What is Paradox: Definition and 1000 Discussions

A paradox is a logically self-contradictory statement or a statement that runs contrary to one's expectation. It is a statement that, despite apparently valid reasoning from true premises, leads to a seemingly self-contradictory or a logically unacceptable conclusion. A paradox usually involves contradictory-yet-interrelated elements that exist simultaneously and persist over time.In logic, many paradoxes exist which are known to be invalid arguments, but which are nevertheless valuable in promoting critical thinking, while other paradoxes have revealed errors in definitions which were assumed to be rigorous, and have caused axioms of mathematics and logic to be re-examined. One example is Russell's paradox, which questions whether a "list of all lists that do not contain themselves" would include itself, and showed that attempts to found set theory on the identification of sets with properties or predicates were flawed. Others, such as Curry's paradox, cannot be easily resolved by making foundational changes in a logical system.Examples outside logic include the ship of Theseus from philosophy, a paradox which questions whether a ship repaired over time by replacing each and all of its wooden parts, one at a time, would remain the same ship. Paradoxes can also take the form of images or other media. For example, M.C. Escher featured perspective-based paradoxes in many of his drawings, with walls that are regarded as floors from other points of view, and staircases that appear to climb endlessly.In common usage, the word "paradox" often refers to statements that are ironic or unexpected, such as "the paradox that standing is more tiring than walking".

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  1. J

    I Does it make sense to speak about the Grandfather paradox in QM?

    Since QM is not deterministic, the future state B is not determined by the previous state A (at time A, B was only a possibility, not a certainty). Then, when we are at time B, and assuming we could move back in time (of course, we cannot do that, but let us make a Gedankenexperiment), it just...
  2. Lars Krogh-Stea

    B Hilbert's paradox of the Grand Hotel - An easier solution?

    I know the paradox is about a hypotethical hotel with hypotethical guests, how else could new people arrive, when everyone (infinite) is already inside;) But maybe the ones outside are aliens, I don't know.. My real point though, is that there's no need for the intricate shuffeling of guests...
  3. PeterDonis

    I Kinematic Decomposition for "Rod and Hole" Relativity Paradox

    In a recent thread, I said that if there was interest, I would post in a separate thread the calculations for the kinematic decomposition of the congruence of worldlines describing the rod in the "rod and hole" relativity paradox discussed in that thread. Since there was interest, I am posting...
  4. B

    B Loschmidt's paradox and free gas expansion

    Following up on a previous discussion: https://www.physicsforums.com/threads/conservation-of-momentum-in-a-closed-system.1009693/post-6570341 An uncontained system of particles interacting only by elastic collision is the same as a gas undergoing free expansion. If, for simplicity, the...
  5. alan123hk

    B The Paradox of Relativity Length Contraction

    A rigid rod with length ## l_0## slides on a smooth and flat tabletop along the length at speed of ## ~\frac {\sqrt{3}}{2}c~ ##, there is a hole of width ##~l_0~##on the table. The observer who is stationary relative to the desktop thinks that the length of the rigid rod ##~ l=l_o...
  6. P

    I SR "Paradox" Thread Continuation: Tilt of Plates & Simultaneity

    The thread: https://www.physicsforums.com/threads/question-about-a-stronger-relativity-paradox-than-the-twin-paradox.1009978/ proposed an interesting SR scenario. The thread could have led to instructive discussion of exactly how relativity of simultaneity resolved a "false paradox" that...
  7. P

    A Klein paradox in the massless case

    I have a question about the Klein paradox in the massless case, for a potential step of height ##V_0## (this is exactly the situation described by Wikipedia). I don't have a problem to understand the "paradox", and I think the Wikipedia's illustration is quite telling. My question is : what...
  8. K

    B Explaining a Stronger Relativity Paradox than the Twin Paradox

    The twin paradox can be explained by changing reference frames. But I’m really curious how this paradox can be explained. In the situation below there are three observers: A: Standing at a moving train platform moving at a speed of c/2 relative to “the ground”. B: Standing at a moving train...
  9. alan123hk

    B Solving the twin paradox with special relativity

    I saw a book that uses special relativity to solve the twin paradox, the inference process is roughly as follows. Suppose a spacecraft sets off from the Earth to travel to a distance black hole and then return to the earth. We divide this process into three stages, that is, the process of...
  10. pinball1970

