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  1. #17
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    Re: Einstein's thought experiments

    Quote Originally Posted by doojie View Post
    He correctly reasons.
    As for the person on the ground, he says that he isn't moving either, that it's the train that is moving, and so he correctly says that while the light is travelling, the train will have moved to the right, and so the light will hit the back of the train before it hits the front.[/quote]

    But there would be no logical reason for him to say that, since he can also reason that the light will travel in relation to the traveler , meaning that they have both come to the same conclusion. :liefde: [/QUOTE]

    The observer on the ground would be wrong if he reasoned this. If the light was moving in relation to the traveler, then for the person on the ground, the light travelling forwards and the light travelling backwards would be moving at different speeds, and light always travels at the same speed for every observer.

    If we were talking about anything moving at normal everyday speeds, then the reasoning would be perfectly correct. It's only once you talk about light that this logic doesn't work.

    The diagram above is confusing to me because of the t=1 and t=2 representation. If the flashlight is turned on and both are in alignment, both will see the flashlight turned on at the same instant. The speed of light being ab absolute frame of reference, both observers will view the light itself as an absolute frame of "everywhereness". That is, the speed of light, being absolute, will seem instantaneous.
    I'm not sure what you're getting at here. The speed of light is absolute in the sense that it is always the same, not that it is instantaneous. It still takes a finite amount of time between when the flashlight is turned on, and when the light hits the walls of the train.

    Assume the traveler is on a flatbed car with no walls, and he comes into alignment with a stationary observer.

    At the exact point of alignment, the traveler hits the flashlight switch so that the beams are going sideways to the observer with no wall to restrain them. To both frames of reference, light speed will be absolute, so that both will see light projecting equally in both directions. Granted, the range of the light will move with the traveler, due to the limitations of the flashlight, but for one instant dictated by the speed of light(time/space), both will see that light within an absolute frame. To say otherwise is to say that light travels at a speed less than it travels. less than absolute within any time frame.

    I'm not trying to be nasty or argumentative, just trying to reconcile this in my mind.
    Your example above will actually make the situation more complicated because it is looking at two dimensions instead of one. If you want to take into account the direction perpendicular to the train tracks it becomes more complicated, since you need to take into account what we're working on already before you can work out what will happen.



    This is fairly difficult to explain over the net, and I could find surprisingly few good sites to explain (perhaps a testament to how hard it is to explain/understand this). Maybe you could have a look at these - they might prove more useful since they can include diagrams...

    http://www.youtube.com/watch?v=wteiuxyqtoM
    http://members.tripod.com/conduit9SR/SR3.html
    http://www.7stones.com/Homepage/Publisher/Rel01.html

  2. #18
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    Re: Einstein's thought experiments

    Here's the thing that keeps haunting me, even after viewing the links you sent(and thanks for taking the time).

    As I remember conclusions from the Michaelson-Morley experiments, light speed was the same regardless of the direction or speed of the object measuring.

    So, if the train is moving forward, and lightning appears to hit simultaneously from the ground observer's point of view, the train occupies a position at that time in which the moving and ground observer are in alignment.

    If light travels 186,000 mps regardless of the direction or speed of the observer, the light will still reach the moving observer at 186,000mps the same as it will hit the ground observer.

    For average situations, at average speeds, the difference would be negligible to the point that there would be no observable difference, since a train traveling at even a hundred mph, subtracted from light traveling at 186,000 maps, will produce such a tiny reference frame that the observers will see no difference.

    Unless you're saying that light has a constant speed to the traveler(186000mps) that would still measure the same to the ground observer.

    However, as I understand Michelson Morley, light still measure the same regardless of the direction, so that light to the rear of the train would measure 186000, reaching the observer at the same time as light to the front, which would also travel at the same speed. That would be consistent with Michelson-Morley, since, assuming an observer were able to take the time to measure the light that hit the train from the back, he would measure it at 186000. If another observer, at the same instant, were to measure the light coming from the front, he would also measure it as 186000.

    That is assuming two travelers in nearly the same position, measuring the light from the rear and the light from the front at exactly the same instant, both would register as 186000(give or take). If both travelers, at that same instant, measured the light reaching them as 186000mps, then the only possible conclusion is that the lightning hit simultaneously from their frame of reference.

