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Comment by ColinWright | Hacker News Reader

  > ... we still can't make up our
  > minds as to how wings work, ...

Interestingly, I recently spoke to an aerodynamicist and mentioned this, and I was severely lectured. In the follow-up he mellowed a little, but I suspect the "we" in your statement refers to laypersons who don't have advanced training in aeronautics, and yet claim to understand about flow pressure, Bernoulli, etc.

I've been convinced that the people who genuinely work with these things and study them properly, actually do know how wings work. You may not, I may not, the mythical "man on the street" may not, but there are people who really, really do.

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No, I've a masters degree in Physics, and did several research projects on turbulent/laminar flow. We have a detailed understanding of a variety of theories which correctly represent the lift effect experienced on a body moving through a flow, yet none of them do it perfectly or completely. We can implement the technology well, but not without the aid of wind-tunnels and brute-force computer simulations in order to validate our designs - had we a perfect mathematical understanding of lift - we would not need to.

Another example from Physics - quantum mechanics. We can describe the properties of a QM system, its outcomes based on inputs, the probabilities of states - yet we have not got the blindingest clue as to how it works. We have theories, but again, none are perfect - for instance see the Pilot Wave formulation of QM, which implies a deterministic universe, and isn't wrong, but isn't generally accepted - even though it describes QM systems well, for what little of it has been formulated in the last century.

Fluid flow is complicated, true - but the basic physical phenomenon of lift is very simple and completely understood: wings redirect air downwards, and the reaction force is what we call lift.

Agreed, but it's the fluid flow bit that complicates it. We absolutely understand lift in the Newtonian sense of a reaction, but wings, we're still learning about. If we weren't, we wouldn't have developed winglets and other such adaptations, as we slowly but surely assemble the theoretical particles into a coherent whole.

Yes, but your original comment in this thread is just a semantic ruse.

Saying we can't decide how wings work is not at all in the same category as saying these guys don't know how Q works.

One "not knowing" is about not knowing exactly precisely how to model something perfectly because it has so much complexity.

The other not "not knowing" is about not having even a basic working theory that doesn't use mumbo jumbo and stands up to scrutiny.

Inventing new engineering solutions to existing problems (e.g. winglets) is totally different from not understanding the situation at all.

If we weren't, we wouldn't have developed winglets and other such adaptations...

Is the development of new programming languages evidence that we don't "understand" computers? In some very abstract sense, yes, but in a more practical sense - no.

No, but I'd say it means we don't understand software.

You know, that's a really good point, I hadn't considered that maybe virtual particles might act like a gas.

Imagine if they permeate everything and are generally nonreactive, but still have pressure and viscosity (however small). Maybe the engine is working like an airfoil, displacing the gas locally and using entrainment to create a pressure imbalance with the surrounding virtual particles to cause a larger flow (like in the Bernoulli and Coanda effects). A broken analogy would be the lift to drag ratio of a wing. If it’s 10:1, then you can lift a 1000 pound glider by pushing on the tail with just 100 pounds of thrust.

Maybe a black body is on par with light pressure because the radiation is concentrated like in a rocket, but other geometries are able to interact more strongly with the quantum foam or whatever you want to call it, because they don’t radiate energy in one direction but instead excite the foam enough that it becomes opaque to radiation and can be interacted with (like what happens when an inert gas becomes a plasma). Say the thruster excited the foam so that it was moving faster on one side of the chamber than the other, then that would work like the top and bottom of a wing (where the air on the top side moves faster) and create a pressure differential. The chamber would move to the side with the highest velocity. This might sound far fetched but there’s some evidence that the wave particle duality can be simplified to particles traveling along waves:

https://www.youtube.com/watch?v=fnUBaBdl0Aw

Probably what we’re eventually going to find is that empty space is filled with these virtual particles, weighing much less than say electrons, and because of the uncertainty principle, they are spread over large volumes. So imagine the vertices being shed off of an airliner’s wing. Even though the wing is only a few feet wide, the vortices might be 100 feet wide, so maybe if you could put special glasses on, you’d see a big vortex being shed around the chamber, and if the foam weighs orders of magnitude less than air, the vortex could be really big. It’s probably even possible to tie the size of the vortex to the “lift to drag” ratio of the chamber. Now it doesn’t seem so far fetched that the engine could work when it’s pushing off such a large number of virtual particles.