    I Solving Hangman's Paradox: Axioms, Deductions & Probabilities

    This started from a fairly innocuous post until fresh42 posted this link. Perok gave me guidance to work through it and I would like views on this. I kind of get it now I think, breaking the statements down into assumptions and deductions, combining the two is not permitted because you are...
  11. S

    I A problem with the solution to the ladder paradox

    Imagine that there is a very tight string connected to the bottom of each barn door. Each barn door opens and closes vertically. If the string breaks the barn blows up. The doors have to close at the same time for the string not to break. If the doors open one at a time, the extra distance...
  12. Buckethead

    B Paradox in the block universe interpretation

    Consider two observers, Alice and Bob, standing on the Earth together with synchronized clocks. Bob asks Alice, “Is this now?” to which Alice replies, “Yes, it is”. They are clearly both in the present moment. Bob climbs a mountain and some time later, due to the lower gravitational field...
  13. C

    I Solving Zeno's Paradox: The Flaw in the Notion of Infinity

    Zeno's paradox claims that you can never reach your destination or catch up to a moving object by moving faster than the object because you would have to travel half way to your destination an infinite number of times. The two conflicting elements in this paradox are: 1) We do reach...
  14. D.S.Beyer

    B Twin Paradox: Orbital Geodesics, Gravity, & Time Dilation

    ~ Shower Thoughts ~ Twin A is in a spaceship, Twin B is in a spaceship. Both in 'deep space'. B follows a highly elliptical geodesic which goes around a planet (or black hole) with strong gravity, very far away. When they meet again, who is younger and why? I genuinely don't know what this...
  15. RH111

    I Pole in the Barn Paradox: Exploring Relativity of Simultaneity

    Hi all, I've been exploring the concept of the Relativity of Simultaneity. However, I cannot grasp the Pole in the Barn Paradox. Here is the video to the version of this paradox I am referring to: https://worldscienceu.com/lessons/11-3-the-pole-in-the-barn-paradox/ In this situation, the...
  16. J

    Crypto-cancer fade out end of civilization for Fermi paradox?

    Imagine civilization gets a positive feedback mechanism for wasting resources, like cryptocurrencies: “one gets $100 banknote if burning $99 worth resources”, leading to exponential growth of waste at individual gains. We can observe exponential growth of their energy consumption (below)...
  17. G

    I Leading Clocks Lag: Twin Paradox Explained

    . . Two distant observers at rest with one another synchronize their clocks utilizing the travel time of light (by first moving the clocks to their locations, then using a light signal to time when the second clock should be turned on, and setting the second clock ahead to account for the...
  18. Demystifier

    A Evolution of the Renner et al Wigner-like paradox

    In the first version of their arxiv preprint, Frauchiger and Renner argued that only the many word interpretation of QM was consistent. https://arxiv.org/abs/1604.07422v1 In the second version, the famous one published in Nature Communications, they radically changed their conclusions. They...
  19. O

    I Twin Paradox: Spacetime Physics Ch.4 & 202 Year Difference

    Spacetime physics chapter 4 describes this wonderfully well chosen set of speeds/distances for the twin paradox. A traveler departs from Earth at a speed of 99/101 (1=speed of light), traveling to a star that is 99light years away. From Earth's perspective, the traveler takes 101 years to go...
  20. Killtech

    I Exploring the Twin Paradox for Light & Sound Waves

    Originally i just wanted to look at how much analogy can be made between light and sound waves using all that math has to offer to depict them in most similar framework possible - just so as to have a different perspective to understand some things better. Anyhow, no matter how well one tries to...
  21. Sophrosyne

    B Twins Paradox Thought Exp: What Happens When Reunited?

    I have been looking through some of the threads about the twins paradox in relativity. It’s clear there’s a lot of confusion on this, and I am yet one more person very confused on this. So I was thinking about a hypothetical experiment, and I will lay out my hypothesis of what might...
  22. MikeandSuch

    How to get away with killing your great grandfather with time travel

    So picture this, its 2500 and you want to time travel back to 2300 to kill your great grandfather, this obviously creates a paradox wherein you prevent yourself from being born right? Well why don't you use your year 2500 technology to create a clone of your grandfather to replace him after (or...
  23. G

    I Pole-Barn Paradox: Is My Understanding Correct?

