How to get interdisciplinary when your program isn't When I was shopping for graduate schools, I was very interested in a particular subdiscipline of mathematical logic. I got into a pretty good state school, but now that I've been here for two years, I am no longer sure that I want to do research in that field anymore.  I still think it's cool- I could fuss with it all day, and I'm sure I could write a thesis on it- but I have lost faith that it is useful. I want to do something that is good for society, and this is decidedly too pure.

We don't have to pick an advisor until the end of this year. I've looked around, but all the professors seem to be working on very narrow problems that don't interest me.  What I would really like to do is integrate computer science into my work, since programming has always been one of my most enjoyed hobbies, and since it plays well with logic in general.  But nobody is doing that here.  Unfortunately, due to family issues, transferring is out of the question.

How does one go about making his research interdisciplinary when he is not in an interdisciplinary program?

Is it a bad idea to just pick an advisor that I get along with, even if his research is not directly applicable to mine, and try to carve my own path? What can I do to make this approach successful?
You need to pick an advisor who will support your interest in going outside of the area of his own work.  More likely than not, that means somebody who you both get along with well and who is tenured, so that they are not feeling pressure to "get work out of you" towards their own career goals.  Be up front about wanting to expand your area of interests.  You may, in fact, find a professor who is interested in supporting you in this, because they are interested in expanding their own interests, and the best way for two professor to begin collaborating is often to be co-advising a student who stands between them.  

Which brings me to my next point: you are likely going to need to start connecting with professors in the other department where you are interested in working as well.  Start sitting in on seminars, get to know people, and see where you can find a connection.  You might or might not end up needing formal co-advising, but you will certainly need to have at least informal mentors who you can go to for advice in the other discipline.

What is certain, however, is that you need to be very proactive about searching out your interests and taking on risky projects that will let you "get your feet wet" and figure out how you relate to the other field.  You are also likely to take longer to graduate.  Interdisciplinary research is a much harder path, but also potentially a much more rewarding one.