Bernoulli's principle

Cecil's Columns/Staff Reports
1

Hi, Cecil. I just wanted to point out, unfortunately with a lack of supporting
evidence (hoping you can find it), that recently a book was published by some
sort of aeronautics guy that explains why Bernoulli was wrong. He went on to
say that the reason that’s taught even in flight schools and physics classes
is because, just like laser light, it just isn’t entirely understood yet. He has
gotten some acclaim for this book’s contents. I really wish I could remember his
name, or the book’s name, but it was published within the past 4 months.
I heard an interview with him on NPR. In reference to the laser comment, there’s
some sort of example of our partial ignorance to what laser light is, where one can
shine the laser through a series of ‘slotted filters’ and a specific filter causes a
phenomenon that is unexpected and currently unexplainable. Anyway. I am not
a scientist, but I wondered what you might think of this. Thanks!

The original URL to the story:

http://www.straightdope.com/classics/a2_104.html

2

Anyone who supports his or her theory by poopooing another different theory is usually suspect. Why drag it in unless his or her theory lacks supporting evidence?

:eek: :eek: :rolleyes: :dubious: : :smiley: :

3

Here is the original airing on NPR

ScienceFriday with Ira Flatow

Audio Clip

4

Here is a cite of the book

Bernoulli’s Principle

5

I’m not sure if this should be considered misleading or outright wrong. The fluid equations which lead to Bernoulli’s law are well-understood, at least in the idealized cases. In non-idealized cases, you get poorly-modeled things like turbulence, but turbulence is not necessary for a qualitative understanding of lift. Even for turbulence, the underlying physics is understood, it’s just that one must deal with so many variables that it’s computationally prohibitive. As for the comparison with laser light, I cannot think of a single aspect of laser light for which there is any indication that we have less than perfect understanding.

It is true that the explanation commonly given for lift in terms of Bernoulli’s law is incorrect. The standard explanation assumes that air travelling over the top and bottom surfaces of a wing must reach the trailing edge at the same time, which is incorrect, and also generally denies the existance of downdraft (required by Newton’s laws), which is also incorrect. In fact, lift occurs as a result of Newton’s laws, just like all other motions in physics, and Bernoulli’s law, rather than being a competitor to Newton’s laws, is a special case of those laws applying to fluids.

6

You are referring to the phenomenon of interference as observed in the famous Young double-slit experiment. The observed effects are completely explainable by quantum physics, if somewhat counterintuitive. In the experiment, a laser is aimed at an opaque baffle with a single, narrow slit cut into it. Due to an effect called diffraction, the edges of this slit cause the beam to spread out. If a screen is placed behind the slit, it will show a characteristic pattern. The beam will spread out into a band, graduating from maximum brightness in the center to a minimum at each end. The strangeness begins when a second slit is cut into the baffle, very close and parallel to the first. Now, instead of the pattern of two bands as described above overlaying each other that we would expect, we get a pattern of alternate light and dark bars. But after some thought, this isn’t so strange after all. It’s exactly the pattern one might expect if light was composed of waves instead of pointlike particles. At some points along the length of the band, a wave from one slit will hit the screen at the same time as one from the other slit when both waves are at their peak. This results in a bright spot, because the two waves reinforce each other–an effect called constructive interference. At other points along the band, the peak of one slit wave will combine with the trough of the other, resulting in a dark spot since the peak and trough cancel each other out. This is called destructive interference. So far so good. Now for the real weirdness. If we are able to adjust the output of our laser so that it emits just one photon at a time, and replace the screen with a photographic film, you might expect that after firing several billion photons at the slits one at a time that the film would record a pointilistic pattern resembling the single-slit case above, since a single photon can’t go through both slits at the same time, right? Wrong! Turns out that when we actually do this, the familiar double-slit interference pattern emerges! This can only mean that each photon is actually passing through both slits simultaneously, and quantum physics tells us that this is, in fact, exactly what is happening. Simply put, QM, tells us that rather than following a well-defined trajectory, photons and other particles follow paths of probability. That they, in fact, follow every possible path through the Universe at the same time. Probability determines along which paths they are most likely to be found, with the highest probabilities being along the paths you’d conventionally expect them to take. It is because of these probability paths, that the photons in our experiment appear to pass through both slits at once.