    If the "front" measured it at less than what the "back" measured, then the speed would be different for each light flash. But it should measure 186000 regardless. If each at one instant measured it the same, then it would have to appear simultaneous, unless there is an accounting for the observer, what the observer "sees".

    But as the observer moves toward the propagation of light, Einstein concludes the light from the front will "hit" first. Yet if light measures 186000mps no matter the speed or direction, the propagation of light will be the same for ground and moving observer.

    For anything less than the speed of light, Einstein's statement would be true, but not where the same measurement must occur regardless of the speed or direction.

  3. #19
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    Re: Einstein's thought experiments

    (See post above this one)
    Of course, if we assume the two obserevers on the train actually saw the light hit at the front instead of the back, the "front" observer would start his measurement before the "back" observer. While both observers, from the same point on a moving train would measure light at 186000 mps, the light from the front would have to be measured before the light from the back, giving two different points in time from the same position on a moving train.

  4. #20
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    Re: Einstein's thought experiments

    Quote Originally Posted by doojie View Post
    (See post above this one)
    Of course, if we assume the two obserevers on the train actually saw the light hit at the front instead of the back, the "front" observer would start his measurement before the "back" observer. While both observers, from the same point on a moving train would measure light at 186000 mps, the light from the front would have to be measured before the light from the back, giving two different points in time from the same position on a moving train.
    Ok, I can see where you are starting to get confused, and I'm not sure that there's an easy way to resolve your confusion. The explanations that I've given, and that are given on the web, overlook many of the fine details.

    When you actually start to work this out properly you have to be very careful with your definitions. Defining how observers measure time, how they measure distance, how they know when something occured etc... these all becomes very important to answering the questions. Most derivations set up imaginary 3D lattices of rods and clocks that I don't think I could explain clearly.

    I reckon the best thing for you would be to pick up a book on special relativity from the nearest library. There are heaps and heaps of them out there, and usually not very long and only assume basic maths. Unfortunately I don't know of any off the top of my head, but there should be lots of good ones available. I think that the questions you're asking of sufficient complexity that I'm only going to make things worse for you by trying to explain it here.

  5. #21
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    Re: Einstein's thought experiments

    Quote Originally Posted by kazza View Post
    Ok, I can see where you are starting to get confused, and I'm not sure that there's an easy way to resolve your confusion. The explanations that I've given, and that are given on the web, overlook many of the fine details.

    When you actually start to work this out properly you have to be very careful with your definitions. Defining how observers measure time, how they measure distance, how they know when something occurred etc... these all becomes very important to answering the questions. Most derivations set up imaginary 3D lattices of rods and clocks that I don't think I could explain clearly.

    I reckon the best thing for you would be to pick up a book on special relativity from the nearest library. There are heaps and heaps of them out there, and usually not very long and only assume basic maths. Unfortunately I don't know of any off the top of my head, but there should be lots of good ones available. I think that the questions you're asking of sufficient complexity that I'm only going to make things worse for you by trying to explain it here.
    I truly appreciate your patience, Kazza. I've always been one of those from the first grade up who couldn't simply accept what I was told.

    However, one more look at the thought experiment with a new twist.

    Same as before, with lightning striking simultaneously according to the stationary observer, etc.

    Here's where we add an extra ingredient:

    At each end of the train is an electronic hammer that will drive a steel spike into the ground the instant lightning strikes at each end.

    The ground observer will see simultaneous lightning, with spikes drive into the ground at that instant.

    The traveling observer will see the front spike driven first, and then the second spike at the back.

    This should yield one of two measurements objectively:

    Either the distance between the two spikes will show the lightning hit simultaneously, or the measurement of the spikes will show them to be slightly off center of the ground observer, and a shorter measurement.

    But BOTH measurements CANNOT occur, since there are only two spikes.

    Measurement can only confirm the ground observer, or the traveling observer, but not both, proving that only one frame of reference is objective.

  6. #22
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    Re: Einstein's thought experiments

    Quote Originally Posted by doojie View Post
    I truly appreciate your patience, Kazza. I've always been one of those from the first grade up who couldn't simply accept what I was told.

    However, one more look at the thought experiment with a new twist.

    Same as before, with lightning striking simultaneously according to the stationary observer, etc.

    Here's where we add an extra ingredient:

    At each end of the train is an electronic hammer that will drive a steel spike into the ground the instant lightning strikes at each end.

