclopedia to a dictionary would increase the average entry length on both books.
Probability
The Monty Hall problem: which door do you choose?
See also: Category:Probability theory paradoxes
Bertrand's box paradox: A paradox of conditional probability closely related to the Boy or Girl paradox.
Bertrand's paradox: Different common-sense definitions of randomness give quite different results.
Birthday problem: What is the chance that two people in a room have the same birthday?
Borel's paradox: Conditional probability density functions are not invariant under coordinate transformations.
Boy or Girl paradox: A two-child family has at least one boy. What is the probability that it has a girl?
Dartboard Puzzle: If a dart is guaranteed to hit a dartboard and the probability of hitting a specific point is positive, adding the infinitely many positive chances yields infinity, but the chance of hitting the dartboard is one. If the probability of hitting each point is zero, the probability of hitting anywhere on the dartboard is zero.[2]
False positive paradox: A test that is accurate the vast majority of the time could show you have a disease, but the probability that you actually have it could still be tiny.
Grice's paradox: Shows that the exact meaning of statements involving conditionals and probabilities is more complicated than may be obvious on casual examination.
Monty Hall problem: An unintuitive consequence of conditional probability.
Necktie paradox: A wager between two people seems to favour them both. Very similar in essence to the Two-envelope paradox.
Nontransitive dice: You can have three dice, called A, B, and C, such that A is likely to win in a roll against B, B is likely to win in a roll against C, and C is likely to win in a roll against A.
Proebsting's paradox: The Kelly criterion is an often optimal strategy for maximizing profit in the long run. Proebsting's paradox apparently shows that the Kelly criterion can lead to ruin.
Sleeping Beauty problem: A probability problem that can be correctly answered as one half or one third depending on how the question is approached.
Three cards problem: When pulling a random card, how do you determine the color of the underside?
Three Prisoners problem: A variation of the Monty Hall problem.
Two-envelope paradox: You are given two indistinguishable envelopes, each of which contains a positive sum of money. One envelope contains twice as much as the other. You may pick one envelope and keep whatever amount it contains. You pick one envelope at random but before you open it you are given the chance to take the other envelope instead.
Infinity and infinitesimals
Burali-Forti paradox: If the ordinal numbers formed a set, it would be an ordinal number that is smaller than itself.
Cantor's paradox: The set of all sets would have its own power set as a subset, therefore its cardinality would be at least as great as that of its power set. But Cantor's theorem proves that power sets are strictly greater than the sets they are constructed from. Consequently, the set of all sets would contain a subset greater than itself.
Galileo's paradox: Though most numbers are not squares, there are no more numbers than squares. (See also Cantor's diagonal argument)
Hilbert's paradox of the Grand Hotel: If a hotel with infinitely many rooms is full, it can still take in more guests.
Skolem's paradox: Countably infinite models of set theory contain uncountably infinite sets.
Zeno's paradoxes: "You will never reach point B from point A as you must always get half-way there, and half of the half, and half of that half, and so on." (This is also a physical paradox.)
Supertasks may result in paradoxes such as
Benardete's paradox: Apparently, a man can be "forced to stay where he is by the mere unfulfilled intentions of the gods".
Grandi's series: The sum of 1-1+1-1+1-1… can be either one, zero, or one-half.
Ross–Littlewood paradox: After alternately adding and removing balls to a vase infinitely often, how many balls remain?
Thomson's lamp: After flicking a lamp on and off infinitely often, is it on or off?
Geometry and topology
The Banach–Tarski paradox: A ball can be decomposed and reassembled into two balls the same size as the original.
Banach–Tarski paradox: Cut a ball into a finite number of pieces and re-assemble the pieces to get two balls, each of equal size to the first. The von Neumann paradox is a two-dimensional analogue.
Paradoxical set: A set that can be partitioned into two sets, each of which is equivalent to the original.
Coastline paradox: the perimeter of a landmass is in general ill-defined.
Coin rotation paradox: a coin rotating along the edge of an identical coin will make a full revolution after traversing only half of the stationary coin's circumference.
