We always knew Magic: the Gathering was a complex game. But now it's proven: you could assemble a computer out of Magic cards.
There's an idea called "Turing completeness", which is used to indicate that a system has a particular degree of complexity. Any Turing-complete system is theoretically able to emulate any other. One way to show that a system is Turing complete is to make a "Turing machine" in it. This isn't the only way of demonstrating Turing completeness, but it is one of the more understandable. In the discussion on this site I assemble a Universal Turing Machine from Magic: the Gathering cards.
Normally, yes, it does. But occasionally in normal gameplay you get a sequence of three or four events in a row that are forced to happen by the cards and the rules of the game. The machine below just extends this idea to millions of forced choices in a row.
The idea of my Magic Turing machine is that the players do nothing at all, except when the game offers them a choice.
Once the in-game "machine" has started, processing continues without requiring any choices from the players, with one category of exceptions: Some of the cards in the machine say "You may [do X]. If you do, [Y happens]." In these cases, the machine arranges that the players will be able to do X, in precisely one way. It just requires the players to always choose to take the game up on any options they're offered.
(For discussion of the possibility of removing the "may" cards, see the Future Directions page.)
I'm glad you asked! On this website you can see the full list of all cards required in the machine, and an explanation of how it works in detail. You can also find out more about the person who created this monstrosity. Have fun!
6th November 2017: Translation into Japanese
Keisuke Kishimoto has translated this site into Japanese! So if you'd find it easier to follow reading in Japanese than English, head over to http://d.hatena.ne.jp/kkishi/20171104/ and enjoy. Many thanks, Keisuke!
20th September 2012: Version 5 Has Arrived
With all the recent attention from the Internet, it's been brought to my attention again how the (2, 3) universal Turing machine used in versions 1-4 is only a proper universal Turing machine in very specific circumstances, involving an infinitely long nonrepeating section of tape, which the combo failed to deliver. This spurred on my efforts to fix this by making a version instead of the (2, 18) universal Turing machine, and today, the results have gone online. Enjoy.
16th September 2012: Hello, Internet!
It seems the Magic Turing machine has been getting a sudden burst of online attention. So if you like to read other people's thoughts, you could go and read about it on: Reddit (2, 3), BoingBoing, Kotaku, Metafilter, Slashdot, Mark Rosewater's blog, or the Wizards of the Coast forums.