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Where s the moving train on the embankment?

  1. Dec 9, 2009 #1
    http://img682.imageshack.us/img682/4396/movingtrain.jpg [Broken]

    How do these observers line up in relation to each other when A' passes A?

    We know that from the train's reference frame B' will be adjacent to C and that from the embankment's frame C' will be adjacent to B, but as there is only one train and one embankment and as each has only a single physical existance and can only physically exist in one place at one time, how do they 'really' line up as they pass.

    Now I can appreciate that time dilation and length contraction are perceived effects, distortions if you like due to the relative velocity of the participants, and that that perceived 'distortion' of time and space is reciprocal (as it must be, being due to their relative velocity).
    And I can also appreciate that, to the observers in their reference frames those 'distortions' are the 'real' measurements they perceive; much as the pitch, of the sound of a passing car, being higher as it approaches and lower as it recedes, is real to an observer, whilst the actual pitch of the sound produced does not vary. It is once again an effect (albeit entirely different in nature) due to the relative velocity.

    Now can we say that if we could calculate their relative positions, rather than 'observing them' from one or other of the reference frames, we would see them align, observer to observer?

    Grimble :confused::confused:
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Dec 9, 2009 #2

    jtbell

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    You need to take into account relativity of simultaneity.

    In the embankment frame, the train moves from left to right and is contracted by a factor of 1/2. The events "A' passes A" and "C' passes B" are simultaneous.

    In the train frame, the embankment moves from right to left and is contracted by a factor of 1/2. The event "B passes C'" precedes the event "A passes A'", i.e. they are not simultaneous.
     
  4. Dec 10, 2009 #3
    Re: where is the moving train on the embankment?

    But in this scenario there are no 'lightning strikes' to associate simultaneity with either frame of reference, so why give preference to either?

    Anyway, that apart, I am thinking of this as a 'thought experiment'.
    We know that from the embankment's frame the train will be length contracted; and that from the train's frame the embankment will be length contracted; but those are two views/perceptions of one state of physical existence and unless we are venturing into the realms of multiple realities, only one set of physical relationships can exist.

    Now it cannot be either of the above for that would give preference to one, so we need another view/reality and that can only be that neither is physically contracted i.e. the length contraction is an effect of their relative velocities.

    This leaves us with the idea that, to a 'neutral observer', B lines up with B' and C lines up with C' when A lines up with A'.

    And in fact, this is what Einstein describes in http://www.bartleby.com/173/9.html" [Broken] and in the following paragraph where he writes
    This is before he associates the relationship with reference to either of the parties and agrees with the fact that neither the train nor the embankment would experience any length contraction with reference to their own frame of reference.

    I am trying to comprehend the physical relationship/juxtaposition of the two entities, not how they appear to each other.

    PS I am not in any way denying the validity of all the experiments, and the fact that time dilation and length contraction are real and used everyday for GPS.

    Grimble
     
    Last edited by a moderator: May 4, 2017
  5. Dec 10, 2009 #4

    HallsofIvy

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    What do you mean by "neutral observer"? Every observer has some frame of reference. And observer moving with the train will see the situation completely differently form an observer on the embankment. And there are observers with non-zero speed relative to both train and embankment that would see the situation completely differently from either.
     
  6. Dec 10, 2009 #5

    Doc Al

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    Re: where is the moving train on the embankment?

    The lightning strikes are just meant to illustrate the relativity of simultaneity--they don't create it. No preference is given to either frame of reference--they are equally valid.

    Every frame of reference will have its own space and time coordinates with with to describe the world. Each is perfectly valid.

    What do you mean by 'physically contracted'? If you want an observer-independent way of describing things, you'll need to use space-time diagrams.

    Huh? Please describe this 'neutral observer'. A lining up with A' is an event; so is B lining up with B', etc. The time order of those events depends on the frame of reference; no observer is 'neutral'.

    That quote has nothing to do with any mythical 'neutral' observer--it's just a description of M' and how an observer there is moving with the train.
     
    Last edited by a moderator: May 4, 2017
  7. Dec 10, 2009 #6

    Ich

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    please, please, please draw spacetime diagrams.
    A train exists during an extended period of time, not at one time. Each observer picks a different bunch of events from this extended existence and calls it "the train now". That's how it works, and I don't see how you could possibly come to grips with it or even ask the right questions if you don't draw the situation on a sheet of paper.
     
