This is a pretty hard question David. Not just in science but in essentially all of the working world, you will find that people become specialized in certain areas. As you get older you will see that everything in the world is infinitely complex; whether it be the countless ways wood can be fastened together by carpenters or the way plants can be grown, planted, and bred by horticulturists, or of course in science. Anywhere you look (it doesn't have to be science--it can be anything, cooking, brick laying, painting, butchering, etc.) the deeper you investigate the more complex and varied the thing will become--and so will any work associated with it.
This is certainly true in science. No matter what kind of science you pick, the deeper you look, the more you find, and you can't know it all, so you have to focus on just one or maybe a few aspects of the field. This is true in anything. Say you wish to play a musical instrument. You will likely end up focussing on just one or two. It would be impossible to know how to play every instrument well. If you want to be good at something you will have to concentrate your energies on that one thing. If you don't mind being mediocre, then you can spread your energy around and be moderately good at a whole bunch of things, but you will not find success in any one thing that way. You will not rise above all the others practicing in those fields. It would be very rare for an individual to be good at everything. This is why people end up specializing.
At best you might be able to find two fields that you can combine, such as biology and physics so you may work on the physics of cell membranes lets say. Or chemistry and biology so you can work on biochemistry of DNA binding sites, for example. Or physics and chemistry so you can work on the physics of transient intermediates in chemical reactions. There are hundreds of examples. One that comes to mind is the sense of smell. Nobody yet knows how it works, but one theory is that it's a combination of physics, chemistry and biology, in that the smell receptor is like a microscopic tunnelling electron spectrometer that can detect the physical vibration of parts of molecules. Nobody has proved this because it would take someone with all three areas of expertise (biology, chemistry and physics) and this sort of person is rare, or does not exist. Maybe that could be you (but it would be a huge amount of hard work).
You are still young and you have lots of time to explore many things. My advice would be to go to university and take courses in lots of different sciences to begin with. You may find after a while that some are simply too difficult because of the unbelievably hard math or gross yucky experiments (grinding up the brains of rats), or you may find that one just grabs your interest over the others. There's lots of time. I would encourage you to change courses, drop courses, try new ones, get summer jobs in different science labs, quit them and try other ones. There's lots to experience and learn before you decide. Don't be scared to quit one thing and try something else.
As far as science careers that *do* involve all the sciences, you can make any science be like this, but it's very hard work. I recently interviewed a paleontologist (a guy who digs up dinosaur bones) who is like you. He is trying to know chemistry (so he can understand the chemical process of fossilization), biology (so he can understand about the life forms in the world these ancient creatures inhabited), physics (so he can understand the geological forces that moved the rocks around where the fossils were found) and now even microbiology and genetics (because he's feeling that the truth behind our origins may be found at the microscopic level of genes). You can read what I wrote about Robert Carroll. He works so hard trying to know all the sciences he hardly has time for anything else.
I suppose there is another way to cover all of science. Become a science writer. However, before you can do that you have to spend many years studying and working in science, or else you cannot write about it with much authority. Also, be warned: as a writer you will probably not end up doing any real science because you will be too busy trying to learn all the different aspects of science so you can write about it. There's not enough time to do both.
In the end, the problem is time. As an adult you will have to make a living, and you have to work to make a living. To make good money, you have to be good at what you do. And to make a lot of money you have to be one of the best at what you do. The only way you can achieve that is by focusing and studying and practicing and working at your chosen specialty. If you do that, you don't have time for anything else. So that is the crux of the problem: time. But right now, at 15, you have tons of time. So my advice is spend your time doing lots of different kinds of science. Spend ten or twenty years at it. After that you will know the answer to your question.
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