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  1. #1 Twin paradox 
    Forum Freshman DevPhysics's Avatar
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    Hello everyone!
    So basically we all know that In physics, the twin paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket and returns home to find that the twin who remained on Earth has aged more.
    So I am confused about the term "aged". Is this "aged" term only the term represented by numbers? I mean, for biologically does one of the twins look older compared to the other one? Or they look the same, but they are at different ages because one of the twins were moving in a high-speed?
    Thank you and have a great day!


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    .. also, if anyone cares to respond .. a question I asked in another thread but got no reply, why does one twin enjoy the privileged position of been considered at rest (relative motion and all) ?


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    Given that the aging effect is a result of biological processes occurring over a period of time, and that time is simply distance travelled in space, the twins will age differently, yes.
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    @marcbo, could you expound a little on what you want to know exactly?
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    Quote Originally Posted by marcbo View Post
    .. also, if anyone cares to respond .. a question I asked in another thread but got no reply, why does one twin enjoy the privileged position of been considered at rest (relative motion and all) ?
    One of the twins doesn't accelerate.
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    Quote Originally Posted by marcbo View Post
    .. also, if anyone cares to respond .. a question I asked in another thread but got no reply, why does one twin enjoy the privileged position of been considered at rest (relative motion and all) ?
    Hi marcbo, sorry I couldn't understand the question. Can you please explain it a bit more?

    Quote Originally Posted by Curiosity View Post
    Given that the aging effect is a result of biological processes occurring over a period of time, and that time is simply distance travelled in space, the twins will age differently, yes.
    So you mean, moving in a high-speed can also affect biological processes and simply biological time?
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    The aging affect is accrued cellular damage in many forms. Given that when travelling at high speeds, time moves differently to that person compared with an observer. At high enough speeds, there would be a non-trivial difference in time between the person going on this high speed journey, and his twin. Hypothetically, to the traveller, 10 years passes, while back on Earth, 12 years passes. More time has elapsed for the twin on Earth, more cellular damage has built up, resulting in the observation of him having aged more.
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  9. #8  
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    Quote Originally Posted by DevPhysics View Post
    So you mean, moving in a high-speed can also affect biological processes and simply biological time?
    Huh?
    Time "affects biological time".
    Time = time.
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  10. #9  
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    Quote Originally Posted by marcbo View Post
    .. also, if anyone cares to respond .. a question I asked in another thread but got no reply, why does one twin enjoy the privileged position of been considered at rest (relative motion and all) ?
    Actually, any single inertial frame in this scenario can be considered as being at rest. You can choose either the Earth frame, the outbound leg frame or the inbound leg frame as being the rest frame. The trick is is that only the Earth twin remains in one inertial frame the entire time. His brother starts in the Earth frame, changes to the outbound frame and then changes again to the inbound frame. Put another way, this twin cannot claim to be at rest both during the outbound leg and the inbound leg. He can pick one or the other as the rest frame, but not both.

    It is this fact that the "traveling" twin changes frames that results in the differential aging. You could work this same scenario out assuming that both twins are traveling at some high fraction of c, one stops, waits for a while, and then speeds up to catch up with his twin, and you get the same result; the twin that "stopped" and then sped up again will have aged less when they meet up again. Or you could have him speed of in the direction they are both already heading, come to a "stop" and wait for the Earth to catch up, and he still will be younger.
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    Quote Originally Posted by Curiosity View Post
    @marcbo, could you expound a little on what you want to know exactly?
    OK, thanks. Two twins, accelerating away from each other. No other reference point. Neither have a privileged position. Both can say the other is younger ? I've never been able to understand this.

    Real simple language, please!

    (I read a book years ago by some proff Dingle .. 'Science at the Crossroads' was it called ? I think he made the same point, though I'm going on memory and most probably am making it quite poorly)
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    Can't seem to edit ..
    Edit: Janus, just saw your reply above .. thanks. Will read it carefully. Any furhter responses are also welcome.
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    Quote Originally Posted by marcbo View Post
    OK, thanks. Two twins, accelerating away from each other. No other reference point. Neither have a privileged position. Both can say the other is younger ? I've never been able to understand this.
    No.
    If BOTH accelerate and then return they'll be the same age as each other.
    But not the same age as someone who stayed put at their mutual start point.
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    Because the twin who stays is not accelerating to such a high degree.
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    Thanks for the above replies. I'm not trying to be wrong or right here - I'm trying to understand a point that I've enquired about many times, but have never had a satisfacttory, or at least, comprehensible (to me) answer to.

    Allow me to restate my scenario thus;

    Two twins - both accelerating relative to each other.
    No returning, no other reference point.
    According to relative motion, either can say the other is at rest OR accelerating.
    Therefore, either can say the other is younger / older.
    Which one IS younger / older ?
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  16. #15  
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    Quote Originally Posted by marcbo View Post
    Thanks for the above replies. I'm not trying to be wrong or right here - I'm trying to understand a point that I've enquired about many times, but have never had a satisfacttory, or at least, comprehensible (to me) answer to.