Gabriel's Horn: or Torricelli's trumpet: A simple object with finite volume but infinite surface area. Also, the Mandelbrot set and various other fractals are covered by a finite area, but have an infinite perimeter (in fact, there are no two distinct points on the boundary of the Mandelbrot set that can be reached from one another by moving a finite distance along that boundary, which also implies that in a sense you go no further if you walk "the wrong way" around the set to reach a nearby point). This can be represented by a Klein bottle.
Hausdorff paradox: There exists a countable subset C of the sphere S such that S\C is equidecomposable with two copies of itself.
Nikodym set: A set contained in and with the same Lebesgue measure as the unit square, yet for every one of its points there is a straight line intersecting the Nikodym set only in that point.
Sphere eversion: A sphere can, topologically, be turned inside out.
Decision theory
Abilene paradox: People can make decisions based not on what they actually want to do, but on what they think that other people want to do, with the result that everybody decides to do something that nobody really wants to do, but only what they thought that everybody else wanted to do.
Apportionment paradox: Some systems of apportioning representation can have unintuitive results due to rounding
Alabama paradox: Increasing the total number of seats might shrink one block's seats.
New states paradox: Adding a new state or voting block might increase the number of votes of another.
Population paradox: A fast-growing state can lose votes to a slow-growing state.
Arrow's paradox: Given more than two choices, no system can have all the attributes of an ideal voting system at once.
Buridan's ass: How can a rational choice be made between two outcomes of equal value?
Chainstore paradox: Even those who know better play the so-called chain store game in an irrational manner.
Decision-making paradox: Selecting the best decision-making method is a decision problem in itself.
Ellsberg paradox: People exhibit ambiguity aversion (as distinct from risk aversion), in contradiction with expected utility theory.
Fenno's paradox: The belief that people generally disapprove of the United States Congress as a whole, but support the Congressman from their own Congressional district.
Fredkin's paradox: The more similar two choices are, the more time a decision-making agent spends on deciding.
Green paradox: Policies intending to reduce future CO2 emissions may lead to increased emissions in the present.
Hedgehog's dilemma: or Lover's paradox Despite goodwill, human intimacy cannot occur without substantial mutual harm.
Inventor's paradox: It is easier to solve a more general problem that covers the specifics of the sought-after solution.
Kavka's toxin puzzle: Can one intend to drink the non-deadly toxin, if the intention is the only thing needed to get the reward?
Motivation crowding theory: Adding incentives for some behavior can sometimes backfire and actually result in less of that behavior.
Morton's fork: a type of false dilemma in which contradictory observations lead to the same conclusion.
Navigation paradox: Increased navigational precision may result in increased collision risk.
Newcomb's paradox: How do you play a game against an omniscient opponent?
Paradox of tolerance: Should one tolerate intolerance if intolerance would destroy the possibility of tolerance?
Paradox of voting: Also known as the Downs paradox. For a rational, self-interested voter the costs of voting will normally exceed the expected benefits, so why do people keep voting?
Parrondo's paradox: It is possible to play two losing games alternately to eventually win.
Prevention paradox: For one person to benefit, many people have to change their behavior – even though they receive no benefit, or even suffer, from the change.
Prisoner's dilemma: Two people might not cooperate even if it is in both their best interests to do so.
Voting paradox: Also known as Condorcet's paradox and paradox of voting. A group of separately rational individuals may have preferences that are irrational in the aggregate.
Willpower paradox: Those who kept their minds open were more goal-directed and more motivated than those who declared their objective to themselves.
Physics
Further information: Physical paradox
File:Tea Leaf Paradox Stirring.ogv
A demonstration of the tea leaf paradox
Cool tropics paradox: A contradiction between modelled estimates of tropical temperatures during warm, ice-free periods of the Cretaceous and Eocene, and the lower temperatures that proxies suggest were present.
Irresistible force paradox: What would happen if an unstoppable force hit an immovable object?
Paradox of place: If everything that exists has a place, that place must have a place, and so on ad infinitum.
Paradox of the grain of millet: When a grain of millet falls it makes no sound, but when a thousand grains fall they do, thus many of nothing become something.
