exp(i k.r) ~ A travelling wave ?? I am studying electrons in solids and the wavefunctions of free electrons, which have the form exp(i k.r), are said to be representing travelling waves. Isn't there supposed to be a term involving time in the exponential such as exp (i (k.r -wt)) so that it is a travelling wave? If exp(i k.r) does not represent a standing wave, then what represents a standing wave?