# What is Twins paradox: Definition and 44 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. ### I Reference systems and apparent counter-explanations on Twin Paradox

In the Twin Paradox, I don't understand how it can be determined which of the two is moving and therefore which is the younger. What I realised is that the fact that the Earth is not an inertial system is irrelevant: just consider a planet that is stationary in an inertial system and start the...
2. ### 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...
3. ### 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...
4. ### B Theoretical Question On The Twins Paradox and Heart Rate

Okay, so if two identical twins put on a heart rate monitor that after let's say 400 million beats were detected (at 80bpm that's about 10 years) the monitor killed them. Now one gets in a spaceship and the other stays on earth. The one in the spaceship travels at say 95% of the speed of light...
5. ### B Lag of a clock coming back to its initial position (Twins Paradox)

Hello, I'm a beginner on SGR and I'm struggeling with this, probably, simple problem. I'm interested on exploring Field Theory ( relativistic) so I started reading http://www.elegio.it/mc2/LandauLifshitz_TheClassicalTheoryOfFields_text.pdf which was suggested and reccomended by a PF's member...
6. ### I Does the "space twin" benefit from length contraction?

Does the “space twin” benefit from length contraction as well as time dilation? In Einstein’s thought experiment, let one of twins travel to a galaxy known to be 10 light years from Earth at a speed of sixty percent of light speed (0.6c). Were it not for time dilation the one-way trip would...
7. ### Twins Paradox: Solving for Total Time Lapse

Homework Statement In the twins paradox, suppose that Florence begins at rest beside Methuselah, then accelerates in Methuselah’s x-direction with an acceleration a equal to one Earth gravity, “1g”, for a time ##T_F/4## as measured by her, then accelerates in the −x-direction at 1g for a time...
8. ### Twin's Paradox: Who is Older A or B?

Assume A and B are at rest on Earth initially. Then B travel to to one light year far away from Earth with an extremely low speed (even slower than a car). So I can assume their ages are the same after B has arrived, right? Then, B returns to Earth with an extremely fast speed (~0.9c) and A and...
9. ### Twins Paradox: Nature Cheats but B Gets Fooled

Dear PF Forum, After all these post'. I want to know that my understanding of twins paradox is close enough. So, Green travels, Blue stays. Blue keeps sending signal, Green bounces the signal. Doppler factor = 1/4, so ##\frac{1}{4} = \frac{1-v}{1+v}; v = 0.6## And at event T0 (from B), Green...
10. ### How to determine which clock runs slower in relativity?

So I was recently reading Stephen Hawkings' "The Universe in a Nutshell" and came across the famous Twins Paradox thought experiment. My question is, since motion is relative, couldn't we extrapolate that either the observer on Earth is stationary and the rocket is traveling near light-speed...
11. ### Exploring Twins Paradox & Length Contraction

Dear PF Forum I want to know about these questions that are still bothering me, Does the universe have preferred frame of refference? Why there's twins paradox? Motion is relative, why 1 clock experiences time dilation while the other doesn't? V = \sqrt\frac{3}{4} ≈ 86.6\% If V define ratio of...
12. ### Twins Paradox: Exploring Clocks in a Closed Room

Dear PF Forum, I have a tought experiment here. I'm asking about twins paradox, but instead of using twins, I'm using clocks to lock them up in a closed room. Sort of Einstein elevator. (unlike Schrödinger, even in tought experiment, I can't imagine locking human being -- or cat -- in a closed...
13. ### The Twins Paradox: Understanding the Slow-Motion Effect

I've read different reasons for the reciprocal illusion experienced by the twins (the slow-motion effect). I've not been able to understand the reason that the traveling twin would see the earthbound twin moving in slo-mo. Can anyone offer a simple explanation -- or is there actually no...
14. ### Time Dilatation: The Twins Paradox

