What is Twin paradox: Definition and 318 Discussions

In physics, the twin paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket and returns home to find that the twin who remained on Earth has aged more. This result appears puzzling because each twin sees the other twin as moving, and so, as a consequence of an incorrect and naive application of time dilation and the principle of relativity, each should paradoxically find the other to have aged less. However, this scenario can be resolved within the standard framework of special relativity: the travelling twin's trajectory involves two different inertial frames, one for the outbound journey and one for the inbound journey. Another way of looking at it is by realising that the travelling twin is undergoing acceleration, which makes him a non-inertial observer. In both views there is no symmetry between the spacetime paths of the twins. Therefore, the twin paradox is not a paradox in the sense of a logical contradiction.
Starting with Paul Langevin in 1911, there have been various explanations of this paradox. These explanations "can be grouped into those that focus on the effect of different standards of simultaneity in different frames, and those that designate the acceleration [experienced by the travelling twin] as the main reason". Max von Laue argued in 1913 that since the traveling twin must be in two separate inertial frames, one on the way out and another on the way back, this frame switch is the reason for the aging difference. Explanations put forth by Albert Einstein and Max Born invoked gravitational time dilation to explain the aging as a direct effect of acceleration. However, it has been proven that neither general relativity, nor even acceleration, are necessary to explain the effect, as the effect still applies to a theoretical observer that can invert the direction of motion instantly, maintaining constant speed all through the two phases of the trip. Such observer can be thought of as a pair of observers, one travelling away from the starting point and another travelling toward it, passing by each other where the turnaround point would be. At this moment, the clock reading in the first observer is transferred to the second one, both maintaining constant speed, with both trip times being added at the end of their journey.

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  1. G

    How to Solve the Twin Paradox Problem Using Lorentz Transformations

    Homework Statement This is a typical twin paradox problem as laid out in Griffith's Introduction to Electrodynamics, problem 12.16. The problem states that, on their 21st birthday, one of two twins - we'll call her Alice - departs Earth for star X at (4/5)c. Upon arriving at star X, she...
  2. PeterDonis

    Using General Relativity to analyze the twin paradox

    In a previous thread, reference was made to an entertaining "defense" of relativity by Einstein, which can be found here: https://en.wikisource.org/wiki/Dialog_about_Objections_against_the_Theory_of_Relativity One of the arguments Einstein makes in this dialog is that the twin paradox can be...
  3. L

    Special Relativity: Twin Paradox Question

    I've already completed most of the question, it's an add on at the end that has stumped me. I've calculated using time dilation the difference in ages between the two twins, Joe traveled at v = 24/25 (c=1 units) to a planet for 7 years in HIS reference frame and returned at v = 12/25 The...
  4. D

    The Paradox of Aging in Relativity: Resolving the Twin Paradox with a Twist

    Hi all,I know the twin paradox has been discussed many times, so I hope you'll bear with me. My version has a slight twist. Suppose twin's A and B start off in a "rest" frame in dead space, frame F, at x = 0. At t = 0, A moves in the direction x with velocity c/2, and B moves in direction -x...
  5. DiracPool

    Twin paradox and the size of the universe

    Ok, let's take the standard twin paradox, Alice leaves on a trip in her rocketship near the speed the light, and comes back to Earth some time later to find herself 5 years younger than her twin, Bob. Now they go out to lunch and strike up a conversation as to how old the universe is. Alice...
  6. C

    Twin Paradox with "home time"

    Some thoughts about the twin paradox (with a new question/paradox at the end): Beginners, like myself, when confronting the twin paradox often want to know where it is that the traveling twin loses so much time. A frustrated poster trying to explain the paradox posted this: He's been given...
  7. F

    Does the Twin Paradox Still Apply with Identical Acceleration Readings?

    I am surprised this has apparently not been discussed here as such in this forum (nor anywhere else on the web). The point is that in the classical space-ship twin paradox, not only the traveling twin will be subject to a proper acceleration, but, according to Einstein's equivalence principle...
  8. D

    Could the Twin Paradox in GR be Generalized to Completely Empty Space?

    Whenever the twin paradox in GR seems to be discussed, it always seems to be done in the presence of a large mass such that the twins can be considered as test particles moving in some metric. I was wondering whether the same problem could be generalised and be proposed in completely empty...
  9. V

    Twin Paradox: Which Twin is Older?

