On Einstein's explanation of the invariance of c

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

  1. RJBeery Natural Philosopher Valued Senior Member

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    In other words, Motor Daddy, you are presuming that there is an ACTUAL midpoint between two events, which there is not. Any observer, moving in any direction whatsoever, could find the midpoint between two events and claim that he is right while all others are wrong. If there were an ACTUAL midpoint between two events then I suppose you could say that you have "zero velocity" relative to those events but that is a far cry from "absolute true zero velocity". Make sense?
     
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  3. Motor Daddy Valued Senior Member

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    What is the midpoint of a 12" ruler?
     
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  5. RJBeery Natural Philosopher Valued Senior Member

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    Wow bro I almost feel like I'm being Punk'd. You're claiming that if the Earth were rotating, we could stand between two synchonized clocks and watch them become unsynced? Man, if we could only find a suitable planet to test such a theory...
     
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  7. RJBeery Natural Philosopher Valued Senior Member

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    The midpoint of a 12" ruler is 6" from the end, unless you're moving parallel to its length.
     
  8. Motor Daddy Valued Senior Member

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    I didn't say they become unsync'd, I said you would see them as reading different. Why? Because one light would have to travel a longer time than the other to reach you if you had anything other than an absolute zero velocity.
     
  9. Motor Daddy Valued Senior Member

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    Your movement has nothing to do with the midpoint of a ruler. The ruler has an absolute midpoint.
     
  10. RJBeery Natural Philosopher Valued Senior Member

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    No sweat, please identify such midpoint in terms of the time it would take a photon to travel from one end to reach it. If c doesn't change, and the midpoint doesn't change either, then this time interval should not change. You will find that your statement is false, because that amount of time will change if the ruler is moving. Measurements come about ultimately by using light (even it that means relying on the EM forces holding the molecules of your ruler in place). You have to believe (well, no you don't, but it would be nice) that there is no other reliable way to take a measurement.

    If you don't believe me then take your ruler and measure out your "absolute midpoint" between the sync'd clocks in your living room as you drive by very fast. You will find that your ruler is giving a different answer than before, and that cannot be blamed on "moving away from light sources".
     
  11. arfa brane call me arf Valued Senior Member

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    We're back on the "how long is a ruler" argument?

    Suppose the ruler has no marks on it that say "inches", no numbers at all, it's a completely blank, but 'rigid' length of something. How do you find its midpoint, what can you use to measure its 'proper' length, hmm?

    Suppose you're a patent clerk with a bit of spare time, and you decide the best way to measure the length of rigid objects is with a beam of light, and you measure how long it takes for such a beam to traverse the length of the ruler so you can attach a number to the 'start' and 'end' of the ruler, according to the distance light travels and the time it takes, relative to the length of the ruler.

    If the ruler isn't in motion that simplifies the situation considerably. So how do you know the ruler isn't in motion relative to the beam of light?

    Well, it isn't hard to arrange such a situation, if you have a 'local' frame of reference where the ruler, the source of light and all the instruments (that you need to measure locations, etc) are 'stationary'--thanks to the existence of a 'fixed' region of space and time, a nominally labeled part of the surface of the earth (i.e. a system of coordinates) whose motion through space and time can be ignored relative to the 'laboratory frame' of reference.

    (ellipsis omitted for clarity)
     
  12. Motor Daddy Valued Senior Member

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    So if I create a new measuring device called a swaglenot, and measure the time it takes light to travel from end to the other, it is always ~186,000 miles in length if it takes light one second to travel the length?
     
  13. RJBeery Natural Philosopher Valued Senior Member

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    No, that's the point. Your swaglenot's length will vary depending on its movement relative to you.
     
  14. Motor Daddy Valued Senior Member

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    Length is not dependent on my motion. 1 second of light travel is ~186,000 miles, regardless of what my motion is. If it takes light 1 second to travel the length of the swaglenot, the swaglenot is ~186,000 miles long.
     
  15. RJBeery Natural Philosopher Valued Senior Member

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    Think about this: do you believe that there is a point on the train in Einstein's experiment that could be labeled the "precise midpoint" between the lightening flashes?
     
  16. RJBeery Natural Philosopher Valued Senior Member

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    Your second statement is true, your first is false. Demanding otherwise is like demanding that the Earth is flat, unfortunately. I want Physics to be intuitive also, believe me.
     
  17. Motor Daddy Valued Senior Member

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    There is a midpoint on the train that is centered between the two ends of the train, yes. That midpoint remains the train's midpoint regardless of the motion of the train.
     
  18. Motor Daddy Valued Senior Member

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    So 186,000 miles is not a constant?
     
  19. arfa brane call me arf Valued Senior Member

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    That's only true if the train is a 'rigid body'. If the train is flexible like a spring you can't define a fixed midpoint. There are no absolutely rigid bodies that we know about (anywhere in the universe), so your definition falls at the first hurdle.

    There is however, a 'rigidity' to the speed that light propagates, which brings us back to "doh"!
     
  20. Motor Daddy Valued Senior Member

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    An inertial train is rigid. There is no change in an inertial train's length.
     
  21. RJBeery Natural Philosopher Valued Senior Member

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    Yes, as Arfa points out, Einstein skipped over the fact that the train SHRINKS lengthwise as it moves on the track. He did this for simplicity but you cannot ignore length contraction. So, no, 186,000 miles is NOT a constant - it changes as you move. In other words, what I measure as 186,000 miles you may not.
     
  22. Motor Daddy Valued Senior Member

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    Wrongamundo, the train is inertial, there is no change in its length. The train has a velocity, but it is inertial. Do you understand the difference between having velocity and being inertial? Evidently not.
     
  23. RJBeery Natural Philosopher Valued Senior Member

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    This is not true, unfortunately. If it's moving fast I will 'see' its length decrease. Is it "really" contracting? Einstein says no one is in a position to say, including the person on the train. Note that this is EXACTLY the OPPOSITE of what you're claiming Einstein said, which is that there is a "true" frame from which to make measurements.
     

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