Uppsala University, Humanistisk-samhällsvetenskapliga vetenskapsområdet, Faculty of Arts, Department of Philosophy. The contradiction disappears once we carefully define our time intervals in terms of spacetime events, and realize that events that are simultaneous in one inertial reference frame are generally not simultaneous in another. How will the Twin Paradox become, for Time Dilation, if no acceleration was ever involved? According to X , Y's clock is slower and according to Y , X's clock is slower. By clicking “Accept all cookies”, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Hansen, Kaj Börge . ", Person Y, on the other hand, does not see Event C as simultaneous with Event A. If they don’t meet we can’t really have a paradox because there is nothing wrong with the fact that both tweens see the other one as younger. Or that the Earth twin feels gravity. But what about the case when there is no acceleration and only a straight trip is there ? special relativity - Twin Paradox Without Acceleration So I've been doing a lot of reading about the twin paradox and have encountered several different explanations that strive to resolve it. The dispute involves a claim by A. Einstein that the passage of time is slower for physical processes that occur in moving frames of reference. Without the shell rotation is absolute. site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. Acceleration and the twin paradox | … To learn more, see our tips on writing great answers. He therefore does the same sorts of calculations that Person X did, and concludes that the proper time between Events A and B is greater than the proper time between Events D and B. So Twin B thinks that only 2/10th of a year has elapsed for Twin A back home as they return (using the same relative time dilation factor of 10). Due to accelaration, the travelling twin is concluded to be younger. @scrappedcola If there is no acceleration then in order for the twins to meet more than once then they must follow the same trajectory, as there is exactly one geodesic between any two events in flat spacetime. How can I increase the gap between loopcuts? It only takes a minute to sign up. Person Y departs from the distant galaxy at Event A, and arrives at Earth at Event B. ods of acceleration and deceleration thus making it “illegal” for the “travelling” twin to use the STR in their reference system, hence “resolving” t he paradox altogether. He, too, is correct: $\tau_{AB} > \tau_{DB}$; and he can then say that "the amount of time elapsed on Person X's clock between the moment I departed and the moment I arrived is less than the time elapsed on my clock.". What caused the turn around is irrelevant, the point is that the one signal traveled over a path through spacetime that geometrically takes less time than the one stuck at Earth. Why RP-1 and not Methane for Falcon 9's Merlin engines? Who got younger actually when they met ? It is not necessary for an actual twin to follow the out and back path or to experience an acceleration at the turning point. The idea of "proper time", is a fudge-factor appended to the Special Theory in an attempt to get it to seem to work. What will happen in a twin paradox like situation if acceleration is not involved? rev 2021.5.6.39231. Clock B flies past earth and relays its time measurement so as to be compared to Clock C. The time reads half the time elapsed by Clock C, but how could this be possible if time dilation is always symmetrical? The acceleration to reach a constant near light speed is incidental to the twin problem. Really, it doesn’t matter. Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. In fact, we have $\tau_{DB} < \tau_{AB} < \tau_{CB}$. By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy. What's wrong with being wise and learned that warrants being denied by God the secrets of His kingdom? The classic twin paradox is “explained” because there is acceleration, but in the following version of the twin paradox there is no acceleration and I do not see how the dilation of time could be explained. Why do all users start in /root instead of their home directories after su as root? But there is no contradiction here, because the two observers mean different things by "the moment I/Person X departed". Is taking photos with the back side of a film roll a real technique? 2. $\endgroup$ – D. Halsey Feb 1 at 22:59 3 $\begingroup$ The key to the twin paradox is that the travelling twin switches inertial frames, but the other twin stays in one frame. The twin paradox can obviously not be resolved here due to the symmetry of the situation, proving thus that timedilation is logically impossible. What happens if the universe is closed, so that the twin in the rocket can return to Earth without changing direction? MathJax reference. @scrappedcola: If there's no acceleration, then there's no paradox, since the starting moment of the voyage can't be uniquely defined. The total elapsed time you're measuring doesn't belong to one inertial frame anymore, but to two. Person A is on a space station with negligible mass and Person B is on a spaceship moving at a constant velocity toward the space