I offer a frame challenge.
Of course you have every right to feel the way you do. Most people don't like talking about "work" outside of work. But really, it's only "work" if you choose to look at it that way. It sounds like people aren't asking you about Excel spreadsheets and tedious numerical calculations. They're asking you about the very mysteries of nature, things mankind has pondered for millennia and slowly come to understand piece by piece.
I also have a background in science and engineering. Personally, I wish more people would ask me questions. The view I see it, you have a golden opportunity to do two things here:
Help your community become less ignorant and more educated. Sure, helping one person understand why the sky is blue might not count for much, but you'd be doing more than that. You'd be showing people that there is a reliable, logical way in which to deduce truth, which is based on experimental observation of nature and generalization of the results. In this age of magical crystals, climate change denial and alternative facts, it's clear that most people don't have any idea how to determine truth from non-truth. Most accept as true whatever they've heard the most times, this is a significant problem in pretty much every society. Some of us have the ability to play a small role in the solution of this problem, and I believe we should.
Become the "wise-man" (or woman) in your group. Why did you learn everything you did in school? Perhaps it was just to make money, which is fine. Perhaps because you found it interesting. Either way, with the knowledge you have now, you have the chance to become someone who is helpful, appreciated and respected. Is that not a higher calling than your professional work which is "not as exciting as it sounds"?
How can I, as politely as possible, escape from these kind of situations without playing dumb?
My answer is don't. You can use these situations as an opportunity to become a respected and appreciated resource for the people in your life.
If you don't want to do this because it sounds like just more work to you, that's a perspective you might want to analyze. For instance, the question of whether earth is round or flat might feel menial to you, but maybe that's because you take it for granted. Historically it was a very controversial topic, mired in religious and philosophical conflict. This question great example demonstrating to people that though our senses tell us one thing, we can systematically deduce a truth that our instincts would have hid from us. In my opinion, the questions you are being asked are profound, and you have a chance to better live up to your potential as a human if you seek to address them. Just my opinion.
If you don't want to answer questions because you're not sure of the answers and will inevitably get stuck on some detail you can't explain, that's fine! No one has all the answers. You can always simply say "I'm not sure about that specific point, but hopefully you see the general idea." Personally, I look at it as a chance to learn. Maybe you feel like learning is a chore, some people do and I used to as well. I'd again challenge you to reevaluate that perspective.
Tl;dr You have a chance to do something with your knowledge far more meaningful than your daily professional tasks, if you choose to view it that way.