[__]

Yes, I was being imprecise by talking about "lack of interest". It's not only lack of interest per se; there are many reasons why someone might not think deeply about a subject. Lack of mental energy due to daily responsibilities is, I'm sure, quite common -- even for geeks.

And it's even more nuanced than that. Someone might be very interested, on an absolute scale, in some subject. But as long as he's even more interested in social reassurance, gossip, networking, status, and so on, he will prefer conversations that revolve around those instead. It's a matter of priority. Yeah, there's a latent physics geek in his brain, but when he asks you about next week's weather, his goal is imprecise small talk, not a discussion of turbulence models.