  8. Dec 10, 2009 #7
    Re: where is the moving train on the embankment?


    The above was in reply to my response to the following post :
    So why are the two scenarios given that way round?

    Grimble
     
  9. Dec 10, 2009 #8

    jtbell

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    Re: where is the moving train on the embankment?

    That's just the way I happened to write them. Where I live, I don't have many opportunities to ride trains, so it's more common for me to be standing on the ground watching them go by, and that's the scenario I thought of first.
     
  10. Dec 10, 2009 #9
    I will do my best and draw some diagrams, but that may take me a day or so!o:):smile:
     
  11. Dec 10, 2009 #10
    Re: where is the moving train on the embankment?

    OK:smile::rofl:
     
  12. Dec 10, 2009 #11

    Ich

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    Only two: embankment frame and train frame. If you succeed, I bet you will answer the question yourself.
    And don't draw fancy trains or something, a train in a spacetime diagram is two parallel lines, nothing more.
     
  13. Dec 10, 2009 #12
    Re: where is the moving train on the embankment?

    Yes of course, I can see and acept that, but they are all different 'views' (i.e. space and time coordinates) of the same single world, that are dependent on the conditions under which they are viewed (measured).
    And yes, of course they are all perfecly valid, for those conditions, but the conditions under which they are viewed do not change the way the molecules of either the train, or the embankment are put together.


    Thank you, yes, I will have a go!o:)


    Sorry, please excuse my ignorance in choosing the way I express myself!:redface:
    I believe I should have used the term "observer-independent"


    I'm sorry, I was reading it that, as Einstein was saying: "But the events A and B also correspond to positions A and B on the train." and: "Let M' be the mid-point of the distance A —> B on the travelling train." and: "Just when the flashes 1 of lightning occur, this point M' naturally coincides with the point M", he was, effectively describing an "observer-independent" view of the scenario...

    Grimble
     
  14. Dec 10, 2009 #13
    Re: where is the moving train on the embankment?

    There is no such thing in this scenario. Perhaps you are thinking of an observer relative to which the train and embankment have equal and opposite motion.


    Matheinste.
     
  15. Dec 10, 2009 #14
    may be, may be a 'neutral' observer has instant-vision.
    (i.e. not an atomic observer, as usual, that depends on 'c' speed to ackowledge the events (and label time coordinate)).
     
  16. Dec 10, 2009 #15
    The entire rod is shortened.

    They will not line up if A does.
     
    Last edited by a moderator: May 4, 2017
  17. Dec 10, 2009 #16
    Re: where is the moving train on the embankment?

    What experiment do you have to conclude this.

    Time dilation and length contraction are in the mainstream logic.
     
  18. Dec 10, 2009 #17
    So, in a spacetime diagram what method do you use for multiple observers in the same frame but at a distance?
     
  19. Dec 11, 2009 #18

    Ich

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    Come on, the longest thread in PF history, and nobody bothered to explain the role of an observer in SR to you?
    What changes if you introduce a gazillion observers hiding at every conceivable place?
     
  20. Dec 11, 2009 #19
    Re: where is the moving train on the embankment?

    I'm sorry, but are you saying that the "way the molecules of either the train, or the embankment are put together" varies according to where they are referenced from, and therefore, if many observers are referencing them from different frames, that those molecules are arranged differently, and that we must therefore have multiple existences?

    Can you point me to the experiments that lead one to conclude this?

    And yes, of course TD and LC are real, I have never said they are not, but they are effects albeit ones that are real to those that experience them.

    Consider, if you will, a rod at rest viewed by many observers in different frames moving with different velocities; which seems the more likely explanation:
    1. that the rod has a separate existence in each frame of reference with a different length depending on the relative velocity
    2. or, that the rod has one, unique existence that is measured differently from the different frames as a function of their individual velocities.

    Surely the way to find the 'real' length of the rod would be to use an observer that was at rest relative to the rod, where the TD and LC, as a function of the velocity was 0? Would it not o:)

    Grimble
     
  21. Dec 11, 2009 #20
    Re: where is the moving train on the embankment?

    Length contraction in SR is stress free, that is no distorting forces are involved, no moleclar changes. I am sure you already knew that.

    As regards "real" length, I think what is usually said is that the rod length is maximal when measured in a frame in which it is at rest. Rest length.

    Matheinste.
     
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