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. S

    Exploring the Twin Paradox: Searching for the Oldest Frame of Reference

    I was just working on my knowledge of the twin paradox, and had a question that I couldn't find an answer to anywhere: If I understand it correctly the paradox is resolved because the two frames are not symmetrical, one is non-inertial, so that frame has it's clock run slower. So if you carry...
  2. DARKSYDE

    Angular momentum and the twin paradox

    If you were spinning on a marry-go-round approaching the speed of light, would you experience time dialation?
  3. B

    Exploring the Twin Paradox: 4 Solutions = 0?

    how about this: silly idea, but think about it: the square root of 1 = {+1,-1} and we all know: t'=t/(1-(v/c)^1/2) in essance, t' must simultaneously have values of {+t',-t'} for all velocities not equal to zero and approaching c. this means that for a relativistic frame of...
  4. Jonnyb42

    Twin Paradox (by acceleration)

    Ok so, supposedly you have two twins in [seperate] spaceships, one approaches the speed of light going away from his twin, then decelerates until going the same high speed but opposite direction, decelerates again to stop next to his twin buddy. The twin that did not move (relative to the very...
  5. A

    Solving the Twin Paradox with Lorentz Transformation

    We use the following formulas for the Lorentztransformation: x’ = [ x / sqrt(1-v**2/c**2)] - [vt / sqrt(1-v**2/c**2)] (1) and t’ = - [ (vx)/c**2 / sqrt(1-v**2/c**2)] + [ t / sqrt(1-v**2/c**2)] (2) The twin paradox reads as follows. Gea and Stella are identical twins. Stella...
  6. R

    Twin paradox and length contraction

    We know that the twin paradox is not a paradox since one of the twins accelerate and time passes more slowly for him relative to his twin on earth. My question is why can't we apply the same reasoning for length contraction? If one of the twin A measures the lengths of sticks in the reference...
  7. I

    Solving the Twin Paradox: Age Difference Between Bill and Ben

    Homework Statement Two twins, Bill and Ben are 30.0 years old and they leave Earth for a distant planet 12.0 light years away. The twins depart at the same time on Earth, and travel in different space ships. Bill travels at 0.93c, while Ben travels at 0.70c. What is the difference between...
  8. T

    Time dilation & symmetrical twin paradox variant

    I'm still trying to understand time dilation, the twin paradox, and the effect of acceleration. Yes, I've read the twin paradox FAQ, but it only gets me so far. Here's a hypo I came up with. I've tried to simplify it by (a) making the observers symmetrical and (b) eliminating the...
  9. G

    Twin paradox thought experiment

    I've been wracking my brain for days trying to comprehend everything that deals with the twin paradox. I have a vague understanding of relativity and its effects, but am massively confused by most of the explanations given on this forum and other places. I think perhaps it extends from my lack...
  10. J

    Understanding the Twin Paradox: Exploring the Effects of Relativity on Aging

    I'm kind of a noob who's posted a question or two here before to settle arguments and such. Please don't kill me if this has been covered before. I was reading some stuff about space travel trying to get some creative ideas for a sci/fi book idea I had. Nothing serious and really nothing to...
  11. E

    Twin paradox without length contraction

    Hi all, apologies if this has been posted a million times before... I'm trying to explain the twin paradox without getting involved with length contraction. One way to think of it is Twin A remains at rest on Earth then twin B goes off at 4c/5 to Alpha centauri 4 light years away, then...
  12. G

    Proving the One Way Twin Paradox: O' Reads T'>0, K Clock Reads More than T

    In the following, "G" stands for "gamma". Clocks O and O' coincide when they mutually read zero. POV is that of K'. Prove: When O' reads T'>0, the coincident K clock reads more than T'. Proof: When O' reads T', O reads T'/G and O' coincides with K clock at x=GvT'. That clock reads...
  13. J

    Twin Paradox: Anne & Joe's 20th Birthday

    Homework Statement Anne and Joe are twins, happily living in an inertial frame. On their 20th birthday Joe decides to take a rocket. (a) According to Anne the rocket moves with constant speed v = \frac{3c}{5}. For 6 months it moves away from Earth and then returns in time for Anne's 21st...
  14. D

    Twin paradox in a closed universe

    I`ve thought about a special sort of twin paradox. I know the usual explanation of the twin paradox but give me please the answer to this special case: Imagine: A static universe (non-expanding) with a closed geometry and a circumference of one lightyear. The twins start their journey in...
  15. M

    Twin paradox - which twin is older?

    Hi everybody! :) can anyone help me with this idea: in the twin paradox, a twin that travels at speed of light in space should be younger than the one that remains on earth. But why can it be other way? Can we say that the twin on the Earth is traveling at speed of light compared to the twin in...
  16. B

    Twin Paradox - Acceleration vs Velocity

    Sorry, I know there are tons of threads about this, but I've been though them all and couldn't answer this satisfactorily myself. You know how it goes. You have 2 twins, A and B, who are magically the same age exactly. They start at the same point in space, each in their own spaceship. They...
  17. N

    Twin paradox and Hafele-Keating experiment

    In a few places I have read things along the line of this: I am getting confused, because some people say this experiment has resolved the paradox and some people think its still a paradox. How exactly has the twin paradox "been verified experimentally" from the HK experiment?
  18. G

    Is gravity the key to solving the twin paradox in general relativity?

