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Question about Einstein's train thought experiment

  1. Jan 24, 2014 #1
    Hi all. I'm a run-of-the-mill layman—not a student. I've just developed an interest in physics lately and have been watching/reading a lot pop physics. So hopefully my question's not too stupid and hopefully the answer's not too far over my head.

    My question is about the thought experiment where one observer sees a super-train pass by at super-train speed. As it goes by, two lightning bolts strike simultaneously (from the first observer's reference frame) on each side of the train car. A second observer riding in the car sees two non-simultaneous lightning strikes, in his or her reference frame.

    I think I get the gist of why their reference frames produce different experiences as to when the lightning strikes occur, but what I'm a little unclear on is why the observer on the train wouldn't register a blue shift in the light emitted from the first strike and a red shift in the second. The train is moving relative to the lightning (or from the other point of view, the lightning is moving relative to the train), so why is their no Doppler effect?

    Thanks for the help!
     
  2. jcsd
  3. Jan 24, 2014 #2

    ghwellsjr

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    Hi Incognito310 and welcome to PF.

    You're absolutely right. In fact, the light that was visible to the first observer will be shifted out of the visible range for the train observer. But, like super fast trains, we over-simplify scenarios like this because we don't want to detract from the issues that we are trying to emphasize. In fact, I believe that Einstein purposely used lightning so that we wouldn't be concerned with which frame the source of the light was stationary in, such as if it came from a flash bulb. He wanted to get as near as possible to an ideal event, with no duration in time and if that were the case, then it wouldn't have a wavelength or any Doppler shift.
     
  4. Jan 24, 2014 #3
    Thanks for the response ghwellsjr! I thought that might be the case, but it's nice to hear it from somebody who would know better than me. I'm still skeptical (not a refection on your response—that's just my nature), but it's nice to know that someone else thinks it's possible a Doppler effect would be detectable in this scenario.

    I've gotta say, one of the most frustrating parts of being a layman with an interest in Physics is the tendency for pop-science resources to overstate the profoundness of some of the apparently counter-intuitive aspects of Relativity and/or Quantum Mechanics.

    Sometimes the exaggeration is obvious, but even non-sensationalistic resources often contain misinformation to drive home a point while abandoning some of the finer points of reality. But the philosophical implications are huge. This could be one of those cases.

    The paper I read about this experiment stressed that there was absolutely no way whatsoever that the two observers could calculate their own motion relative to each other or the lightning. Philosophically speaking, that puts the common-sense Universe that most of us know and love on shaky ground.

    But if a Doppler effect really would be present in the case of lightning strikes it would hold sway in this thought experiment regardless of what the light source is. Whether it be lightning, flash bulbs, fireflies, or a theoretical "ping" on a plank-time scale, the light sources, in order to have an effect on reality, have to actually exist within it. I've never heard of light without a wavelength before. Is such a thing really possible/detectable?

    Unless I'm mistaken, no matter how small their duration in time is, the light sources would have a real presence in a non-fiction account of this scenario, and whether they're stationary to one observer or the the other (or in motion for both), if the Doppler effect is really detectable for the observers, even though they will all experience it very differently, they all actually still share the same reality.

    If Einstein really did pick lightning for his thought experiment to skirt the notion that the light sources actually have their own time frame, that's too bad. The physical effects he predicted trough this thought experiment are obviously true and profound but he may have added philosophical ambiguity to time dilation in this instance where it didn't really need to be.

    I'm not one to question Einstein, so I still have to wonder if the Doppler effect would really be measured in this scenario. It seems reasonable to think so, and thanks ghwellsjr for supporting the merit of the assertion. But if it's true, it seems that trough it's omission in the description of this thought experiment, a philosophical paradox of dueling realities between two observers has arisen where simple geometry would suffice.
     
  5. Jan 24, 2014 #4

    jtbell

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    The relativistic Doppler effect (as opposed to the purely classical Doppler effect) is well established. You can think of relativistic Doppler as basically being classical Doppler modified by time dilation.

    http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/reldop2.html
     
  6. Jan 24, 2014 #5

    ghwellsjr

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    Can you provide a reference to that paper? It's hard (or impossible) to know what they meant without seeing the full context of their statements. In general though, I would say that it is possible for observers to determine what's going on around them, if we grant them certain reasonable assumptions and measurement options.

    No, but what difference would it make to our understanding of Einstein's thought experiment?

    I don't see how including additional details would change anything or why you are concerned. Is it that the paper you read asserts that the lack of knowledge about the relative speeds of the observers and/or the lightning has anything to do with Time Dilation? You should be aware that neither observer can see the Time Dilation of the other one, all they can see are the Doppler effects. The Time Dilation has to do with the assignment of the propagation of light to one frame or the other and that's why it comes out differently in different frames.
     
  7. Jan 24, 2014 #6
    Thanks jtbell! That's definitely over my head at this point, but as a newbie/layman, any well established mathematical proof will be. It's comforting to see it anyway. Hopefully one day I'll progress beyond pre-algebra and pop-science and be able to comprehend it. I guess its back to Khan Academy with me for now.
     
  8. Jan 24, 2014 #7
    If the duration is much less than the wavelength of the light divided by the speed of light (in the frame where the strike has duration at a fixed location), then the Doppler shift should not be detectable. Correct?
     
