Hacker News Reader

Comment by lb1lf | Hacker News Reader

My signals processing professor apparently knew one joke, and one joke only.

"A LOT (Polish airline) airplane is about to land in New York City; as they align for final approach, the first officer notifies the passengers that those seated on the right can now see the Statue of Liberty. A number of passengers get up from their seats left of the aisle and lean over the people seated on the right to get a glimpse of the statue. Plane promptly crashes.

Why? There were too many Poles in the right half of the plane.

I'll lead myself out.

View on HN

There was nearly a disaster earlier in that flight too, when the pilots got sick from food poisoning and the cabin crew had to ask if any of the passengers knew how to fly a plane. As it turned out, one of the passengers had flown many years previously, and offered to try.

When he reached the cockpit, however, he took one look at all of the controls on the modern aircraft and realized that he wasn't going to be able to operate it. When the chief steward asked why, he replied, "I am just a simple Pole in a complex plane".

I'll follow lb1lf out.

Why did the man from Gdansk marry the woman from Zakopane? 'Opposite Poles attract'.

It has a grain of truth to it. Poland could donate MiGs to Ukraine, but other countries with US fighters could not, because Ukrainian soldiers didn't know how to fly them.

Actually not true. Many Ukrainian fighter pilots have received training in the US, where they flew F-16:s. (That is, before the current phase of the conflict.)

The hard part is actually the maintenance crews. A modern fighter plane requires constant intensive mainentance, they generally spend much more time being worked on than they spend actually in the air. And this maintenance work requires a lot of specific skills that don't necessarily translate well or at all between aircraft types.

And a steady supply of parts.

Now this made me laugh like crazy

Did we have the same professor? Mine loved that joke too.

The students in the classroom next door always knew when Prof Lipovski told his joke because everybody along the hallway could hear the loud groan emanating from our room.

Heard a varient at Berkeley from a professor of Applied Math, which you would probably no longer hear.

Generic airline, specifically Polish people were asked to move to the left side of the plane. Because "Poles in the right half plane cause instability."

(As for application in economics, economist Steve Keen, for one, explicitly models time delays and dynamics, with results worth learning from, AFAIK.)

I think all systems professors have the same joke book and there's only one page in it :)

The version my professor told was, “Why is politics in Poland so volatile? Because all the Poles on the right are unstable!”

And you know why the contour integral of Western Europe is zero? Because all the poles are in Central Europe.

(sorry)

As a resident of post-Brexit Britain, I was horrified as I read this expecting it to have the opposite punchline and to turn into an anti-immigration joke.

For those that don't know, Polish (and other Eastern European) migration was a major issue raised by some in the Brexit campaign.

A more horrific and offensive version of the joke, which I actually saw recently as output from fortune [1], adds a part about the contour integral of the whole of Europe still being zero because the Poles are removable.

[1]: https://en.wikipedia.org/wiki/Fortune_(Unix)

The article saying zeros (where the transfer function is 0) when it should be saying poles (where the denominator of the rational function is zero), right?

Controls engineer here. The author’s control theory knowledge is correct - a RHP zero indicates that corrective action will begin in the wrong direction (which the article compares well with countersteering on a bike). The technical term is a “non minimum phase system”. It’s possible this will lead to instability, but in general the long term stability of a system is determined by the location of its poles (which is more applicable to the author’s ice cream example). Poles in the RHP will cause a system to blow up.

My gripe with the article is that the author tries to wow you with some obscure technical points about a system which is unmodeled and he does not understand, to wave his hands at a vague conclusion. If he had made the same point using common English phrases that encapsulate the idea (“positive feedback loop”, “we have to let it get worse before it gets better”, etc), then it would be a lot clearer how wispy his argument is.

The real answer is Ilyushin made terribly finicky engines and people skimped on maintenance but that’s another story.

I hate to be the guy to ask you to explain the joke, but why would that make the plane crash?

I do know what a “pole” refers to, but I don’t follow the crash bit.

A little bit broader than you asked for, but:

A dynamic control system is modeled by a set of dynamic equations, usually expressed as partial derivatives. To analyze the behaviour of such a system, the equation is solved or approximated in the complex time domain. The relevant part of the solution is where the real part of time is positive, i.e. the right-half plane.

A pole is a coordinate for which the dynamic equations have no solution (y = 1/z has a pole at z=0), which results in undefined or uncontrolled behaviour.

(also see https://en.wikipedia.org/wiki/Zeros_and_poles)

Honestly, this comment better than the article.

I don't think a vague but more precise mathematical explanation of the terms zero and pole are even that difficult to understand, x has a zero, 1/x has a pole, people kind of know what that means if you look at a graph of a pole, I don't think a rigorous definition of pole is that far off - a pole of f is just a zero of 1/f.

Instead we get waffle like:

> Again roughly speaking, zeros describe mathematically how a system reacts to some input in the short term, while poles describe how a system reacts in the long term.

I know it's "roughly" speaking, but isn't it too rough?

A control system with a pole (in the control systems sense) in the right hand half of the plane will be unstable. If the system changes during operation, the pole can move around the coordinate plane.* As it goes from the left (stable) towards the right (unstable) the system will begin to oscillate as the pole crosses the Y axis.

In the joke, a Pole (person of Polish origin) was in the left of the (air) plane. When they crossed the aisle to look out the window, the (air) plane became unstable and crashed (pole in the right hand plane).

