a vortex pulls to the center

[miriam_e]

Why, when you stir a cup of tea, do the tea leaves all move to a neat pile in the center? Why doesn't it simply mix everything up and distribute the leaves evenly throughout the volume?

Comments

Re: nope it wasn't a rhetorical question

[miriam-e.livejournal.com]

I have a feeling it might all be due to the way the fluid circulates. If the fluid moves inward along the bottom of the cup and spirals upwards then that would push the leaves along, piling them up on each other in the center. I notice that the center of the top of the cup of fluid is lower than the edges so if the fluid is rising from the center at the bottom, it isn't going all the way to the top. There might be something like the atmospheric circulation on Earth. I can imagine the top and bottom of the cup being like mirror images of each other, with fluid circulating in at the top and at the bottom of the cup, and about halfway up the cup it moves outward. I drew a quick sketch -- a picture works so much better than words. As you suggest, I'll try an experiment. If this is how it circulates it should make possible a simple test. I need to think on it. Perhaps a clear glass and a small amount of very fine sand that is suspended in the liquid, might work...

Then, of course, is the next question: why would it circulate like that rather than the reverse, or some other pattern?

The infinitely running sprinters is an example of Zeno's Paradox, actually a family of paradoxes that all use the same concept. They come from ancient Greek times. I think I first read about it in Douglas Hofstadter's amazing book "Godel Escher Bach". He plays with a lot of wonderful, crazy concepts in that book.

Re: nope it wasn't a rhetorical question

[miriam-e.livejournal.com]

Zeno's paradox (http://en.wikipedia.org/wiki/Zeno%27s_paradox)