    Maybe someone can explain the following twin paradox. There are two twins A and B. Twin A is shot straight up into the air with a very short and powerful cannon. Twin B remains on earth. Twin A goes up a distance and then falls back to earth. Which twin is now older?
  10. J

    Acceleration-free twin paradox

    I find the discussion about acceleration confusing. I want to avoid that. So try this scenario: Lucy is traveling at .6c relative to Bob. Just when they reach their point of closest approach, one kilometer, they synchronize clocks. As their distance increases, each of them sees a red shift...
  11. adoion

    Was Einstein lucky when not considering twin paradox as paradox?

    hi, Einstein did not even consider the twin paradox as problematic at all, he argued that it is a simple consequence of his special relativity? obviously he never gave a explanation of why the two twins don't age the same he instead left it to others to do so. was Einstein just having a hunch...
  12. D

    Twin Paradox Questions: Exploring the Observer's Perspective

    Hello all, I have not studied general relativity only special relativity, I apologize therefore if my questions seem low level. Thank you for your help. Three questions: The way in which I learned that the twin paradox is resolved is through the illustration of the way in which the observer...
  13. DiracPool

    Acceleration and the twin paradox

    After many years of agonizing over it, I have still failed to come to terms with the twin paradox. Here's a brief review of my understanding and a few questions: A standard story is as follows: Twin A leaves the Earth for planet Zolan 10 light years away. Twin B stays on Earth. Let's say...
  14. kmm

    Time Dilation, Twin paradox etc.

    I know there have been a lot of other threads on this topic but I wanted to get some feedback on my thoughts on this. Time dilation is a result of Einstein's second postulate of SR, although the first postulate is what gives rise to the apparent twin "paradox". The common solution to the paradox...
  15. L

    Twin paradox spaceship problem

    Homework Statement Anna is born in a spaceship, Bob is born on Earth (at the same time t=t'=0) just as Anna's spaceship passes Earth at 0.9c (EVENT A) Planet Z is 45 ly away at rest in Bob's frame. Anna flies past plant Z (event B) Meanwhile, Carl was born at the same time (t=t'=t''=0) on a...
  16. M

    Twin Paradox Explained: Earth & Space Time Effects

    A person in the Earth (stationery frame) observes the clock in the spaceship (which almost moving at the speed of light) runs slow right? and an astronaut in the space-ship observes the clock in the earth, to him the clock also appears slow the same rate as the person observes the clock in the...
  17. V

    Too Simple & Short Solution of Twin Paradox

    Consider two points A and B (Fig.1) at rest in S frame having a distance L0 between them (a long scale may be imagined as placed between A and B). ------ (1) The ‘traveler’ twin (S-frame) starts from A at time t = 0 and suddenly accelerates to attain a constant velocity v in the view...
  18. bcrowell

    Your preferred way of explaining this point about the twin paradox?

    A student asked me a question in class last week about the twin paradox, and I found that although I knew a valid answer, it wasn't an answer that my class had the background to understand, and I wasn't immediately able to come up with one that would work for them. The standard difficulty in...
  19. Grimble

    Exploring the True Nature of Relativity & The Twin Paradox

    It is said that Relativity proves that if one travels near the speed of light that time slows down, i.e. that the twin who travels ages less than the twin who stays behind (Twin Paradox) and that distances/lengths contract, but only in the direction of travel. Yet a simple look at these two...
  20. A

    Understanding the Twin Paradox: A Quick Question Answered

    Hey guys, I just want to ask a quick question that confuses me a bit regarding the twin paradox. During acceleration, the moving twin very quickly 'runs over' across a large segment of the worldtube of the stationary twin. But, what is the perspective of the stationary twin during the...
  21. R

    Twin paradox without acceleration - A different question

    Hi, I noticed this topic already being discussed but didn't want to hijack the thread with a different question. I don't know if this has been asked before but if it has I couldn't find it. I was watching something recently about quantum physics and entanglement and it mentioned something...
  22. stevmg

    Does Time Dilation Occur for Both Twins in the Twin Paradox?

