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Why do people talk as if the Twin Paradox is problematic?

  1. Jun 7, 2004 #1
    I have read a few threads that talk about scientist having to “solve” the twin paradox as if there is a problem with it… What is it that I am misunderstanding?

    Since time and length contractions have been verified with multiple experiments then what is the problem with the twin paradox?

    At the end of the journey one twin is younger then the other… Is there some problem with this that I am unaware of? Why do people talk about a problem within the twin paradox?

    Thank you,
  2. jcsd
  3. Jun 7, 2004 #2
    syano asked, "Why do people talk about a problem within the twin paradox?"

    For one (or both) of two reasons:

    1) They don't understand SR

    2) They've read in books, by authors whose credentials as physicists seem sufficient, that the twin paradox can only be explained with GR because it involves acceleration. This is nonsense, but as late as the 1960s it was still showing up in physics text books, written by PhD physicists, teaching at major universities.
    Last edited: Jun 7, 2004
  4. Jun 7, 2004 #3
    I can't see a problem with it either.
    I can understand people trying to find a cause for relativistic effects but not why they argue with experimetally verified equations and the predictions that come from them.
  5. Jun 7, 2004 #4
    It depends on which frame of reference you wish to describe events from. But to give a precise answer you'd have to state where you read this and what the context was. It might have been something as simple as a statement of something to solve and thus any such "something to solve" might be called a "problem". Otherwise I'm not sure where you heard that and what context it was stated in. There is really no problem inherent to relativity. There is at best some misunderstanding of the physics.

    That is incorrect. It is far from nonsense. At best it is a difference of opinion of how "GR" is defined. As defined by Einstein GR is physics in non-inertial frames. As such the traveling observer, i.e. the observer who turns around and comes back, uses GR to explain what he observers. The temporal effects can then be explained in terms of gravitational redshift.

    And this has zero to do with how old a text was. Even some modern texts explain the events as observed by the traveling twin in terms of gravitational redshift. In fact one such text is Cosmological Physics, John A. Peacock, Cambridge University Press, (1999) . In fact the portion of that text which explains this in terms of GR is located online at


  6. Jun 7, 2004 #5
    From a tyro's point of view:

    The twin paradox is, simply stated, that in SR the time dilation can be attributed to either twin, and so there should be no age difference between the twins should their inertial, non-accelerated courses bring them back in contact with each other.

    I probably really hacked that up. Sorry if it's obscure. Let me put this another way: Unless you have one twin accelerate, travel a distance at a significant fraction of the speed of light, decelerate, turn around, accelerate again, travel back to the starting point, decelerate, and meet the other twin, then there is a paradox involved.

    In SR, time dilation is attributed to velocity. But, the time dilation works in both directions; observers in both inertial reference frames see the other as experiencing a time dilation. So, since the time dilation is complementary, then if the only effect we were considering was velocity (as opposed to acceleration/gravity), then each twin would see the other as aging more slowly than himself as they traveled away from each other, and so if it were possible for the two twins to meet again without decelerating or accelerating, we would have to explain how the apparent time dilation is "undone" when the two inertial frames again syncrhronize.

    I'm sure someone else can add some more information to clear this up. I've probably muddied the waters rather than helping. I welcome the assistance of anyone who can spot my inadequacies and clear them up.

    But I hope this helps somewhat.

    (BTW, if I understand it correctly, there is a triplets paradox, wherein two of the three triplets hop into spaceships, accelerate in opposite directions, achieve near-light speeds, slow down, turn around, come back, and meet up, all together again. If I am not mistaken, the two traveling triplets out to be different ages, but will not be, while both of the travelers will have the same age difference from the homebody triplet.)
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  7. Jun 7, 2004 #6
    I will add a little to one eye's post. The so called paradox comes about because the situation starts out symmetrical. Two twins pass each other at velocity v, but neither can determine which one is moving (at this point). The argument the proceeds that as they depart each sees the other twins clock running slow. Well, that is usually based upon signals being sent from one to the other (they don't really see the other twin's clock running slow - so the statment is misleading) ... what they see is that the signals received by the other are separated by a greater period because they are receding from one another - then the jump to the paradox is made, specifically that it is impossible for both clocks to be running slow with respect to each other.

