user1247

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May 4	comment	I read a book saying bernoulli's flight equations didn't have as much impact on lift as most people think @Mike Dunlavey, I don't see anything in your link (I have no idea why you would consider it authoritative, btw) that contradicts anything I have said.
May 3	comment	I read a book saying bernoulli's flight equations didn't have as much impact on lift as most people think @Michael Brown, in other words, there is not some magic "lower air pressure on top, higher on bottom" produced by the camber that causes lift. The main principle is simple: use angle of attack to direct air downward. As you say yourself, the Bernoulli effect is secondary, or at the very least both pictures exist. But if you invoke Bernoulli as the primary explanation, you can leave one with completely wrong impressions such that a wing with 0 angle of attack (and atypical tailing edge) that does not direct air downwards and yet could somehow produce lift by making air go faster over the top.
May 3	comment	I read a book saying bernoulli's flight equations didn't have as much impact on lift as most people think @Michael Brown, I think you over-reacted to my use of the term "ram". I am in no way advocating the "bullet picture." The use of the term "ram" is meant to emphasize the principle reason air is being directed downward. It is due to the wing's angle of attack. The camber is meant more to keep the flow laminar than to produce via Bernoulli a lower pressure on top due to air travelling faster. I think we essentially see things the same, however I think that to emphasize the word "bernoulli" at all when discussing wing lift can give a completely wrong impression to the lay person.
May 3	comment	I read a book saying bernoulli's flight equations didn't have as much impact on lift as most people think @Michael Brown, you are just plain wrong. What matters, as I said in my post, is "directing air flowing over the wings downward". This is obvious by conservation of momentum. There can not possibly ever be any other explanation, unless you don't believe in Newtonian mechanics. The camber of the wing plays a minor role, but to first order the role of the wing shape, in addition to the angle of attack, is merely to direct air downwards and thus produce positive lift by momentum conservation. Bernoulli's principle is one way of thinking about an aspect of this, but to focus on it is naive.
May 3	comment	I read a book saying bernoulli's flight equations didn't have as much impact on lift as most people think @Mike Dunlavey, actually, no, you are both wrong. To some extent this comes down to the semantics of whether you refer to my description as a dual description of a Bernoulli effect. But it is plain wrong to imply that the camber of a wing (which would produce lift via Bernoulli) is primary. This is, indeed, why a plane has no trouble flying upside-down. This is simply a fact. The camber is not very important. What is important is directing air downwards (by whatever means). What I said in my answer is absolutely and unequivocally correct.
May 2	comment	I read a book saying bernoulli's flight equations didn't have as much impact on lift as most people think If you downvote, please comment. My answer is correct and canonical.
May 2	answered	I read a book saying bernoulli's flight equations didn't have as much impact on lift as most people think
Apr 24	comment	Interference and which-path information Very nice! Thank you.
Apr 23	comment	Interference and which-path information Wait, but I thought that was what I was talking about to begin with: the natural self-evolution of the photon causes it to spread and interfere. The idler photon is entangled with the signal photon, and so if the signal photon goes through the L slit, the idler photon must go through the corresponding R slit (by momentum conservation). Therefore the idler photon carries which-path information, and by your story there can be no interference even due to the natural self-evolution of the photon's wave function. So unless I am missing something your description is not consistent.
Apr 23	comment	Interference and which-path information This looks like a really good answer, thanks! One big question that still bothers me is why, in the lone interfering photon example, it is not in actuality entangled in some way with the device used to create it. After all, the photon had to come from somewhere! Say you produce the photon from a laser -- then isn't at least one atom in the laser entangled with the photon, which would imply, by your logic, that the photon can never interfere!?
Apr 23	comment	Interference and which-path information Thanks for your edit. I will have to digest this as it is different from what I have learned before, which is that decoherence is the result of correlation (due to entanglement) between the observer's state and the environment. In the MWI, for example, I understand this as both the observer and the environment being in many states, but conservation laws require that both states be consistent with each other due to entanglement, and this selects out subsets of observer-environment combinations of pure states.
Apr 22	comment	Interference and which-path information Do you have a reference for that? I don't think you are correct for two reasons: 1) Every resource I can find (see wiki:en.wikipedia.org/wiki/Quantum_decoherence, for example) disagrees with you, seeming to require interaction with the environment in order to induce decoherence. And 2) If what you say were true then no photon would ever show interference in a double slit experiment, since it is surely entangled in some way with a particle in the past. It seems as though you are saying that the schrod. eq. does not apply to entangled particles (wave funct diffusion -> interference)?
Apr 21	comment	Interference and which-path information OK, so to summarize what you are saying: if the idler is entangled with the signal photon and goes off in another direction then no matter what there is no interference, even if which-path information is destroyed. Is that correct? Is this true even if the idler photon never becomes entangled with the environment and thus the observer? (I thought that was the mechanism behind decoherence...)
Apr 21	comment	Does entanglement not immediately contradict the theory of special relativity? @Michael Brown, furthermore, I am not unconvinced that FTL communication is impossible (I am not a crackpot but a mainstream physicist). But I have never gotten a satisfying response to my above conundrum. The general proof that you are trying to provide would only cement my general agreement about FTL communication, but would not at all address the flaw in specific thought experiment or address the conceptual confusion.
Apr 21	comment	Does entanglement not immediately contradict the theory of special relativity? @Michael Brown. I think I do understand the proof. It does not apply because the expectation value of the position measurement after the interference screen is zero regardless of whether interference takes place or not. Therefore your proof does not at all constrain FTL communication through changes in the form of the interference pattern that results from ensembles of measurements which preserve the expectation value.
Apr 21	comment	Does entanglement not immediately contradict the theory of special relativity? @Michael Brown, that is not relevant to my point. I understand completely that FTL is impossible for individual measurements. My point is that for an ensembles of measurements, the interference pattern can be turned on and off FTL, if one assumes instantaneous copenhagen collapse of the wave function. I have never had explained to me why this doesn't unambiguously show that the wave function cannot possibly collapse instantaneously.
Apr 21	comment	Interference and which-path information My point is that your current answer "To get interference, the momentum change must be indistinguishable in principle" is ambiguous. What if the inteference is measurable in principle, but I choose not to, thus causing the information to eventually be lost, long after the interference pattern has been seen or not seen. Furthermore, your answer does not address the fact that (AFAIK) loss of interference requires the entanglement between the idler photon and the environment/observer, and the fact that the idler's momentum could be observable in principle yet not be entanlged with the environ
Apr 21	comment	Interference and which-path information (continued)... in other words, I could measure which-path information, so in principle it is available, however if I choose not to measure it the which path information will be destroyed 1000 years from now. Is the interference pattern seen now or not? This is different from the quantum eraser experiment in that no information is needed from the idler photon in order to reconstruct the interference pattern. The interference pattern is either seen now or not.
Apr 21	comment	Interference and which-path information Thanks for your edit. So, the question I want to really drive home here is: is the interference pattern destroyed if the idler photon's momentum is measurable in principle, even if the idler photon is totally isolated from the environment and the observer? In other words, the idler photon can be travelling through a vacuum, not entangled with the macroscoping environment or the observer. In 1000 years it will hit a wall which will destroy which-path information. Does the fact that the which-path information is measurable in principle in the intervening 1000 years relevant?
Apr 20	comment	Interference and which-path information I'm not at all asking about an explanation for what happens when you try to detect which-path information on a single photon in the double-slit experiment. That is not relevant to this question. This question is about entangled pairs of photons. The wall in question is not the same wall that has the slits. The signal photon goes through the slits. Somewhere far away from the slits the entangled idler photon hits a wall. Different wall.