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{{short description|Thought experiment in special relativity}}
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[[File:Mark and Scott Kelly at the Johnson Space Center, Houston Texas.jpg|thumb|upright=1.15|During the [[ISS year-long mission]], astronaut [[Scott Kelly (astronaut)|Scott Kelly]]&nbsp;(right) aged about {{convert|8.5|ms|ms|disp=output only|abbr=off|frac=2}} less than his Earthbound twin brother [[Mark Kelly|Mark]]&nbsp;(left) due to relativistic effects.<ref name="Lichfield 2015">{{cite web |date=2015-03-15 |title=Astronaut Scott Kelly will return from a year in space both older and younger than his twin brother |url=https://qz.com/370729/astronaut-scott-kelly-will-return-from-a-year-in-space-both-older-and-younger-than-his-twin-brother |access-date=2024-10-15 |url-status=live
 | archiveurl  = https://archive.today/20241023010252/https://qz.com/370729/astronaut-scott-kelly-will-return-from-a-year-in-space-both-older-and-younger-than-his-twin-brother
 | archivedate = 2024-10-23 }}</ref>]]
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{{Special relativity sidebar}}
In physics, the '''twin paradox''' is a [[thought experiment]] in [[special relativity]] involving twins, one of whom takes a space voyage at relativistic speeds 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<ref>{{cite book |title=The Modern Revolution in Physics |edition=illustrated |first1=Benjamin |last1=Crowell |publisher=Light and Matter |year=2000 |isbn=978-0-9704670-6-5 |page=23 |url=https://books.google.com/books?id=OMs-_JK-wncC}} [https://books.google.com/books?id=OMs-_JK-wncC&pg=PA23 Extract of page 23]</ref><ref>{{cite book |title=Modern Physics |edition=3rd |first1=Raymond A. |last1=Serway |first2=Clement J. |last2=Moses |first3=Curt A. |last3=Moyer |publisher=Cengage Learning |year=2004 |isbn=978-1-111-79437-8 |page=21 |url=https://books.google.com/books?id=uTM8AAAAQBAJ}} [https://books.google.com/books?id=uTM8AAAAQBAJ&pg=PA21 Extract of page 21]</ref> and naive<!-- please do not remove this word, it appears in both cited sources: --><ref>{{cite book |title=From Special Relativity to Feynman Diagrams: A Course of Theoretical Particle Physics for Beginners |edition=illustrated |first1=Riccardo |last1=D'Auria |first2=Mario |last2=Trigiante |publisher=Springer Science & Business Media |year=2011 |isbn=978-88-470-1504-3 |page=541 |url=https://books.google.com/books?id=R-qIh6kd8d0C}} [https://books.google.com/books?id=R-qIh6kd8d0C&pg=PA541 Extract of page 541]</ref><ref>{{cite book |title=Gravitation and Spacetime |edition=3rd |first1=Hans C. |last1=Ohanian |first2=Remo |last2=Ruffini |publisher=Cambridge University Press |year=2013 |isbn=978-1-139-61954-7 |page=176 |url=https://books.google.com/books?id=JVQhAwAAQBAJ}} [https://books.google.com/books?id=JVQhAwAAQBAJ&pg=PA176 Extract of page 176]</ref> 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 frame of reference|inertial frames]], one for the outbound journey and one for the inbound journey.<ref>{{cite book |title=Foundations of Modern Cosmology |edition=illustrated |first1=John F. |last1=Hawley |first2=Katherine A. |last2=Holcomb |publisher=Oxford University Press |year=2005 |isbn=978-0-19-853096-1 |page=203 |url=https://books.google.com/books?id=s5MUDAAAQBAJ}} [https://books.google.com/books?id=s5MUDAAAQBAJ&pg=PA203 Extract of page 203]</ref> Another way to understand the paradox is to realize 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 actually 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".<ref name='Debs_Redhead'>{{cite journal |author1=Debs, Talal A. |author2=Redhead, Michael L.G. |title=The twin "paradox" and the conventionality of simultaneity |journal=American Journal of Physics |volume=64 |issue=4 |year=1996 |pages=384–392 |doi=10.1119/1.18252 |bibcode=1996AmJPh..64..384D}}</ref> [[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.<ref>{{Cite book |author=Miller, Arthur I. |year=1981 |title=Albert Einstein's special theory of relativity. Emergence (1905) and early interpretation (1905–1911) |location=Reading |publisher=Addison–Wesley |isbn=0-201-04679-2 |pages=[https://archive.org/details/alberteinsteinss0000mill/page/257 257–264] |url=https://archive.org/details/alberteinsteinss0000mill/page/257 }}</ref> Explanations put forth by [[Albert Einstein]] and [[Max Born]] invoked [[gravitational time dilation]] to explain the aging as a direct effect of acceleration.<ref name=Jammer>{{cite book |title=Concepts of Simultaneity: From Antiquity to Einstein and Beyond |page=165 |author=Max Jammer |author-link=Max Jammer |url=https://books.google.com/books?id=vuTXBPvswOwC&pg=PA165 |isbn=0-8018-8422-5 |publisher=The Johns Hopkins University Press |year=2006}}</ref> However, it has been proven that neither general relativity,<ref>{{cite book |title=Gravity from the Ground Up: An Introductory Guide to Gravity and General Relativity |edition=illustrated |first1=Bernard |last1=Schutz |publisher=Cambridge University Press |year=2003 |isbn=978-0-521-45506-0 |page=207 |url=https://books.google.com/books?id=P_T0xxhDcsIC}}[https://books.google.com/books?id=P_T0xxhDcsIC&pg=PA207 Extract of page 207]</ref><ref>{{cite web |url=http://math.ucr.edu/home/baez/physics/Relativity/SR/acceleration.html |title=Can Special Relativity Handle Acceleration? |first=John |last=Baez |date=1996 |access-date=30 May 2017}}</ref><ref>{{Cite web | url=https://www.scientificamerican.com/article/how-does-relativity-theor/ | title=How does relativity theory resolve the Twin Paradox?| website=[[Scientific American]]}}</ref><ref>David Halliday et al., ''The Fundamentals of Physics'', John Wiley and Sons, 1997</ref><ref>Paul Davies  ''About Time'', Touchstone 1995, ppf 59.</ref> nor even acceleration, are necessary to explain the effect, as the effect still applies if two astronauts pass each other at the turnaround point and synchronize their clocks at that point. The situation at the turnaround point can be thought of as where a pair of [[observer (special relativity)|observers]], one travelling away from the starting point and another travelling toward it, pass by each other, and where the clock reading of the first observer is transferred to that of the second one, both maintaining constant speed, with both trip times being added at the end of their journey.<ref>{{cite web|url=https://www1.phys.vt.edu/~jhs/faq/twins.html|title=Frequently Asked Questions About Special Relativity - The Twin Paradox|website=Virginia Tech Physics|author=John Simonetti|access-date=2020-05-25}}</ref>