    Allow me to restate my scenario thus;

    Two twins - both accelerating relative to each other.
    No returning, no other reference point.
    According to relative motion, either can say the other is at rest OR accelerating.
    Therefore, either can say the other is younger / older.
    Which one IS younger / older ?
    If they both accelerate at the same rate for the same time to the same speed then they'll both age exactly the same. There will be no difference.
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    Quote Originally Posted by marcbo View Post
    Thanks for the above replies. I'm not trying to be wrong or right here - I'm trying to understand a point that I've enquired about many times, but have never had a satisfacttory, or at least, comprehensible (to me) answer to.

    Allow me to restate my scenario thus;

    Two twins - both accelerating relative to each other.
    No returning, no other reference point.
    According to relative motion, either can say the other is at rest OR accelerating.
    Therefore, either can say the other is younger / older.
    Which one IS younger / older ?
    Unless they come back together, there is no answer to this question ( or to be more precise, there are an infinite number of equally correct answers). Different reference frames will get different results as to which is older and by how much and not one of them is more correct than the other. The only way you get an answer where all reference frames agree as to the relative ages of the two is if you bring them back together at the end.

    Unless the twins are separated and then rejoined, there is no one answer to this question.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Quote Originally Posted by Dywyddyr View Post
    If they both accelerate at the same rate for the same time to the same speed then they'll both age exactly the same. There will be no difference.
    At the risk of being pecked... Aren't they the same age regardless? I mean just because your cells breakdown faster than the other doesn't mean you've lived less time, does it? (Molecular breakdown doesn't necessarily = age) One twin lives 85 Earth years and the other lives 360 Earth years because of the speed he is traveling. If the the other twin didn't die at year 85 he would also be 360 years old. So, how are they older and younger than each other?

    So. I'm older than my buddy that died 6 years ago even though we were born on the exact same date and the exact same time?
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    Time will pass more slowly for the one travelling at a higher rate. The passage of time is different for the twins. While from a certain point of view, you could say the travelling twin is 360 earth years old, that is inaccurate, because he hasn't experienced 360 years worth of time, only 85.
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  20. #19  
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    Quote Originally Posted by grmpysmrf View Post
    Quote Originally Posted by Dywyddyr View Post
    If they both accelerate at the same rate for the same time to the same speed then they'll both age exactly the same. There will be no difference.
    At the risk of being pecked... Aren't they the same age regardless? I mean just because your cells breakdown faster than the other doesn't mean you've lived less time, does it? (Molecular breakdown doesn't necessarily = age) One twin lives 85 Earth years and the other lives 360 Earth years because of the speed he is traveling. If the the other twin didn't die at year 85 he would also be 360 years old. So, how are they older and younger than each other?

    So. I'm older than my buddy that died 6 years ago even though we were born on the exact same date and the exact same time?
    For one time itself passes more slowly.
    They'll both have the same "calendar age" but ONLY if you look at one calendar.
    Their calendars (and clocks) will disagree.
    One will be physically younger because his calendar will say that he's only lived 85 years, not 360.
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    Quote Originally Posted by Janus View Post
    Quote Originally Posted by marcbo View Post
    Thanks for the above replies. I'm not trying to be wrong or right here - I'm trying to understand a point that I've enquired about many times, but have never had a satisfacttory, or at least, comprehensible (to me) answer to.

    Allow me to restate my scenario thus;

    Two twins - both accelerating relative to each other.
    No returning, no other reference point.
    According to relative motion, either can say the other is at rest OR accelerating.
    Therefore, either can say the other is younger / older.
    Which one IS younger / older ?
    Unless they come back together, there is no answer to this question ( or to be more precise, there are an infinite number of equally correct answers). Different reference frames will get different results as to which is older and by how much and not one of them is more correct than the other. The only way you get an answer where all reference frames agree as to the relative ages of the two is if you bring them back together at the end.

    Unless the twins are separated and then rejoined, there is no one answer to this question.
    Firstly, to the OP Devphysics, apologies if I may have sidetracked your thread. If you feel I have, please say so and I'll try to take this elsewhere.

    Next, to the other above posters who have replied, thanks.

    But Janus, I'll reply to yours above, as it seems to go closest to my question - not that the others haven't, they probably have, but I seem to 'grok' with yours the most.

    Your 1st comment fascinates me .. Unless they come back together, there is no answer to this question...

    Please allow me to press my question further.

    They never come back together. However, they have means to instantaneous communication - spooky action at a distance .. quantum communication, which I just recently learnt by coincidence in another thread, seems to be a reality;

    Human Consciousness Video Neuroscience Debate The Other Side

    Breakthrough in quantum communication -- ScienceDaily

    Even if such science is in a nascent stage at the moment, let's assume that it is developed sufficiently in the future, for the twins to communicate (heck, 150 years ago, we were at the horse and buggy stage, but now lookk up .. there's the International Space Station).

    So, at some point, during the continuing acceleration, one twin instantaneously communicates with the other, and compares ages. Time dilation, not being a hypothetical but a real thing, MUST have occured up until that point, right ?

    So which twin can claim to be the older or younger ?
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    It's okay.
    Well for your last question, I am not pretty sure but you know time dilation also depends on the velocity of the object. The faster you go, the slower the time will go. So I would say, If we know which twin travels faster, we can also know which twin is younger.
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    Quote Originally Posted by DevPhysics View Post
    It's okay.
    Ta.

