I also work on this field so I do have an idea of what's possible if you apply machine learning techniques to improve relevance rankings. But to my intuition,

I also work on this field so I do have an idea of what's possible if you apply machine learning techniques to improve relevance rankings. But to my intuition, basic search doesn't feel like a machine learning task. After reading some of the responses to my post however I'm trying to come up with a meaningful reason why I wouldn't consider IDF to be machine learning, given that it is updated as more documents enter the corpus and your system "learns" to re-rank existing result sets based on these new documents.