DrClaude said:
Make no mistake, the professors know very well what is going on. It is often simpler just to ignore it and focus on the students that are actually listening.
Maybe a way to counter such behavior would be to take some interesting sidetracks like current topics related to the course. That's easier in physics etc.
More important is to make it part of examinations or whatever kind of grading is used. Like a small question about those sidetracks.
In that way the students (or at least the students I've been following lectures with) could/should be encouraged to do some extra research if there's time for that. Maybe even ask some questions later on in the course if they found some spare time.
I believe this could work. The best course I've had was given by a professor that used very little printed materials. He would give out a print out of the slides he would use but that's it. Other than that he only used the blackboard and we would have nothing else.
Very often he'd wander of to whatever was relative to the discussion was.
Some other professors, I've noticed, seem to be teaching because they have to. One of those teachers was responsible for an analysis course concerning sequences and series. Because it was difficult (1st year course) and we couldn't ask questions, I stopped going to the lectures and did the whole thing on my own in the library with some extra references.(As did a few others)
This is the other side of the spectrum. This would, in my opinion, encourage the behavior described in previous posts.
Towards infinite.curve, I wouldn't say it's solely the student's fault, the teacher could have something to do with it.