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. R

    Twin Paradox: Time & Speed Explained

    hi .. i don't understand in the twin paradox and in general and special relativity why time changes if speed changes thank you
  2. I

    Twin paradox on another planet

    Hello! I'm new here, and already did a search. Sorry if the answer is elsewhere, I just couldn't find it! I'm writing a sci-fi novel and have been brushing up on general and special relativity. I'm running into a small problem with the twin paradox, and how to describe it from another planets...
  3. JimiJams

    Time Dilation and The Twin Paradox?

    Hey everyone, I've searched the threads for an answer to my question but came up short. Basically what I want to know is how does time dilation relate to the twin paradox. I'm a newb to relativity and just started reading about it last night. I hit this bump and I'm trying to get over it...
  4. H

    The Twin Paradox and the Equivalence Principle

    I'm having a little trouble understanding the equivalence principle explanation of the twin paradox. I understand that the resolution to the paradox according to the equivalence principle is that the non-traveling twin has a higher gravitational potential energy in the pseudo-gravitational...
  5. V

    Who Ages Faster in the Twin Paradox?

    Hi, I am a little confused with this paradox. I asked my professor about it and he didnt really give a convincing answer. So the scenario basically seems to be some twins on earth(or anywhere) at rest, and then one leaves at relativistic speed for some time then comes back to see that his/her...
  6. M

    Is the Twin Paradox Truly Unresolvable from Speedo's Perspective?

    According to Lorentz, time is always dilated.So think of two twins, Speedo and Goslo. - Goslo stays on Earth and drinks tea. - Speedo gets into a rocket, zooms off into outer space and zooms back.Imagine YOU are Speedo. You zoom off, and when you return to Earth, you find that Goslo is 20...
  7. W

    Simple Twin Paradox Resolution Using Rocket Man Traveling from Earth to Planet Claire

    I worked through a simple twin paradox problem, and I think I have the correct solution. However, I wanted to confirm that my numbers are correct. There is nothing fancy about the solution, but it resolves the "paradox" completely for me. Every other resolution I have looked at either leaves...
  8. M

    Twin Paradox Explained: Long Outbound & Short Inbound Leg

    Hello, All I read the article: http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_paradox.html But, I cannot understand how just asymmetry (without acceleration) can cause less age of moving observer. The article says: I cannot understand why do Terence see an...
  9. I

    Time curvature and twin paradox.

    Can the twin paradox provide us with insight into time curvature? If my twin boards a ship that can travel near the speed of light, special relativity says that on arrival back on Earth, my twin should be younger that I am. Has my twin experienced a time curvature?
  10. K

    How does the duration of acceleration affect the Twin Paradox?

    [later edit: Sorry for the title -- I am glad to see this was interesting enough to spark discussion.] Twin A takes off and leaves Twin B behind. Rather than switching on the reverse-thrust, slowing down, and beginning the journey back home (an acceleration that would be distinctly detectable...
  11. S

    Acceleration doesn't cause the Twin Paradox?

    Acceleration doesn't "cause" the Twin Paradox? In a recent review of a physics textbook, the reviewer is critical of the author of the book because the the author doesn't correct the persistent notion of many students that it is the acceleration of one of the twins that "causes"[reviewer's...
  12. X

    Twin paradox and time dilation

    I like astrophysics,so i read books relating to it.I am also determined to make my career in the same.At present i am reading a book ,which includes brief about Einstein's 'Relativity and special relativity' theory.It also explains to some extent the 'Twin paradox' and 'Time dilation'...
  13. S

    Does Earth's Movement Affect the Twin Paradox in Space Travel?

    If in the twin paradox instead of the twin traveling to the distant star let's say he stays and the Earth along with that star moves (imagining a rod joining Earth and star moves) and the star reaching this twin goes back at the same speed(along with earth) now i guess the twin whos stationary...
  14. G

    Twin Paradox: How Much Time Has Passed on Earth?

    Let's say there are two synchronized clocks on Earth. One leaves the Earth going at .8c and travels in uniform motion for one year. At the one year mark the clock is handed off to another spaceship coming towards Earth at the same speed of .8c. The clock eventually makes it back to Earth...
  15. S

    Twin Paradox: Solving Confusing Explanations

    This question about the twin paradox(twins A,B). I hope everyone knows the statement. I have read explanations for this but they differ. Some authors say twin(say B) who takes the trip accelerates so he can't be an inertial frame of reference. Some say it has nothing to do with acceleration and...
  16. L

    How does special relativity handle acceleration?

    I'd just like someone to clarify that my argument is correct here. Assume twin A stays on Earth and B goes 2 light years away and then back to Earth at a velocity of 0.8c From Earth frame of reference: Twin A makes the calculation that 2/0.8=2.5 years will be needed for the outward...
  17. D

    Understanding The Twin Paradox

    The twin paradox seems not to be understand by everybody. But if you see it like this (see below), everybody would understand it. Why are we getting older on Earth ? Our (relative) time system is the system we compare time to the axes of the Earth. If we start in 1 point on the surface...
  18. A

    Twin Paradox: Accelerating, Turning Back & Decelerating

    hi... In the twin paradox, we consider that one of the twins went out of the surface of Earth ACCELERATING , then suddenly TURNED BACK and then DECELERATED back towards the Earth with some relativistic velocity………. Why is it so? What if the twin goes on a round trip around the Earth and...
  19. W

    Twin Paradox: Who is Older - B or C?