==History==
{{Further|History of special relativity#Time dilation and twin paradox}}
In his famous paper on [[special relativity]] in 1905, [[Albert Einstein]] deduced that for two stationary and synchronous [[clock]]s that are placed at points A and B, if the clock at A is moved along the line AB and stops at B, the clock that moved from A would lag behind the clock at B. He stated that this result would also apply if the path from A to B was polygonal or circular.<ref group=A>{{Cite journal |author=Einstein, Albert |title=On the Electrodynamics of Moving Bodies |journal=Annalen der Physik |volume=17 |pages=891 (end of §4)|year=1905 |url=http://www.fourmilab.ch/etexts/einstein/specrel/www/ |bibcode=1905AnP...322..891E |doi=10.1002/andp.19053221004 |issue=10|doi-access=free }}</ref> Einstein considered this to be a natural consequence of special relativity, not a [[paradox]] as some suggested, and in 1911, he restated and elaborated on this result as follows (with physicist [[Robert Resnick]]'s comments following Einstein's):<ref group=A>{{Cite journal |author=Einstein, Albert |title=Die Relativitäts-Theorie |journal=Naturforschende Gesellschaft, Zürich, Vierteljahresschrift |volume=56 |pages=1–14 |year=1911 |url=https://archive.org/details/vierteljahrsschr56natu}}</ref><ref>{{Cite book
|title=Introduction to Special Relativity
|first1=Robert
|last1=Resnick
|publisher=John Wiley & Sons, Inc.
|year=1968
|isbn=0-471-71725-8
|chapter=Supplementary Topic B: The Twin Paradox
|page=[https://archive.org/details/introductiontosp0000resn/page/201 201]
|url=https://archive.org/details/introductiontosp0000resn
|url-access=registration
|location=place:New York
|lccn=67031211}}. via [[August Kopff]], [[Hyman Levy]] (translator), ''The Mathematical Theory of Relativity'' (London: Methuen & Co., Ltd., 1923), p. 52, as quoted by [[Gerald James Whitrow|G.J. Whitrow]], ''The Natural Philosophy of Time'' (New York: Harper Torchbooks, 1961), p. 215.</ref>

{{blockquote|Einstein: If we placed a living organism in a box ... one could arrange that the organism, after any arbitrary lengthy flight, could be returned to its original spot in a scarcely altered condition, while corresponding organisms which had remained in their original positions had already long since given way to new generations. For the moving organism, the lengthy time of the journey was a mere instant, provided the motion took place with approximately the speed of light.<br>
Resnick: If the stationary organism is a man and the traveling one is his twin, then the traveler returns home to find his twin brother much aged compared to himself. The paradox centers on the contention that, in relativity, either twin could regard the other as the traveler, in which case each should find the other younger—a logical contradiction. This contention assumes that the twins' situations are symmetrical and interchangeable, an assumption that is not correct. Furthermore, the accessible experiments have been done and support Einstein's prediction.}}

In 1911, [[Paul Langevin]] gave a "striking example" by describing the story of a traveler making a trip at a [[Lorentz factor]] of {{nowrap|''γ'' {{=}} 100}} (99.995% the speed of light). The traveler remains in a projectile for one year of his time, and then reverses direction. Upon return, the traveler will find that he has aged two years, whereas 200 years have passed on Earth. During the trip, both the traveler and Earth keep sending signals to each other at a constant rate, which places Langevin's story among the Doppler shift versions of the twin paradox. The relativistic effects upon the signal rates are used to account for the different aging rates. The asymmetry that occurred because only the traveler underwent acceleration is used to explain why there is any difference at all,<ref>{{cite book |title=Space, Time and Einstein: An Introduction |author1=J.B. Kennedy |edition=revised |publisher=Routledge |year=2014 |isbn=978-1-317-48944-3 |page=39 |url=https://books.google.com/books?id=bdjfBQAAQBAJ}} [https://books.google.com/books?id=bdjfBQAAQBAJ&pg=PA39 Extract of page 39]</ref><ref>{{cite book |title=Basic Relativity |author1=Richard A. Mould |edition=illustrated, herdruk |publisher=Springer Science & Business Media |year=2001 |isbn=978-0-387-95210-9 |page=39 |url=https://books.google.com/books?id=lfGE-wyJYIUC}} [https://books.google.com/books?id=lfGE-wyJYIUC&pg=PA39 Extract of page 39]</ref> because "any change of velocity, or any acceleration has an absolute meaning".<ref group=A>{{citation |first = P. |last = Langevin |author-link = Paul Langevin
|title =  The evolution of space and time
|journal = [[Scientia (Italian journal)|Scientia]] |volume = X |pages = 31–54 |year = 1911|url=http://amshistorica.unibo.it/diglib.php?inv=7&int_ptnum=108&term_ptnum=302}} (translated by J. B. Sykes, 1973 from the original French: "[http://amshistorica.unibo.it/diglib.php?inv=7&int_ptnum=10&term_ptnum=39&format=jpg L'évolution de l'espace et du temps"]).</ref>

[[Max von Laue]] (1911, 1913) elaborated on Langevin's explanation. Using [[Hermann Minkowski]]'s [[spacetime]] formalism, Laue went on to demonstrate that the [[world line]]s of the inertially moving bodies maximize the [[proper time]] elapsed between two events. He also wrote that the asymmetric aging is completely accounted for by the fact that the astronaut twin travels in two separate frames, while the Earth twin remains in one frame, and the time of acceleration can be made arbitrarily small compared with the time of [[Inertial frame of reference|inertial motion]].<ref group=A name="Laue1913">{{cite journal |last=von Laue |first=Max |author-link=Max von Laue |title=Zwei Einwände gegen die Relativitätstheorie und ihre Widerlegung (Two Objections Against the Theory of Relativity and their Refutation) |journal=Physikalische Zeitschrift |volume=13 |year=1911 |pages=118–120}}</ref><ref group=A>{{cite book |last=von Laue |first=Max |title=Das Relativitätsprinzip (The Principle of Relativity) |edition=2 |year=1913 |publisher=[[w:de:Vieweg Verlag|Friedrich Vieweg]] |location=Braunschweig, Germany |oclc=298055497}}</ref><ref group=A>{{cite journal |last=von Laue |first=Max |title=Das Relativitätsprinzip (The Principle of Relativity) |journal=Jahrbücher der Philosophie |volume=1 |year=1913 |pages=99–128}}</ref> Eventually, Lord [[Tony Giffard, 3rd Earl of Halsbury|Halsbury]] and others removed any acceleration by introducing the "three-brother" approach. The traveling twin transfers his clock reading to a third one, traveling in the opposite direction. Another way of avoiding acceleration effects is the use of the relativistic Doppler effect {{crossreference|(see [[#What it looks like: the relativistic Doppler shift|{{nowrap|§{{tsp}}What}} it looks like: the relativistic Doppler shift]] below)}}.

Neither Einstein nor Langevin considered such results to be problematic: Einstein only called it "peculiar" while Langevin presented it as a consequence of absolute acceleration.<ref group=A>"We are going to see this absolute character of the acceleration manifest itself in another form." ("Nous allons voir se manifester sous une autre forme ce caractère absolu de l'accélération."), page 82 of Langevin1911</ref> Both men argued that, from the time differential illustrated by the story of the twins, no self-contradiction could be constructed. In other words, neither Einstein nor Langevin saw the story of the twins as constituting a challenge to the self-consistency of relativistic physics.