    Now Einstein predicted that if one of the clocks were decelerated and brought back (reunited with the other clock) the two clocks would not read the same. To avoid the paradox he postulated that the turn around clock had to have experienced a force (an acceleration) but the other did not - therefor since SR only deals with inertial systems (those that do not undergo acceleration) there is no paradox

    The problem however, gets more intriguing - if time dilation is real, there is no need to ponder the effect of an acceleration at the turn around point. (For example, if the outbound twin, instead of turning around and undergoing an acceleration, simply transfers his clock reading to an inbound 2nd traveler, when the 2nd traveler encounters the first twin, there will be a time discrepency. This is known as the triplet scenaro - it does not appear to be resolvable unless time dilation is actual - not apparent. As near as can be determined, Einstein considered time dilation as apparent, and therefore, to arrive at a time loss for the traveler, something else, such as acceleration, was required to avoid the paradox.
    Last edited: Jun 7, 2004
  8. Jun 7, 2004 #7


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    I was a little gradeschool pipsqueak when an older kid down the block borrowed a laser overnight from his high-school physics class. It was a large, boxy affair back in those days, not like the modern battery-powered laser pointers. I think he even had to have an extension cord running from inside his house to the laser which was mounted on a tripod. At any rate, I watched him goof around with the laser, and we got to talking about physics. He told me, as best he could, about the twin paradox. My kneejerk reaction was: ah come on, time does not work like that. They've got to be wrong.

    Humans evolved in a world where a kind of Galilean/Newtonian intuition was good enough for us to kill bison and to drive the poor sabre-toothed tiger to extinction, so I guess we ought to be pretty proud of even our naive intuition about physics. :rolleyes:
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  9. Jun 8, 2004 #8


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    Wrong, The time delay caused by the increasing distance is factored out and is not considered a part of time dilation. Time dilation is the time rate difference left over after you've accounted for said signal delay.
    Wrong, SR can deal with acceleration.
    I've already in another thread showed you how SR deals with the triplet scenerio and how it predicts a difference in the clocks, without relying on absolute time dilation.
  10. Jun 8, 2004 #9

    Well, perhaps you can help me, then: The Microsoft Encarta article on Special Relativity posits a Gedanken wherein a spaceship passes by a "stationary" observer. The article reads thus:

    "A consequence of [the constancy of c] is that there must be no such thing as absolute time either. Inside the red ship we have a clock that uses light to measure time. A pulse of light strikes a mirror, and returns to its source. The light travels .6 meters in two nanoseconds. In the astronaut's frame of reference, the pulse travels a total distance of one meter [v=240,000 km/s] along a diagonal path. The astronaut observed the light travel the same speed as always -- .3 meters per nanonsecond. According to his watch, the pulse takes 3.3 nanoseconds to make the trip. The astronaut concludes correctly that the clock on the moving ship runs slowly." ((c) 1999 Microsoft.)

    This appears to disagree with what you are saying. The Encarta article seems to say that the apparent time delay is exactly and only the result of increasing distance. (Which would make sense, since time delay is a direct result of and proportional to v.)

    Maybe I mistook you. Would you please apply yourself to this? Thanks.
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  11. Jun 8, 2004 #10
    Maybe it would be better if the term "paradox" were removed from this and another title given, like "spacetime asymmetry consequences" of the relativity theories instead. Then it would merely be a matter of experimental confirmation or disconfirmation of the theories, rather than having to carry the stigma of being some kind of dysfunction of thought.
  12. Jun 8, 2004 #11


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    Each observer will see (with his eyes) that the face of the distant clock is running at a slower rate than that of his local clock. This is the Doppler Effect. (Clock ticks separated by a lightlike vector.) I don't feel this is so paradoxical. Each observer equipped with an identically constructed light source will see the light from the other observer to be redshifted, indicating that they are separating from each other.

    What could be regarded as paradoxical (of time dilation) involves the comparison of clock ticks that are simultaneous according to an observer. (Here, the clock ticks are separated by a spacelike vector orthogonal to the observer.)

    The resolution should read
    "the turn around clock had to have experienced a force (an acceleration) but the other did not - therefore since the situation is no longer symmetrical there is no paradox"
  13. Jun 8, 2004 #12
    pmb_phy said, "That is incorrect. It is far from nonsense."