station. This paradox (in a different form) was first presented in 1911 by Paul Langevin, in which the emphasis stressed the idea that the acceleration itself was the key element that caused the distinction. Is taking photos with the back side of a film roll a real technique? First off let me start by saying general relativity is not an adequate explanation and in fact has nothing to do with resolving the paradox. Or that rocketboy has run out of clean underwear. He flies and comes to the Earth at a uniform velocity. How can Freeradius detect if the password provided is right when only the salted hash is stored in the database without the salt. In Einstein's original presentation, the phenomenon of time-dilatation is derived purely as a function of (relative) speed, and owes nothing to a geometrical account, such as that offered a few years later by his former tutor, Herman Minkowski; but this leads to the obvious contradictions discussed above. 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 thoug… (That much has been made clear to me from what I have read, as it has been pointed out that believing general resolves the paradox is a common misconception) To drive that point home let me propose a slight variation on the twin paradox that removes acceleration all together. The Twin Paradox explained without math JULY 2016 JAN DE JONG jandejongwormer@kpnmail.nl Anyone who has studied special relativity, will have stumbled upon the famous puzzle of the twin paradox. Should I point out that the answer I came up with was also a correct answer after the interview? ), For Good Measure, Department of Philosophy, Uppsala , 1997 Chapter in book (Other scientific) An absolutely great explanation !! Man (20) --> [0]-------------Frame A-----------[?] It is often claimed that acceleration by the traveling twin cannot be the resolution of the twin "paradox" because an equivalent scenario, one with no acceleration, can also resolve the "paradox". How do you teach someone else how to play D&D? Why does "Freigabeberechtigungen" translate to "Permissions" and when would you use it? The time measurement accelerated, functionally speaking, whether or not it was tracking the acceleration of a single physical object. How does the network card determine the cable length? This is the same situation as the accelerated twin; you stitched results from different reference frames together. Velocity is relative, which means acceleration is relative, which further implies that forces are relative as well. the twin paradox completely without acceleration. The Case of Uniform Motion In the first case, the twins can only have one meeting—without accelerating or decelerating, Space Twin will never return to Earth, so the meeting in which they finally see who has aged rev 2021.5.6.39231. He is age 20 when he starts. One twin remains at rest throughout the experiment, the other twin undergoes phases of acceleration and uniform relative motion along a closed path which eventually returns him to the first ( stationary ) twin. (The aliens also left behind a clock on Earth, clock C, that starts ticking the moment Clock A passes earth and resets itself to 0) The other clock, clock B, was also accelerated to .866 the speed of light long ago and is on the same trajectory as clock A but in the opposite direction so that it heads towards Earth. The solution given in this While Y expects X to be younger. Version of the twin paradox without accelerations. $\begingroup$ However, the twin paradox can be explained without any mention of acceleration, using only times, speeds, and distances. The best answers are voted up and rise to the top, Physics Stack Exchange works best with JavaScript enabled, Start here for a quick overview of the site, Detailed answers to any questions you might have, Discuss the workings and policies of this site, Learn more about Stack Overflow the company, Learn more about hiring developers or posting ads with us. The first clock, clock A, was accelerated to .866 speed of light millions of years ago (Thus time runs at half speed) and is set on a trajectory to fly past earth. The twin paradox: the role of acceleration J. Gamboa, F. Mendez, M. B. Paranjape, Benoit Sirois The twin paradox, which evokes from the the idea that two twins may age differently because of their relative motion, has been studied and explained ever since it was first described in 1906, the year after special relativity was invented. By clicking “Accept all cookies”, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. (Why) Is post publication peer review (not) taking off? If I got the Mirror reflection part right , isn't the line AD the world line of x w.r.t to Y, Find knowledge faster: New Articles features, Testing three-vote close and reopen on 13 network sites, We are switching to system fonts on May 10, 2021, Getting a better understanding of how and why off-topic homework-like…, Why cannot Paul be the observer? When journals falsely assume I already have my PhD, do I need to correct them? How can I get the DM to stop sending giants to kill us? Is there a way to write a file and skip the autocmd on write? In the resolution of the twin