ANDREA SEABROOK, host:
Speaking of messy things all in a tangle, let's talk for a moment about Christmas lights, the garden hose, that gold necklace. No matter how carefully you put these things away, when you take them out again they're hopelessly knotted up. So this week on Science Out of the Box, we investigate the hows and whys of knots.
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SEABROOK: Douglas Smith is a psychics professor at the University of California in San Diego, and he's just published a paper on this thorny, or should I say, knotty problem.
Hi there, Douglas Smith.
Professor DOUGLAS SMITH (Physics, University of California, San Diego): Hi.
SEABROOK: So why does my jewelry knot itself up whenever I put it away?
Prof. SMITH: Well, you might call it a kind of an example of Murphy's Law. But we, you know, we have done kind of specific scientific studies to try to answer this question. We got interested in this because - actually one of my students was interested in mathematical field of knot theory. So mathematicians have kind of been thinking about knots for a very long time, for over a hundred years, but it kind of occurred to us that, you know, knots in the real world and the physics of how knots actually come to form - so annoyingly often, you know, in life - hadn't really been well studied.
SEABROOK: So what did you find?
Prof. SMITH: Well, we basically did experiments, and we found that just a little bit of kind of random jostling motion would very quickly tend to get a piece of string knotted. And - then, there are also many, many different kinds of knots that can form increasingly complex. We put a length of string in a box and we kind of spun the box the way the drum of your clothes dryer would spin and tumble your socks. We tumbled a piece of string and we kind of watched it and asked how long does it take to become knotted, how does that depend on, you know, how many times you rotate the box and the length of the string and the flexibility of the string and this. So we did - we tried to very carefully control the experiment to understand the phenomena.
SEABROOK: And you're not just playing around here with knots? These are not - this is not sort of a Boy Scout endeavor. You're actually trying to understand, as I gather, something about more organic things - DNA, proteins.
Prof. SMITH: Well, that's right. On the one hand, you can think of it as a kind of a fun, little physics project that a sort of nerdy physicist would like to do that's how…
SEABROOK: Nerdy physicist. No.
Prof. SMITH: Oh, yes. That's - you know, we definitely are. But on the other hand, there are real life situations where you have this. And so, one of the things that we were interested in is the tendency of DNA molecules, which are essentially also strings, just very tiny strings inside your body to get tangled and knotted and sometimes that has serious consequences, like when a cell copies its DNA and is going to try to divide into two cells. If the two copies of the DNA get themselves tangled or knotted, then those two sells can't divide.
And we could try to jump to studying DNA but it's difficult. So we thought to ourselves, well, there's nobody really even understands how a common piece of string ends up getting in a knot, we better try to understand that first and then move on, you know, potentially later to look at DNA.
SEABROOK: Not so really a basic human problem. They're really kind of universal.
Prof. SMITH: People seem to have fascination with them, believe it or not.
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SEABROOK: Douglas Smith is an assistant professor of physics at the University of California in San Diego. He and his student, Dorian Raymer, published this paper called "Spontaneous Knotting of an Agitated String."
Thanks so much for talking with us.
Prof. SMITH: Thank you.