Well, if brute-force computers simulations correctly represent a wing, then we do know how they work. We may need wind-tunnel tests even if we have a perfectly realistic model, because we may not know if our simulations converge.

In the case of the wing, all of the underlying physics is completely understood. It might be difficult to calculate and have complex behaviors.

By contrast, this "reactionless drive" posits both a completely unknown underlying physical mechanism and a major violation of physics-as-we-know-it (i.e. action without reaction).

It doesn't imply a deterministic universe any more than vanilla many-worlds. Sure, the pilot wave is deterministic. But when you actually measure the position of the particle? You get one of many possible results, with probability determined by the pilot wave. Big whoop.

We understand QM perfectly well in the only meaningful sense - we can predict the results of any experiment. There's plenty of arguing about "what it means" or "what's really happening", but that's not really physics in my book.

Exactly. I am constantly astounded by professional scientists who try to find a deeper meaning in mathematical models whose only purpose is to predict experimental results. Human brains work on intuition, but when dealing with imperfect models such as Quantum Mechanicns (QM), there is no intuition to be had. If Quantum Mechanics or General Relativity were perfect, we wouldn't actively be looking for a unifying theory to resolve their mutual inconsistencies.

Once we have a theory of everything, then we can begin trying to form an accurate intuition underlying the model. Until then, there is no point trying to intuit probabilistic models which are designed to predict experimental results, and nothing else.

With pilot wave when you measure (impossibly accurately...) you get the position - bit a statistically derived one. Pilot wave does imply a clockwork universe - albeit one so complex that it cannot be deterministically predicted, thus statistical QM works just fine.

Is argue that understanding the "basement" is very much physics - sure, it can err into the philosophical, but it's still definitely a "real" physical process which drives this.

Does Feynman's "if you can't explain it to a laymen in a few paragraphs then you don't truly understand it" no longer hold?

I'd really like the source for that quote, because it's dumb for obvious reasons, and I don't know Feynman to be dumb.

In fact he said pretty much the opposite to the interviewer asking about magnets in this well known video: https://www.youtube.com/watch?v=MO0r930Sn_8

Hmm. The closest I can find is this:

"Once I asked him to explain to me, so that I can understand it, why spin-1/2 particles obey Fermi-Dirac statistics. Gauging his audience perfectly, he said, "I'll prepare a freshman lecture on it." But a few days later he came to me and said: "You know, I couldn't do it. I couldn't reduce it to the freshman level. That means we really don't understand it." David L. Goodstein, "Richard P. Feynman, Teacher," Physics Today, volume 42, number 2, February 1989, p. 70-75, at p. 75

I think Feynman is drawing a distinction between being able to calculate an accurate prediction, and "understanding" things intuitively enough to describe with plain English.

That particular version of a quote is listed on Wikiquotes as unsourced, and therefore unreliable. [1] However, he did say, "I couldn't do it. I couldn't reduce it to the freshman level. That means we don't really understand it." [2]

[1] http://en.wikiquote.org/wiki/Talk:Richard_Feynman#Unsourced

[2] http://en.wikiquote.org/wiki/Talk:Richard_Feynman#Teaching_q...

I have never heard the quote with the middle section you included "to a laymen in a few paragraphs"

I have frequently heard the quote without that section, that section seems flawed in that a layman would need to have all the underlying concepts explained to them also, indicating that application of the quote would not scale, as more underlying concepts were necessary to include.

I also generally subscribe to the idea that, if you can't adequately explain something to someone unfamiliar with the idea, then you don't properly understand it yourself. But my inability to explain something to someone says nothing about whether the phenomenon "is understood" in the more general sense.

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