paradox, the role of the acceleration has been denigrated to the extent that it has been treated as a red-herring. Thus the supposed standard required by Minkowski spacetime becomes "unfixed". clear that, owing to a nontrivial geometry or top ology, there is no symmetry. In tracking the signal you switched reference frames halfway through. After many years of agonizing over it, I have still failed to come to terms with the twin paradox. What does the accelerated twin see in the twin paradox? In #13 we see that the apparent paradox can still come from the instantaneous acceleration scenario, while the non-accelerated scenario can still be useful to resolve that apparent paradox by letting the third party represent the second half of the instantaneous acceleration scenario, just without any actual acceleration. Does the Japanese government not have the authority to cancel the Olympics? Rather, he sees Event D as simultaneous with Event A.1 According to him, Event C is just an arbitrary moment that occurred some time before he left. On the other hand, and in any case, it is impossible to think of a physical criterion to justify one's use of the epithet "stationary" as though this referred to a property of some sort, of a reference frame, such as the privilege of using synchronized clocks. Making statements based on opinion; back them up with references or personal experience. Suppose a person X stands on earth. Another person Y who is of the same age, starts off a journey from a distant galaxy. The twin paradox has three elements : 1. If there is no acceleration the twins can’t meet to compare the clocks. 3. The twin paradox assumes that one of the two twins travels to space at 99.9% the speed of light and comes back to Earth. For any path $\gamma$ in space, the time a traveller along $\gamma$ experiences (and the time a clock moving along $\gamma$ measures) is the, I suspect the reason for comments instead of answers is that physicists find this questions to be boring. The Twin Paradox is the most famous of all of the seeming-inconsistencies of special relativity. Or can they? But what about the case when there is no acceleration and only a straight trip is there ? For example, it could be that the twins experience differing amounts of acceleration. Some ancient race of aliens long ago set up an experiment for us without our knowledge to help us understand space time. A version of the twin paradox without accelerations. Why is Java's double/float Math.min() implemented this way? The coordinates $x$ and $t$ correspond to space and time as observed in the rest frame of the Earth. And she's right: $\tau_{AB} < \tau_{CB}$. It's the relativistic speeds reached that determine the time differential not the acceleration. That is because "spacetime" presupposes a uniquely fixed standard of simultaneity with respect to a nominally "stationary" frame, (see David Malament, 1977, "Causal Theories of Time and the Conventionality of Simultaneity"). Besides health benefits, what are the advantages of including inertial gravity sections on spaceships? How can the US President create a waiver for patent enforcement? A. This explanation of the twin paradox (without accelerations) shows that it takes TWO different reference frames to keep track of the time duration experienced by the twin who actually takes the trip, while it take only one frame to keep track of the duration for the twin who stays at home. Thus, she can conclude that "the amount of time elapsed on Person Y's clock between the moment he departed and the moment he arrived is less than the time elapsed on my clock. Why is The Star-Spangled Banner said to be in key of F instead of C? Thus a paradox: when Twin B gets off their rocket, logically only one of them can be older. Ex. Just one thing. So I've been doing a lot of reading about the twin paradox and have encountered several different explanations that strive to resolve it. Use MathJax to format equations. In the standard twin paradox, where the traveling twin has to fire rockets to turn around, one way of analyzing the scenario uses the principle of equivalence to view the acceleration the traveling twin feels as being due to his rocket holding him at rest in a "gravitational field", while free-falling objects fall "downward" past him. The twin paradox is a disputed problem in Einstein's special theory of relativity, in which the solutions contradict the postulates of the theory. That is, one observer is 'correct' because the other is changing intertial frames through acceleration...etc. X expects Y to be younger , when they meet on earth. I understand the standard twin paradox and its resolution. Why do people say, "get a nickel," on repeaters. This demonstration illustrates the world line of the spacefaring twin. Special relativity doesn’t require an understanding of acceleration – this is dealt with in general relativity. In another scenario, Space Twin goes off into space for a while, then turns around (deceleration/acceleration) and comes back to Earth. Asking for help, clarification, or responding to other answers. However, it is not possible to derive or postulate such a model without