    Well for you last question, I am not pretty sure but you know time dilation also depends on the velocity of the object. The faster you go, the slower the time will go. So I would say, If we know which twin travels faster, he/she will be younger. And you don't need a reference like earth(doesn't move). If you take one twin as a reference, the faster one will always be the faster one. This means he/she is moving in a velocity that is closer to the speed of light(c). and For him/her the time will go slower.
    (my opinion)
    I think I've canvassed these. Janus seems to have had a good handle on my question. I'm looking forward to his reply.
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    You can ignore the last phrase. I also edited my post. But simply, I think if you know which twin is faster, then he/she will be younger.
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    Insanity: doing the same thing over and over again and expecting different results.
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    Quote Originally Posted by DevPhysics View Post
    You can ignore the last phrase. I also edited my post. But simply, I think if you know which twin is faster, then he/she will be younger.
    OK.
    I don't know which twin is 'faster'. Neither do they. See all my comments, above. Let's see what Janus says.
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  26. #25  
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    Quote Originally Posted by marcbo View Post
    Quote Originally Posted by Janus View Post
    Quote Originally Posted by marcbo View Post
    Thanks for the above replies. I'm not trying to be wrong or right here - I'm trying to understand a point that I've enquired about many times, but have never had a satisfacttory, or at least, comprehensible (to me) answer to.

    Allow me to restate my scenario thus;

    Two twins - both accelerating relative to each other.
    No returning, no other reference point.
    According to relative motion, either can say the other is at rest OR accelerating.
    Therefore, either can say the other is younger / older.
    Which one IS younger / older ?
    Unless they come back together, there is no answer to this question ( or to be more precise, there are an infinite number of equally correct answers). Different reference frames will get different results as to which is older and by how much and not one of them is more correct than the other. The only way you get an answer where all reference frames agree as to the relative ages of the two is if you bring them back together at the end.

    Unless the twins are separated and then rejoined, there is no one answer to this question.
    Firstly, to the OP Devphysics, apologies if I may have sidetracked your thread. If you feel I have, please say so and I'll try to take this elsewhere.

    Next, to the other above posters who have replied, thanks.

    But Janus, I'll reply to yours above, as it seems to go closest to my question - not that the others haven't, they probably have, but I seem to 'grok' with yours the most.

    Your 1st comment fascinates me .. Unless they come back together, there is no answer to this question...

    Please allow me to press my question further.

    They never come back together. However, they have means to instantaneous communication - spooky action at a distance .. quantum communication, which I just recently learnt by coincidence in another thread, seems to be a reality;

    Human Consciousness Video Neuroscience Debate The Other Side

    Breakthrough in quantum communication -- ScienceDaily

    Even if such science is in a nascent stage at the moment, let's assume that it is developed sufficiently in the future, for the twins to communicate (heck, 150 years ago, we were at the horse and buggy stage, but now lookk up .. there's the International Space Station).

    So, at some point, during the continuing acceleration, one twin instantaneously communicates with the other, and compares ages. Time dilation, not being a hypothetical but a real thing, MUST have occured up until that point, right ?

    So which twin can claim to be the older or younger ?
    Quantum entanglement cannot be used to transfer information at greater than c speeds. I know that many popular science articles tend to paint things as if it can, but be assured, if it were true or even the scientists working with it thought it even theoretically could, it would have made bigger headlines than the recent neutrino flap.

    If you really want to grasp what SR is all about, then you need to forget about schemes about instantaneous communication. Instantaneous communication leads to paradoxes in SR. (For one, it would allow you to receive a reply to a transmission before you sent the transmission.)

    You see, not only do frames in relative motion not agree on time rates, they don't always agree on what "instantaneous" is. There is another aspect of SR we have not yet discussed, it is the relativity of simultaneity. The two frames will not agree on which events are simultaneous.

    To illustrate we'll consider the following scenario:

    You have two frames in relative motion with respect to each other. One has two clocks separated by some distance along the line of relative motion. In this frame the clocks are synchronized to each other and always read the same. Now if we consider what the other frame says about these clocks we find that it will not agree that the two clocks are synchronized. The one clock will be ahead of the other. So for example, if the motion of the other frame is from left to right with respect to him, the clock to the left will be ahead of the other clock to the right. The greater the distance between the clocks, the greater the difference between the clock readings.

    So now consider what happens if one clock tries to send an "instantaneous" signal to the other clock. In this frame, the signal leaves and arrives when the clocks read the same no matter which direction the signal is sent. But what happens in the other frame? Does this frame also consider the transmission as instantaneous? If so, then then a signal sent from the right clock to the left clock leaves the left clock when it reads one time and arrives at the right clock when it reads some later time. A signal sent from left to right, arrives at the right clock when it reads some earlier time. Now this disagrees with what the clock frame says happens. So if we try to consider the signal as instantaneous in both frames, the frames disagree as to what the clocks read when the signals were sent and received. This makes no sense. If we were to give an observer in each frame the ability to physically record the events and then bring these records together in one frame, you would end up with two contradictory physical records of the same events.

    So instead, let's assume that the two frames both agree that both clocks have the same reading at transmission and reception. This eliminates the contradictory records when they meet up at the end, however it opens a new can of worms. In the second frame, one clock reads earlier than the other. For example if clock A reads 12:00, clock B reads 12:05. if a signal is sent from A to B when it reads 12:00, and must arrive at Clock B when it reads 12:00, then in the second frame, it must arrive at Clock B when clock A reads 11:55, before it was sent!.

