Why do strings and headphone cables spontaneously wrap around each other when placed in proximity?

Submitted by sfsolarboy t3_xt82za in askscience

It seem as if some invisible force pulls them together but I know that gravity is too weak to be the answer. I've always wondered this but today when my ear bud cables twisted around each other it annoyed me enough to finally ask. Of course I've asked the search engines, phrasing the query in many different ways, but can't seem to find the answer to this exact phenomenon. It's like some malicious magical force that exists solely to bedevil us (no, I don't believe in magic, but I like the song).

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BurnOutBrighter6 t1_iqpsh4f wrote

As others have said, yes it's due to entropy. But I want to get to the "why" a little more.

It's probability. There are thousands, millions, maybe infinite different tangled states a cord can be in, and exactly one non tangled state. Being tangled is the "natural state" simply because it's by far the most likely state.
Every time you put the cord in your pocket and scrunch it around, you're picking another random configuration. Since "untangled" is vanishingly unlikely when picking randomly from all possible configurations, it "spontaneously" comes out tangled ~every time.

The exact phenomenon you're looking for is Spontaneous Knotting of an Agitated String by the way. Here's the landmark paper on it:

Spontaneous knotting of an agitated string - Raymer and Smith 2007

Math blog explaining the above paper.

Video of Raymer doing a talk on the topic.

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danby t1_iqtgbe9 wrote

> It's probability. There are thousands, millions, maybe infinite different tangled states a cord can be in, and exactly one non tangled state

This can not possibly be true. Or rather I feel you're conflating topology (does or doesn't contain a knot) with physical pose (the real physical position of a string).

Consider protein folding. No protein forms true knots when they fold but we know there are [nearly] infinitely large number of states any non-trivial protein chain can adopt. And so it must be with regards pieces of string; there are surely an infinite number of unknotted poses a string can adopt and an infinite number of poses which also contain knots.

So yes there is only one topology that has no knots (by definition of the problem) but a string with that topology can still explore nearly infinite numbers of physical states. And this is somewhat reflected in the paper you've linked. In their tumbling experiment their string only knots about 33% of the time. Because there is a very, very large set of unknotted physical states that can be explored.

In the paper they also describe the sequence of braiding steps that generates knots, As this is an ordered set of moves this surely indicates the knotting can't be a purely entropy driven process. Certainly whatever energy surface the string is moving over can't be purely smooth and downhill. Which is likely also why in their tumbling experiment only a third of the tests results in knots, as there is some manner of "energy" barrier that must be surmounted to get to a knotted state.

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Virukel t1_iqptfyn wrote

Ah!! Quick! He's getting to close to the secret rules that run the universe, like where the second sock in the dryer goes! He's on to us!

Joking aside, I both love that you asked this, and the responses that are posted. So much of the world we take at face value, "Because that's just how it is." I love the moment when a person decides to stop accepting it as a given and asks why a thing is. At worst the world becomes a little weirder, as the limits of human understanding become a little clearer, but often I've found you get a fun little bit of knowledge that hardly anyone bothers to go for.

Kudos!

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Crowonthepost t1_iqpo3rw wrote

I had always heard this referenced as a case for entropy. Their natural state is to be tangled. We impose order on them by making them straight. Left unattended though they will always move towards disorder.

I understand your question was more about the physical forces at work. But I always thought this was interesting!

Edit: autocorrect

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BurnOutBrighter6 t1_iqprns7 wrote

Yes it's a case for entropy.

>Their natural state is to be tangled. We impose order on them by making them straight.

Even more than this, the "why" can be explained with probability. There are thousands, millions, maybe infinite different tangled states a cord can be in, and exactly one non tangled state. Being tangled is the "natural state" simply because it's by far the most likely state.

Every time you put the cord in your pocket and scrunch it around, you're picking another random configuration. Since "untangled" is vanishingly unlikely when picking randomly from all possible configurations, it "spontaneously" comes out tangled ~every time.

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Helios4242 t1_iqpo84j wrote

The honest answer is entropy. There's no guiding force, but things being "ordered" and "nice" are unlikely to happen by chance. If you leave a house unattended for centuries, what happens? They turn into ruins. Order requires upkeep. So when cords get moved around and aren't bound, there are more ways for them to get tangled than to stay untangled. It's just probability.

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sfsolarboy OP t1_iqpzybr wrote

So far none of the replies have explained why two pieces of string when dangled next to each other will twist together before one's very eyes.

Just saying "entropy" feels like a philosophical answer but I was hoping to learn what is drawing the electrons toward each other. What is actually happening?

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_mick_s t1_iqqh4ux wrote

What others are talking about is more tendency for strings to tangle, but I think you're talking about something different.

I think what happens is that wire is already twisted internally so it wants to untwist, but since there's another wire next to it that's also untwisting they twist around each other instead.

It might help if you make a photos of what you're describing.

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breadandbits t1_irqh1yf wrote

I think this is correct. It would be dangerous for this to occur with climbing ropes, so they are made with equal spirals in opposite directions so that stretching does not impart twist. It can be referred to as being torque balanced. Stranded electrical wire is generally twisted in the same direction, so every time you and/or gravity pulls on it, it twists. You’ll notice that your headphones always twist in the same direction.

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Goobamigotron t1_iqsatdi wrote

When they are dangling freely they turn around each other using ambient physics, where Their weight is so much lighter than everything around them. The chances that they dangle in parallel are nearly 0 so they spiral around each other.

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NonSequiturSage t1_it6eg41 wrote

Only an amateur scientist here.

Some knots come loose accidently, others are difficult to untie. Look to the design.

Luck can appear to be made by those who observe and plan carefully.

One can be amused or bitter about perceived perverseness of the universe, or the luck of an individual.

Is science knowledge like an onion of indeterminate thickness?

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