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B How relativity deals with simultaneity as requirement

  1. Feb 23, 2016 #1
    OK, guys and galls, I'm back

    I admit that the ladder paradox topic that I started was put quite stupidly, and I wouldn't have the chance to make my point, even if Peter didn't lock it.
    But... we had quite a long conversation in private with him, I learned quite few things, and I believe I made may point before him.
    So, thanks Peter.

    I hope I won't disappoint you, if I tell you that I come again with the relative simultaneity in mind, this time with different approach.

    In our conversation with Peter I introduced the idea of inserting one more simultaneous event in the work of the (garage) system. I can show it to you, but for now I'd prefer to ask an easier question.

    The set:

    We have a spaceship A. The work of its engine depends on the simultaneity of two events.
    So, two simultaneous events are keeping the engine running.
    The moment the simultaneity is lost, the engine stops running and obviously the spaceship stops moving.
    The simultaneity is visible for the passing by other spaceships as two big poles set apart from each other, parallel to the direction of spaceship movement, simultaneously sinking half way into two holes and then simultaneously going up, simultaneously touching two switches on the end of their paths.
    If needed we can set time interval, but I don't see it as necessary... for now.
    The spaceship A is moving through space and sees another spaceship named B, traveling parallel and coming its way. Spaceship A measures the velocity as v=0.7c .

    From spaceship B, they see a funny spaceship (A) with two poles moving up and down, on its front deck. Obviously because of the relativity of simultaneity, in the reference frame of B the two simultaneous event on the front deck of A are not simultaneous.

    The question is:
    does B measures same relative speed like A, v=0.7 ?

    The obvious answer is “yes”.
    On another hand, since in the reference frame of B simultaneity does not happen, A should be loosing speed or not moving at all.
    But if the relative velocity is v=0.7c for both frames, then relativity is observational, not real.

    When in our conversation with Peter I decided to add one more simultaneous event in the garage-ladder paradox, It appeared as an effect from the work of the system and then I clicked: but in many systems, simultaneity is required for the system to work, or is an effect from the work of a system. How does relativity deals with that?

    So, how does the Theory of Relativity deals with the above?
  2. jcsd
  3. Feb 23, 2016 #2


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    As I said in your last thread, whatever analysis B performs of the "simultaneity detector" mounted on A will show that it does not detect simultaneity. It detects whether or not the two events happened a certain time apart - the time apart that events simultaneous to A appear to B. As you were told in the last thread, simultaneity is not a "thing" that you can detect. Thus the rocket engine is expected to function by both.

    As a side note, if the rocket engine is functioning the rocket will be accelerating. If the engine is not functioning the ship will be moving at constant speed.
  4. Feb 23, 2016 #3
    Ha-ha :biggrin:
    I cannot get off Earth.
    OK, let say that simultaneity is responsible for another process, say it keeps on a light bacon on the front deck of A
    Simultaneity can't be detected but can be verified, which is quite the same. Semantics - the way we use the words.
    OK I'll have to bring up my modified garage-ladder experiment.
    Here it is:

    A third simultaneous event is introduced in the ladder paradox, by attaching a rod to each door, which are welded perpendicularly on the inside of the doors, in a way that both rods touch when the doors are simultaneously closed (the small added red lines on the graphics)
    (The rods are not on the way of the ladder, and can be placed in a way not to touch the floor.)
    The single event "touching of the rods" verifies that both doors are closed simultaneously.
    touch - simultaneity
    no_touch - no_simultaneity
    In the same time as a single event it must be present in both frames of reference.

    As seen on the left graphic (garage frame of reference with shorter ladder), the rods touch in the frame of the garage, when the doors are closed.
    As an event, it should be present in the ladder frame of reference, but it is not.
    Last edited: Feb 23, 2016
  5. Feb 23, 2016 #4


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    As I said in my first post in your last thread, you cannot have perfectly rigid object even in principle. When the door starts to open a mechanical wave starts propagating through the attached rod. The same happens in the other rod when the other door opens. In the rest frame of the garage the two waves travel the same distance at the same speed and meet in the middle. In the ladder frame they travel different distances at different speeds, but still meet in the middle. So in both frames the rods move apart from their meeting point at the same time. But this does not mean that the doors opened at the same time except in the garage frame.

    Edit: I have my doors opening instead of closing. The argument stands.
  6. Feb 23, 2016 #5
    For great lengths that propagation will work to the extent that you claim, but I doubt it will work for the garage rods with the most rigid material available.
    Furthermore, note that while the end of the back door rod departs later from the point of the touch, the left rode will arrive later too, thus missing the touch, for the single fact, that the front door need to give time for the ladder to pass through.
    You are missing a fraction of time.
  7. Feb 23, 2016 #6


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    Believe me - I have done the maths many times. There is no missing time.

    Relativity would hardly have stood for over a century with such an obvious flaw.
  8. Feb 23, 2016 #7


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    In fact, your thought experiment is a more complex version of the one Einstein used to derive relativity in the first place...
  9. Feb 23, 2016 #8
    I'd like to take it on faith and believe, but I'm following the science claims.
    Here is what science says about the process of ladder passing through (right side of the image):
    The first event is the front of the ladder approaching the exit door of the garage.
    The door closes, and then opens again to let the front of the ladder pass through.
    At a later time, the back of the ladder passes through the entrance door, which closes and then opens.

    At which point the touching event happens?
  10. Feb 23, 2016 #9


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    The mechanical propagation speed doesn't depend on the distance.
  11. Feb 23, 2016 #10


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    What is the "touching event" and does "at what point" mean "at what point in time" ("at what time") or "at what point on the door"?

    At one point you say "then relativity is observational, not real". What do you mean by "real" as opposed to what is "observed"?
  12. Feb 23, 2016 #11
    Sure, it doesn't.
    But time of propagation depends on the distance.
  13. Feb 23, 2016 #12


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    You need to stop over-complicating your scenarios. Adding acceleration for the spaceship adds complexity to the analysis which is not necessary. You should stick to objects that are all moving inertially.

    Also, in the previous thread and in our private discussion, you were adding doors that swing through an angle, and rods that touch or don't touch, etc. This also adds unnecessary complexity: the doors and rods were moving in a different direction from the relative motion of the ladder and the garage. Also (I didn't bring this up in our discussion, but it's relevant), using rods means you have to model the material properties of the rod and deal with the limitations of SR on rigid bodies: applying a force at one end of the rod does not instantaneously move the other end, it only starts a force being transmitted through the rod, which can only move at the speed of light.

    In your OP scenario here, you have two different motions: the motions of the objects whose "simultaneity" is driving ship A, and the relative motion of ship A and ship B. That, again, is unnecessary complication.

    You need to strip this down to the absolute simplest scenario that identifies the issue you think you see. Until you do that, and do it in a PM to me so I can review it before it becomes public, this topic is off limits for you; if you post about it publicly again you will receive a warning. Thread closed.
  14. Feb 23, 2016 #13


    Staff: Mentor

    Oh, and just for the record: no, you didn't.
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