    Lets say we have triplets on Earth A B and C. B and C goes from Earth together in the same direction at some relativistic speed let's say lorentz factor one milion. Now after one year passes from the point of A, brother B decides to stop. Now after one more year from the point of A passes...
  20. phinds

    How does the twin paradox work in a circular orbit?

    There have been a couple of posts over the last few months that posit a relativistic-speed path in a circle around the Earth and I want to make sure I correctly understand the ramifications. It's the twin paradox in a circle. SO ... here's a scenario that I think will solidify it for me: This...
  21. P

    How does the Twin Paradox demonstrate the effects of general relativity?

    Here is an explanations due to Daniel F. Styer, Prof Physics at Oberlin Daniel's original is at https://mail.google.com/mail/?ui=2&view=bsp&ver=ohhl4rw8mbn4". He uses general relativity and the equivalence principle. The equivalence principle is not entirely true -- it IS possible to...
  22. J

    Twin Paradox Explanation Needed

    So in the twin paradox, the traveling twin ages less in the end. According to time dilation, each twin sees time passing in the other twin's frame of reference slower than his own. However, if the twin on Earth is ultimately older, doesn't time have to appear to be moving faster for the twin...
  23. P

    Twin paradox: who decided who is the younger one

    Hello, Still puzzled about the twin paradox (one guy stays on the ground, the other goes travelling). If we see the two twins as points in 3D space, the only thing that changes, is the distance between two. That is, the two guys are in totally symmetrical positions, and there should be no...
  24. A

    How Does Acceleration Affect Time Dilation in Twin Space Travel Scenarios?

    We think of two very similar scenarios: 1. Two twins are hoovering with their spacecraft s at an earthlike distance A from the Sun and with no velocity with respect to the Sun. Then one of the twins suddenly accelerates to 100 km per second in some direction. He is now the accelerated twin...
  25. A

    What is the outcome of the twin paradox in a new version?

    We think of two very similar scenarios: 1. Two twins are hoovering with their spacecraft s at an earthlike distance A from the Sun and with no velocity with respect to the Sun. Then one of the twins suddenly accelerates to 100 km per second in some direction. He is now the accelerated twin...
  26. A

    How Does Time Dilation Explain the Twin Paradox?

    Hello, I was talking with my Physics professor the other day about something I could do in Special Relativity, since I'm only in Physics III and haven't experienced much of it, yet find it very interesting. He mentioned that I could do stuff with the Twin Paradox. Now I know the basic idea...
  27. J

    Twin Paradox with no asymmetry

    Twin "Paradox" with no asymmetry Here is another variation of the twin paradox. Suppose we let both twins start their journey from a space station far from any heavenly bodies, so that the whole experiment can be carried out in free space. The twins Jack and John are equipped with identical...
  28. A

    More thoughts on the twin paradox. One persons view has to be an illusion?

    Everyone knows the situation. The one twin takes off in a rocket, and both twins see the others clock as running slow. Upon the return, its the one who has experienced acceleration that ends up being incorrect. So at some point, the "moving" twins view must show time on his brothers Earth clock...
  29. B

    Is the Twin Paradox Truly Unsolvable?

    can twin paradox be solved,if no, what r the difficulties scientist face to solve it?? or is there a solution on it.
  30. S

    How is twin paradox resolved in case of no/zero acceleration?

    how is twin paradox resolved in case of no/zero acceleration/deceleration? two twins one on Earth and other on a ship moving at constant velocity (say 80% speed of light). the other twin does not turn around ..thus no acceleration/deceleration. how we resolve the paradox now? twin A will say...
  31. C

    What time does an observer see when watching a clock on a rotating circle?

    Hi, I'm new here. This question may already have an answer but I didn't find it. Sorry if there's already one. It's a static version of the twin paradox, without travel and without twins. We have 2 circles C and C' that are superposed. C is fixed, with time t and co-ordinate x along the circle...
  32. B

    How would a trampoline solve the twin paradox in relativity?

    So we have covered relativity a couple weeks ago and I've been thinking about the twin paradox. It is when one twin travels on a high speed ship to some distant planet and back. But since all frames of reference are equivalent, each twin measures the other to be younger. My book explained that...
  33. K

    Twin paradox alone kills special relativity

    twin paradox alone kills special relativity: the story: A stays on the planet, B round-travels by spaceship; when back, based on spectral relativity, B youngs. analysis: when special relativity is be applied, the property difference between planet and spaceship is never used; so...
  34. P

    Twin Paradox in Relativity: Examining Motion, Time and Distance

    According to what I have researched in Relativity (thanks to you guys) there is no such thing as absolute motion, time, or distance. I had a question about this paradox mentioned. Paraphrase: Two people that are exactly 20 years old are on earth. One decides to fly to planetX and some...
  35. D

    The Twin Paradox: Mike & Angela's Frame of Reference

    I am confused about the twin paradox. Let's imagine Mike and Angela are twins. Mike stays on Earth, and Angela rockets off, away from Earth at close to the speed of light, with the intention of making a round trip back to Earth at the end of her voyage. My question is this: why wouldn't...
  36. K

    Is anyone good at special relativity? the twin paradox?