==Specific example==
Consider a space ship traveling from Earth to the nearest star system: a distance {{nowrap|''d'' {{=}} 4 light}} years away, at a speed {{nowrap|''v'' {{=}} 0.8''c''}} (i.e., 80% of the speed of light).

To make the numbers easy, the ship is assumed to attain full speed in a negligible time upon departure (even though it would actually take about 9 months accelerating at [[Standard gravity|1&nbsp;''g'']] to get up to speed). Similarly, at the end of the outgoing trip, the change in direction needed to start the return trip is assumed to occur in a negligible time. This can also be modelled by assuming that the ship is already in motion at the beginning of the experiment and that the return event is modelled by a [[Dirac delta]] [[Distribution (mathematics)|distribution]] acceleration.<ref name='Minguzzi' />

The parties will observe the situation as follows:<ref>{{cite book
|title=Textbook Of Engineering Physics, Part I
|first1=Mahesh C.
|last1=Jain
|publisher=PHI Learning Pvt
|year=2009
|isbn=978-8120338623
|page=74
|url=https://books.google.com/books?id=DqZlU3RJTywC&pg=PA74}} [https://books.google.com/books?id=DqZlU3RJTywC&pg=PA74 Extract of page 74]
</ref><ref>{{cite book
|title=Introduction to Relativity
|first1=P. L.
|last1=Sardesai
|publisher=New Age International
|year=2004
|isbn=8122415202
|pages=27–28
|url=https://books.google.com/books?id=CX5GyYwnVLcC&pg=PA27}} [https://books.google.com/books?id=CX5GyYwnVLcC&pg=PA27 Extract of page 27]
</ref>

=== Earth perspective ===
The Earth-based mission control reasons about the journey this way: the round trip will take {{nowrap|''t'' {{=}} 2''d''/''v'' {{=}} 10 years}} in Earth time (''i.e.'' everybody who stays on Earth will be 10 years older when the ship returns). The amount of time as measured on the ship's clocks and the aging of the travelers during their trip will be reduced by the factor <math>\alpha = \scriptstyle{\sqrt{1 - v^2/c^2}}</math>, the reciprocal of the [[Lorentz factor]] (time dilation). In this case {{nowrap|''α'' {{=}} 0.6}} and the travelers will have aged only {{nowrap|0.6 × 10 {{=}} 6 years}} when they return.

=== Travellers' perspective ===
The ship's crew members also calculate the particulars of their trip from their perspective. They know that the distant star system and the Earth are moving relative to the ship at speed ''v'' during the trip. In their rest frame the distance between the Earth and the star system is {{nowrap|''α d'' {{=}} 0.6 × 4 {{=}} 2.4 light}} years ([[length contraction]]), for both the outward and return journeys. Each half of the journey takes {{nowrap|''α d'' / ''v'' {{=}} 2.4 / 0.8 {{=}} 3 years}}, and the round trip takes twice as long (6 years). Their calculations show that they will arrive home having aged 6&nbsp;years. The travelers' final calculation about their aging is in complete agreement with the calculations of those on Earth, though they experience the trip quite differently from those who stay at home.

=== Conclusion ===
{| class="wikitable"
|+ Readings on Earth's and spaceship's clocks
! Event || Earth<br>(years) !! Spaceship<br>(years)
|-
| Departure || 0 || 0 
|-
| End of outgoing trip =<br>Beginning of ingoing trip || 5 || 3
|-
| Arrival || 10 || 6
|}
No matter what method they use to predict the clock readings, everybody will agree about them. If twins are born on the day the ship leaves, and one goes on the journey while the other stays on Earth, they will meet again when the traveler is 6&nbsp;years old and the stay-at-home twin is 10&nbsp;years old.

==Resolution of the paradox in special relativity==

The paradoxical aspect of the twins' situation arises from the fact that at any given moment the travelling twin's clock is running slow in the earthbound twin's inertial frame, but based on the relativity principle one could equally argue that the earthbound twin's clock is running slow in the travelling twin's inertial frame.<ref name='Ohanian'>{{cite book|last=Ohanian|first=Hans|title=Special relativity: a modern introduction|date=2001|publisher=Physics Curriculum and Instruction|location=Lakeville, MN|isbn=0971313415}}</ref><ref name='Harris'>{{cite book|last=Harris|first=Randy|title=Modern Physics|date=2008|publisher=Pearson Addison-Wesley|location=San Francisco, CA|isbn=978-0805303087}}</ref><ref name='Rindler'>{{cite book|last=Rindler|first=W|title=Introduction to special relativity|date=2006|publisher=Oxford University Press|location=Oxford, UK|isbn=9780198567318}}</ref> One proposed resolution is based on the fact that the earthbound twin is at rest in the same inertial frame throughout the journey, while the travelling twin is not: in the simplest version of the thought-experiment, the travelling twin switches at the midpoint of the trip from being at rest in an inertial frame which moves in one direction (away from the Earth) to being at rest in an inertial frame which moves in the opposite direction (towards the Earth). In this approach, determining which observer switches frames and which does not is crucial. Although both twins can legitimately claim that they are at rest in their own frame, only the traveling twin experiences acceleration when the spaceship engines are turned on. This acceleration, measurable with an accelerometer, makes his rest frame temporarily non-inertial. This reveals a crucial asymmetry between the twins' perspectives: although we can predict the aging difference from both perspectives, we need to use different methods to obtain correct results.

===Role of acceleration===
Although some solutions attribute a crucial role to the acceleration of the travelling twin at the time of the turnaround,<ref name='Ohanian'/><ref name='Harris'/><ref name='Rindler'/><ref name='Weidner'>{{cite book|last=Weidner|first=Richard|title=Physics|url=https://archive.org/details/physics0000weid|url-access=registration|date=1985|publisher=Allyn and Bacon|location=Needham Heights, MA|isbn=0205111556}}</ref> others note that the effect also arises if one imagines two separate travellers, one outward-going and one inward-coming, who pass each other and synchronize their clocks at the point corresponding to "turnaround" of a single traveller. In this version, physical acceleration of the travelling clock plays no direct role;<ref name="Einstein, A. 1923 pp. 38">Einstein, A., Lorentz, H.A., Minkowski, H., and Weyl, H. (1923). [[Arnold Sommerfeld]]. ed. ''The Principle of Relativity.'' Dover Publications: Mineola, NY. pp. 38–49.</ref><ref name='Kogut'>{{cite book |title=Introduction to Relativity: For Physicists and Astronomers |first1=John B. |last1=Kogut |publisher=Academic Press |year=2012 |isbn=978-0-08-092408-3 |page=35 |url=https://books.google.com/books?id=9AKPpSxiN4IC}} [https://books.google.com/books?id=9AKPpSxiN4IC&pg=PA35 Extract of page 35]</ref><ref name='Minguzzi'/> "the issue is how long the world-lines are, not how bent".<ref name='Maudlin'>{{cite book|last=Maudlin|first=Tim|title=Philosophy of physics : space and time|date=2012|publisher=Princeton University Press|location=Princeton|isbn=9780691143095|pages=77–83}}</ref> The length referred to here is the Lorentz-invariant length or "proper time interval" of a trajectory which corresponds to the elapsed time measured by a clock following that trajectory (see Section [[#Difference_in_elapsed_time as_a_result_of_differences_in_twins'_spacetime_paths|Difference in elapsed time as a result of differences in twins' spacetime paths]] below). In Minkowski spacetime, the travelling twin must feel a different history of accelerations from the earthbound twin, even if this just means accelerations of the same size separated by different amounts of time,<ref name='Maudlin'/> however "even this role for acceleration can be eliminated in formulations of the twin paradox in curved spacetime, where the twins can fall freely along space-time geodesics between meetings".<ref name='Debs_Redhead'/>

