As an undergraduate, how can I explore research topics that may interest me without having a background?
This is a good question and I'm sure you're going to get a lot of "try doing X" style answers - however, I think it's important to zoom out for a second and remember that there are two forces which decide what topics researchers investigate.

The first comes from within - what you are most motivated and passionate to research, giving you a strong pull factor in that direction. Alternatively, some topics you will find totally and utterly boring, and there is a strong push factor there. When you are young, it can feel like this force is the only force guiding your learning, since you pick your elective modules, you choose which Wikipedia articles to read, etc. This is probably why you feel that choosing a good topic is going to be really important going forwards.

The second force comes from academia itself. Some topics are "sexy" and funding opens up for those topics, pulling researchers in. Other topics go out of fashion, pushing researchers away. This force is created by the hundreds of thousands of people working and funding academic research right now, and is totally out of your control. The significance of this force grows as you mature through your academic career, so while right now you probably barely feel it (beyond the fact that the electives you can choose from are decided by academics), in the future you will be acutely aware of it and how much of an influence it has on your research area. If you ever listen to someone accomplished explain their academic career path, they'll always tell you it was "unusual" because they started in X and ended up doing Y. "Unusual" career paths are so common place that these days i'd only be surprised if someone said they started working on X when they were 25 and now at 60 their still working on X. That would be unusual. Of the hundreds of researchers I have ever met, I can only think of 2 or 3 people like that.

So long story short, I wouldn't bother researching around to find other topics you may or may not be interested in, because it simply doesn't work like that - particularly in practical/applied Physics or life sciences. You will get funding and find mentors only in the topics society/academia finds most important. To go against the flow in this regard early on is to commit career suicide, since everything you do will be that much harder. No mentors, no funding, no one wants to accept your poster presentation, no one wants to publish your paper, no one asks you to review anything, etc. So from that point of view, if you want to make smart early-career decisions, look at new/expanding areas of research and find a way to make it your own. Learn to enjoy it because you are the best at it and you are energized by the progress you make, not necessarily the subject matter itself. Keep the things you love as hobbies, or save them until tenure :)