The moving rows: Suppose two rows are moving past a stationary row in opposite directions, if a member of a moving row moves past a member of the stationary row in an indivisible instant of time, they move past two members of the row that is moving in the other direction in this instant of time.
Astrophysics
Algol paradox: In some binary star systems the partners seem to have different ages, even though they are thought to have formed at the same time.
Faint young Sun paradox: The contradiction between existence of liquid water early in the Earth's history and the expectation that the output of the young Sun would have been insufficient to melt ice on Earth.
GZK paradox: Extreme-energy cosmic rays have been observed that seem to violate the Greisen–Zatsepin–Kuzmin limit, which is a consequence of special relativity.
Paradox of youth: Compared to theory, there is an overabundance of young stars close to the supermassive black hole in the Galactic Center.
Olbers' paradox: see § Cosmology
Classical mechanics
Achilles and the tortoise: If the tortoise is ahead of Achilles, by the time Achilles reaches the tortoise's current position, the tortoise will have moved a bit further ahead, which goes on indefinitely.
Archer's paradox: An archer must, in order to hit his target, not aim directly at it, but slightly to the side. Not to be confused with the arrow paradox.
Arrow paradox If we divide time into discrete 0-duration slices, no motion is happening in each of them, so taking them all as a whole, motion is impossible.
Hydrostatic paradox: A massive battleship can float in a few litres of water.
Aristotle's wheel paradox: Rolling joined concentric wheels seem to trace the same distance with their circumferences, even though the circumferences are different.
Carroll's paradox: The angular momentum of a stick should be zero, but is not.
D'Alembert's paradox: Flow of an inviscid fluid produces no net force on a solid body.
Knudsen paradox: Based on the Navier–Stokes equations, one would expect the mass flux in a channel to decrease with increasing Knudsen number, but there is a distinct minimum around Knudsen number 0.8.
Denny's paradox: Surface-dwelling arthropods (such as the water strider) should not be able to propel themselves horizontally.
Dichotomy paradox: To reach its target, an airborne arrow must first reach an infinite number of midpoints between its current position and the target.
Elevator paradox: Even though hydrometers are used to measure fluid density, a hydrometer will not indicate changes of fluid density caused by changing atmospheric pressure.
Feynman sprinkler: Which way does a sprinkler rotate when submerged in a tank and made to suck in the surrounding fluid?
Norton's dome: Are there non-deterministic systems in Newtonian mechanics?
Painlevé paradox: Rigid-body dynamics with contact and friction is inconsistent.
Tea leaf paradox: When a cup of tea is stirred, the leaves assemble in the center, even though centrifugal force pushes them outward.
Upstream contamination: When a fluid is poured from a higher container onto a lower one, particles can climb up the falling water.
Cosmology
Bentley's paradox: In a Newtonian universe, gravitation should pull all matter into a single point.
Boltzmann brain: If the universe we observe resulted from a random thermodynamic fluctuation, it would be vastly more likely to be a simple one than the complex one we observe. The simplest case would be just a brain floating in vacuum, having the thoughts and sensations you have.
Fermi paradox: If there are, as various arguments suggest, many other sentient species in the Universe, then where are they? Shouldn't their presence be obvious?
Heat death paradox: If the universe were infinitely old, it would be in thermodynamical equilibrium, which contradicts what we observe.
Olbers' paradox: Why is the night sky dark if there is an infinity of stars, covering every part of the celestial sphere?
Electromagnetism
Faraday paradox: An apparent violation of Faraday's law of electromagnetic induction.
Quantum mechanics
Aharonov–Bohm effect: A charged particle is affected by an electromagnetic field even though it has no local contact with that field
Bell's theorem: Why do measured quantum particles not satisfy mathematical probability theory?
Double-slit experiment: Matter and energy can act as a wave or as a particle depending on the experiment.
Einstein–Podolsky–Rosen paradox: Can far away events influence each other in quantum mechanics?
