Correct but you still missed the point. The point is the view of the embankment observer. In his view the light is chasing the engine and MUST be red shifted since it takes light longer to reach the engine than it does in the train frame. Granted they would detonate based on the train frame alone but the issue is that the light should not trigger the bombs in the embankment view. This misses the point doesn't it. See above. The issue is the embanment observers view not the detector view. It is red shifted in the observers view. Nope but do you think that an observer on the street wearing an acustical filter that didn't hear the siren would expect the ambulance driver to hear one? Come on. The issue is the view of the embankment not of the detector.
Using the gedanken of a light repeater in the embankment frame which detects the light pulse and re-emits it without change ... the engine detector has no effect whatsoever on Doppler shift detected in the embankment frame. All that matters is the relative velocity of the repeater (i.e. a stationary point in the light path, in the embankment frame) with respect to the light source in the carriage frame.
This is absolutely correct, in fact, it is even understated. Not only is the Doppler effect frame-variant it is even variant at different locations and times within a single frame. -Dale
By the way, you have said something similar to this several times now, but it is simply incorrect. If light takes longer then the distance is greater, that has nothing to do with the Doppler effect. In fact, the light going from the flash towards the engine (if sampled with one of Zephyr's repeaters at rest in the embankment frame) is blue-shifted, not red-shifted. Observers at rest at other points in the embankment frame will detect different degrees of Doppler shift, some blue and some red, but all will agree on the time it takes for the flash to reach the detector. -Dale
Correct. But my challenge was that the embankment observer would have cause to believe it would not detonate because HIS view is that the light is red shifted. I have at no time said it would not detonate onoy that from the embankment observers perspective it would not.
So it is not the embankment observer that is important. It is the detector that is important. In the train frame: the relative velocity between the flash and the detector is zero so there is no Doppler shift. The relative velocity between the flash and the embankment observer causes a Doppler shift, but who cares, it has nothing to do with the bomb. In the embankment frame: the relative velocity between the flash and the detector is zero so there is no Doppler shift. The relative velocity between the flash and the embankment observer causes a Doppler shift, but who cares, it has nothing to do with the bomb. Same result, both frames agree on the same Doppler shift between any two objects. There is absolutely no conflict here, as has been pointed out nearly a dozen times by at least three people now. -Dale
Notin my scenario. It is the view of the embankment that is important. For whatever reason Physics Monkey, Funkstar and others that are educated and should know better have launched negative innuendo and ignored physics facts. FACT: In the embankment frame the veloicty of light is "c" and the velocity of the train is "v". There IS a relative velocity between the light flash and the detector of Vr = c - v and that is clearly red shifted from the train frame where the relative velocity of the flash and detector is simply "c". In the embankment frame the detector will not trigger the bomb. PERIOD.
Frames do not have red shift. Particular points in frames do. It depends only on the source of the light and the point you're measuring at; nothing to do with where the light would've gone if you didn't measure it. That's another shift.
Why are we still talking about this? This problem is so trivial and the suggestions of MacM are so outrageous that I wonder why anyone is still talking with him about it. He's repeatedly ignored several correct explanations in this thread just as he ignored my explanation in the original thread. We all know that the bomb will explode in every frame, so let's just drop it since MacM isn't going to change his mind. As a professional physicist and a lover of science, seeing this continue hurts me, so I beg you all, let this embarrassing discussion end.
Why not? Just because you on the embankment see a different frequency of light than the pass-frequency of the filter, doesn't mean that the filter does so, too.
It depends where you are in the frame. There's no frequency ratio that applies to the whole frame. If your frame moves right and you're to the right of the source, you'll see red shift. If you're to the left you'll see blue shift. If you're somewhere else you'll see some kind of in-between shift. And the existence of the engine detector can have no effect on how you perceive the light pulse, since, assuming you're standing between the emitter and the detector, you see it before the detector does. The detector's affecting what you see would require FTL communication.
Certainly but I specified I was standing watching the train and flash arrive. Never said it did. I said the embankment see the flash as blue shifted but red shifted to the detector since the flash has a relative velocity to the detector jof Vr = c - v. Do you disagree with that? Never said it did. Do you disagree that the embankment (as specified) sees the flash as being red shifted to the detector?