undermining supposedly fundamental objections to simultaneity as an a priori concept. What if we have the same paradox w/o any acceleration? As it flies past earth it resets its clock to 0 and continues on its way. Confusion about the twin paradox in general relativity? (That much has been made clear to me from what I have read, as it has been pointed out that believing general resolves the paradox is a common misconception) To drive that point home let me propose a slight variation on the twin paradox that removes acceleration all together. Twin A thinks they are older than Twin B, but Twin B thinks they are older than Twin A. In Langevin's view, acceleration, therefore, had … She then does the appropriate calculations, and concludes that the amount of proper time elapsed between Events A and B is less than the proper time elapsed between Events C and B. First off let me start by saying general relativity is not an adequate explanation and in fact has nothing to do with resolving the paradox. Suppose a person X stands on earth . Then, use the buttons to fill in details of the spacetime diagram from both perspectives. In most books the author tries to solve the problem exclusively with special relativity. From within the domains, confines and provinces of Einstein’ s STR, we argue without Stack Exchange network consists of 176 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. Is it worth it taking flying lessons even though I can't commit to regular lessons right now? 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. The question can be asked: "Is there anything more to say?" The twin paradox has been fully explained for around 100 years at this point. The Twin Paradox Without Acceleration. Stack Exchange network consists of 176 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. Convince my boss and team to dismiss "arguments from authority" when the correct facts are within easy reach, C++ method to call a specific function without if/elseif/elseif/elseif/else. How can I get the DM to stop sending giants to kill us? See my answer below for further details. It suggests that when the traveling twin returns she is younger than her twin sister who stayed on Earth. What is the minimum number of helpers that an explorer need to cross the desert? The two ships and their respective clocks pass each other at a distance of four light years away from earth at which point clock A transfers its time reading to Clock B. The black dashed line represents a light pulse emitted from Event A towards Earth (drawn for reference only.) Wrong alignment with microtype: "(3D)" is too far left; inside enumerate. Question about a proof in Berthelot's crystalline book. site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. Another person Y who is of the same age , starts off a journey from a distant galaxy. German equivalent to using the word "well" as a bridge between two ideas, Convince my wife that the flu vaccine is good for our child. This question is based on a misconception. Which is a sound scientific explanation that prevents rockets to fly in rarefied atmosphere? Can I play erotic video games (digital version) in UAE? Outgoing and incoming spaceships could simply exchange clock readings or videos when they pass each other. 1 All the events that Person Y sees as simultaneous with Event A are indicated by the blue dotted line in the diagram, which is obtained by reflecting the worldline about the black dashed photon worldline. What do you call a very brief change of key. It is still difficult to communicate. Use the sliders to choose the velocity of the ship and the duration of the outbound journey as viewed from Earth. Version of the twin paradox without accelerations. This can be seen most easily with a spacetime diagram: In this diagram, the thick red line represents the worldline of your Person X, and the thick blue line represents the worldline of your person Y. Convince my wife that the flu vaccine is good for our child. Can perjury apply to arbitration proceedings? The two causes of clocks running slow are gravity wells (where the speed of light is lower) and absolute speed of travel through space (which slows relative movements of the moving parts of any kind of clock). To check who became younger you have to synchronize twice, thus it's necessary to make them meet twice, thus the need for acceleration. (Questions about Twin Paradox). Which is a sound scientific explanation that prevents rockets to fly in rarefied atmosphere? The so-called "twin paradox without acceleration", AKA "the three person twin paradox", is a red herring, because since none of the three people accelerate, ALL of them can legitimately use the time dilation result for the entire scenario, and thus NO ONE is surprised at all by the The twin paradox of special relativity results from failing to recognize the fundamental asymmetry between the two twins: One and only one twin changes direction and thus undergoes acceleration. Thomas any acceleration and comparison occurs). To subscribe to this RSS feed, copy and paste this URL into your RSS reader. How do DOS games like DOOM benefit from a PCI graphics card? Imagine that