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Whether a person will die or not is relative

  1. Mar 13, 2007 #1
    "Whether a person will die or not" is relative

    Barn and pole paradox is a famous controversial topic.
    (IMHO, Barn and pole is not paradox. There's no paradox
    in this universe.)
    However, if the case is modified as follow:

    Imagine that the two barns become two big sharp knives, and they
    are separated by a distance of 1.5 meters. The pole becomes
    a human who is flying like the person in rocket man movie.
    His speed is 0.8c

    Now, assume that the front knife and back knife is cutting down
    and up on a table with same frequency. The rocket man body is also 1.5meter long.
    When he's flying above the table in order to pass through the knives,
    he will find that the distance between two knives relative to himself to be:

    1.5meters x squareroot(1-(0.8c^2)/c^2) where c is speed of light
    = 1.5meters x 0.6
    = 0.9 meter

    Distance between two knives is much shorter than his body so he'll possibly die.

    Whereas, for a stationary person (say Mr. X) who is just standing still relative to table,
    he'll find that the rocket man body length is 0.9 meter, so the rocket man will not
    be cut into pieces.

    Now, the next point is that if I want the rocket man die, I need not
    kill him directly. But I just act like a rocket man to fly in the same
    speed at somewhere else. Then my wish can come true.

    Furthermore, after I finished flying trip and saw rocket man being cut into pieces,
    I asked Mr. X if the rocket man died. What would he answer ?
    I think he would also answer me that the knives cut the rocket man
    and he would also claim that the space distance between two knives was shorter than
    the rocket man.

    Therefore, reality must also be relative according to different reference frame.
    But this is too hard to imagine though it can be understood by theory.
    Anyone has similar feeling?
    Last edited: Mar 13, 2007
  2. jcsd
  3. Mar 13, 2007 #2
    You've not understood the "barn paradox". Your rocket man will disagree with Mr. X's interpretation that the knives both cut "simultaneously", giving him time to nimbly fly through unharmed, so there is nothing relative about the ultimate result.
  4. Mar 13, 2007 #3
    Nimbly? Hmmm, the margin of error is rather small here.
    I for one would not volunteer for this experiment. :rofl:
  5. Mar 13, 2007 #4
    "Whether a person will die or not" is relative

    I know that simultaneous condition is also changed relative to rocket man,
    so I also mentioned that the two knives are in same frequency
    but did not want to mention whether they're in same pace or not.
    Actually I think the most important point is not if the two knives
    are in same pace simultaneously.
    The result that I'm interested is the probability that relative to
    myself (flying) or the rocket man himself will die possibly because the gap
    between two knives shorten relative to the rocket man.
    Another result is that in case if the rocket man died, Mr. X would also
    agree that the rocket man died because rocket man and I were the observer but not Mr. X.
    Last edited: Mar 13, 2007
  6. Mar 14, 2007 #5


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    I think your questions have been answered already. But I'll repeat the answers here just in case there is some question.

    Everyone will agree about whether the rocket man dies or not.

    Everyone will agree that both knives move at the same frequency (cycles per second). (They won't necessairly agree about what that frequency is, as another poster points out, due to time dilation).

    However, not everyone will agree whether the knives move "in unison". Some will see the knives move in unison, others will not. This is known as "The relativity of simultaneity".
    Last edited: Mar 14, 2007
  7. Mar 14, 2007 #6
    See, this is critical (unlike the frequency you mentioned.. which, incidentally, different observers will disagree about due to time dilation). If Mr. X thinks the slashes of both knives are in phase (such that both occur while the length-contracted rocket man is safely between those two points of danger) than the rocket man will see them so far out of phase as to give him enough time to cross your butcher-table.

    Seriously, study the textbook a bit harder before you suggest new principles of physics!
  8. Mar 14, 2007 #7
    pervect, thanks for your repeated explanation with patience.
    To my understanding, do you mean that in case if the rocket man doesn't die,
    then all observers in any relative speed will get the same result that
    rocket man is still alive.

    I'm still not convinced that a man will have the same fate (dies or not)
    relative to any observer with different speeds.

    I'll consider the moving train paradox is the best example.
    If the man in the middle of train emits a light to both front door and back
    door of the train, then he'll see the light reaching both front and back door
    simultaneously so that both doors open at the same time (as doors are light sensor
    control). But relative to outside observers two doors are not opened simultaneously.