Here, have an aspirin.

7

It’s a bit of a stretch to baldly state that “the photons follow every path thru the Universe” and “they go thru both slits”. It’s pretty well established that we can know nothing about their actual paths, we only know their starting and ending points.

Now the likelyhood of them ending at point X does depend on the state of the whole Universe, that’s the truly mind-blowing aspect. How that can actually be is really hard to explain, perhaps unexplainable. Some recent brainy folk, like Carver Mead, do have original and plausible extensions to QM to “explain” this. Too bad the explanations are inscrutable to anybody without several PhD’s in Math.

8

That’s one possible explanation. Here’s another I read earlier to today over at the JFER message board:

The author (a physicist) is talking about electrons but I think exactly the same thing applies as for photons.

9

It isn’t that much of a stretch, and is, in fact, the current most-favored explanation in the world of higher physics. I suggest you read Brian Greene’s The Elegant Universe for a more detailed, plain-language treatise of this and other aspects of QM and Superstring theory. It really is an excellent read.

I don’t know this Carver mead you speak of, but a Google search on the name tells me only that he is a pioneer in semiconductor design. Perhaps you’d care to share this theory to which you refer?

10

jeebus, he’s one of Feynman’s brightest students, AND the guy that figured out IC’s get faster and cooler the smaller they’re made (quite opposite to the conventional wisdom of the time).

From Amazon.com: http://tinyurl.com/64bra
In this book Carver Mead offers a radically new approach to the standard problems of electromagnetic theory. Motivated by the belief that the goal of scientific research should be the simplification and unification of knowledge, he describes a new way of doing electrodynamics–collective electrodynamics–that does not rely on Maxwell’s equations, but rather uses the quantum nature of matter as its sole basis. Collective electrodynamics is a way of looking at how electrons interact, based on experiments that tell us about the electrons directly. (As Mead points out, Maxwell had no access to these experiments.)

The results Mead derives for standard electromagnetic problems are identical to those found in any text. Collective electrodynamics reveals, however, that quantities that we usually think of as being very different are, in fact, the same–that electromagnetic phenomena are simple and direct manifestations of quantum phenomena. Mead views his approach as a first step toward reformulating quantum concepts in a clear and comprehensible manner.

The book is divided into five sections: magnetic interaction of steady currents, propagating waves, electromagnetic energy, radiation in free space, and electromagnetic interaction of atoms. In an engaging preface, Mead tells how his approach to electromagnetic theory was inspired by his interaction with Richard Feynman.

11

Most likely the only time you will ever see that combination of words in a coherent sentnece.

12

grg88, apologies if this is asking too much of you, but in what way, if any, does this “collective electrodynamics” differ from the widely-held theory of quantum electrodynamics propounded by Feynman himself?

As for explaning interference, there is admittedly no way to prove Feynman’s notion of sum over histories (the idea that a particle follows every path simultaneously), but it appears to be perfectly consistent with all experiments, and does succinctly explain the phenomenon of multiple-slit interference. What’s wrong with calling it an explanation of interference?

13

Well , Im’’ just an armchair physicist but here goes,

Feynman’s stuff is brilliant, but it’s based on the basic notions of probability, Maxwell and of electrons as point particles.

As you probably know, any theory that has infinitesmally small particles is going to give you a bunch of infinities, so you have to kludge-up the theory by some dippy dividing everything by infinity. Feynman and others spent decades figuring out how to get sensible results with all those infinities popping up all over the place.

And Maxwell’s equations are obviously NFG when you consider relativity.

And probability means of course things are not deterministic.

Poor Richard flailed around for most of the 1950’s trying to reconcile low-temperature and coherent physics with rather meager results.

On The Other Hand,

Mead starts off with electrons being waves, most easily demonstrated these days with superconductivity.

And with not a trace of Maxwell, he derives all the regular electromagnetic interactions from basic coherent quantum effects. And being coherent, there’s no probablility involved.