===Relativity of simultaneity===
[[Image:Twin Paradox Minkowski Diagram.svg|thumb|right|333px|[[Minkowski diagram]] of the twin paradox. There is a difference between the trajectories of the twins: the trajectory of the ship is equally divided between two different inertial frames, while the Earth-based twin stays in the same inertial frame.]]
For a moment-by-moment understanding of how the time difference between the twins unfolds, one must understand that in special relativity there is no concept of ''absolute present''. For different inertial frames there are different sets of events that are simultaneous in that frame. This [[relativity of simultaneity]] means that switching from one inertial frame to another requires an adjustment in what slice through spacetime counts as the "present". In the spacetime diagram on the right, drawn for the reference frame of the Earth-based twin, that twin's world line coincides with the vertical axis (his position is constant in space, moving only in time). On the first leg of the trip, the second twin moves to the right (black sloped line); and on the second leg, back to the left. Blue lines show the ''planes of simultaneity'' for the traveling twin during the first leg of the journey; red lines, during the second leg. Just before turnaround, the traveling twin calculates the age of the Earth-based twin by measuring the interval along the vertical axis from the origin to the upper blue line. Just after turnaround, if he recalculates, he will measure the interval from the origin to the lower red line. In a sense, during the U-turn the plane of simultaneity jumps from blue to red and very quickly sweeps over a large segment of the world line of the Earth-based twin. When one transfers from the outgoing inertial frame to the incoming inertial frame there is a jump discontinuity in the age of the Earth-based twin<ref name='Ohanian'/><ref name='Harris'/><ref name='Kogut'/><ref name="Wheeler, J. 1992 pp. 38, 170">Wheeler, J., Taylor, E. (1992). ''Spacetime Physics, second edition.'' W. H. Freeman: New York, pp. 38, 170-171.</ref><ref name="Einstein et al 1923">Einstein, A., Lorentz, H.A., Minkowski, H., and Weyl, H. (1923). Arnold Sommerfeld. ed. ''The Principle of Relativity.'' Dover Publications: Mineola, NY. p. 38.</ref> (6.4 years in the [[#Specific example|example]] above).

==A non-spacetime approach==
As mentioned above, an "out and back" twin paradox adventure may incorporate the transfer of clock reading from an "outgoing" astronaut to an "incoming" astronaut, thus eliminating the effect of acceleration. Also, the physical acceleration of clocks does not contribute to the [[kinematical]] effects of special relativity.  Rather, in special relativity, the time differential between two reunited clocks is produced purely by uniform inertial motion, as discussed in Einstein's original 1905 relativity paper,<ref name="Einstein, A. 1923 pp. 38"/> as well as in all subsequent kinematical derivations of the Lorentz transformations.

Because spacetime diagrams incorporate Einstein's clock synchronization (with its lattice of clocks methodology), there will be a requisite jump in the reading of the Earth clock time made by a "suddenly returning astronaut" who inherits a "new meaning of simultaneity" in keeping with a new clock synchronization dictated by the transfer to a different inertial frame.<ref name="Wheeler, J. 1992 pp. 38, 170"/>

If, instead of incorporating Einstein's clock synchronization (lattice of clocks), the astronaut (outgoing and incoming) and the Earth-based party regularly update each other on the status of their clocks by way of sending radio signals (which travel at light speed), then all parties will note an incremental buildup of asymmetry in time-keeping, beginning at the "turn around" point. Prior to the "turn around", each party regards the other party's clock to be recording time differently from his own, but the noted difference is symmetrical between the two parties. After the "turn around", the noted differences are not symmetrical, and the asymmetry grows incrementally until the two parties are reunited. Upon finally reuniting, this asymmetry can be seen in the actual difference showing on the two reunited clocks.<ref>William Geraint Vaughan Rosser (1991). ''Introductory Special Relativity'', Taylor & Francis Inc. USA, pp. 67-68.</ref>

==The equivalence of biological aging and clock time-keeping==
All processes—chemical, biological, measuring apparatus functioning, human perception involving the eye and brain, the communication of force—are constrained by the speed of light. There is clock functioning at every level, dependent on light speed and the inherent delay at even the atomic level. Biological aging, therefore, is in no way different from clock time-keeping.<ref>{{cite book |title=Spacetime Physics |edition=2nd, illustrated |first1=Edwin F. |last1=Taylor |first2=John Archibald |last2=Wheeler |publisher=W. H. Freeman |year=1992  |isbn=978-0-7167-2327-1 |page=[https://archive.org/details/spacetimephysics00edwi_0/page/150 150] |url=https://archive.org/details/spacetimephysics00edwi_0|url-access=registration }}</ref> This means that biological aging would be slowed in the same manner as a clock.

==What it looks like: the relativistic Doppler shift==

In view of the frame-dependence of simultaneity for events at different locations in space, some treatments prefer a more phenomenological approach, describing what the twins would observe if each sent out a series of regular radio pulses, equally spaced in time according to the emitter's clock.<ref name='Kogut'/> This is equivalent to asking, if each twin sent a video feed of themselves to each other, what do they see in their screens? Or, if each twin always carried a clock indicating his age, what time would each see in the image of their distant twin and his clock?

Shortly after departure, the traveling twin sees the stay-at-home twin with no time delay. At arrival, the image in the ship screen shows the staying twin as he was 1&nbsp;year after launch, because radio emitted from Earth 1&nbsp;year after launch gets to the other star 4&nbsp;years afterwards and meets the ship there. During this leg of the trip, the traveling twin sees his own clock advance 3&nbsp;years and the clock in the screen advance 1&nbsp;year, so it seems to advance at {{frac|3}} the normal rate, just 20&nbsp;image seconds per ship minute. This combines the effects of time dilation due to motion (by factor {{nowrap|''α'' {{=}} 0.6}}, five years on Earth are 3&nbsp;years on ship) and the effect of increasing light-time-delay (which grows from 0 to 4&nbsp;years).