    In either case we get a paradox when we try and apply instantaneous communication within SR.

    Now I know that the above can sound quite bizarre. The idea that something that whether or not two events happen at the "same time" is not as straight forward as one would expect can be a hard pill to swallow. We are somewhat conditioned by our environment to consider simultaneity as an absolute. But this is only because we are only used to dealing with relative speeds where the fact that it isn't is not noticeable. Because of this absolute simultaneity is more or less our "default".

    You might want to check out the Special Relativity Primer sticky. It goes into a little deeper into how SR builds to these conclusions from its original postulates.
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by grmpysmrf View Post
    Quote Originally Posted by Dywyddyr View Post
    If they both accelerate at the same rate for the same time to the same speed then they'll both age exactly the same. There will be no difference.
    At the risk of being pecked... Aren't they the same age regardless? I mean just because your cells breakdown faster than the other doesn't mean you've lived less time, does it? (Molecular breakdown doesn't necessarily = age) One twin lives 85 Earth years and the other lives 360 Earth years because of the speed he is traveling. If the the other twin didn't die at year 85 he would also be 360 years old. So, how are they older and younger than each other?

    So. I'm older than my buddy that died 6 years ago even though we were born on the exact same date and the exact same time?
    For one time itself passes more slowly.
    They'll both have the same "calendar age" but ONLY if you look at one calendar.
    Their calendars (and clocks) will disagree.
    One will be physically younger because his calendar will say that he's only lived 85 years, not 360.
    Thanks
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    Janus #25, thanks. Will consider and reply.
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    Quote Originally Posted by marcbo View Post
    Janus #25, thanks. Will consider and reply.


    As Janus has stated, while the twins observe each other, each perceives the other frame (observer, clock, etc) to function at a slower rate. This is the principle of relativity that produces symmetrical or reciprocal observations, as a result of relative speed.
    Based on the relation t=x/v, a faster speed results in less time for a given distance. Consider a Minkowski diagram with anaut A moving at a constant speed a. If anaut B moves away and returns, His speed profile (which is what the diagram plots) will be longer. If you avoid viewing the diagram in a ‘road map’ context, the faster leg loses more time than is gained by the slower leg, relative to the A clock, since the time dilation is not a linear function. The accumulated time for the closed trip determines the aging. This requires a comparison of clocks at a common site.
    The simple ‘twin’ example only appears to depend on acceleration, but when examined in detail, time dilation is a function of speed, v/c, as expressed in the gamma factor. For the general case, both twins could accelerate for different time periods at various speeds, which would require integration of the curved speed profile.
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    Hi Janus .. apologies for the late reply and thanks for making this effort to explain this to me. Unfortunately, your explanation, though rudementry to most here, goes well beyond my personal grasp of the matter. Nevertheless, I feel I should be able to understand - at my level.

    I stated;

    Two twins - both accelerating relative to each other.
    No returning, no other reference point.
    According to relative motion, either can say the other is at rest OR accelerating.
    Therefore, either can say the other is younger / older.
    Which one IS younger / older


    You did not deny the physics of that so far, but replied ..

    Unless they come back together, there is no answer to this question

    Come back together ? To compare clocks, surely.

    But then I proposed a means by which the clocks can be compared with each other, without them physically coming back together. You would not deny that as the twins were moving away from each other for an appreciable time, the time dilation effect had happened, was happening, and would continue to happen. So it's not as though it's some hypothetical thought bubble - it was a real happening, according to physics.

    Now, of course, I did read carefully what you said about the dubious nature of quantum entanglement. It does not seem to be so dubious. The ScienceDaily article I linked, seemed to suggest it was all but a reality - even if nascent. And I don't think that ScienceDaily is what would be considered as woo 'round here, is it ? Additional research also shows many similar reports from other respected, or in the least, non woo sites.

    You then spoke of .. leading to paradoxes .. etc. Well, OK, but still, that is no proscription to considering the matter. I'm sure physicis, scientists, quantum physicists, cosmologists, et al, consider at least a dozen paradoxes and absurdities before breakfast :-)

    So, I am still quite curious to know if there is answer to my question, without the need to bring in all the other stuff that makes me feel like just walking away from the question, frustrated once again.
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    Quantum communications leap out of the lab : Nature News & Comment

    Janus, another very interesting report on 'spooky action at a distance'

    I also wanted to add, if you still believe it to be 'pie in the sky', fair enough.
    But can you then hypothesise what the answer to my question might be ? Like, as in a thought experiment ?

    Cheers!
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    Quote Originally Posted by marcbo View Post
    Hi Janus .. apologies for the late reply and thanks for making this effort to explain this to me. Unfortunately, your explanation, though rudementry to most here, goes well beyond my personal grasp of the matter. Nevertheless, I feel I should be able to understand - at my level.

    I stated;

    Two twins - both accelerating relative to each other.
    No returning, no other reference point.
    According to relative motion, either can say the other is at rest OR accelerating.
    Therefore, either can say the other is younger / older.
    Which one IS younger / older


    You did not deny the physics of that so far, but replied ..

    Unless they come back together, there is no answer to this question

    Come back together ? To compare clocks, surely.

    But then I proposed a means by which the clocks can be compared with each other, without them physically coming back together. You would not deny that as the twins were moving away from each other for an appreciable time, the time dilation effect had happened, was happening, and would continue to happen. So it's not as though it's some hypothetical thought bubble - it was a real happening, according to physics.

