Relativity of Simultaneity Gendankin

Discussion in 'Physics & Math' started by MacM, Feb 3, 2006.

  1. MacM Registered Senior Member

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    I raised this issue in another thread but it did not receive any physics resolution and I feel it merits its own thread for consideration.

    ************************ Gendankin ************************
    Given the following:

    A train has bombs mounted at the engine and caboose, wired to go off when struck by a flash of specific light given off by a strobe mounted midway between the engine and caboose. The phototriggers are further protected by neutral optical filters. That is red or blue shifted light will not pass through and trigger the device.

    (In the other thread it was suggested that the red and blue shift might somehow be offset by length contraction or something like that but I note that length contraction is a unidirectional change and red shift, blue shift is bi-directional change.)

    In the train frame it is clear that light will reach the triggers simultaneously and the bombs go of simultaneously and it doesn't matter if the train is in motion or not.

    However, from the embankment frame view the light flash sent to both ends of the train, when in relative motion according to SRT, not only do not arrive simultaneously but are both red or blue shifted for the forward engine and caboose car respectively.

    Now you have the situation where according to relativity not only are the explosions simultaneous in one frame and not in the other but in this case they occur in one frame and do not occur in the other.

    I await your comments.
     
    Last edited: Feb 4, 2006
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  3. quadraphonics Bloodthirsty Barbarian Valued Senior Member

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    The "neutral optical filters" also undergo Doppler shifts that exactly match the shifts in the light reaching them, from the embankment's perspective. The absorbtion spectra of moving matter (and thus the passband of any optical filter) undergoes the exact same Doppler shift as light. Thus, both frames agree that both filters pass the light and that both explosions happen, but disagree about their simultaneity. The filters have no effect on the experiment, and so are a red herring.
     
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  5. MacM Registered Senior Member

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    Sounds feasible at first glance, however the trailing flash is red shifted to observers on the embankment but blue shifted to the photodetector and the foward flash is blue shifted to the embankment but red shifted to the photodetector?

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  7. quadraphonics Bloodthirsty Barbarian Valued Senior Member

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    You're mixing frames. Nothing is shifted in the photodetector's frame, since neither it or the light source are moving in that frame. The correct statement is that, from the embankment frame, BOTH filters have red-shifted passbands for light travelling the opposite direction as the train, and blue-shifted passbands for light travelling in the same direction as the train. Thus, the apparent frequencies of the light pulses and the passbands of their respective filters always line up exactly, regardless of which frame the measurements are done in.

    Again, the important point is that the filters undergo the same Doppler effects as the light pulses.
     
  8. CANGAS Registered Senior Member

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    I understand the underlying principle of Doppler light wave shift. I know something about exactly what the working mechanism of color filters is.

    I know that light waves are waves of energy. I know that light filters are ( pretty much ) massive objects.

    So, how do the atoms constituting the crystal matrix of solid light filters become Doppler ( AntiDoppler? ) shifted to exactly counteract the ( attempted ) shift of the light waves?
     
  9. MacM Registered Senior Member

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    Sorry Quad..., that doesn't cut it. I am standing on the embankment as the train approaches and I am facing towards the on coming train at the instant the train (engine photodetector) passes my location I see the light flash as blue shifted.

    From "My" frame the light is chasing the engine and must be red shifted to the detector.

    I agree that in the train frame the light and detector are in the same frame and there is no doppler but that is the point. My claim is that photons observed in other frames are not the same photons in the rest frame (which accounts for the apparent invariance of light velocity) but SRT claims that the embankment view is as real as the inertial rest frame and that the explosion events will therefore be shifted by simultaneity since the light does not arrive at the same instant as it does in the train's frame.

    Part of that scenario is the doppler shift in that frame. If you want to claim a simultaneity shift of the explosions based on the doppler shifted light from the embankment view then you must account for the red and blue filter for photons in that frame view.
     
  10. MacM Registered Senior Member

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    Fair question. I don't believe I have ever heard of doppler shifted footballs either.

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  11. DaleSpam TANSTAAFL Registered Senior Member

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    There are two effects here that I think everyone is lumping together and calling "Doppler". There is the time dilation (a.k.a. transverse Doppler) and the normal Doppler. The time dilation slows everything in the moving frame, so both the filters and sources are redshifted by the same amount. The normal Doppler effect is irrelevant to the scenario. Both frames agree on the result.

    -Dale
     
  12. MacM Registered Senior Member

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    Can you clairfy this comment?

    As I said if I am standing on the embankment watching the train arrive, looking in the direction the flash is arriving from, the flash will be blue shifted.

    The photodetector sees the flash either without doppler (the train frame) or it is red shifted to the photodetector in the embankment frame view since you claim the light is chasing the detector which is moving away from the origin of the flash.
     
  13. quadraphonics Bloodthirsty Barbarian Valued Senior Member

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    Right.

    Right. In the embankment frame, the filter/detector is moving, and so has a Doppler shift applied to its absorbtion spectrum. Since the filter/detector has the exact same speed as the light source, the Doppler shift of the light pulse and the Doppler shift of the filter spectrum match up exactly, and the light passes through.

    Right.

    Right, the "neutral" filters become either red or blue filters when they are moving (depending on whether they are moving towards or away from the incident light). Thus, in the embankment frame, you see a blue light pulse go through a blue filter and set off the explosion at the front of the train, and a red light pulse go through a red filter and set off the explosion at the back of the train.
     