Partly. You could argue that since the flash has a relative velocity to the emitter that that causes a blue shift. There are several ways of looking at it, as I've seen in this thread. You can consider that if you don't intercept the light, there is no shift since the source an detector are not moving w.r.t. each other. Or you could imagine sticking out a re-emitter in the embankment frame, which detects a blue shifted pulse and re-emits it, which is red-shifted back to its original frequency as it hits the detector. If you look at the relativistic Doppler formula you'll see that putting -v instead of v into the equation gives the inverse ratio, so using the re-emitter gedanken: the re-emitter is moving towards the source at v and away from the detector at v, giving the total frequency shift ratio (as seen by the detector) of 1.
Blue shifted to an observer watching it arrive toward the engine. However, the "c - v" fact of relative velocity between the flash and the detector in the embankment frame mandates red shift. So I would say that this thread has not been a complete waste then. I would accept that but that isn't what was proposed. An observer on the embankment is not a re-emitter. The issue was strictly the view of the embankment observer being that the light was red shifted (Vr = c - v) to the detector and should not therefore be able to trigger the bomb.
Considering the number of off topic and unrelated comments this thread has generated by supposedly educated persons, it will be interesting to see if they can (or dare) actually address the issues with yes or no responses. Lets approach this step by step. *********************** Thread Test ********************** 1 - Does the flash of light have a relative velocity to the detector(s) in the train frame of v = c? Yes or No. 2 - Can we equip the detectors with an optical frequency filter what will only accept light from the train source which is not red or blue shifted? Yes or No. 3 - In the embankment frame does the flash of light have a relative velocity to the caboose of c + v? Yes or No. 4 - In the embankment frame does the flash of light have a relative velocity to the engine of c - v? Yes or No. 5 - Does c + v or c - v represent a doppler shift which also represents the reason for relativity of simultaneity? Yes or No. 6 - Can you seperate the claim of relativity of simultaneity from the doppler shift of the light in the embankment frame? Yes or No. 7 - With optical filters are in place can light that is doppler shifted relative to the detectors detonate the bombs? Yes or No. Please do not elaborate or qualify your responses. Just answer Yes or No. ******************************************************** This will narrow the issue and disagreement of answers can be posted
Yes. Yes. Meaningless question. Either you're in the caboose frame or the embankment frame - not both. As for 3. No. Yes. No. (If you want explanations for my answers at some stage, just ask.)
James R, First thanks for almost responding as requested. I'll skip the agreements and concentrate on your other comments. So your response is that you cannot measure a relative velocity between the light flash and the caboose? Ditto for #4 relative to the engine. I guess that says it all. Yours is not a physics response but a guarded reply in hopes of misdirecting or avoiding the question. Clearly the light flash has a velocity of "c" in both frames and in the embankment frame the caboose has a velocity of v(+) toward the flash while the engine has a velocity of v(-) or away from the flash. The relative velocity in the embankment frame between the flash and the caboose is c+v and to the engine is c-v. These are doppler shifted values. If you deny that then you are simply mistaken or lying to CYA. See below. Your replies raise an interesting issue. If in the embankment frame there is a relative velocity other than "c" between the light flash and the detectors and that relative velocity is not the same as relativity of simultaneity then we now have a third condition of light to detector and detonation timing. I suggest you reconsider your answers above. Good. Then we agree on the posted claim and purpose of this thread. No need thanks.
Well, that's where the magic comes in ... the emitter is just a thought tool to see how the light 'looks' in the embankment frame. It doesn't change anything - since relativity is not an emission theory, what frame the repeater is in makes no difference whatsoever, so long as, in that frame, it reproduces the frequency it receives. If you don't want to consider the re-emitter, it gets a bit fuzzier, but you can still say that the source is chasing the light pulse => blue shift, but the light pulse chasing the target => red shift, so there's more than one in the embankment frame. The idea of a re-emitter just helps to quantify both so you can see that they do, in fact, cancel out.
