The Entropy of Christmas Lights

Our guests get into a bit of a tangle...
21 December 2017

Interview with 

Hugh Hunt, University of Cambridge

Christmas lights

Christmas lights


For most of us, it’s a trip to the attic to dust off the decorations and get to those lovely twinkling lights out. Now, you’re absolutely SURE, if you’re anything like Chris Smith, that you put them away in a highly organised, nice neat pile last year. But no! They’re tangled in one big lump. It’s not rocket science it’s just physics. Entropy, specifically, as Hugh Hunt explained... 

Hugh - This idea of tangled mess of things is disorder. If you’ve got things really nicely ordered, then so long as you’re very careful, they stay ordered. But once they become disordered it’s really hard to get them back again so, for instance, if you spill some water or if you break a plate. This idea of entropy is, it’s a measure of disorder and if you’ve got something like a gas like air, then it turns out that the equations to calculate the entropy are not too difficult. But when it comes to Christmas lights the equations aren’t quite so straightforward.

I’ve got a balloon here, I can blow it up… and the air is in that balloon, and if I’m very careful the way I let it out, I could get all the energy to come out carefully. Now if I’m careful about it I can get a lot of the energy back again, but if I blow up my balloon and I’m not so careful, I’ve just lost all the energy all at once. What’s happened is it’s a very disordered way of getting the air out of the balloon.

Chris - Can you explain to me what this has got to do with my Christmas lights?

Hugh - What you’ve got to think about is that you’ve got to be careful like with the balloon. If you want to get things back how they were to start with you have to be careful. Entropy can be measured but you can use common sense and be very careful. Wrap your lights up carefully so that you get them back to how…

Chris - Many people say that they did wrap their lights up carefully. Is what you’re saying that, basically, the whole system is that there’s one organised way to have your lights, but there are many many ways to have your lights in a disorganised way so, the odds are, that they’re not going to stay organised for very long unless you take enormous steps to make sure?

Hugh - The worst thing you can do is put more than one set of lights into the same bag because sure as…

Chris - Yes, been there done that.

Hugh - … they’re going to be get tangled up with each other and that’s the worst bit

Georgia - Well speaking of, we actually have some tangled lights here that the Naked Scientist producers have found in their house. So we thought for a fun Christmas activity we could use you guys to help us untangle our lights, and we’ll have a little competition.

Hugh - Is that the only reason you have us along?

Georgia - Yeah, pretty much.

Chris - So everyone’s getting a set of fairy lights.

Hugh - God! This one’s terrible.

Chris - Everyone is busy beavering away trying to get these lights untangled and some people are having some success. Philipe; you’re looking like you’re being reasonably successful there, you’ve got most of the knots out of yours. These genuinely just came out of people’s cupboards, didn’t they, Georgia? These were just found like this?

Alex, it looks like you’re making them more tangled up.

Alex - No. I’ve nearly done one of them.

Chris - She’s nearly got one big tangle out. Hugh’s got the worst. You’ve actually added disorder to yours.

Hugh - But the thing is that I can’t find the bloody end. And I think that’s the thing you were saying Chris, if the chain is really long then there’s many many more end states. If you’ve got a really short piece of string, it’s really hard to tangle a short piece of string.

Chris - David’s there first. He did have the shortest strand of lights.

David - Did I?

Chris - Yeah.

David - No.

Georgia - Philipe’s done as well.

Hugh - So, all I would say is that once they’re untangled you’ve got to get something like an old cereal box or something and wrap them around the outside of the cereal box. Give them something that they can stay attached to.Stuffing them into a bag, they’ll be exactly like this again next year.

Chris - So the bottom line is that entropy is all about things becoming more disorganised. There are lots of ways that the fairly lights can arrange themselves to be disorganised; there are very few ways they can arrange themselves to be organised.

Everyone’s still going at this Georgia.

And, as a result, unless you take steps to make sure they stay well organised then you’re going to have trouble Hugh? So to put them round a piece of cardboard or something which confers order on them and stops them having those degrees of freedom? We now have wrapped fairy lights that are flashing.

Hugh - I know. I saw that. You are very sparkly! It’s all very well to have a system but, if the person who finds the lights the following year doesn’t know your system…

Chris - That is true. There’s a great gift idea in this Alex?

Alex - Self-untangling fairy lights?

Chris - Fairy lights that don’t get tangled and then some system that stops them tangling up again - that confers order and reverses the effects of entropy.

Alex - Absolutely.


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