# 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.

View More On Wikipedia.org
1. ### B What does the term "age" mean in descriptions of the twin paradox?

In other words: Does a reference system, in the relationship between the displays of its synchronised clocks, have a specific property that is independent of any reference system? Is this particular relation of events in the form of displays of spatially distant clocks at rest the same from the...
2. ### B Thought experiment: The twin paradox, observed from far away

Consider the classic twin paradox scenario involving twins A and B, who start at the same location. Twin B embarks on a journey, traveling 1 light-year away from A at a speed of 0.86c, before returning. Upon reunion, A and B agree that A has aged more than B. Now, let's introduce an observer...
3. ### I Twin paradox, virtual clock on ship with Earth time, discontinuity

Conventional Twin Paradox. A ship with speed v = 0.8c makes a round trip from Earth and back. It lasts 6 years and on earth 10 years have passed. The ship carries its own clock and also a computerized clock that always shows the time on earth at that moment. It is about knowing the T - T'...
4. ### B A scenario to help understand the twin paradox in SR

I am looking at some of the threads on the twin paradox, and getting even more confused. I have been trying to run through the details of what each twin is seeing, and was wondering if I could get some help. I am just trying to imagine how each twin is “seeing” the other twin at each step as the...
5. ### B The twin paradox and black holes

Does the twin paradox hold around a black hole (or maybe less extreme gravitational fields)? In a gravitational fields like that of the Earth it seems to apply. If two particles fall together, with synchronized clocks, and one of them rests on a platform for a while, after which it accelerates...
6. ### I Twin Paradox: Aging Slower with Continuous Acceleration?

I understand that the travelling twin (T, say) is subjected to acceleration and deceleration while the stay-at-home twin (S) is in inertial frame all the time. It is this asymmetry which results in the travelling twin aging less than the other, when they two meet up. Since acceleration is the...
7. ### I Why Are the Two Diagrams in the Twin Paradox Not Symmetric?

In the image below from the website http://www.mysearch.org.uk/website1/html/250.Twins.html , they try to explain why the two situations are not symmetric, but I don't understand their approach. Even if the website is not giving a sufficient explanation, I would still like to know why the two...

9. ### B My paraphrased explanation of the Twin Paradox

So I've been trying really hard to understand the theory of relativity at its most basic level lately, and recently I dove down the rabbit hole of the Twin Paradox. This has led me through a series of youtube videos, each one claiming to present a different solution, explain why the other...
10. ### I Twin Paradox: Calculating Stan's Time Elapsed According to Moe

I know, I know, yet another Twin Paradox thread. (My apologies if this is already on the Forum somewhere. I found a similar discussion but with no answer to my question. I'd appreciate a link if someone has already given an answer to it.) I'm trying to construct the time elapsed for Stan...
11. ### B Understanding twin paradox without math

Using a simple time clock in the horizontal position you can see the solution to the twin paradox. In this graph the Time clock tics once every half year on earth. 5 years pass on Earth 4 years pass on the spaceship. You don't have to worry about mysterious simultaneity or time jumps...
12. ### I Twin Paradox with accelerated Motion

Alice rests at ##X=L+1## in the inertial frame (T, X). Bob is at rest in the Rindler frame (t, x) at ##x=1## and has the proper acceleration ##\alpha=1##. In the rest frame of Alice, Bob moves from event ##E_1=(-T_2, L+1)## over the distance of ##L## in negative X-direction to event ##(0, 1)##...
13. ### I Lorentz Contraction & Twin Paradox

The twin paradox is connected to the special relativity but I wonder simply if one might construct the paradox (or something very similar) based on the Lorentz’ (and FitzGerald) work alone? Several ingredients in the paradox, time dilation and Lorentz contraction, are often mentioned with...
14. ### I Resolving the Twin Paradox: Non-Straight Paths and Proper Time in Space-Time

Is the twin paradox settled by saying that any non-straight path between two events (points) in space-time has less proper time that a straight path between the two events? So the twin in the frame which has a longer trajectory between the two pints(curved) will have less elapsed time?

Replace the twins in the twin experiment by identical radioactive samples containing the same starting number ## N_0 ## of atoms. One (A) stays on Earth while the other (B) makes a round trip at high speed. When back the traveling sample is more radioactive because its half-life ## \Delta t1/2...
16. ### I Twin paradox for (accelerated) dummies?

Applying the Lorentz transformation to velocity and acceleration, we can easily obtain that Aγ3 = α, where A is the acceleration measured by the stationary observer and α is the proper acceleration of the relative moving object. From this point, the equations for a constant accelerated motion...
17. ### B Einstein's Twin Paradox: Confusing Solution?

In Einstein's twin paradox,the solution comes like this: The twin on the spaceship de-accelerates his spaceship first and then accelerates in the reverse direction.This means his reference frame is not inertial hence he doesn't measure greater time interval as the other twin does...
18. ### B Twin Paradox (3 objects version)

Consider 3 objects, A, B, and C, in relative motion along the x direction. EVENT 1 B passes A while moving at a constant v = 3/5c relative to A. Both clocks set to t = 0. EVENT 2 C passes B when B’s clock reads t = 5 while C is moving at a constant v = 3/5c towards (and relative) to A. C’s...
19. ### I Can time run backwards in an accelerating frame?

I'm trying to make sure I understand how the traveling twin tracks the time of his stationary earthbound sibling and the time of another stationary observer who's farther away. From what I've understood until now, it's pretty straightforward with the earthbound twin: In the traveler's frame, the...
20. ### 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...
21. ### 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...
22. ### 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...

. . 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...
24. ### 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...
25. ### 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...
26. ### 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...

