Students and researchers alike have long understood that physics is challenging. But only now have scientists managed to prove it. It turns out that one of the most common goals in physics—finding an equation that describes how a system changes over time—is defined as "hard" by computer theory. That's bad news for physics students who hope that a machine can solve all their homework problems, but at least their future jobs in the field are safe from automation.So there you are! :)
I'll update this when I have the exact reference to the PRL paper.
Edit (3/22/2012) Here is a link to a review of this paper and the exact reference to it in PRL: