When I first saw the headline, I assumed it was going to be about the CDO packing problem discussed at http://rjlipton.wordpress.com/2009/10/22/helping-wall-str

When I first saw the headline, I assumed it was going to be about the CDO packing problem discussed at http://rjlipton.wordpress.com/2009/10/22/helping-wall-street-cheat-with-theory/ http://rjlipton.wordpress.com/2009/10/22/helping-wall-street... . The basic idea is that if you're buying a Collateralized Debt Obligation, you can't tell whether the people structuring the deal have intentionally given you an unfair share of bad loans, even when you know a lot about each of the loans in particular. There's also a fairly trivial argument for the incomputability of the EMH: for each Turing machine, issue bonds that compound interest in perpetuity and pay it off if the Turing machine halts. Since the value of the bond is positive if the Turing machine halts and zero if it doesn't, in an efficient market investors will price the bonds in a way that solves the halting problem.