It’s difficult to get across, even to students of physics. The first thing I’d like to say is that people have the impression that all physicists are out there searching for a “grand unified theory of everything,” or “quantum gravity” or something. This isn’t even close to true. For starters, most physicists are experimentalists, working on things far removed from particle physics. Maybe they do condensed matter physics, nuclear physics, quantum optics, or something completely different. These branches of physics will probably not be extremely affected by the discovery of a grand unified theory. However, each of these fields have their own theoretical physicists. Not all theoretical physicists are working on things like string theory. They are found in all branches of physics. You may wonder, what is it that they are doing? Aren’t their respective fields “done”? Not at all. Even when we have a theory of basic physics, how these laws interact in larger phenomena can still be hard to determine. Very often, effects are discovered in the laboratory and then only later theoretically explained!
The number of physicists working on quantum gravity is about 1%. What is it that they are doing, exactly? Aren’t they all trying to find a grand unified theory? Well, not exactly. Theoreticians spend a lot of time on theories that in no way describe our world, and usually never will. They study universes that have very different properties from our own, or particles that definitely do not exist. The point of studying these things, firstly, is that they are interesting! They have very interesting properties and motivate a lot of interesting new physics. Sometimes, in these made up universes, we can find properties that look a lot like our own, even if they have other properties that do not match up. But by studying what sorts of theories have these properties, we can try to reproduce these effects elsewhere.
Theoretical physicists also study certain things an extreme amount, hoping that they shed light on far off branches of physics. The Black Hole Information Paradox is a great example that I am studying. Basically, stuff can fall into a black hole and never be seen again. Stephen Hawking showed that black holes can radiate off their mass in “hawking radiation” and eventually disappear. So what happens to that information that went into the black hole? Does it just disappear? Physicists have many ideas for what happens, although there is still a lot of work to be done, with new interesting papers coming out all the time.
Why are they studying these black holes? Many are studying them because, perhaps, by studying this black hole we can learn more about the interaction of gravity and quantum field theory. We assume that both are manifestations of one theory. If we truly understand what is happening here, we might hopefully be able to unite quantum mechanics and gravity into one coherent framework.