Extinction paradox: In the small wavelength limit, the total scattering cross section of an impenetrable sphere is twice its geometrical cross-sectional area (which is the value obtained in classical mechanics).[3]
Hardy's paradox: How can we make inferences about past events that we haven't observed while at the same time acknowledge that the act of observing it affects the reality we are inferring to?
Klein paradox: When the potential of a potential barrier becomes similar to the mass of the impinging particle, it becomes transparent.
Mott problem: Spherically symmetric wave functions, when observed, produce linear particle tracks.
Quantum LC circuit paradox: Energies stored on capacitance and inductance are not equal to the ground state energy of the quantum oscillator.[citation needed]
Quantum pseudo-telepathy: Two players who can not communicate accomplish tasks that seemingly require direct contact.
Quantum Zeno effect: (Turing paradox) echoing the Zeno paradox, a quantum particle that is continuously observed cannot change its state
Schrödinger's cat paradox: According to the Copenhagen interpretation of quantum mechanics, a cat could be simultaneously alive and dead, as long as it remains unobserved.
Uncertainty principle: There is a fundamental limit to the precision with which certain pairs of physical properties of a particle, known as complementary variables, such as position and momentum can be known. This is often confused with a similar effect in physics called the observer effect
Relativity
Bell's spaceship paradox: About the stress on a rope under the effects of length contraction.
Black hole information paradox: Black holes violate a commonly assumed tenet of science that information cannot be destroyed.
Ehrenfest paradox: On the kinematics of a rigid rotating disk.
Ladder paradox: Introductory relativity problem about a ladder, a barn, and simultaneity.
Mocanu's velocity composition paradox: Which formula should be used to transform velocities between non-collinear reference frames in special relativity?
Paradox of radiation of charged particles in a gravitational field: An accelerated charge should radiate, yet such radiation is not observed for stationary particles on gravitational fields.
Supplee's paradox: The buoyancy of a relativistic object (such as a bullet) appears to change when the reference frame is changed from one in which the bullet is at rest to one in which the fluid is at rest.
Tachyonic antitelephone: Einstein's thought experiment about how faster-than-light communication could cause a causality paradox.
Trouton-Noble or Right-angle lever paradox: Does a torque arise in static systems when changing frames?
Twin paradox: The theory of relativity predicts that a person making a round trip will return younger than his or her identical twin who stayed at home.
Thermodynamics
Gibbs paradox: In an ideal gas, is entropy an extensive variable?
Loschmidt's paradox: Why is there an inevitable increase in entropy when the laws of physics are invariant under time reversal? The time reversal symmetry of physical laws appears to contradict the second law of thermodynamics.
Maxwell's demon: The second law of thermodynamics seems to be violated by a cleverly operated trapdoor.[4]
Mpemba effect: Hot water can, under certain conditions, freeze faster than cold water, even though it must pass the lower temperature on the way to freezing.
Biology
Antarctic paradox: In some areas of the oceans, phytoplankton concentrations are low despite there apparently being sufficient nutrients.
C-value enigma: Genome size does not correlate with organismal complexity. For example, some unicellular organisms have genomes much larger than that of humans.
Cole's paradox: Even a tiny fecundity advantage of one additional offspring would favor the evolution of semelparity.
Gray's paradox: Despite their relatively small muscle mass, dolphins can swim at high speeds and obtain large accelerations.
Hormesis: Exposure to small doses of toxins can have beneficial effects.
Lek paradox: Persistent female choice for particular male trait values should erode genetic variance in male traits and thereby remove the benefits of choice, yet choice persists.
Lombard's paradox: When rising to stand from a sitting or squatting position, both the hamstrings and quadriceps contract at the same time, despite their being antagonists to each other.
Paradox of enrichment: Increasing the food available to an ecosystem may lead to instability, and even to extinction.
Paradox of the pesticides: Applying pesticide to a pest may increase the pest's abundance.
Paradox of the plankton: Why are there so many different species of phytoplankton, even though competition for the same resources tends to reduce the number of species?
Sherman paradox: An anomalous pattern of inheritance in the fragile X syndrome.
Taxonomic boundary paradox: The concept for a taxon can overlap in the past.
