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  1. #1 twins paradox? 
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    I am having an issue understanding the twins paradox and time dilation. If someone could point out where I have gone wrong it would be appreciated.

    A light clock is often used to describe time dilation. It consists of a pulse of light bouncing between two mirrors, every time it does a round trip a second passes. Therefore a light clock that is in motion relative to an observer will tick slower than a stationary one, apparently due to the photons within the clock having a greater distance to travel through the clock.

    So lets imagine the twins paradox occurring and each twin is given a light clock.

    Before the Space Twin departs, both twins synchronise clocks.

    The Space Twin then takes off, whizzes around at close to the speed of light and then returns to earth to compare the difference on their clocks.

    Earth Twin says "100 ticks passed for me because I was looking at my clock the whole time and could see that the photons were travelling the optimum route through the clock at speed C"

    Space Twin says "100 ticks passed for me because I was looking at my clock the whole time and could see that the photons were travelling the optimum route through the clock at speed C"

    I don't see how this is consistent with the twins paradox, or the nature of light clocks slowing down due to time dilation.


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  3. #2 Re: twins paradox? 
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    Quote Originally Posted by Monkey.Man
    I am having an issue understanding the twins paradox and time dilation. If someone could point out where I have gone wrong it would be appreciated.

    A light clock is often used to describe time dilation. It consists of a pulse of light bouncing between two mirrors, every time it does a round trip a second passes. Therefore a light clock that is in motion relative to an observer will tick slower than a stationary one, apparently due to the photons within the clock having a greater distance to travel through the clock.

    So lets imagine the twins paradox occurring and each twin is given a light clock.

    Before the Space Twin departs, both twins synchronise clocks.

    The Space Twin then takes off, whizzes around at close to the speed of light and then returns to earth to compare the difference on their clocks.

    Earth Twin says "100 ticks passed for me because I was looking at my clock the whole time and could see that the photons were travelling the optimum route through the clock at speed C"

    Space Twin says "100 ticks passed for me because I was looking at my clock the whole time and could see that the photons were travelling the optimum route through the clock at speed C"

    I don't see how this is consistent with the twins paradox, or the nature of light clocks slowing down due to time dilation.
    Both twins do not count the same number of light ticks.

    The traveling twin sees fewer ticks.

    One simple resolution of the twin paradox is that the traveling twin accelerates, therefore does not represent an inertial reference frame and hence the equations of special relativity do not apply. The stationary twin, on the other hand does represent an inertial reference frame and the simple equations of special relativity do apply.


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  4. #3  
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    That tiny paragraph has been very helpful for me in understanding the importance of acceleration with all these events. That had been bothering me for some time so thanks a million.

    But under the basis that both twins would be correct in their statement that since the departure the photons had been travelling at speed C along the optimum route through the box, I am finding it hard to understand why they would have experienced a different number of ticks.
    It just makes light clocks an unhelpful analogy for me because the distance that photons travel through the clock doesn't seem to directly coincide with the difference in time.
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  5. #4  
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    So if it is during the acceleration leg of the journey that the time difference occurs and it is due to the principal that special relativity does not explain the behaviour or physics for bodies that are not in uniform motion, what theory do we need to look at to predict the change in time for bodies that are not in uniform motion? Does general relativity cover this? It seems to me that the twins paradox is therefore not a special relativity problem.

    What I would like ideally is to understand what exactly is going on with the Spacetwin's light clock during the acceleration leg of the journey, and why. And especially why it contradicts the spacetwin's perception of the photons having travelled the optimum route through the clock at speed C for the entire journey, even during the acceleration.
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  6. #5  
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    Quote Originally Posted by Monkey.Man
    So if it is during the acceleration leg of the journey that the time difference occurs and it is due to the principal that special relativity does not explain the behaviour or physics for bodies that are not in uniform motion, what theory do we need to look at to predict the change in time for bodies that are not in uniform motion? Does general relativity cover this? It seems to me that the twins paradox is therefore not a special relativity problem.

    What I would like ideally is to understand what exactly is going on with the Spacetwin's light clock during the acceleration leg of the journey, and why. And especially why it contradicts the spacetwin's perception of the photons having travelled the optimum route through the clock at speed C for the entire journey, even during the acceleration.
    This isn't what DrRocket meant.

    A large problem here is that you haven't come to terms with the basics of SR before trying to tackle the Twin Paradox. For example, have you studied the "Relativity of Simultaneity" yet? It is avital element to understanding Relativity and the Twin paradox.

    Check out the Sticky "Special Relativty Primer" thread at the top of the physics page, it will walk you through the basics.
    "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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  7. #6  
    . DrRocket's Avatar
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    Quote Originally Posted by Monkey.Man
    So if it is during the acceleration leg of the journey that the time difference occurs and it is due to the principal that special relativity does not explain the behaviour or physics for bodies that are not in uniform motion, what theory do we need to look at to predict the change in time for bodies that are not in uniform motion? Does general relativity cover this? It seems to me that the twins paradox is therefore not a special relativity problem.

    What I would like ideally is to understand what exactly is going on with the Spacetwin's light clock during the acceleration leg of the journey, and why. And especially why it contradicts the spacetwin's perception of the photons having travelled the optimum route through the clock at speed C for the entire journey, even during the acceleration.
    General relativity does address accelerated motion and. what is equivalent, gravitiation. It is a much more mathematicall difficult theory than special relativity. Special relativity can be handled with algebra, but general relativity requires differential geometry.

    It is also possible to handle acceleration in special relativyt, but only from the perspective of an inertial reference system.

    What general relativit tell you is that time is a local notion and therefore that the question of "when does the time difference occur" in the twin paradox is not a valid question. In general relativity all that you can compare is the result of two world lines that coincide at a point of comparison.
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