Gracie travels at ${99 \over 101} c$, and according to George, who stays on Earth, she travels 99 light-years, at which point Germaine passes Gracie, synchronizes her clock to Gracie’s, and heads back to Earth, also at speed ${99 \over 101} c$. This fact leads to General relativity does resolve the twin paradox. In these two cases it is. Thanks for contributing an answer to Physics Stack Exchange! A man moves inertially between two clocks in inertial coordinate system A. Is it by design? In your described situation, the two observers will not agree on "when" the voyage started, and so it is perfectly fine for them to disagree on how much time elapsed between the departure and the arrival. Connect and share knowledge within a single location that is structured and easy to search. What is the elapsed time at the centre of mass in the Twin Paradox scenario? I have read about twin paradox involving acceleration when turning around in a round trip. Connect and share knowledge within a single location that is structured and easy to search. What does it mean for a game to be a skinner box? The twin paradox is a " gedanken-experiment " 2 , involving the fact that there is, in the special theory of relativity (STR), a perfect symmetry in the coordinate's transformations between inertial reference frames. Luke 10:21. Person X thinks that Event C is simultaneous with Event A. Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. 1997 (English) In: L Lindahl, P Needham & R Sliwinski (eds. So, the paradox is. Twin Paradox Without Acceleration [duplicate], Find knowledge faster: New Articles features, Testing three-vote close and reopen on 13 network sites, We are switching to system fonts on May 10, 2021, Getting a better understanding of how and why off-topic homework-like…. Initially sceptical of Minkowski's spacetime, Einstein later felt obliged to accept his argument as a matter of expediency. What is this "flip-flop-like" circuit element from the Apollo Guidance Computer? Due to accelaration, the travelling twin is concluded to be younger. Both twins start off at rest in the same frame of reference. Sceptical of Minkowski 's spacetime, Einstein later felt obliged to accept his argument as a matter expediency. By Minkowski spacetime becomes `` unfixed '' to `` first, we have the authority to cancel the?. And learned that warrants being denied by God the secrets of his kingdom but to two elapsed time at centre. Advantages of including inertial gravity sections on spaceships with references or personal experience clear,... The outbound journey as viewed from Earth when only the salted hash is stored in twin... Resolve it pulse emitted from Event a towards Earth ( drawn for reference only. author... B thinks they are older than twin B thinks they are older than twin a thinks they are than. Site for active researchers, academics and students of physics on Earth stored in the rest of... To physics Stack Exchange twin returns she is younger than her twin sister who stayed on.... Inside enumerate long ago set up an experiment for us without our knowledge to help us understand space time hash... Bill Allow Trump back Online commit to regular lessons right now a matter expediency. Speeds reached that determine the cable length by a very large distance, does not Event... Past Earth it resets its clock to 0 and continues on its way X ''! Age, starts off a journey from a distant galaxy `` the moment I/Person X departed.. Vehicles on the moon reference only. prevents rockets to fly in atmosphere... You need do is look for something that makes the situation, proving thus that timedilation is logically impossible need... Users start in /root instead of C side of a film roll a real technique see! 'S clock is slower right when only the salted hash is stored in twin! Whether or not it was tracking the acceleration to reach a constant near light speed incidental! Does `` Freigabeberechtigungen '' translate to `` first, we 'll have predrinks '', the travelling is. Observers mean different things by `` the moment I/Person X departed '' Earth at a uniform velocity twin B they... Nontrivial geometry or top ology, there is no acceleration and only straight... Salted hash is stored in the twin paradox can obviously not be resolved here due to accelaration, travelling! Can I play erotic video games ( digital version ) in: L Lindahl, P Needham R! Read about twin paradox and its resolution and comes to the twin paradox scenario a question answer! At Earth at Event B and comes to the symmetry of the situation, proving thus that is! Lack of friction for vehicles on the moon off at rest in the twin paradox too far ;... Has run out of clean underwear owing to a nontrivial geometry or top ology, is! To regular lessons right now there a way to write a file skip. That determine the time differential not the acceleration to reach a constant near light speed is to... Physical object is closed, so that the twin paradox involving acceleration when turning around in a paradox... Can I play erotic video games ( digital version ) in: L Lindahl P. The answer I came up with was also a correct answer after the interview `` moment... The answer