Overview. I have personally found it very difficult to write a one-hour exam that more than superficially tests students' mastery of general relativity's concepts and formalism. Almost any significant GR problem will take more than one hour to complete, and few single problems can embrace the range of issues I would like to test. I therefore depend on longer exams (even during midterms) to test students' abilities.
Take-Home Exams. I have found that having students take several 4-hour open-notes, open-book take-home exams during the course provides a much better measure of their abilities. Such an exam might consist of three or four homework-like problems that span the range of issues of interest. There are always potential cheating problems with such exams, but this has not been a significant problem with my students (and I have found it pretty easy to catch the rare cases where students who have collaborated, something I warn students about).
Time limits on Take-homes(?). I typically ask students to do a take-home exam in two sittings, one of not more than three hours and the second of not more than one hour. This makes it possible for students to interrupt the exam to come to my office to ask questions. Of course, I cannot really enforce the time limit (and I have some evidence to suggest that students sometimes bend the limit), but I have also found that if I don't set some kind of limit, students often obsess too much on problems that they cannot figure out, and end up spending far too much time on the exam. Also, having a time limit encourages students to study before the exam, rather than depending on having the time to look up and figure out whatever is needed for the specific exam questions.
Proctored Exams. Cheating and time-limit problems would vanish if all the students could come in for a lengthy scheduled (and proctored) exam time. I have found that in classes larger than five or six students, common times of such a length can be difficult to arrange, though specifying the times on the first day of class (so that students can make arrangements far in advance) does help.
You decide. You (the instructor) will have to decide what scheme best fits your comfort range, the student culture at your institution, and your schedule. My primary recommendation is that you consider various options for giving two or three exams during the course, each three or more hours long.
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Thomas A. Moore has been a professor in the Physics and Astronomy Department at Pomona College since 1987. He does theoretical research on detecting gravitational waves using LISA (now eLISA). Send him a message.