    Two twins A and (evil) B. Twin A remains "stationary" while evil B goes at near light speed to-and-fro. Ignoring General Relativity (acceleration/deceleration) evil B experiences time dilation and as result, when they meet up again, B's clock is behind A's clock. Look at it from B's point...
  23. T

    Twin paradox without accelerative frame.

    I've asked this question a few times. This time i have tiny change in the question. Instead of people I'm using stopwatch & i have removed the accelerative frame. If you have any doubts I'll clear it but do not skip the topic. All i want is an answer that can be agreeable for all...
  24. N

    Understanding the Twin Paradox: Time Dilation in Relativity

    Hey! So the formula for time dilation is: \gamma \Delta t_0 = \Delta t, where ##\Delta t_0## is the time elapsed for a traveller ##S'## who has a relative constant velocity ##u## compared to a stationary observer ##S##, while ##\Delta t## is the time elapsed for the stationary observer...
  25. C

    [Question] About the radio signal for Twin Paradox

    There is an example for the radio signal for Twin Paradox from the book "Classical Dynamics of Particle and Systems, Thornton, Marion". Refer "1.jpeg", example 14.5, it said that "However, Frank continues to receive a signal every 3 years for the 8 years it takes the last signal Mary sends when...
  26. A

    Momentum conservation in the twin paradox

    I was thinking about one of the many resolutions of the so-called 'twin paradox' within SR framework. I realize many methods of resolving this have been proposed, but I am curious about the particular solution that uses 'acceleration' of the traveling twin, to create the required asymmetry for...
  27. P

    Can a twin paradox occur if one twin never changes direction or stops?

    Hi Everyone, I've managed to confuse myself with a mutilated thought experiment. A half twin paradox. Imagine a twin, Paul travels away from Earth at 8/10c while peter stays at home. Because of time dilation paul reaches a planet by his clock, in one year. Peter registers Pauls arrival at...
  28. C

    Exploring the Twin Paradox: Acceleration & Aging

    http://en.wikipedia.org/wiki/Twin_paradox I read a little bit about this, and am a little bit confused. I always learned about reference frames. When I jump, you can say that I am moving away from the earth, or the Earth is moving away from me. Why is this any different for near-light travel...
  29. H

    Twin Paradox & Neutron Speeds: Questions Explored

    I have make many threads about this topic because I am really interested. I reading that from an pdf,but I have some "idiot" questions too. 1)A guy told me that If you run in other speed from an other you see his clock slower. I have hear about paradox of twins,that this with biggest speed...
  30. Z

    Modified Twin Paradox in Special Relativity

    The problem statement Suppose the universe is a 2-torus, thus there exists a spatial path that you will go back to where you start eventually. Let Observer A and Observer B travel on this closed path. Observer B is traveling at a constant speed of 0.9c relative to Observer A, where c is the...
  31. R

    Understanding the Twin Paradox: A Simple Explanation

    Please explain me simply about twin paradox..
  32. Z

    A question on a tweek of the Twin Paradox

    now the resolution of the twin paradox as put down in textbooks i have read relies on one observer being accelerated and one not so one can claim to be inertial and thus priveledged - this i always felt to be a cheat - even edward teller uses it in conversations on the dark secrets of physics -...
  33. A

    Gravitational Time Dilation and Twin Paradox ?'s

    Disclaimer: I am relatively illiterate on the subject so please try to keep explanations generally understandable. Aerospace undergrad programs get zero exposure to relativity I suppose. [Gravitational Time Dilation] First I want to know if I am correct in saying that the gravitational...
  34. T

    Resolving the Twin Paradox in Relativity

    Suppose you have two people who are in identical orbits around a large star. The only difference between them is the direction they are traveling. At a certain point where they meet ( there are two such points ), they sync clocks. After a few orbits, they meet again and again compare clocks...
  35. H

    Twin Paradox: Exploring Time Dilatation & Relativity

    Hi, my first post on the forums. I've known about twin paradox for a while, so when we learned special relativity at school in September, it wasn't anything weird. Math is elegant, unlike quantum physics. But a few weeks ago, I started wondering something. It might be just an ordinary logical...
  36. jaumzaum

    Spacetime diagram - Twin paradox

    I was studying the twin paradox (of Einstein special relativity) and everything was working well until I get to the traveler's spacetime diagram. First let me introduce the paradox for you to understand the diagram. Pam is the twin sister of Joe. Pam goes out Earth in 2007 in a spaceship...
  37. Ibix