    The ground observer will see simultaneous lightning, with spikes drive into the ground at that instant.

    The traveling observer will see the front spike driven first, and then the second spike at the back.

    This should yield one of two measurements objectively:

    Either the distance between the two spikes will show the lightning hit simultaneously, or the measurement of the spikes will show them to be slightly off center of the ground observer, and a shorter measurement.

    But BOTH measurements CANNOT occur, since there are only two spikes.

    Measurement can only confirm the ground observer, or the traveling observer, but not both, proving that only one frame of reference is objective.
    You're really thinking like a physicist now :)

    There is something else going on that we haven't discussed yet, and that's length contraction. The ground observer would see a train that is shorter than the train when it is at rest (again, this isn't just a visual effect, the train actually is shorter). It's another thing that will be covered in a book on relativity, and it follows from the same basic principles as we've been discussing above.


    So the ground observer sees the two stakes driven into the ground simultaneoulsy, but they are spaced closer together than the length of the stationary train, because the moving train has been length contracted.

    The observer on the train says that the train is the normal length, but the front stake is driven in first, followed by the rear stake, so that is why they are closer together than the length of the train.



    There are lots of thought experiments along the lines of these, and they are usually given to undergraduates as exam questions. For example, if the train is 100 meters long, but when it's moving it's length contracted to 50 meters long, what happens if it passes into a 60 meter shed and you close both doors while it's in there?

    Usually the answers aren't obvious, and it only makes sense once you sit down and carefully work out the maths at each stage of the problem. Our brains just weren't built to understand this sort of thing intuitively (well, unless you're Einstein).

  7. #23
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    Re: Einstein's thought experiments

    hey!! when it comes to physicist(the physical) i prefer olivia newton john for my PLEASURES/speculations of mind!? :freak3: :spin2: :
    i do not endorse/recommend any advertising on scam.com associated with my name /posts or otherwise. thank you

  8. #24
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    Re: Einstein's thought experiments

    Quote Originally Posted by kazza View Post
    Usually the answers aren't obvious, and it only makes sense once you sit down and carefully work out the maths at each stage of the problem. Our brains just weren't built to understand this sort of thing intuitively (well, unless you're Einstein).
    do you suppose that ALL kinds of problems RESPOND to sitting down and CARE-FULLY working them out!? finding the RELATIVITY!? :freak3: :spin2: :
    i do not endorse/recommend any advertising on scam.com associated with my name /posts or otherwise. thank you

  9. #25
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    Re: Einstein's thought experiments

    Quote Originally Posted by kazza View Post


    There are lots of thought experiments along the lines of these, and they are usually given to undergraduates as exam questions. For example, if the train is 100 meters long, but when it's moving it's length contracted to 50 meters long, what happens if it passes into a 60 meter shed and you close both doors while it's in there?
    What happens is the train busts out the wall of the shed, because you couldn't possibly close the shed faster than a train moving at near light speed, right?
    A real, honest, falsifiable claim made b.y Seer of dreams:(2011)
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    I believe there will be a nuclear war in October of this year.
    Oh Cnance.... Full of shit as always.

  10. #26
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    Re: Einstein's thought experiments

    [quote=kazza;756807]You're really thinking like a physicist now :)




    The observer on the train says that the train is the normal length, but the front stake is driven in first, followed by the rear stake, so that is why they are closer together than the length of the train.
    But if the front stake is drive in first, and then the rear, obviously there will be the appearance, to the traveling observer that the train is shorter.

    However, the front spike merely marks the point where the traveling observer saw the light, and the rear spike will mark where he saw the light from the rear. In fact, both work out to equivalent positions because the traveling observer is now in a different time frame from the stationary observer, which will make the spikes seem closer only according to his new position in space.

    However, if the same observers were stationed on the ground at positions where each observer saw the light, and repeated the experiment, the same result would occur. being now at different stationary positions, the light will "hit" them at the same time. The slower the train, the closer the spacing. At near light speed, the spacing would be very far apart.



    There are lots of thought experiments along the lines of these, and they are usually given to undergraduates as exam questions. For example, if the train is 100 meters long, but when it's moving it's length contracted to 50 meters long, what happens if it passes into a 60 meter shed and you close both doors while it's in there?
    You find that one size doesn't fit all!

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