  14. quadraphonics Bloodthirsty Barbarian Valued Senior Member

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    Recall that the Doppler effect applies equally to waves emitted from moving sources and to waves measured by moving observers. So, suppose I have an optical filter that only passes blue light. I place it on a table in front of me, shine a red light on it, and nothing comes through. Now suppose that this optical filter is moving towards me at a large speed; I shine a red light at it and, due to the Doppler effect, it appears to be blue to the filter and passes through. From my perspective, of course, the light is still red, and so the net effect is that the moving filter seems to have a shifted absorbtion spectrum. This is exactly the principle used in astronomy to calculate the speeds of celestial bodies.

    The reason that the shift in the filter passband corresponds exactly to the shift in the light pulse in the train experiment is that both the filter and the light source are moving at the same speed relative to the embankment. Thus, since the Doppler shift is a function of speed, they both undergo the same amount of shift.
     
  15. MacM Registered Senior Member

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    Bingo. You seem to agree with me but have confused yourself such that you don't know it.

    You are acknowledging that the photodetector is not functioning in the embankment frames perspective. Such that what you see as an observer on the embankment has no bearing on the physics and function of the system.

    From the embankment perspective the oncoming light is blue shifted and the light is chasing the train engine which is receeding from the source origin point of the flash hence it must be red shifted. But this view is not physical reality.
     
  16. CANGAS Registered Senior Member

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    I finally get it about the filter shift. Dr CANGAS thanks everyone for all the patients.
     
  17. DaleSpam TANSTAAFL Registered Senior Member

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    The distance between the source and the detector is not changing in either frame, so neither frame anticipates a Doppler shift.

    -Dale
     
  18. quadraphonics Bloodthirsty Barbarian Valued Senior Member

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    No I'm not. The embankment frame sees two photons, one red and one blue, travel in opposite directions, pass through filters that match their respective colors, hit the detectors, set off the bombs and blow up the train. The *only* difference between the observations in the embankment frame and those in the train frame are that the light pulses and filters have different (matched) colors, and the bombs do not go off at the same time.

     
  19. 2inquisitive The Devil is in the details Registered Senior Member

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    by quadrophonics:

    "No I'm not. The embankment frame sees two photons, one red and one blue, travel in opposite directions, pass through filters that match their respective colors, hit the detectors, set off the bombs and blow up the train. The *only* difference between the observations in the embankment frame and those in the train frame are that the light pulses and filters have different (matched) colors, and the bombs do not go off at the same time."
    =============================================================

    The filters will BOTH be blue-shifted in the embankment frame. Photons will be blue shifted in one direction of propagation, and RED shifted in the other, in the embankment frame.
     
  20. 2inquisitive The Devil is in the details Registered Senior Member

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    I should clarify my last post. The filters will both be blue shifted if the embankment observer is watching the train approach his location. The filter located at the engine will be red shifted and the filter located at the caboose will be blue shifted when the embankment observer is located between the moving train's engine and caboose. And of course, both filters will be red shifted after the train has passed his location and he is looking at the receeding filters. The Doppler shift of the photons and filters will match ONLY when the embankment onserver is located midway between the engine and caboose of the passing train.
     
  21. Zephyr Humans are ONE Registered Senior Member

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    Put another way: light emitted from an object moving in your frame is dopplered in your frame. Light received by an object moving in your frame is reverse-dopplered in your frame. The overall frequency effect (what the receiver receives) is the same in any frame.
     
  22. quadraphonics Bloodthirsty Barbarian Valued Senior Member

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    The relevant velocity here is the relative velocity of the filter and light pulse, not the filter and observer. Since the light pulse is always emitted between the two filters, regardless of where the observer stands, the color of each light pulse always matches that of the filter it hits.

    This does bring up an interesting point, however. When I said that a moving neutral filter appears to be a blue filter for the observer, I was referring to light moving in the same direction as the filter. For light moving in the opposite direction, the Doppler effect is reversed, and it becomes a red filter. So the color of a moving filter depends on which side the light is coming from; it's blue in one direction, and red in the other. Perhaps it's misleading to speak of an observer perceiving a moving filter as having a particular color; it only has that particular color for light emitted at the same position as the observer. It's better to emphasize the general principle that the color of a moving filter depends on its velocity relative to the light source due to the Doppler effect.

    So, in the end, which light pulse is which color does depend on whether the observer is standing in front of or behind the train. But in either situation, the relative velocities of the light pulses and filters work out such that the colors match up.
     
  23. 2inquisitive The Devil is in the details Registered Senior Member

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    I agree with you, DaleSpam. The ONLY photons we can 'see' are those that move directly to us or measuring device. STR constructs gedankins in which paths and frequency shifts of photons are 'assumed' to definable from our remote location. We don't see those photons, we only see the photons that come in our direction from the emitting source. The distance between the source and the detector is not changing and the photons in the train frame are not Doppler shifted. The filters are not Doppler shifted in either frame. The photons we see from the filters were not Doppler shifted when they left the filters, we see them as Doppler shifted because we are approaching or receeding from light itself. Spectrum shift is due to the relative motion of the observer wrt the light beam itself. This is MY interpretation, not necessarily in concert with theory.
     

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