    I had ever imagined in my mind that if the doors sensors are not controlled
    for opening doors but it's a self destruction unit by explosion. If both
    sensors get signals at the same time (or "same time" means within certain short time range),
    then destruction unit explodes and all people are killed. However, relative to outside
    people, the lights arrive to both ends at different timing, so it will not explode.

    I would greatly appreciate if you could explain since it involves two realities.

    Since I believe Einstein Relativity is correct, thus, could we say that the same group
    of people in train have different fates relative to different observers. Actually
    I remember that Einstein had also mentioned different realities.
    I had also investigated the Lorentz invariance ago, but find that it's not able to
    explain this phenomenon, unless we accept different realities can exist.
  9. Mar 14, 2007 #8
    That is unfortunate, since it is the case that all observers must agree on whether the man lives or dies.

    No, it does not involve two realities.

    Fates have nothing to do with it.

    All I can say is that you are on the wrong track and you are wasting your time on it!
    Last edited: Mar 14, 2007
  10. Mar 14, 2007 #9
    OK, I don't talk about fate.
    I just wish to understand the modified train paradox
    but it seems that no one is able to explain the result for observers
    inside and outside the train.
    I don't find a real scientist here though I agree that I'm still learning.
    Very disappointed with this forum.
  11. Mar 14, 2007 #10


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    I'm not quite sure why you're not conviced, but I don't think I have much more to add other than what I've said.

    It seems pretty straightforwards to me, relativity is not so strange as to have two different observers observe totally different things. This is a pretty basic property of reality.

    Even quantum mechanics is not that weird - while you can perhaps have a cat in a superposition of live and dead states, when you open the box, everyone will agree about whether that cat is alive or dead. You'll never get one observer saying "live" and another observer saying "dead".
  12. Mar 14, 2007 #11
    pervect, I totally agree what you said.
    That's also something that I want to say:

    ...whether that cat is alive or dead. You'll never get one observer saying
    "live" and another observer saying "dead"...

    I mean for two realities we can only choose one here.
    All observers here will only agree the same reality even though
    the results are different to different observers.
    Actually I understand this...
    However, what I wish to express is Quantum Mechanics seems to always
    give a better explanation for what we want in future.
    Anyway thanks for your patience to explain again.
  13. Mar 14, 2007 #12


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    There ARE "real scientist" here. Just because you cannot understand what they have explained to you doesn't mean there aren't any.

    You are also new here. I would put it to you that you haven't been around long enough to be able to draw an accurate conclusion about anything. If you intend to stay here for any period of time, I would suggest that you refrain from making such comments.

  14. Mar 14, 2007 #13


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    That doesn't make a whole lot of sense - people see what they see and there is only one reality. It seems you are implying that they will see something and agree with someone else that it isn't reality.
  15. Mar 14, 2007 #14
    I hope to see here real scientists that really fulfil Einstein's teaching
    that the personality and respecting other people is the first requirement
    to be a real scientist.
  16. Mar 14, 2007 #15
    You misunderstand my meaning, and perhaps I didn't express in proper way.
    I mean that all people will agree with the same result in harmony.
  17. Mar 14, 2007 #16


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    I'm not going to get into an argument about the definition of "real scientist", especially when I'm guessing that you haven't familiarized yourself with the working of science and who are actually in this forum. All I'm saying is that you shouldn't make such statements about something that you apparently do not have ample "data" on. This is not a trademark of a "real scientist" either, and even Einstein would agree with that since, even as a theorist, he paid a lot of attention to VALID experimental evidence.

    "Respecting people" goes both ways. What you have said shows very little respect to people on here, and especially to those who have offered you help in responding to your post.

    Last edited: Mar 14, 2007
  18. Mar 14, 2007 #17


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    I would sugggest that you read more about the relativity of simultaneity. I mentioned this before, I'll give you the wiki URL for further reading.


    It appears to me that you have realized that your conclusion (about one observer thinking the person is alive and the other that he is dead) is wrong, now the remaining task is for you to understand relativity well enough to understand in more detail where you went off the rails.

    To do this you need to understand the relativity of simultaneity. This will entail a certain amount of reading of sources and thinking on your part to succeed.
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