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B Pressure vessel

  1. Dec 13, 2017 #21
    well some ppl are saying length contraction is real and some are saying its not, its analogous to a measurement error. Obviously if it is not real then it can be ignored

    if a space ship whizzes past me has it actually contracted ie the distance between the atoms that make up the space ship decreased, or not, or the distance between the quarks that make up the atoms decreased, or not. "Length" being the average distance between atoms that make up the space ship? or we dont know?

    there are 4 choices
    yes it has contracted
    no it has not contracted
    we dont know
    it depends
     
    Last edited by a moderator: Dec 13, 2017
  2. Dec 13, 2017 #22

    Orodruin

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    ”Some people” is not a valid reference. Please link to these statements. It is also likely that the statements have all been true, but that your incomplete understanding of SR have led you to conclude that they are contradictory.

    Will an object be shorter than its proper length in a frame where it is moving? Yes. This is what length contraction means.

    Does this mean that the proper length of the object changes when it is moving? No. The object remains the same and looks no different in its rest frame just because it happens to move at relativistic speeds relative to some arbitrary observer.

    An object’s length is not the same in all frames.
     
  3. Dec 13, 2017 #23
    so the answer is we dont know as there is no experiment that can be carried out to determine it if actually contracts, as opposed to "appearing" to contract as a result of the fact that we have nothing faster than the speed of light to measure with. It is an artifact of the measuring equipment. For instance lest say in the future physicists discover a previously unknown phenomena that travels 100 trillion times faster than light, would SR would cease to exist?

    The fact that it appears to be all different lengths shouldn't we be blaming the equipment rather than assuming space and time somehow warp
     
    Last edited: Dec 13, 2017
  4. Dec 13, 2017 #24

    Orodruin

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    No, this is an incorrect representation of what I said. I said that ”actual” contraction as in an object getting a shorter proper length would fundamentally violate not only a basic principle of SR but also classical mechanics. Having access to things faster than light to measure with has nothing to do with it. Signal delay due to a finite speed of light is easily taken into account.

    You are currently moving (and hence length contracted) at very close to the speed of light relative to a cosmic proton hitting the atmosphere. Do you feel any different just because I told you that?

    No. This assertion is just absurd. Please learn the actual theory and the predictions it makes (this does not mean to learn about the theory through popular science presentations) if you want to be able to draw your own accurate conclusions about what it implies.
     
  5. Dec 13, 2017 #25
    so what you are saying here is your measuring equipment tells you it contracted, but in reality it didnt? as a result of the limitations of the measuring equipment
     
  6. Dec 13, 2017 #26

    PeterDonis

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    Ah, of course. So the idea that length contraction makes the pressure go up is at best a heuristic; it can be a good thought starter but it won't give you a correct analysis once you dig into the details.
     
  7. Dec 13, 2017 #27

    PeterDonis

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    There is no point in continuing this discussion if you are going to continue to misunderstand and misrepresent what we say.

    As @Orodruin commented earlier, this topic is really not a "B" level topic; it requires at least an "I" level background to be able to go into details. The best "B" level answer that can be given is what has already been said: there is a tensor called the stress-energy tensor that represents energy, momentum, and stress; the components of this tensor do change when you change frames; but the change cannot be simply described as "pressure goes up because of length contraction"; it's more complicated than that, and the complications require an "I" level discussion.

    Thread closed.
     
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