In this article ("The problem with physics", Tony Rothman, ABC science) the author says in the 5$^\textrm{th}$ paragraph:
For example, one needs only first-semester equations to describe reasonably well the behaviour of a gyroscope; engineers can then go off and build gyrocompasses that guide aircraft or missiles to their destinations. But if you merely ask: "At what, exactly, is the gyroscope pointed?" you are plunged headlong into one of physics' deepest questions, one that led Einstein to develop his general theory of relativity — and that, even today, has no definitive answer. I know of no undergraduate textbook that acknowledges the question."
What is the issue he is referring to?