*My control systems professor loved to explain using an example of a driver as a control system. The system (car + human driver) seeks to minimize error against the lines on the road. If the driver starts drinking, one of the system's poles will move right. The car will start overshooting first, then will start weaving, before finally crashing when the driver is too drunk.

tl; dr: The system is unstable as it has positive feedback loops. You can "first day of class" think of it as the implied series 1/(s-p) -- p the pole, s the Laplace variable -- exists and converges. And in particular you want that series to converge on the imaginary axis which means it does not diverge in the frequency domain (Fourier transform). Essentially, that means that you have regions of frequencies where your system diverges/amplifies them excessively and thus breaks down: is unstable. Filters do the opposite.

P.S. The other note is that for real linear time invariant systems the region of convergence of the series/Laplace transform of the system must be positive for the system to be causal -- and thus real and implementable. So the joke could also have been modified to get a magical and unstable plane.

> You can "first day of class" think of it as the implied series 1/(s-p) -- p the pole, s the Laplace variable -- exists and converges. And in particular you want that series to converge on the imaginary axis which means it does not diverge in the frequency domain (Fourier transform)

If this was the first day of any class I took, I would have dropped it before the second day.

That makes me think of calculus in freshman's year. The first week the prof explained all maths we had learned in highschool, and then seemingly continued that same pace every week, it was rough. Especially for some of the smart kids who had never experienced learning material coming at them faster than they could take it in. The types who opened their textbook the night before the exam and would ace it in highschool got a real test of character.

That's why letting kids cruise in high school is a terrible mistake. So many school systems are uninterested in making sure everyone is challenged, especially in maths.

University is good because you'll meet lots of people who are smarter than you are.

I don't even know how kids cruised in high school and still got their diplomas.

I flunked out of high school despite getting A's and B's on all my tests because I never did my homework which was always at least 50% of our grade.

Trigonometry was so interesting to me as a 15 year old that I decided to make it my internet alias. 95+% on every test (even trig identities!), still got a C in the class, with the teacher taking me aside 1-on-1 to tell me "I'm breaking the rules to give you this C when I'm supposed to be giving you an F because you only did 2 of the 30 homework assignments."

In my highschool maths grades were always 100% based on tests.

I get the idea of motivating students to do their homework failing them when they test perfect doesn't make sense.

I like it when the homework allows the students to skip questions on the test. That way you reward the work but still let's the students catch up if they didn't do the homework.

I like what my Calculus teacher in college did. Homework was only 3% of your grade, but if you did at least 80% of the homework, then you could redo any questions you got wrong on the tests and midterm.

That's why I feel it's good to learn a bit about Laplace transform for anybody doing anything technical. It has so many applications you can hardly get away from it.

Acausal systems are implementable. They're common in signal processing and even in some rather exotic control systems:

https://www.researchgate.net/publication/253627368_Feedforwa...

Because if your poles are not in the left half [of the] plane, the system is unstable.

The weight imbalance from everybody rushing to the right made the plane unstable.

Interestingly, I recently spoke to a loadmaster who told me that left/right side weight distributions are far less important than forward/aft, to the point that for the majority of aircraft they're loading, imbalance to the left or right side of the plane aren't accounted for at all.

Yes you want the center of mass to be more forward than the center of lift. Otherwise, a small deviation in the pitch becomes hard to impossible to recover -- the system is unstable and stalls extremely easily. If that is a fighter plane of course you want that, so you make sure the opposite is true, and have a computer counteract that during normal flight.

I laughed the first time I made an airplane with the centre of mass behind the centre of lift in Kerbal Space Program.

this is the only answert to the question above and it's being downvoted? welcome to hacker news ...

I didn't downvote it, because it's not a bad attempt at an answer if you don't know the context, but the actual joke is completely different and about calculus and control systems. The right side of the plane is double meaning for the right half of the complex plane in dynamical systems analysis, which is the area where the real part of every point is positive. If your poles are on the right half of the complex plane, then the system is unstable and the output will tend towards exponential growth (going out of control) for any change in input.

The question is about the literal meaning of the joke, he knows what a pole is within the context of calculus. He didn't understand within the context of the joke what lead to the crash.

It's a joke. There is no cause within the literal context. The humor comes from word play.

After the plane crashed, one survivor was stranded in the wilderness. Miles from civilization, he cried and screamed until he got hoarse. Then he mounted the horse and rode back to civilization. Back at home, he found himself locked out of his house, since he lost his house keys with his luggage on the plane. He sat on the porch and sang various lamentations, until he found the right key and unlocked the door.

The joke relies on the fact that keys unlock doors, this is the cause, just like the prior one relies on the assumption that people rushing to one side crashes the plane.

If there are 10 comments explaining what a key is in the context of music and I add another that says "I understand what the key refers to but how was the door opened", one can only assume I lack the knowledge that keys open doors.

Sometimes people understand the more complex behaviours but somehow miss the simpler explanations, it's happened to me before.

But the plane didn’t crash because of weight imbalance - if the Poles all went to the left half of the plane, it wouldn’t have crashed, because that would make the system stable!

There are enough actual LOT crashes he can form a model on his own…

That joke is at least 38 years old, as I heard it in my controls class in 1984 :)

Reply on news.ycombinator.com