On Einstein's explanation of the invariance of c

Discussion in 'Pseudoscience Archive' started by RJBeery, Dec 8, 2010.

  1. AlexG Like nailing Jello to a tree Valued Senior Member

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    Err, no. The invariance of c holds for every frame. The paradoxes are avoided by the Lorentz transformation.

    Well, if there is only a single event, with what would it be simultaneous?
     
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  3. Pete It's not rocket surgery Registered Senior Member

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    I think you are using a different meaning for "frame".

    What do you mean by "perceive the simultaneity"?
     
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  5. Pete It's not rocket surgery Registered Senior Member

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    That's correct, and Alex was wrong in the quoted paragraph.
    In Einstein's thought experiment, everyone agrees that both flashes reach both observers at the same time.
    This is true generally:
    In any scenario, if two events happen in the same place at the same time in one reference frame, then they must happen at the same place and the same time in all reference frames.

    Even more generally, the spacetime interval between two events is frame independent. Eg if two events are separated by a spacetime interval of zero in one reference frame, then that will be true in all reference frames.

    No, there's no problem with that visualisation at all in special relativity.
    I think that only becomes a problem in quantum theory.
     
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  7. AlexG Like nailing Jello to a tree Valued Senior Member

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    This not so. To quote from Relativity, The Special and General Theory, Chapter 9, The Relativity of Simultaneity

     
  8. Pete It's not rocket surgery Registered Senior Member

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    Yes, the simultaneity of the strokes of lightning is frame dependent. The two lightning bolts struck at different locations.

    No, the simultaneity of the flashes reaching the observers is not frame dependent. That happened at a single location.
     
  9. Lady Historica Banned Banned

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    Invariance is more about mass energy equivalence than it is a difference between frames of reference. You have to follow your wave particle duality as it transferes between mediums to understand the invariant speed. The velocity is changable, but even when it hits a brick wall it is going the same relative speed.
     
  10. RJBeery Natural Philosopher Valued Senior Member

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    My point, and the original issue that bothered me which I've only just been able to succinctly identify, is that it is contradictory to say that c is invariant for a given frame and that an observer in that frame can run away from or towards a light beam. Claiming to alter the distance between you and an incoming beam to adjust its travel time to you is akin to a Galilean velocity calculation.
     
  11. Pete It's not rocket surgery Registered Senior Member

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    I think you're conflating observer and frame. If an observer that is running toward or away from anything in some reference frame, then that reference frame is not that observer's rest frame.
     
  12. RJBeery Natural Philosopher Valued Senior Member

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    Yes, of course. I didn't understand what you meant before about my use of the word frame, but obviously frame+observer's location is vital when discussing simultaneity. In this case, though, what I wrote holds for a given frame rather than an observer because simultaneity isn't referenced.
     
  13. Motor Daddy Valued Senior Member

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    Light travels independently of frames. If a midpoint observer is on a train with a velocity, and a light source at each end of the train emits light simultaneously towards the midpoint, the observer at the midpoint will not be at the point where the two beams of light meet, even though he remains centered between the sources at all times.
     
  14. Pete It's not rocket surgery Registered Senior Member

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    I'm not following what you mean.
    What holds for a given frame? What does it mean to reference simultaneity?

    That's right, MD.
     
  15. Motor Daddy Valued Senior Member

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    Right, so what right does Einstein's chapter 9 train observer have, to say that the strikes were not emitted simultaneously, just because the strikes didn't impact him simultaneously? Just because he remained centered between the sources at all times doesn't mean the strikes must have occurred at different times if they impact him at different times. The strikes could have been simultaneous and he had a velocity, which he did, as clearly stated in the example..
     
  16. Pete It's not rocket surgery Registered Senior Member

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    Your objection has been repeatedly answered in your previous thread, MD:
    [thread=101682]The Relativity of Simultaneity[/thread]

    You might like to review the end of our conversation in that thread.
     
  17. Motor Daddy Valued Senior Member

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    I stand by my statement in post #103!

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  18. RJBeery Natural Philosopher Valued Senior Member

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    OK now slow down dammit! And Pete you added to your post after I replied

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    This is only true if by "simultaneously" you mean that the embankment observer claims so. We've been over this topic plenty of times.

    Pete: We're saying the same thing in two different ways, I think...an observer in a frame, acknowledging that "he's the one moving" with respect to a light source, nevertheless cannot claim* to be moving towards or away from the beam of light regardless of his relative motion. Claiming that moving towards/away from the beam of light is not possible for that inertial observer and claiming that c is invariant for that inertial observer is the same thing.

    *he cannot make the claim, but others can make the claim about him.
     
  19. Motor Daddy Valued Senior Member

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    That's because the embankment observer has a true zero velocity and the train doesn't. The train's midpoint observer is clueless as to his velocity. All he knows is that he doesn't have a velocity relative to his frame (the train). That doesn't speak to his true absolute velocity. The embankment observer can honestly say he has a true zero velocity.
     
  20. RJBeery Natural Philosopher Valued Senior Member

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    Yeah, um, the embankment observer is cruising in a big circle at about 1000 miles per hour. What if neither guy said the lights were simultaneous but some dude in a space capsule said so?
     
  21. Pete It's not rocket surgery Registered Senior Member

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    What does "invariant for that inertial observer" mean?
    Does it mean "light moves at c relative to that observer"?
    That would mean "light moves at c in that observer's rest frame", right?

    But if an observer acknowledges that they are moving, then clearly they're not considering their rest frame, but some other reference frame.

    It sounds like you're trying to make the observer confuse their rest frame with a reference frame in which they are moving.
     
    Last edited: Dec 13, 2010
  22. Motor Daddy Valued Senior Member

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    The lights each traveled the same distance to reach the embankment observer, and reached him simultaneously. The same can't be said for the train's observer.

    Light travels independently of all frames, so a space capsule dude would be wrong if he said the embankment observer had anything other than a zero velocity.
     
  23. Motor Daddy Valued Senior Member

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    Post #80 is also noteworthy.

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