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I Does the "space twin" benefit from length contraction?

  1. Dec 22, 2017 #121
    bah, we make and read em.
  2. Dec 22, 2017 #122
    But if you were present at all drunk driver collisions wouldn't somebody get suspicious?
  3. Dec 22, 2017 #123
    You and Homer can put on identical wrist watches that are guaranteed accurate. You and Homer will synchronize your watches before each run of the experiment (space trip). Do each run of the experiment using different acceleration profiles for the turnaround section, maintaining the same average relative velocity for each leg of the trip, and traveling the same proper length each time, as computed by Homer. Assume that there is a flag floating in the vacuum of space (marking the turnaround point) that is at rest relative to Homer, which he has bounced radio waves off of to determine proper length. Notice that during the accelerations phases, you do not notice anything "strange" happening to your wrist watch that would key you into something "weird" happening. At the end of each trip, you and Homer compare the time elapsed on your wrist watches (each person's proper time). Ask Homer if he noticed anything "weird" happen to his wrist watch at any point when you were gone. Notice that the results are the same every time and therefore the acceleration can not be the cause of the differential aging (difference in proper times).

    If that doesn't convince you, go on several more trips, but this time make the proper length 1/2 the original proper length. Use the same average relative velocity and the same acceleration profiles for the turnaround as for the first set of experiments. Compare watch readings after these experiments and see that the results are 1/2 the difference in proper times compared the first set of experiments.

    If that doesn't convince you, read through @PeroK post #45 a few times to see that the clock's proper time traced out the path through spacetime without acceleration on any physical object. The same path you would have made with an instantaneous turnaround neglecting acceleration. And then conclude that acceleration is not necessary to explain the results.
  4. Dec 22, 2017 #124

    Mister T

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    The point is that someone must be, and just because they are doesn't mean they're the cause.

    Acceleration may be present, that doesn't mean it's the cause. Even if acceleration were always present in every version in which there's a difference in ages, that doesn't mean it's the cause. That's true regardless of your suspicions.
  5. Dec 22, 2017 #125
    This reminds me of the old story where the farmer noticed that every time his rooster crowed the sun came up, which woke him up because of the bright light. Wanting to be able to sleep in, he decided to kill his rooster.

    Correlation does not prove causation.
    Last edited: Dec 22, 2017
  6. Dec 23, 2017 #126
    I am reminded that George Gamow long ago in "Mr Tompkins in Wonderland" fingered the brakeman:

    “Yes, of course,” thought he, “if all were relative,the traveler would appear to his relatives
    as a very old man, and they would appear very old to him, although both sides might in fact
    be fairly young. But what I am saying now is definitely nonsense: One could not have relative whiskers!
    So he decided to make a last attempt to find out how things really are, and turned to a solitary
    man in railway uniform sitting in the buffet.
    “Will you be so kind, sir, ” he began, “will you be good enough to tell me who is responsible
    for the fact that the passengers in the train grow old so much more slowly than the people
    staying at one place? ”
    “I am responsible for it ”, said the man, very simply.
    “Oh! ”exclaimed Mr Tompkins, “So you have solved the problem of the Philosopher's Stone
    of the ancient alchemists. You should be quite a famous man in the medical world. Do you
    occupy the chair of medicine here? ”
    “No,” answered the man, being quite taken aback by this, "I am just a brakeman on this
    railway. ”
    “Brakeman! You mean a brake man . . .” exclaimed Mr Tompkins, losing ground beneath him
    “You mean you just apply the brakes when the train comes to the station? ”
    “Yup, that's all I do, and every time the train gets slowed down, the passengers gain in their
    ages relative to other people… Of course, the engine driver who accelerates the train does his
    part too.”

    The "learned professor" discusses it further later with Mr Tompkins but you are invited to read (or re-read) this lovely book.
  7. Dec 25, 2017 #127

    Wes Tausend

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    Time must never be thought of as preexisting in any sense; it is a manufactured quantity. --Hermann Bondi
  8. Dec 25, 2017 #128


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    Staff: Mentor

    Popularizations, even when written by well-regarded professionals like Gamow, should be approached with great caution. They belong to a different genre and are written for different purposes than serious presentations of the actual physics.
  9. Dec 26, 2017 #129
    I certainly agree, but the good popularizers (Sagan, Feynman, Gamow, Asimov.......) are very careful not to say things that are incorrect just to make it "simple".
    Thanks largely to your clarity of explanation, I believe the remaining disagreements here are semantic. The "common man" definition of the twin problem requires both the existence of time dilation (fully calculable using Special Relativity alone!!) and an acceleration (to differentiate the younger twin). Choice of only one of them to be "the cause" is a semantic issue.
  10. Dec 26, 2017 #130


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    But you can easily construct experiments with the same outcome that have no acceleration. So "it's a semantic issue", if true at all, could only be true about this version of the experiment, and not a general truth about physics
  11. Dec 26, 2017 #131
    What do you mean "same outcome"?? Please construct for me a complete experiment involving the birth of twins and the later meeting (for handshake) of same twins that does not include perceptible acceleration of the apparently younger twin.
  12. Dec 26, 2017 #132


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    The three clocks experiment that has been discussed several times produces the exact same difference in accumulated time. The twins and the handshake aren't the important part of the experiment - for the purposes of the paradox they are just clocks and can be replaced by any other clock.
  13. Dec 26, 2017 #133
    It is a simple request. You said it was easy......I think it is not.
  14. Dec 26, 2017 #134


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    I've already done so, and so has PeroK. See #22, the second half of #45, and #51 for example.
  15. Dec 26, 2017 #135


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    Instead of transferring clock readings, you could (theoretically at least) replicate one twin on a moving space ship. A la the transporter on Star Trek.

    That would obviate the need for acceleration.

    And, if you quibble about this, remember that we are not a fixed set of atoms from birth.
  16. Dec 26, 2017 #136
    I was making a serious request.. .
  17. Dec 26, 2017 #137


    Staff: Mentor

    In flat spacetime, you can't. So what? You can construct a thought experiment in which exactly the same worldlines--curves in spacetime--are involved, without any observer feeling any perceptible acceleration. You are singling out the acceleration by singling out the condition that a single twin has to follow the "traveling" worldlines--out and then back in. But that is not at all necessary to the key purpose of the thought experiment, which is to demonstrate that different paths through spacetime between the same two events can have different lengths. It's just the way the thought experiment happened to be originally formulated.

    At this point nothing useful is being added to the discussion. Thread closed.
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