I have a whole backlog of questions here and my answers to many of them are far too snappy to justify a full post, so I’m going to economise and do a three-in-one. HOLD ON TO YOUR BUTTS.

Gerry asks

I’ve noticed that when I have a string of some sort (be it a usb cord or an audio cable that I use to plug in my phone to my aux in my car stereo, or any other string-like item) it has a tendency to tangle. This happens all the time (depending on the environment) is there a branch of physics or science/math that investigates this phenomenon? (Chaos maybe?) Not to be confused with the Quantum physical term of entanglement or spooky action)

There absolutely is (well, kind of). It’s called knot theory, which is one of those things bored mathematicians do in their spare time. You can keep a string straight and unknotted if you don’t allow it to cross itself, or cross with other strings (usually by tying it up in a coil). As soon as it crosses, though, you put it in a mode where it can enter a knot state, and since there are vastly more knot states where the string is tangled (trillions) than there are knot states where it is straight (one), the odds of your string remaining unknotted are trillions to one against. All it takes is the slightest movement to instigate a transition through various knot states, and since we’re talking about stuff like tremors transmitted through floorboards, changes in the temperature of the surrounding air causing the string to expand/contract, your cat sleeping on them etc. etc. it’s impossible to avoid this without physically containing the string as in the coil example. Basically, as soon as your string/wire/cable/whatever starts moving it’ll start shifting through knot states. Strings are always moving. Strings always knot.

Joshomon asks

This is a map of meteor craters: http://www.scienceclarified.com/landforms/images/ueol_02_img0082 Two features spring to the eye: the clustering in Europe, America and Australia, and the lack of impacts around the equator. The first should be easier to explain: more densely inhabited, developed countries. But it’s not entirely satisfying. Meteor impacts are measured on a long timescale, and the period in which America and Australia in particular have been developed is short. Presence of geologists and astrophysicists is also interesting, but frankly impact craters are quite big, so I can’t see a reason why eg Chinese impact craters would be obscure. The lack of equatorial impacts is also interesting, There’s only one impact crater between Khartoum and Rio de Janeiro, and even accounting for the lack of population and scientific community that seems very unlikely. So what gives? Are meteor impacts that clustered, or is there an obvious factor in the myopic recording that I’m missing? In either case, why?

 It’s a combination of geographic bias, crater erosion and camouflage. This is a better map of known impact craters. Zoom in on South America. You’ll notice that the south of the continent has plenty of craters, but the north, covered in rainforest, does not. That’s not to say that the craters aren’t there – they almost certainly are – but out chances of finding them under that dense jungle canopy are practically nil. We know of few impact craters in Russia despite it being the largest country on Earth because a large portion of its land area is made up of sparsely-populated taiga; even when we know the rough location of an asteroid impact we can’t find the precise spot where it struck because the place is so sodding huge. I’m less up on my Chinese geography but I imagine there are similar reasons why there are few known impact craters there as well.

 Meanwhile, the clusters in America and Australia are easily explained away by the fact that these are the geological Meccas of the world and are positively teeming with archaeologists, geologists and other scientists¹ who are combing the landscape for interesting features. Impact craters aren’t always easy to find despite their size, either; classical crater-shaped craters are incredibly rare, and the passage of time (and the size of the impact) is more likely to make your crater look like this – i.e., nothing like an impact crater. They take time and effort to identify, which is another reason why we’ve only found the low-hanging fruit so far. That impact craters are clustered over certain areas of the Earth’s surface merely reflects the immense difficulty in finding them rather than a real phenomenon, and our current data isn’t even remotely representative of the full picture.

Taz asks

Last night I was watching ‘Contact’ (http://www.imdb.com/title/tt0118884/) and was wondering what your thoughts were on this film. I might suck at search because I’ve found no article about this on your blog and I am curious. Both in the film being film and how many errors they have made. And the philosophical question behind the film. I hope you’ll write an entry about that film!

Contact is a bit tricky to talk about because (wisely) the film is mainly dealing with the philosophical question of talking to aliens rather than the scientific question of how we’re going to achieve this. It’s based on a book by Carl Sagan so most of the core concepts are (annoyingly) fairly sound, and the only actual mistakes it makes are nit-picky ones that can be put down to artistic licence.The one thing I might draw the film up on is the idea that the television broadcast of Hitler addressing the 1936 Olympics is going to make it all the way out to Vega, 26 light years away. You could get a coherent signal out that far if you focused it into a directed beam (probably), but the thing about television broadcasts is that they are broadcasts – i.e., they are omnidirectional and spread out in a sphere from the point of origin. This means that the strength of the signal is going to be fading exponentially the further away it gets from Earth and at 26 light years distance it is going to be very faint indeed, to the point where you’d need absurdly sensitive detection equipment to distinguish it from normal background radiation. I don’t think it’s very likely it’d survive in a form where an extraterrestrial species could find it, make sense of it, and beam it back to us.

Otherwise most of the film is out of my purview – the wormhole device is alien Star Trek technology so I can’t deconstruct that, and aside from a woeful misunderstanding of the numbers involved in the Drake equation the bits and pieces of astronomy that make it through into the movie are reasonably solid, which gives me little to talk about. Sorry!

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