    Now, of course, I did read carefully what you said about the dubious nature of quantum entanglement. It does not seem to be so dubious. The ScienceDaily article I linked, seemed to suggest it was all but a reality - even if nascent. And I don't think that ScienceDaily is what would be considered as woo 'round here, is it ? Additional research also shows many similar reports from other respected, or in the least, non woo sites.

    You then spoke of .. leading to paradoxes .. etc. Well, OK, but still, that is no proscription to considering the matter. I'm sure physicis, scientists, quantum physicists, cosmologists, et al, consider at least a dozen paradoxes and absurdities before breakfast :-)

    So, I am still quite curious to know if there is answer to my question, without the need to bring in all the other stuff that makes me feel like just walking away from the question, frustrated once again.
    Yes, you can do the experiment you are proposing. But you need to do it as follows:

    Have the traveling twin accelerate from rest from a marker on the ground where you have a clock. The ground clock , clock1, and the traveling twin clock, CLOCK, start at the time the twin starts accelerating.
    Some distance away, there is another clock, clock2, on the ground, synchronized with clock1. When the traveling twin approaches the second marker , it starts decelerating and it stops right next to it. Thus, we can compare the time on the clock CLOCK with the time . You will find that , where is the speed of the traveling twin wrt. the ground. The exact calculations can be found here.
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    Quote Originally Posted by marcbo View Post
    Quantum communications leap out of the lab : Nature News & Comment

    Janus, another very interesting report on 'spooky action at a distance'
    Note that quantum entanglement (aka 'spooky action at a distance', even though there is nothing spooky about it; nor any action...) is a real thing. This article is about using it for securing communications (which still take place at light speed or lower).
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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    Quote Originally Posted by marcbo View Post
    Two twins - both accelerating relative to each other.
    No returning, no other reference point.
    According to relative motion, either can say the other is at rest OR accelerating.
    Therefore, either can say the other is younger / older.
    Which one IS younger / older
    Note that speed is relative, but acceleration isn't. You never tell the difference between you moving at a constant velocity and your twin remaining stationary, or vice versa (or any combination).

    But you can tell if you are accelerating or not. You will not be pressed back into your seat when your twin puts his foot down. This means it is possible to define, objectively, which twin experiences less time in their journey.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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    [QUOTE=xyzt;577103]
    Quote Originally Posted by marcbo View Post

    Yes, you can do the experiment you are proposing. But you need to do it as follows:

    Have the traveling twin accelerate from rest from a marker on the ground where you have a clock. The ground clock , clock1, and the traveling twin clock, CLOCK, start at the time the twin starts accelerating.
    Some distance away, there is another clock, clock2, on the ground, synchronized with clock1. When the traveling twin approaches the second marker , it starts decelerating and it stops right next to it. Thus, we can compare the time on the clock CLOCK with the time . You will find that , where is the speed of the traveling twin wrt. the ground. The exact calculations can be found here.
    Hi xytz. (bolded) That's not the experiment I am proposing.
    The experiment I am proposing is;

    Two twins - both accelerating relative to each other.
    No returning, no other reference point.


    Thanks for your contribution.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by marcbo View Post
    Quantum communications leap out of the lab : Nature News & Comment

    Janus, another very interesting report on 'spooky action at a distance'
    Note that quantum entanglement (aka 'spooky action at a distance', even though there is nothing spooky about it; nor any action...) is a real thing.
    Yes, agree it is a real thing. That may sooner or later, be developed for instantaneous communication.

    This article is about using it for securing communications (which still take place at light speed or lower).
    Yes, I mistook the article for something similar to the previous article I linked. This one is about something different altogether. Thanks.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by marcbo View Post
    Two twins - both accelerating relative to each other.
    No returning, no other reference point.
    According to relative motion, either can say the other is at rest OR accelerating.
    Therefore, either can say the other is younger / older.
    Which one IS younger / older
    Note that speed is relative, but acceleration isn't. You never tell the difference between you moving at a constant velocity and your twin remaining stationary, or vice versa (or any combination).
    Yes, agree.


    But you can tell if you are accelerating or not. You will not be pressed back into your seat when your twin puts his foot down. This means it is possible to define, objectively, which twin experiences less time in their journey.
    Yes, agree.

    Now, recall the conversation you and I and others had in another thread, about me driving up the freeway and observing a pendulum move slightly towards the rear of the car. I was there told, in no uncertain terms, that such movement is only momentary, and that the pendulum would return to it's initial position (I can go find the tread if you like). Eventually we had consensus on this.

    So why would it be any different for our twins - particularly in a sealed space ship ?

    In any case, for the purposes of my thought experimet, we can add..

    - The twins are asleep, therefore do not sense any such effect.
    - There is no .. umm, what's it called .. accelaremeter on board.

    Both real, physical possibilities. What then ?