Of course, the observed frequency of the transmission is also {{frac|3}} the frequency of the transmitter (a reduction in frequency; "red-shifted"). This is called the [[relativistic Doppler effect]]. The frequency of clock-ticks (or of wavefronts) which one sees from a source with rest frequency ''f''<sub>rest</sub> is
:<math>f_\mathrm{obs} = f_\mathrm{rest}\sqrt{\left({1 - v/c}\right)/\left({1 + v/c}\right)}</math>
when the source is moving directly away. This is ''f''<sub>obs</sub> = {{frac|3}}''f''<sub>rest</sub> for ''v''/''c'' = 0.8.

As for the stay-at-home twin, he gets a slowed signal from the ship for 9&nbsp;years, at a frequency {{frac|3}} the transmitter frequency. During these 9&nbsp;years, the clock of the traveling twin in the screen seems to advance 3&nbsp;years, so both twins see the image of their sibling aging at a rate only {{frac|3}} their own rate. Expressed in other way, they would both see the other's clock run at {{frac|3}} their own clock speed. If they factor out of the calculation the fact that the light-time delay of the transmission is increasing at a rate of 0.8&nbsp;seconds per second, ''both'' can work out that the other twin is aging slower, at 60% rate.

Then the ship turns back toward home. The clock of the staying twin shows "1&nbsp;year after launch" in the screen of the ship, and during the 3&nbsp;years of the trip back it increases up to "10&nbsp;years after launch", so the clock in the screen seems to be advancing 3&nbsp;times faster than usual.

When the source is moving towards the observer, the observed frequency is higher ("blue-shifted") and given by
:<math>f_\mathrm{obs} = f_\mathrm{rest}\sqrt{\left({1 + v/c}\right)/\left({1 - v/c}\right)}</math>
This is ''f''<sub>obs</sub> = 3''f''<sub>rest</sub> for ''v''/''c'' = 0.8.

As for the screen on Earth, it shows that trip back beginning 9&nbsp;years after launch, and the traveling clock in the screen shows that 3&nbsp;years have passed on the ship. One year later, the ship is back home and the clock shows 6&nbsp;years. So, during the trip back, ''both'' twins see their sibling's clock going 3&nbsp;times faster than their own. Factoring out the fact that the light-time-delay is decreasing by 0.8&nbsp;seconds every second, each twin calculates that the other twin is aging at 60% his own aging speed.

[[Image:rstd4.gif|thumb|Light paths for images exchanged during trip<br>Left: Earth to ship. Right: Ship to Earth.<br>
Red lines indicate low frequency images are received, blue lines indicate high frequency images are received]]
The ''x''–''t'' (space–time) diagrams at right show the paths of light signals traveling between Earth and ship (1st diagram) and between ship and Earth (2nd diagram). These signals carry the images of each twin and his age-clock to the other twin. The vertical black line is the Earth's path through spacetime and the other two sides of the triangle show the ship's path through spacetime (as in the Minkowski diagram above). As far as the sender is concerned, he transmits these at equal intervals (say, once an hour) according to his own clock; but according to the clock of the twin receiving these signals, they are not being received at equal intervals.

After the ship has reached its cruising speed of 0.8''c'', each twin would see 1&nbsp;second pass in the received image of the other twin for every 3&nbsp;seconds of his own time. That is, each would see the image of the other's clock going slow, not just slow by the {{nowrap|''ε''}} factor 0.6, but even slower because light-time-delay is increasing 0.8&nbsp;seconds per second. This is shown in the figures by red light paths. At some point, the images received by each twin change so that each would see 3&nbsp;seconds pass in the image for every second of his own time. That is, the received signal has been increased in frequency by the Doppler shift. These high frequency images are shown in the figures by blue light paths.

===The asymmetry in the Doppler shifted images===
The asymmetry between the Earth and the space ship is manifested in this diagram by the fact that more blue-shifted (fast aging) images are received by the ship. Put another way, the space ship sees the image change from a red-shift (slower aging of the image) to a blue-shift (faster aging of the image) at the midpoint of its trip (at the turnaround, 3&nbsp;years after departure); the Earth sees the image of the ship change from red-shift to blue shift after 9&nbsp;years (almost at the end of the period that the ship is absent). In the next section, one will see another asymmetry in the images: the Earth twin sees the ship twin age by the same amount in the red and blue shifted images; the ship twin sees the Earth twin age by different amounts in the red and blue shifted images.

==Calculation of elapsed time from the Doppler diagram==
The twin on the ship sees low frequency (red) images for 3 years. During that time, he would see the Earth twin in the image grow older by {{nowrap|3/3 {{=}} 1 year}}. He then sees high frequency (blue) images during the back trip of 3&nbsp;years. During that time, he would see the Earth twin in the image grow older by {{nowrap|3 × 3 {{=}} 9 years.}} When the journey is finished, the image of the Earth twin has aged by {{nowrap|1 + 9 {{=}} 10 years.}}

The Earth twin sees 9 years of slow (red) images of the ship twin, during which the ship twin ages (in the image) by {{nowrap|9/3 {{=}} 3 years.}} He then sees fast (blue) images for the remaining 1&nbsp;year until the ship returns. In the fast images, the ship twin ages by {{nowrap|1 × 3 {{=}} 3 years.}} The total aging of the ship twin in the images received by Earth is {{nowrap|3 + 3 {{=}} 6 years}}, so the ship twin returns younger (6&nbsp;years as opposed to 10&nbsp;years on Earth).

===The distinction between what they see and what they calculate===
To avoid confusion, note the distinction between what each twin sees and what each would calculate. Each sees an image of his twin which he knows originated at a previous time and which he knows is Doppler shifted. He does not take the elapsed time in the image as the age of his twin now.
*If he wants to calculate when his twin was the age shown in the image (''i.e.'' how old he himself was then), he has to determine how far away his twin was when the signal was emitted—in other words, he has to consider simultaneity for a distant event.
*If he wants to calculate how fast his twin was aging when the image was transmitted, he adjusts for the Doppler shift. For example, when he receives high frequency images (showing his twin aging rapidly) with frequency <math>\scriptstyle{f_\mathrm{rest}\sqrt{\left({1 + v/c}\right)/\left({1 - v/c}\right)}}</math>, he does not conclude that the twin was aging that rapidly when the image was generated, any more than he concludes that the siren of an ambulance is emitting the frequency he hears. He knows that the [[Doppler effect]] has increased the image frequency by the factor 1 / (1 − ''v''/''c''). Therefore, he calculates that his twin was aging at the rate of
:<math>f_\mathrm{rest}\sqrt{\left({1 + v/c}\right)/\left({1 - v/c}\right)}\times \left(1 - v/c\right) = f_\mathrm{rest}\sqrt{1 - v^2/c^2}\equiv\epsilon f_\mathrm{rest}</math>
when the image was emitted. A similar calculation reveals that his twin was aging at the same reduced rate of ''εf''<sub>rest</sub> in all low frequency images.