    Is anyone good at special relativity? the "twin paradox?" Homework Statement In the problem a pilot accelerates away from Earth for 10 years, then decelerates the same way for 10 years, then turns around and repeats the whole process to return to earth. His acceleration is given by...
  37. K

    Special Relativity - Twin Paradox

    Homework Statement Consider the following space flight. As seen by the pilot the ship accelerates for 10 years with an acceleration given by a=2g[1-exp(-rt)] where g = 9.8 m/sec^2 and r=1/(3 years). She then decelerates the same way for 10 years (begins with 0 acceleration and increases...
  38. T

    Twin Paradox: Explaining Symmetry & Age Difference

    Hi, i want to ask about the basic explanation of twin paradox. In the explanation it says one twin has to accelarate to come back and the symmetry is broken and so one twin is older than other. Could you explain this symmetry and aging relation?
  39. V

    Why Does the Twin Paradox Not Invalidate the Theory of Relativity?

    Bob sets on a voyage with a velocity 0.8c for total 6 years, leaving back Dave on the space station. On return Dave finds him younger by 4 years. Reciprocal results in time measurements in relativity make the theory redundant but here we see application of time dilation equation. This is...
  40. G

    Twin Paradox (thorough explanation needed)

    Hi, I'm an undergraduate physics student trying to comprehend why the twin paradox is not a paradox. The standard reply usually amounts to this: the dude in the spaceship has to turn around at some point to come back to Earth. So he accelerated during his journey (by changing his direction...
  41. O

    Twin paradox, what about ageing?

    I may not be right exactly but however i stand corrected where necessary. Twin paradox experiment explains time dilation in space time. My understanding is like this, assuming twins at 10yrs each. If one makes a space travel and the other is at stationed on Earth (both being at initial...
  42. K

    Solving Twin Paradox: Earth's Motion in Astronaut's Frame

    Again on twin paradox! Hi! I'm trying to definitely solve the twin paradox (after 5 years of efforts :) ) In every physics textbook, it is studied the motion of the twin in an inertial frame (so the proper time is that measured in the twin frame), so we can express time delta t in the Earth...
  43. D

    Twin Paradox Revisited: Exploring Time Dilation

    Folks, apologies for reviving a thread that I know has been picked over, but I am a bit confused. I understand the explanation that "solves" the Twin Paradox -- Twin B in the rocketship flying close to light speed undergoes acceleration and deceleration on his outbound trip away from Earth...
  44. I

    Twin paradox initial acceleration

    Hi, I've seen many variations of explanations of the twin paradox using special relativity, but i haven't yet seen an explanation that bothers to take into account the initial acceleration of the traveling twin away from planet Earth. Is it safe to say that this can be neglected...
  45. Buckethead

    Yet another Twin Paradox thread

    I've read most of the Paradox within a Paradox thread but rather than hijack it to get to my point I thought I'd start a new thread. First, if this has already been addressed from this angle my apologies, but it is next to impossible to read through all the TP threads to try and find one from...
  46. C

    Paradox within the twin paradox

    1) So let's say we have twin A and twin B. 2) Twin A stays on Earth. 3) Twin B goes on a trip nearly at the speed of light. 4) Twin A sees twin's B clock moving slower. 5) Twin B sees twin's A clock moving slower. 6) When twin B returns, twin A is older. #6 Implies that twin B's clock...
  47. N

    Varitaion on the twin paradox that just doesnt work out for me

    So if you were to take the twin paradox, but make it so instead of the ship turning around and going to earth, the ship bends space through higher dimensions to return to Earth with no movement, what happens then? I'm sure this has been thought up of before, but i can't find an explanation that...
  48. H

    Doppler effect and the twin paradox

    Hi. I am learning special Relativity and I am seeing that the answer to the twin paradox involves the Doppler effect, and I am struggling to understand what is going on. First let me explain what I know about the Doppler effect. If an object produces waves (might it be light waves or sound...
  49. A

    Is the Twin Paradox Real or Just Apparent in Relativity?

    Friends, I will be happy if anybody throws light on the following concepts: (1). In the case of time dilation it is said that a clock in a moving frame appears to go slow to an observer in a resting frame.It leads to the famous twin paradox in which 'A' who spends some time in a spaceship...
  50. L

    Twin Paradox in a Flat Toroidal Universe: Time Dilation and Inertial Frames

    In four dimensions, a flat torus is an object that has zero curvature but still has closed geodesic curves. What this means is that if you try to measure geometry locally, you will find that it is perfectly Euclidean. Nevertheless, if you travel on a straight line, you'll eventually end up...
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