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Comment by Marticus | Hacker News Reader

Well, from looking at it, it's actually very predictable.

Once one arm crosses what I'll call the lower bound (at roughly 80+ degrees from the bar being parallel to the ground), the water empties so rapidly that the tube filling up is not nearly fast enough, so the pendulum tilts back to the other side.

There is also an "upper" bound on each side, which is during the filling phase. If the arm doesn't switch sides and "stops" roughly within, say, 15 degrees from parallel, the water will fill up to the point that it will become so heavy at that side that it will tip, satisfy the lower bound, and switch sides.

Otherwise the "current" side will oscillate between those two bounds until it breaches (usually) the upper bound.

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It depends on what you mean by "predictable". Chaos does not necessarily mean you can't tell where something is going next. It means you can't reliably predict it over the long term.

If you were to watch the pendulum, you might be able to reliably make statements like "it's going to tip to the right in a few seconds" (particularly when it's near one of the bounds), but you won't be able to reliably predict which direction it will be tipped 3 minutes from now, or whether it will spend 8 seconds or 28 seconds tipped to the right, or whether its next 10 motions will be RLLRLRRRLL or RRRLLRLLRL. Any tiny error you make in measuring or predicting will be magnified over and over until the error is bigger than the signal/calculations.

We're all familiar with another chaotic phenomenon: weather. Tomorrow's temperature and prevailing atmospheric conditions can be predicted fairly well, but you can't make reliable weather projections more than a few days out. It's intrinsic to the system; a tiny effect like a butterfly flapping its wings half a world away is enough to make your predictions worthless a couple weeks out.

> Well, from looking at it, it's actually very predictable.

Yes, that's it of course! you have single handedly out proven hundreds of man years of scientific study from watching a 5min video!

>Well, from looking at it, it's actually very predictable.

People are very predictable too. On the whole they walk on the surface of the Earth and don't deviate from it more than a few km.

I deviated from that 'few km' by 5,000km. I also regularly deviated a several hundred km to France with my family. My in-laws regularly deviate a thousand km to the east coast of Canada. One of my bosses regularly deviates to the southern USA.

It's a very misleading assumption to say people don't deviate far from where they live. I know here in Canada it's not only common, but considered abnormal if people don't regularly go to cottage country, frequently travelling a minimum of 100km to get there.

Just because city-dwellers naively believe the city has everything they need, doesn't mean the 4/5ths of the population that permanently live outside of large cities behave like those in the cities.

Even when I lived in the UK, it was rare to know people who didn't travel far frequently.

The claim was about deviation from the surface of the Earth, not about deviation away from some particular area on that surface. I doubt very much you have been anything like an Earth radius from Earth's surface.

> I deviated from that 'few km' by 5,000km.

Reread what you're responding to; he's talking about people staying within that distance of the surface of the earth, not people staying within that distance of their home city. The list of people who have been more than ~25 km away from the surface of the earth is very, very small.

It was a good answer to the previous post: staying within 25 km of the earth's surface (with a few exceptions) means we're all "predictable" in some sense, but we're all very unpredictable in other senses -- much like the pendulum is predictable in some ways and not in others.

I think you'll find that it varies according to how fast it's going when it gets to one of the bounds that you think you've observed. It's not as predicatable as you seem to think.

Not really, because it has hard boundaries, that is to say it can never "level" at perfectly parallel to the ground (obviously), and cannot achieve perfectly perpendicular alignment to the ground.

Speeds with such limited motion are actually quite limited themselves along the circle.

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