    You misread my post. What I said was that claiming GR is NECESSARY to explain the twin paradox, is nonsense. The fact that it's POSSIBLE to explain it with GR should come as no surprise (after all, GR includes SR). But it's not necessary. The apparent symmetry leading to this so called paradox is easily broken with arguments base purely on SR.

    As to the link you gave me where the paradox is explained using GR, it uses an approximation (the traveler's speed is much less than c) to arrive at exactly the result predicted using SR and ignoring the acceleration. Not too impressive!
  14. Jun 8, 2004 #13


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    Read the article again. It deals with a light beam that bounces back and forth between a lightsource and mirror that is traveling with the ship. For an observer in the ship the light travels straight back and forth. For an observer that the ship is traveling relative to, the light has to travel a diagonal path to strike the mirror and return. This has nothing to do with any increase of distance between the ship and the outside observer. It would be just as true if the distance between the ship and observer were decreasing or remained constant (such as when the ship is traveling in a circle around the observer).

    Yogi's statement implies that time dilation is the product of the increasing distance between the observers. While this effect is real, it is due to Doppler shift and not due to Relativistic time dilation. While in real life you would see a combination of both, in most Gedankens dealing with Relativity we ignore the Doppler effect because it is not the point of interest.
  15. Jun 8, 2004 #14
    Yes Janus - there is the Doppler shift - but read what I said - it is not correct to interpret the increased spacing between the received pulses as indicative of the fact that the other clock is running slow - that is the common bogus approach given by most of those who publish simple explanations - the fact of the matter is, if you simply start with the statement that two twin pass each other at velocity v, there is no basis for assuming either clock is running faster than the other - how could there be - the fallacy is in asserting that there is something left over after you have factored out the Doppler - which one is running faster???

    I would also take direct issue with you Janus on when the symmetry is broken - it is at the beginning - when one twin takes off and the distance traveled by him is measured in the earth rest frame - we now have two terms that are proper in the earth frame - the proper time as measured by an earth clock and the proper distance as measured in the earth frame to the turn around point. If we specify a third data point - e.g., the time lapsed by the clock which escorts the traveling twin when he arrives, then there is only one factor left to calculate - that is the effective distance as measured by the traveling twin. While we all agree that the interval is invarient, the space term and the time increment as measured by the traveling twin to the turn around point in the frame of the traveling twin will be different from the space term and the time increment as measured in the earth frame. The symmetry is broken as soon as you identify the proper distance that is traveled in the earth system.

    I have never seen you explain the triplet paradox - when I have raised this issue before you have not answered it directly (at least I never saw your post if you did - and the last time the subject got interesting, you locked the thread)
  16. Jun 8, 2004 #15
    Let me ad a little more - I do not now, nor have I ever considered the signaling method of resolving SR problems as being either useful or correct - I couldn't care less what the earth observer might deduce from getting a squint at a passing clock or signals sent from afar - So tell me Janus, or anyone else, will the traveling twin's clock have accumulated less time that the earth clock when the twin reaches the turn around point. If the answer is yes - there is no need to consider forces, GR, changing inertial system - whatever. The problem is resolved because time loss is actual, not apparent, and upon returning home the lost time will simply be double that accrued at the turn around post. Ok Posters - here is you chance to commit... Yes if the clock in the earth frame reads different at the time the turnaround is commenced... No if you agree with Einstein, that time dilation is apparent and some other factor(s) such as acceleration, forces, GR, different inertial system, etc must be involved to explain the time difference upon reunion.
  17. Jun 8, 2004 #16


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    Exactly right. In fact, relativistically speaking, it doesn't even make sense to say one clock is running faster than another.

    Such statements only make sense relative to a frame of reference, and there most certainly is a basis for stating that each clock is running slow in the rest frame of the other clock.

    This question does not make sense... until you specify in which reference frame you want to compare.

    The triplet paradox (that I'm familiar with) is only confusing if you've already been confused by the twin paradox and are trying to salvage the paradoxical nature. :smile:

    The asymmetry is much more apparent in the triplet "paradox"; the earthbound path consists of a single clock in a single reference frame, and the spacebound path consists of two distinct clocks in two different reference frames.
  18. Jun 8, 2004 #17


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    The turnaround twin accumulates less proper time on his total journey - out and back. Think of a spacetime diagram with time vertical and the relevant space direction horitzontal. Let it be taken relative to the nonturning twin's restframe, so his worldline will appear as a vertical line along the time-axis - conveying that he let time pass without (relative to his own rest frame) moving any distance.