We’ve all seen this little fellas up there. Two brothers, twins, in their 60’s. And they decide that one of them is going to take a trip into space, let’s say for a year, at 99% the speed of light. The other brother stays there, waiting for his twin to return, and looking at the light clock that...
15. ### Twins paradox and time dilatation

I have already read about the twins paradox a lot of times, but what puzzles me is that how the Universe would "now" what twin had to be younger since, if the spaceship travels at high speed in relation to Earth, the planet would move the same with the ship as reference.
16. ### Frame-dependency of aging rate in the twin paradox

Hi, from what I understand about the twin paradox, Is the resolution essentially that since the traveling twin undergoes acceleration when the ship reverses direction, so since the velocity of the twin is no longer constant, the inertial reference frames are no longer valid? Then does that also...
17. ### How Does the Twin Paradox Affect Aging in Space Travel?

Everyone knows that paradox. In true we can say who person is standing,so If we say that the person in Earth is standing,then the clock of astronaut will run slower. Else,if we say that astronaut is standing and that Earth is moving,we see the clock of Earth's person to run slower. So in true...
18. ### Confirm: Smooth Twin Paradox Intuition

I'd like someone to confirm whether I am on the right track here. Most formulations of the twin paradox involve a sharp turn-around with infinite acceleration. I suppose that there is an SR-only description of a non-infinite acceleration - a kind of 'smooth' version of the twin paradox. But my...
19. ### Twins Paradox: Why One Twin is Older When Reunited

Hi All. Layman question. In the twin's paradox, why one of the brothers is older when they meet again?. if movement is relative. What determines which of them ends up older? Hope i explained myself. Thanks.
20. ### Strange Twins Paradox: A Crackpot Physics Mystery | Discover the Solution Here!

Hi, I did a search for "crackpot physics" and found https://www.physicsforums.com/showthread.php?t=111647 and then this forum. A friend of mine found some strange twins paradox on the net but he can't remember where. Neither of us can figure out the solution, but it must be crackpot...
21. ### Twins Paradox - but with a different spin

While attempting to wrap my head around the classical Twin Paradox, I asked myself the following. So far I have not been able to reach a well reasoned answer... which is no big surprise. :) Twins (A and B) are oriented in space such that A could be said to be standing on B but facing in the...

The twin who has left Earth and travels close to the speed of light in his spaceship, lands back on Earth only to find everyone has died of old age thousands of years ago. This I'm told is because the perception of time and time itself, within the inertial frame of reference of the spaceship...
23. ### Time dilation and twins paradox

"The confusion that arises in problems like Example 26.1 lies in the fact that movement is relative: from the point of view of someone in the pendulum’s rest frame, the pendulum is standing still (except, of course, for the swinging motion), whereas to someone in a frame that is moving with...
24. ### Resolve to the Twins Paradox – Frame of Origin

I have read many arguments concerning the issue of the famed “Twin Clock Paradox”. Very many arguments go through considerably complex explanations so as to defend Special Relativity or dispute it. But the explanation is actually much simpler. In every proposed twins paradox, there is a Frame...
25. ### Exploring the Reverse Twins Paradox: A Hypothetical Scenario

Reverse Twins paradox ? This may have come up before but I haven't seen it and don't know the answer. What if the twins started out in two ships moving together at relativistically significant velocity relative to earth. One of the twins then simply goes to Earth and back while the other...
26. ### Twins Paradox and acceleration

Why do most explanations of the Twins Paradox claim that the twin on the spaceship ages less because he is the one who undergoes acceleration and/or changes direction, causing asymmetry between the points of view of each twin? It seems clear to me that the twin on the spaceship would age less...
27. ### What is the answer to the Twins Paradox confusion in relativity?

