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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    Dammit guys, just when I claim to have everything resolved in my head you have to start chiming in...

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    My point is that you cannot hasten towards nor ride ahead of a beam of light. It's distance to you at the time of it's emission is the sole determinant of its travelling time before you see it. Please see my motorcycle/street lamp comments above.
    In the rest frame of the light source? How is there a single rest frame of the light source if there are two local sources moving relative to each other?
    Yes, those are good animations, thanks. However you might notice that they are using length contraction...in the top picture the distance between the red posts is the same length as the train and in the bottom picture it is clearly shorter. I've already claimed that I understand length contraction explains the relativity of simultaneity; I said it wasn't possible if one were to rely on "adjusting the distance" between oneself and the light.
     
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  3. Neddy Bate Valued Senior Member

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    According to the embankment reference frame, the passenger on the train IS hastening toward one beam of light, and riding ahead of the other beam of light.

    However, according to the train reference frame, there is no hastening. Thus, what you have written above holds true in that frame.



    Length contraction will ultimately be required in order to fully explain what happens in both reference frames. But if you only look at it from the embankment frame, you can make sense of it without invoking length contraction.
     
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  5. RJBeery Natural Philosopher Valued Senior Member

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    Do you feel the same way if you did the following, as mentioned in the OP?
    Even restricting our dealings to the embankment frame, I don't think "moving towards or away from the light" explains the phenomena, nor do I believe it's physically proper to claim so.
     
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  7. AlexG Like nailing Jello to a tree Valued Senior Member

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    I think what you've missed is this:

    The 'you' which you are referring to is the passenger on the train, not the observer on the embankment.

    Once the photon is emitted, the movement of the source is immaterial. We put the lamps on the train. They flash. There is a photon from the front of the train approaching the observer on the train. There is a photon from the back of the train approaching the observer on the train. The observer on the train sees the photons as reaching him simultaineously. But the observer ON THE EMBANKMENT sees the photon from the front of the train reach the observer on the train before the photon from the back of the train.

    The point is not 'what does the observer on the train see as simultaneous'. The point is that the observer on the train and the observer on the embankment do not agree on 'simultaneous'.
     
    Last edited: Dec 9, 2010
  8. Dinosaur Rational Skeptic Valued Senior Member

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    Arguments/questions suggesting problems relating to Special Relativity are almost all equivalent to denying the results of the Michaelson-Morley experiment. It is that experiment (not an opinion) of Einstein which is the basis of the view that the velocity of light in a vacuum is constant & independent of the motion of the emitting source and/or the motion of the measuring devices.

    Using a natural language like English to describe experiments relating to either Special or General Relativity is an exercise in futility & obfuscation. A natural language is not capable of dealing with logic, mathematics, & scientific analysis. It requires the addition of some axioms, some undefined primitive words/phrases, & some mathematical notation.

    A proper analysis of Relativity-related concepts requires establishing synchronized clocks & the use of the concept of events taking place at (x, y, z, t) coordinates. This requires a lot of careful logic & analysis. It requires using the notion of the Interval between events. It requires consistency with the results of the Michaelson-Morley experiment.

    In the absence of the above, it is both futile & silly to use one's intuition relating to the conclusions of Special Relativity.
     
  9. Neddy Bate Valued Senior Member

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    According to the embankment frame, the passenger on the train changes position during the time light signals are "in transit". It should be perfectly proper to acknowledge this fact, should it not?
     
  10. Janus58 Valued Senior Member

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    Of course it does.

    Here's a simply example that more or less runs things in reverse.

    Start with two clocks. From a point halfway between them, emit a light pulse as a spherical wave front. The front will hit both clocks at the same time, starting them simultaneously.

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    This is how things appear in the rest frame of the clocks.

    Now imagine exactly the same scenario, only this time you are watching from a frame to which the clocks have a relative motion. (it does not matter if which you consider as "moving", you or the clocks.

    From your new perspective, the same pulse again expands as a spherical wave front with its origin being the mid point between the clocks. However, the clock to the left is moving to the right to meet the part of the front moving to the left, and the clock to the right is moving to the right, running ahead of the light moving to the right. The clock on the left will meet up with its light before the clock on the right meets up with its light, and the left clock starts first.

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    Just because an inertial observer cannot measure the speed of light as being anything but c relative to himself does not mean that he cannot measure light as moving at different speeds with respect to frames that are moving with respect to himself.
     
  11. Lady Historica Banned Banned

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    \(\sum_{\iota = 1}^{N} \frac{\alpha M_{e} + \beta M_{\mu} + \gamma M_{\tau}} {(\sqrt{e - \alpha^{2}} + \sqrt{\mu - \beta^{2}} + \sqrt{\tau - \gamma^{2}})^{2}} C^{2}\)

    You have to set your invariant variables of mass before you play with them at light speed.
     
  12. Pete It's not rocket surgery Registered Senior Member

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    Hi RJBeery,
    Yes, they will.
    The position and motion of the flashes is independent of the position and motion of the lamps.

    There are actually three separate locations here that might be confused with each other by one or more posts in this thread:
    1) The location of the physical source of the flash, ie the lamp
    2) The location at which the flash was emitted
    3) The location of flash itself

    Of these, it is the second which is most relevant in Einstein's thought experiment, and is the reason he chose lightning strikes as the source.

    Note that the embankment observer measures the speed of the flash not by its distance from the lamp (1), but by its distance from where it was emitted (2).

    Note also that the constant speed of light postulate means that the speed of the flash is independent of the motion of the lamp.
     
