I get asked that question frequently. Whenever I do an outreach project with either high school or college students, or even sometime even when I talk to the general public, I often get asked the question on what I think was the most important thing that I learned in becoming a scientist, or in this case, a physicist.
Now, I've mentioned a while back that I believe that the transition from being a "student" to being a scientist is when you learn the difference between what is "interesting" and what is "important. This is because what is interesting need not always be important, and making that realization is the first step towards becoming a scientist. So certainly, that realization and the ability to distinguish between the two is an important lesson in becoming a scientist.
But is that the most important thing that I've learned in becoming a scientist? When students asked me that question, they often expect that I would say that learning quantum mechanics, or electromagnetism, etc. would be the most important thing. So it comes as a surprise to many when I told them what I believe is the most important thing that I learned in becoming a scientist: Learning To Learn!
One of the things about being a scientist is that we always have to learn new things all the time! There's always something that we haven't heard of, something that is new, something we have never quite fully understand, or something puzzling. You are always faced with tying to find out about something. What we have acquired along the way, starting from undergraduate years to graduate school to postdoctoral work, and even through our early careers, is the ability and skill to learn. I'm not just talking about reading a book or paper and trying to understand something. I'm talking about knowing WHERE to look, WHO to ask, WHAT do I need to do to understand that, HOW do *I* understand something? We all work in different ways. Knowing how I, personally, comprehend something is very important, because I have consciously tried to discover when I can make something click in my head, and when it can't. How the material is presented, how I organize my thoughts in my head, how I work things out on paper, etc. are all my own personal preferences and skills that I know help me to understand something. In other words, at some level, I know what makes me tick and how I can grasp something. To me, this is the most valuable and important thing I learned in becoming a scientist.
So why is this the most important thing I learned? Besides the fact of what I've mentioned earlier about scientist always having to learn new things, it is also an important factor career wise. Not many of us are lucky enough to know what we want to do, and get to do it. Often, we have to make career changes, often changing field of studies due to one reason or another. Switching fields is not as uncommon as one would think, and in such cases, you definitely are faced with new knowledge to understand and comprehend. This is where the ability to learn becomes invaluable because in many situation, you are almost starting from scratch. This is where the skill that you used to obtain that PhD could be employed to get you up to speed in another field. The subject matter may be different, but if you've honed your skills properly, it is the same set that you will need to use to learn the new subject matter. It certainly happened to me when I switched from condensed matter physics to accelerator physics.
So yes, learning how to learn is the most important thing I learned in becoming a scientist.