    If the fixed stars/galaxies did not exist, would there be a twin paradox?
  19. Loren Booda

    Is special relativity sufficient to treat the twin paradox?

    I cannot conceive of a complete path which the "away" twin can follow, relative to the "home" twin, that does not involve significant acceleration. A straight line from home to star then back home in reverse, a centrifugal circle from home (0 degrees) to star (180 degrees) to home (360...
  20. Jonnyb42

    Relativity and time (twin paradox)

    I have thought about it and I have found to be confused about something, regarding the twin paradox. As I know it, the twin paradox (set up to emphasize my confusion) is as follows: Two spaceships floating in space pointing away from each other, let's say A and B. When B accelerates and...
  21. T

    Why Does the Traveling Twin's Clock Show Less Time in the Twin Paradox?

    Hi, I can see that this topic has been much discussed, but I haven't seen a thread on it with the particular spin I want to give it (just the journey out from Earth). I understand the traditional view of the twin paradox (I think!): Two twins a and b are on Earth and each has a clock. The...
  22. U

    Twin Paradox: Can Barbara Reach XEON Alive?

    My query is this. In the twin paradox let's assume that one twin Barbara keeps on moving at a relative speed of "c" away from the stationary Earth twin Alex. If I were to be able to be at both Barbara's rocket and the Earth at the same instant ( meaning I could travel through space...
  23. Jorrie

    Understanding the False Twin Paradox: An Alternative Explanation

    The "False Twin Paradox" I have no problems with the resolution of the classical 'Twin Paradox', but the following "False Twin Paradox" does have me puzzled. (If it was discussed and explained elsewhere, please just point me there.) Instead of the usual accelerating twin at the turnaround...
  24. I_am_learning

    How exactly to calculate the age difference in Twin paradox

    How exactly to calculate the age difference in "Twin paradox" In so much of the text I have seen the "twin paradox" is resolved by simply showing how the condition is asymmetrical, they make no calculation on how this asymmetry results in age difference. I have seen some claiming that its...
  25. M

    How is the twin paradox solved in the first model of the expanding universe?

    I already posted about this under a different thread (Time dilation for clock thrown up and caught back) but, after reading some of the replies, figured that that was not the right way of attacking my real problem, which is figuring out what happens to time during a Big Crunch. So I will restate...
  26. P

    Understanding Einstein's Twin Paradox, past the usual level?

    I heard some scientists say special relativity may not be relative to your frame of reference, but rather possibly distant galaxies or great sources of gravity? I heard they sent a jet around the world with an atomic clock, and also decaying sub atomic particles down a tube, to test special...
  27. L

    Twin Paradox (I know not again)

    I am new to the forums so hello everyone. I have spent time trying to understand the twin paradox, and I have only partially grasped it. What I specifically don't grasp is...what really happens when the traveler changes inertial frames from leaving Earth to heading towards Earth? I know...
  28. J

    The Twin Paradox: Exploring Time Dilation

    Can anyone prove the twin paradox
  29. R

    Acceleration and twin paradox question

    In the twin paradox, the twin that experiences the most acceleration does not age as much. If I synchronize two digital watches, then put one in a centrifuge for a long time, one watch will experience much more acceleration than the other. If they remain synchronized, why is the acceleration...
  30. S

    Twin Paradox- a quick(ish) question

    Hi all, I'm new here, I was on another forum asking this question but there was a lack of a response, so I hope you guys can help me out! Ok, so after a few weeks of grappling with the twin paradox, I finally accept that the twin that travels on the rocket and back is the one that ages less...
  31. B

    Twin Paradox in outgoing frame

    Homework Statement Show that as calculated in the rest frame comoving with the twin on the outgoing trip, the ratio of the two ages of the twins is the same: i.e. the twin on Earth has age gamme times the other twin Homework Equations Lorentz Transforms The Attempt at a Solution We...
  32. T

    Twin Paradox: Explaining the Relativity of Aging

    I know very little about relativity. I have heard that it implies that----- if A and B be two tweens. If A goes in a space trip and then return to B at earth, he will be younger than B. What I am confused is, It must be same thing whether A went away from B in his space trip on the ship or...
  33. C

    The Twin Paradox: Exploring Age Difference & Time Travel

    The idea is as follows: My (imaginary) twin and I were born at the same time, thus just as old. Now, my twin made his own spaceship in the backyard, one day, and decided to go for a ride. It had only room for one person, so I had to stay behind. Now, my twin went out of the Solar System and...
  34. D

    Twin Paradox with acceleration

    Homework Statement Assume a rocket ship leaves the Earth in the year 2100. Castor, one of a set of identical twins born in 2080, remains on Earth to work at Mission Control, while the other twin, Pollux, travels in the rocket. Ignore the motion of the Earth relative to the fixed stars. The...
  35. C

    Twin paradox: telling who accelerates (why isn't it arbitrary?)