    I want to make sure I have this right, and whatever I have wrong I would like to fix it. Part I: So in particular, I’m referring to a fast moving pole along the x-axis of an observer who is at rest with respect to the barn (the barn is a few feet away from the observer), and when the pole is...
  24. B

    B Anthropological Influence on Culture: Exploring the Observer Paradox

    When an anthropologist analyzes a culture he/she might influence it and so does not get an accurate understanding of that culture. Does that mean that there was no specific culture before the anthropologist arrived? NO! Similarly, why would someone say that a particle has no position before...
  25. B

    Life vs Entropy: Investigating an Unexpected Paradox

    I understand entropy as a movement from order to less order and that the universe's entropy is increasing. So I was wondering, life takes molecules and organizes them into organisms, so isn't life a reversal of entropy?
  26. J

    B Velocity Addn Paradox: SR Postulate Explained

    Velocity addition paradox: In observer A's frame, observers B and C move away from him at the speed of light c, B to the left and C to the right. In B's, frame A and C are both moving away from him at c, i.e. at the same speed. In both A's and C's frames they are moving at different speeds...
  27. Buckethead

    B The Twins Paradox: Exploring Time Dilations' Effects

    In a thread I started awhile back, in the common twins paradox scenario, it was indicated to me that the actual time (paraphrasing) on Earth for any given time in the ship is basically “undefined” (as it can’t be verified) and/or time dilated (ticking slower) for the trip out and then shifted to...
  28. BWV

    Can Coal Deposits Explain the Fermi Paradox?

    Reading McGhee's excellent book on the Carboniferous period and he explains in detail the unique circumstances behind the creation of 90% of the world's coal deposits - receding ocean in an ice age that created low lying swamps, high concentrations of oxygen in the atmosphere that would have...
  29. J

    Energy paradox in classical electrodynamics?

    Consider two massive charged objects at rest with a large horizontal distance ##d## between them (object ##1##: mass ##m_1##, charge ##q_1## and object ##2##: mass ##m_2##, charge ##q_2##). I apply a constant vertical force ##\vec{f_1}## upwards to object ##1## so that it gains an acceleration...
  30. astrodummy

    I Solving the Flickering Light Paradox

    This post builds on a previously closed thread here: https://www.physicsforums.com/threads/lightbulb-paradox.141191/#post-1145674 I will not describe the problem here without the copyright owner's permission. I, like the OP in the original thread, am keen to see if anyone else who has this text...
  31. Biggles 1984

    Engineering Paradox: Weight & Thrust/Weight

    Engineers are confronted with two apparently true but contradictory statements for airplanes in flight: a) Lift must equal the weight of the airplane (Lift = Weight), based on Newtons 2nd Law of motion (i.e. Force = ma); where gravity is used to calculate weight (i.e. Weight = mass x...
  32. O

    B Paradox of Simultaneous Flashing in a Moving Triangle of Lights

    There are 3 lights in the form of a triangle... A, B, and C are lights and are stationary with respect to each other. S1, S2, S3 are spaceships. B S1 S2 A S3...
  33. SEYED2001

    I Why does "control" mean "coherency" in Wigner's friend paradox?

    There we read: "note that if Wigner did not know this phase due to the lack of control of it, he would describe the “spin + friend’s laboratory” in an incoherent mixture of the two possibilities". Why is this the case? Given that the author has propoede neither a citation nor a proof for this...
  34. Kairos

    B Twin Paradox: Radioactive Decay Tested

    If two identical radioactive masses were subjected to the "twin paradox" experiment of Langevin, would the mass that traveled be really less radioactive than the one that did not? Radioactive decay is supposed to be independent of physical conditions and to only depend on the isotope.
  35. A

    I Gibbs paradox: an urban legend in statistical physics

    Hi, I recently discovered that there is no real paradox in the question of the mixing of classical distinguishble particles. I was shocked. Most books and all my professors suggest that an extensible entropy could not be defined for distinguishble particles. I believe that many of you will be...
  36. Marilyn67

    I Explaining the Twin Paradox: A Big Problem?

