You did not fully follow your professor when s/he lectured on communications theory. Sampling an analogue signal at twice the highest frequency of interest, will result in zero output at exactly that frequency, as you identified. This effect is known as sampling uncertainty. However, if you sample an analogue signal of 0.9999999....*f(max) at 2*f(max), you will indeed get the desired result, no uncertainty there. The farther your signal to be sampled is away from f(max), the quicker the desired output signal will be available. If we just could figure out what will happen when the input frequency is h i g h e r than f(max)....... Trust Shannon