    I'm sure this question has been asked thousands of times before, but I can't see how nature determines which twin accelerates (or is subject to a gravitational field) and which one doesn't. People say one twin will feel the acceleration and the other won't, but suppose neither twin had an...
  36. P

    Does the Earth's Rotation Affect Time Dilation in Einstein's Twin Paradox?

    I know that the frame of reference determines that it is the stay-at-home twin who ages faster than the twin traveling at some fraction of light speed. Some years back, an experiment in which one of a pair of synchronized atomic clocks was placed on a commercial airliner and flown around the...
  37. M

    What is the figure of 6 years representing in resolving the Twin Paradox?

    Resolving the "Twin Paradox" http://mentock.home.mindspring.com/twins.htm I've been trying to follow this unusual explanation to resolve twin paradox, which uses the lorentz relativistic velocity transformation equation to get the speed bob zooms off after Ann at,15/17 C. I can understand...
  38. A

    Exploring the Reverse Twin Paradox: Predicting Age Differences at .6c Speeds

    HI My question is what would the outcome be if the twins started out in 2 ships traveling at uniform .6c relative to Earth and one of them then took her ship and traveled to Earth and back? Which twin would be older?
  39. Artlav

    Twin paradox and the nature of time?

    I'm trying to make sense of the way time dimension is related to the other 3, the example is twin paradox: Observer 1 is moving away at 0.866c from observer 2, who is standing still, then turns around at a defined point and goes back at the same speed, arriving back to observer 2 position...
  40. S

    Twin Paradox and Time Travel (Special Relativity)

    Homework Statement Identical twins Speedo and Goslo join amigration from the Earth to Planet X. It is 20.0 light years away in a reference frame in which both planets are at rest. The twins, of the same age, depart at the same time on different spacecraft s. Speedo's craft travels steadily at...
  41. C

    Twin Paradox: Einstein's Explanation and Alternative Interpretations

    It is my understanding that the twin paradox arose from the fully reciprocal nature of special theory which shows that if a clock is moving past me in outer space that clock is ticking over at a slower rate than my clock but that from the point of view of a person accompanying that clock it is...
  42. M

    Time Dilation in Twin Paradox: Exploring the Puzzling Reality

    I can't fully understand why a person who makes a journey into space in a high-speed rocket will return home to find his age less than an identical twin who stayed on Earth. It makes since for the twin who stayed on earth, but for the twin who traveled into space, he sees himself at rest and...
  43. A

    Twin Paradox: Question on the Effects of Speed and Gravity on Time

    I have a question about the Twin Paradox. I don't know if I'm right or wrong, but that's why I figured I would come here to ask. The way I understand things is that the twin on Earth's clock would end up being faster than the twin's clock out in deep space if the twin out in space was...
  44. D

    Revised Twin Paradox: What's Wrong?

    Thought experiment / story: Albert the Alien sits atop a non-accelerating comet traveling 0.5c. He approaches earth, barely missing it, and passes on by. After he has passed Earth, he looks back toward it with his ultra-sensitive telescope and sees the face of Hyphy the Human. Hyphy the...
  45. J

    Special relativity: twin paradox

    I am just a little bit unsure about something I read on the "twin paradox". It talks about Dick and Jane who are twins, each 20yrs old. Dick departs at a speed of 0.80c to a star 20 light years away. So I'm sure you all know how the story goes, Dick is 50yrs old when he returns and Jane is 70...
  46. Q

    Can the Symmetric Twin Paradox be Tested with Atomic Clocks?

    So, I was thinking about a variation on the Twin Paradox, and was hoping someone could help me work through it. The motivation is the usual explanation for the Twin Paradox, namely that one twin accelerates and so breaks the symmetry. This begs the question of what happens when both twins ride...
  47. F

    How Much Older Is Frank When Mary Returns from Barnard's Star?

    [SOLVED] Twin Paradox problem Here is a problem about twin paradox that I can't quite figure out. I got part a, but I can't get part b. I think that the 6 years spent doing research is kinda throwing me off. Help! The International Space Federation constructs a new spaceship that can...
  48. K

    Does Travel Distance Affect Aging in the Twin Paradox?

    Suppose one twin travel to a distance L and turn around, another twin travel to a distance L/2 and turn around. When they reunite at home, the twin travel longer will age less? Since they both experience acceleration, so acceleration is not the cause of age difference. Can acceleration break...
  49. G

    Who Ages More in the Inverse Twin Paradox?

    suppose we have a stationary observer 'A' at the origin. at t=0 rockets 'B' and 'C' pass the origin moving at gamma=10 but rocket B stops. when rocket 'C' reaches some point along the x-axis rocket 'B' accelerates to gamma=10 as measured by rocket 'C'. when rockets 'B' and 'C' meet its over...
  50. G

    Understanding the Twin Paradox: Time and Length Contractions Explained

    suppose we start with a long line of stationary, evenly spaced and perfectly synchronized clocks along the x axis. if the stationary twin is at the origin and at t=0 the other twin passes the origin moving at relativistic speed with gamma=2 then from the point of view of the stationary twin the...
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