Sorry if this is a stupid question, but I'm rusty on this and none of the
H&R or Serway level books give me a good answer. My students asked, "If
free space is totally empty, why does it have impedance [of 377 ohms]?" I
was about to answer that free space isn't really empty and launch into
something about a sea of virtual particles, but then I thought I'd better
ask the list first.