    Closed flat space twin paradox

    I'm not sure if this belongs here or in Cosmology - possibly either would do, I think. I was reading the thread about open/closed/flat universes that's currently ongoing in Cosmology, and a related question occurred to me. According to post #7 you can have geometrically flat universes that...
  38. 8

    Resolving the Twin Paradox with Symmetry and LabVIEW

    i can't get my head around the twin paradox at all. so twins A and B move relative to each other and as a result, according to special relativity, from twin A's perspective twin B will of aged less but from twin B's perspective twin A will of aged less. when they meet up which one will of...
  39. D

    Twin Paradox With A Beacon

    So once again, I'm trying to wrap my head around the asymmetry in the twin paradox problem. Here's my setup. The two twins start on a very massive but small planet such that they always have an unobstructed view of each other. 1. They each attach a beacon to themselves that will briefly flash...
  40. T

    Time dilation in relativity of simultaneity related to twin paradox

    Hi All, I think that I understand (as far as you can!) the ideas behind explaining why two twins have different ages if one travels at near light speed away for a time and then returns to his twin on Earth (the return journey changing the inertial reference frame). I have a related...
  41. morrobay

    Twin Paradox Without Acceleration ?

    What would be the example of the twin paradox where there is no acceleration at all ? Only different periods of constant velocity for both twins. Or of periods with different velocities for both twins.
  42. V

    What happens during acceleration in twin paradox?

    Hi, I just recently studying special relativity in my class. It's fun and I kind of understand what's going on. But the professor skips the acceleration/deceleration part because it's for advance level. So what happens during the acceleration and deceleration (assuming the object won't get...
  43. R

    Does the Twin Paradox Involve More Than Just Velocity?

    I know the twin paradox has been answered a lot on here, sorry for bringing it up again. I thought i about had it figured out but then i read a few things that confused me again. For starters, its starting to sound like time dilation won't happen unless there is a complete round trip, that...
  44. A

    Is asymmetric time dilation in twin paradox possible in SR?

    I am trying to understand if SR can explain a real, measurable velocity time dilation as seen in experiments/observations like GPS satellite or Bailey et. al. Let us say we have twins sitting in their identical, individual spaceships in space, close to each other, and far away from any large...
  45. G

    The Twin Paradox: Triplets Edition

    A variant of the classic twin paradox. There are three triplets, who have the boring names of Adam, Bob and Charles. In Charles' "rest" frame, Adam and Bob get into identical rocketships, jet off in opposite directions and return a year later. To Charles, Adam and Bob's paths are...
  46. R

    Twin Paradox Problem: Do Twins Age Differently?

    twin paradox problem!? ok it's about the twin paradox the guy on spaceship goes at 90% of C (say) but since both the twins see each other's clocks equally slow and if one sees other at 90%C then the other one also sees his twin at 90%C in opposite direction. then how on returning the twin on...
  47. S

    Ehrenfest Paradox + Twin Paradox

    1. The Ehrenfest Paradox After watching some of the posts here, I am not sure if I can conclude - The circumference-radius ratio is INDEED not 2π, but with a factor γ (which means Euclidean geometry is not necessarily correct in relativity, and the radius 'cuts through the circumference' ?) ...
  48. J

    Attempt on Twin Paradox via Minkowski diagrams

    This is an attempt to solve the twin paradox via two Minkowski diagrams with a few questions attached. Note that there might be mistakes in this drawing i will fix in the course of this thread in case someone notices any. First, let me explain the two Minkowski diagrams in the drawing first...
  49. E

    A Twin Paradox Question, totally lost.

    Homework Statement Planet W is 12ly from earth. Anna and Bob are both 20 years old. Anna travels to planet W at 0.6c, quickly turns around, and returns to Earth at 0.6c. How old will Anna and Bob be when Anna gets back? Homework Equations The Attempt at a Solution I've tried...
  50. T

    A Universe with no Twin Paradox

    People are intrigued by Einstein's Special Theory of Relativity. They are fascinated by the Twin Paradox and the slowing down of moving clocks. However it is often forgotten that Einstein's SR, in spite of being weird to the day to day speeds we are accustomed to, saved Physics from something...
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