  13. Motor Daddy Valued Senior Member

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    Correct, and it is also independent of the motion of the observer, ie, light has an absolute velocity, which is ~186,000 mi/sec, which doesn't change because someone or something is in relative motion.

    1. Moving towards a light means the distance and time the light has to travel to impact you is less than what it would have been if the light would have had to travel the original distance that was between you and the source at time of emission. That doesn't change the speed of light, as 10 ft/sec is 10 feet in one second, or 5 feet in 1/2 sec, or 2.5 feet in 1/4 sec. The distance and time is less, but the speed doesn't change.

    2. Most important to note, you can not move away from light in motion towards you. You can travel in the same direction as the light, but the distance is constantly decreasing, not increasing as would imply by "moving away." The only way you can move away from a light is to travel faster than the speed of light.

    3. In Einstein's chapter 9, the scenario is about the simultaneity (or lack thereof) of the times the strikes were EMITTED, not the simultaneity or lack thereof of the times they impacted the observer. IT IS THE TIME THE STRIKES WERE EMITTED that is in question. The strikes were emitted simultaneously, and impacted the train observer at different times due to his absolute velocity. It is also of note that the embankment observer has a true zero velocity. That is a fact!
     
    Last edited: Dec 9, 2010
  14. Pete It's not rocket surgery Registered Senior Member

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    MD, you have unfinished business in your previous thread: [post=2624104]What time is it?[/post]
     
  15. Tach Banned Banned

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    Actually , the above is not quite correct, the ballistic theory of light survives the MMX. What it does not survive is Ives-Stilwell/ Sagnac/ DeSitter dual star observation. All these three experiments are closely related.

    You are absolutely right on this.
     
    Last edited: Dec 9, 2010
  16. RJBeery Natural Philosopher Valued Senior Member

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    OK so I'm hearing that it's equivalent to consider either Lorentz Transforms or relaxing the invariance of c for other frames by a given observer when explaining SR? Doesn't that allow for absolute frames and aether models?
     
  17. AlexG Like nailing Jello to a tree Valued Senior Member

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    Compared to what?
     
  18. RJBeery Natural Philosopher Valued Senior Member

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    It's a fact bro, you can't argue with a fact.
     
  19. Motor Daddy Valued Senior Member

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    Compared to the absolute velocity of light in space.

    See, if two sources equal distance from a midpoint emit light towards the midpoint simultaneously, and the two lights strike the midpoint simultaneously, the midpoint had a true zero velocity. It's not even debatable, it is a rock solid fact. It is simple distance and time.

    One second of light travel is equal to one second of light travel. The lights travel the same distance and time towards each other and meet. If an observer was at the meeting place the entire time of light travel, the observer had a true zero velocity, as light travel in a vacuum (ie, distance and time) is independent of all.
     
    Last edited: Dec 9, 2010
  20. AlexG Like nailing Jello to a tree Valued Senior Member

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    Same old crap. Doesn't understand relativity, and never will, therefore simply denies it.
     
  21. Pete It's not rocket surgery Registered Senior Member

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    That's what I hear from MotorDaddy as well, but in my experience he is not a reliable source when describing special relativity.

    For your original post, the bottom line is that Einstein's use of lightning as the source of the flashes was to subtly emphasise that the location of the emission point changes over time in different reference frames. Ie the flash emission is an instantaneous event, and the motion or otherwise of the flash source is irrelevant for this particular experiment.

    If a flash is emitted from a lamp on the embankment, then the lamp marks the emission location in the embankment reference frame... but not in the train reference frame. In the train reference frame, the lamp moves away from the emission point, so the distance the flash travels in that frame should not be measured from the lamp.

    Similarly,
    If a flash is emitted from a lamp on the train, then the lamp marks the emission location in the train reference frame, but not in the embankment reference frame. In the embankment reference frame, the lamp moves away from the emission point, so the distance the flash travels in that frame should not be measured from the lamp.

    Does that make sense?
     
    Last edited: Dec 10, 2010
  22. AlexG Like nailing Jello to a tree Valued Senior Member

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    The whole point of chapter 9 of Relativity is to demonstrate that simultaneous, (i.e. AT THE SAME TIME), is different in moving frames. And that by extension, time is different in moving frames.
     
  23. RJBeery Natural Philosopher Valued Senior Member

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    I see the issue! The invariance of c holds only for the observer's frame, and MUST be relaxed for other frames, or else paradoxes arise. This is explained by saying that the other frames are moving relative to the light, even though we ourselves could never do such a thing.

    I'm not sure you agree with the above, Pete, but I'm also not sure you have a choice. You said it didn't matter if the lamps were on the train or the embankment, and I agree; however, if they were on the train equivalency would allow us to treat the embankment as "moving" during the travel time of the incident photons, thereby destroying the observer's perception of simultaneity. Since the embankment observer would perceive the simultaneity regardless of the placement of the lamps then his ability to "run towards" or "run away from" the light is not possible.
    This paragraph bothered me for a bit and now I know why. The relativity of simultaneity only exists for multiple, remote events. The photons reaching the passenger is a single remote event for the embankment observer. Give the passenger a 2-sided photon detector with a little flag that only raises when and if photons are detected a the same time...if what you're saying were true then the embankment observer would be witnessing a physically impossible act when the flag raised with only at single photon being detected. I think most Relativity problems are associated with our "envisioning" the little photons flying through space even though they are not detectably there (at least, that's an issue that I catch myself being guilty of frequently).
     

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