Temporal paradox (paleontology): When did the ancestors of birds live?
Health and nutrition
French paradox: The observation that the French suffer a relatively low incidence of coronary heart disease, despite having a diet relatively rich in saturated fats, which are assumed to be the leading dietary cause of such disease.
Glucose paradox: The large amount of glycogen in the liver cannot be explained by its small glucose absorption.
Hispanic paradox: The finding that Hispanics in the United States tend to have substantially better health than the average population in spite of what their aggregate socio-economic indicators predict.
Israeli paradox: The observation that Israelis suffer a relatively high incidence of coronary heart disease, despite having a diet very low in saturated fats, which are assumed to be the leading dietary cause of such disease.
Meditation paradox: The amplitude of heart rate oscillations during meditation was significantly greater than in the pre-meditation control state and also in three non-meditation control groups[5]
Mexican paradox: Mexican children tend to have higher birth weights than can be expected from their socio-economic status.
Obesity survival paradox: Although the negative health consequences of obesity in the general population are well supported by the available evidence, health outcomes in certain subgroups seem to be improved at an increased BMI.
Peto's paradox: Humans and other small-to-medium-sized mammals get cancer with high frequency, while larger mammals, like whales, do not. If cancer is essentially a negative outcome lottery at the cell level, and larger organisms have more cells, and thus more potentially cancerous cell divisions, one would expect larger organisms to be more predisposed to cancer.
Pulsus paradoxus: A pulsus paradoxus is an exaggerated decrease in systolic blood pressure during inspiration. It can indicate certain medical conditions in which there is reduced cardiac output, such as cardiac tamponade or constrictive pericarditis. Also known as the Pulse Paradox.[6]
Second wind: The "second wind" is a sudden period of increased wakefulness in individuals deprived of sleep that tends to coincide with the individual's circadian rhythm. Although the individual is more wakeful and aware of their surroundings, they are continuing to accrue sleep debt and thus, are actually exacerbating their sleep deprivation.
Chemistry
Faraday paradox (electrochemistry): Diluted nitric acid will corrode steel, while concentrated nitric acid will not.
Levinthal paradox: The length of time that it takes for a protein chain to find its folded state is many orders of magnitude shorter than it would be if it freely searched all possible configurations.
SAR paradox: Exceptions to the principle that a small change in a molecule causes a small change in its chemical behavior are frequently profound.
Time travel
Bootstrap paradox (also ontological paradox): You send information/an object to your past self, but you only have that information/object because in the past, you received it from your future self. This means the information/object was never created, yet still exists.
Predestination paradox: A man travels back in time to discover the cause of a famous fire. While in the building where the fire started, he accidentally knocks over a kerosene lantern and causes a fire, the same fire that would inspire him, years later, to travel back in time. The bootstrap paradox is closely tied to this, in which, as a result of time travel, information or objects appear to have no beginning.
Temporal paradox: What happens when a time traveler does things in the past that prevent him from doing them in the first place?
Grandfather paradox: You travel back in time and kill your grandfather before he conceives one of your parents, which precludes your own conception and, therefore, you couldn't go back in time and kill your grandfather.
Polchinski's paradox: A billiard ball can be thrown into a wormhole in such a way that it would emerge in the past and knock its incoming past self away from the wormhole entrance, creating a variant of the grandfather paradox.
Hitler's murder paradox: You travel back in time and kill a famous person in history before they become famous; but if the person had never been famous, then he could not have been targeted as a famous person.
Linguistics and artificial intelligence
Bracketing paradox: Is a "historical linguist" a linguist who is historical, or someone who studies "historical linguistics"?
Code-talker paradox: How can a language both enable communication and block communication?
Moravec's paradox: Logical thought is hard for humans and easy for computers, but picking a screw from a box of screws is an unsolved problem.
Movement paradox: In transformational linguistics, there are pairs of sentences in which the sentence without movement is ungrammatical while the sentence with movement is not.
Sayre's paradox: In automated handwriting recognition, a cursively written word cannot be recognized without being segmented and cannot be segmented without being recognized.