It’s like the classic chicken joke: There’s this farmer whose chickens have stopped laying eggs for some reason. He [inexplicably] calls in a physicist [instead of a chicken expert] to help solve his problem. The physicist does some calculations, and finally tells the farmer, “I’ve got a solution! But it’ll only work for spherical chickens in a vacuum.”

Both the joke and the comic are funny because in physics, the problems, formulas, etc, are based on ideal conditions (like Rob said). It doesn’t just refer to physics professors giving problems or examples in class though. There are several jokes involving this phenomena, including one that I don’t feel like typing out about finding the volume of a cow by approximating it as spherical. It’s a common nerd joke that physics problems don’t actually apply to realistic situations, while the engineers are left to sort out the messy real-world applications (you’re welcome).

When the professor wakes up, there is atmosphere in the room, but black-hat removes the air in the room when the professor wakes up.

The image text refers to another “ideal environment” where the study of waves and particles are often examined without the forces that would act on them.