So this is a VERY different starting point. Unfortunately we now have 70+ years of books, history, professors, and students that have learned things the “old way”, so poor Dr. Carver is going to have a heck of a time backing up the Ship of Physics and getting it going up the right tributary.

Feynman’s “minimum-action” and sum over histories approach to physics was another way of redefining EVERYTHING, but he probably wisely, didnt try to rewrite all the books and retrain everybody.

14

I do not intend to hijack this thread; we can move this to a new thread if you wish. But since I see my name in this cite, I thought I would put my 2 cents in.

First of all, as I understand it, Bernoulli is the major component of shower curtain movement. However, it is terribly misleading to say that “Airplanes fly because air flows faster over the top of the wing.” That is like saying cars move forward because the wheels turn. There may be some truth in the statement, but it is not the full story.

Airplanes fly because of the Laws of Motion identified by Newton.

A wing moving through a fluid will accelerate that fluid (A=F/M; F=MA) – Newton #2. This creates an equal and opposite force on the wing – Newton #3. The fluid that is accelerated forward (in the direction of the wing) creates Drag. The fluid that is accelerated downward creates mostly Lift, but some Drag.

A mass of air is supported by the pressure of the air underneath it. A wing separates this mass of air from its support. Mass at rest {even a fluid mass} tends to remain at rest. – Newton #3. The inertia of the air that is deflected under the wing builds pressure and friction. The inertia of the air over the wing and behind the height of contour lowers pressure and friction.

The greater pressure of the air under the wing pushes the wing into the low pressure area above the wing- Lift.

So why does air flow faster over the top of the wing?

The lowered pressure over the wing decreases friction so there is minimal forward motion (drag) imparted to the air over the wing. On the other hand, the increase pressure and friction under the wing causes the air to be dragged forward. Air flowing faster over the wing is a result of Newton, not the cause of lift.

Where does Bernoulli fit in? Bernoulli’s formula is actually a derivative of Newton’s F=MA. It applies ONLY to fluids flowing parallel to a surface. Bernoulli does not address fluids changing direction. If you remember derivatives from your Calculus class, you know that Newton also gets credit for Calculus. Bernoulli is but a Vector of Newton.

Airplanes fly because they accelerate air downward: Action - Reaction. Newton.
Ron Wyett (Jefferson)

15

On the contrary, rewriting all of physics in terms of path integrals would be a pretty accurate description of Feynman’s ambitions in his younger days. Furthermore, to a remarkable extent he both succeeded in that project and persuaded everybody else to think in similar ways.

16

I learned the Bernoulli Principle when I learned to fly. Heck, I learned it when I was about six years old. It’s still being taught. When I argued in favour of Bernoulli instead of downwash in an old thread, I got my ass handed to me on a platter. So I wrote to a well-known aerodynamicist who writes for a well-known aviation magazine. Here is his reply, with a few lines of unimportant stuff deleted:

17

Then please explain to a layperson:

How can planes fly upside down?

best to all,
plynck

18

If you take an airplane in level flight, and flip it completely over, it will not fly (or at least, will not stay in level flight), since the lift which was previously holding it up against gravity will now be working with gravity. But if you then angle the engine somewhat towards the ground, you can get a little bit of lift back that way. Depending on the plane, that little bit of lift might be enough to keep it flying level. From the point of view of the pilot, who sees the ground above him and the sky below him, the plane is trying to “dive”, against gravity.

Of course, another way to fly inverted, for any plane, is to just start off with enough altitude, and just lose altitude while you’re inverted. I don’t recommend you do this for an extended period of time, though.

19

Unless of course you happen to be flying an alien spacecraft. They can stop on a dime I tell ya. :wink:

North

(just looking around…I’ll go back next door now)

20

Short Answer to laypersons with Bonus Answers for anyone interested:

Add Power, Push the Nose Up
Up is no longer up; it is now away from you.
Push the yoke/stick forward.

This causes the wing to more strongly deflect the air.
Action equals Reaction.
The Reaction is increased Lift.

Longer answer to follow.

Peace through Liberty