    BTW, thanks for keeping your reply at my level. I hope you continue to do this.
    Last edited by marcbo; July 2nd, 2014 at 09:31 PM. Reason: spell
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    [QUOTE=marcbo;577237]
    Quote Originally Posted by xyzt View Post
    Quote Originally Posted by marcbo View Post

    Yes, you can do the experiment you are proposing. But you need to do it as follows:

    Have the traveling twin accelerate from rest from a marker on the ground where you have a clock. The ground clock , clock1, and the traveling twin clock, CLOCK, start at the time the twin starts accelerating.
    Some distance away, there is another clock, clock2, on the ground, synchronized with clock1. When the traveling twin approaches the second marker , it starts decelerating and it stops right next to it. Thus, we can compare the time on the clock CLOCK with the time . You will find that , where is the speed of the traveling twin wrt. the ground. The exact calculations can be found here.
    Hi xytz. (bolded) That's not the experiment I am proposing.
    The experiment I am proposing is;

    Two twins - both accelerating relative to each other.
    No returning, no other reference point.


    Thanks for your contribution.
    Then, your experiment is nonsense since the twins cannot compare clocks. You are wasting your time making up BS.
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    Quote Originally Posted by marcbo View Post
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by marcbo View Post
    Two twins - both accelerating relative to each other.
    No returning, no other reference point.
    According to relative motion, either can say the other is at rest OR accelerating.
    Therefore, either can say the other is younger / older.
    Which one IS younger / older
    Note that speed is relative, but acceleration isn't. You never tell the difference between you moving at a constant velocity and your twin remaining stationary, or vice versa (or any combination).
    Yes, agree.


    But you can tell if you are accelerating or not. You will not be pressed back into your seat when your twin puts his foot down. This means it is possible to define, objectively, which twin experiences less time in their journey.
    Yes, agree.

    Now, recall the conversation you and I and others had in another thread, about me driving up the freeway and observing a pendulum move slightly towards the rear of the car. I was there told, in no uncertain terms, that such movement is only momentary, and that the pendulum would return to it's initial position (I can go find the tread if you like). Eventually we had consensus on this.

    So why would it be any different for our twins - particularly in a sealed space ship ?
    It is not different, it is exactly the same, the pendulum in the accelerated car will measure FEWER ticks than the one on the grouns. We don't use pendulums for measuring time in the case of the Twins Paradox, we use atomic clocks. The outcome is always the same, the non-inertial twin accumulates less time than the inertial one.
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    Quote Originally Posted by marcbo View Post
    But you can tell if you are accelerating or not. You will not be pressed back into your seat when your twin puts his foot down. This means it is possible to define, objectively, which twin experiences less time in their journey.
    Yes, agree.

    Now, recall the conversation you and I and others had in another thread, about me driving up the freeway and observing a pendulum move slightly towards the rear of the car. I was there told, in no uncertain terms, that such movement is only momentary, and that the pendulum would return to it's initial position (I can go find the tread if you like). Eventually we had consensus on this.
    Go back and read the relevant thread/ posts.
    The pendulum only returns to its initial position when acceleration stops.
    I.e. when the car settles down to a constant speed - zero acceleration.
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    Quote Originally Posted by marcbo View Post
    Yes, agree it is a real thing. That may sooner or later, be developed for instantaneous communication.
    No. Because there is no communication involved. You might as well say that one day the colour green will be developed for instantaneous communication. It makes as much sense.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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    Quote Originally Posted by marcbo View Post
    ...
    In any case, for the purposes of my thought experimet, we can add..

    - The twins are asleep, therefore do not sense any such effect.
    - There is no .. umm, what's it called .. accelaremeter on board.

    Both real, physical possibilities. What then ?
    ...
    If you fall asleep in a bus going from Frankfurt to Dresden, you still get to Dresden whether you were awake for the journey or not.

    Having sleeping twins and a lack of accelerometers in your example makes no difference to the physical facts of what occurs.

    If one of them accelerated and the other did not, then regardless of whether they were aware of it or not, regardless of whether they measured it or not, then the situation is not symmetrical.
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    Quote Originally Posted by pzkpfw View Post
    If you fall asleep in a bus going from Frankfurt to Dresden, you still get to Dresden whether you were awake for the journey or not.
    Unless you're the driver...
    "[Dywyddyr] makes a grumpy bastard like me seem like a happy go lucky scamp" - PhDemon
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    Quote Originally Posted by marcbo View Post
    Hi Janus .. apologies for the late reply and thanks for making this effort to explain this to me. Unfortunately, your explanation, though rudementry to most here, goes well beyond my personal grasp of the matter. Nevertheless, I feel I should be able to understand - at my level.

    I stated;

    Two twins - both accelerating relative to each other.
    No returning, no other reference point.
    According to relative motion, either can say the other is at rest OR accelerating.
    Therefore, either can say the other is younger / older.
    Which one IS younger / older


    You did not deny the physics of that so far, but replied ..

    Unless they come back together, there is no answer to this question

    Come back together ? To compare clocks, surely.

    But then I proposed a means by which the clocks can be compared with each other, without them physically coming back together. You would not deny that as the twins were moving away from each other for an appreciable time, the time dilation effect had happened, was happening, and would continue to happen. So it's not as though it's some hypothetical thought bubble - it was a real happening, according to physics.

    Now, of course, I did read carefully what you said about the dubious nature of quantum entanglement. It does not seem to be so dubious. The ScienceDaily article I linked, seemed to suggest it was all but a reality - even if nascent. And I don't think that ScienceDaily is what would be considered as woo 'round here, is it ? Additional research also shows many similar reports from other respected, or in the least, non woo sites.