I am having trouble understanding how the explanation of the paradox solves the problem. What if people in two different frames were moving wrt one another with constant velocity. Say for instance, these people never turn around to meet up once again and are constantly moving at a constant...
28. ### The twins paradox and explanation

Hello, I'm reading through my textbook and it claims the explanation for the twins paradox is that the space-traveler must experience acceleration during his journey... but could it not be said that, relative to him, the rest of the universe is accelerating in the opposite direction?
29. ### Geodesic applied to twins paradox

Regarding the twins paradox: do all closed trajectories require nonzero acceleration at some point, or can a closed geodesic fulfill overall the special relativistic requirement of constant velocity?
30. ### Can Relativity Explain the Confusion of the Twins Paradox?

I am having trouble understanding how the explanation of the paradox solves the problem. What if people in two different frames were moving wrt one another with constant velocity. Say for instance, these people never turn around to meet up once again and are constantly moving at a constant...
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Why do most explanations of the Twins Paradox claim that the twin on the spaceship ages less because he is the one who undergoes acceleration and/or changes direction, causing asymmetry between the points of view of each twin? It seems clear to me that the twin on the spaceship would age less...
32. ### How Does the Twin Paradox Affect Time Perception in Accelerating Spaceships?

imagine that two spaceships are far away from each other,both at rest,then one of them accelerate to nearly C towards the other,assume that after the accelerating process,their clocks are just the same,when their distance is almost 0(no decelerating),they change the infermation,then what will...
33. ### The Twins Paradox in curved spacetime

The Schwarzschild Metric on a spatial plane passing through the center of a spherically symmetric (non-spinning) center of gravitational attraction is: d \tau^2 = (1- \frac{2M}{r})dt^2 - \frac{dr^2}{(1- \frac{2M}{r})} - r^2d \phi^2 If there are two spaceships at a distance r_1 from a...
34. ### Einstein's space elevator and the twins paradox

A PBS program said that the theory of general relativity holds acceleration and gravity observationally equivalent. (Einstein's space elevator example was given.) Here is a thought experiment based on that mental picture. A "gravity switch" will make the following thought experiment easier to...
35. ### How does the Twins Paradox challenge our understanding of ageing?

Hi, I'm a Biology teacher, constantly getting into discussions with the physics teachers at my school regarding the effect of traveling at speed on ageing. They have explained to me (countless times) all the examples and experiments that show that time passes more slowly for objects traveling...
36. ### 4D spacetime Light cone Twins paradox

I'm an "on-my-own-free-time" arm-chair student of physics. Lol. So if this question is way off the mark my apologies. Feel free to let me know where I’m off base. Anyway... For me, a great visual example of the twin paradox was found at this site...
37. ### Closed Universe Twins Paradox: No Acceleration?

If the universe was closed and such that if one continued in a straight line they would arrive where they began eventually. Wouldn't the twins paradox still be a paradox as neither party is accelerating (are they?) and this would prove that such a universe doesn't exist?
38. ### Twins Paradox: What Do Each Twin Observe During Trip?

Twin A is at rest with respect to Twin B who is traveling close to c and after taking off for a bit Twin B turns and returns to earth. Now the thing that I am wondering is what does each twin observe during the whole trip. I might have it wrong and that's why I made this thread. Lets say...
39. ### Is the Langevin Twins Paradox Validated by Other Experiments?

Paradox was verified by Hafele-Keating in 1971. They placed atomic clocks in commercial airplanes and then compared to a reference clock. Recently some people/groups are considering their data as questionable & unreliable. Does somebody know later experiment that also verified the paradox in a...
40. ### Unreliability of Hafele & Keating Experiment: A Critical Analysis

Does someone have any opinion regarding the following paper attending to prove that Hafele & Keating experiment (that actually verified the time distorsion caused by speed & gravitation in General relativity) to be highly unreliable? Hafele & Keating Tests; Did They Prove Anything? A. G...
41. ### Exploring the Twins Paradox: Understanding the Confusion

why is it a paradox? it makes sense to me.
42. ### Twins paradox need clarification

hi all i m new to this forum but would like to know a lot abt relativity. I read through a dialogue in one of the threads on twins paradox. What exactly do you mean by saying one of the twins will be younger than the other? As far as i understand biologically neither of them is younger. Pls...
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