Ask HN: Going to meet one of the world's top string theorist – what should I ask?

I agree with the other commenters. Don't try to interview him, unless that is why this was setup. I have to imagine it was setup with some intent because either you have a passion for physics or something where your friend felt it wouldn't be a waste of Mr Witten's time.

Seriously, after reading his wiki page, he obviously seems like one hell of an intelligent person. But something else struck me, he's been married since 1979 to the same person who is also a professor of physics, plus they have kids that all appear to have done quite well. I'd be more curious to see how they balanced their life and were successful in raising kids that have passion and are doing well, at least from their basic bios.

Everyone probably asks him physics questions, but chatting with him about the everyday things to me could be way more interesting, and wouldn't feel interviewee. But that may just be me. I would want to ask him what he feels is his best accomplishment professionally and personally. That is always interesting to hear.

Unless you're a physicist yourself I don't see much value in asking physics questions. You'd probably get out more from any introductory text.

Ask him about his interests beside physics. Philosophy. Art. Whatever.

But most of all, don't pester him. If he seems receptive to those "big questions", great! If not, just do normal smalltalk.

If it's a friendly lunch, I would advise to treat it mostly as a friendly lunch and not too much as an interview.

He has a very interesting background -- as an undergraduate he studied history and linguistics, and began studies in economics before switching to applied math, and then physics. Clearly he has some of the "wide-eyed wanderer" in his personality, in addition to remarkable intellect. I'm betting he will direct the conversation more than you might assume, and will be able to speak with interest about any subject you might wish to discuss.

Don't worry about it -- be yourself.

Just out of curiosity, what did you talk about? (assuming the lunch is already over)

Can he concatenate a string?