    Now the worlline of the other twin will appear as two slanted lines; one going out from the origin to the turn around point, and the other slanting back from the turn around point to the time-axis at the point where the twins meet again. Together with the length on the time-axis between the start and finish events (which is the nonturning twin's worldline) , these two lines form a triangle. Now you have to know that in relativity, the direct path through time is LONGER in proper time than the indirect or partly spatial one. Just the opposite of what you would think of course, but it's a solid result of relativity. So it's the fact that the turnaround twin took a partly spacelike excursion while the other twin didn't that makes the difference in their ages, which is just the proper time that each has incurred along their respective worldlines.
  19. Jun 8, 2004 #18
    yogi said, "So tell me Janus, or anyone else, will the traveling twin's clock have accumulated less time that the earth clock when the twin reaches the turn around point."

    hurkyl replied, "This question does not make sense... until you specify in which reference frame you want to compare."

    Why? An event occurs: traveler arrives at distant planet. That event has spacetime coordinates in both frames. yogi's question is, "Are the time coordinates of that event the same in both frames, or are they different?"
  20. Jun 8, 2004 #19


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    Read the article posted by OneEye. The fact that the bouncing light as measured in the ship's frame travels a shorter distance than that as measured by the external observer, yet both observers measure the light as moving at c means that the same light takes longer to reflect back and forth as measured by the outside observer. Thus if the light takes 1 sec according to the ship's time, it takes more than one sec by the external observer's time, and time progresses more slowly for the ship as measured by this observer. None of these observations rely on Doppler shift, and still occur even if you ignore the Doppler effect.

    The reply is here. It is the reply right after it and you acknowledeged it:


    I very carefully show how, when you take all the triplet's measurements into account, they all agree as to the time difference between the Earth clock and the clock that had the time transfered to it from the outbound triplet's clock. There is no paradox, and no reason to assume absolute time.
  21. Jun 8, 2004 #20
    Thank you for all of the explanations.

    The differences of thought you guys mention in this post has caused some confusion for me.

    There are three parties of thought here right? One, are folks like Janus who sees no paradox in the “Twin Paradox” and is a firm believer in relativity. Two, are folks like Yogi who are skeptical of relativity and sees problems within the Twin Paradox. And three, are folks like Jdevel who are just trying to get a clearer picture. Am I correct on this?

    Allow me to explain my thoughts on the subject please.

    It seems simple to me. But I am thinking my logic may be way off because of the simplicity of it.

    Say a certain type of house takes 64 man hours to build. And you hire 2 workers who can each work 8 hours a day. So it will take 4 days to build the house. I am sure no one disagrees with this. Now say the house is built in 2 days. If the house is built in 2 days then something had to change. Either there were more workers added, or the 2 original workers worked more then 8 hours a day.

    That’s very simple to understand. My understanding of relativity’s time contractions is very similar to that.

    Simple explanation I have read:
    (I’m sure you guys have read this too)
    A train traveling 40 feet per second has a man on it that throws a ball 30 feet per second towards the other side of the cart he is on. The man on the train threw the ball at the precise time he passed another man standing outside of the train. The man on the train saw the ball move at 30 feet per second while the man outside the train saw the ball moving 70 feet per second.

    This is easy to understand as well. One man sees the ball flying at one speed and the other man saw the ball flying at a different speed.

    The speed of a ball is not constant. But the speed of light is constant. So if you take the same example but use a flashlight, instead of a ball, then each man will see the light move at 186,000 miles per second.

    Since the man standing outside of the train did not see the light travel at 186,000 miles per second plus 40 feet per second then something had to change (just like something had to changes with the analogy of workers building the house). Either the distance the ball traveled or the time interval.

    So if the speed of light is constant then time dilation and length contraction is neat little phenomenon.

    So why is there a problem with this phenomenon when talked about in terms of the Twin Paradox? And is there anything wrong with my logic on the examples I mentioned above?

    Thanks again,
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