  9. Jan 24, 2014 #8
    It's the first four pages of this PDF:
    http://www.vicphysics.org/documents/teachers/unit3/EinsteinsTrainGedanken.pdf

    From what I read, I got the impression the author was stating each observer had different and incomparable experiences with the dual lightning strikes and that there would be no way to deduce which observer was in motion relative to each other or the lightning strikes.

    I understand that neither one can see the time dilation of the other, but it stands to reason that if they could measure Doppler effects of the lightning strikes, that they could calculate whether or not they were moving in relation to the other observer and the lightning.

    So if I were stationary on the platform and saw simultaneous lightning strikes as the train passed, with no Doppler shift, I would know that the lightning's presence, however brief, was stationary relative to me. I could then deduce what the observer on the super-fast train would have seen—two lightning strikes in succession.

    Now if I were on the train and saw two lightning strikes in succession but detected a Doppler shift, I would be able to deduce that I was in motion relative to both the lightning and the other observer. I'd be able to figure out that the observer on the platform was stationary in relation to the lightning strikes.

    I think that contradicts the statement in the paper that says:
    "Remember that this is not a case of what various observers saw first, it was a case of what they deduced actually happened."

    If the Doppler effect is measured, doesn't it then actually become a case of what each observer saw first? If I'm on the train and I see a blue-shifted light, followed by I red-shifted light, can I deduce that the strikes actually occurred simultaneously relative to each other? If so, then even though I saw two flashes in succession, I would agree with the observer on the platform that the flashes occurred simultaneously.

    To be clear I'm not in anyway one of those non-physisists I see from time to time trying to debunk anything. I know these are well-established and thoroughly vetted concepts. My own ignorance is grand enough that I'm a little embarrassed to even be discussing it in a public forum. In the laymen's world we get a lot of misleading information about physics, so I'm just trying to get as firm a grasp on general concepts as I can while I try to learn the maths necessary to get a truly clear understanding.
     
  10. Jan 24, 2014 #9

    ghwellsjr

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    The smallest packet of light that we can detect is a photon which has a wavelength in any given frame and therefore will be Doppler shifted in any other frame.
     
  11. Jan 24, 2014 #10
    Thanks. Help me out some more. How can one determine which observer will observe a Doppler shift? Is it just that the shift does not occur in the frame that the strike physically occurred, and it does occur in other frames?

    Chet
     
  12. Jan 25, 2014 #11

    ghwellsjr

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    This is actually a pretty good paper because it emphasizes what each observer actually sees and makes a distinction between what they can deduce. My only real complaint is their constant use of the term "actually happened". I think that phrase has lead you into the idea that there actually exist a knowable reality beyond what observers can actually see.

    Each observer does have different experiences but that doesn't mean they're incomparable or in conflict with each other. And I didn't see anywhere that the paper claims that observers can't deduce relative speed of the other observer or the lightning. Can you point to the page number and location on the page that you got that inference?

    Try looking at some other recent threads in which I have posted diagrams. Things might make a little more sense to you then.
     
  13. Jan 25, 2014 #12
    ghwellsjr. Please, did you not notice my previous post. Help me out. If a lightning strike hits both the train and the ground next to it, how does one determine which set of observers (train or ground) experiences a Doppler shift? Is it assumed that the lightning strike is at rest in the ground frame?

    Chet
     
  14. Jan 25, 2014 #13

    Saw

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    I think that the mirrors where the lightning reflects play the role of the light source. If the lightning reflects on mirrors both at the ground and at the earth, you will have two sources and two light rays. Each ray will be Doppler-shifted in the frame where its source is not at rest
     
  15. Jan 25, 2014 #14

    ghwellsjr

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    A lightning strike is actually a discharge between two objects with a difference of charge so it could go between two objects with a relative speed so there would be no way to say which one it was at rest with respect to but that's not really important to the point I was making. The point I was making is that whatever is in the visible range for one observer would be shifted out of the visible range for the other observer at super high speeds. Both will see a Doppler shift compared to the other.
     
  16. Jan 25, 2014 #15
    Thanks. Still a little confused, but, oh well.

    Chet
     
  17. Jan 25, 2014 #16

    ghwellsjr

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    What is your point of confusion?
     
  18. Jan 25, 2014 #17
    For a single strike that hits both the ground and the train simultaneously at a given location, what do the people on the ground to the rear of the strike observe for the frequency, relative to the frequency observed by the people on the train to the rear of the strike? I hope that articulates my point of confusion more accurately.

    Chet
     
  19. Jan 25, 2014 #18

    ghwellsjr

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    A lightning strike isn't just one frequency, it's a fairly wide spectrum. We don't really know what it is and we don't care but we can still say that the people on the train approaching the light source more rapidly than the people on the ground will see it Doppler blue shifted compared to what the people on the ground see and the people on the ground will see it Doppler red shifted compared to what the people on the train see.
     
  20. Jan 25, 2014 #19
    When you say that the train is approaching the light source, how do we know it is approaching the light source if the lightning struck both the ground and the train simultaneously at a given location? Also, is the duration of the strike irrelevant? Also, if the lightning were at a single frequency (rather than spectral), would it matter whether during the duration, the strike was at rest in the train frame or in the ground frame?:confused:
     
  21. Jan 25, 2014 #20

    ghwellsjr

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    As long as the people on the train and the people on the ground and the light source are along the same line and the light is at one end, we only care about the relative motion between the two sets of people. Whichever group is moving faster toward the end where the light is coming from will see it Doppler blue shifted compared to the other group.

    Yes.

    No.
     
    Last edited: Jan 25, 2014
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