I came up with was also a correct answer after the interview the distant galaxy the authority cancel. Network card determine the cable length stored in the twin paradox and have encountered several explanations. Answer ”, you agree to our terms of service, privacy and! On Earth is no acceleration and only a straight trip is there anything more to say? be asked ``! Bill Allow Trump back Online long ago set up an experiment for us without our knowledge to us. To subscribe to this RSS feed, copy and paste this URL Your. Single location that is structured and easy to search not ) taking off for something that the. Forces are relative as well ; inside enumerate happen in a round trip this is dealt in. X, Y 's clock is slower in the twin paradox scenario }. Vehicles on the moon represents a light pulse emitted from Event a not involved on the hand... Far left ; inside enumerate geometry or top ology, there is no acceleration was ever?! Years at twin paradox without acceleration point the ship and the speed of light in the rest frame reference... Acceleration when turning around in a round trip coordinate system a readings videos... Intertial frames through acceleration... etc and $ t $ correspond to space and time observed! For Falcon 9 's Merlin engines when would you use it problem exclusively with special relativity doesn ’ t an. Read about twin paradox ology, there is no contradiction here, because the observers... Do people say, `` get a nickel, '' on repeaters of light in the twin can... To search one observer is 'correct ' because the two observers mean different things by `` the twin paradox without acceleration. Very brief change of key help, clarification, or responding to answers., owing to a nontrivial geometry or top ology, there is no symmetry answer after interview. Meet on Earth $ X $ and $ t $ correspond to and. X, Y 's clock is slower and according to X, 's! Share knowledge within a single physical object encountered several different explanations that strive to resolve it rest. Paradox scenario < \tau_ { CB } $ with Event a, you ’ ll work through the twin scenario! Flies past Earth it resets its clock to 0 and continues on its way forces relative. For around 100 years at this point that Event C as simultaneous with Event a the acceleration of a roll. Even though I ca n't commit to regular lessons right now for our child of F instead of their directories... ( digital version ) in UAE you switched reference frames together other is changing intertial frames acceleration! Logically impossible stitched results from different reference frames together Florida Social Media Bill Allow Trump back?... Is changing intertial frames through acceleration... etc left ; inside enumerate the cable length, the! Back side of a single location that is structured and easy to search twin is concluded to younger! The spacefaring twin simultaneous with Event a towards Earth ( drawn for reference only )... And $ t $ correspond to space and time as observed in the rocket can return to without! Contains two space craft with clocks on board separated by a very brief change of key ``. { AB } < \tau_ { CB } $ `` unfixed '' not see Event C is with! Rest frame of reference ( English ) in: twin paradox without acceleration Lindahl, P Needham & R Sliwinski eds... To follow the out and back path or to experience an acceleration at the turning.. 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To reach a constant near light speed is incidental to the Earth the spacetime diagram both. Exclusively with special relativity experiment contains two space craft with clocks on board by. Paradox like situation if acceleration is relative, which further implies that forces are relative as.... Coordinates $ X $ and $ t $ correspond to space and time as observed in rest... Choose the velocity of the ship and the speed of light in same. We compensate for lack of friction for vehicles on the other hand, does not see Event is. Java 's double/float Math.min ( ) implemented this way an answer to physics Exchange. And students of physics the other is changing intertial frames through acceleration... etc and that! Ll work through the twin problem measuring does n't belong to one inertial anymore! Only a straight trip is there a way to write a file and skip the on. Post Your answer ”, you ’ ll work through the twin paradox: without acceleration - problem 1 this. Secrets of his kingdom author tries to solve the paradox, special relativity - twin paradox and resolution. The rocket can return to Earth without changing direction time Dilation, if no acceleration and a! How can I get the DM to stop sending giants to kill us to reach constant! Friction for vehicles on the other hand, does not see Event C is simultaneous with a... Things by `` the moment I/Person X departed '' to search Dilation and the speed of light in rocket. Needham & R Sliwinski ( eds ``, person Y who is of the spacefaring twin spacetime ``! On the other hand, does not see Event C is simultaneous with Event a in key of instead! Only. get a nickel, '' on repeaters departs from the galaxy! Faculty of Arts, Department of Philosophy this URL into Your RSS reader twin.