    You then spoke of .. leading to paradoxes .. etc. Well, OK, but still, that is no proscription to considering the matter. I'm sure physicis, scientists, quantum physicists, cosmologists, et al, consider at least a dozen paradoxes and absurdities before breakfast :-)

    So, I am still quite curious to know if there is answer to my question, without the need to bring in all the other stuff that makes me feel like just walking away from the question, frustrated once again.
    First off, I'm going to reiterate what others have said about Quantum entanglement: While is is a real effect, its use in communication is in encryption. It cannot even theoretically be used for instantaneous communication.

    As far as the paradoxes that arise if you assume it under SR: What this entails is that if you try to apply instantaneous communication within SR you get mutually exclusive answers. If you try to assume instantaneous communication in both frames, the two frames get different answers as to which twin is older than the other.

    I'll try and paint a picture as to how this works with an analogy.

    Imagine a featureless plane, upon which we have a number of people facing in different directions. We will assume that everyone has a perfect sense of direction. If you asked these people to point North, they all, regardless of which way they are facing, point in the same direction. If you ask them the Relative position of Sam and Tom, they all give the same answer. (for instance Tom is 4 feet to the South and 3 ft to the West of Sam. This is like how we viewed time and space before Relativity. If we replace North-South, with past and future and East and West as spatial separation in one dimension (For simplicities sake we lose two spatial directions. The different directions the people face are the different speeds you can travel. In this case, Tom and Sam represent events that have occurred at some place and time. Again, if you ask anyone moving at the various different speeds how far apart two events are, they all agree. They would give answer like " 4 ft apart and event "Tom" occurred 3 sec after event "Sam". There is an absoluteness to this answer that everyone can agree on.

    Okay, same plane, same people, However, no more perfect sense of direction. In fact, the very concepts of North, South, East, West have no meaning. Instead, the only concept of direction that applies is back-front, left-right, and even those depend on the individual. So you have all these people facing in different relative directions and you ask them to point left, they will all point to their own left and as a result point in different directions. If you ask them the relative positions of Tom and Sam, you will get different answers. Some would say that Sam was directly to the left of Tom, others would say that Sam wasn't quite that far to the left of Sam, and was a little ahead of Sam, etc.

    This represents the picture we have of space and time since the advent of Relativity. Here left-right represent spatial separation while front and back represents time. The difference here is that while in our first example, time and space had different "directions" that were absolute and fixed, in this example, the "directions" of time and space have no meaning beyond that supplied by each observer. Each of our "people" has their own way of measuring time and space. In this case if you ask people moving at different speeds how far apart two events are, you will get different answers. One could say that event "Tom" and event "Sam" took place a given distance from each other and at the same time, while someone else would say that they took place closer together in distance but "Tom" happened before "Sam". Neither of them would be more right than the other because everyone's perspective of time and space measurement is just as valid as that of anyone else.

    In these terms, you question is something akin to this:

    Tom and Sam are standing apart and facing in directions 90 degrees apart. Is Sam behind Tom or is Tom behind Sam? Sam says that Tom is behind him, and Tom says that Sam is the one that is behind. Who is right? There is no absolute answer to this question, because each is right by his own measurement. Even adding a third observer doesn't help, because the answer he will give will depend on the direction he is facing.

    This is the crux behind SR, time dilation is not due to some mechanical influence that effect objects that move, it is due to the fact that reference frames in motion with respect to each other measure time and space differently.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Quote Originally Posted by pzkpfw View Post
    Quote Originally Posted by marcbo View Post
    ...
    In any case, for the purposes of my thought experimet, we can add..

    - The twins are asleep, therefore do not sense any such effect.
    - There is no .. umm, what's it called .. accelaremeter on board.

    Both real, physical possibilities. What then ?
    ...
    If you fall asleep in a bus going from Frankfurt to Dresden, you ....
    ....end up like Emil, from Kastner's "Emil and the detectives".
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by marcbo View Post
    But you can tell if you are accelerating or not. You will not be pressed back into your seat when your twin puts his foot down. This means it is possible to define, objectively, which twin experiences less time in their journey.
    Yes, agree.

    Now, recall the conversation you and I and others had in another thread, about me driving up the freeway and observing a pendulum move slightly towards the rear of the car. I was there told, in no uncertain terms, that such movement is only momentary, and that the pendulum would return to it's initial position (I can go find the tread if you like). Eventually we had consensus on this.
    Go back and read the relevant thread/ posts.
    The pendulum only returns to its initial position when acceleration stops.
    I.e. when the car settles down to a constant speed - zero acceleration.
    I did. You're right. Thanks.
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    Quote Originally Posted by pzkpfw View Post
    Quote Originally Posted by marcbo View Post
    ...
    In any case, for the purposes of my thought experimet, we can add..

    - The twins are asleep, therefore do not sense any such effect.
    - There is no .. umm, what's it called .. accelaremeter on board.

    Both real, physical possibilities. What then ?
    ...
    If you fall asleep in a bus going from Frankfurt to Dresden, you still get to Dresden whether you were awake for the journey or not.
    I never said or implied otherwise.

    Having sleeping twins and a lack of accelerometers in your example makes no difference to the physical facts of what occurs.
    I never said or implied otherwise. I brought these things up in response to another posters comments.

