Whenever I write a new blog post, I discover something new. Usually it’s a fun maths fact, but this time I stumbled upon a website for aspiring millionaires: http://most-expensive.com/. You’ll find there answers to all your burning questions about most expensive things : celebrity homes for sale (it’s Billy Joel’s Sagaponack Village Home, if you’re wondering), letters (Francis Crick’s explanation of the DNA molecule addressed to his 12 year old son) or hotel suites in Paris (Royal Suite in Dorchester Collection’s Hôtel Plaza Athénée – I’ll try to persuade my university that it’s exactly where I should stay during my next visit to EDF, my French industrial partner). All that information was almost useful (or at least provided me with a nice way to procrastinate), but today I’ll focus on a different object – a diamond soccer ball designed for the 2010 FIFA World Cup in 2010 by a local jewellery house Shimansky. **The most expensive** soccer ball, of course – let’s call it MESB from now on.

What could be better than possessing such a jewel? Possessing two of them, for the same price! Don’t run to your local LIDL, it’s not the South African week yet. However, it’s Polish week, which makes things even better.

One of the coolest mathematical theorems is the so-called Banach-Tarski paradox. I’m very proud to say that Stefan Banach and Alfred Tarski were outstanding Polish mathematicians, the latter even graduated from my alma mater, the University of Warsaw.

Now you understand the first part of this theorem’s name – but why a “paradox”, not a “theorem”?Because it says that it’s possible to double the volume of a 3D object – without adding extra points. No, it’s not a mistake, it’s not magic – it’s maths! This means that if you had MESB on your desk, you could split it into a **finite** number of non-overlapping pieces, just to glue them back together to get **two** identical copies of your ball. Please don’t try it before you finish reading though!

The Banach-Tarski paradox takes advantage of the existence of non-measurable sets. These are collections of points that we can’t measure, not because our measuring devices aren’t good enough, but because it was proven that we just can’t define a sensible measure for such a set. It’s a bit like trying to measure the total weight of these cats – you’ll fail no matter how hard you try.

The key idea of Banach and Tarski was to divide the ball in to a **finite** number of non-overlapping **non-measurable** pieces; actually, only five sets are enough (so you don’t even have to spend too much time cutting your precious ball), where one of them is the central point.

Can we do the same with MESB? No, because all physical objects are measurable as they’re made of finite collections of atoms – yes, you too. Don’t worry, I’m not going to leave you without any real-world examples.

Take a balloon with some gas inside, release the gas into a container and fill two other balloons with the released gas – this will give you two balloons with identical volume, half of the volume of our initial balloon. Now halve the air pressure, so that both balloons double their size. You obtained two identical copies of the initial balloon without adding any extra atoms! Hm, but I told you that Banach-Tarski paradox doesn’t work in the physical world… So I have to admit that I lied a bit in this experiment. Yes, new balloons have the same volume, but the gas density inside is only half of the initial density, so they aren’t **identical** copies.

In my humble opinion Banach-Tarski paradox is one of the most mind-boggling maths facts. If you think the same, please leave a comment. But if it’s too much to handle in this heat, just read about the most expensive ice cream sundae…