MacM is trying to show SRT as internally inconsistent, so discussion in this topic is mostly meant to assume SRT and work from there...
You are absolutely correct. And I think this is why your "two shifts" idea winds up getting the right answer. I just think it adds a lot of unnecessary complexity and confusion to a rather simple situation. I also note that, with your light sampler-repeater, both frames would still calculate the exact same Doppler shifts. They would disagree about which things were moving and which were stationary, but not about any of the Doppler shifts. The bottom line is: In either approach both frames agree on all Doppler shifts and both frames agree that the flash passes the filter. -Dale
I said that. Here is where you miss my posted information. For the light to take longer to reach the detector in the embankment frame the light must be red shifted. Yes or No? The light as seen by the embankment is blue shifted. Yes or No?
Which detector? Where on the embankment? It's simplest if you're considering a point between emitter and detector that just happens to be stationary w.r.t. embankment...
Engine. Correct and facing the on coming flash. I stipulated that position at the outset. The next point however is philosophical. That is for the light to take longer to reach the detector (chasing the receeding engine) it must be red shifted to the detector.
Well, it depends on whether the embankment observer knows that the light came from a moving source or not. If he knows that the light source is on the train with the photodetector, he can of course immediately conclude that in the rest frame of the detector, the velocity of the source is zero, and so no Doppler computations are required (except maybe to calculate what color the light pulse is in the train frame, since he's seeing a shifted version). This is certainly the most direct, easy way to see the answer. However, suppose that the observer doesn't know that the light was emitted by a moving source. Suppose he has his eyes closed until the instant after the light pulse is emitted so, for all he knows, it was a blue (or red) pulse emitted by a source that is stationary in the embankment frame. Unless I'm mistaken, this is the scenario MacM was discussing, and so that's what I was addressing. In this situation, the embankment frame observes a relative velocity between the source and filter which shifts the filter color to match the light pulse. The upshot is that both scenarios have the same result: the light passes through the filter and sets off the bombs. The lesson is that the conclusions of relativity are the same regardless of which frame one assigns the light source to. You can imagine it to be stationary in either frame and, as long as you are careful with the Doppler calculations, it all works out the same in the end.
You seem to think that there is some kind of inconsistency here, in that the light cannot be both blue shifted and red shifted. However, you're talking about two different shifts, one with respect to an observer who is stationary in the embankment frame, and one who is moving (the detector). These observers are not in the same reference frame, and so they do not need to agree on the color of the light pulse.
So that the number of wavelengths is preserved despite a longer path? Interesting idea, but I'm not sure it's necessary... (sorry, it's late in my timezone ... will get back to this 'tomorrow'.)
Let me see if I follow MacM's idea: In the train frame the filter lets a specific frequency of light pass, and this frequency is not the frequency that the embankment frame sees, due to the Doppler effect. Therefore, concludes MacM, the train blows up in the train frame, but not in the embankment frame, because in the embankment frame the light no longer has pass-frequency. This, of course, is nonsense. The filter doesn't care what frequency the light has relative to someome else. As an analogy, consider an ambulance driver with perfect pitch. If the ambulance's siren has a specific frequency, the driver will flash the lights. Now, MacM suggests that because the siren has a different pitch to a man on the sidewalk, the driver won't flash the lights.
That is correct. This isn't. It is a fact that in the embankment frame the light flash must chase the engine and takes longer to reach the photodetector than it does in the trains frame. That requires that the light be red shifted relative to the detector. I know and understand that photons in the train frame are of the neutral color and arrive before photons in the embankment frame which are of different color and timing. The point is this dictates different photons are being observed, not photons with some magical power to exist at muiltiple velocities simultaneously.
MacM, in the future you might try spelling "gedanken" right. I don't know what a "gendankin" is, but it certainly isn't a thought experiment. After all, you've clearly put no thought into this one. The funny thing is that you rage in your little corner of the internet, yet all your nonsense is nothing but "a tale told by an idiot, full of sound and fury, signifying nothing."
MacM, Oops, I wasn't tuned into the Doppler arguments and was considering the time of arrival of the flashes at the bombs. Certainly a detector can be modified to ignore all frequencies within any range that suits the current purpose, but once a light beam has been passed through that is the end of matter. There were some references to arrival of the flash associated with the Doppl;er shift detected, but this has nothing to say to the speed of light. It seems that this thread is more of an Engineering problem than a physics problem. Geistkiesel
No. Only relative to objects at rest in the embankment frame. Just because you on the embankment see the light redshifted, doesn't mandate that the photodetecter does too. You're forcing your pitch of the siren on the ambulance driver. The real world isn't like that: The ambulance driver hears a steady pitch, no matter how many pedestrians (all hearing the classic "EEEEoooow!") he passes. So, because the ambulance driver and the pedestrian hears a different pitch for the siren, they're no longer hearing the same sound? Hell, because the ambulance has different velocities wrt. the driver and the pedestrian, they're no longer the same car? I simply can't agree. It isn't a useful viewpoint, especially since that if you take the viewpoint that they are, you get results consistent with reality. What you are arguing seems to be a philosophical viewpoint, not a physical (or even mathematical) one. It's ironic, really. You berated James R (and several others) that relativity lead to "multiple realities", yet you now seem to advocate the ultimate "multiple realities" viewpoint: nothing is related between frames!
Damn and I thought it was understood that theembankment observer was standing still. :bugeye: You do love to switch sides don't you. Anyother time you would be keen on pointing out that sound is not invariant since it travels in a medium. Yes it is ironic how the SRTist's pick and choose when and where multiple realities exist.
It is. However, you claimed that the light was red-shifted relative to the detector. The detector isn't at rest in the embankment frame, and therefore doesn't see the redshift. The bomb blows up regardless of the embankment observer. It doesn't matter, because this isn't a relativity problem. Even in a purely Newtonian universe with no relativistic effects at all, your argument is still that the bomb doesn't blow up in the embankment frame: The embankment observer still won't see the same frequency of light as the detector. Think about the ambulance siren some more. Do you really think that a pedestrian on the sidewalk could make the siren change for the driver? Why do you think an observer on the embankment can make the light change for the detector?
It's pretty simple, really. Doppler shift is not invariant. Observers in motion relative to the source of the light all see different Doppler shifts based of their velocity wrt the source. Measure the Doppler shift and you measure the relative velocity. Light leaves its source at 'c' in a vacuum. Use a Special Theory co-moving frame, one in which no purtabations affect the photons, and one will measure a distance to the source based on a constant 'c' and travel time of the photon. The calculation will not be correct unless one can possibly find an area of the vacuum unaffected by gravity and other influencies, such as magnetospheres, solar wind, and gas in the vacuum. NASA uses Newtonian physics, times the travel of the signal based on UTC time, and then accounts for all effects on the signal travel time as mentioned above. Their 'time' is normally kept by hydrogen maser clocks on Earth accurate to +/- 1 second in 30 million years. Gravity affects time in many different ways. Enviromental conditions affect clock stability and accuracy. Doppler shift of the signal is effected by both gravity and enviroment factors, as well as relative velocity, of course. I can find nothing to support clocks 'beat slower' due to relative velocity at NASA's websites on the Deep Space Network (DSN), telemetry, or time/frequency standards. Everything I have read points to the opposite conclusion.