    If one of them accelerated and the other did not, then regardless of whether they were aware of it or not, regardless of whether they measured it or not, then the situation is not symmetrical.
    Agree. And one would be older / younger than the other, yes ?
    Last edited by marcbo; July 3rd, 2014 at 10:00 PM. Reason: format
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by marcbo View Post
    Yes, agree it is a real thing. That may sooner or later, be developed for instantaneous communication.
    No. Because there is no communication involved. You might as well say that one day the colour green will be developed for instantaneous communication. It makes as much sense.
    I hear what you say. But never say never.

    There's a lot of speculation / experimenting out there, as some research shows ..


    Physics News :: Quantum "spooky action at a distance" travels at least 10,000 times faster than light
    Quantum "spooky action at a distance" travels at least 10,000 times faster than light


    Quantum entanglement - Wikipedia, the free encyclopedia
    Special Theory of Relativity - Another theory explains quantum entanglement using special relativity.[29] According to this theory, faster-than-light communication between entangled systems can be achieved because the time dilation of special relativity allows time to stand still in light's point of view. For example, in the case of two entangled photons, a measurement made on one photon at present time would determine the state of the photon for both the present and past at the same moment. This leads to the instantaneous determination of the state of the other photon. Corresponding logic is applied to explain entangled systems, i.e. electron and positron, that travel below the speed of light.

    Anyhow, I get the message. See my reply to Janus following.

    But, I also cannot escape the message that todays magic, can be tomorrows science / reality. And it seems there's a lot of 'toil and trouble' on this one.
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    Hi Janus. Thanks for this - I really appreciate the effort you go to, and your non antagonistic, non 'holier than thou' tone.

    Quote Originally Posted by Janus View Post
    First off, I'm going to reiterate what others have said about Quantum entanglement: While is is a real effect, its use in communication is in encryption. It cannot even theoretically be used for instantaneous communication.
    Yes, to date. Who knows ? See above reply to Strange.

    As far as the paradoxes that arise if you assume it under SR: What this entails is that if you try to apply instantaneous communication within SR you get mutually exclusive answers. If you try to assume instantaneous communication in both frames, the two frames get different answers as to which twin is older than the other.
    I have bolded and underlined the above, because there, RIGHT THERE, is the answer I've been seeking all these posts. It's that simple. And it means that if instant communication does become a reality (impossible in most scientists minds, I know) it disproves relativity.

    I'll try and paint a picture as to how this works with an analogy.
    Imagine a featureless plane, upon which we have a number of people facing in different directions. We will assume that everyone has a perfect sense of direction. If you asked these people to point North, they all, regardless of which way they are facing, point in the same direction. If you ask them the Relative position of Sam and Tom, they all give the same answer. (for instance Tom is 4 feet to the South and 3 ft to the West of Sam. This is like how we viewed time and space before Relativity. If we replace North-South, with past and future and East and West as spatial separation in one dimension (For simplicities sake we lose two spatial directions. The different directions the people face are the different speeds you can travel. In this case, Tom and Sam represent events that have occurred at some place and time. Again, if you ask anyone moving at the various different speeds how far apart two events are, they all agree. They would give answer like " 4 ft apart and event "Tom" occurred 3 sec after event "Sam". There is an absoluteness to this answer that everyone can agree on.

    Okay, same plane, same people, However, no more perfect sense of direction. In fact, the very concepts of North, South, East, West have no meaning. Instead, the only concept of direction that applies is back-front, left-right, and even those depend on the individual. So you have all these people facing in different relative directions and you ask them to point left, they will all point to their own left and as a result point in different directions. If you ask them the relative positions of Tom and Sam, you will get different answers. Some would say that Sam was directly to the left of Tom, others would say that Sam wasn't quite that far to the left of Sam, and was a little ahead of Sam, etc.

    This represents the picture we have of space and time since the advent of Relativity. Here left-right represent spatial separation while front and back represents time. The difference here is that while in our first example, time and space had different "directions" that were absolute and fixed, in this example, the "directions" of time and space have no meaning beyond that supplied by each observer. Each of our "people" has their own way of measuring time and space. In this case if you ask people moving at different speeds how far apart two events are, you will get different answers. One could say that event "Tom" and event "Sam" took place a given distance from each other and at the same time, while someone else would say that they took place closer together in distance but "Tom" happened before "Sam". Neither of them would be more right than the other because everyone's perspective of time and space measurement is just as valid as that of anyone else.
    These are great analogies, and I actually understood them. It almost seems we were better off in the former scenario rather than the latter. But yes, we're in the latter.

    In these terms, you question is something akin to this:

    Tom and Sam are standing apart and facing in directions 90 degrees apart. Is Sam behind Tom or is Tom behind Sam? Sam says that Tom is behind him, and Tom says that Sam is the one that is behind. Who is right? There is no absolute answer to this question, because each is right by his own measurement. Even adding a third observer doesn't help, because the answer he will give will depend on the direction he is facing.
    Yes, I understand.

    This is the crux behind SR, time dilation is not due to some mechanical influence that effect objects that move, it is due to the fact that reference frames in motion with respect to each other measure time and space differently.
    So there is no physical / mechanical influence / force that actually physically causes time dilation ?
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    Quote Originally Posted by marcbo View Post

    So there is no physical / mechanical influence / force that actually physically causes time dilation ?
    Time dilation/length contraction are projective effects, they happen when we project time/length from one frame onto another.
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