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There is a didactic gap in the explanation of P.O.R

  1. May 5, 2015 #1
    Hello,

    Here is a quote from the Wikipedia article 'Principle of relativity':

    http://en.wikipedia.org/wiki/Principle_of_relativity

    i'v added a string of asterisks around the word 'A consequence'. i feel that there is not enough elaboration about this 'consequence' thing. Was the consequence obtained either by mathematical or by experimental results of the first part (i.e. the validity of the principle of relativity)? Or is it that what follows the word 'consequence', is just an axiomatic statement? Am i using the right word in this context? Axiomatic? There is not enough elaboration as well about the implicit relation, brought up in this sentence, between the term absolute speed and the term inertial reference frame. Do these terms have to be mutually exclusive?

    Here is the quote:

    "The principle requires physical laws to be the same for any body moving at constant velocity as they are for a body at rest. ******A consequence****** is that an observer in an inertial reference frame cannot determine an absolute speed or direction of travel in space, and may only speak of speed or direction relative to some other object."

    If one has a small tub full of water, then maybe by observing the water movement and ripples, after acceleration has ended not long ago, one can tell the change of speed relative to the past speed that the inertial reference frame he was inside, was going through. Now you will say, but that previous speed or direction, was or has to be in relation to another frame of reference and if then you also had that water tub set-up, then the previous speed or direction before that has to be in relation to another frame of reference and so on and on.

    But if all matter behaves like this water in the tub, for much longer time than just the waves of water inside a tub can indicate - In the sense that matter contains constant radiation, that tells us about the movement history of that matter, in the same scale of past time events as say the cosmic background radiation goes back, but this kind of radiation is within matter and not outside matter - Then would that be considered 'absolute'? How accurate does such a hypothetical in-matter radiation measurement have to be, in order for it to be called: 'An absolute determination of speed or direction of travel in space'?

    Thanks.
     
    Last edited: May 5, 2015
  2. jcsd
  3. May 5, 2015 #2

    Dale

    Staff: Mentor

    It is Wikipedia. If you feel there is not enough elaboration then add some elaboration.

    All matter does not behave like a tub and even if it did it still wouldn't give an absolute velocity.
     
    Last edited: May 5, 2015
  4. May 5, 2015 #3

    Nugatory

    User Avatar

    Staff: Mentor

    You cannot, because these observations are also consistent with the hypothesis that there has been no acceleration but a few seconds or minutes ago someone was stirring the water. You would have to start with a tub of water in a specific state (past history irrelevant because whatever that history was, the tub is now in that specific state) and then observe it evolving differently from that state forward. The experimental fact is that accelerations can be detected in this way, but motion in at constant speed relative to some external inertial observer cannot.
     
  5. May 5, 2015 #4
    Let's call this - 'The absolute position' of the universe at the very very very very first moment of the big bang. Does it also have to be relative to something else? Could that state of the universe be called 'An absolute-non movement state'?
     
    Last edited: May 5, 2015
  6. May 5, 2015 #5

    Dale

    Staff: Mentor

    No. It was expanding very rapidly. Certainly not non-movement.

    When people speak of absolute motion they mean a state of motion in which the laws of physics are different. Not merely where some configuration of matter is at rest.

    Please do not propose your own terminology.
     
  7. May 6, 2015 #6
    As i said, there is a didactic gap, at least at my level of being able to understand:

    The ****Consequence**** is relating to the impossibility of measuring something they call 'absolute'. Yet this measurement that is concluded as impossible, is by definition of the ****Consequence**** itself, done with no relation to any other frame of reference. So how exactly is the consequence related to the difference or to the non-difference of the laws of physics in comparison between frames of reference?
     
    Last edited: May 6, 2015
  8. May 6, 2015 #7

    russ_watters

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    Staff: Mentor

    Seems clear enough to me: all frames are equally valid, therefore no frame is special. Its just opposite sides of the same coin.

    As for logic vs experiment: the statement is one of logic, but it has been experimentally verified.

    The real issue here appears to me to be that you just don't believe it.
     
  9. May 6, 2015 #8

    russ_watters

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    Staff: Mentor

    That doesn't make sense. All speeds are measured between two frames of reference/objects. A frame of reference or object does not have a "speed" on its own.
     
  10. May 6, 2015 #9

    Dale

    Staff: Mentor

    So then edit it yourself. You are talking about a Wikipedia article, so you are free to supply any gaps you see.

    Personally, I don't see what your confusion is. "Absolute velocity" refers to velocity in the one unique "absolute" reference frame where the laws of physics are uniquely defined. The principle of relativity asserts that no such frame exists. As a fairly obvious and direct consequence "absolute velocity" does not exist.

    In any case, this complaint and discussion belongs on the Wikipedia discussion pages, not here.

    Thread closed.
     
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