    Hello, For many years, solving the twin paradox has not been a problem for me. I understood that it is the change of Galilean frame of reference that is responsible for the age difference, and this clearly appears on a Minkowski diagram. Okay. However, since yesterday, in wanting to explain...
  37. I

    B Twin Paradox: Tom & Jerry Agree on Clock Time But Who is Older?

    I know this has been asked so many times, but would someone please answer why to this particular variation to the question. Tom is moving at constant velocity past Jerry on Earth (assume no acceleration). At the moment they pass each other they agree on the time seen on a clock. Tom thinks...
  38. Omega0

    Paradox for time travel to the past solved?

    Hello, I am currently reading "The first Fifteen Lifes of Harry August" from Catherine Webb and really like it so far. I did not even read the half or so but there was an interesting idea I would like to discuss about. As far as I understand Harry August does return after dying to the same...
  39. Kostik

    B Paradox about black hole evaporation

    Paradoxical scenario. Suppose Jack and Jill are sitting safely a kilometer above the event horizon (EH) of a large black hole. Now suppose: Jack decides to head toward the center of the black hole, traveling at an easy pace (say 10 km per hour). Jill sees Jack (with her ultrasensitive infrared...
  40. Buzz Bloom

    I EPR Paradox: Exploring Uncertainty & Experimental Results

    While I have have heard about it for years, I have just read a more-or-less clear description of the EPR "paradox" in David Lindley's Where Does the Weirdness Go? (1996), page 91, "The fatal blow?". Here is a summary (paraphrasing what I read) as I understand it. A pair of particles (say A and...
  41. F

    Does Fluid Dynamics Explain Pressure Gradients in a Frictionless Tube?

    Let's start with a horizontal tube with a constant diameter. I'm not sure if it's important, but let's assume it's frictionless. I will have some fluid flowing in this tube and if it's important, we can make the fluid incompressible, inviscid, irrotational, etc. To create a flow in the tube...
  42. Philip Koeck

    I Experimental test of the Mixing Paradox

    Can anybody point to an experiment that shows that the total entropy increases if two different gas mix at constant pressure and temperature, whereas if two volumes of identical gases mix the total entropy doesn't change?
  43. R

    Gibbs "Paradox" and the Entropy of mixing

    (not a paradox nowadays, but it was an issue for years) https://en.m.wikipedia.org/wiki/Gibbs_paradox It's not a question about a formula. I don't understand the motivation in physics to claim Gibbs mixing "paradox", the discontinuity point. What bothers the physicist to ask for a continuous...
  44. Pyter

    I What happens when twins synchronize clocks and one teleports?

    Hello all. Recently this twin paradox variant occurred to me, and I can't wrap my head around it: Alice and Bob are in the same (roughly inertial, for our purposes) reference frame, separated by a sideral distance. Let's say Alice is on Earth and Bob on Pluto. They synchronize their clocks at...
  45. B

    I What is the paradox in the EPR paradox?

    Hi Which of these understandings is correct ? A stationary particle separates into 2 particles A & B Is it that : (a) One can independently measure accurately to the desired accuracy the momentum of A and the position of B and thus obtain both position AND momentum of either particle thus...
  46. P

    I Possible paradox in the uncertainty principle?

    In the Heisenberg's uncertainty principle ## \triangle x \triangle p \geqslant \frac{\hbar}{2} ## what happens when the uncertainty in position becomes very small is that the uncertainty in momentum becomes very large. But what happens when the spread of the uncertainty in momentum becomes...
  47. Hiero

    I Twin Paradox Time Dilation in Positively Curved Universe

    I’m sure the resolution is something to the effect of “we can only apply special relativity in flat spacetime” but I’m hoping someone can explain in more detail. Disclaimer: I don’t know general relativity. So in a positively curved universe, if you keep traveling (let us neglect expansion)...
  48. Imaxx

    I The Mirror Paradox: Exploring Special Relativity Theory

    Let's imagine that there are two people who has the same biological age. One is inside a spaceship, currently crossing a Quasar(A) in a constant velocity near the speed of light toward the Earth and the other is on Earth(B). In this situation, what will happen when A moves to the Earth and meets...
  49. G

    B A Meritable Veritable Time Paradox?

    So I stumbled upon this paradox and was wondering what cosmologists had to say about it. For the most part, I take an instrumental view of science. Of course, I accept all scientific histories and truisms as rational touchstone, though I also think we can never be too precocious about the fate...
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