01 November 2013

Magic and maths

Last week we went to the "Maths and Computing Magic Show" - an illustrated talk by Peter McOwan, who has been doing magic tricks since he was 10, and is now a Professor of Computer Science in the School of Electronic Engineering and Computer Science at Queen Mary, University of London ... an interesting combination of skills.

The magic&maths shows are one of the science outreach programmes he's involved in. You can bet this catches the imagination of schoolkids - I felt like one myself, watching the tricks and hearing about how they were done and the maths behind them, and what other applications that maths has.
Do try this at home
For instance, the Princess Card Trick, first performed by Harry Hardin in 1905. In its simplest manifestation, The magician show packet of cards, normally four or five cards. The spectator is requested to mentally remember one of the card . The cards are then turned back towards viewers and the magician pockets a single card. The faces of the remaining cards are shown and magically the spectator chosen card is gone. What we saw, adapted to a powerpoint presentation, was a shot of 6 cards - then, a shot of five cards and one lying face down, torn up. Yes, everyone agreed the missing card was the one they'd chosen - amazing!

But ... though the cards in the second shot looked like the original cards - 6s, 7s, 8s in red and black - they were not the very same cards as in the first lot, so of course the chosen card was missing!

What this demonstrates is how the brain completes a pattern, how visual attention takes short cuts - the data shown (the exact cards, number and suite) is too much to take in perfectly at a glance, it must be compressed. By needing to focus on the one card, this reading of the data is even more difficult. So the viewer is tricked ... but isn't this how any magic trick works?

The practical application of what's going on in that card trick is in MP3 players. Your brain does some data compression - and the same sort of compression is at work to fit a lot of info into an MP3 data file (for the record, it's explained here). Your brain gets enough info without having to process all the data in a complete file - because it would be ignoring some of it anyway.

Peter McOwan demonstrated some other tricks, with participation of volunteers from the audience, keeping us guessing as to how some were done. Here are some links for downloading free books and tricks -

This event was part of the Inside Out Festival 2013, which was organised by The Culture Capital Exchange. The festival will be returning for a fifth year in October 2014.

One trick Peter McOwan didn't do was to saw a woman in half - but he mentioned that this trick was first performed in Finsbury Park - this was in January 1921 at the Empire Theatre , and the practitioner was one P.T. Selbit (who had changed his name from Tommy Tibbles). Selbit's version was Sawing Through a Woman (not sawing her in half), but it gave rise to the later versions. Here he is at work -
Who would have thought that "magic" has a history ... but then, doesn't everything have a history?

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