===Simultaneity in the Doppler shift calculation===
It may be difficult to see where simultaneity came into the Doppler shift calculation, and indeed the calculation is often preferred because one does not have to worry about simultaneity. As seen above, the ship twin can convert his received Doppler-shifted rate to a slower rate of the clock of the distant clock for both red and blue images. If he ignores simultaneity, he might say his twin was aging at the reduced rate throughout the journey and therefore should be younger than he is. He is now back to square one, and has to take into account the change in his notion of simultaneity at the turnaround. The rate he can calculate for the image (corrected for Doppler effect) is the rate of the Earth twin's clock at the moment it was sent, not at the moment it was received. Since he receives an unequal number of red and blue shifted images, he should realize that the red and blue shifted emissions were not emitted over equal time periods for the Earth twin, and therefore he must account for simultaneity at a distance.

==Viewpoint of the traveling twin==
During the turnaround, the traveling twin is in an [[accelerated reference frame]]. According to the [[equivalence principle]], the traveling twin may analyze the turnaround phase as if the stay-at-home twin were freely falling in a gravitational field and as if the traveling twin were stationary. A 1918 paper by Einstein presents a conceptual sketch of the idea.<ref group=A>Einstein, A. (1918), "[[s:Translation:Dialog about Objections against the Theory of Relativity|Dialog about objections against the theory of relativity]]", ''Die Naturwissenschaften'' '''48''', pp. 697–702, 29 November 1918</ref> From the viewpoint of the traveler, a calculation for each separate leg, ignoring the turnaround, leads to a result in which the Earth clocks age less than the traveler. For example, if the Earth clocks age 1&nbsp;day less on each leg, the amount that the Earth clocks will lag behind amounts to 2&nbsp;days. The physical description of what happens at turnaround has to produce a contrary effect of double that amount: 4&nbsp;days' advancing of the Earth clocks. Then the traveler's clock will end up with a net 2-day delay on the Earth clocks, in agreement with calculations done in the frame of the stay-at-home twin.

The mechanism for the advancing of the stay-at-home twin's clock is [[gravitational time dilation]]. When an observer finds that inertially moving objects are being accelerated with respect to themselves, those objects are in a gravitational field insofar as relativity is concerned. For the traveling twin at turnaround, this gravitational field fills the universe. In a weak field approximation, clocks tick at a rate of {{nowrap|<var>t'</var> {{=}} ''t'' (1 + ''Φ'' / ''c''<sup>2</sup>)}} where ''Φ'' is the difference in gravitational potential. In this case, {{nowrap|''Φ'' {{=}} ''gh''}} where ''g'' is the acceleration of the traveling observer during turnaround and ''h'' is the distance to the stay-at-home twin. The rocket is firing towards the stay-at-home twin, thereby placing that twin at a higher gravitational potential. Due to the large distance between the twins, the stay-at-home twin's clocks will appear to be sped up enough to account for the difference in proper times experienced by the twins. It is no accident that this speed-up is enough to account for the simultaneity shift described above. The general relativity solution for a static homogeneous gravitational field and the special relativity solution for finite acceleration produce identical results.<ref>{{cite journal |last=Jones |first=Preston |title=The clock paradox in a static homogeneous gravitational field |journal=Foundations of Physics Letters |volume=19 |issue=1 |pages=75–85 |date=February 2006 |author2=Wanex, L.F.  |arxiv=physics/0604025 |doi=10.1007/s10702-006-1850-3 |bibcode=2006FoPhL..19...75J|s2cid=14583590 }}</ref>

Other calculations have been done for the traveling twin (or for any observer who sometimes accelerates), which do not involve the equivalence principle, and which do not involve any gravitational fields. Such calculations are based only on the special theory, not the general theory, of relativity. One approach calculates surfaces of simultaneity by considering light pulses, in accordance with [[Hermann Bondi]]'s idea of the [[k-calculus]].<ref>{{cite journal |author1=Dolby, Carl E.  |author2=Gull, Stephen F  |name-list-style=amp |title=On Radar Time and the Twin 'Paradox' |journal=American Journal of Physics |volume=69 |year=2001 |pages=1257–1261 |arxiv=gr-qc/0104077 |bibcode=2001AmJPh..69.1257D |doi=10.1119/1.1407254 |issue=12|s2cid=119067219 }}</ref> A second approach calculates a straightforward but technically complicated integral to determine how the traveling twin measures the elapsed time on the stay-at-home clock. An outline of this second approach is given in a [[Twin paradox#Difference in elapsed times: how to calculate it from the ship|separate section below]].

==Difference in elapsed time as a result of differences in twins' spacetime paths==
{{Further|Hyperbolic motion (relativity)}}

[[Image:TwinParadoxProperAcceleration.svg|thumb|right|250px|Twin paradox employing a rocket following an acceleration profile in terms of coordinate time T and by setting c=1: Phase 1 (a=0.6, T=2); Phase 2 (a=0, T=2); Phase 3-4 (a=-0.6, 2T=4); Phase 5 (a=0, T=2); Phase 6 (a=0.6, T=2). The twins meet at T=12 and τ=9.33. The blue numbers indicate the coordinate time T in the inertial frame of the stay-at-home-twin, the red numbers the proper time τ of the rocket-twin, and "a" is the proper acceleration. The thin red lines represent lines of simultaneity in terms of the different momentary inertial frames of the rocket-twin. The points marked by blue numbers 2, 4, 8 and 10 indicate the times when the acceleration changes direction.]]

The following paragraph shows several things:
*how to employ a precise mathematical approach in calculating the differences in the elapsed time
*how to prove exactly the dependency of the elapsed time on the different paths taken through spacetime by the twins
*how to quantify the differences in elapsed time
*how to calculate proper time as a function (integral) of [[coordinate time]]

Let clock ''K'' be associated with the "stay at home twin".
Let clock <var>K'</var> be associated with the rocket that makes the trip.
At the departure event both clocks are set to 0.

:Phase 1: Rocket (with clock <var>K'</var>) embarks with constant [[proper acceleration]] ''a'' during a time ''T''<sub>a</sub> as measured by clock ''K'' until it reaches some velocity ''V''.
:Phase 2: Rocket keeps coasting at velocity ''V'' during some time ''T''<sub>c</sub> according to clock ''K''.
:Phase 3: Rocket fires its engines in the opposite direction of ''K'' during a time ''T''<sub>a</sub> according to clock ''K'' until it is at rest with respect to clock ''K''. The constant proper acceleration has the value −''a'', in other words the rocket is ''decelerating''.
:Phase 4: Rocket keeps firing its engines in the opposite direction of ''K'', during the same time ''T''<sub>a</sub> according to clock ''K'', until <var>K'</var> regains the same speed ''V'' with respect to ''K'', but now towards ''K'' (with velocity −''V'').
:Phase 5: Rocket keeps coasting towards ''K'' at speed ''V'' during the same time ''T''<sub>c</sub> according to clock ''K''.
:Phase 6: Rocket again fires its engines in the direction of ''K'', so it decelerates with a constant proper acceleration ''a'' during a time ''T''<sub>a</sub>, still according to clock ''K'', until both clocks reunite.

