Subtitles section Play video Print subtitles [Professor Masur]: But I thought I would mention one other problem that arises that you can ask about polygons. You could ask is there what's called a periodic orbit? When I say orbit, that might be "start off at a point, you bounce off the walls and after some long number of times; can you always find a path that repeats itself?" So let me give one very, very classical example of that and that goes back to the square – if you start, let's say on the bottom, and move off at a right angle, 90 degrees, you move to the top. Now when you get to the top, because these are parallel, this angle is also 90 degrees, and so the billiard path says you rebound where angle of reflection equals angle of incidence, and that means that when you come to the side you will bounce off exactly the way that you came in; you will come to the side and bounce right back. You come back along this line and then you will bounce and… repeat yourself – so that's what's called periodic. There are other periodic lines. So, for example, you could start at a 45-degree angle, everything would be 45 degrees, and you would come back right where you started and then repeat yourself. In fact, if you started any point, at any angle that is a rational number – a quotient of two integers –if you started with an angle of 27 degrees starting here, you will come back after a certain number of bounces. That's true for a square; and, in fact, one can prove for any rational polygon, were all the angles are fractions of 180 degrees, or P over Q times 180 degrees, again I like to write it like that, you can always find a periodic orbit. [Brady]: From any point? [Professor Masur]: No not from any point. There's some periodic orbit, from somewhere. Some periodic orbit from somewhere. There might be; so there will be some direct… Some point, in some direction, where there will be a periodic orbit. [Brady:] For example on this side there are none, but there is one sitting over here somewhere. [Professor Masur:] Well usually what happens if… It, it… In most cases, that… That's possible; in fact you can always find periodic orbits that will hit every side. In most cases a periodic orbit will hit a side. There might be other points on that side, where the orbit in that same direction, is not periodic. So in other words, you start at a point on a side and move in a direction it might close up, meaning periodic, but if you move the point a little bit and headed off in the same direction, what could happen is that it isn't periodic; what might happen is it hits a corner and then you don't know what to do. That's different from the square. In the square no matter where I started, if I went at 45 degrees, it would be periodic, but in a general, rational polygon that may not be the case. I want to talk about triangles now that are not necessarily rational, but just triangles. If they are acute, which means all of the angles are 90 degrees or less, what you could do is: you could take the vertex of the triangle and drop a perpendicular down to the opposite side to get a point, and then drop this perpendicular down to the opposite side and get a point so these are right angles. This triangle joining those points gives a periodic orbit; meaning if I start here I go to the line towards here this angle will equal that angle and I'll bounce like that. And then i'll get to this point and this angle will equal this angle and I'll bounce like that. And I come back, this angle equal this angle, and I'll repeat myself. This example was has been known for hundreds of years. I don't think it was ever thought of as billiards, but anybody in high school or even junior high could do this in… In trigonometry. When you do obtuse triangles, you can't drop a perpendicular because if I started at this vertex and drop the perpendicular, it wouldn't be on that side. That's because this angle is bigger than 90 degrees and, in fact, for something as very simple as in a triangle, obtuse triangle, one does not know whether there are periodic orbits. So this is a famous unsolved problem in the subject of, what this is called is, dynamical systems. Well so again if the obtuse triangle is rational, then there are periodic orbits. So I'm talking about obtuse triangles. I think I gave an example – 90 times square root of two – that particular one I would not be able to tell you if it had a periodic orbit. There are some… Some reason where people, by very, very hard work, have shown where if this angle is less than a hundred degrees, then there are periodic orbits. But that was very, very hard work and if it was a hundred and ten degrees; again I'm assuming not rational; then we don't know the answer. If it's rational we always know, if all the angles, if this is a hundred and eleven, and this is 36. What's left? 33 degrees there will be. But if one of the angles is an irrational number then we don't know. [Brady:] So what, does greatness await if someone can crack that? [Professor Masur:] Well, it's… It's um… There is a very prominent, one of the most prominent mathematicians in the United States or in the world in the subject of dynamical systems whose name is Professor Katok at Penn State has listed this as one of the five outstanding problems in dynamical systems. —But really there's nothing you could do. You can try desperately to solve it, but if it hasn't been solved for a hundred years, you probably aren't going to. And you know it's only given to one person, so to speak, to solve a particular one of these problems. So we're used to it and here's an atmosphere of resignation.

B1 periodic angle orbit rational bounce perpendicular Problems with Periodic Orbits - Numberphile 1 0 林宜悉 posted on 2020/03/27 More Share Save Report Video vocabulary