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 :)

I was in your shoes approximately 7 years ago when I graduated with a degree in a biology-related field. I am now studying this as a Ph.D. student, but encountered similar challenges of not even knowing the state of the field at the time of my graduation. There are some ways to overcome this at various stages of your pre-graduate and post-graduate career.

1. Volunteer in a physics lab, a related engineering lab, or even expand your search to tangential fields like biophysics and even math. The point is to get you exposure to an actual research environment. You might look up questions related to picking a good lab to get experience as an undergraduate, and how to land a spot. Many undergraduate programs maintain websites where professors post openings in their labs.
2. Find a mentor, for instance one of your physics professors. Let them know that you are interested in pursuing this as a graduate student, but that you feel you do not know where to begin. Many people will be very accommodating. If you join a research lab (point 1), your advisor there can also serve this function. They can help break down research papers, assuming you've made a good attempt on your own.
3. Take seminar classes where you review research papers under the guidance of a professor and/or teaching assistant. This would be an excellent place to ask very basic questions and get a solid foundation.
4. If you are like me, maybe you will graduate with slightly too little experience/knowledge and decide to work full time as a technician or assistant in a lab for one to a few years. This can actually be a good setting to see what it is like to have to get up and commit a whole day to research for an extended period of time, in spite of failure, in spite of crummy co-workers, or whatever the situation might be. Many graduate students who come straight from undergrad go through a difficult period of acclimating to full-time research, which can often mean quite a bit more than 40 hours/week. You will also get to see how academic research labs work or do not work. As a technician I spoke to all the surrounding labs to learn about their research and get a sense for how their labs worked as small businesses. For some people, doing this cements their interest in graduate school, and for others, they recognize that it might not be a good fit for them, or maybe they decide to go to graduate school for something else.

In the end, like fish in the sea, there are more research topics that would interest you than you could possibly pursue in one lifetime. Thus, the point is often just to find one that suits you and pursue it fully. Then there are the techniques that you use. As you gain experience, you will find that even within one field, there are different techniques for studying the same thing, and maybe you prefer that over something else. I like applying machine learning and high performance computing to my research questions because it intrinsically interests me, gives me options for work outside of academia, and has not been applied much in my subfield, which limits chances of getting "scooped," but did not figure this out until my second year in graduate school.

I also found out that I really like hardware programming, DAQ, and design work, but had no background before graduate school. Thus, graduate school is also a place where you learn about yourself, and you do not necessarily need to have all of the answers - just enough to be dangerous. You can enter with an open mind because it is mostly a learning experience. You need to publish, and if you want to stay in academia, maybe publish an "important" (read: widely cited; read: popular) paper, but you will also likely post-doc, which is another stage for personal growth and exploration.

Physics is also widely generalizable to many areas, from biology to social sciences and finance, and if you have experience with high performance computing, even more areas, although this is not necessary. One of my thesis committee members is a physicist who is applying his training to genomics and has changed his entire field multiple times. Just work on getting exposure, keep an open mind, and good luck!

The best way to find out about research topics is to talk to researchers. Researchers love to talk about what they do, especially if you are interested. Luckily, you are in an academic department, which is a great place to interact with researchers.

Do any of your professors participate in active research? Ask them what projects they are working on. Ask them if they've always researched the same area. Ask them what they worked on as graduate students. Who taught your electromagnetism class? Go ask them if they use any EM in their research.

Are there graduate students? Ask them what they're working on.

Are there colloquia given by outside researchers at your institution? These talks are specifically designed to teach you something about the speaker's current research. You can hear about what people do. Bring a pen and paper. Write down key words to look up later. If a talk seemed interesting (even if two thirds was way over your head), hang around for a few minutes after and ask the speaker a question. Say "I'm a senior thinking about grad school. How many grad students are in your group? Are you looking for more?"

When researchers use jargon or say something you don't understand ask them to clarify. Don't be afraid, they know you're an undergraduate. They won't think you're not smart enough for asking. They'll think you're insightful and interested.

Later go look up some key words from the discussion on the internet. Don't look up the latest papers. Look it up on Wikipedia or HyperPhysics or Stack Exchange. If you're still interested, go back and ask more questions.