Knowing that the clock ''K'' remains inertial (stationary), the total accumulated [[proper time]] Δ''τ'' of clock <var>K'</var> will be given by the integral function of [[coordinate time]] Δ''t''
:<math>\Delta \tau = \int \sqrt{ 1 - (v(t)/c)^2 } \ dt \ </math>
where ''v''(''t'') is the ''coordinate velocity'' of clock <var>K'</var> as a function of ''t'' according to clock ''K'', and, e.g. during phase 1, given by
:<math>v(t) = \frac{a t}{ \sqrt{1+  \left( \frac{a t}{c} \right)^2}}.</math>

This integral can be calculated for the 6 phases:<ref>C. Lagoute and E. Davoust (1995) The interstellar traveler, ''Am. J. Phys.'' '''63''':221-227</ref>
:Phase 1 <math>:\quad c / a \ \text{arsinh}( a \ T_a/c )\,</math>
:Phase 2 <math>:\quad T_c \ \sqrt{ 1 - V^2/c^2 }</math>
:Phase 3 <math>:\quad c / a \ \text{arsinh}( a \ T_a/c )\,</math>
:Phase 4 <math>:\quad c / a \ \text{arsinh}( a \ T_a/c )\,</math>
:Phase 5 <math>:\quad T_c \ \sqrt{ 1 - V^2/c^2 }</math>
:Phase 6 <math>:\quad c / a \ \text{arsinh}( a \ T_a/c )\,</math>

where ''a'' is the proper acceleration, felt by clock <var>K'</var> during the acceleration phase(s) and where the following relations hold between ''V'', ''a'' and ''T''<sub>a</sub>:
:<math>V = a \ T_a / \sqrt{ 1 + (a \ T_a/c)^2 }</math>
:<math>a \ T_a = V / \sqrt{ 1 - V^2/c^2 }</math>

So the traveling clock <var>K'</var> will show an elapsed time of
:<math>\Delta \tau = 2 T_c \sqrt{ 1 - V^2/c^2 } + 4 c / a \ \text{arsinh}( a \ T_a/c )</math>
which can be expressed as
:<math>\Delta \tau = 2 T_c / \sqrt{ 1 + (a \ T_a/c)^2 } + 4 c / a \ \text{arsinh}( a \ T_a/c )</math>

whereas the stationary clock ''K'' shows an elapsed time of
:<math>\Delta t = 2 T_c + 4 T_a\,</math>
which is, for every possible value of ''a'', ''T''<sub>a</sub>, ''T''<sub>c</sub> and ''V'', larger than the reading of clock <var>K'</var>:
:<math>\Delta t > \Delta \tau\,</math>

==Difference in elapsed times: how to calculate it from the ship==
[[Image:TwinParadoxProperAcceleration2.svg|thumb|right|250px|Twin paradox employing a rocket following an acceleration profile in terms of proper time τ and by setting c=1: Phase 1 (a=0.6, τ=2); Phase 2 (a=0, τ=2); Phase 3-4 (a=-0.6, 2τ=4); Phase 5 (a=0, τ=2); Phase 6 (a=0.6, τ=2). The twins meet at T=17.3 and τ=12. {{br}}This is a different voyage than the one shown above, as both schemes take the same assumed total ''point-of-view time'': T=12 (stay-at-home), resp τ=12 (ship), so the results of the calculated ''other-one's times'' must be different: τ=9.33 (ship), resp T=17.3 (stay at home).]]

In the standard proper time formula
:<math>\Delta \tau = \int_0^{\Delta t} \sqrt{ 1 - \left(\frac{v(t)}{c}\right)^2 } \ dt, \ </math>

Δ''τ'' represents the time of the non-inertial (travelling) observer <var>K'</var> as a function of the elapsed time Δ''t'' of the inertial (stay-at-home) observer ''K'' for whom observer <var>K'</var> has velocity ''v''(''t'') at time ''t''.

To calculate the elapsed time Δ''t'' of the inertial observer ''K'' as a function of the elapsed time Δ''τ'' of the non-inertial observer <var>K'</var>, where only quantities measured by <var>K'</var> are accessible, the following formula can be used:<ref name='Minguzzi'>E. Minguzzi (2005) - Differential aging from acceleration: An explicit formula - ''Am. J. Phys.'' '''73''': 876-880 [https://arxiv.org/abs/physics/0411233 arXiv:physics/0411233] (Notation of source variables was adapted to match this article's.)</ref>
:<math>\Delta t^2 = \left[ \int^{\Delta\tau}_0 e^{\int^{\bar{\tau}}_0 a(\tau')d \tau'} \, d \bar\tau\right] \,\left[\int^{\Delta \tau}_0 e^{-\int^{\bar\tau}_0 a(\tau')d \tau'} \, d \bar\tau \right], \ </math>
where ''a(τ)'' is the [[proper acceleration]] of the non-inertial observer <var>K'</var> as measured by himself (for instance with an accelerometer) during the whole round-trip. The [[Cauchy–Schwarz inequality]] can be used to show that the inequality {{nowrap|Δ''t'' &gt; Δ''τ''}} follows from the previous expression:
:<math>\begin{align}
\Delta t^2 & = \left[ \int^{\Delta\tau}_0 e^{\int^{\bar{\tau}}_0 a(\tau')d\tau'} \, d \bar\tau\right] \,\left[\int^{\Delta \tau}_0 e^{-\int^{\bar\tau}_0 a(\tau')d \tau'} \, d \bar\tau \right] \\
& > \left[ \int^{\Delta\tau}_0 e^{\int^{\bar{\tau}}_0 a(\tau')d\tau'} \, e^{-\int^{\bar\tau}_0 a(\tau') \, d \tau'} \, d \bar\tau \right]^2 = \left[ \int^{\Delta\tau}_0 d \bar\tau \right]^2 = \Delta \tau^2.
\end{align}</math>

Using the [[Dirac delta function]] to model the infinite acceleration phase in the standard case of the traveller having constant speed ''v'' during the outbound and the inbound trip, the formula produces the known result:
:<math>\Delta t = \frac{1}{\sqrt{1-\tfrac{v^2}{c^2}}} \Delta\tau .\ </math>

In the case where the accelerated observer <var>K'</var> departs from ''K'' with zero initial velocity, the general equation reduces to the simpler form:
:<math>\Delta t = \int^{\Delta\tau}_0 e^{\pm\int^{\bar{\tau}}_0 a(\tau')d \tau'} \, d \bar\tau , \ </math>
which, in the ''smooth'' version of the twin paradox where the traveller has constant proper acceleration phases, successively given by ''a'', −''a'', −''a'', ''a'', results in<ref name='Minguzzi'/>
:<math>\Delta t = \tfrac{4}{a} \sinh( \tfrac{a}{4} \Delta\tau) \ </math>
where the convention ''c'' = 1 is used, in accordance with the above expression with acceleration phases {{nowrap|''T''<sub>a</sub> {{=}} Δ''t''/4}} and inertial (coasting) phases {{nowrap|''T''<sub>c</sub> {{=}} 0.}}

==A rotational version==
Twins Bob and Alice inhabit a space station in circular orbit around a massive body in space. Bob suits up and exits the station. While Alice remains inside the station, continuing to orbit with it as before, Bob uses a rocket propulsion system to cease orbiting and hover where he was. When the station completes an orbit and returns to Bob, he rejoins Alice. Alice is now younger than Bob.<ref name=Hobson>{{cite book |title=General Relativity: An Introduction for Physicists |author=Michael Paul Hobson, [[George Efstathiou]], Anthony N. Lasenby |url=https://books.google.com/books?id=xma1QuTJphYC |page=227 |isbn=0-521-82951-8 |year=2006 |publisher=Cambridge University Press}} See [https://books.google.com/books?id=xma1QuTJphYC&pg=PA227 exercise 9.25] on page 227.</ref> In addition to rotational acceleration, Bob must decelerate to become stationary and then accelerate again to match the orbital speed of the space station.

