What is the main topic of the conversation?

W: Did you hear the thunder last night?
   M: Did I? It was really loud.
   W: I didn’t see any lightning though. I thought that thunder and lightning always occur together.
   M: They do. It’s not possible to have one without the other since thunder is a direct result of lightning.
   W: Really?
   M: Yes. Thunder is produced by the rapid expansion and contraction of highly heated air along the path of an electrical discharge.
   W: In other words, lightning quickly heats the air and causes thunder. But if the lightning is there, why can’t we see it?
   M: Because it sometimes occurs very high in the clouds.
   W: I wonder whether it’s possible to see the lightning and not to hear the thunder.
   M: Yes. It’s possible if the lightning flashes are far enough away. Sound doesn’t travel as far as light.