A Very General Argument

The general arguments of the left panel can place only an upper limit on the sizes of variable sources. It is quite possible that the signals causing the variation travel at less than light speed in particular cases, so that the size of the energy-producing region is less than the upper limit imposed by these arguments. But this is a very powerful argument, because it depends only on a general principle (finite speed of light) and not on the internal details of the source in question.