==No twin paradox in an absolute frame of reference==
Einstein's conclusion of an actual difference in registered clock times (or aging) between reunited parties caused Paul Langevin to posit an actual, albeit experimentally indiscernible, absolute frame of reference:

In 1911, Langevin wrote: "A uniform translation in the aether has no experimental sense. But because of this it should not be concluded, as has sometimes happened prematurely, that the concept of aether must be abandoned, that the aether is non-existent and inaccessible to experiment. Only a uniform velocity relative to it cannot be detected, but any change of velocity ... has an absolute sense."<ref>Langevin, P. (1911), "The evolution of space and time", Scientia, X: p.47 (translated by J. B. Sykes, 1973).</ref>

In 1913, [[Henri Poincaré]]'s posthumous ''Last Essays'' were published and there he had restated his position: "Today some physicists want to adopt a new convention. It is not that they are constrained to do so; they consider this new convention more convenient; that is all. And those who are not of this opinion can legitimately retain the old one."<ref>Poincaré, Henri. (1913), ''Mathematics and science: last essays'' (''Dernières pensées'').</ref>

In the relativity of Poincaré and [[Hendrik Lorentz]], which assumes an absolute (though experimentally indiscernible) frame of reference, no paradox arises due to the fact that clock slowing (along with length contraction and velocity) is regarded as an actuality, hence the actual time differential between the reunited clocks.

In that interpretation, a party at rest with the totality of the cosmos (at rest with the barycenter of the universe, or at rest with a possible ether) would have the maximum rate of time-keeping and have non-contracted length.  All the effects of Einstein's special relativity (consistent light-speed measure, as well as symmetrically measured clock-slowing and length-contraction across inertial frames) fall into place.

That interpretation of relativity, which John A. Wheeler calls "ether theory B (length contraction plus time contraction)", did not gain as much traction as Einstein's, which simply disregarded any deeper reality behind the symmetrical measurements across inertial frames. There is no physical test which distinguishes one interpretation from the other.<ref>Wheeler, J., Taylor, E. (1992). Spacetime Physics, second edition. W. H. Freeman: New York, p. 88.</ref>

In 2005, [[Robert B. Laughlin]] (Physics Nobel Laureate, Stanford University), wrote about the nature of space: "It is ironic that Einstein's most creative work, the general theory of relativity, should boil down to conceptualizing space as a medium when his original premise [in special relativity] was that no such medium existed ... The word 'ether' has extremely negative connotations in theoretical physics because of its past association with opposition to relativity. This is unfortunate because, stripped of these connotations, it rather nicely captures the way most physicists actually think about the vacuum. ... Relativity actually says nothing about the existence or nonexistence of matter pervading the universe, only that any such matter must have relativistic symmetry (i.e., as measured)."<ref>Laughlin, Robert B. (2005). A Different Universe: Reinventing Physics from the Bottom Down. Basic Books, NY, NY.  pp. 120–121.</ref>

In ''Special Relativity'' (1968), A. P. French wrote: "Note, though, that we are appealing to the reality of A's acceleration, and to the observability of the inertial forces associated with it.  Would such effects as the twin paradox (specifically -- the time keeping differential between reunited clocks) exist if the framework of fixed stars and distant galaxies were not there?  Most physicists would say no. Our ultimate definition of an inertial frame may indeed be that it is a frame having zero acceleration with respect to the matter of the universe at large."<ref>French, A.P. (1968). Special Relativity. W.W. Norton, New York. p. 156.</ref>

==See also==
<!--Note to editors: Links should generally not be repeated if they are in the main body of the article-->
*[[Bell's spaceship paradox]]
*[[Clock hypothesis]]
*[[Ehrenfest paradox]]
*[[Herbert Dingle]]
*[[Ladder paradox]]
*[[List of paradoxes]]
*[[Supplee's paradox]]
*[[Time dilation]]
*''[[Time for the Stars]]''

==Primary sources==
{{Reflist|group=A}}

==Secondary sources==
{{Reflist|30em}}

==Further reading==
;The ideal clock
The ''ideal clock'' is a clock whose action depends only on its instantaneous velocity, and is independent of any acceleration of the clock.

* {{cite book |author=Wolfgang Rindler |title=Relativity: Special, General, and Cosmological |chapter=Time dilation |page=43 |chapter-url=https://books.google.com/books?id=0J_dwCmQThgC&pg=PA43 |isbn=0-19-856731-6 |publisher=Oxford University Press |year=2006}}

;Gravitational time dilation; time dilation in circular motion
*{{cite book|title=Cosmological Physics|author=John A Peacock|url=https://books.google.com/books?id=t8O-yylU0j0C&pg=PA7|page= 8|isbn=0-521-42270-1|year=2001|publisher=Cambridge University Press}}
*{{cite book|title=Modern Cosmology|author1=Silvio Bonometto |author2=Vittorio Gorini |author3=Ugo Moschella |url=https://books.google.com/books?id=jtyCC-b0SToC&pg=PA12|page= 12|isbn=0-7503-0810-9|year=2002|publisher=CRC Press}}
*{{cite book|title=Advanced Electromagnetism and Vacuum Physics|page= 180|author=Patrick Cornille|url=https://books.google.com/books?id=y8sSFTDkQ20C&pg=PA180|isbn=981-238-367-0|publisher=World Scientific|year=2003}}

==External links==
{{Wikibooks|Special relativity}}{{commons category|Twin paradox}}
*[http://www.physics.adelaide.edu.au/~dkoks/Faq/Relativity/SR/TwinParadox/twin_paradox.html Twin Paradox overview] {{Webarchive|url=https://web.archive.org/web/20150924072818/http://www.physics.adelaide.edu.au/~dkoks/Faq/Relativity/SR/TwinParadox/twin_paradox.html |date=24 September 2015 }} in the [[Usenet]] Physics FAQ
*[http://www.phys.unsw.edu.au/einsteinlight/jw/module4_twin_paradox.htm The twin paradox: Is the symmetry of time dilation paradoxical?] From [http://www.phys.unsw.edu.au/einsteinlight Einsteinlight: Relativity in animations and film clips].
*[https://math.ucr.edu/~jdp/Relativity/TwinParadox.html FLASH Animations:] ''from John de Pillis.'' (Scene 1): "View" from the Earth twin's point of view. (Scene 2): "View" from the traveling twin's point of view.
*[https://www.relativitycalculator.com/twin_clock_paradox.shtml Relativity Science Calculator - Twin Clock Paradox]

{{Relativity}}

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