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.
28. ### 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...
29. ### 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...
30. ### 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...
31. ### 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)...

33. ### I The Twin Paradox implies that the Universe as a whole is a special frame

The Twin Paradox implies that the universe as a whole is a special frame of motion according to "Relativity Simply Explained" by Martin Gardner. I want to be sure than I haven't misunderstood something. I don't find the explanation completely clear. If the universe as a whole is a special and...
34. ### I Help Solve Twin Paradox Calc: A & B 1 Lyr Apart

A and B 1 lyr apart and sync watches, C moving at .5 C, when at B, C sync watch with B, what time does A and C's watch show when meet. (I was told answer already here but could not find).
35. ### I Twin Paradox Freely Falling Observers

Here I consider the twin paradox for two observers moving in the Schwarzschild spacetime, i.e., the spacetime where one has one radially symmetric non-rotating star, leading to the invariant line element (written in the usual Schwarzschild coordinates) \mathrm{d} s^2=\left (1-\frac{R}{r}...
36. ### I A version of the twin paradox without accelerations

Bob is standing on Earth and Alice is on a distant planet at a constant distance from Earth. Their watches are already synchronized in the following sense: Suppose Alice's planet is a light-year away from Earth. Bob emits a light signal to Alice at time t = 0 according to Bob's watch. When Alice...
37. ### B Twin paradox explained for laymen

As a layman I also have conceptual difficulties with the twin paradox. It would allow me to move on if some one could quickly answer the rather obvious question: would there be any time dilation if the Earth was removed entirely from the thought experiment so that the twins are in a symmetrical...
38. ### B Why the stay-at-home twin is not considered to be accelerating?

I am not a physicist. I need your kind help in removing my following doubt about twin paradox. What I have been able to understand about twin paradox is this- 1. Special relativity deals with non-accelerating (inertial) motion. 2. The traveling twin (A) moves at a high speed in relation to the...
39. ### I Twin Paradox Explained: No Math Required

Hello I have been trying to understand the twin paradox (without math) but I’m still trying to grasp the idea. I have seen and read enough tutorials to know that acceleration is not needed for the twin paradox to be solved. For anyone who doesn’t know the twin paradox without acceleration...
40. ### B Can time pass slower on another planet, causing the Twin Paradox?

It is a long message indeed but there is one doubt I want to ask, As from the above video the time measured by the person which is outside the Frame of reference let's say t' depends on the position and time measured by the person within the Frame of reference. But how do we know that whose...
41. ### I Twin Paradox: Who Is Right, A or B?

In this version of the twin paradox one twin, A, is located on earth, and the other twin, B, is located on a distant planet, which is at a fixed location in A's frame of reference. At the beginning time t = 0 the twins are stationary and their clocks are synchronized at 0. A gets on a rocket...
42. ### B Relativity: Twin Paradox - Is Age Determinable?

(I swore to myself I would never ask a relativity question again...oh well) I don't know why I can't find anything about this in a search so I guess I'll just have to ask. Is a twin that takes off to Mars to stay, younger than a stay at home twin or is it ambiguous? I would think that this...
43. ### I Why Is the Green Observer Always the Bent One in the Twin Paradox?

I feel a little guilty writing this post because I'm sure there are people here who are tired of answering questions about the twin paradox, hence the FAQ post on the subject, but there's something which is still nagging me. First I have a question about the FAQ post itself. Toward the bottom of...

Thanks. This made a lot of things clear to me. But there's one last thing that I want to check: Twins paradox is not a paradox, right? It says that one twin goes to space at near light speed and the other stays back on Earth. Then the one in space returns like f.eg. after 30 years. The twin...
45. ### I Extended Twin Paradox: Explaining Time Lapse and Symmetry Principle

Suppose an observer (O) sees a traveler (T1) pass by at time t=0, moving a speed 3c/5. Five years later (according to O), T1 returns. If we assume that T1 traveled at 3c/5 for half the journey and instantaneously reversed direction, returning at the same speed, we can calculate that T1 aged only...
46. ### B Time dilation and Einstein's theorem

Hello! Einstein's theorem is in the last sentence of the following quote (bold) [1]: "If at the points A and B of K there are stationary clocks which, viewed in the stationary system, are synchronous; and if the clock at A is moved with the velocity v along the line AB to B, then on its...
47. ### I Solve General Relativity "Paradox" w/ Automatic Diff. ODE Solver

After making a couple of comments on this StackExchange question, and pointing yet again to this article, a thought occurred to me. I have been working on an Automatic Differentiation based ODE solver and equation analyzer, mentioned in this thread. Why not use it to solve equation (7) in the...
48. ### I Understand Twin Paradox, Leading Clocks Lag & Doppler Effect

I have an obvious understanding failure here, so hopefully someone can help me clear this up. Thanks for reading this obnoxious drivel.Leading clocks lag So, if two clocks are fixed to a the ends of a barn, and they are set off with light pulses from the midpoint, in the frame of the barn the...
49. ### I Twin Paradox: Travel at Fractional Speed & Instantly Return ##v##

A traveler visits a location (or doesn't!) ##x## light years away at fractional [EDITED] speed ##v## and instantly returns at the same speed. After this her clock has progressed by ## \frac {2 x} {v} \sqrt {1 - v^2}## years. [EDITED] That really is all there is to be said. If the poster...
50. ### I A variation of the twin paradox

I've read the twin paradox and if I am correct the resolution is that one twin accelerates and decelerates so he comes back younger. But I have a different scenario that I would like to ask: What if you have two twins equally distant from a point in space and completely at rest relative to each...